A. M. Zaitsev
Optical Properties of Diamond Data Handbook With 285 Figures and 21 Tables
Dr.Sc. Alexander M. Zaitsev Ruhr-Universität Bochum, Institut für Geologie, Mineralogie und Geophysik, Universitätsstr. 150, D-44780 Bochum, Germany e-mail:
[email protected] Front cover: Electroluminescence of a comb-structured planar p-i-n diode made on a diamond substrate activated with the H3 centers. The size of the picture is about 100×100 µm2 (courtesy of Dr. A. A. Melnikov).
Foreword
The unique properties of diamond are responsible for its pre-eminence as a gemstone, and give it a glamour and attraction unprecedented for any other mineral. As the first member of group IV of the periodic table of elements, carbon, in its crystalline form as diamond, has also fascinated scientists for at least 300 years. Many experimental techniques have been employed in the study of diamond, and of these, optical spectroscopy has proven one of the most fruitful. The absorption line at 415 nm, characteristic of "Cape Yellow" diamonds, was first documented by Walter in 1891. Further work on this absorption, now known as "N3", by the Indian school under Sir C. V. Raman in the 1930s and 1940s led to a basic understanding of the system, which they observed in both absorption and luminescence. The N3 center is a structural defect in the diamond, and the absorption of light occurs by exciting electrons in this defect from one well-defined energy state to another. When the electron returns to the original energy level, luminescence is produced. Detailed studies of natural diamonds over the subsequent 60 years have discovered large numbers of absorption and emission lines, characteristic of different defects. In 1904 Sir William Crookes showed that a colorless diamond could be turned green by long exposure to radium salts. The spectroscopic study of defect centers created by radiation damage in diamond, and the way in which these centers anneal following heat treatment, began in earnest in the 1950s and continues to the present day, creating new series of absorption and luminescence lines which are not normally found occurring naturally. In 1955 the dream of converting graphite to diamond became a reality, and of the 100 tones of diamond now produced annually, around 80% is manufactured by the high-pressure, high-temperature (HPHT) route. This "synthetic diamond" or "manmade diamond" has all the desirable mechanical properties associated with diamond which makes it suitable for industrial applications, but the optical properties are quite different from those of the majority of natural diamonds. At present there is considerable interest in the spectroscopy associated with nickel and cobalt, the metals normally used in the synthesis of diamond. Here again we find a large number of absorption and luminescence lines which are not normally seen in natural diamond. Diamond can also be produced from the gas phase to produce "thin film diamond" by chemical vapor deposition (CVD); in fact, developments of the CVD process over the last 15 years now enable polycrystalline diamond wafers more than 150 mm in diameter and more than 1 mm thick to be produced on a routine basis.
VI
Foreword
Because this material is grown in a quite different way from natural diamond or HPHT diamond, some of the defects it contains are unique to CVD diamond. As a result of the intensive research carried out on all forms of diamond, particularly during the last 50 years, a huge number of absorption and luminescence systems have been documented. Some are in the regular scientific journals, others in conference proceedings or special supplements to journals which are more difficult to obtain; some work is found only in Ph.D. theses, and there is a large body of important research on diamond which has been carried out by Russian scientists but not translated into English. Recognizing the need for a single source of reliable information, Dr Alexander Zaitsev has gathered together all of the known references dealing with optical centers in diamond. This is an ongoing task; new optical centers continue to be discovered while the work is being compiled, but publication of the present volume is a formidable achievement for which the author must be congratulated. It is a book which anyone working on the spectroscopy of defects in diamond will want on their desk, and to which they will make frequent reference.
Professor Alan T. Collins Wheatstone Physics Laboratory King's College London
to my wife Inga
Preface
Optical properties play an important role in the investigation, characterization, and application of diamond. These optical properties have always made diamond so attractive in its natural beauty, and it still keeps its commercial value today. The unique optical properties of diamond are also one of the main criteria for its industrial use, being the second best property after mechanical hardness. Many outstanding physical properties of diamond make it an attractive material for optical and optoelectronic applications. Diamond has the widest optical transparency band of all known solids, which ranges from 0.22 µm (fundamental absorption edge) to the far-infrared. Only the intrinsic vibrational absorption band of moderate intensity between 2.5 and 7 µm disturbs the perfection of diamond's transparency in the infrared region. Being transparent in the ultraviolet, visible and infrared spectral regions, diamond provides many opportunities for lattice defects to reveal the optical activity of their electronic and vibrational transitions. The large bandgap energy (5.49 eV) is a particularly favorable condition in the case of luminescence, because the radiative electronic transitions require that both the ground and excited electronic states lie within the bandgap. The high mechanical hardness and thermal conductivity of diamond greatly support its optical applications making diamond optics very stable and resistant in many respects. When discussing the optical properties of a material its optical centers should be considered carefully, because their properties and abundance determine almost all optical performance of the material. Besides, the content of optical centers is the main parameter of the optical characterization of the material. So far more than 150 vibrational and more than 500 electronic optical centers have been detected in diamond within the spectral range of 20 to 0.17 µm; that is, between the vacuum ultraviolet and the mid-infrared regions. To fill up this large spectral range diamond possesses many optically active defects of various origins including intrinsic and impurity-related, point and extended defects. Both types of intrinsic point defects (vacancy- and interstitial-related) in diamond can form optical centers. Many impurities are known to form optically active defects in diamond: H, He, Li, B, N, O, Ne, P, Si, As, Ti, Cr, Ni, Co, Zn, Zr, Ag, W, Xe and Tl. Many of the optical centers related to these impurities have been created artificially using doping during growth and, in particular, ion implantation. These centers have never been seen before in pristine natural diamonds. The reason for that is the very short and strong sp3 hybridized covalent C-C electronic bonds preventing thermodynamical equilibrium (or quasi-equilibrium) incorporation of impurities (even hydrogen) into the diamond lattice. The only remaining possibility is the use of forced methods of
X
Preface
impurity insertion, such as ion implantation. Another reason for the high efficiency of the ion implantation as a method of optical activation of diamond is its inevitable creation of radiation damage. Since the majority of optically active defects in diamond are complexes involving impurity atoms bound to some intrinsic structural defects (vacancies and/or interstitial atoms), defect production is an essential advantage of ion implantation. The impurities mentioned above are not equally active in creating optical centers in diamond. Some of them, like Si, form only one optical center characteristic of the specie. Others, like N, produce a great number of optical centers throughout the whole optical range of diamond. Nitrogen is an impurity of special importance for diamond. Firstly, nitrogen is responsible for vast majority of impurity-related optical centers. Secondly, many of the most intense and most interesting optical centers for practical applications are known to be nitrogen-related. Nitrogen can form optically active defects in many ways: single isolated nitrogen atoms, multi-atom nitrogen complexes, and complexes of nitrogen atoms with intrinsic lattice defects and with other impurities. Thus the presence of nitrogen in diamond in almost any form immediately changes its optical properties. One of the consequences of high optical activity of nitrogen is the physical classification of diamond based primarily upon nitrogen-related optical absorption. An important optically related feature of diamond is its high Debye temperature (about 2000 K). Actually this is the highest Debye temperature of those known for any solids. Owing to the high Debye temperature, a remarkable excitation of phonons in the diamond lattice, and consequent electron-phonon coupling with lattice modes in optically active defects, occurs at elevated temperatures. As a result, many optical centers in diamond interact predominantly with local and quasilocal vibrations of the corresponding defects and retain their spectral structures and radiative transition probabilities unaffected to relatively high temperatures. For instance, the H3 or 575 nm nitrogen-related centers exhibit a strong luminescence intensity at temperatures above 500°C. Being a nondirect bandgap semiconductor, diamond, at first glance, does not appear to be a promising material for light-emitting optoelectronic applications. Indeed, band-to-band radiative transitions in diamond require the participation of phonons, which strongly reduces their probability and makes band-to-band intrinsic luminescence ineffective. However, fortunately the conduction band of diamond has a local minimum in the center of the Brillouin zone, lying at about 7.2 eV above the maximum of the valence band. This peculiarity has a great effect on the probability of the extrinsic radiative transitions occurring at the centers, the excited electronic levels of which lie in proximity to the conduction band. Namely, the wave function of the excited states of such optical centers may possess a considerable local maximum at k = 0. This means that the impulse relaxation required for a quantum assisted electronic transition can easily occur inside the defect via interaction with its quasilocal vibrations. Thus the probability of radiative recombination over such optical centers can be high and even dominant. The light emission efficiency of extrinsic optical centers in diamond is expected to be like that in GaP, which has a bandgap structure analogous to that of diamond, and which is known to be an effective semiconductor for light-emitting diodes.
Preface XI
The physical background of the optical properties of diamond has already been discussed in detail in a number of books and review articles. There is no need to repeat it here. The aim of the present handbook is to present in a systematic manner the experimental and theoretical data on optical properties of diamond accompanied by short explanations and models. This is a handbook, which is supposed to provide a short and quick way to search for concrete information, rather than to give a general view of the subject. It will help diamond researchers to find a reference or assess the level of knowledge of a particular optical feature. Very often the data obtained by different authors are contradictory. This handbook presents the different points of view equally without any prejudice. The presentation of the information is based on the presumption that all of the experimental data have been obtained correctly (unless the opposite is admitted by the authors themselves). Readers have to form their own opinion based on the facts available and to decide which data or interpretation seem to be more correct. However, in some cases the author of the present handbook gives his opinion; this is marked with (*). The current understanding of the optical properties of diamonds is sometimes considered as excellent. However this statement is valid only for a few well-known "classical" optical effects (provided they are located in a perfect diamond lattice) and in the case of the basic effects on some main intrinsic and nitrogen-containing defects. The present handbook clearly demonstrates that the number of questions and problems in optics of diamond is growing much faster that the acquisition of reliable data and the elaboration of thought-out answers.
Bochum, July 2000
Alexander M. Zaitsev
Acknowledgements
I thank all the researchers who have contributed and continue to contribute to the optical science of diamond and, consequently, who made it possible for me to write this collection of diamond optical data. In particular I am grateful to Drs V. S. Varichenko and A. A. Melnikov who provided me with many experimental data published in the former USSR, and which had not been known to western researchers until now. I am always thankful to my diamond teachers Prof. V. S. Vavilov and Prof. A. A. Gippius, who triggered and stimulated my optical studies of diamond. I thank Prof. A. T. Collins for kindly writing the foreword for this handbook, and I express my admiration of his personal outstanding contribution to the optics of diamond. And of course I am always grateful to my parents, who helped me for so many years and still help in many ways.
Contents
Abbreviations, Definitions and Methods .......................................................
XIX
1 Refraction.......................................................................................................
1
1.1 Value and Spectral Dependence........................................................ 1.1.1 Natural Diamonds................................................................... 1.1.2 HPHT Synthetic Diamonds.................................................... 1.1.3 CVD Diamond Films.............................................................. 1.2 Dependence on Temperature, Pressure and Defects.......................... 1.2.1 Temperature Dependence....................................................... 1.2.2 Pressure Dependence.............................................................. 1.2.3 Influence of Defects................................................................ 1.3 Birefringence..................................................................................... 1.3.1 Elasto-Optical Constants........................................................ 1.3.2 Influence of Defects and Impurities.......................................
1 1 4 5 6 6 7 8 8 8 9
2 Reflection and Transmission.........................................................................
13
2.1 Reflection........................................................................................... 2.1.1 Natural and HPHT Synthetic Diamonds................................. 2.1.2 CVD Diamond Films.............................................................. 2.1.3 Influence of Defects and External Forces............................... 2.2 Transmission......................................................................................
13 13 15 15 16
3 Vibronic Absorption......................................................................................
19
3.1 Intrinsic Features................................................................................ 3.1.1 One-Phonon Region................................................................ 3.1.2 Multi-Phonon Region............................................................. 3.2 Defect-Induced Vibrational Bands....................................................
19 19 23 27
4 Scattering........................................................................................................
69
4.1 Rayleigh Scattering............................................................................ 4.2 Raman Scattering............................................................................... 4.2.1 General Properties................................................................... 4.2.2 Raman Features......................................................................
69 69 69 73
XVI
Contents
4.3 Miscellaneous....................................................................................
121
5 Optical Electronic Transitions......................................................................
125
5.1 Optical Bands..................................................................................... 5.2 Optical Continua................................................................................ 5.3 Electron-Phonon Coupling at Optical Centers...................................
125 359 372
6 Coloration of Diamond..................................................................................
377
6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8
Red..................................................................................................... Yellow................................................................................................ Green.................................................................................................. Blue.................................................................................................... Brown................................................................................................. White.................................................................................................. Dark and Black................................................................................... Miscellaneous.....................................................................................
377 379 381 383 384 386 386 386
7 Physical Classification of Diamond..............................................................
389
7.1 Type I................................................................................................. 7.1.1 Type Ia.................................................................................... 7.1.2 Type Ib.................................................................................... 7.1.3 Type Ic.................................................................................... 7.2 Type II................................................................................................ 7.2.1 Type IIa................................................................................... 7.2.2 Type IIb................................................................................... 7.2.3 Type IIc................................................................................... 7.3 Miscellaneous.....................................................................................
389 389 390 391 391 391 392 392 392
8 Interaction with Energetic Light Beams......................................................
395
8.1 Laser Treatment.................................................................................. 8.1.1 ArF Laser (6.42 eV, 193 nm)............................................... 8.1.2 KrF Laser (5.0 eV, 248 nm)................................................. 8.1.3 Frequency Quadrupled Nd:YAG Laser (4.66 eV, 266 nm) 8.1.4 XeCl Laser (4.02 eV, 308 nm)............................................. 8.1.5 Copper Vapor Laser (2.43 eV, 510 nm)............................... 8.1.6 Ar+ Laser (2.99 eV, 415 and 2.54 eV, 514 nm)................... 8.1.7 Frequency Doubled Nd:YAG Laser (2.33 eV, 532 nm)...... 8.1.8 Ruby Laser (1.786 eV, 694 nm)........................................... 8.1.9 Nd:YAG Laser (1.17 eV, 1.06 µm)..................................... 8.1.10 CO2 Laser (0.117 eV, 10.6 µm)........................................... 8.2 Synchrotron Irradiation...................................................................... 8.3 Miscellaneous.....................................................................................
395 396 397 398 398 399 399 399 400 400 401 401 402
Contents XVII
9 Thermostimulated Luminescence and Tunnel Luminescence................... 9.1 9.2 9.3
405
TSL and TL Features....................................................................... 405 Optical Centers in TSL..................................................................... 409 Miscellaneous................................................................................... 410
10 Photoconductivity.......................................................................................... 413 10.1 10.2 10.3
Thresholds and Peaks....................................................................... Microwave Photoconductivity......................................................... Miscellaneous..................................................................................
413 423 426
11 Related Data..................................................................................................
429
11.1 11.2 11.3 11.4 11.5 11.6 11.7
Mechanical Properties...................................................................... Edge Electronic Transitions............................................................. Thermal Properties........................................................................... Lattice Structure and Defects........................................................... Anisotropy and Polarization............................................................. Electrical Properties......................................................................... Luminescence Excitation................................................................. 11.7.1 Cathodoluminescence........................................................ 11.7.2 Photoluminescence............................................................. 11.7.3 X-Ray Luminescence......................................................... 11.7.4 Electroluminescence.......................................................... 11.7.5 Iono- and γ-Luminescence................................................. 11.8 Impurities......................................................................................... 11.9 Gem Diamonds................................................................................. 11.10 Miscellaneous...................................................................................
429 431 432 434 436 437 439 439 440 441 442 443 444 445 445
References...........................................................................................................
447
Abbreviations, Definitions and Methods
a
lattice parameter (for diamond a = 0.3568 nm)
A
absorption (optical)
ATR
attenuated total reflectance
BE
bound exciton
CAS
calorimetric absorption spectroscopy
CL
cathodoluminescence CL is a luminescence excited with energetic electrons (usually with energy of a few tens of kiloelectronvolt). A disadvantage of electron excitation in cw mode is its very low efficiency of intracenter excitation of optical centers. Since the cross-section of interaction of free electrons with bound electrons in crystals is relatively small (about 0.25 nm2 in diamond), the probability of direct excitation of optical centers with cw electron beams (of a density 1 W/cm2) is negligible even for concentrations of optical centers as high as 1018 cm-3. The main pathway of excitation of optical centers by fast electrons is via the recombination of nonequilibrium charge carriers. As a result, CL can be applied only for optical centers which are effective recombination centers (like the H3 or N3 nitrogen related centers). In contrast, ineffective recombination centers (like the 638 nm nitrogen-related center) can be hard to excite in CL. A technical advantages of CL is the easy and inexpensive interband excitation of wide bandgap semiconductors, like diamond, as well as the opportunity of depth resolved investigations varying the energy of the exciting electrons.
CVD
chemical vapor deposition
d
dipole moment
dg
degeneracy ratio
D-A
donor-acceptor The quantum energy of D-A radiative recombination is described by the expression:
XX
Abbreviations, Definitions and Methods
hν = E g − E A − E D +
e2 + J (rDA ) , 4πεε 0 rDA
where J is the Coulomb integral (important only for very small rDA), rDA = a (m − b) / 2 is the D-A interspacing, m is the shell number, and b may have two values: 0 and 5/8 (Thomas et al. 1964, Williams 1968a). DL
delayed luminescence
DOS
density of states
DRFTI
diffuse-reflectance Fourier-Transform Infrared spectroscopy
E, E
electric field
EA
energy level of acceptor counted from the top of the valence band
Ebx,B
binding energy of exciton localized at a defect B The binding energy can be assessed approximately as Ebx,B ~ 0.1EB, where EB is the ionization energy of the defect B (Sternschulte et al. 1999a).
EC
energy of the conduction band minimum
ED
energy level of donor counted from the bottom of the conduction band
Edg
direct bandgap energy
Eg
indirect bandgap energy
ET
thermal activation energy
EV
energy of the valence band maximum
EA
electro-absorption
eh
electron-hole
EL
electroluminescence (excitation by electric current)
EPL
excitation of photoluminescence
f0
frequency factor
FE
free exciton
FTIR
Fourier transform infrared spectroscopy
FWHM
full width at half magnitude
GL
gammaluminescence (induced by γ-rays)
HFCVD
hot filament assisted CVD deposition
IL
ionoluminescence (ion beam induced luminescence)
Abbreviations, Definitions and Methods XXI
IRA
IR optical absorption The method is based on the measurement of the quantity of light absorbed by atomic vibrations. Since the method requires carefully polished sample surfaces, it is hardly applicable for insitu studies of growing CVD diamond films.
IRE
IR emission The method is based on measurements of the irradiation emitted by atomic vibrations from a hot sample (usually at a temperature of 500°C). The emitted light is a product of gray-body emission and the emission from the vibrational states of the sample. The method does not require polishing surfaces of the sample. It is particularly convenient for insitu studies of CVD diamond growth. IRE is a completely nonintrusive method (Ayres et al. 1998). IR emission shows all of the features of the corresponding IR absorption spectra of diamond in the one-phonon, two-phonon, C-H stretch, nitrogen defect, and C-H bending regions. The IRE spectra are the most prominent for transparent and white color diamonds.
J-T
Jahn-Teller
k
wave vector
kn
wavenumber
k
extinction coefficient
kB
Boltzmann constant
Kc
momentum of the conduction band minimum (k = 0.76 X)
L
luminescence
LA
longitudinal acoustic
LHeT
liquid helium temperature
LNT
liquid nitrogen temperature
LO
longitudinal optical
LVM
local vibration mode
MIRIRS
multiple internal reflection infrared spectroscopy
MPCVD
microwave plasma assisted CVD deposition
MWPC
microwave photoconductivity MWPC does not require electrical contacts applied to the sample and possesses a high areal selectivity (determined by the size of exciting light spot and the diffusion length of the nonequilibrium charge carriers) (Zaitsev et al. 1992). In general the MWPC spectra of type IIa diamonds at the bandgap edge spectral region are characterized by two regions (Fig. A.1): the phonon-absorption region
XXII
Abbreviations, Definitions and Methods
(anti-Stokes region, from 237 to 226 nm) and the phonon-emission region (Stokes region, shorter than 226 nm). The intensity of the phonon-absorption region Ia is commonly weaker than that of the phonon-emission region Ie because of the very low probability of high-energy phonon excitation at room temperature. 1
0.8
L s= 0 µm
0.6
Ls = 1 µm 0.4
L s= 10 µm
0.2
0 200
210
220
230
240
250
WAVE LENGTH, nm
WAVELENGTH, nm
Fig. A.1. MWPC spectra calculated for 100 µm thick diamond with different rates of surface recombination. Ls is the thickness of the layer depleted due to surface recombination. The dashed line shows the absorption edge of diamond (Zaitsev et al. 1997b)
The shape and the intensity of the MWPC spectrum is determined by the absorption coefficient of the exciting light α(λ), the bulk lifetime of the nonequilibrium charge carriers τb, the surface recombination rate s (corresponding lifetime τs) and the mobility of the excited charge carriers µ. The shape is especially sensitive to the α(λ) and s values. The mobility and the bulk lifetime significantly influence only the MWPC intensity, not the shape. For the phononemission region, when the excitation occurs near to the surface within a few microns (thickness d) (Kania et al. 1990), surface recombination would strongly affect the MWPC intensity provided the d value is comparable with the diffusion length of the charge carriers L = (Dτ)0.5. For the phonon-absorption region the absorption of the light occurs in a layer of thickness 1 mm. Since the L value does not exceed a few microns even in high-quality diamonds characterized by τb ∼ 10-8 s and by diffusion coefficients D of 25 cm2/s, the surface recombination does not affect noticeably the MWPC intensity Ia in the phononabsorption region. So far the Ia value is determined mainly by τb, that is Ia∼τb. In contrast, the surface recombination strongly controls the MWPC intensity in the phonon-emission region Ie, where the light excitation depth is comparable to L or even less, that is Ie ∼ τbτs/(τb+τs). In the case of negligible surface recombination s ∼ 0,
Abbreviations, Definitions and Methods XXIII
the ratio Ie/La should be of the order of 103. In reality, this value seldom attains even 102, ranging in most cases from 1 to 10 (including CVD diamond films). This means that surface recombination is a common parameter controlling the concentration of the nonequilibrium charge carriers in diamond in the proximity of the surface. Routine measurement of the Ie and Ia intensities can be performed by averaging the MWPC value over the spectral region from 215 to 220 nm and from 230 to 235 nm respectively (Fig. A.2, A.3).
ABSORPTION, %; MW PC INTENSITY, arb. units
120 Eex + TO Eex + TA 100
Eex + 43 meV
80
Ie
60
Ia
40
20
0 205
210
215
220
225
230
235
240
WAVELENGTH, nm
Fig. A.2. MWPC (solid line) and optical absorption (dashed line) spectra of a natural type IIa diamond. The spectral area for measurements of intensities of the phonon-absorption region (Ia) and phonon-emission region (Ie) are indicated (Zakharov 1997)
ABSORPTION, %; MW PC INTENSITY, arb. units
120 E ex + TA 100
Eex - TA E ex - TO/LA
Eex + TO E ex + 43 meV
80
60
40
20
0 210
215
220
225
230
235
240
WAVELENGTH, nm
Fig. A.3. MWPC (solid line) and optical absorption (dashed line) spectra of a natural type IIa diamond with relatively strong anti-Stokes area (Zakharov 1997)
XXIV
Abbreviations, Definitions and Methods
The bulk lifetime can be assessed by the Ia intensity and the surface recombination rate from the ratio Ie/Ia≈1000τs/(τb+τs). The wavelength of the MWPC maximum indicates roughly the depth of the depletion area resulting from surface recombination. NEA
negative electron affinity
n
refractive index
ODMR
optically detected magnetic (electron spin) resonance
pij
nonzero elements of elasto-optical tensor
P
pressure
PAS
photo-acoustic spectroscopy
PC
photoconductivity
PCCVD
polycrystalline CVD (diamond)
PDS
photothermal deflection spectroscopy
PL
photoluminescence Photoluminescence is a luminescence excited with light quanta. The main process of PL excitation is the resonance interaction of electrons of optical centers with an electromagnetic wave. An advantage of PL is its capability of intensive intracenter excitation, which does not require the creation of nonequilibrium free charge carriers. Therefore optical centers can be readily excited in PL with quanta of energy much lower than Eg. Since the cross-section of interaction of quanta with bound electrons in crystals is very large (about 1 µm 2 for visible light), relatively low light fluxes (of 1 W/cm2) can effectively excite intracenter luminescence of optical centers with concentrations as low as 1015 cm-3.
PLE
photoluminescence excitation (spectroscopy) An important advantage of PLE spectroscopy is its capability of measuring the absolute energy position of electronic states of luminescence centers in the forbidden energy gap.
POM
polarization-optical method The polarization-optical method has been developed for the evaluation of internal mechanical stress in diamonds (Orlov 1973; Varshavskii 1968; Orlov et al. 1973; Lang 1967a). The method deduces the internal crystal lattice perfection, Π, from the brightness of the birefringence pattern, relative to that of a reference sample. The sensitivity of the birefringence method on diamond is evaluated to be about 0.003° of the lattice distortions (Wilks et al. 1991).
PS
piezospectroscopy
Abbreviations, Definitions and Methods XXV
Piezospectroscopy studies the splitting of narrow optical lines (usually ZPLs) in crystals under mechanical stress. For cubic crystals (like diamond) the number of splitting-off components and their polarization in absorption and luminescence are given in Kaplyanskii (1964), Hughes and Runciman (1967), Stoneham (1975), Davies and Nazare (1980b), Mohammed et al. (1982b). In the ideal case the local symmetry of a defect, at which the "optical" electronic orbital is localized, can be distinguished unambiguously by analysing the splitting patterns when applying stress along the main crystallographic directions (Table A.1). The splitting pattern can also reveal the degeneracy of the related electronic levels.
Table A.1. Number of splitting components of ZPL of an optical center in a diamond lattice under uniaxial mechanical stress along the main crystallographic directions. Unpolarized light incident normal to the stress axis. Two inequivalent directions k for [110] stress are indicated. Symmetry of defect Cubic (Td)
Symmetry of electronic levels A1↔T1, A2↔T2 E↔T1, T2 T1↔T2 T1↔T1, T2↔T2 Γ6↔Γ8, Γ7↔Γ8 Γ8↔Γ8 Tetragonal A↔B (D2d) A↔E; B↔E E↔E Trigonal A↔A (C3v) A↔E E↔E Rhombic I A↔B, A↔A, B↔B [110] (C2v) Rhombic I A↔B [100] (D2) Monoclinic A↔A I (C1h) Monoclinic A↔A, B↔B, A↔B II (C2) Triclinic A↔A
[100]
[111]
[110]
2 3 3 3 2 4
2 2 4 3 2 4
3; 2[001], [1-10] 5; 4[001]; 3[1-10] 6; 4[001], [1-10] 6; 4[001], [1-10] 2 4
2 3 3 1 2 4 2
1 2 2 2 3 5 2
2; 1[001] 4; 3[001], [1-10] 4; 2[001] 2 4; 3[1-10] 8 3
3
1
3
2
3
4
3
2
4
3
4
6
The unusually large number of split components (exceeding the numbers given in Table A.1) is explained by the existence of forbidden transitions due to the breaking of selection rules by deformation. An alternative explanation is the structural transformation of the diamond lattice in the proximity of defects under
XXVI
Abbreviations, Definitions and Methods
high stress, causing the corresponding optical center to be surrounded by a noncubic lattice experiencing uniaxial distortion (Zaitsev and Gippius 1988). It is supposed that such local phase transitions occur readily at "soft" vacancy-type defects, the electronic bond deformations at which are believed to be the highest. It is interesting to note that at low stress the splitting of ZPL of "soft" centers usually starts from two components irrespective of their symmetry. Pure orientational degeneracy can be distinguished from pure electronic and electronic-orientational degeneracy by measuring temperature dependency of intensities of the splitting components. Breaking of the orientational degeneracy results in temperatureindependent components, whereas the components resulting from the electronic degeneracy are thermolized. PSC
photostimulated current
R
reflection (optical)
RT
room temperature
RTA
rapid thermal annealing
Rp
projected range of ions introduced by ion implantation
r
Raman scattering cross-section The r value shows the intensity of Raman scattering with absorption (hνincident = hνscattered + hω, anti-Stokes scattering) or emission (hνincident = hνscattered - hω, Stokes scattering) of phonons. The reduced Raman spectrum is Ireduced(ω)= Imeasured(ω)/{[1+n(ω, T)]/ω}, where n(ω, T) is the Bose-Einstein distribution. The standard proof of the Raman scattering nature of the features observed in Raman spectra is performed by changing the quantum energy of the excitation laser light (hν1 to hν2). Then the genuine Raman features do not change their relative spectral position with respect to the laser line (and change absolute spectral positions by the energy hν1 - hν2), whereas the PL features do not change their absolute spectral positions irrespective of the excitation quantum energy. However, for broad Raman bands a slight shift (up to a few tenths of cm-1) can be observed due to the change in the PL background slopes excited by light with different quantum energies. When evaluating the relative intensities and widths of Raman features, the different polarization degrees of these features must be taken into account. To overcome the difficulties connected with this effect it has been proposed that the Raman scattering intensity be taken as the sum of the intensities measured parallel and perpendicular to the incident beam polarization (Prawer et al. 1994). Raman scattering in some diamond materials, for instance in CVD diamond films with a high content of nondiamond
Abbreviations, Definitions and Methods XXVII
inclusions, occurs as in compounds. In this case the Raman scattering intensity IR for a compound with an optical absorption µ can be given as: IR =
I0S S + 2 µ , 1 − exp − S + 2µ L
where S is the scattering efficiency, I0 is the incident light intensity, and L is the thickness of the excited layer. µ is assumed to be the same for the incident and scattered light (Mermoux et al. 1996; Loudon 1964a). Polarized Raman spectroscopy can be used for the characterization of the alignment of the CVD diamond with respect to the underlying silicon substrate (Jubber and Milne 1996). The temperature of the sample can be measured from the Stokes/anti-Stokes intensity ratio (Kulisch et al. 1996): T=
hν ph I k ln S I aSt
ν ex + ν ph ν ex − ν ph
4
,
where νex is the excitation light frequency, νph is the scattering phonon frequency, and h and k are the Planck and Boltzmann constants. The accuracy of the measurements is ±50 K for the temperature range 800 to 1200 K. S
Huang-Rhys factor S=a
ω max
∫ 0
g (ω ) [2n(ω ) + 1] , ω2
where n(ω) is the phonon population factor, and g(ω) is a function determining the density of vibrations interacting with the optical center (Maradudin 1966). The S value can be easily evaluated from the ratio of the ZPL intensity to the total intensity of the vibronic band: S = ln(I total / I ZPL ) . The S value may differ for different vibrational modes interacting with the same optical center. For a particular modeω the S value is measured as: S = ( hν ZPL − hν max / hω ) ,
where hνmax is the quantum energy of the maximum of the vibronic band related to the mode ω.
XXVIII
Abbreviations, Definitions and Methods
S
spin moment
SP
mechanical "softness" Mechanical softness of an optical center is a parameter showing the rate of change of spectral position ∆hνZPL of its ZPL in crystal under pressure P: SP = ∆hνZPL/P. The value of the mechanical softness is determined by the relative strength of interatomic bonding in optically active defect in comparison with regular host crystal lattice. There is a tendency that optical centers related to the vacancycontaining defects have greater SP (above 1 meV/GPa) than those related to the interstitial type defects (SP < 1 meV/GPa) (Zaitsev et al. 1995).
ST
thermal "softness" Thermal softness of an optical center shows the rate of temperature broadening of its ZPL. Being determined as a derivative d(FWHM ZPL)/dT, ST is a function of temperature (usually ST increases with temperature). The value of the thermal softness can be measured as ZPL broadening with temperature increase from LNT to RT reduced to this temperature difference: ST = (FWHMRT - FWHM LNT)/(TRT – TLNT). There is a tendency that the optical centers related to the vacancy type defects have greater ST (above 0.01 meV/K) than those related to the interstitial type defects (usually below 0.01 meV/K). Physical background of the thermal softness is the unharmonicity of vibrations interacting with the optically active defect. It is believed that mechanically "soft" vacancy related defects exhibit greater vibrational amplitudes and, consequently, greater unharmomicity (Zaitsev et al. 1995; Mainwood 1999). Nonhomogeneous broadening may considerably reduce the measured ST value, therefore the true ST value should be measured in perfect crystals.
SERS
surface enhanced Raman scattering The SERS method is used for the detection of Raman active species localized at the surface. A considerable background characteristic of SERS spectra taken from diamond is probably due to inelastic scattering at electron-hole pairs (Lopez-Rios 1996; Monreal et al. 1987; Burstein et al. 1982).
SFG
sum-frequency generation
SR
synchrotron radiation
T
temperature
TD
Debye temperature TD for diamond is of 2219 to 2246 K (Nazare and Neves 2001).
TLNT
liquid nitrogen temperature
Abbreviations, Definitions and Methods XXIX
TRT
room temperature
TA
transversal acoustic
TL
tunnel luminescence TL is the optical emission resulting from direct electronic transitions between spatially separated electron and hole centers. The TL intensity does not depend on temperature (the TL process has no activation energy) because the electron levels involved in TL transitions are of the same energy. The decay of TL can be described by Bekkerel's law I(t) ∼ I(0)/(1+bt)χ. The parameter χ1 means a recombination on paired defects (Yelisseyev 1977). TL of synthetic diamonds occurs mainly on paired defects (Vins et al. 1989). TL can be separated from TSL using rapid cooling of the sample during the intensity measurements of the delayed luminescence upon switching off the excitation source. Usually cooling down to 80 K for a few tens of seconds is sufficient. TL, in contrast to TSL, does not show a rapid intensity decrease with cooling.
TO
transversal optical
TSL
thermostimulated luminescence TSL is one of the most effective optical methods for investigation of deep traps in diamond. The order of TSL kinetics m can be found from the dependence lg(I/Nm) ∼ 1/T. Second-order kinetics points to the recombination character of TLS. First-order kinetics describes processes with negligible retrapping of the thermally released carriers. The intensity of the first-order TSL for the linear increase of temperature q = dT/dt = constant is given by the expression (McKeever 1985; Vittone et al. 1999): sT I TSL (T ) = n 0 s exp[− ( E / kT ) ]exp − ∫ dT exp[− E / kT ] , q T0
where n0 is the initial density of trapped carriers, f0 is the frequency factor, ET is the trap depth energy, T0 is the initial temperature. At constant temperature the first-order TSL decays exponentially. XL
X-ray excited luminescence
ZPL
zero-phonon line
α
Debye-Waller factor
Λ
thermal conductivity
ΛT
temperature conductivity
XXX
Abbreviations, Definitions and Methods
λ
wavelength
Π
internal crystal lattice perfection Π is a parameter characterizing the birefringence effect (see POM). Birefringence is one of the most reliable methods for detecting internal strains in a diamond lattice. It is a standard technique for the inspection and selection of diamonds prior to mechanical treatment. The birefringence effect can also be used for the selection of diamonds suitable for fabrication of radiation detectors (Bienemann-Kuespert et al. 1967). The visibility criterion for a birefringence image from an edge dislocation is given by (Enckevort and Seal 1988a): qbG tan( u )λ I / I 0 (1 −ν )
≥
1 , 4
where q is the stress optical constant, λ is the light wavelength, G is the shear modulus of diamond, ν is the Poisson ratio, b is the Burgers vector, u is half the cone-angle of light in the specimen volume, and I0 and I are the incident and transmitted beam intensities. Dislocations with small 1/2 edge components, ending on {111}, give visible birefringence images when viewed end-on. Only edge dislocations with asymmetric contrast, in the presence of a long-range stress field, can be imaged when viewed side-on. The decoration of dislocations can give only a symmetrical image. The birefringence induced by external pressure is characterized by three constants: q1111 (the change in refractive index for light vibrating perpendicular to the cubic planes, caused by the stress applied to those planes), q1122 (the change in refractive index for light vibrating parallel to the cubic planes, caused by the stress applied to those planes) and q1212 (relates to the shift stress with respect to the direction of the resultant indicatrix). σ
mechanical stress
µ
absorption coefficient The µ value is approximately given by the expression: µ = {(1/tsample)ln(Iincedent/Itransmitted) - C}, where I is the intensity of light, tsample is the thickness of the sample, C is a parameter accounting for nonabsorption losses (for instance reflection at the surfaces, scattering, etc.). In general C is a function of the light wavelength. The µ value is proportional to the concentration N of the absorbing defects, enabling an absolute relationship between µ and N to be established (Collins 1997).
ε
dielectric constant
Abbreviations, Definitions and Methods XXXI
γ
thermal resistivity
τ
lifetime or total decay time
τr
radiative lifetime
τn
nonradiative lifetime
φ
luminescence efficiency (φ = τn/τr)
ϕ
work function
hν
quantum energy
hω
lattice vibration energy (energy of phonons or local vibrations)
χ
recombination parameter of TL
ωD
Debye frequency of diamond lattice, ωD = 150 meV
ωR
resonance frequency This parameter is usually used to characterize quasilocal vibrations resulting from heavy impurity atoms. According to Brout and Visscher (1962) the frequency ωR of the local vibration and the width ∆ωR of the resonance peak are given as: ωR = ωD
MC p MC , ; ∆ω R = ω D 6 nM I − M C 3(nM I − M C )
where MC and MI are the masses of a carbon atom and impurity atom respectively. Although these formulas were derived assuming the isotopic character of the impurity atom, they also describe satisfactorily the quasilocal vibrations of many chemically different impurities in diamond.
Units Used: 1[ppm] = 1.76×1017 [cm-3] 1[eV] = 1239.5/λ[nm] in air, or = 1239.8/λ[nm] in vacuum kn[cm-1] = 107/λ[nm]
1
Refraction
1.1
Value and Spectral Dependence
1.1.1
Natural Diamonds
The experimental data on the refractive index of natural diamonds are summarized in Table 1.1. Table 1.1. Refractive index, n, of natural diamond at room temperature at various wavelengths (Orlov 1973; Fedoseev et al. 1981; Bokii et al. 1986; Novikov et al.1987; Field 1992; Davies 1994a; Feldman and Robins 1991; Bienemann-Kuespert et al. 1967; Schrauf 1884; Walter 1891; Anderson B. et al. 1940; Martens 1902; Kurdumov et al. 1994; Balzaretti and da Jornada 1996)
λ [µm]
n
0.225 0.226 0.2265 0.2288 0.2313 0.2329 0.25 0.2573 0.2749 0.2837 0.2881 0.2981 0.3 0.313 0.3404 0.3467 0.35 0.3611 0.3969 0.397
2.729 2.7151 2.7150; 2.7151 2.7042 2.6947 2.6882 2.627; 2.6333 2.6144 2.5785 2.5633 2.5567 2.5428 2.545; 2.5407 2.5254 2.5000 2.4952 2.490; 2.4928 2.4852 2.4653 2.4648
2
1 Refraction
Table 1.1. Continued 0.4 0.4102 0.4308 0.4358 0.4410 0.4416 0.45 0.4678 0.4800 0.4860 0.4861 0.48613 0.5 0.5270 0.5338 0.5350 0.5460 0.5461 0.55 0.5641 0.5780 0.5890 0.5892 0.58929 0.5893 0.6 0.64385 0.644 0.65 0.65428 0.6560 0.65628 0.6563 0.6708 0.6876 0.7 0.75 0.7590 0.7628 0.8 0.9 1.0 1.2 1.5 2.0 2.5 3.0 4.0
2.463; 2.4641 2.4592 2.4512 2.44902 2.4482 2.4478 2.4454 2.4410 2.4372 2.4354 2.4354; 2.43488 2.43554 2.432; 2.4324 2.4269 2.4257 2.4278 2.4237 2.42309 2.4230 2.4237 2.4186; 2.419 2.41723; 2.4173; 2.41734; 2.4175 2.4176 2.41726 2.4195; 2.41681 2.415, 2.4159 2.4111 2.4109 2.4105 2.40990 2.4099 2.40990 2.4103; 2.40967 2.4135 2.4077 2.405; 2.4062; 2.407 2.4028 2.4024 2.4024 2.400 2.396 2.394 2.390 2.386 2.383 2.3786 2.382; 2.3782 2.3773
1.1 Value and Spectral Dependence 3
Table 1.1. Continued 5.0 6.0 7.0 8.0 9.0 10.0 12.0 14.0 15.0 16.0 18.0 20.0 25.0
2.381; 2.3767 2.3763 2.3761 2.3759 2.3758 2.380; 2.3756 2.3755 2.3753 2.380 2.3752 2.3751 2.380; 2.3750 2.380; 2.3749
The data in Table 1.1 can be approximated by the expression (Peter 1923; Collins 1993b): n2 = 1 +
0.3306 λ2 4.3356 λ2 , + 2 2 λ − (175.0) λ − (106.0) 2 2
(λ is given in [nm] units), or (for the range 0.5 to 6.5 eV) by the expression (Robins et al. 1995): n(hν ) = 2.377 + 0.0086(hν ) 2 +
4.7 ×10 −5 (hν ) 4 , 1 − 0.0155(hν ) 2
where hν is the quantum energy in [eV] units. The mean value of n for natural diamond at a wavelength of 547 nm is given as 2.4236 (Bokii et al. 1986). The average value of n over the visible spectrum is given as 2.418 (Driscoll and Vaughan 1970). The refractive indices of diamonds of different types varies negligibly. For instance, the n value of type I and type II natural diamonds differ by no more than 1% (Robertson et al. 1934; Champion and Prior 1958). The dispersion curve of the refractive index of different type of diamonds in the ultraviolet spectral range is shown in Fig. 1.1. The differences between the two curves are explained by the lattice distortion caused by high nitrogen content rather than by the increased absorption in the UV region due to nitrogen (Walker and Osantkowski 1964). The average value of the dispersion of diamond in the visible spectral range can be given as dn/dλ ≈ -8.5×10-5 nm-1 (Berman 1965). The dispersion of diamond measured as the difference of the refractive indices at the Fraunhofer B (686.7 nm) and G (430.8 nm) lines is 0.044 (Bruton 1978). The dielectric constant of diamond at a temperature of 300 K is ε = 5.70 ± 0.05 (Fontanella et al. 1977; Davies 1994a). The temperature and pressure dependencies of the dielectric constant are given by the expressions:
4
1 Refraction
ε = 5.70111 - 5.35167×10-5 T + 1.6603×10-7 T2, (1/ε)(dε/dT)P = 8.09×10-6 K-1, (1/ε)(dε/dP)T = -0.72×10-12 Pa-1.
5 11.7 eV
7.4 eV 7 eV
REFRACTIVE INDEX
4
3
type IIa 2 21.5 eV 1
type I 0 0
5
10
15
20
25
30
QUANTUM ENERGY, eV
Fig. 1.1. Refraction index of type IIa and type I natural diamonds in the UV spectral region (Philipp and Taft 1962; Walker and Osantkowski 1964)
1.1.2
HPHT Synthetic Diamonds
The refractive index of synthetic diamond at a wavelength of 580 nm (experimental error of ±0.0004) varies from 2.4183 to 2.4216 (for octahedral crystals), from 2.4182 to 2.4238 (for cubo-octahedral crystals with dominant cubic facets), and from 2.4167 to 2.4192 (for cubo-octahedral crystals with dominant octahedral facets) (Novikov et al. 1987; Kurdumov et al. 1994; Vishnevskii and Malogolovets 1973). The mean values of n for synthetic diamond at wavelengths of 547 and 550 nm are (experimental accuracy ±0.0004) 2.4259, 2.4298 (for octahedral crystals), 2.4243 (cubo-octahedral crystals), 2.4227 (for cubic crystals) (Bokii et al. 1986; Novikov 1968; Vishnevskii and Malogolovets 1973). At a wavelength of 580 nm these are: 2.4208 (for cubo-octahedral crystals with dominant cubic facets) and 2.4181 (for cubo-octahedral crystals with dominant octahedral facets) (Kurdumov et al. 1994). The different n values for differently shaped crystals are explained by the different structural perfection of diamonds (the role of the impurity content is believed to be negligible) (Novikov 1968). The dispersion curve of synthetic diamond can be described by the expression (Kurdumov et al. 1994; Voronkova et al. 1965):
1.1 Value and Spectral Dependence 5
1 0.002678 . = 0.21413− n −1 λ2 2
1.1.3
CVD Diamond Films
The refractive index of good-quality CVD diamond films does not differ from that of natural diamond by more than a few percent (Robins et al. 1995). The refractive index of flame-grown epitaxial diamond films is comparable to that of natural IIa diamond, to within an error of ±10% (Schermer et al. 1994). The refractive index of CVD diamond films is nearly constant in the IR spectral region from 6 to 21 µm with a value of about 2.475. With a reduction of wavelength from 6 to 5 µm the refractive index falls from 2.475 down to 2.275 and then it increases again from 2.3 to about 2.5 with a further decrease of wavelength down to 3 µm (Clement 1997). The refractive index of CVD diamond films in the visible and near IR spectral range (250 to 900 nm) does not change much with wavelength, showing a small decrease by about 0.2 in the spectral range from 300 to 500 nm (Yin et al. 1997). The absolute value of the refractive index depends strongly on the quality of the CVD diamond films, varying from n ∼ 1.9 for poor-quality films (the Raman spectra of such films are dominated by the features of nondiamond phases) to n ∼ 2.3 for good-quality films (diamond line dominates the Raman spectra). In the IR region (500 to 5000 cm-1) the refractive index of CVD diamond films varies with the film quality only from 2.3 to 2.4 (Yin et al. 1997). The refractive index of fine-grained (grain size of 200-300 nm) CVD diamond films is slightly lower than that of natural diamond (Stenzel et al. 1993). This may be caused by the presence of low-density nondiamond phases. The refractive index of air-oxidized CVD diamond films can fall to 1.5 due to the formation of pores (the volume fraction of diamond in such films is lowered by 40 to 60%) (Khomich et al. 1995b). Annealing at 850°C increases the n value of poor-quality CVD diamond films by only 2 to 7% (Yin et al. 1997). The extinction coefficient k of good-quality CVD diamond films in the IR region (from 500 to 5000 cm-1) is 0.0076. This value is reduced below 0.001 in poor-quality films. In the visible/UV region the k value changes from 0.035 (at λ = 250 nm) down to 0.01 (at λ = 900 nm) in good-quality films and from about 0.06 (at λ = 250 nm) down to 0.03 (at λ = 900 nm) in poor-quality films. Annealing at 850°C reduces the k value of the poor-quality CVD diamond films only by about 4 % (Yin et al. 1997). The dielectric function ε of PCCVD diamond films can be found from the equation (Yin et al. 1997; Bruggeman 1935): vd
εd − ε ε −ε ε −ε + vnd nd + vv v =0, ε d + 2ε ε nd + 2ε ε v + 2ε
6
1 Refraction
where εd, εnd and εv = 1 are the complex dielectric functions and vd, vnd and vv are the volume fractions (vd, + vnd + vv = 1) for the diamond, nondiamond carbon and void components respectively; ε = ε1 -iε2 (ε1 = n2 - k2, ε2 = 2nk) is the measured dielectric function.
1.2
Dependence on Temperature, Pressure and Defects
1.2.1
Temperature Dependence
The general temperature dependence (thermo-optical coefficient) of refractive index n of diamond at normal pressure is given by Fontanella et al. (1977) by the expression: (1/n)(dn/dT)P = +4.04×10-6 K-1. The experimental values of n at room and low temperatures for type II diamond are given by Fontanella et al. (1977). The data are summarized in Table 1.2.
Table 1.2. Refractive index of natural diamond at various temperatures T, K 340 320 300 280 260 240 220 200 180 5.5
n 2.38791 2.38767 2.38747 2.38729 2.38714 2.38701 2.38692 2.38684 2.38678 2.38668
The temperature dependence of the refractive index of type IIa natural diamond in the range from 25 to 1200°C can be approximated by the relation (Rawles and D'Evelyn 1995): (1/n)(dn/dT) = 3.2×10-6 + 3.76×10-8 T - 3.78×10-11 T2 + 1.50×10-14 T3. In the temperature range from -100 to +400°C the temperature dependence of the refractive index can be presented as (Bienemann-Kuespert et al. 1967): n(T) = n20°C + 9.9×10-6 T[°C] - 7.8×10-8 T2[°C] (at wavelength 435.8 nm); n(T) = n20°C + 8.7×10-6 T[°C] - 7.0×10-8 T2[°C] (at wavelength 546.1 nm);
1.2 Dependence on Temperature, Pressure and Defects 7
n(T) = n20°C + 8.4×10-6 T[°C] - 7.0×10-8 T2[°C] (at wavelength 589.3 nm). The reduction in the refractive index at these wavelengths with temperature decrease from 180 K to 100 K is negligible (Fig. 1.2).
2.436
REFRACTIVE INDEX
2.434 2.432 2.430 2.428 2.426 2.424 2.422 0
200
400
600
800
TEMPERATURE, K
Fig. 1.2. Change of the refractive index of natural diamond at a wavelength of 546.1 nm with temperature (Bienemann-Kuespert et al. 1967)
1.2.2
Pressure Dependence
The general pressure dependence of the refractive index n of diamond at room temperature is given by Fontanella et al. (1977), Balzaretti and da Jornada (1996) by the expressions: (1/n)(dn/dP)T = -0.36×10-12 Pa-1, or d[n(P)/n(0)]/dP = -(3.6±0.1)×10-4. The reduction in n at a wavelength of 589.3 nm with hydrostatic pressure is also given by Schmidt et al. (1968) as: dn/dP ≈ -0.42×10-12 Pa-1. In diamond due to the symmetry conditions there is no transverse effective charge eT* determining the difference of LO and TO vibrations, that is (Balzaretti and da Jornada 1996): νLO2-νTO2 = [16π(eT*)2]/[n2µa3] ≈ 0.
8
1 Refraction
1.2.3
Influence of Defects
The refractive index of the area within a {111} stacking fault in type I diamond is described by a uniaxial indicatrix with extraordinary component (normal to the fault plane) with a value of 2.45, and an ordinary component with of a value of 2.38 (Enckevort 1990). Nitrogen does not strongly influence the refractive index of diamond in the visible spectral region. Therefore the refractive index for types I and II natural diamonds may differ by not more than 1% (Robertson et al. 1934; Champion and Prior 1958). The refractive index of synthetic diamond is reduced by high-temperature treatment. An explanation of the effect is the annealing of structural lattice defects and micro-inclusions (Novikov 1968). The refractive index of natural diamonds in the visible range is increased by 20 keV carbon ion implantation, and attains a value of 2.65 at a dose of 2×1015 cm-2. The dose dependence is not a linear one, and exhibits a rapid increase from 2.42 to 2.52 at doses above 1014 cm-2. This effect depends strongly upon the type of diamond. Annealing at about 750°C reduces the refractive index to that characteristic of nonimplanted diamond (Hines 1965). The refractive index of the graphitized buried layer produced in natural diamond by 350 keV He + ion implantation with doses above 2.8×1016 cm-2 (this figure is the critical dose of graphitization for 350 keV He+ ions) at a wavelength of 700 nm is 2.21±0.06, what is close to the index of dispersed graphite (ngraphite = 2.05) (Khmelnitskiy et al. 1996). The refractive index of CVD diamond film is considerably influenced by N-atom beam treatment, using doses above 1018 cm-2 (Jubber et al. 1995). Neutron irradiation up to a dose of 1.2×1017 cm-2 does not influence the refractive index of diamond (at a wavelength of 589.3 nm) noticeably. Neither are any changes observed after annealing at temperatures up to 750°C. Any change in n at a wavelength of 546.1 nm after neutron irradiation with a dose of 2×1015 cm-2 is evaluated to be below 0.004% (Bienemann-Kuespert et al. 1967; Pelsmakers and Schepers 1958; Denning 1964).
1.3
Birefringence
1.3.1
Elasto-Optical Constants
Diamond is an isotropic crystal with regard to its stress-birefringence behavior. The stress birefringence constant of diamond is 3×10-13 Pa-1 (Poindexter 1955). The uniaxial stress-induced birefringence for the and crystallographic directions is given by the expressions:
1.3 Birefringence 9 nΙΙ − n ⊥ = −0.5n 3
( p11 − p12 ) σ 100 , (c11 − c12 )
nΙΙ − n⊥ = −0.5n 3
p44 σ 111 . c 44
The hydrostatic stress-induced change in the refractive index is given by: ∂n / ∂P = 0.5n 3
( p11 + 2 p12 ) σ. (c11 + 2c12 )
Experimental values of the components of the elasto-optical tensor at various wavelengths are given in Table 1.3. Table 1.3. Values of the components of the elasto-optical tensor at various wavelengths (Grimsditch et al. 1979; Grimsditch and Ramdas 1975) λ [nm] 250 366.3 457.9 488 500 514.5 546.1 589.3 632.8 700
p11
p12
p11 + 2p12
p11 - p12 -0.365
p44 -0.2
p44/(p12 - p11)
p44/p12
0.551 0.572
4.8 3.85
0.572
3.66
p12/(p12 - p11)
-0.2033
-0.249
+0.043
-0.164 -0.1586
- 0.31
-0.175
-0.292
-0.172
-0.31
-0.17
0.156
0.58
The experimental dispersion curves of p44 and p11-p12 values in the spectral range of 250 to 700 nm are given by Davies (1994a); Grimsditch et al. (1979); and Grimsditch and Ramdas (1975) (Fig. 1.3). There is almost no dispersion at wavelengths from 700 to 500 nm, the values being p44 ≈ -0.17, p11-p12 ≈ -0.31. At 250 nm these figures increase to about -0.2 and -0.365 respectively.
1.3.2
Influence of Defects and Impurities
The reasons for birefringence of diamond are plastic deformation, elastic deformation near inclusions, growth striations, growth sector boundaries, dislocations, grain boundaries, and diamond-substrate boundaries (Field 1992; Johnson et al. 1964; Friedel 1924; Moore 1979; Lang 1967a; Brewster 1835; Palyanov et al. 1991; Paljanov et al. 1997). Birefringence can occur in diamonds which are composed of regions of different types (e.g. type I or type II) (Kaiser and Bond 1959; Harrison and Tolansky 1964; Bienemann-Kuespert et al. 1967). The birefringence of diamond can be calculated by taking into account the contribution due to the lowest direct gap E0' (Grimsditch et al. 1979). Fragments of natural diamond (chips) show the highest birefringence, whereas dodecahedral diamonds are the least birefringent ones (Moore 1979). Slip bands result in a stripe-like birefringence pattern (Wilks and Wilks 1991). The experimental values of the parameters of pressure-induced birefringence have been found for natural diamond under stresses of up to 3000 kg/cm2 for a
10
1 Refraction
wavelength of 540 nm: 2q1212 = 2.98×10-14 cm2/dyn and q1111 - q1122 = (3.06 to 3.08)×10-14 cm2/dyn. Almost total isotropy of the stress-induced birefringence has been found by Poindexter (1955); and Bienemann-Kuespert et al. (1967). -0.30
p11 - p12
-0.32
-0.34
-0.36 200
300
400
500
600
700
600
700
WAVELENGTH, nm -0.17
p44
-0.18
-0.19
-0.20 200
300
400
500
WAVELENGTH, nm
Fig. 1.3. Dispersion of elasto-optical constants of natural diamond (Grimsditch et al. 1979)
The birefringence contrast resulting from partial dislocations is expected to be stronger than that arising from sheets of stacking faults (Field 1992). However neither partial dislocations nor stacking fault sheets are seen unless the background birefringence, due to long-range strains, is very low (Field 1992). A "wavy" character of birefringence pattern suggests a higher dislocation density than that of the "tatami" pattern (Sumida and Lang 1981). Many high-dislocation natural diamonds show an orientation-dependent anisotropy of birefringence (Sumida and Lang 1981).
1.3 Birefringence 11
There is no definite tendency for preferential birefringence of diamonds of any type. This is an indication that nitrogen impurity does not directly influence the birefringence of diamond (Bienemann-Kuespert et al. 1967). However there is a trend such that natural diamonds of average size, with an enhanced birefringence, are ultraviolet-transmitting. In contrast, diamonds with a low birefringence are ultraviolet-opaque. Nitrogen is responsible for this effect. It is known that lownitrogen diamonds (type IIa, transparent in UV) have a very deformed stressed crystal lattice. This trend, however, does not apply to microdiamonds (Moore 1979). Birefringence in flame-grown epitaxial CVD diamond films results mostly from a cobbled surface structure (Janssen et al. 1991; Schermer et al. 1994). The birefringent units seen in patterns of PCCVD diamond films are comparable with the crystallite size, thus suggesting that a stress gradient exists within each of the grains (Field 1992). Some synthetic diamonds grown by the temperature gradient method show a weak cloud-type birefringence, which does not relate to the growth sectors. In synthetic diamonds the highest birefringence is observed at tenit inclusions, the lowest being at fluid inclusions (Paljanov et al. 1997; Paljanov 1997). The strain-induced birefringence patterns of synthetic diamonds do not change during annealing to 1200°C at pressures ranging from 1 to 50 kbar (Jackson and Webb 1995). Neutron irradiation does not cause any definite change in the birefringence constants of diamond up to a dose of 2×1015 cm-2 (Bienemann-Kuespert et al. 1967; Denning 1964). Diamonds with strong birefringence are usually mechanically weaker than those exhibiting low birefringence (Bell et al. 1975).
2
Reflection and Transmission
2.1
Reflection
2.1.1
Natural and HPHT Synthetic Diamonds
The available experimental data on the reflectance of diamond are summarized in Table 2.1. Table 2.1. Reflectance, R, of natural diamond at various wavelengths λ (Orlov 1973; Fedoseev et al. 1981; Bokii et al. 1986; Novikov et al.1987; Davies 1994a; Feldman and Robins 1991; Philipp and Taft 1964; Kurdumov et al. 1994) λ [µm] 0.180 0.185 0.188 0.191 0.194 0.197 0.200 0.203 0.207 0.210 0.214 0.217 0.221 0.225 0.25 0.3 0.35 0.4 0.5 0.6 0.7 0.8 0.9
R 0.268 0.260 0.254 0.248 0.243 0.239 0.235 0.231 0.228 0.225 0.222 0.219 0.216 0.2150 0.2012 0.1899 0.1823 0.1785 0.1741 0.1717 0.1703 0.1696 0.1690
14
2 Reflection and Transmission
Table 2.1. Continued 1.0 1.2 1.5 2.0 3.0 5.0 10.0 15.0 20.0 25.0
0.1687 0.1681 0.1676 0.1671 0.1670 0.1669 0.1668 0.1668 0.1668 0.1668
The spectral dependence of the reflectance of diamond, at quantum energies up to 34 eV, is given by Walker and Osantkowski (1964); and Philipp and Taft (1964) (Fig. 2.1). Three broad maxima at about 7.3, 12.2 and 16 eV as well as a minimum at about 8 to 9 eV (especially pronounced at low temperatures, antiresonance structure) are the features of the spectrum that have been found. These features are common to both type I and type IIa diamonds. The 7.3 eV peak is attributed to the direct Γ25′ to Γ15 transition between the conduction and the valence band. The 12 eV transition occurs at the X zone boundary from the X4 point in the V-band to the X1 point in the C-band. The 16 eV feature is a transition from Γ25' in the V-band to Γ12' in the C-band (Collins 1998; Saslow et al. 1966). At low temperatures the 7.3 eV peak sharpens and shifts slightly towards higher energies (Philipp and Taft 1964; Phillips 1965; Bienemann-Kuespert et al. 1967). The reflectivity at a quantum energy of 100 eV has been extrapolated by a value of 0.1% (Walker and Osantkowski 1964). 10
3
10
2
10
1
10
0
REFLECTANCE, %
12.6 eV
10
16.5 eV 7.1 eV
-1
0.1
1
10
100
QUANTUM ENERGY, eV
Fig. 2.1. Reflectivity of natural diamond. The curve is compiled from the data taken from (Walker and Osantkowski 1964; Philipp and Taft 1964)
2.1 Reflection 15
The following bands are observed in the diffusion reflectance spectra of some synthetic diamonds: 315, 350-360 (FWHM ∼ 20 nm), 380-390 (FWHM ∼ 30 nm), 430 (FWHM ∼ 30 nm), 470 (FWHM ∼ 40 nm), 503-520 (FWHM ∼ 20-30 nm), 620-640 (FWHM ∼ 40-50 nm), 650-680, 700, 950 nm. The bands at 350 and 470 nm are attributed to single nitrogen atoms (Babich et al. 1973). The reflectance of diamond in oil has values of 0.525 at a wavelength of 671 nm, and 0.544 at a wavelength of 440 nm respectively (Bienemann-Kuespert et al. 1967).
2.1.2
CVD Diamond Films
The reflectance of hot-filament PCCVD diamond films (grain size of 5 to 10 µm, boron content B/C in the reactant gas of 0 to 100 ppm) grown on Si substrates is 0.2 within the spectral range of 50 to 1000 cm-1. The films grown from gas with a B/C content of 1000 ppm exhibit an increase in the reflectance, from 0.2 to 0.7, with decreasing wavenumber from 200 to about 70 cm-1. This is accounted for by the effect of over-damped charge carriers (Grosse 1979). The plasma frequency and the damping constant correspond to values of 1400 and 1500 cm-1 respectively. Lowboron films (B/C ratio in the reactant gas of 10 to100 ppm) exhibit a peak at 800 cm-1, with a width of 10 cm-1, caused by the formation of a SiC layer at the Si-diamond interface (Okano et al. 1990). The reflectance of as-grown PCCVD diamond films (initial roughness: mean r.m.s. and peak-to-valley values are above 1260 and 6000 nm, respectively) attain a value of 0.5% in the visible range; and 30% at a wavelength of 25 µm. After polishing by ion-sputtering with a planarizing overcoat (residual roughness: mean r.m.s. and peak-to-valley values are 35 and 217 nm, respectively) the reflectance of the films increases up to 20% in the visible range and to 40% at a wavelength of 25 µm (Grogan et al. 1992) (Fig. 2.2). The roughness of CVD diamond films covered with a thin metal layer can be determined from the reduction in the specular reflectivity Rr(Θ) due to the surface roughness (Schaefer and Klages 1991; Bennett and Bennett 1967; Porteus 1963): Rr (Θ) = R0 (Θ) exp[−(4pσ cos Θ / λ ) 2 ] ,
where R0 is the reflectivity of the smooth surface, Θ is the angle of incidence, and σ is the r.m.s. roughness. The reduction in specular reflectivity of CVD diamond films increases from 17 to 99.5% when the roughness of the film surface changes from 22 to 120 nm (Schaefer and Klages 1991).
2.1.3
Influence of Defects and External Forces
Ion damage increases the reflection of diamond in the visible spectral range. Carbon ion implantation of energy 20 keV at a dose of 1015 cm-2 increases the reflection of
16
2 Reflection and Transmission
natural diamonds by 10% at a wavelength of 450 nm and by 18% at a wavelength of 650 nm (Hines 1965). High mechanical deformation increases the reflection of diamond strongly. Diamond loaded by a diamond indenter (quasi-uniaxial stress) becomes nontransparent and highly reflective in the visible spectral region (Gogotsi et al. 1998).
0.7
REFLECTANCE
0.6 0.5 0.4 0.3 0.2 0.1 0.0 0
200
400
600
800
1000
-1
WAVENUMBER, cm
Fig. 2.2. IR reflectance of a CVD diamond film grown on a Si substrate from a gas mixture containing boron at a B/C ratio of 1000 ppm. The curve is recalculated using the data taken from Okano et al. (1990)
2.2
Transmission
The transparency of CVD diamond films is affected mainly by two factors: light scattering due to surface roughness, and absorption on nondiamond sp2 bonded carbon phases. The transmittance of 1 µm-thick PCCVD diamond film (grown on a Si substrate) with initial roughness of 95 nm considerably decreases (down to 20%) at wavelengths below 850 nm. This value falls further to a few percent at a wavelength of 400 nm. However the transmittance of such films may increase up to 80% (at a wavelength of 633 nm) when their roughness is lowered down to 22 nm. 0.5 µmthick films with a 30 nm roughness exhibit a transmittance of 65% in the visible spectral range (Ilias et al. 1996; Gicquel et al. 1993). PCCVD diamond films exhibit a slightly higher optical transmittance over the UV, visible and IR spectral regions when the light penetrates through the rough side
2.2 Transmission 17
of the films. This effect is attributed to the total internal reflection of the light (Snail and Craigie 1991). The IR transmission of unpolished PCCVD diamond films increases linearly with decrease of wavenumber from 4000 to 200 cm-1. In some textured films the transmission may increase from 25 to 80% (Haq et al. 1994). The transmittance of free-standing PCCVD diamond film, in the spectral region 500 to 4000 cm-1, after mechanical polishing, may range from 57 to 66%. This value falls to 35% after thermochemical polishing on a hot iron plate (Choi et al. 1994). The spectral emittance and transmittance of polished CVD diamond films in a spectral region from 1 to 22 µm do not vary over the 4.2-295 K temperature range. The spectral emissivity of CVD diamond in this spectral region is below 2% (Clement 1997). The transmission of 1 mm thick synthetic diamonds with 13C isotope content of 0.1% at wavelengths 193 and 210 nm is 0.0073 and 0.015, respectively (Deshmukh et al. 1994). The transmittance of white light in type I diamonds falls with the dose of fast neutron irradiation (Bienemann-Kuespert et al. 1967). It is 99% for a dose of 2×1013 n/cm2, 95% for a dose of 2×1014 n/cm2, and 75 to 77% for a dose of 2×1015 n/cm2.
3
Vibronic Absorption
Diamond is a purely covalent crystal possessing, consequently, no dipole moment. Due to this the ideal diamond lattice does not absorb light in the one-phonon spectral region. The intrinsic absorption of perfect diamonds is observed only in two- and three-phonon regions: that is, in the spectral region spreading from single to triple Raman frequencies (1332 to about 4000 cm-1) (Bokii et al. 1986; Davies 1994a; Koidl and Klages 1992; Willingham et al. 1991; Klein et al. 1992; Hardy and Smith 1961; Charette 1966; Kluev 1971; Kluev et al. 1972a). This absorption is about two orders of magnitude weaker than that in the one-phonon bands of the ionically bonded crystals. A weak one-phonon absorption caused by lattice defects is also usually seen in most diamonds. Vibronic spectroscopy of diamond is performed using mostly IRA and IRE methods.
3.1
Intrinsic Features
3.1.1
One-Phonon Region
One-phonon dispersion curves, energies of phonons at critical points of the Brillouin zone of diamond, as well as one-phonon density of states of perfect diamond, graphite and diamond-like carbon can be found in (Warren et al. 1967; Wehrner et al. 1967b; Bou and Vandenbulcke 1991; Yarnell et al. 1964; Al-Jishi and Dresselhaus 1982). The data are summarized in Table 3.1 and presented in Fig. 3.1. Any distortion of the diamond lattice induces its one-phonon absorption at wavelengths above 7.502 µm (wavenumbers below 1333 cm-1). The spectra of onephonon absorption are shown in Fig. 3.3a. To a first approximation, the one-phonon absorption corresponds to the phonon density of states of diamond which is modified by a frequency-dependent factor characteristic of the disturbing defects (Walker 1979; Kurdumov et al. 1994). Most type I diamonds absorb in the one-phonon spectral region (the most likely reason being substitutional nitrogen). In contrast, most of type II diamonds do not show a noticeable one-phonon absorption (Robertson et al. 1934; Kaiser and Bond 1959). One-phonon intrinsic absorption is always observed in PCCVD and epitaxial diamond films due to lattice distortions and imperfections. The strength of this absorption in good-quality films can be as low as 0.1 cm-1 (Harris 1995). In epitaxial
20
3 Vibronic Absorption
flame-grown films one-phonon absorption is observed from about 800 to 1410 cm-1 (Davies 1994a; Janssen et al. 1992). One-phonon absorption can be most intense in -textured CVD diamond films (Haq et al. 1994). The addition of oxygen to the gas growth mixture considerably reduces the one-phonon absorption of CVD diamond films (Haq et al. 1994). The one-phonon absorption strength varies only slightly with the thickness of CVD diamond film (Haq et al. 1994). The mean absorption coefficient of free-standing CVD diamond IR windows in the 6 to 21 µm spectral region is 0.79 cm-1 (Clement 1997).
Table 3.1 Critical-point phonon frequencies in diamond as measured in optical absorption (Solin and Ramdas 1970; Sobolev et al. 1969c; Klein et al. 1992; Hardy and Smith 1961; Nazare and Neves 2001) Symmetry point Γ (Γ25') X (X1) X (X4) X (X3) L (L2') L (L3') L (L1) L (L3) K K (K4) K (K2) K (K1) K K (K3) W W (W1) W (W2)
Phonon branch ∆'2(O), ∆5(O) ∆'2(O), ∆1(A) ∆5(O) ∆5(A) Λ1(O) Λ3(O) Λ1(A) Λ3(A) Σ1(O) Σ2(O) Σ3(O) Σ1(A) Σ3(A) Σ4(A) Z(U) Z(M) Z(L)
Phonon energy [cm-1] (meV) 1332.5 (165) 1185 (147), 1191 (148) 1069 (133), 1072 (133) 807 (100), 829 (103) 1242±37 (154), 1252 (155), 1256 (156) 1206 (149), (150), 1220 (151) 1006 (125), 1033 (128) (68), 563 (70), 553 (69) 1230 (152), 1239 (154) 1109 (137), 1111 (138), 139 1045 (130), 1042 (129) 988 (122), 992 (123) 980 (121), 978 (121) (93), 764 (95) 1179 (146), 1146 (142) (123), 999 (124), 1019 (126) 908 (113), 918 (114)
Irradiation with fast electrons, neutrons or ions induces one-phonon absorption, which anneals out at temperatures above 1100°C, due presumably to vacancyinterstitial annihilation (Field 1992; *) (Fig. 3.3). Irradiation with 2 MeV electrons at a dose of 2×1019 cm-2 induces one-phonon absorption with an intensity of 15 cm-1 (Smith 1961). Neutron irradiation with doses above 1019 cm-2 induces in diamond intensive intrinsic one-phonon absorption, the spectrum and intensity of which are almost independent of the type of diamond. The main bands of the spectrum are 1420, 1535 and 1570 cm-1 (see also Raman Scattering). However such irradiation does not noticeably change the far-infrared absorption in the region below 700 cm-1 (Malogolovets et al. 1978c).
VIBRATIONAL DENSITY OF STATES, arbitrary
3.1 Intrinsic Features 21 optic
a
Diamond
acoustic
0
200
400
600
800
1000
WAVENUMBER, cm
1200
1400
-1
100
VIBRATIONAL DENSITY, arb. units
b
Graphite
1564
50
614 428
1743 952 1283
0 0
500
1000 WAVENUMBER, cm
1500
2000
-1
VIBRATIONAL DENSITY, arb. units
100
c
three-fold bound atoms
50
0 0
500
1000 WAVENUMBER, cm
1500 -1
2000
22
3 Vibronic Absorption
VIBRATIONAL DENSITY OF STATES
d
two-phonon DOS overtones
1600
1800
2000
2200
2400
WAVE NUMBER, cm
2600
2800
-1
VIBRATIONAL DENSITY, arb. units
100
e
four-fold bound atoms
50
0 0
500
1000 WAVENUMBER, cm
1500
2000
-1
Fig. 3.1. Ab initio (a, e) and tight-binding molecular-dynamic (b, c, d) simulations of vibrational density of states of diamond, graphite, four-fold bound atoms in diamond-like carbon and three-fold bound atoms in diamond-like carbon (Wang and Ho 1993; Windl et al. 1993). The sharp maximum at the highest energy of the phonon density of states of diamond is a unique feature. Such a feature does not occur for the other A4-semiconductors. Almost the whole two-phonon spectrum is formed by optical vibrations. The acoustic phonon region (below 1600 cm-1) is almost absent from the two-phonon spectrum
3.1 Intrinsic Features 23
VIBRATIONAL DENSITY OF STATES, arbitrary
157 153 146 meV (∆ optic and acoustic at X point, Z at W point) 136 meV (Σ2 optic at K point, ∆5 optic at X point) 127 meV (Λ1 acoustic at L point, Z at W point)
162
119 meV (Σ3 acoustic at K point) 166.7
99 meV (Σ4 acoustic at K point, ∆5 acoustic at X point) 73 meV (Λ3, acoustic at L point) quasilocal modes of heavy atoms (M>40)
cut-off 168 meV
165.1 meV, Raman phonon (Γ-point)
0
50
100
150
200
PHONON ENERGY, meV
Fig. 3.2. Phonon density of the diamond lattice with indication of the main features. Note that the Raman phonon does not have the highest energy of the diamond phonon spectrum. A sharp peak at an energy of 166.7 meV is due to LO phonons with k-vectors in the L and X directions. The energy range of quasilocal vibrations of heavy atoms (over 40 a.m.u.) in the diamond lattice is shown in a range of 10 to 60 meV
3.1.2
Multi-Phonon Region
Two-phonon intrinsic absorption of diamond occurs at wavelengths above 3.751 µm (wavenumbers below 2666 cm-1). Selection rules for two-phonon optical absorption in diamond are given in Table 3.2.
Table 3.2. Selection rules for two-phonon processes in absorption in diamond Symmetry point Γ L
X W
Phonons no TO+LO TO+TA LO+LA LA+TA TO+L TO+TA TA+L TO+L TO+TA L+TA
24
3 Vibronic Absorption
2
B center O(G) TO(L) A center B SO
L(W)
1
S3O LA(L) subst. N
platelets L(X)
TO(W)
TO(X)
S1A
A/B centers
LO(L)
TA(L) 8H polypype TA(X)
S 4A
subst. N 21R? platelets A center O(G) S 1O TO(L) LO(L)
S3A TO(W) TA(W) subst. N
TO(X)S2 O L(W) LA(L) L(X)
1
B center A/B centers
S 3O
500
600
700
800
900
3
1000
1100
1200
1300
1400
-1
WAVENUMBER, cm
1196 (148 meV) 1117 (138 meV) 1242 (154 meV) 630 (78 meV) 1089 (135 meV) 1328 (165 meV)
1015 (126 meV) 817 (101 meV)
600
800
1000
1200
1400
-1
WAVENUMBER, cm
Fig. 3.3. (a) Absorption spectra of CVD diamond films (1, 2) and natural type IIa diamond (3) in the one-phonon absorption region (Klein et al. 1992). (b) Intrinsic one-phonon absorption in a natural type IIa diamond irradiated with 5.6 GeV Xe ions at a dose of 2×1014 cm-2. Compare the marked features of the spectrum with those of the theoretical vibrational DOS of the diamond lattice in Fig. 3.1 and Fig. 3.2
3.1 Intrinsic Features 25
The experimental two-phonon absorption spectrum of diamond is presented in Fig. 3.4. The calculated two-phonon density of vibrational states is given in Fig. 3.1. The main peaks of the intrinsic two-phonon absorption spectrum of diamond are (the absorption intensities are given for type IIa diamond if the other is not indicated): 2.79 µm (3580 cm-1), absorption coefficient of 1.7±0.3 to 3±2 cm-1; 3.07 µm (3260 cm-1); 3.91 µm (2560 cm-1), 2TO, absorption coefficient of 4.6±0.3 cm-1 (5.2±0.1 cm-1 in type IaB and 5.05±0.1 cm-1 in type IaA diamonds); 4.11 µm (2430 cm-1), TO+LO(L), absorption coefficient of 4.6±0.3 cm-1 (5.2±0.1 cm-1 in type IaB and 5.05±0.1 cm-1 in type IaA diamonds); 4.62 µm (2170 cm-1), LO+LA(L), absorption coefficient of 12.8±0.3 cm-1 to 13.8±0.5 cm-1 (14.3±0.3 cm-1 in type IaB and 14.9±0.2 cm-1 in type IaA diamonds); 4.92 µm (2030 cm-1), TO+TA, absorption coefficient of 12.8±0.3 cm-1 to 13.8±0.5 cm-1 (14.3±0.3 cm-1 in type IaB and 14.9±0.2 cm-1 in type IaA diamonds); 5.04 µm (1980 cm-1), LO+TA(X), absorption coefficient of 12.8±0.3 cm-1. Usually the strength of the intrinsic absorption of the diamond lattice at a wavenumber of 1995 cm-1 is 12.3 cm-1 (Field 1992; Loubser and Ryneveld 1996; Nadolinny et al. 1999). The two-phonon absorption features at critical points of the Brillouin zone observed in diamond are given in Table 3.3 (Klein et al. 1992; Hardy and Smith 1961; Wehrner et al. 1967a). The absorption coefficient of the two-phonon intrinsic bands increases with temperature at a rate of 0.009 cm-1/K in the temperature range from 80 to 420 K (Charette 1959).
Table 3.3. Critical point two-phonon absorption observed in diamond Wavelength [µm] 5.509 5.343 5.080 5.018 4.938 4.899 4.804 4.731 4.642 4.591 4.524 4.411 4.245 4.104 3.935 3.756
Energy [meV] 225 232 244 247 251 253 258 262 267 270 274 281 292 302 315 330
Assignment LO(L)+TA(L) TO(X)+TA(X), Σ2O+Σ4A Σ1A+Σ3A L(X)+TA(X), 2Σ1A Σ3O+Σ3A Σ3O+Σ1A, 2TO(W) Σ2O+Σ3A, 2Σ3O Σ2O+Σ1A Σ2O+Σ3O L(W)+TO(W) Σ1O+Σ3A LO(L)+LA(L), L(X)+TO(X) Σ1O+Σ2O 2TO(L) 2LO(L) 2O(Γ) (IR inactive)
26
3 Vibronic Absorption
L(X)+TA(X)
a
S1 O+S4A
S2O+S 3O L(W)+TO(W) L(X)+TO(X)
S1A+S3A S1O+S 3O LO(L)+LA(L) TO(X)+TA(X)
S1 O+S2 O
S2O+S 4 A 2L(X) 2TO(L) LO(L)+TO(L) 2LO(L)
LO(L)+TA(L) S2O+S 3A
TO(L)+TA(L) S1 A+S 4A S3A+S4A
2O(G)
1200
1400
1600
1800
2000
2200
WAVENUMBER, cm
2S1A
2600
2800
S2O+S 3O
L(X)+TA(X) S1A+S 3A S3 A+S1 A S1O+S 4A 2S3A
2400
-1
b 2TO(X)
2TO(W)
L(W)+TO(W)
S1O+S 3A
L(W)+TA(W) 2LA(L) TA(W)+TA(W)
S1O+S 1A
S2O+S3A S2O+S1A
TO(L)+LA(L)
1850
1900
1950
2000
2050
2100
2150
2200
2250
2300
-1
WAVENUMBER, cm
c
13
TRANSMISSION, arbitrary
0.07% C
13
99% C
two-phonon region three-phonon region
0
1000
2000
3000
WAVENUMBER, cm
4000
5000
-1
Fig. 3.4. (a), (b) Absorption spectra of a CVD diamond film in the two-phonon region (Klein et al. 1992). (c) Transmission spectra of synthetic diamonds with a 13C isotope content of 0.07% and 99%. Note the shift of the bands towards lower energies and the relatively strong intensity of the third-order phonon absorption in the 13C enriched diamond (Anthony and Banholzer 1992)
3.1 Intrinsic Features 27
The reference intensity of the optical absorption of diamond in the two-phonon spectral range can be taken as 12.31 cm-1 (at a wavenumber of 2000 cm-1) (Palik 1985). For comparison, free-standing CVD diamond IR windows show an absorption coefficient of 11.7 cm-1 at this wavenumber (Clement 1997). The intrinsic two-phonon lattice absorption of diamond is not changed noticeably by plastic deformation, for instance, after indentation by a diamond pyramid loaded with 45 kg at a temperature of 1800°C (Phaal 1965; Bienemann-Kuespert et al. 1967). Electron irradiation with energy of 0.75 MeV does not influence the twophonon diamond absorption up to a dose of 1018 cm-2 (Phaal 1965; BienemannKuespert et al. 1967).
3.2
Defect-Induced Vibrational Bands
1.161 µm (8615 cm-1); FWHM of 150 cm-1; a line observed in natural hydrogen-rich diamonds of light gray color. The feature is attributed to hydrogen-related vibration (Fritsch et al. 1991a) (Fig. 3.5).
0.85
ABSORPTION COEFFICIENT, cm
-1
4494
0.80
0.75 8255
8615
7500 7850
0.70
5880
4157
5555
6070
4704
0.65 4000
5000
6000
7000
8000
9000
-1
WAVENUMBER, cm
Fig. 3.5. Absorption spectrum of a hydrogen-rich natural diamond (Fritsch et al. 1991a)
28
3 Vibronic Absorption
1.211 µm (8255 cm-1); FWHM of 150 cm-1; a line observed in natural hydrogen-rich diamonds of light gray color. The feature is attributed to hydrogen-related vibration (Fritsch et al. 1991a) (Fig. 3.5). 1.274 µm (7850 cm-1); FWHM of 180 cm-1; a line observed in natural hydrogen-rich diamonds of light gray color. The feature is attributed to hydrogen-related vibration (Fritsch et al. 1991a) (Fig. 3.5). 1.333 µm (7500 cm-1); FWHM of 120 cm-1; a line observed in natural hydrogen-rich diamonds of light gray color. The feature is attributed to hydrogen-related vibration (Fritsch et al. 1991a) (Fig. 3.5). 1.647 µm (6070 cm-1); see 3.219 µm (3107 cm-1) line (Fig. 3.5). 1.701 µm (5880 cm-1); see 3.219 µm (3107 cm-1) line (Fig. 3.5). 1.800 µm (5555 cm-1); see 3.219 µm (3107 cm-1) line (Fig. 3.5). 2.028 µm (4932 cm-1); a weak narrow line observed in (Fuchs et al. 1995) (Fig. 3.6).
13
C CVD diamond films
10
ABSORPTION COEFFICIENT, cm
-1
13
C
8
6 C-H stretch 3317 4 4830 2
3532
1281
4932
6873 7378
5572 0 1000
2000
3000
4000
5000
WAVENUMBER, cm
6000
7000
8000
-1
Fig. 3.6. LNT absorption spectrum of a 13C homoepitaxial CVD diamond film deposited on a 12 C diamond substrate (Fuchs et al. 1995). The features at 3317 and 7378 cm-1 are the 3323 and 7366 cm-1 centers
2.070 µm (4830 cm-1); a weak narrow line observed in (Fuchs et al. 1995) (Fig. 3.6).
13
C CVD diamond films
3.2 Defect-Induced Vibrational Bands 29
2.126 µm (4703 cm-1); a narrow line observed in natural hydrogen-rich diamonds of light gray color (Fritsch et al. 1991a) (Fig. 3.5). 2.223 µm (4498 cm-1); see 3.219 µm (3107 cm-1) line (Fig. 3.5, 3.7).
1.0 1363
ABSORBANCE, arb. units
0.8 3107
0.6
1010
2496
1404
465
0.4 754 1547
3236 2787
0.2
4496 4168
0.0 1000
2000
3000
WAVENUMBER, cm
4000
5000
-1
Fig. 3.7. Absorption spectrum of a brilliant cut diamond with nitrogen impurities characteristic of type IaAB and also containing hydrogen impurities (Ferrer and NoguesCarulla 1996)
2.406 µm (4157 cm-1); a sharp line observed in some hydrogen-rich natural diamonds (Fritsch et al. 1991a) (Fig. 3.5). 2.401 µm (4168 cm-1); a narrow line observed in natural hydrogen-rich diamonds of light gray color (Fritsch et al. 1991a). The feature relates to the 3107 cm-1 center (Fig. 3.7). 2.740 µm (3650 cm-1); FWHM of 200 cm-1; a broad band observed in single-crystal CVD diamond films grown in an acetylene combustion flame. The feature is attributed to part of the three-phonon absorption of the diamond lattice (Janssen et al. 1991). 2.831 µm (3532 cm-1); a weak sharp line observed in 13C hydrogen-containing CVD diamond films. In fully deuterated CVD diamond films the line disappears. The feature is attributed to hydrogen-related vibration. Nitrogen is not supposed to be involved in this vibration (Fuchs et al. 1995) (Fig. 3.6).
30
3 Vibronic Absorption
2.9 to 3.3 µm (3000 to 3500 cm-1); a broad band observed in CVD boron-doped diamond films. The feature is attributed to O-H stretching vibrations of molecular water physisorbed on the diamond surface (Chia-Fu Chen et al. 1994b; Chia-Fu Chen and Sheng-Hsiung Chen 1995). 2.9 to 3.2 µm (3100 to 3500 cm-1); several sharp lines at 3107, ~3137, 3145, ~3181, 3310, 3343, 3372, and 3394 cm-1 observed in natural diamonds showing Ib type character. The intensity of these absorption features increases by about 3% at a temperature of 90 K (Bienemann-Kuespert et al. 1967). The lines in the 2.9 to 3.0 µm spectral range are probably due to stretching vibrations of N-H bonds (Field 1992; Woods and Collins 1983) (Fig. 3.8, 3.13).
ABSORBANCE
2.0
1.6 3107 3144
1.2
1170 1405 687
0.8
0
1000
2000
3000
4000
5000
-1
WAVENUMBER, cm
Fig. 3.8. IR absorption spectrum of a green-to-yellow type IaAB+Ib natural diamond exhibiting sharp lines due to hydrogen-related vibrations (Reinitz et al. 1998)
2.9 µm (3400 cm-1) and 6.1 µm (1640 cm-1); two lines observed in type Ia diamonds. The features are attributed to inclusions of water (Bokii et al. 1986; Galimov et al. 1979). 2.946 µm (3394 cm-1); a line observed in some natural diamonds showing type Ib character (Woods and Collins 1983) (Fig. 3.9). 2.959 µm (3380 cm-1); a broad feature observed in CVD diamond films grown from gas mixture with oxygen added (Fig. 3.13). 2.991 µm (3343 cm-1); a relatively sharp line observed in some natural diamonds showing type Ib character (Woods and Collins 1983) (Fig. 3.9).
3.2 Defect-Induced Vibrational Bands 31
3.009 µm (3323 cm-1); a Lorentzian-shape line observed in CVD diamond films of high quality (Fuchs et al. 1995b). The line shifts to 3.015 µm (3317 cm-1) in 13C CVD diamond films. In fully deuterated CVD diamond films the line disappears. The feature is attributed to hydrogen-related vibration. Nitrogen is not supposed to be involved in the vibration (Fuchs et al. 1995) (Fig. 3.6). 3.020 µm (3310 cm-1); a weak relatively broad feature observed in some natural diamonds showing type Ib character (Woods and Collins 1983) (Fig. 3.9). 3.030 µm (3300 cm-1); a line observed in α−C:H diamond-like films deposited at temperatures below 100°C and in CVD diamond films treated by an N-atom beam. The feature is attributed to triple-bond configuration sp1 C-H stretching vibrations (Dischler et al. 1993; Jubber et al. 1995).
1.6
ABSORPTION COEFFICIENT, cm
-1
3107 3145 3181
1.2
3310 3343 3394
0.8
0.4
0.0 2800
3000
3200
WAVENUMBER, cm
3400
-1
Fig. 3.9. RT absorption spectrum of a type Ib natural diamond (Woods and Collins 1983)
3.089 µm (3237 cm-1); a weak Lorentzian-shape line observed in natural highquality diamond (Fritsch and Scarratt 1989; Fuchs et al. 1995b) (Fig. 3.7). 3.091 µm (3235 cm-1); an intense line observed in gray-violet hydrogen-rich diamonds. The line is attributed to hydrogen-related vibration (Fritsch et al. 1991a). Possibly this very feature is observed in Raman spectra (see 3234 cm -1 Raman line) (*). 3.144 µm (3181 cm-1); a relatively weak line observed in some natural diamonds showing type Ib character (Woods and Collins 1983) (Fig. 3.9).
32
3 Vibronic Absorption
3.175 µm (3150 cm-1); FWHM of 100 cm-1; a band observed in DRFTI in amineterminated diamonds. The feature is attributed to the asymmetrical σN-H vibrational mode (Miller and Brown 1995). 3.180 µm (3145 cm-1); a sharp line observed in some natural diamonds showing type Ib character (Woods and Collins 1983) (Fig. 3.8, 3.9). 3.202 µm (3123.6 cm-1); a sharp Lorentzian-shape line observed in high-quality homoepitaxial CVD diamond films. The line is supposed to be an analog of the 3.219 µm line observed in natural diamonds. In 13C CVD diamond films the spectral position of the line shifts to 3.211 µm (3114.5 cm-1). In fully deuterated films the line disappears. The feature is attributed to hydrogen-related vibration involving only one carbon atom. Nitrogen is not believed to be involved in this vibration (Fuchs et al. 1995a; Fuchs et al. 1995b). A tentative atomic model of the feature assumes hydrogen to be located in vacancies or dislocation lines (Fuchs et al. 1995b). 3.219 µm (3107 cm-1); a narrow Lorentzian-shape line observed in natural and CVD hydrogen-rich diamonds. The line may be very intense in diamonds of a light gray color. The absorption strength of the line in type Ia diamonds has been recorded as high as 13 cm-1 (Woods and Collins 1983; Fritsch et al. 1991a). The line intensity is related to the concentration of cloud-like inclusions confined to the cuboid growth sectors (Field 1992; Welbourn et al. 1989). This line is absent from the spectra of PCCVD diamond films (Dischler et al. 1993). The line is readily observed in highquality homoepitaxial CVD diamond films (Fuchs et al. 1995b). In 13C:14N diamonds the line shifts to 3.228 µm (3098 cm-1). The line is always accompanied by a weak Lorentzian-shape line at 3.228 µm (3098 cm-1), the intensity of which is 1% of that of the 3.219 µm line. There are lines the intensity of which correlate with the 3.219 µm line: at 1.647 µm (6070 cm-1) tentatively attributed to the first overtone of the 3.219 µm line, at 1.701 µm (5880 cm-1) attributed to a combination band, and at 1.800 µm (5555 cm-1) attributed to the third overtone of the 7.117 µm line (Field 1992; Fritsch et al. 1991a) (Fig. 3.7, 3.9). The feature is attributed to carbon-hydrogen vibration in cis-form of the disubstituted ethylene group −CH=CH− (Field 1992), or in the vinilidene group >C=CH2 (vibration of the sp2 bonds) (Field 1992; Woods and Collins 1983). The vibration is probably localized at interfacial surfaces forming C-H bonds (Davies 1994a). This vibration is interpreted as a stretching one (s). The corresponding bend vibration (b) gives a line at 1405 cm-1 (7.117 µm). The s and b vibrations produce the combinations 2b (first overtone at 3.589 µm (2786 cm-1)), 3b (second overtone at 2.399 µm (4168 cm -1)) and s+b at 2.223 µm (4498 cm-1) (Davies et al. 1984; Davies 1994a; Woods and Collins 1983; Collins et al. 1988c; Mainwood et al. 1994; Fritsch et al. 1991a). The satellite at 3.228 µm (3098 cm-1) is attributed to 13C-H vibration (Woods and Collins 1983; Fuchs et al. 1995; Fritsch and Scarratt 1989). The vibration involves only one carbon atom. A tentative atomic model of the feature is hydrogen atoms locating in vacancies or dislocation lines (Fuchs et al. 1995b).
3.2 Defect-Induced Vibrational Bands 33
3.22 µm (3106 cm-1); a weak band formed in strained type I diamonds by electron irradiation (Phaal 1965). The feature can naturally occur in some nitrogencontaining diamonds of mixed type (Fig. 3.8). 3.228 µm (3098 cm-1); see 3.219 µm (3107 cm-1) line. 3.289 µm (3040 cm-1); FWHM of 35 to 70 cm-1; a band observed in some PCCVD diamond films. The band is relatively strong in films grown on a Si substrate under negative bias. The feature is attributed to amorphous sp2 C-H stretch vibrations of symmetry CS:A (graphitic forms of carbon, or =CH2 groupings) (Dischler et al. 1993; John et al. 1994). 3.279 µm (3050 cm-1); FWHM of 50 cm-1; a band observed in DRFTI in amineterminated diamonds. The feature is attributed to a symmetrical σN-H vibrational mode (Miller and Brown 1995). 3.3 µm (3000 cm-1); a broad band ranging from about 3300 to 2800 cm-1. The band is observed in diamonds of any type when irradiated with neutrons and subsequently annealed at a temperature of 800°C. The band intensity depends strongly upon the irradiation dose. The band may also be observed in as-irradiated type Ib diamonds. The annealing at 800°C may cause partial reduction of the band (Bokii et al. 1986; Malogolovets et al. 1978c). 3.306 µm (2972 cm-1) and 3.365 µm (3025 cm-1) doublet. Widths of the lines are 30 cm-1. The doublet is observed in PCCVD undoped and boron-doped diamond films. The feature is detected in single-crystal epitaxial CVD diamond films deposited onto (110)-oriented substrates. The lines are attributed to olef. sp2 CH2 configuration of the C-H stretching vibrations with symmetry C2V:A1 and B1 (Dischler et al. 1993; Janssen et al. 1991; Chia-Fu Chen et al. 1994b; Janssen et al. 1992). 3.333 µm (3000 cm-1); FWHM of 30 cm-1; a broad line observed in PCCVD diamond films. The feature is attributed to olef. sp2 C−H stretch vibrations of symmetry CS:A (Dischler et al. 1993; John et al. 1994). 3.346 µm (2989 cm-1); a weak peak observed in some CVD boron-doped diamond films. The feature is attributed to stretching vibrations of C-H bonds (Chia-Fu Chen and Sheng-Hsiung Chen 1995). (Fig. 3.13). 3.391 µm (2949 cm-1); FWHM of 10 cm-1; a weak feature observed by MIRIRS method on hydrogenated (100) surface on natural type IIa diamond. The feature may reduce in intensity upon annealing at temperatures from 800 to 1000°C (Russell et al. 1999). 3.411 µm (2932 cm-1); FWHM of 5 cm-1; a weak feature observed by MIRIRS method on hydrogenated (100) surface on natural type IIa diamond. The feature may
34
3 Vibronic Absorption
reduce in intensity upon annealing at temperatures from 800 to 1000°C (Russell et al. 1999). 3.425 µm (2920 cm-1); a weak narrow line observed in natural hydrogen-rich diamonds of a light gray color (Fritsch et al. 1991a). 3.41 µm (2930 cm-1); a broad band ranging from 2820 to 2950 cm -1. The band is observed in α−C:H carbon films. The band is attributed to hydrogen bonded to sp3 coordinated carbon (as in methane) (LeGrice et al. 1990a; Herzberg 1962). 3.368 to 3.382 µm (2957 to 2969 cm-1); FWHM of 25 to 85 cm-1; observed in CVD diamond films. The feature is attributed to asymmetrical stretching vibrations of sp3bonded CH3 asymmetrical configurations (Wild et al.1989a; John et al. 1994; DehbiAlaoui et al. 1990; Dehbi-Alaoui et al. 1991; Janssen et al. 1992). 3.384 µm (2955 cm-1); FWHM of 20 cm-1; observed by the SFG method from the (100)-2×1 surface of epitaxial CVD diamond films. The feature is attributed to C−H stretching vibrations of sp3-hybridized bonds (Ando et al. 1994). 3.4161 to 3.426 µm (2919 to 2927 cm-1); FWHM of 30 to 50 cm -1; a band observed in PCCVD diamond films. The feature is ascribed to sp3 asymmetrical CH2 (Dischler et al. 1993; John et al. 1994). 3.421 µm (2923 cm-1) and 3.504 µm (2854 cm-1); a doublet always observed in CVD diamond films. The spectral position of the bands may change within ranges from 2840 to 2862 cm-1 and from 2904 to 2927 cm-1, respectively, depending upon the origin and structural perfection of diamond. FWHM of each band is 30 to 50 cm-1. Is a predominant hydrogen-related feature in both high-quality and lowquality MPCVD diamond films (Ertz et al. 1995). This doublet actually characterizes type IIc diamond (Fig. 3.10). The origin of this doublet is established unambiguously. It is attributed to the asymmetrical and symmetrical C-H stretching modes respectively of sp3-bonded methylene groups −CH2−. The symmetry of the vibrations is C2V:A1 and B1 (Dischler et al. 1993; John et al. 1994; Field 1992; Janssen et al. 1991; Bi et al. 1990; Wild et al. 1989b; Janssen et al. 1992; Ertz et al. 1995; Charette 1959; Kislovskii and Spitsyn 1989; McNamara et al. 1995). Absorption at wavenumbers above 2960 cm-1 is related to vibrations of sp2 coordinated C-H bonds (Zhu et al. 1993). A similar doublet is characteristic of hydrogen absorption in saturated hydrocarbons. The hydrogen atmosphere (not methane) is the main source of this H-related absorption in CVD diamond films (Fuchs et al. 1995b). In CVD diamond films the hydrogen atoms giving rise to this absorption doublet are believed to be located at interfaces and surfaces like grain boundaries, voids, etc. (McNamara et al. 1995). These hydrogen-related bands are especially strong in ATR spectra. There is a strong dependence in revealing the bands on crystallographic orientation of CVD
3.2 Defect-Induced Vibrational Bands 35
diamond films. The bands are very pronounced in (111) textured CVD diamond films and less intense in (110) textured films (Haq et al. 1994). This absorption is also very strong in brownish PCCVD diamond films (Sussman et al. 1994a; Sussmann 1993). The 2854 cm-1 band is readily observed in single-crystal epitaxial CVD diamond films when deposited onto (110) and (111)-oriented substrates. However it is absent from the spectra of films grown on (100)-oriented diamond substrates. Addition of oxygen to the growth gas mixture considerably reduces this hydrogen-related absorption in CVD diamond films (Haq et al. 1994). The intensity of the absorption is reduced with increasing CVD diamond film thickness (perhaps due to an increase in the average grain size and a reduction in the percentage of grain boundaries, where the defects responsible for this absorption feature are supposed to be preferentially localized) (Haq et al. 1994). The intensity of these bands in CVD diamond films increases as the CH4/H2 ratio of the feeding gas increases (Yin et al. 1997).
20
ABSORPTION COEFFICIENT, cm
-1
2840
15 2915
10 1739
5
0 1000
1500
2000
2500
WAVENUMBER, cm
3000
3500
4000
-1
Fig. 3.10. Absorption spectrum of a free-standing flame-grown CVD diamond film. The spectrum is replotted using the data of Janssen et al. (1991)
A fine structure is observed at wavenumbers of 2820, 2831, 2845, 2884 and 2923 cm-1 in 12C:H CVD diamond films. In 12C:H: 15N CVD diamond films an additional peak at 2834 cm-1 appears. In 13C:H films the corresponding structure is observed at wavenumbers of 2810, 2820, 2834, 2845, 2870, 2908 cm-1. In 12C:D deuterated CVD diamond films three additional relatively broad bands peaking at 2075, 2120 and 2170 cm-1 can be distinguished (possibly, this is an effect of lifetime broadening due to resonance of the C-D stretching vibrations with the intrinsic twophonon band) (Fuchs et al. 1995b). All of the lines coincide, with an accuracy of ±10 cm-1, with those present in α−C:H carbon films. After deuteration of the pre-
36
3 Vibronic Absorption
hydrogenated diamond surface at increased temperatures the spectral range of the bands shifts towards 2030-2250 cm -1 (this energy is attributed to C-D stretching vibrations). Upon deuteration the high-energy band of the doublet (symmetrical band) is reduced faster as compared to the low energy band (asymmetrical band). The peak at 2835 cm-1 shifts with deuteration towards lower energies, while the peak at 2821 cm-1 does not change its position noticeably. The bands disappear upon deuteration at temperatures above 650°C (Charette 1959). Annealing at 850°C does not change the absorption spectra of these peaks in CVD diamond films (Yin et al. 1997; Zhu et al. 1993). The portion of sp2 bonding (vibrations at wavenumbers below 2960 cm-1) over sp3 bonding (vibrations at wavenumbers above 2960 cm-1) increases in the films grown at high CH4 concentrations (Yin et al. 1997). The mode 2923 cm-1 appears to be Raman active (see below). The concentration of the CH2 groups responsible for this absorption doublet in intentionally undoped HFCVD diamond films is 1000 ppm (Kislovskii and Spitsyn 1989). The total C-H band absorption intensity can be calculated according to the relation (Ertz et al. 1995): 0.7 ×10 21 cm −2 nH = 2900cm −1
3000cm −1
∫ µ (ν )dν .
2800cm −1
The C-H bond concentration NC-H can be found as follows (Yin et al. 1997; Basa and Smith 1990): NC-H = 2.303 KC-H SC-H/, where KC-H = 1.35×1021 cm-2 is the cross-section for the C-H stretch mode, SC-H is the area of the C-H absorption band, and = 2910 cm-1 is the average wavenumber of the band. The NC-H value can be apportioned in CVD diamond films between nondiamond (Vnd) and void (Vv) components as following (Yin et al. 1997): NC-H[cm-3]= 0.36×1022 Vnd + 10.8×1022 (Vv)2/3. The concentration of hydrogen NH in type IIc diamonds can be found from the following relation (Janssen et al. 1991; Enckevort 1994): NH[ppm] = 103 µ2840cm-1[cm-1]. 3.432 µm (2922 to 2904 cm-1); FWHM of 30 to 50 cm-1; observed in PCCVD diamond films and epitaxial single-crystal CVD diamond films deposited onto (110)-oriented substrates. The feature is attributed to the sp3 CH configuration of C−H symmetric stretching vibrations. The vibration symmetry is C3V:A1 (Dischler et al. 1993; Janssen et al. 1992). Probably this very feature is observed by the SFG and MIRIRS methods on (100) diamond surfaces and plasma-hydrogenated (100)-2×1 diamond surfaces (also natural type IIa diamonds) (Ando et al. 1994; Russell et al. 1999). When measured by the MIRIRS method the feature is strongly polarized (Russell et al. 1999).
3.2 Defect-Induced Vibrational Bands 37
3.449 µm (2899 cm-1); FWHM of 6 cm-1; observed by the MIRIRS method on hydrogenated (100) surfaces of natural type IIa diamonds. The feature reduces in intensity upon annealing at temperatures from 800 to 1000°C. It is ascribed to C-H asymmetric stretch vibration of the monohydride dimer on H-C(100)2×1 reconstructed surface (Russell et al. 1999). 3.478 µm (2875 cm-1) and 3.370 µm (2967 cm-1); FWHM of 35 to 45 cm-1; observed in PCCVD diamond and epitaxial CVD diamond films deposited onto (111)- and (110)-oriented substrates. The 2875 cm-1 band may change its spectral position from 2862 to 2884 cm-1. These bands are absent from the spectra of the films deposited onto (100)-oriented substrates. The features are attributed to symmetrical and asymmetrical C−H stretching vibrations of the sp3-bonded methyl groups −CH3 with symmetries C3V:A1 and E, respectively (Dischler et al. 1993; Gheeraert and Deneuville 1992b; John et al. 1994; Field 1992; Janssen et al. 1991; Dehbi-Alaoui et al. 1990; Dehbi-Alaoui et al. 1991; Janssen et al. 1992; Jubber et al. 1995). 3.480 µm (2870 cm-1); a broad band in the spectral range from 2820 to 2950 cm-1 observed in CVD diamond films grown at methane/hydrogen ratio above 6%. The feature can be observed in boron-doped CVD diamond films annealed at a temperature of 900°C. The feature is attributed to C-H stretching vibrations of hydrogen bonded to sp3 configured carbon (LeGrice et al. 1990a; Chia-Fu Chen et al. 1994b; Chia-Fu Chen and Sheng-Hsiung Chen 1995). 3.497 µm (2860 cm-1); FWHM ~ 25 cm -1; a band observed by the SFG method from a clean (111) diamond surface dosed to a hydrogen coverage of ~ 5% when reconstruction from (2×1) to (1×1) begins (Chin et al. 1992). 3.504 µm (2854 cm-1); see 3.421 µm (2923 cm-1). 3.521 µm (2840 cm-1); FWHM of 40 cm-1; a band observed by the SFG method from plasma-hydrogenated (111)-1×1 diamond surfaces. The feature is attributed to C−H symmetrical stretching vibrations of CH3 species (Ando et al. 1994). 3.527 µm (2817 to 2837 cm-1, commonly at 2820 cm-1); FWHM of 10 to 25 cm-1; a narrow band observed in CVD diamond films. It is pronounced in thick PCCVD films (Dischler et al. 1993; John et al. 1994). The band appears in spectra of H-terminated unreconstructed (111) surfaces (Dischler et al. 1993). It can be pronounced in spectra of single-crystal epitaxial CVD diamond films deposited onto (110)- and (111)-oriented substrates. However the line is absent from the spectra of the films grown on (100)-oriented substrates (Miller and Brown 1995; Janssen et al. 1991; Janssen et al. 1992). The feature is depressed by surface chlorination (Miller and Brown 1995). The annealing at 1200°C increases the intensity of the band (Nesladek et al. 1999). The feature is thought to be associated with hydrogen locating at crystallite grain boundaries: H-(111) (Dischler et al. 1993; John et al. 1994; Ertz et al. 1995). It is attributed to C(111)-H surface stretching vibrations of
38
3 Vibronic Absorption
symmetry C3V:A1 (C−H stretching vibrations of CH2 groups). The hydrogen directly bonded to the crystalline diamond at sites like dislocations is supposed to be responsible for this feature (Dischler et al. 1993; Miller and Brown 1995; Janssen et al. 1991; Janssen et al. 1992). McNamara et al (1994) suggest that the 2820 cm-1 band is related to N-H vibrations. 3.534 µm (2830 cm-1); FWHM of 8 of 15 cm-1; a narrow band observed by the SFG method from (111) surfaces of natural type IIa diamonds after covering with atomic hydrogen. The feature is attributed to the C−H stretch mode on the ideal (111) diamond surface (Chin et al. 1992; Ando et al. 1994). 3.559 µm (2810 cm-1); a weak line observed in some diamond films of high quality (Fuchs et al. 1995b).
13
C homoepitaxial CVD
3.589 µm (2786 cm-1); see the 3.219 µm (3107 cm-1) line. 3.606 µm (2773 cm-1); a weak line observed in some PCCVD diamond films (Dischler et al. 1993). 3.711 µm (2695 cm-1); a weak band observed in single-crystal epitaxial CVD diamond films deposited onto (110)- and (111)-oriented substrates. The feature is absent from the spectra of (100)-oriented films. This line strongly correlates with the 2949 cm-1 band. The feature is attributed to C−H stretching vibrations of CH2 groups. This is possibly a combined overtone of the 1252 cm-1 peak and the upper part of the one-phonon band at 1280 cm-1 (Janssen et al. 1991; Janssen et al. 1992). 3.720 µm (2688 cm-1); a weak narrow line observed in some flame-grown CVD diamond films exhibiting strong hydrogen-related absorption (Phillips et al. 1992). 3.92 µm (2550 cm-1); a broad band observed in synthetic diamonds within a spectral range from 2400 to 2700 cm-1. The band is enhanced by irradiation (Bokii et al. 1986). 4.08 µm (2450 cm-1); a band attributed to the O=C=O stretching vibrations localized at CO2 molecules physically adsorbed onto the diamond surface (Chia-Fu Chen and Sheng-Hsiung Chen 1995). 5.06 µm (1975 cm-1); a relatively broad band observed in some synthetic diamonds after electron irradiation and subsequent annealing at a temperature of 450°C (Collins et al. 1988c) (Fig. 3.11, 3.12). 5.198 µm (1924.0 cm-1); a weak band observed in heavily neutron irradiated natural diamonds after subsequent annealing at temperatures of 300 to 400°C. The band retains its spectral position in 12C:15N diamonds. It shifts to 5.408 µm (1849.0 cm-1) in 13C: 14N diamonds. The feature is ascribed to a vibration involving only carbon atoms. Very tentatively it is a carbon interstitial center. Possibly this vibration
3.2 Defect-Induced Vibrational Bands 39
(239 meV) interacts with the 5RL center (Field 1992; Collins et al. 1988c; Briddon and Jones 1993; Davies 1994a; Mainwood et al. 1994). The atomic model of this center is >C=C=C< group (Field 1992; Davies 1994a) (Fig. 3.11). 5 µm (∼2000 cm-1); a line observed in some synthetic diamonds doped with Al (Klimenkova et al. 1975c). 5.388 µm (1856 cm-1); FWHM of 10 cm-1; a line observed in type IaA natural and synthetic nitrogen-containing diamonds after irradiation and subsequent annealing at temperatures above 500°C (Field 1992; Davies 1994a; Collins and Woods 1987b). The center is stable at temperatures above 1400°C. In 12C:14N+15N diamonds two additional lines appear at 5.456 µm (1833 cm -1) and 5.495 µm (1820 cm-1). The feature is attributed to a radiation defect involving two nitrogen atoms (Field 1992) (Fig. 3.12).
1975
1924
1570
1706 1496
1400
1500
1600
1700
1800
WAVENUMBER, cm
1900
2000
-1
Fig. 3.11. Absorption spectrum exhibiting the tail of the two-phonon band of 12C synthetic diamond irradiated with 2 MeV electrons and subsequently annealed at 450°C (Collins et al. 1988c).
5.750 µm (1739 cm-1); a sharp peak observed in absorption in combustion flamegrown CVD diamond films and in fine grained diamond powders (Novikov 1968). Possibly this very feature is observed as a relatively broad band with FWHM of 60 cm-1 in DRFTI in diamond powders (Miller and Brown 1995). This band is strongly induced in diamonds by indentations with diamond tips as a result of mechano-chemical reactions between diamond and air under high pressure (Gogotsi et al. 1998). The feature may originate either from a C=O stretching vibration of
40
3 Vibronic Absorption
carbonyl groups (Janssen et al. 1991), or from the oxygen sorbed on the diamond surface (Novikov 1968) (Fig. 3.10). 5.8 µm (1720 cm-1); a line observed in some synthetic diamonds doped with As (Klimenkova et al. 1975c). 5.861 µm (1706.1 cm-1); a line observed in irradiated type Ib diamonds (including synthetic HPHT diamonds) after annealing at temperatures of 300 to 450°C. This line can be observed in diamonds exposed to very heavy neutron irradiation and subsequent annealing at a temperature of 600°C. The line is found in CVD diamond films treated by an N-atom beam (Jubber et al. 1995). Possibly this very center is generated in synthetic diamonds by doping with Ga and Si (Kluev et al. 1972c). The line anneals out at a temperature of 800°C (Collins 1991a). The line shifts to 5.961 µm (1677.7 cm-1) in 12C:15N diamonds and to 5.974 µm (1674.0 cm-1) in 13C:14N diamonds. The feature is attributed to stretching vibrations of double bonded >C=N− groups (Field 1992; Collins et al. 1988c; Davies 1994a; Mainwood et al. 1994; Jubber et al. 1995). This center is supposed to be responsible for the electrical conductivity of ion irradiated diamonds (Kluev et al. 1972c) (Fig. 3.11).
1975
1833 1820
1800
1856
1850
1900 WAVENUMBER, cm
1950
2000
-1
Fig. 3.12. Absorption spectrum of a synthetic diamond after irradiation with fast neutrons at a dose of 5×1017 cm-2 and subsequent annealing at 1400°C for 3 hours. The diamond was doped with 15N and 14N isotopes at a ratio of 7:3 (Collins and Woods 1987b)
6 µm (1670 cm-1); a line observed in some synthetic diamonds doped with As (Klimenkova et al. 1975c).
3.2 Defect-Induced Vibrational Bands 41
6.060 µm (1650 cm-1); a peak observed in A and DRFTI in boron-doped CVD diamond films and diamond powders. The feature is attributed to O-H bending vibrations of water absorbed on polar carbonyl groups on the diamond surface (Chia-Fu Chen et al. 1994b; Chia-Fu Chen and Sheng-Hsiung Chen 1995; Miller and Brown 1995) (Fig. 3.13).
N-H 3380 (O-H)
1380 (C-H bend)
2980 (C-H stretch) 1080 (B-H)
1650 (O-H bend) C=O=C
620 O-B-O
0
1000
2000
3000
WAVENUMBER, cm
4000
-1
Fig. 3.13. Absorption spectrum of a CVD diamond film compressed with KBr powder. The film was deposited from a gas mixture with addition of 30 sccm of CO2 (Chia-Fu Chen et al. 1994b)
6.1 µm (1640 cm-1); see 2.9 µm (3400 cm-1). 6.2 µm (1610 cm-1); a band observed in fine grained diamond powders. The band possibly originates from the oxygen absorbed on the diamond surface. The feature is attributed to the triple hydroxyl groups (Novikov 1968). 6.329 µm (1580 cm-1); a feature observed in some natural brown diamonds (Collins and Mohammed 1982b) (Fig. 3.14). 6.369 µm (1570.3 cm-1); a narrow line appearing in types Ia, Ib and IIa diamonds immediately after irradiation (with electrons or neutrons). This line may become a dominating feature in diamonds of any type after neutron irradiation with doses above 1019 cm-2 (Malogolovets et al. 1978c). The line intensity increases after annealing at temperatures of 300 to 500°C. Note that this is the same temperature range when the 1531 cm-1 center anneals out. The 1570.3 cm-1 line anneals out at temperatures of 600 to 750°C. The line does not change its spectral position in
42
3 Vibronic Absorption
12
C:15N diamonds. In 13C:14N diamonds the line shifts to 6.630 µm (1508.8 cm-1) exhibiting the shift factor (12/13)0.5 and implying vibrations of only carbon atoms. This feature is very tentatively attributed to a split interstitial carbon center (two adjacent carbon atoms) (Bokii et al. 1986; Field 1992; Collins et al. 1988c; Davies 1994a; Woods 1984; Mainwood et al. 1994; Collins 1991a; Bienemann-Kuespert et al. 1967; Mainwood 1999). Numerical modeling of the split carbon interstitial predicts a vibration of energy 1590 cm-1 active in IR absorption. Interesting, that the calculations predict a low mechanical softness of the feature: SP = 0.5 meV/GPa (Breuer and Briddon 1995; Mainwood 1999) (Fig. 3.11). 6.464 µm (1547 cm-1); a sharp peak observed in some type IaAB natural diamonds containing hydrogen (Ferrer and Nogues-Carulla 1996) (Fig. 3.7). 6.477 µm (1544 cm-1); a feature observed in some natural brown diamonds (Collins and Mohammed 1982b) (Fig. 3.14).
1278
ABSORPTION COEFFICIENT, cm
-1
1.5
1240 1169 1329
1.0 1439
1580 1544
0.5 1022
0.0 800
1000
1200
1400
WAVENUMBER, cm
1600
-1
Fig. 3.14. RT IR absorption spectrum of a brown natural diamond exhibiting a particularly strong 2.6 eV absorption band in the visible spectral region (Collins and Mohammed 1982b)
6.534 µm (1530 to 1535 cm-1); a line appearing in type Ia, Ib and IIa diamonds immediately after electron or neutron irradiation. The line is particularly strong in type Ib diamonds. This feature is considered to be a dominating feature in diamonds of any type after neutron irradiation with doses above 1019 cm-2 (Malogolovets et al. 1978c). The center is readily formed by electron irradiation in strained type I diamonds (Phaal 1965). The center is stable to a temperature of 400°C. It anneals out rapidly at temperatures above 400°C (Malogolovets et al. 1978c). The line does not change its spectral position in 12C:15N diamonds and it shifts to a wavelength of
3.2 Defect-Induced Vibrational Bands 43
6.799 µm (1470.7 cm-1) in 13C:14N diamonds. It has been shown that only carbon atoms are involved in the corresponding vibration. Very tentatively the feature is attributed to a nitrogen interstitial atom surrounded by carbon atoms (Bokii et al. 1986; Collins et al. 1988c; Briddon and Jones 1993; Davies 1994a; Woods 1984; Mainwood et al. 1994; Collins 1991a; Bienemann-Kuespert et al. 1967; Malogolovets et al. 1978c) (Fig. 3.15).
1130
1332 1050
1344
920
1530
800
1000
1200 WAVENUMBER, cm
1400
1600
-1
Fig. 3.15. Absorption spectrum in the one-phonon region of a irradiated with 2 MeV electrons (Collins et al. 1988c)
12
C synthetic diamond
6.623 µm (1510 cm-1); a narrow line appearing in synthetic and nitrogen-containing natural diamonds after irradiation and subsequent annealing at temperatures above 600°C. The line intensity correlates with the concentration of single nitrogen atoms. A possible origin of this feature is a complex combining interstitial nitrogen atoms bound to vacancies (Bokii et al. 1986; Malogolovets et al. 1978c). 6.658 µm (1502 cm-1); a narrow line observed only in type Ib diamonds after irradiation and subsequent annealing at a temperature of 750°C. The line anneals out at a temperature of 1000°C (Collins 1991a). The line changes its spectral position to 6.761 µm (1479.1 cm-1) in 12C:15N diamonds and to 6.789 µm (1473.0 cm-1) in 13 14 C: N diamonds. The feature is attributed to C−N vibration. This center is possibly a different charge state of the defect producing the absorption line at 6.896 µm (Field 1992; Collins et al. 1988c; Briddon and Jones 1993; Davies 1994a; Mainwood et al. 1994; Bienemann-Kuespert et al. 1967) (Fig. 3.16).
44
3 Vibronic Absorption
1344 1450
1502
1300
1350
1400 WAVENUMBER, cm
1450
1500
-1
Fig. 3.16. Absorption spectrum of a 12C synthetic diamond after neutron irradiation and subsequent annealing at 800°C. The spectrum shows the spectral range just above the onephonon absorption region (Collins et al. 1988c)
6.68 µm (1496 cm-1); a weak relatively broad line observed in some synthetic diamonds after electron irradiation and subsequent annealing at temperatures of 450°C (Collins et al. 1988c) (Fig. 3.11). 6.7 to 9.1 µm (1500 to 1100 cm-1); a broad complicated band observed in brownish CVD diamond films; tentatively attributed to C-H-bend or -wag vibrational modes (Sussman et al. 1994a). 6.752 to 6.944 µm (1481 to 1440 cm-1); a group of lines observed in some CVD diamond films. These lines are accompanied by a group of lines in the spectral region from 8.834 µm (1132 cm-1) to 10 µm (1000 cm-1) and at 14 µm (700 cm-1). All these features have been attributed to deformational modes of sp2-hybridized C−H bonds (Field 1992; Dehbi-Alaoui et al. 1990; Dehbi-Alaoui et al. 1991). 6.826 µm (1465 cm-1); a band observed in CVD diamond single crystalline films. The feature has been attributed to an H−C−H bending mode of CH2 groups (Field 1992; Janssen et al. 1991; Janssen et al. 1990). 6.873 µm (1455 cm-1); a band observed in type Ib synthetic diamonds after neutron irradiation and annealing at temperatures above 600°C. The line intensity correlates
3.2 Defect-Induced Vibrational Bands 45
with the concentration of single nitrogen atoms. The feature is attributed to a vacancy-related defect (Malogolovets et al. 1978c). 6.895 µm (1450.3 cm-1); possibly the H1a center (Clark et al. 1956c). This line is observed in both types Ia and Ib diamonds (including HPHT synthetic diamonds) after irradiation and subsequent annealing at temperatures above 500°C. The line intensity correlates with the concentration of single nitrogen atoms (Malogolovets et al. 1978c). The intensity of the line is enhanced by annealing at a temperature of 1000°C (Collins 1991a). The line anneals out at a temperature of 1500°C (Collins 1980). The spectral position of the line changes for 7.0126 µm (1426.0 cm-1) in 12 15 C: N diamonds and for 7.025 µm (1423.4 cm-1) in 13C: 14N diamonds. Studies of the center in diamonds with different combination of isotopes 12C, 13C, 14N, and 15N unambiguously point to a vibration of one nitrogen and two equivalent carbon atoms C-N-C. The involved nitrogen atom possibly occupies an interstitial position (Field 1992; Davies 1994a; Collins et al. 1988c; Woods 1984; Mainwood et al. 1994; Antsygin et al. 1996; Kiflawi et al. 1996). The numerical modeling of the defect suggest a quasi-bond-center interstitial nitrogen atom giving rise to the dominant vibrational modes at 1170, 1335 and 1450 cm-1 (Mainwood 1999). Another model of the center comprises interstitial nitrogen atoms bound to vacancies (Bokii et al. 1986) (Fig. 3.16). 6.9 µm (1450 cm-1); a band observed in type Ia diamonds and coats of natural diamonds in the spectral region from 6.8 µm (1470 cm-1) to 7 µm (1430 cm-1) (Angress and Smith 1965). The band is accompanied by features at 9.132 µm (1095 cm-1) and 11.4 µm (880 cm-1). A possible origin of the band is inclusions of carbonates (Bokii et al. 1986; Galimov et al. 1979). 6.94 µm (1440 cm-1); FWHM of 100 cm-1; a band observed in DRFTI spectra of diamond powders. Possibly this band is also observed in some natural brown diamonds (Collins and Mohammed 1982b). The feature is attributed to oxygencontaining groups locating at the diamond surface (Miller and Brown 1995) (Fig. 3.14). 6.99 µm (1430 cm-1); a band observed in nitrogen-containing diamonds. In pure type Ia diamonds this band correlates with the N3 center (Nepsha et al. 1971). 6.993 µm (1430 cm-1); a weak line observed in low-nitrogen type Ib natural diamonds. This is a naturally occurring center (Bokii et al. 1986; Woods 1984). 7.042 µm (1420 cm-1); a narrow line appearing in synthetic diamonds immediately after irradiation. This feature is dominant in diamonds of any type after neutron irradiation with doses above 1019 cm-2 (Malogolovets et al. 1978c). Possibly this line is induced in synthetic diamonds by doping with Ga and Si (Kluev et al. 1972c). The feature is attributed to an intrinsic defect containing interstitial atoms (Bokii et al. 1986).
46
3 Vibronic Absorption
7.04 µm (1420 cm-1); a broad band observed in transmission spectra of some CVD diamond films grown on a Si substrate (Gheeraert and Deneuville 1992b) (Fig. 3.17). 7.072 µm (1414 cm-1); a sharp peak observed in DRFTI spectra of amine-terminated diamonds. The feature is attributed to a δNH2 (scissors) vibrational mode (Miller and Brown 1995). 7.092 to 11.1 µm (900 to 1410 cm-1); a complicated absorption band observed in a spectral range similar to that of the absorption of the boron acceptors. Peaks at 1055, 1120, 1525 and 1332 cm-1 are superimposed on this band. The feature is tentatively attributed to a combination of H−C−H bending modes and a defect-induced onephonon absorption (Janssen et al. 1992) (Fig. 3.18). 7.117 µm (1405 cm-1); a very temperature stable hydrogen-related feature observed in diamonds of different origin. This line does not anneal out at temperatures as high as 2350°C (Brozel et al. 1978) (see 3.219 µm (3107 cm-1) line) (Fig. 3.7, 3.8). 7.14 µm (1400 cm-1); a weak sharp line observed in type IIa diamonds implanted with C+ and B+ ions at LNT at subsequent annealed by the RTA method at a temperature of 1100°C (Sandhu et al. 1989).
TRANSMISSION, arb. units
0.9
0.8
460
0.7
1075
0.6
700 1420 800
0.5 500
1000
1500
2000
-1
WAVENUMBER, cm
Fig. 3.17. Transmission spectrum of a CVD diamond film deposited on a Si substrate from a gas mixture containing 1.2% of CH4 (Gheeraert and Deneuville 1992b)
3.2 Defect-Induced Vibrational Bands 47
120
ABSORPTION COEFFICIENT, cm
-1
1332
100 1252
80
1120 1055
60
40
20
0 800
1000
1200
1400
1600
-1
WAVENUMBER, cm
Fig. 3.18. Absorption spectrum of a flame-grown epitaxial CVD diamond film deposited on a (110)-oriented diamond substrate (Janssen et al. 1992)
0.08 1332
OPTICAL DENSITY
0.06
13
C
0.04 1281 1340 1330
0.02
1355
12
C
1397
0.00 1200
1250
1300
1350
WAVENUMBER, cm
1400
1450
-1
Fig. 3.19. Absorption spectra of homoepitaxial CVD diamond films grown from carbon isotopes (Fuchs et al. 1995b)
12
C and
13
C
48
3 Vibronic Absorption
7.168 µm (1397 cm-1); a weak line observed in some CVD diamond films (Fuchs et al. 1995b) (Fig. 3.19). 7.215 µm (1386 cm-1); a weak peak observed in some natural type Ib diamonds (Woods and Collins 1983) (Fig. 3.20). 7.25 µm (1380 cm-1); a sharp peak observed in nominally undoped and boron-doped CVD diamond films. The feature is attributed to bending vibrations of C-H bonds (Chia-Fu Chen et al. 1994b; Chia-Fu Chen and Sheng-Hsiung Chen 1995). Note that a local vibration of energy 1380 cm-1 is predicted at substitutional hydrogen atoms in the diamond lattice (Kurdumov et al. 1994; Malogolovets and Nikityuk 1981) (Fig. 3.13). 7.273 to 7.366 µm (1358 to 1380 cm-1); a sharp line the spectral position of which may vary within the indicated spectral range. This is the most characteristic feature of the B' center (alternative names of this center are B2-aggregates or platelets). The B' line is observed in nitrogen-containing type Ia diamonds. The center is never observed alone, but only together with the B-aggregates (Brozel et al. 1978). The feature is virtually absent from the spectra of type IaA diamonds (Collins 1997). The spectrum of the B' center has been isolated from that of the B center in (Sobolev et al. 1969b). The B' line shows no spectral shift in 12C:15N diamonds. The peak becomes stronger and narrower after neutron irradiation. However irradiation does not cause any spectral shift of the line (Bienemann-Kuespert et al. 1967). This line may be induced in type IIa diamonds by fast neutron irradiation with doses above 1017 cm-2 (Bienemann-Kuespert et al. 1967). There is a correlation between intensities of the B' absorption and the X-ray or electron microscopy spikes characteristic of the platelets. Hoever for some diamonds the spike intensity does not follow the B' peak intensity observed in A (Sobolev et al. 1968a; Evans 1979). The precise position of the line depends on the size of the platelets: the larger the platelets the greater the wavelength of the line maximum. The B' line is accompanied by a peak at 7.0 µm (1430 cm-1), which is not a sample-dependent one, and a weak broad band at 30.3 µm (330 cm-1). The intensity ratio of the 1430 and ∼1370 cm -1 bands depends on the platelet size (Bokii et al. 1986; Sobolev 1978). The size of the platelets may be as great as 100 µm. The large platelets are often seen in CL (yellow-green emission) as regular squares oriented in the (100) planes (Sobolev 1978). Platelets can be formed in synthetic type Ib diamonds by HPHT treatment at temperatures above 2700°C (Evans and Qi 1982a). Heating at temperatures of 2200 to 2300°C for a few minutes reduces the line intensity by 40 to 80%. The reason of this reduction is a decomposition of the platelets into smaller units, possibly the A-, B- and N3 centers (Evans 1979; Wilks J. and Wilks E. 1991). The B' center intensity correlates with the N3 center intensity (Sobolev et al. 1969b). In type Ia diamonds the intensity of the B' peak is proportional to the B-aggregate absorption strength (Field 1992; Woods 1986), however the strongest B' absorption is observed in natural type Ia diamonds containing about 20 to 40% of the A-aggregates of nitrogen and 60 to 80% of the B-aggregates of nitrogen,
3.2 Defect-Induced Vibrational Bands 49
whereas pure type IaA and IaB diamonds exhibit very low content of the B' centers (Brozel et al. 1978). Concentration of the B' defects (platelets) in natural diamonds may attain a value of only 1015 cm-3 (Kluev et al. 1982). The concentration of platelets NP can be found using the following relation (Sumida and Lang 1988): NP[platelet area in µm2/volume in µm3] = (9±2)×10-3?PeakB'[cm-2].
1.6
ABSORPTION COEFFICIENT, cm
-1
1140 1270 1240
1.2
0.8
1332.5 1090
1344
1358
1045
0.4
1386 970
0.0 900
1000
1100
1200
WAVENUMBER, cm
1300
1400
-1
Fig. 3.20. Absorption spectrum of a diamond showing type Ib character (Woods and Collins 1983)
The B' centers (platelets) probably do not involve nitrogen directly (Davies 1994a; Sobolev et al. 1968a; Evans and Rainey 1975; Sumida and Lang 1988; Woods et al. 1993a; Woods and Collins 1983; Szigeti 1963). The relation of nitrogen to the platelets can be given by two statements: (i) the platelets oriented in (100) planes do not comprise all the nitrogen impurity present in diamond; (ii) the platelets are composed not only of nitrogen (Lang 1979). A possible origin of the B' absorption is the C-C bond vibrations within the platelets, which can be formed from interstitial carbon atoms (Woods 1986). According to an early model the platelets are composed of a double layer of nitrogen atoms segregated in the (100) plane and replacing a single layer of carbon in this plane (the 1430 cm-1 line originates from the most deformed edge area of the platelets) (Sobolev 1978; Lang 1964; Hutchinson et al. 1982). The one-nitrogen-layer model (the N-C model) is discussed by Davies (1970a); Berger and Pennycook (1982); Humble et al. (1984). There is also a model of the platelets solely composed of carbon (Cowley et al. 1984; Humble 1982; Berman et al. 1975). A proof that platelets do contain nitrogen impurities is given by Pennycook (1983). Formation of the B'-aggregates in diamond has been considered as a polymerization process of CN2 groups (Sobolev 1978) (Fig. 3.7, 3.20).
50
3 Vibronic Absorption
7.353 µm (1360 cm-1); a line naturally occurring in low-nitrogen type Ib natural diamonds. This line can also be observed in boron-doped CVD diamond films. This feature is attributed to a B-H deformation vibration (Bokii et al. 1986; Chia-Fu Chen et al. 1994b; Chia-Fu Chen and Sheng-Hsiung Chen 1995) (Fig. 3.7, 3.20). 7.380 µm (1355 cm-1); a weak line observed in some CVD diamond films (Fuchs et al. 1995b). (Fig. 3.19). 7.440 µm (1344 cm-1); see 8.850 µm (1130 cm-1) (Fig. 3.15, 3.16). 7.46 µm (1340 cm-1); a weak line observed in some CVD diamond films. The line shifts to 1281 cm-1 in 13C CVD diamond films (Fuchs et al. 1995b) (Fig. 3.19). 7.508 µm (1332 cm-1); a sharp line appearing at the Raman frequency. The line is induced by the B-aggregates of nitrogen and by neutron irradiation (Bokii et al. 1986). This is a predominant feature in diamonds of any type after neutron irradiation with doses above 1019 cm-2 (Bokii et al. 1986; Malogolovets et al. 1978c). The line is particularly strong in neutron irradiated type Ib diamonds (Malogolovets et al. 1978c). The line is does not anneal at temperatures to 800°C (Malogolovets et al. 1978c). This feature can be particularly strong in synthetic diamonds grown in the presence of a nickel-containing catalyst (Davies 1994a; Collins and Spear 1982a; Collins and Spear 1983b). A vibration of energy 1340 cm-1 is predicted theoretically for quasi bond-center nitrogen interstitial atom (Mainwood 1999). A possible origin of this line is the substitutional nitrogen atoms in the ionized form (Ns+) (Lawson et al. 1998; Nadolinny et al. 1999). Possibly this line also relates to the X center (see below) (Fig. 3.15, 3.18, 3.20). 7.508 µm (1332 cm-1); the X center; a complicated absorption band in the spectral range from 7.435 to 13.3 µm (750 to 1345 cm-1) (Fig. 3.21). The center is observed in spectra of synthetic nickel-containing diamonds grown by the temperature gradient method. This center is also a common feature of boron-containing diamonds and electron irradiated diamonds, though it appears to be much stronger in the nickel-containing diamonds. The dominating features of this center are a sharp line at 1332 cm-1 and two broad less intense maxima at 1045 and 945 cm-1. The feature at 1332 cm-1 is broader in nickel-containing diamonds. The center intensity is proportional to the concentration of compensated boron. The center is strongly reduced or, possibly, completely annealed at a temperature of 2150 K (Yelisseyev and Nadolinny 1993). One of the models of this center is a defect containing nitrogen and boron atoms (Novikov 1968). The center is tentatively assigned to the one-phonon absorption of positively charged single substitutional nitrogen atoms (N+) (Lawson and Kanda 1993b; Lawson et al. 1998). According to this model the concentration of N+ centers can be evaluated from the 1332 peak intensity as follows (Lawson et al. 1998): NN+[ppm] = (5.5 to 7)×µ1332[cm-1].
3.2 Defect-Induced Vibrational Bands 51
1332
5
ABSORBANCE
4
3
2
945
1045
1
0 800
1000
1200 WAVENUMBER, cm
1400 -1
Fig. 3.21. The X center absorption in a synthetic diamond grown from a nickel catalyst with nitrogen getter added (Lawson and Kanda 1993b)
7.69 µm (1300 cm-1); a weak line observed in some CVD diamond films (Fuchs et al. 1995b). 7.69 µm (1300 cm-1); FWHM of 20 cm-1; a weak band observed in IR emission spectra of CVD diamond films of different quality (from white to black color) (Ayres et al. 1997). 7.69 µm (1300 cm-1); FWHM of 80 cm-1; a band observed in DRFTI spectra of diamond powders. The feature is attributed to oxygen-containing surface functional groups (Miller and Brown 1995). 7.752 µm (1290 cm-1); a broad complicated band usually observed in nitrogencontaining boron-doped diamonds and in synthetic diamonds grown from a Ni-MnC-B system. This band is called the D' center (not related to the D center with the line at 8.26 µm). The 1290 cm-1 line is accompanied by the bands peaked at 990 and 1065 cm-1. Usually closely related bands at 1290-1320 and 1335 cm -1 are observed together with the D' center. The D' center may be most intensive in high-nitrogen synthetic diamonds. The diamonds exhibiting the D' center are characterized by an increased Mn and Ni content (Malogolovets and Vishnevskii 1975a). The center is nonparamagnetic, but all of the diamonds exhibiting the D' center are characterized by the Ni-related EPR center (Malogolovets and Vishnevskii 1975a). The intensity of the D' center may be as high as 0.3 of that of the absorption at a wavelength
52
3 Vibronic Absorption
2810 cm-1 (Nachalnaja et al. 1980). It has been noticed that increase in the D' center content increases the hardness of synthetic diamonds (Novikov 1968). The D' center is attributed to the vibration of two neighboring B and N substitutional atoms. The center considerably stimulates the lattice vibration at 1332 cm-1 due to polarization of the B−N bond (transition of electron from N to B) (Bokii et al. 1986; Kluev et al. 1974a; Malogolovets 1983; Novikov 1968; Nachalnaja et al. 1980). The atomic structure of the D' center is thought by Malogolovets and Vishnevskii (1975a) to contain also atoms of metals. The boron concentration NB in synthetic diamonds can be evaluated from the intensity of the D' center with an accuracy of 20% using the following relation (Malogolovets 1981; Novikov 1968): NB ≈ 1.6×1017µ1290. The concentration ND' of the defects responsible for the D' center can be found from the intensity of its absorption as follows: ND' = 0.9×1018(µ1290 - 0.34µ1135) (Malogolovets 1981; Malogolovets 1983; Novikov 1968). 7.75 µm (1290 cm-1); a band starting at about 1000 cm -1 and sharply falling down at the Raman energy of 1332 cm-1 (at RT). The band is observed in boron-doped CVD diamond films and in type IIb single-crystal diamonds (Chrenko 1973; Smith and Taylor 1962; Ertz et al. 1995; Mort et al. 1991). A similar absorption band can be induced in CVD diamond films by other impurities (Bi et al. 1990). The intensity of the band falls with temperature (Bienemann-Kuespert et al. 1967). The 1290 cm-1 band is supposed to be one-phonon vibronic absorption induced by substitutional boron atoms. A local vibration of energy 1280 cm-1 is predicted for substitutional boron atoms in the diamond lattice (Kurdumov et al. 1994; Malogolovets and Nikityuk 1981). Substitutional boron atom in the negative charge state is expected to have a resonant mode of energy 1288 cm-1 for (Goss et al. 1999). 7.800 µm (1282 cm-1); the A-aggregate of nitrogen (the A center). The 1282 cm-1 peak is the most intensive feature of the one-phonon absorption of the A-aggregates of nitrogen occurring within a range from 1050 to 1330 cm-1 (Davies 1994a; Davies 1971; Woods 1986; Kaiser and Bond 1959; Sobolev 1978) (Fig. 3.22). The absorption coefficient µA of the 7.8 µm band can be as high as 85 cm-1 (Bokii et al. 1986). The further spectral features of the A-aggregate spectrum are at 8.313, 9.149 and 20.8 µm (Sutherland et al. 1954; Kaiser and Bond 1959; Phaal 1965). The bands shift to shorter wavelength by only 0.01 µm when measured at a temperature of 5 K (Kaiser and Bond 1959). The absorption intensity at 7.8 µm in natural diamonds can attain a value of 80 cm-1 (Kurdumov et al. 1994). The A center is paramagnetic when in an ionized state (van Wyk and Loubser 1983). In addition to the 7.8 µm band the A-aggregates give rise to an absorption continuum at quantum energies above 3.7-4 eV, and to a photoconductivity continuum at energies above 4 eV (Denham et al. 1967; Tatarinov 1986; Konorova and Shevchenko 1967). The peak 7.8 µm (1282 cm-1) shifts to 7.886 µm (1268 cm-1) in 12C:15N diamonds (Davies 1994a; Collins et al. 1987a; Evans 1979). The A center is the most characteristic naturally occurring absorption feature of type IaA natural diamonds. The A center is very seldom in synthetic diamonds grown by the traditional method of HPHT synthesis (Charette 1962). The center can
3.2 Defect-Induced Vibrational Bands 53
be produced in type Ib diamonds by radiation damage and subsequent heating at temperatures above 1500°C (Collins 1980; Collins 1978c; Collins 1991a), or by sole heating at temperatures above 1700°C (Chrenko et al. 1977; Evans and Qi 1982a). The A center is formed in synthetic diamonds during growth at temperatures above 1350 (Collins 1991a; Kluev et al. 1975; Kanda et al. 1988; Kanda and Watanabe 1998). The A center is a very temperature stable one. It starts to anneal out only at temperatures above 2300°C being dissociated into C-defects (single nitrogen atoms); however only 0.5% of the A-aggregates decompose at this temperature (Brozel et al. 1978). In the temperature range 2000 to 2350°C the A-aggregates can be formed from the C-defects. The mutual transformation of the C- and A centers (rate of the A↔C reaction and its direction) at high temperatures is determined by their initial concentration: the reaction is directed to the equilibrium concentrations of the A-and C centers at given temperature. At temperatures above 2500°C the A-aggregates are converted progressively into B-aggregates, N3 centers and platelets (Evans and Qi 1982b).
1282
6
ABSORBANCE
5
4 1215 3
2 1
0 800
1000
1200
1400 -1
WAVENUMBER, cm
Fig. 3.22. Absorption spectrum of the A center in a synthetic diamond of pure IaA type (Lawson and Kanda 1993b)
The impurity-related nature of the A center was first proposed by Burstein and Oberly (1950). The defects responsible for the A center consist of two neighboring substitutional nitrogen atoms relaxed away from each other by about 1/3 of the normal C-C distance (N-N bond length of 0.214 nm) (Jones et al. 1992; Mainwood 1994; Sobolev et al. 1967b). The bands at 7.8, 8.3 and 9.1 µm originate from three C-N stretching vibrations, while the 20.8 µm band corresponds to a bending C-N vibration (Kaiser and Bond 1959). In synthetic diamonds the A-aggregates locate
54
3 Vibronic Absorption
mostly at the interfaces between the growth pyramids (Palyanov et al. 1997a). An alternative model of the A-aggregate of nitrogen is a spherical macroscopic atomic association of size 4 nm, possibly, of weakly bonded nitrogen atoms (Bokii et al. 1986; Naletov et al. 1977). However, this large spherical defect area may merely relate to the area of mechanical strains surrounding the pair of nitrogen atoms. There is a model regarding the A center as a single nitrogen atom, in particular a N+ ion (Tatarinov 1986; Nepsha et al. 1982). Electrical activity of the A center has not been proved unambiguously yet: the center is believed to be a deep donor (Denham et al. 1967; Konorova and Shevchenko 1967), or acceptor (Tatarinov 1986). The nitrogen concentration NA in the A-aggregates can be evaluated using the following expressions: NA [ppm] = (10 to 33.3) µ1282[cm-1] (Field 1992; Woods et al. 1990a; Kaiser and Bond 1959; Kiflawi et al. 1993); NA [ppm] = 16.3 µ1282[cm-1] (Kiflawi et al. 1994; Boyd et al. 1994) (this relation appears to be the most reliable one *); NA [atom%] = 3.3×10-3 µA[cm-1] (Davies and Summersgill 1973c); NA [cm-3] = 5.8×1018 µ1282[cm-1] (Bokii et al. 1986; Kaiser and Bond 1959; Sobolev 1978). In diamonds containing considerable amounts of A- and B-aggregates the A-aggregate concentration can be found as follows (Kurdumov et al. 1994; Davies 1980): 2.72(µ1282 / µ1175 ) − 1 1 − 0.41(µ1282 / µ1175 ) . 2.72(µ1282 / µ1175 ) − 1 1+ 1 − 0.41(µ1282 / µ1175 )
N diam µ1282 −3
N A [cm ] = 0.0033
An approximate decomposition of the 1282 cm-1 peak into the parts related to the A- and B-aggregates can be given as µA/µB = 11.4/1 (Evans and Qi 1982b). The absorption coefficient related to the A-aggregates in type Ia+Ib diamonds can be given as µA = 1.1µ1282 - 0.2µ1135 (Malogolovets 1984; Antsygin et al. 1995; Kluev et al. 1979b), or µA = 1.2µ1282 - 0.49µ1175 (Bokii et al. 1986; Kurdumov et al. 1994). The B-defects also contribute to the absorption at a wavelength of 7.8 µm with a conversion coefficient ranging from 64 to 103.8 ppm of nitrogen per 1 cm -1 of absorption coefficient (Woods et al. 1990a; Kiflawi et al. 1993). The change of the lattice constant of diamond ∆a versus strength of the Aaggregate absorption and A-aggregate concentration is given by the relations (Kaiser and Bond 1959; Suvorovtsev et al. 1999): ∆a [nm] = 5×10-7µA[cm-1]; ∆a/a = 0.18×10-6 NA[ppm]. The presence of the A-aggregates at a concentration of 3×1020 cm-3 increases the lattice constant of diamond by 0.000028 nm (Kurdumov et al. 1994; Lisoivan and Sobolev 1974). The A-aggregates of nitrogen are very effective recombination centers of nonequilibrium charge carriers and deep traps of free charge carriers. Fig. 3.23
3.2 Defect-Induced Vibrational Bands 55
shows that they can even dominate as recombination centers in type IIa natural diamonds (Fahrner et al. 1998). 7.8 µm (1280 cm-1); the strongest peak of the IR absorption spectrum induced in type IIa diamonds by fast neutron (dose above 1017 cm-2) irradiation. The feature is also observed in CVD diamond films (Fuchs et al. 1995b). Possibly this band is naturally occuring in some brown diamonds (Collins and Mohammed 1982b). The feature is attributed to TO phonon absorption (Bienemann-Kuespert et al. 1967) (Fig. 3.6, 3.14, 3.19).
Free Exciton CL Intensity, arb. units
7.9 and 8.95 µm (1265 and 1117 cm-1); two bands observed in fine grained diamond powders. The bands originate from the oxygen absorbed on the diamond surface. The features are attributed to C=O vibrations of ether-type bonds (Novikov 1968).
10
3
10
2
10
1
10
0
-1
10
10
16
10
17
18
10
10
19
20
10 -3
Concentration of A-aggregated nitrogen, cm
Fig. 3.23. CL intensity of free excitons versus concentration of the A-aggregates of nitrogen in several natural diamond substrates used for the fabrication of p-i-n light emission diodes. The law IFE ∼ NA-aggr-2 is a hint of the dominant electron-hole recombination via the A-aggregates. The vertical arrow points to an approximate concentration at which the A-aggregates become the dominant recombination centers (Fahrner et al. 1998)
7.905 µm (1265 cm-1); a weak line observed in some (Fuchs et al. 1995b).
13
C CVD diamond films
7.94 µm (1260 cm-1); a sharp line appearing in type IIa diamonds implanted by C+ and B+ ions at LNT at subsequent annealed by RTA at a temperature of 1100°C (Sandhu et al. 1989).
56
3 Vibronic Absorption
7.987 µm (1252 cm-1); a sharp peak observed in homoepitaxial CVD diamond films deposited onto (110)- and (111)-oriented diamond substrates. This peak strongly correlates with the 2949 cm-1 band. The feature is possibly induced by hydrogen (tentatively a H−C−H bending mode) (Janssen et al. 1992). A local vibration with a wavenumber of 1250 cm-1 localizing at a substitutional hydrogen atom in the diamond lattice is predicted by Kurdumov et al. (1994); Malogolovets and Nikityuk (1981) (Fig. 3.18). 8.000 µm (1250 cm-1); the F center; a complicated band in the spectral range from 1100 to 1335 cm-1 with maxima at 975, 1155, 1250 and 1332 cm -1 observed in nitrogen-containing diamonds (Clark and Davey 1984b) (Fig. 3.24). 8.065 µm (1240 cm-1); a band observed in brown hydrogen-rich natural diamonds showing type Ib character. The feature appears to correlate with the 2.6 eV absorption band (Collins and Mohammed 1982b; Woods and Collins 1983) (Fig. 3.14).
-1
154 (1250 cm )
-1
157 (1267 cm )
-1
144 (1155 cm )
-1
164 (1332 cm )
-1
121(976 cm )
110
120
130
140
150
160
170
QUANTUM ENERGY, meV
Fig. 3.24. Simulated absorption spectrum of the F center. The spectrum has been isolated from the experimental absorption spectrum of a natural diamond by subtraction of the spectra of the A- C- and E centers (Clark and Davey 1984b). The main features of the F center appear to be the bands at 976, 1155, 1250, 1267 and 1332 cm-1
7.87 µm (1240 cm-1); a broad peak observed in natural diamonds of type Ib character (Woods and Collins 1983) (Fig. 3.20).
3.2 Defect-Induced Vibrational Bands 57
8.197 µm (1220 cm-1); a broad band within the spectral range from 900 to 1550 cm-1 with features at 7.506 µm (1332 cm-1), 5.090 µm (1100 cm -1), 9.804 µm (1020 cm-1) and 10.8 µm (926 cm-1) observed in polycrystalline natural and synthetic diamonds containing inclusions of lonsdaleite and graphite. The band is attributed to the absorption of the diamond lattice distorted by inclusions of lonsdaleite (Bokii et al. 1986; Kluev et al. 1978; Galimov et al. 1980; Kurdumov et al. 1994; Lukjanovitch et al. 1978). 8.251 µm (1215 cm-1); a feature of the one-phonon absorption of the A-aggregates of nitrogen (Davies 1994a; Davies 1971; Woods 1986). (Fig. 3.22). 8.26 µm (1210 cm-1); the D center; a complicated absorption spectrum in a spectral range from 1100 to 1335 cm-1 with the most intensive band ranging from 1324 to 1332 cm-1. The D center is observed only in diamonds containing the B-aggregates of nitrogen. There is a direct correlation between absorption intensities of the platelets and the D center. The D center is tentatively attributed to nitrogencontaining defects (Clark and Davey 1984b). The D center may relate to diamond lattice vibrational modes stimulated by the platelets (Field 1992; Woods 1986; Clark and Davy 1984a; Collins 1997) (Fig. 3.25).
-1
165 (1332 cm )
-1
150 (1210 cm )
135
140
145
150
155
160
165
170
QUANTUM ENERGY, meV
Fig. 3.25. Simulated absorption spectrum of the D center. The spectrum has been isolated from the experimental absorption spectrum of a natural type Ia diamond by subtraction of contributions of the A- and B-aggregates of nitrogen (Clark and Davey 1984b). A characteristic feature of the D center appears to be a broad band at 1210 cm-1
58
3 Vibronic Absorption
8.3 µm (1200 cm-1); a very complicated structured band in the spectral range from 500 to 1550 cm-1 consisting of a number of narrow lines (the most intense are those at 1545, 1495, 1430, 1010, 870, 770, 710 cm-1). The band is observed in natural hydrogen-rich diamonds of light gray color (Fritsch et al. 1991a). Local vibrations of energies 1380 and 1250 cm-1 localizing at substitutional hydrogen atoms in the diamond lattice are predicted by Kurdumov et al. (1994); Malogolovets and Nikityuk (1981) (Fig. 3.26). 8.3 µm (1200 cm-1); a band observed in type Ib diamonds after neutron irradiation. The band is not observed in nitrogen-free diamonds. The feature anneals out at temperatures above 800°C (Malogolovets et al. 1978c). 8.3 µm (1200 cm-1); a weak shoulder appeared in absorption spectra of type IIa diamonds implanted by C+ and B+ ions at LNT and subsequently annealed by the RTA method at a temperature of 1100°C (Sandhu et al. 1989). 8.4 µm (1190 cm-1); a band induced by radiation damage in low-nitrogen diamonds irradiated with neutrons (irradiation dose above 1017 cm-2) (Bokii et al. 1986; Bienemann-Kuespert et al. 1967).
ABSORPTION COEFFICIENT, cm
-1
5
4
1430 1495 1010
3
1545 3105
870 770
2
3235 4495 710
1
0 0
1000
2000
3000
WAVENUMBER, cm
4000
5000
-1
Fig. 3.26. Absorption spectrum of a light gray hydrogen-rich natural diamond (Fritsch et al. 1991a).
8.511 µm (1175 cm-1); the B-aggregate of nitrogen (the B center). In some publications the center is labeled as B1 or B1 (Fig. 3.27). The 8.511 µm band is the most intensive peak of one-phonon absorption of the B-aggregates of nitrogen
3.2 Defect-Induced Vibrational Bands 59
within a spectral range from 850 to 1330 cm -1. The absorption intensity at 8.511 µm in natural diamonds can be as high as 40 cm-1 (Bokii et al. 1986; Kurdumov et al. 1994). The B-aggregates are naturally occurring defects in almost all type Ia natural diamonds. The spectral peculiarities of the absorption spectrum of the B center are a shoulder at 9.124 µm (1100 cm-1) and features at 7.013, 7.289, 7.508, 8.540, 9.970, 12.8 and 30.5 µm (Woods 1986; Sutherland et al. 1954; Phaal 1965; Woods 1989). However the features at 7.013, 7.298 and 30.5 µm are possibly also attributed to the platelets (Jones et al. 1992). The intensity and spectral position of the B center absorption does not depend on temperature to 647 K (Bienemann-Kuespert et al. 1967; Collins R. and Fan 1954). The intensity of the B center increases by 3% when diamond is cooled down to 90 K (Bienemann-Kuespert et al. 1967). 1.2
Type IaB
1170
ABSORPTION, arb units
0.9
1332 0.6
1010 0.3
0.0 800
900
1000
1100
1200
1300
1400
-1
WAVENUMBER, cm
Fig. 3.27. Absorption spectrum induced by the B-aggregates of nitrogen (Lawson et al. 1998). This is a characteristic IR absorption spectrum of type IaB natural diamonds
A similar feature is observed in some combustion-grown CVD diamond films (Davies 1994a). The B centers can be formed in type Ib diamonds by radiation damage and subsequent heating at temperatures above 2200°C (Allen and Evans 1981; Evans and Qi 1982b). Aggregation of the C centers into the B-aggregates (formation of the A-aggregates of nitrogen is possibly a necessary intermediate stage) occurs at temperatures above 2400°C with an activation energy considerably above 5 eV (Evans and Qi 1982b). The B center is not affected by heating to a temperature of 2300°C, being much more stable than the A center (Evans 1979). The A-aggregates of nitrogen can be directly converted into the B-aggregates without radiation damage at temperatures around 2500 to 2600°C (Evans et al. 1981; Evans and Zengdu 1980; Evans and Qi 1982a). Some decomposition of the B-aggregates of nitrogen into single substitutional nitrogen atoms in natural type IaB
60
3 Vibronic Absorption
diamonds occurs at 2240°C (Brozel et al. 1978). Formation of the B-aggregates has been thought to be a dissociation of the A-aggregates into C-defects with subsequent gathering the C-defects into the B-aggregates (Evans 1979). B center absorption can be considerably stimulated by plastic deformation and subsequent electron irradiation (Phaal 1965). The impurity-related nature of the B center was proposed by Burstein and Oberly (1950) and confirmed by Davies (1977a). The most widely used model of the B center is a cluster of four nitrogen atoms surrounding a vacancy, the nitrogen atoms relaxing outwards by about 1/10 of the C-C bond length from the vacancy (N-C length is 0.149 nm) (Davies 1994a; Davies 1971; Woods 1986; Sutherland et al. 1954; Robertson et al. 1934; Jones et al. 1992; Mainwood 1994). The alternative models of the B-aggregates of nitrogen are (i) the dislocation loops stabilized by nitrogen (Sobolev and Lisoivan 1972b), (ii) macroscopic defects of size 8 nm (Bokii et al. 1986), or (iii) nitrogen aggregates in octahedral (111) planes (Sobolev and Dubov 1979a; Sobolev and Dubov 1975b; Sobolev 1978). There is also a model of the B center as negatively charged boron atoms B-, vibrations of the B--C bonds giving the characteristic B center absorption (Tatarinov 1986). Formation of the B-aggregates is considered as a polymerization process of CN2 groups (Sobolev 1978). The following relation can be used for the evaluation of nitrogen concentration NB in the B-aggregates: NB[cm-3] = (2.4 to 7.6)×1018 µ1175[cm-1] (Sobolev and Lisoivan 1972b). There is an expression for the evaluation of nitrogen content in the B-aggregates by measuring absorption at a wavelength of 1282 cm-1 (plateau area) (Woods et al. 1990a; Boyd et al. 1995; Evans and Qi 1982b). This relation appears to be the most reliable one: NB[ppm] = (79.4 to 103.8) µ1282[cm-1]. In diamonds containing considerable amount of A- and B-aggregates, the B-aggregate concentration can be found as follows (Kurdumov et al. 1994; Davies 1980): N B [cm −3 ] = 0.012
N diam µ1282 , 2.72(µ1282 / µ1175 ) − 1 1+ 1 − 0.41( µ1282 / µ1175 )
and the real intensity of the B-aggregate absorption is given as (Bokii et al. 1986; Kurdumov et al. 1994): µB = 1.2µ1175 - 0.51µ1282. There is a good correlation between the B-aggregate absorption and intensities of the N9- and N10 centers (Bokii et al. 1986; Sobolev et al. 1969b): µB = 0.16 µ236nm = 2.66 µ240nm. Interesting, that a 1179 cm -1 vibrational feature is predicted for the bond-center nitrogen interstitial atom in the diamond lattice (Mainwood 1999). 8.55 µm (1170 cm-1); a sharp line observed in some natural type Ia diamonds. The feature is ascribed to hydrogen-related vibration (Reinitz et al. 1998) (Fig. 3.8).
3.2 Defect-Induced Vibrational Bands 61
8.55 µm (1169 cm-1); a feature observed in some natural brown diamonds (Collins and Mohammed 1982b) (Fig. 3.14). 8.70 and 9.09 µm (1150 and 1100 cm-1); one-phonon absorption features tentatively ascribed to dislocation loops (Boyd et al. 1995). 8.834 to 10 µm (1000 cm-1 to 1132 cm-1); see 6.752 to 6.944 µm (1481 to 1440 cm-1). 8.850 µm (1135 cm-1); the C center, in some publications called the N-aggregate (Sobolev 1978; Smith et al. 1959) (Fig. 3.16, 3.20, 3.28). The C center is readily observed in any high-nitrogen diamonds. Traces of the C center are detected in almost all natural diamonds (Sobolev 1978). The C center is the main IR absorption feature of nitrogen-containing HPHT synthetic diamonds. The C center can be observed in CVD diamond films (Davies 1994a). The 8.850 µm (1135 cm-1) line is the main absorption line induced by single substitutional nitrogen atoms within the spectral range of 1000 to 1335 cm -1 (Davies 1994a; Dyer et al. 1965a). The spectral position of this peak may vary between 1119 and 1135 cm-1 (Charette 1962; Charette 1961b). The sharp line at 7.440 µm (1344 cm-1) is another prominent feature associated with the C center (Field 1992; Collins et al. 1988c; Davies 1994a; Dyer et al. 1965a; Clark et al. 1992a; Collins and Woods 1982d; Mainwood et al. 1994; Charette 1962). Usually the 1135 cm-1 peak is considerably sharper in synthetic diamonds than in type Ib natural diamonds (Collins 1980). There is a relation between the intensities of the main peaks (Lawson et al. 1998): µ1344[cm-1] = 0.572 µ1130[cm-1]. There is a band at 9.1 µm (1100 cm-1) in the absorption spectrum of the C center, which is also believed to relate to the B-aggregates of nitrogen. The absorption coefficient related to the C center in type Ib+Ia diamonds can be given as µc = 1.1µ1135 - 0.32µ1282 (Malogolovets 1984; Antsygin et al. 1995; Kluev et al. 1979b). The spectral position of the 1344 cm-1 peak shifts to 7.740 µm (1292 cm-1) in 13C diamonds. No shift of the 1344 cm-1 peak occurs in 12C:15N diamonds (Collins et al. 1993b; Collins and Woods 1982d); that is the nitrogen atom, if involved, is virtually in a stationary position (Field 1992). In 12C:15N diamonds the band 1135 cm-1 shifts to 8.969 µm (1115 cm-1) (Davies 1994a; Collins and Woods 1982d; Kurdumov et al. 1994; Malogolovets 1984; Kluev et al. 1974b; Samoilovich et al. 1974). The concentration of the C centers in yellow coats of some natural diamonds attains a value of 1020 cm-3 (Podolskich et al. 1985). In synthetic diamonds grown with Si3N4 the C center concentration can be as high as 2×1020 cm-3 (Nachalnaja et al. 1980). In high-nitrogen synthetic diamonds the absorption coefficient µ1135 may attain a value of 80 cm-1 (Kurdumov et al. 1994). Usually synthetic diamonds grown with a pure nickel catalyst contain about 300 ppm of the C-defects (Kanda and Watanabe 1998). The C center content in different growth sectors of synthetic diamonds is very different: {111} - rich; {110} - poor; {113} - rather poor; {100} highly variable (Woods and Lang 1975; Lang 1979). The intensity of the C center in synthetic HPHT diamonds grown by the conventional method and by the
62
3 Vibronic Absorption
temperature gradient method varies usually from 5 to 60 cm-1 and from 0.5 to 1.3 cm-1 respectively. The lower nitrogen content in the latter diamonds is a result of their much lower growth rate. At growth rates below 0.1 µm/min the C center is fully absent from the IR spectra, that is the C center concentration is lower than 1017 cm-3 (Charette 1962; Vins 1988). In synthetic diamonds the C center comprises only a very small part (0.1 to 1%) of the total nitrogen content (measured by neutron activation analysis) (Vachidov et al. 1975a). The intensity of the C center in synthetic diamonds is suppressed by introducing Ti, Al, Ga, Si, Mg or As impurities into the growth media (Kluev et al. 1972c; Rotner et al. 1983; Bakul et al. 1975). Ti is the most effective nitrogen getter in diamond: the C center can be fully suppressed in diamonds grown from Ti-containing media (Malogolovets et al. 1975b). In synthetic diamonds grown from media containing Ga, B and P the C center content (usually < 1017 cm-3) amounts to below 0.01% of the total nitrogen content (Vachidov et al. 1975a). 5 1130
ABSORBANCE
4
3
1344
2
1
0 800
1000
1200
1400 -1
WAVENUMBER, cm
Fig. 3.28. Absorption spectrum of the C center in a synthetic diamond of pure Ib type (Lawson and Kanda 1993b)
The impurity-related (nitrogen) nature of the C center was first proposed by Burstein and Oberly (1950). The C center originates from single substitutional nitrogen atoms. The center is characterized by C3v symmetry due to relaxation of the nitrogen atom from one of its carbon neighbors by about 1/4 of the regular C-C distance (Mainwood 1994). The peak at 1135 cm-1 is probably a quasilocal vibration at substitutional nitrogen atoms (Kurdumov et al. 1994). The theory predicts local vibrations of energy 1130 to 1140 cm-1 and 1344 cm-1 at a substitutional nitrogen atom in the diamond lattice (Kurdumov et al. 1994; Malogolovets and Nikityuk 1981; Briddon et al. 1991). In contrast, the band at 1344 cm-1 is attributed to a vibration of the carbon atom located at the C-N bond containing the unpaired electron (Collins et al. 1993b; Collins and Woods 1982d; Malogolovets 1986a).
3.2 Defect-Induced Vibrational Bands 63
The concentration of the isolated substitutional nitrogen atoms NC can be evaluated from the absorption strength µC at a wavelength of 8.850 µm by the following expressions: NC[ppm] = (22 to 45)µ1344[cm-1] (Mita et al. 1993; Field 1992; Woods et al. 1990b; Collins 1980; Lawson et al. 1998). The most reliable data appears to be the relation (*): NC[ppm] = 25µ1135[cm-1] (Chrenko et al. 1971; Kiflawi et al. 1994). NC[cm-3] = (0.7 to 4.4)×1018µC[cm-1] (Bokii et al. 1986; Kurdumov et al. 1994; Sobolev et al. 1969d; Kluev et al. 1972b). For synthetic diamonds with nitrogen concentration up to 5×1019 cm-3, NC can be calculated according to the relation NC[cm-3] = 1.6×1018µC[cm-1] (Novikov 1968; Nachalnaja et al. 1984). There are the following correlations between the IR absorption band and the UV absorption continuum attributed to isolated substitutional nitrogen (Bokii et al. 1986; Chrenko et al. 1971; Sobolev et al. 1969a; Kurdumov et al. 1994; Sobolev et al. 1969d): µ477 nm = 1.4µC; µ270 nm = (21 to 45)µC. The C center is believed to be stable to temperatures of about 1700°C. However in synthetic diamonds grown by the temperature gradient method at a temperature of 1750°C the C-defects transform completely into A-aggregates of nitrogen (Antsygin et al. 1996). Radiation damage strongly stimulates the aggregation of the C centers into the A-aggregates: an irradiation with 2 MeV electrons at a dose of 2×1017 cm-2 and subsequent annealing at a temperature of 1500°C for 7 h converts 30% of the nitrogen into the A-aggregates (Nconverted ≈ 170 ppm, that is one vacancy for 300 nitrogen atoms!). The dose rate of this conversion can be given as (Collins 1981b): NC-converted/NC ≈ (0.8 to 0.35)×log(10-17D[cm-2]). The activation energy of the transformation of the C-defects into the A-aggregates in synthetic diamonds at temperatures around 1900°C has been found to be about 250 kJ/mol (Chrenko et al. 1977) or 3.6 eV (Kluev et al. 1982). The reduction in the C-defect concentration NC at HPHT annealing are described by two expressions: (i) NC/N0 = exp(-kt3/2), where the ln(k) value reduces from -7 to -8.25 with temperature decreasing from 2000 to 1800°C (Kluev et al. 1982), and (ii) kt = (NC-1-N0-1), where k = 1.47×10-6 min-1ppm-1 at 1700°C and k = 1.9×10-4 min-1ppm-1 at 2100°C for nonirradiated diamond; k may attain a value of 10-5 at 1500°C after 2 MeV electron irradiation with a dose of ∼1022 cm-2 (Collins 1980; Chrenko et al. 1977; Evans and Qi 1982a). The aggregation of the C-defects into the A-aggregates at temperatures below 1800°C can be described by relation dNC/dt = -kNC2, where the activation energy and the rate constant k of the process have been found to be 5 eV and (1.4 to 4)×10-2 atom%/min at 1700°C (under a stabilizing pressure of 7 GPa). The aggregation of the C centers at higher temperatures proceeds at a higher rate: k ≈ 5.3×10-2 atom%/min at 2600°C (under stabilizing pressure of 9.5 GPa) (Evans and Qi 1982b). Some decomposition of the B-aggregates into the C-defects in natural type IaB diamonds occurs at 2240°C (Brozel et al. 1978). The change of the lattice constant of diamond due to the C-defects is given by the relation ∆a/a = 0.12×10-6NC (Lang 1994). The C center is paramagnetic. Its EPR analog is the P1 center (Bokii et al. 1986). Increase in the C center concentration reduces the hardness and increases the plastic flow of synthetic diamonds (Novikov 1968).
64
3 Vibronic Absorption
8.93 µm (1120 cm-1); a broad band observed in some CVD diamond films (Janssen et al. 1992) (Fig. 3.18). 8.9 to 9.0 µm (∼1115 cm-1); a relatively weak absorption peak induced by neutron irradiation in low-nitrogen diamonds (irradiation dose above 1017 cm-2) (Bokii et al. 1986; Hardy and Smith 1961; Bienemann-Kuespert et al. 1967). 8.95 µm (1117 cm-1); see 7.9 µm. 9.091 µm (1100 cm-1); a band appearing in type IIa diamonds implanted by C+ and B+ ions at LNT and subsequently annealed by RTA at 1100°C. The band intensity increases with the boron ion dose (Sandhu et al. 1989). 9.091 µ (1100 cm-1); a weak feature observed in some type IaB diamonds. This line is believed to be caused by the dislocation loops remaining from decomposition of the platelets (Collins 1997). Probably this feature is observed at a wavelength of 9.7 µm in some coated natural diamonds (Angress and Smith 1965) (Fig. 3.29). 9.1 µm (1090 cm-1); a feature characteristic of the A- and B-aggregates of nitrogen (Sutherland et al. 1954). It can be particularly strong in coats of some natural diamonds (Angress and Smith 1965) (Fig. 3.20, 3.29).
ABSORPTION COEFFICIENT, cm
-1
70
0.159 (7.8 µm)
60 50
0.15 0.137 (9.05 µm)
0.125 (9.9 µm)
40 30
0.108 (11.5 µm)
0.06 (20.65 µm)
20
0.128 (9.68 µm)
10 0 0.06
0.08
0.10
0.12
0.14
0.16
0.18
QUANTUM ENERGY, eV
Fig. 3.29. One-phonon absorption in a coated natural type I diamond (Angress and Smith 1965)
9.132 µm (1095 cm-1); see 6.9 µm (1450 cm-1).
3.2 Defect-Induced Vibrational Bands 65
9.26 µm (1080 cm-1); a peak observed in absorption spectra of boron-doped CVD diamond films. The feature is attributed to B-H deformation vibrations (Chia-Fu Chen et al. 1994b; Chia-Fu Chen and Sheng-Hsiung Chen 1995) (Fig. 3.13). 9.3 µm (1075 cm-1); a feature observed in transmission spectra of some CVD diamond films grown on a Si substrate (Gheeraert and Deneuville 1992b) (Fig. 3.17). 9.39 µm (1065 cm-1); see the D center (1290 cm-1). 9.4 µm (1065 cm-1); an intrinsic radiation damage center appearing in neutron irradiated diamonds (irradiation dose of 1018 cm-2) (Bokii et al. 1986; Hardy and Smith 1961). 9.434 µm (1060 cm-1); a band appearing in type IIa diamonds implanted by C+ and B+ ions at LNT and subsequently annealed by RTA at 1100°C. The band intensity increases with the boron ion dose (Sandhu et al. 1989). 9.434 µm (1060 cm-1); a band observed in type Ib diamonds after neutron irradiation. The band is not observed in nitrogen-free diamonds. This feature anneals out at temperatures above 800°C (Malogolovets et al. 1978c). 9.48 µm (1055 cm-1); a broad feature observed in some CVD diamond films (Janssen et al. 1992) (Fig. 3.18). 9.524 µm (1050 cm-1); the E center; observed in nitrogen-containing diamonds (Clark and Davey 1984b) (Fig. 3.30). 9.524 µm (1050 cm-1); a band observed in low-nitrogen synthetic diamonds grown in a TiO2-containing medium (Novikov 1968; Malogolovets et al. 1979). The feature is attributed to local C-O resonance vibration at a substitutional oxygen atom. The theoretically predicted wavenumber of this vibration is 1100 cm-1 (Kurdumov et al. 1994; Malogolovets 1984; Malogolovets and Nikityuk 1981) (Fig. 3.15). 9.57 µm (1045 cm-1); a weak broad peak observed in natural diamonds of type Ib character (Woods and Collins 1983) (Fig. 3.20). 9.68 µm (1033 cm-1); a weak peak observed in coats of some natural diamonds (Angress and Smith 1965) (Fig. 3.29). 9.78 µm (1022 cm-1); a feature observed in some natural brown diamonds (Collins and Mohammed 1982b) (Fig. 3.14). 9.9 to 9.92 µm (∼1010 cm-1); an absorption feature induced by neutron irradiation in low-nitrogen diamonds (irradiation dose above 1017 cm-2) (Bokii et al. 1986; Hardy and Smith 1961; Bienemann-Kuespert et al. 1967). This band is strongly enhanced
66
3 Vibronic Absorption
in synthetic diamonds by doping with Al, Ga and Si (Kluev et al. 1972c). This is possibly the same peak naturally observed in type I diamonds (including hydrogencontaining diamonds of IaAB type). The feature is particularly strong in coats of some natural diamonds. A possible origin of the band is LA phonon assisted absorption (Angress and Smith 1965; Ferrer and Nogues-Carulla 1996; BienemannKuespert et al. 1967; Kluev et al. 1972c). (Fig. 3.7, 3.29).
-1
130 (1050 cm )
120
130
140
150
160
170
180
QUANTUM ENERGY, meV
Fig. 3.30. Simulated absorption spectrum revealing the contribution of the A-, C- and E centers. The spectrum has been isolated from an absorption spectrum of a natural diamond by subtraction of the contribution of the F center (Clark and Davey 1984b). The main feature of the E center is that at 1050 cm-1
10.2 µm (980 cm-1); a band observed in CVD diamond films grown on Si substrates. The feature is attributed to absorption at the SiC interface (Celii et al. 1991). 10.2 µm (980 cm-1); see the D center (1290 cm-1). 10.3 µm (970 cm-1); a weak peak observed in natural diamonds of type Ib character (Woods and Collins 1983) (Fig. 3.20). 10.42 µm (960 cm-1); the M-X center (M stands for metal); a complicated absorption band in the spectral region from 17.3 to 17.7 µm (565 to 578 cm-1) observed in nitrogen-containing synthetic diamonds grown with Ni-Mn, Co-Mn, Co-Fe and Ni-Fe metal catalysts. The band is characterized by features at 10.9 µm (920 cm-1), 11.63 (860 cm-1), 12.2 µm (822 cm-1), 19.6 µm (500 cm-1) and 20.6 µm (485 cm-1). The M-X center is not observed in natural diamonds. The M-X center is attributed to internal epitaxial metal layers in the diamond lattice forming MC4 chains with local Td symmetry and characterized by two IR active vibrational modes
3.2 Defect-Induced Vibrational Bands 67
at around 500 and 900 cm-1 (valence and deformation modes of metal-carbon bonds, or possibly of metal-nitrogen bonds). These two modes can be further split due to distortions caused by impurities and intrinsic defects (Bokii et al. 1986; Kurdumov et al. 1994; Malogolovets and Vishnevskii 1976; Malogolovets et al. 1978b; Malogolovets and Nikityuk 1981; Novikov 1968). For the Ni-X center the characteristic features have spectral positions at 435, 507, 565, 320, 870, 920 and 960 cm-1 (for substitutional Ni atoms theory predicts vibrations with wavenumbers of 510, 705, 800, 866, 935, 1070 and 1190 cm-1) (Malogolovets and Nikityuk 1981; Malogolovets et al. 1978b). 10.85 µm (920 to 950 cm-1); an intrinsic radiation damage center appearing in nitrogen-containing diamonds irradiated with neutrons (irradiation dose of 1018 cm-2) or electrons. The center is not observed in nitrogen-free diamonds. The center anneals out at temperatures above 800°C (Bokii et al. 1986; Collins et al. 1988c; Malogolovets et al. 1978c) (Fig. 3.15). 11 µm (910 cm-1); FWHM of 25 cm-1; a band observed in synthetic diamonds grown from As-containing media. Possibly this band was also reported by Kluev et al. (1972c). The center is tentatively attributed to As-containing defects (Bokii et al. 1986; Kluev et al. 1974a). 11.4 µm (880 cm-1); see 6.9 µm (1450 cm-1). This relatively narrow band is observed in coated natural diamonds (Angress and Smith 1965) (Fig. 3.29). 11.63 µm (860 cm-1); a weak line appearing in type IIa diamonds implanted with C+ and B+ ions at LNT and subsequently annealed by RTA at 1100°C (Sandhu et al. 1989). 12.1 µm (830 cm-1) and 13.7 µm (732 cm-1); two bands naturally observed in some type Ia diamonds. A possible origin of these features is inclusions of nitrites (Bokii et al. 1986; Galimov et al. 1979). 12.42 µm (805 cm-1); FWHM of 30 cm-1; a band appearing in type IIa diamonds implanted by C+ and B+ ions at LNT and subsequently annealed by RTA at a temperature of 1100°C (Sandhu et al. 1989). 12.5 µm (800 cm-1); FWHM of 60 to 100 cm-1; the most intensive feature of a complicated band ranging from 600 to 1000 cm-1. This band is observed in CVD diamond films. It is also observed in FTIR spectra recorded at early growth stages of CVD diamond films deposited on Si substrates (Weringhaus et al. 1996; Jubber and Milne 1996). The band intensity strongly increases in the films grown at temperatures above 940°C. The feature is attributed to a stretching vibrational mode of Si−C bonds formed at the interface between the Si-substrate and the CVD diamond film (TO mode of silicon carbide, possibly β-SiC) (Gheeraert and Deneuville 1992b; Spitzer et al. 1959; Jubber and Milne 1996). The thickness of this
68
3 Vibronic Absorption
SiC layer may vary from 10 to 100 nm (Kislovskii and Spitsyn 1989; Weringhaus et al. 1996) (Fig. 3.17). 12.8 µm (780 cm-1); a feature assigned to the B-aggregates of nitrogen (Bokii et al. 1986). 13.3 µm (754 cm-1); FWHM about 90 cm-1; a band observed in some type IaB natural diamonds (Ferrer and Nogues-Carulla 1996) (Fig. 3.7). 14 µm (700 cm-1); see 6.752 to 6.944 µm (1481 to 1440 cm-1) (Fig. 3.17). 14.5 µm (687 cm-1); a sharp line of a hydrogen-related vibration observed in some natural type Ia diamonds (Reinitz et al. 1998) (Fig. 3.8). 15.2 µm (660 cm-1); a band observed in low-nitrogen synthetic diamonds grown in a TiO2-containing medium. The feature is attributed to the vibration of a CO2 group (Novikov 1968; Malogolovets et al. 1979). Possibly this feature may appear in natural type IaA diamonds (powder sample, grain size below 250 nm) after annealing at 700°C in an oxygen atmosphere (Jaeger 2000). 16.1 µm (620 cm-1); a peak observed in boron-doped CVD diamond films. The feature is attributed to O-B-O vibrations (Chia-Fu Chen et al. 1994b; Chia-Fu Chen and Sheng-Hsiung Chen 1995). Possibly this feature may appear in natural type IaA diamonds (powder sample, grain size below 250 nm) after annealing at 700°C in an oxygen atmosphere (Jaeger 2000) (Fig. 3.13). 20.6 µm (484 cm-1); a relatively broad band attributed to the A-aggregate of nitrogen (Bokii et al. 1986; Sutherland et al. 1954). This band may be relatively strong in coated natural diamonds (Angress and Smith 1965). The intensity of the feature may increase after annealing at 700°C in an oxygen atmosphere (Jaeger 2000) (Fig. 3.29). 21 µm (476 cm-1); a small peak observed in type IIa and IIb diamonds at elevated temperatures (above 450°C). The peak intensity increases with temperature (Bienemann-Kuespert et al. 1967). 21.01 µm (476 cm-1); a peak assigned to TA phonon absorption. This feature is readily observed in type I diamonds and coats of natural diamonds (Angress and Smith 1965). 21.5 µm (465 cm-1); a band observed in some natural type Ia diamonds (Ferrer and Nogues-Carulla 1996) (Fig. 3.7). 21.7 µm (460 cm-1); a band observed in transmission spectra of some CVD diamond films grown on a Si substrate (Gheeraert and Deneuville 1992b) (Fig. 3.17).
4
Scattering
4.1
Rayleigh Scattering
Rayleigh scattering of light in diamonds is caused by microscopic defects. In natural diamonds strong elastic scattering is due to dislocations (Wilks and Wilks 1991). PCCVD diamond films exhibit especially strong Rayleigh scattering due to their polycrystalline structure. In CVD diamond films for mean particle radii inferior to 60 nm, the intensity of Rayleigh scattered light is proportional to the square of the particle volume V2 and quantum energy hν (Moulin and Bonnot 1995). The Rayleigh scattering effect increases drastically with temperature (a remarkable increase occurs at a temperature of 570°C) (Bienemann-Kuespert et al. 1967). {111} faceted PCCVD diamond films show normally greater Rayleigh light scattering as compared with {100} faceted films. This difference is due to more pronounced steep-sided irregularities of the (111) faceted films. The effect is especially strong at wavenumbers above 2000 cm-1. Films of thickness ranging from 6.5 to 18 µm may keep only 5% of a visible laser beam without noticeable deflections (Hsien-Wen Ko et al. 1996).
4.2
Raman Scattering
4.2.1
General Properties
The general spectrum of the Raman scattering of diamond is shown in Fig. 4.1. Diamond has a relatively large Raman scattering cross-section: rsp3 = 9×10-7 cm-1sr-1. However it is much smaller than that of graphite: rsp2 = 5×10-5 cm-1sr-1 (Wada et al. 1980; Dresselhaus and Kalish 1992). There is a general rule that the Raman scattering in a covalent bonded crystal is higher than in ionic bonded ones (Placzek 1934; Behringer and Brandmuller 1956). Because of the long wavelength of visible light as compared to the Brillouin zone dimensions, only the zone center phonons (the 1332 cm-1 line) are normally observed in the first-order Raman spectrum of diamond. The Raman active phonon of diamond corresponds to the vibrations of the two interpenetrating cubic sublattices of the crystal against one another (Eckhardt et
70
4 Scattering
al. 1963). The one-phonon density of states of diamond is shown in Fig. 3.1 and Fig. 3.2. There is a strong maximum at 1335 cm-1 produced by LO phonons with energy 15 cm-1 above the Raman energy, revealing that the center-zone phonon is not the most energetic one. In fact the LO branch has a minimum at the Brillouin-zone center (Windl et al. 1993).
third order second order first order 1332
2467 2667
3825
3300
x100
x20
0
1000
2000
3000
WAVENUMBER, cm
4000
5000
-1
Fig. 4.1. General Raman spectrum of a gem-quality natural diamond excited at RT at a wavelength of 228.9 nm (Bormett et al. 1995). The ranges of the first, second and third orders of the Raman scattering are shown
The condition for observing all one-phonon states of the diamond lattice is the reduction in the correlation length in the crystal down to about 3 nm. This can happen in very fine grained nanocrystalline CVD diamond films (Nistor et al. 1997; Ravet et al. 1993), or in any diamond irradiated with light ions. The damage caused by energetic ions is characterized primarily by nanoclusters of point defects, which can resemble a fine grained polycrystal. For instance, the first-order Raman scattering spectrum of a free-standing CVD diamond film heavily irradiated with protons exhibits all the features of the phonon density of states of the diamond lattice (Fig. 4.2). Second-order Raman scattering in an ideal diamond lattice should occur, in principle, in the spectral range from 0 to 2668 cm-1 (Table 4.1). However, up to now it has been detected experimentally at RT only in the high-energy part of the spectral range from about 1600 to 2690 cm -1 (Solin and Ramdas 1970) (Fig. 4.1, 4.3, 4.4). The second-order Raman spectrum consists mainly of overtone and combination bands of phonons at high symmetry points in the Brillouin zone. Peaks and discontinuities in the spectrum are also observed at frequencies corresponding to the critical points in the phonon density function.
4.2 Raman Scattering 71
162
153 53 75
137 94 124
0
50
168
100
150
200
RAMAN SHIFT, meV
Fig. 4.2. One-phonon Raman spectrum of a PCCVD diamond film damaged at RT with 1 MeV protons at a dose of 1017 cm-2. The spectrum was taken using the 514 nm line of an Ar laser. The spectrum is close to the one-phonon density of states of diamond (see Fig. 3.1 and Fig. 3.2) (Zaitsev et al. 2000)
2458 3
RAMAN SCATTERING, counts
2519
2333
10
2254 2667
2177
2
10
2025 1817 1864
1
10
1600
1800
2000
2200
2400
2600
2800
-1
WAVENUMBER, cm
Fig. 4.3. Second-order Raman scattering of a perfect type IIa natural diamond. The labels of the features are taken from (Solin and Ramdas 1970). Note the logarithmic scale of the intensity axis
72
4 Scattering
2365
13
2462
2565
2243
C
2667
2095 2330
12
1500
1750
2000
2250
C
2500
2750
3000
-1
WAVENUMBER, cm
Fig. 4.4. Second-order Raman spectra of 12C natural and 13C synthetic diamonds
Table 4.1. Selection rules and active modes for two-phonon processes active in Raman scattering (Johnson 1965; Novikov et al.1987;Windl et al. 1993) Symmetry point Γ L
Overtones and combinations LO1+LO2 (sharp peak) 2O 2TO 2LO 2LA 2TA LO+TO TA+LA
X
2TO 2L 2TA TO+L TO+TA L+TA
W
2O2, 2A2 2O1 2A1 O2+O1 A2+A1 O1+A1
Active in Raman Γ1+, Γ12+, Γ25+ Γ1+, Γ12+, Γ25+ Γ1+, Γ12+, Γ25+ Γ1+, Γ25+ Γ1+, Γ25+ Γ1+, Γ12+, Γ25+ Γ12+, Γ25+ Γ12+, Γ25+ Γ1+, Γ12+, Γ25+ Γ1+, Γ12+, Γ25+ Γ1+, Γ12+, Γ25+ Γ25+ Γ12+ Γ25+ Γ1+, Γ12+, Γ25+ Γ1+, Γ12+, Γ25+ Γ1+, Γ12+, Γ25+ Γ12+, Γ25+ Γ12+, Γ25+ Γ1+, Γ12+, Γ25+
Experimentally measured energy [cm-1] 2670 2667 2422 2504 2011 1126 (theory) 2458 1569 (theory) 2138 (theory) 2370 1614 (theory) 2254 1864 1992 (theory) 1998 2358 (theory) 1817 2177 1907 (theory) 2178 (theory)
4.2 Raman Scattering 73
The second-order Raman scattering spectrum can be compared to the twophonon density of states of diamond shown in Fig. 3.1 and Fig. 3.2. The very characteristic peak at a wavenumber of 2670 cm-1 results from the corresponding peak of DOS at 1335 cm-1 (Windl et al. 1993). The second-order Raman scattering is excited especially strongly with UV light. It is an order of magnitude stronger for excitation at 228.9 nm as compared to that excited at 488 nm (Bormett et al. 1995). This effect is possibly a manifestation of resonance enhancement of Raman scattering when approaching the bandgap energy (Calleja et al. 1978). Three-phonon Raman scattering from diamond is experimentally observed at wavenumbers from 3650 to 3760 cm-1 in CVD diamond films and at around 3825 cm-1 in type IIa natural diamond. Cut-off of the third-order phonon density of states of diamond is at about 4000 cm-1 (Wang et al. 1996; Bormett et al. 1995) (Fig. 4.1).
4.2.2
Raman Features
The Raman bands observed in diamond materials are listed below. The first figures in the list show spectral positions of the band maxima at room temperature. 118 to 124 cm-1, FWHM of 5 cm-1; a narrow feature observed in diamonds treated by hot transition metals. It appears, for instance, after thermochemical polishing of diamond on hot nickel or iron. The band correlates with the 1617 cm-1 feature (*). (Fig. 4.5, 4.6, 4.7, 4.8, 4.9, 4.10, 4.11). 142 cm-1, FWHM of 4 cm-1; a relatively strong line observed in some microscopic black inclusions in natural diamonds. The feature may consist of two components at 142 (main) and 137 cm-1 (Fig. 4.12). 250 cm-1; a band with FWHM of 100 to 150 cm-1. The feature is generated in diamonds exposed to neutron irradiation with doses in the range of 1021 cm-2. The intensity of the feature strongly increases upon annealing at temperatures above 600°C. The band attains maximum intensity after 900°C annealing. Traces of the feature can be seen after annealing at a temperature of 1150°C. In the neutronirradiated diamonds the band is strongly polarized (Blank et al. 1999). 321 cm-1, FWHM of 10 cm-1; a feature observed in meteoritic carbon (Knight et al. 1991) (Fig. 4.13). 400 to 490 cm-1, FWHM of 100 cm-1; a band generated in diamonds exposed to neutron irradiation with doses in the range of 1021 cm-2. The feature anneals out at temperatures above 1000°C. Annealing in the range 600 to 900°C makes the band narrower and shifts it to 490 cm-1. The band is strongly polarized (Blank et al. 1999) (Fig. 4.14).
74
4 Scattering
RAMAN SCATTERING, counts
10
5
124
10
224
4
865.3 290 405
10
3
0
200
400
600
800
WAVENUMBER, cm
1000
1200
-1
Fig. 4.5. RT Raman spectrum of a dark CVD diamond film thermochemically treated by low carbon steel at a temperature of 1000°C in the Ar atmosphere. Note the logarithmic scale of the intensity axis
4
10
RAMAN SCATTERING, counts
1579
2669
1493 1617 1334
3
10
123
2465
2920 3238
2
10
1960
0
1000
2000
3000
4000
-1
WAVENUMBER, cm
Fig. 4.6. Raman spectrum of pure pyrolytic polycrystalline graphite taken at RT with He-Ne laser excitation at a wavelength of 632.8 nm. The dominant feature at 1579 cm-1 is the G-line. Note the logarithmic scale of the intensity axis
4.2 Raman Scattering 75
PL of the 738 nm Si-center 1332
260
391
1523
1051
520 Si?
702
862 978 SiC?
1186
124
0
300
600
900
1200
1500
1800
-1
WAVENUMBER, cm
RAMAN SCATTERING, arb. units
Fig. 4.7. Raman spectra taken from imperfections of a few micrometer size on the surface of polycrystalline CVD diamond films exhibiting very strong PL of the 738 nm Si center. The films have been thermochemically polishing on low carbon steel at a temperature of 700°C in an Ar-atmosphere
10
6
10
5
1331 123
10
2659
1579 1402 1617
1130
10
3
10
2
10
1
3234
2460
4
2922 3181
1830 1954
1
2 0
1000
2000
3000
4000
-1
WAVENUMBER, cm
Fig. 4.8. Raman spectra of the surface of good quality CVD diamond films after reaction with low carbon steel at a temperature of 1000°C (1), and pure Ni at a temperature of 900°C (2), both in an Ar atmosphere. Note the logarithmic scale of the intensity axis
76
4 Scattering
291.6 225.6
122
409.9
611 657
II
497 866.8 245.2
I 0
200
400
600
800
WAVENUMBER, cm
1000
1200
-1
Fig. 4.9. RT Raman spectra of a dark CVD diamond film (mechanical grade quality) treated by low carbon steel at a temperature of 1000°C. The spectra have been taken after He-Ne (632.8 nm) laser beam exposure during 10 s (I) and in 200 s (II). All the features in the spectral range below 1000 cm-1 are strongly stimulated by light
450 to 500 cm-1, FWHM of 300 to 550 cm-1; a weak broad Lorentzian or Gaussianshaped band observed in carbon films and CVD diamond films. This band is particularly remarkable in silicon-containing films. The feature is tentatively attributed to Si-Si vibrational modes (Bachmann and Wiechert 1991; Bachmann and Wiechert 1992; Zhang X. et al. 1998; Vassell et al. 1997). Probably the same band is observed in heavily boron-doped CVD diamond films. The alternative assignment of the band is a Raman scattering on acoustic phonons of a distorted diamond lattice (Zhang et al. 1996, *) (Fig. 3.1, 3.2, 4.2, 4.14, 4.16). 500 to 520 cm-1, FWHM of 100 to 200 cm-1; a broad band observed in spectra of meteoritic carbon (Knight et al. 1991) (Fig. 4.13). 516 cm-1, FWHM of 25 cm-1; a weak line appearing in CVD diamond films with a high silicon content at initial stages of thermochemical reaction with steel or Ni at moderate temperatures (about 700°C) (Fig. 4.15). 520 cm-1, FWHM of 3.5 cm-1; a Lorentzian-shaped narrow line occasionally observed from the substrate side of as-grown CVD diamond films deposited onto silicon substrates. This is the Raman peak of crystalline silicon adhered on the diamond surface (Bachmann and Wiechert 1992).
4.2 Raman Scattering 77
1577
a
220
1330 283
118
398 599
0
650
500
1000
1500
WAVENUMBER, cm
2000
-1
645
b
216 285
1326 1575 1900
0
500
1000
1500
2000
2500
-1
WAVENUMBER, cm
Fig. 4.10. Low-energy nondiamond Raman features related to some carbonaceous phases formed on the diamond surface thermochemically treated by hot low carbon steel at elevated temperatures. The intensity of most of them increases during exposure with a CW He-Ne laser at a power density of 2×106 MW/cm2. (a) The lower spectrum has been obtained after 10 s exposure. The upper spectra, after successively 50 s, 120 s, and 300 s. (b) The lower spectrum has been obtained after 10 s exposure; the upper spectra show successive intensity increases in time steps of 60 s each. Note that, firstly, the intensity of the amorphous diamond line (1326 cm-1) and graphite line (1577 cm-1) do not change their intensities with the light exposure, and, secondly, the light stimulated increase of the Raman scattering is observed only in a spectral range below 1900 cm-1
78
4 Scattering
3234 cm and 124 cm band intensities, arb. units
4
10
3
10
2
-1
10
1
-1
10
0
10
1
10
10
2
3
10
10
4
-1
1617 cm band intensity, arb. units
Fig. 4.11. Intensities of the Raman bands at 3234 cm-1 (? ) and 124 cm-1 (? ) versus intensity of the 1617 cm-1 band in polycrystalline CVD diamond films thermochemically treated with steel and nickel at temperatures in the range from 700 to 880°C
1330
142
0
200
400
600
800
1000
WAVENUMBER, cm
1200
1400
1600
-1
Fig. 4.12. RT Raman spectrum (He-Ne laser excitation) taken from an inclusion of 5 µm size in a natural type Ia diamond
4.2 Raman Scattering 79
1576
321 1350 500 900
0
500
1000
1500
WAVENUMBER, cm
2000
-1
Fig. 4.13. Raman spectrum of meteoritic carbon (Knight et al. 1991)
260
1320 490
0
500
1000
1500
2000
-1
WAVENUMBER, cm
Fig. 4.14. Raman spectrum of a natural type IIa diamond irradiated with fast neutrons at a dose of 2×1021 cm-2 and subsequently annealed at 900°C (Blank 1999)
550 cm-1, FWHM of 70 to 150cm-1; a band observed in highly boron doped MPCVD diamond films (boron concentration above 1500 ppm) (Pruvost et al. 2000). The band is very pronounced in films with boron concentration of 104 ppm. With increasing boron concentration the spectral maximum of the band may be shifted
80
4 Scattering
down to 460 cm-1. The feature is attributed to a one-phonon diamond band activated due to relaxation of the selection rules as a result of high boron concentration (Gheeraert et al. 1993). This very feature is possibly observed in heavily damaged diamonds (Fig. 4.2, 4.16).
1331
738 nm (the Si-center)
516.3 627.4
x100
1582
0
1000
2000 WAVENUMBER, cm
3000
4000
-1
Fig. 4.15. Raman spectrum of a high-silicon PCCVD diamond film thermochemically treated by Ni in vacuum at a temperature of 700°C. The luminescence background of the spectrum shows a very strong emission of the Si center
627 cm-1, FWHM of 25 cm-1; a weak line appearing in CVD diamond films with a high silicon content at initial stages of thermochemical reaction with steel or Ni at moderate temperatures (about 700°C) (Fig. 4.10, 4.15). 657 cm-1; a band appearing in the spectra of CVD diamond films treated by transition metals at high temperatures. The spectral position and width of the band may change upon laser irradiation with a power of 1 MW/cm2 (Fig. 4.9, 4.10, 4.17). 680 cm-1, FWHM of 30 cm-1; a weak band appearing in spectra of diamond tips used for indentations of diamonds and in the diamond samples subjected to indentations. The feature is ascribed very tentatively to the carbon phases bc8 and r8 (Gogotsi et al. 1998) (Fig. 4.18). 700 cm-1, FWHM ∼ 400 to 800 cm-1; a very broad band observed in SERS spectra of CVD diamond films deposited onto Ag substrates at early growth stages. The band is attributed to a diamond-like carbon phase with four- (predominant), three- and two-fold bonding (Lopez-Rios 1996).
4.2 Raman Scattering 81
520
a
300
960
1210 1323
200
400
600
800
1000
1200
1400
1600
-1
WAVENUMBER, cm
1320
b
1280 1070 1450
465
1220
1
1283
2 500
1000
1500
2000
-1
WAVENUMBER, cm
Fig. 4.16. (a) Raman spectrum of a CVD diamond film grown from a gas mixture with B:C ratio of 104 ppm (Gheeraert et al. 1993). (b) Raman spectra of heavily boron-doped PCCVD diamond films with hole concentrations of 5.7×1019 cm-3 (1) and 1.8×1021 cm-3 (2). The diamond Raman line shows a strong broadening and downshift to lower energies (Zhang et al. 1996)
800 cm-1, FWHM of 300 cm-1; a relatively weak broad Gaussian-shaped feature observed in carbon films and CVD diamond films. The band is particularly remarkable in silicon-containing materials. The feature is tentatively attributed to SiC vibrational modes (Zhang X. et al. 1998; Vassell et al. 1997).
82
4 Scattering
292
408
607 655
1
493 865
2 671
0
200
400
600
800
1000
WAVENUMBER, cm
1200
1400
-1
Fig. 4.17. Raman spectra of two CVD diamond films thermochemically treated with low carbon steel at a temperature of 1000°C in a Ar+4%H2 atmosphere
865 cm-1, FWHM of 3 cm-1; a narrow line observed in some CVD diamond films heavily treated with hot transition metals at high temperatures (above 900°C) (Fig 4.5). 1332
1732
1544 681
615
0
200
400
600
1270 1189 959 1450
800 1000 1200 1400 1600 1800 2000 -1
WAVENUMBER, cm
Fig. 4.18. Raman spectra of diamond indenter tips after indentations of diamond samples. All the Raman features except the diamond Raman line at 1332 cm-1 are a result of phase transformations occurring under high pressure (Gogotsi et al. 1998)
4.2 Raman Scattering 83
900 cm-1; a line observed in thin CVD diamond films grown on Si substrates. The feature is believed to be two TO zone-center phonon scattering from the Si substrate (Wagner et al. 1989). 940 to 980 cm-1, FWHM of 50 to 150 cm-1; a feature observed in meteoritic carbon and some CVD diamond films grown by the cyclic deposition technique. The band may be particularly strong in nanocrystalline CVD diamond films of a few tens of nanometers thick grown from a gas mixtures with increased CH4 content. The band may be composed of two narrower bands peaked at 940 and 980 cm-1 (Knight et al. 1991; Cline et al. 1992; Boegli et al. 1995). 978 cm-1; a very weak peak observed in some CVD diamond films grown on Si substrates. The spectral position of the feature may shift down to 700 cm-1, which is probably caused by internal mechanical stress (in the GPa range). The line is attributed to a LO phonon of the SiC interlayer or SiC inclusions (Huong 1991; Weringhaus et al. 1996) (Fig 4.7). 1000 cm-1, FWHM of 10 cm-1; a sharp weak feature observed in SERS spectra taken from flame-grown PCCVD diamond films. The feature is attributed to the trigonal ring breathing vibration of benzene derivatives (Okada et al. 1992; Ishida et al. 1986). 1030 to 1070 cm-1, FWHM of 60 to 100 cm-1; a weak band observed in heavily boron-doped CVD diamond films (boron concentration above 1500 ppm) (Pruvost et al. 2000). The feature is tentatively attributed to diamond precursors (Zhang et al. 1996) (Fig. 4.16). 1090 cm-1, FWHM of 70 cm-1; a weak feature appearing in the spectra of some CVD diamond films after thermochemical treatment by nickel at moderate temperatures (about 700 to 800°C) (Fig. 4.19). 1100 cm-1; a strong peak observed in diamond nanocrystals of 5 nm size. The line is tentatively assigned to a surface phonon mode of diamond (Prawer et al. 1998). 1120 to 1140 cm-1; a feature occasionally observed in some CVD diamond films (McNamara et al. 1992; Shroder et al. 1990; Bou and Vandenbulcke 1991; Nemanich et al. 1991). 1120 to 1150 cm-1, FWHM of 30 to 100 cm-1; a Lorentzian-shaped band observed in CVD diamond films. The spectral position of the band may vary in the mentioned range. The width of the band increases with increase in the energy of the band maximum. The feature is readily observed in films grown by the sequential deposition technique (Olson et al. 1993). This band is a characteristic feature of low-quality CVD diamond films grown by the cyclic deposition technique (Cline et al. 1992). The band is also present in nanocrystalline CVD diamond films (Hogmark et al. 1996). This band is a feature of SERS spectra taken from flame-grown PCCVD diamond films (Okada et al. 1992). Possibly this band is observed in ion
84
4 Scattering
implanted diamond (Prawer et al. 1998). The band appears in diamonds after indentations with diamond tips (the band may have a shoulder on the low-energy slope indicating an overlapping with a band peaking at about 1050 cm-1) (Gogotsi et al. 1998). The relative intensity of the band is enhanced in diamond films grown at evaluated methane to hydrogen flow ratios (Loh and Cappelli 1993). The band is pronounced in highly phosphorous-doped films (Bohr et al. 1995). The band is a stable feature of CVD diamond films deposited at temperatures of 600 to 750°C by the d.c. arc discharge method (Konov et al. 1995). The width of the band may strongly increase with improving film quality (Olson et al. 1993). The band is most intense in the Raman spectra excited with IR lasers. It is only weakly excited with UV light (for instance by 244 nm laser excitation) (Leeds et al. 1997). The feature disappears from spectra of CVD diamond films after their oxidation in air at temperatures above 620°C (Khomich et al. 1995b). The band is attributed to ultrafine nano-crystalline diamond or even amorphous diamond (Yarbrough and Roy 1988; Bachmann and Wiechert 1991; Gerber et al. 1994; Knight and White 1989b; Beckman et al. 1994; Eto et al. 1992; Mehlmann et al. 1994; Knight et al. 1991; Green et al. 1991; Khomich et al. 1995b; Bachmann and Wiechert 1992; Rats et al. 1995; Obraztsov et al. 1995; Sanchez et al. 1996; Bou et al. 1992; Nistor et al. 1997). For instance, it might be a disordered sp3 bonded carbon, which can be considered as a precursor of the diamond phase (Field 1992; Nemanich et al. 1988; LeGrice et al. 1990a; Okada et al. 1992; Nistor et al. 1997). Another interpretation is forbidden phonons, the selection rules of which are broken at nanocrystalline grains (Partlow et al. 1990; Knight and White 1989b; Jackman et al. 1995). This feature is also tentatively assigned to a surface phonon mode of nanocrystalline diamond with 1 to 2 nm cluster size (Prawer et al. 1998) (Fig. 4.20, 4.44a). 1140 to 1200 cm-1, FWHM of 30 to 200 cm-1; a band observed in low-quality CVD diamond films and in ion implanted diamonds. A weak similar band can be observed in diamonds treated by hot transition metals (*). The feature is attributed to hydrogenous carbon (possibly a C-C stretching vibration mode of polymer chains like polyenes) (Bou and Vandenbulcke 1991; Sato et al. 1992; Okada et al. 1992). 1166 to 1300 cm-1, FWHM of 150 cm-1; a broad band observed in MPCVD diamond films (Bou and Vandenbulcke 1991). 1180 to 1220 cm-1, FWHM of 150 cm-1; a broad Gaussian-shaped band observed in the spectra of poor-quality PCCVD diamond films with great amounts of sp2 carbon. There is a resonance enhancement of the band at 514.5 nm laser excitation. The band is attributed to microcrystalline or amorphous diamond. It is possibly caused by an optical phonon of wavenumber 1236 cm-1 characteristic of graphite (Nemanich et al. 1988; Bou and Vandenbulcke 1991). 1190 cm-1, FWHM of 20 cm-1; a feature observed in some low-quality CVD diamond films grown by the cyclic deposition technique (Cline et al. 1992). This
4.2 Raman Scattering 85
feature is possibly generated in diamonds deformed under high mechanical stress (Gogotsi et al., 1998) (Fig. 4.18, 4.21).
2
RAMAN SCATTERING, counts
10
1090 1
10
400
600
800
1000
1200
1400
1600
-1
WAVENUMBER, cm
Fig. 4.19. Raman spectrum (taken at RT) of a CVD diamond film after thermochemical interaction with pure Ni at a temperature of 720°C in an Ar atmosphere. Note the logarithmic scale of the vertical axis
1333
1471 1430
1540
1355 1131
1100
1200
1300
1400
1500
WAVENUMBER, cm
1600
1700
-1
Fig. 4.20. Raman spectrum of a CVD diamond film grown on a Mo substrate at a temperature of 820°C from a 1% CH4/H2 gas mixture (Loh and Cappelli 1993)
86
4 Scattering
1560
1332
1190
2
1
800
1000
1200
WAVENUMBER, cm
1400
1600
-1
Fig. 4.21. Raman spectra of CVD diamond films deposited by cycling growth for total cycle times 150 ms (1) and 1 s (2). Growth fraction is 0.5 (Cline et al. 1992)
1195 cm-1, FWHM of 100 cm-1; a band observed in nanocrystalline CVD diamond films. The band is especially intense in fine grained films. The feature is attributed to C-N vibrations. The presence of this band is an indication of an enhanced incorporation of nitrogen in nanocrystalline diamond (Nistor et al. 1997; Bergman et al. 1994a; Bousetta et al. 1994; Locher et al. 1994) (Fig. 4.22). 1200 to 1300 cm-1; a small peak observed in ion implanted diamonds. The feature is attributed to a nondiamond disordered carbon phase (Lai et al. 1995). 1225 cm-1, FWHM of 100 to 200 cm-1; a broad band observed in MPCVD diamond films heavily doped with boron (boron concentration above 1500 ppm) (Pruvost et al. 2000). The band is particularly strong at doping concentrations of 104 ppm. Position of the band maximum does not change with change of the boron concentration. The feature is also observed in ion implanted diamonds (Prawer et al. 1998; Zhang et al. 1996; Wurzinger et al. 1997). This band appears possibly also in boron-doped CVD diamond films after neutron irradiation (Popovici et al. 1996). A similar band appears in CVD diamond films after 193 nm laser ablation in air (Chan et al. 1996). The relative intensity of the band strongly increases with reduction in quantum energy of the excitation light (Locher et al. 1995) (Fig. 4.23). The feature is attributed to a one-phonon intrinsic Raman band activated due to relaxation of the selection rules by very high boron concentration (Gheeraert et al. 1993; Brunet et al. 1998; *). A tentative model of this feature involves vibrations localized at interstitial boron atoms (*). More reasonably the band is attributed to scattering on single optical phonons of the distorted diamond lattice (Zhang et al. 1996) (Fig. 3.1, 3.2, 4.2, 4.16, 4.24).
4.2 Raman Scattering 87
1334
1488 1349
1579
1140 1195
1000
1200
1400 WAVE NUMBER, cm
1600
1800
-1
Fig. 4.22. Raman spectrum of a nanocrystalline CVD diamond film grown in a Ar/CH4/H2 gas mixture. The spectrum can be well deconvoluted into six bands peaked as indicated. The band at 1195 cm-1 is assigned to C-N vibrations (Nistor et al. 1997)
1242 cm-1, FWHM of 25 to 50 cm-1; a line observed in SERS spectra of CVD diamond films. The feature is assigned to defects giving rise to scattering on phonons outside the center of the Brillouin zone (Lopez-Rios and Gomez-Rodriguez 1995; Lopez-Rios 1996). 1265 to 1275 cm-1, FWHM of 20 cm-1; a weak band appeared in spectra of diamond tips used for indentations of diamonds. The feature is tentatively attributed to hexagonal (6H) and rhombohedral (21R) diamond polytypes (Spear et al. 1990; Gogotsi et al. 1998) (Fig. 4.18). 1280 cm-1, FWHM of 30 to 50 cm-1; a band observed in heavily boron-doped CVD diamond films. The feature is tentatively attributed to vibrations of B-C bonds (Zhang et al. 1996). Note, that theory predicts a resonant mode of energy 1288 cm-1 for substitutional boron atom in the negative charge state (Goss et al. 1999) (Fig. 4.16). 1313 cm-1, FWHM of 30 to 40 cm-1; a band observed in meteoritic carbon, from the surface of cut diamonds, after thermochemical treatment by hot transition metals at temperatures above 550°C, in activated carbon powder (*). The spectral position of the band maximum may range from 1311 to 1350 cm-1 and its width may increase up to 100 cm-1. The feature is ascribed tentatively to lonsdaleite inclusions (Field 1992; Knight et al. 1991) (Fig. 4.25).
88
4 Scattering
excitation quanta 1.55 eV
excitation quanta 3.05 eV
1100
1200
1300
1400
WAVENUMBER, cm
1500
-1
Fig. 4.23. Raman spectra of a PCCVD diamond film doped with boron at a concentration of 1.4×1020 cm-3 for two phonon energies of the exciting light (Locher et al. 1995). The intensities of the diamond Raman line of the two spectra are fitted to equal values. It is seen that the relative intensities and shapes of the broad features strongly depend on the energy of the exciting quanta
1250
1120
765 510
200
400
600
800
1000
1200
1400
-1 WAVE NUMBER, cm cm-1 WAVENUMBER,
Fig. 4.24. Reduced Raman spectrum of a diamond implanted with 3.5 MeV He ions at a dose of 1017 cm-2 (solid line) compared with the calculated density of phonon states (dashed line) (Prawer et al. 1998; Wang and Ho 1993)
4.2 Raman Scattering 89
1315
RAMAN SCATTERING, counts
2500
1596
2000 1160
1500
2590
2900
1000
500
0 0
1000
2000
3000
4000
-1
WAVENUMBER, cm
Fig. 4.25. RT Raman spectra of activated carbon powder (at RT). The spectra were taken with He-Ne laser excitation
1313 to 1326 cm-1, FWHM of 2 to 8 cm-1; a sharp line observed in CVD diamond films. The line intensity may exceed that of the diamond Raman line (Huong et al. 1992). The feature is especially intense in isolated crystallites of the films (Sanchez et al. 1996). The feature is attributed to stacking faults oriented in (111) planes (formation of lonsdaleite hexagonal diamond polytypes) (Stuart et al. 1993a; Stuart et al. 1993b; Fayette et al. 1995; Spear et al. 1990; Sanchez et al. 1996; Huong et al. 1992). This line is observed in diamonds highly deformed by diamond tip indentations (Gogotsi et al. 1998). The hexagonal diamond line is also observed on diamond cut surfaces ( Knight and White 1989a). Since the Raman scattering cross-section of hexagonal diamond is considerably smaller than that of cubic diamond, the concentration of lonsdaleite detected in Raman spectra is relatively high (Fig. 4.26). 1320 cm-1, FWHM of 15 to 20 cm-1; a narrow band observed in highly disordered and highly doped diamonds. This band is pronounced in heavily boron doped diamonds. Spectral maximum of the band shifts down to 1306 cm-1 in CVD diamond films with boron concentration of 2400 ppm (Pruvost et al. 2000). The band is tentatively ascribed to the feature at of the diamond one-phonon density of states (*). (Fig. 3.1, 3.2, 4.2). 1330 to 1333 cm-1, FWHM of 30 to 100 cm-1; a band observed in CVD diamond films. A similar band appears in spectra of CVD diamond films after 193 nm laser ablation in vacuum (Chan et al. 1996). This band may strongly appear in spectra of
90
4 Scattering
diamonds treated by hot transition metals (in this case the band has almost pure Lorentzian shape*). The band is attributed to intermediate carbon defects in diamond crystallites controlling the confinement length of diamond phonons (Gheeraert et al. 1992a) (Fig. 4.8).
1325 (hexagonal)
1330 (cubic)
1300
1310
1320
WAVENUMBER, cm
1330
1340
-1
Fig. 4.26. Raman spectrum of MWCVD diamond powder containing cubic and hexagonal phases (Huong et al. 1992)
1332.5 cm-1 (4×1013 Hz, 165 meV), FWHM of 1.5 to 40 cm-1; the diamond Raman line corresponding to first-order Raman scattering in an undisturbed diamond lattice. The line results from scattering on triply degenerate TO(X) phonons of F2g symmetry. When narrow, the line has the Lorentzian-shape. (Fig. 4.27). The spectral position of the diamond Raman line in the Stokes and anti-Stokes regions are different, being at about 1333 and 1325 cm-1 respectively (Eckhardt et al. 1963). At room temperature the intensity of the Stokes line is 426 times stronger than the anti-Stokes line (Bienemann-Kuespert et al. 1967). The Raman efficiency of the diamond phonon is (2.7 to 6.06) 10-7 cm-1sr-1 (Bou and Vandenbulcke 1991; Wada and Solin 1981; McQuillan et al. 1970). The intensity of the Raman emission IR from a type IIa natural diamond sample of 2 mm thickness excited with a ruby laser of intensity IL can be evaluated by the relation: IR[mW] ≈ 5×10-4 IL[MW/cm2]; the Raman gain per unit length of the active medium is 6.9×10-3 IL[MW/cm2] (McQuillan et al. 1970; *). The diamond Raman line was first observed by Ramaswamy (1930) and Bhagavantam (1930a). This line is observed in the Raman spectra of any diamonds. The line can also be detected in diamond-like carbon with sp3 fraction larger than 80% (Basca et al. 1993; Lifshits 1997). The line taken from flame-grown epitaxial ({100}-oriented natural type IIa substrate) CVD diamond films is almost
4.2 Raman Scattering 91
indistinguishable from that of good-quality type IIa diamond (Schermer et al. 1994). The most intense and narrow diamond Raman line can be found in textured CVD diamond films (Haq et al. 1994).
13
C
1100
1200
12
C
1300
1400
1500
-1
RAMAN SHIFT, cm
Fig. 4.27. The diamond Raman line recorded at RT in HPHT synthetic diamonds synthesized from 12C and 13C carbon isotopes. Excitation with 514.5 nm Ar-laser line (Zaitsev et al. 1996a)
In CVD diamond films doped with boron with concentrations above 4×1020 cm-3 the line weakens abruptly: at a concentration of 8×1020 boron/cm-3 the line becomes an order of magnitude weaker than that at a concentration of 4×1020 cm-3 (Brunet et al. 1998; Zhang et al. 1996). Small boron concentrations (< 400 ppm) and nitrogen (N2/CH4 ≈ 10%) present in CVD diamond films increase and narrow the line, while higher nitrogen concentrations suppress the line (Bachmann and Wiechert 1992; Nishimura et al. 1989; Bohr et al. 1996; Wang et al. 1992). CVD diamond films grown in the presence of oxygen (up to 2.2%) in the gas system show an intense and narrow Raman line (Muranaka et al. 1991c). The line intensity taken from (111)-oriented CVD diamond strongly increases during heating at 700°C in air, the effect being much weaker for (100)-oriented CVD diamond. This effect is suppressed when the films are treated in an inert atmosphere (Li-Chyong Chen et al. 1995). In CVD diamond films the line becomes more pronounced after oxidation in air at temperatures above 620°C due to removal of the ultra-fine-grained diamond phase (Khomich et al. 1995b). Ion implantation reduces the line: 90 keV boron ion implantation suppresses the line completely at doses above 5×1015 cm-2 (Fontaine et al. 1994; Yagyu et al. 1995). The reduction in the line intensity in ion implanted diamonds is inversely proportional to the product of the nuclear and electronic stopping powers of the primary ions (Fig. 4.28).
92
4 Scattering
RAMAN SCATTERING INTENSITY, counts
10
8
10
7
10
6
10
5
Diamond Raman
13
C ions 11.1 MeV/amu Sn Se
10
4
10
3
10
2
10
1
10
0
PL background
0
50
100
150
200
250
300
DEPTH, µm 12
Sn maximum
10
FWHM, cm
-1
8
6
4
2
0 0
50
100
150
200
250
300
DEPTH, µm
Fig. 4.28. Depth distribution of intensities of the diamond Raman line (? ), photoluminescence background (¦ ), nuclear (Sn) and electronic (Se) stopping powers (upper plot), and FWHM of the diamond Raman line (lower plot) in a type IIa natural diamond irradiated with 11.1 MeV 13 C ions at a dose of 2.5×1015 cm-2. Excitation with the 514.5 nm Ar laser line. The intensity decrease of the diamond Raman line follows closely the dependence 1/(SeSn)
The intensity of the diamond Raman line in PCCVD diamond films decreases with the decrease in the grain size: the line almost vanishes at a grain size of 100 nm (Nambada et al. 1992). The line intensity in CVD diamond films falls with decrease in the growth temperature and it completely vanishes at the temperature, when the cauliflower structure appears (Stiegler et al. 1996). The spectral position of the diamond Raman line may change considerably depending on the perfection of the diamond sample. In natural diamonds of different
4.2 Raman Scattering 93
origin the line can be found in the range from 1331 to 1346 cm-1 (Huong 1991; Burton et al. 1995b). The spectral position of the diamond Raman line does not change with the change of the excitation wavelength from 244 to 780 nm (Leeds et al. 1997). CVD diamond films grown on (110)-oriented substrates show the diamond Raman line slightly shifted to lower energies (Schermer et al. 1994). In CVD diamond films grown at a temperature of 200°C the line may shift down to 1310 cm-1 (Hiraki 1997). The main sources of uncertainty of the spectral position of the diamond Raman line measured in CVD diamond films are the temperature shift, the domain size effect, and the splitting due to nonhydrostatic stress (Kulisch et al. 1996). There is a trend: the higher the frequency of the diamond Raman line, the more the diamond material is distorted from the cubic structure (Huong 1991). Diamond films produced on Cu substrates by high-temperature high-dose C+ ion implantation reveal the line peak at 1326 cm-1 (Hoff et al. 1993). In boron doped diamonds the line shifts towards lower wavenumbers with increasing boron concentration. In highly boron-doped CVD diamond films with hole concentration above 1021 cm-3 the line may shift down to 1283 cm-1 (Gheeraert et al. 1993; Nachalnaja et al. 1994; Locher et al. 1995; Okano et al. 1990; Zhang et al. 1996; Pruvost et al. 2000) (Fig. 4.16). Note that in CVD diamond films doped with boron at a concentration of 4×1020 cm-3 the line has been found to shift positively by +1.8 cm-1 (Brunet et al. 1998). However at higher boron concentrations the line shifts towards lower energy abruptly: at a concentration of 8×1020 boron/cm-3 the line spectral position is at around 1310 cm-1 (this drastic change is probably caused by preferential incorporation of boron atoms in interstitial positions at concentrations above ∼5×1020 cm-3 *) (Brunet et al. 1998). The strong downward shift of the diamond Raman line in heavily boron doped diamond may be possibly due to mixing up with the band 1320 cm-1. The shift of the line with decrease in the crystalline domain size L in CVD diamond films is negative and becomes a noticeable effect (∼ 0.5 cm-1) at L ∼ 30 nm (Johnston et al. 1992; LeGrice et al. 1990b). Polycrystalline diamond (grain size of 10 nm) obtained from graphite at a temperature of 3000 K and pressure above 11 GPa by direct conversion shows the diamond Raman line shifted to 1324 cm-1. This shift is explained by the phonon confinement effect (Yusa et al. 1998; Yoshikawa et al. 1993). In PCCVD diamond films oxidized in oxygen at 700°C the line (as measured in situ) splits (possibly due to reduction in the grain size) giving the most pronounced lines at 1309, 1311, 1313, 1319, and 1322 cm-1 (Johnston et al. 1992) (Fig. 4.29). Single substitutional nitrogen cause a small shift of the diamond Raman line: by +1 to +2 cm-1 at a concentration of 2×1020 cm-3 (Nachalnaja et al. 1994). The line position taken from triangular faceted and square crystallites of HFCVD diamond films (grown on a Pt substrate) has been found at 1333.7 and 1330.8 cm-1 respectively (Harris et al. 1996). In synthetic diamonds with nitrogen concentration of 800 ppm (the C- and A-defects) the diamond Raman line shifts by about 0.5 to 0.7 cm-1 (Suvorovtsev et al. 1999). The position of the line in diamond powders prepared by the shock-loading technique may range from 1315 to 1326 cm-1, which is explained by the presence of lonsdaleite inclusions (Knight and White 1989a; Field 1992). In very fine submicron diamond powders the line is strongly broadened and shifted down to 1318 cm-1 (this effect occurs possibly due to overheating of the
94
4 Scattering
particles up to 600°C by the exciting laser beam) (Nachalnaja et al. 1994). In diamond nanopowder prepared by mechanical milling the line shifts down to 1314 cm-1 (Niwase et al. 1995). After 2 MeV He+ ion implantation at a dose of 5×1016 cm-2 the diamond Raman line shifts down by -5 cm-1 (tensile strain up to 2.7 GPa) (Weiser et al. 1996). After 3.5 MeV He+ ion irradiation at a dose of 1017 cm-2 the line shifts down to 1316 cm-1 (Prawer et al. 1998; Jamieson et al. 1995). In diamonds as-implanted with 1.4 MeV H+ ions at a dose of 5×1017 cm-2 ( channeling conditions) the diamond Raman line, when detected in proximity to the irradiated surface, is shifted by -8.2 cm-1 and broadened up to 10 cm-1 (average tensile stress of 4 GPa) (Dooley et al. 1993). In CVD diamond films grown on a Si substrate the line sifts upwards from 1331 to 1338 cm-1 after 120 keV B+ ion implantation carried out at RT with a dose of 5×1014 cm-2 (Harper et al. 1991). The line is shifted towards lower energies after neutron irradiation (Gorelik et al. 1990). In CVD diamond films irradiated with reactor neutrons (2.7×1020 cm-2 of thermal neutrons and 3.1×1020 cm-2 of neutrons with energy above 0.1 MeV) the line shifts down to 1320 cm -1 and broadenes to ∼50 cm-1 (Popovici et al. 1996). The spectral shift of the line towards lower wavenumbers and its broadening in radiation damaged diamonds vary linearly with defect density.
1311 1313 1319
1309 1322
1280
1290
1300
1310
1320
WAVENUMBER, cm
1330
1340
1350
-1
Fig. 4.29. Raman spectrum of a CVD diamond film oxidized at a temperature of 700°C for 60 minutes (Johnston et al. 1992)
The spectral width of the diamond Raman line in perfect natural diamond may be as low as 1.5 cm-1. Typical FWHM value of the line in polycrystalline CVD diamond films is 7 cm-1. In homoepitaxial CVD diamond films grown on (100)-oriented substrates the line can show a FWHM of 1.8 cm-1 (Field 1992; Sails et al. 1994; Khomich et al. 1995a; Haq et al. 1994). In disordered man-made diamonds (also in CVD diamond films, grown at high temperatures) the width of the
4.2 Raman Scattering 95
line can attain a value of 40 cm-1 (Knight and White 1989b; Huong 1992; Fabisiak et al. 1992; Fabisiak et al. 1993; Abello et al. 1992; Jackman et al. 1995). In CVD diamond films grown at a temperature of 200°C the line broadens above 20 cm-1 (Hiraki 1997). In CVD diamond films the line shape is asymmetric and can be adequately fitted with two or three Lorentzian curves (Li-Chyong Chen et al. 1995). The FWHM of the line in HFCVD diamond films correlates with the CH3 content in the growth gas mixture (Harris et al. 1996). The increase in FWHM and decrease in intensity of the diamond Raman line in CVD diamond films correlate with the increase in intensity of the dangling bonds detected in ESR (Fabisiak et al. 1993). In sintered diamond compacts the line width ∆ω increases with the reduction in the grain size d: ∆ω ∼ 20 cm-1 for d ∼ 2 µm, ∆ω ∼ 7 cm-1 for d ∼ 75 µm (Evans et al. 1984). In CVD diamond films the width of the line ∆ω increases linearly with decrease in the crystalline domain size L according to the following relation: ∆ω[cm-1] ≈ 70/L[nm] (the effect is explained by the phonon confinement model accounting for the phonon diffusion from the particles). The line broadenes also with growth rate of the diamond films (Bachmann and Wiechert 1992; Ascarelli et al. 1995; Obraztsov et al. 1995; Nistor et al. 1997). The diamond line may be especially broad at the early stages of film growth (Bernardez and McCarty 1993). Diamond films produced on Cu substrates by high-temperature high-dose C+ ion implantation reveal the line with FWHM of 19 cm-1 (Hoff et al. 1993). After 2 MeV He+ ion implantation at a dose of 5×1016 cm-2 the line broadens to 12 meV (Weiser et al. 1996). In ion implanted diamonds the maximum width of the line is observed in the most damaged area subjected to the maximum nuclear stopping of the ions (Fig. 4.28). In CVD diamond films at boron concentrations above 1020 cm-3 the line acquires an asymmetrical Lorentzian+Fano shape (this effect has been found also in boron-doped 13C films), the asymmetry increasing with decrease in quantum energy of the excitation laser (Gheeraert et al. 1993; Nachalnaja et al. 1994; Locher et al. 1995; Okano et al. 1990). At boron concentrations above 4×1020 cm-3 the line broadens abruptly: at a concentration of 8×1020 boron/cm-3 the line acquires a FWHM up to 45 cm-1 (this drastic change is probably caused by preferential incorporation of boron atoms in interstitial positions at concentrations above ∼5×1020 cm-3 *) (Brunet et al. 1998; Zhang et al. 1996; Wurzinger et al. 1997). In polycrystalline CVD diamond films the width of the line taken from (111) facets of single crystallites is usually larger that that from the (100) facets, pointing to a better structural quality of the latter (Lee et al. 1995; Baik et al. 1993). The same behavior is observed in heavily boron-doped PCCVD diamond films: a strong broadening of the line taken from (111) facets of crystallites and no remarkable broadening on (100) facets. This boron stimulated broadening indicates preferential boron incorporation into {111} growth sectors of crystallites (Wurzinger et al. 1997). However the diamond Raman line taken from (100) facets of individual crystallites of PCCVD diamond films show an increasing width and asymmetry with decreasing crystallite size (Bou and Vandenbulcke 1991). In SERS spectra the line may be observed as asymmetrical with a tail down to about 1250 cm-1 (Lopez-Rios and Gomez-Rodriguez 1995). Such an asymmetry is usually observed in disordered diamond deposits, where the momentum selection rules are no longer valid (LopezRios 1996). In CVD diamond films grown in the presence of oxygen (up to 2.2% in
96
4 Scattering
the gas system) the diamond Raman line reveals a relatively small width: FWHM of 4 cm-1 (Muranaka et al. 1991c). The line width ∆ω in nitrogen containing synthetic diamonds depends on the nitrogen content N as follows (Suvorovtsev et al. 1999): ∆ω[cm-1] = 1.60 + 0.00152NC centers[ppm]; ∆ω[cm-1] = 1.57 + 0.00097NA centers[ppm]. Behavior under mechanical stress. The diamond Raman line shifts linearly with strain: ∆ν/ν0 ≅ -(1.0)S, where 1.0 is the approximate mode Grueneisen parameter (Boppart et al. 1985). In free-standing PCCVD diamond films the broadening of the diamond Raman line with the peak energy being fixed is caused by random in-plane compressive and tensile stress at a rate of ±5×1010 dyn/cm2 per FWHM of 10 cm-1 (Wagner et al. 1992). The uniaxial stress splits the line into a doublet and a singlet (however, only the singlet can be observed in back-scattering geometry). In isolated single crystallites of CVD diamond films the line can be split into two or three components with separation up to 7 cm-1 as a result of directional strain fields (Stuart et al. 1993a; Stuart et al. 1993b; Chen et al. 1995; Fayette et al. 1995) (Fig. 4.30). Due to internal mechanical stress induced by the mismatch in the thermal expansion coefficients of diamond and substrate and, probably, by inclusions of the graphitic phase the position of the line in PCCVD diamond films can be shifted up or down in a range from 1329 up to 1345 cm -1 or can be split into two or three components in a range from 1328 to 1370 cm-1 (Huong 1991; Burton and Meaden 1995a; Bergman et al. 1995; Drory 1995; Abello et al. 1992; Bohr et al. 1995; Yoshikawa et al. 1989; Ralchenko et al. 1995a; Kant et al. 1995). The intensities of the three split components are nearly equal (Ralchenko et al. 1995a). The doublet component of the splitting is about 10 times more sensitive to temperature variation than the singlet one (Ralchenko et al. 1995a). No obvious correlation between the line width (stress-induced) and the crystallite size of CVD diamond films has been found (LeGrice et al. 1990b; Ascarelli et al. 1995; Kulisch et al. 1996; Bachmann et al. 1994b). The shift of the diamond Raman line ∆ν in CVD diamond films grown on nondiamond substrates (provided a good adhesion is achieved) roughly depends on the mismatch of the thermal expansion coefficients at the film-substrate interface ∆α = αsubstr- - αdiam and difference ∆T between the deposition temperature and the measurement temperature (Fabisiak et al. 1992): ∆ν [cm-1] ≈ 1.3×103 ∆α[K-1] ∆T[K]. The diamond Raman line positions at RT (TRT) in PCCVD diamond films deposited at various temperatures T on various substrates are given in Table 4.2 (Ralchenko et al. 1995a). The reason for the line shift from its normal position is the thermal stress σth which can be evaluated from the following expression (with the assumption of a good adhesion) (Kulisch et al. 1996): σ th [GPa ] =
(
)
Ed (T [ K] − TRT [K]) α d [K −1 ] − α s [ K −1 ] , 1 −ν d
where Ed, νd and αd are Young’s modulus, Poissons ratio and the thermal expansion coefficient of the diamond deposit, respectively, and αs is the thermal expansion
4.2 Raman Scattering 97
coefficient of the substrate. In CVD diamond films deposited onto porous Si substrates the line shift depends linearly on the thickness of the porous Si layer: ∆dpSi: ∆ν ≈ +5 cm-1 at ∆dpSi = 0 and ∆ν ≈ -2 cm-1 at ∆dpSi = 10 µm (Heiderhoff 1997). The line position and its FWHM in CVD diamond films deposited onto Si change from 1336 to 1332 cm-1 and from 18 to 6 cm-1 respectively with deposition temperature increase from 800 to 1200°C (Obraztsov et al. 1995).
a 1331.9 1350 1361.5
1200
1300
1400
1500
-1
WAVENUMBER, cm
b 1330.5
1338.3
1250
1300
1350
WAVENUMBER, cm
1400
-1
Fig. 4.30. Raman spectra (taken at RT) from two small (of 2 µm size) single crystallites imbedded between larger crystallites in a CVD diamond film. Strong internal stress results in three-fold (a) and two-fold (b) splitting of the diamond Raman line
98
4 Scattering
Table 4.2. Spectral shift of the diamond Raman line in CVD diamond films grown on various substrates Tdeposition [°C]
∆ω [cm-1]2)
SiO2
Film thickness [µm] 1-2
690
-1.0÷-3.7
σ [GPa] + tensile - compressive +2.3
Si
1-2
900 – 950
+1.2÷1.6
-1.0
Si
∼100-300
+0.6
-0.3
+0.35
-0.1
Substrate
Si
Si
850 – 1000
+0.5 ÷ 11.5
SiC
∼10
900 - 950
+0.9÷2.6
-1.6
Cu
∼10
900 - 950
+4.5÷4.7
-2.9
WC-Co(6%)
∼10 20 t 40
900 - 950 laser deposit
+3.8÷6.2 +3.5
-3.8
Mo
∼10
900 - 950
+5.6÷6.7
-4.1
Mo
∼10 ?
+0.5÷5.5
21.1 kbar
Steel 60S2
∼10
900 - 950
+4.5÷8.9
-5.5
Ni
∼10
900 - 950
+17.5÷18.01)
-6.9
Steel R18
∼10
900 - 950
+6÷12
-7.4
Fe-Ni alloy
∼10
900 - 950
+26.0÷27.51)
-10.6
Ni
1-2
900 - 950
+29.5 1)
-11.4
{111}cBN
-7.5
Ti6Al4V alloy
∼1
700 - 900
+13.5
Pt(111)
∼1.5
850-880
+12
650 - 800
+7.5
1000
-1.1
alumina (100)-oriented diamond implanted with 2 MeV He+ ion at a dose of 5×1016 cm-2 1)
∼5
position for the doublet component; 2) ω0 = 1332.5 cm-1
-7
+0.5*
References
(Ralchenko et al. 1995a) (Ralchenko et al. 1995a) (Kant et al. 1995) (Li-Chyong Chen et al. 1995) (Huong et al. 1992) (Ralchenko et al. 1995a) (Ralchenko et al. 1995a) (Ralchenko et al. 1995a) (Badzian et al. 1997a) (Ralchenko et al. 1995a) (Bou and Vandenbulcke 1991) (Ralchenko et al. 1995a) (Ralchenko et al. 1995a) (Ralchenko et al. 1995a) (Ralchenko et al. 1995a) (Ralchenko et al. 1995a) (Koizumi et al. 1990) (Drory and Hutchinson 1995) (Tachibana et al. 1996) (Kulisch et al. 1996) (Weiser et al. 1996)
4.2 Raman Scattering 99
A uniaxial stress of 121 kbar along the [110] direction shifts the line (unresolved splitting) by +8.9±1.0 cm-1 (Gupta et al. 1989). The splitting of the line as functions of uniaxial stress along the and directions are (Davies 1994a; Grimsditch et al. 1978; Anastassakis et al. 1990; Nazare and Neves 2001): δω = 2.2 ± 0.02 cm-1/GPa, δω = 0.73 ± 0.010 cm-1/GPa. By uniaxial stress the hydrostatic component gives a contribution (Davies 1994a; Parsons 1976; Nazare and Neves 2001): ∆ωh = (3.2 to 3.6) cm-1/GPa. The singlet component is shifted by uniaxial stress σ along and directions as follows (the corresponding shifts for the doublet are not appreciably different) (Yoshikawa et al. 1989; Muranaka et al. 1991c): ∆νsinglet[cm-1] = -0.93×10-10 σ[dyn/cm2], ∆ν singlet[cm-1] = -0.38×10-10 σ[dyn/cm2]. By biaxial stress σ in polycrystalline diamond grown on nondiamond substrates the line splits for singlet and doublet shifted at rates (Ralchenko et al. 1995; Ager III and Drory 1993): ∆ωsinglet [cm-1] = -0.93 σ[GPa], ∆ωdoublet [cm-1] = -2.60 σ[GPa]. At low stress, when the splitting is not resolved, the line shifts according to: ∆ω [cm-1] = -(1.62 to 2.05) σ[GPa] (Ralchenko et al. 1995; Bergman et al. 1995; Ager III 1995). The shift and splitting of the diamond Raman line for biaxial stress of different orientations are given in Table 4.3 (Grimsditch et al. 1978; Ager III and Drory 1993; Ager III 1995). The hydrostatic shift rate of the diamond Raman line at pressures up to 40 GPa has a value within the range +(2.58 to 3.6) cm-1/GPa (Boppart et al. 1985; Sharma et al. 1985; Davies 1994a; Grimsditch et al. 1978; Anastassakis et al. 1990; Mitra et al. 1969; Parsons 1976; Hanfland et al. 1985; Goncharov et al. 1985; Tardieu et al. 1990; Kulisch et al. 1996; Schiferl et al. 1997; Nazare and Neves 2001). It has been measured as +1.693 cm-1/GPa at pressures up to 200 GPa (Vohra et al. 1994).
Table 4.3. Spectral shift of split components of the diamond Raman line in biaxially stressed diamond Biaxial stress plane Singlet shift [cm-1/GPa] Doublet shift [cm-1/GPa] (100) -1.64 -2.37 (111) -0.67 -2.86 (110) -0.90 -2.25 (112) -0.90 -2.80 average1) -0.93 -2.60 1) average values over crystallographic orientations. These figures can be used for polycrystalline diamond films.
Very strong mechanical deformations may strongly suppers the diamond Raman line. For instance, the line intensity strongly decreases in diamonds subjected to indentations with diamond tips (Gogotsi et al. 1998).
100
4 Scattering
Temperature Behavior. The energy of the diamond Raman line reduces with temperature (Solin and Ramdas 1970; Herchen and Capelli 1991; Herchen et al. 1992) (Fig. 4.31).
1340 1330
RAMAN SHIFT, cm
-1
1320 1310 1300 1290 1280 1270 0
500
1000
1500
2000
TEMPERATURE, K
Fig. 4.31. Temperature variation of spectral position of the diamond Raman line in natural diamond. The curve is an extrapolation of the data presented by Herchen et al. (1992)
There are experimental data on the spectral position of the line at various temperatures: at 85 K - (1333.8 to 1333.2) cm-1; at 1130 K - 1316.4 cm-1; at 976 K 1316.0 cm-1 (Bienemann-Kuespert et al. 1967). At a temperature of 14.9 K the line shifts upwards to 1333.5 cm-1 (Badzian et al. 1988). The temperature dependence of the spectral position of the diamond Raman line can be given by the following expressions: ω[cm-1] = -1.124×10-5 (T[K])2 -6.71×10-3 (T[K]) + 1334.5 for temperatures from 300 to 2000 K (Herchen et al. 1992); ω[cm-1] = ω0 -0.26×10-4 (T[°C])2 -0.65×10-2 (T[°C]) for temperatures from 0°C to 600°C (Sato Y. 1988); ω[cm-1] = -1.075×10-5 (T[K])2 -7.77×10-3 (T[K]) + 1334.5 for temperatures below 300 K (Herchen and Capelli 1991; Ralchenko et al. 1995a; Nazare and Neves 2001); ω[cm-1] = 1333.5 exp[-3.36 ∫α(T)dT] - 1.21×10-2(T - TRT) (Herchen and Capelli 1993; Zouboulis and Grimsditch 1991; Laikhtman and Hoffman 1997). The temperature shift of the line in 12C and 91%13C diamond films in the temperature range from 200 to 1330K is given by the expressions (Schiferl et al. 1997): ω12C[cm-1] = ωRT(12C)[cm-1] + 0.467 – 7.56×10-4 (T[K]-200)1.5; ω91%13C[cm-1] = ωRT(91%13C)[cm-1] + 0.45 – 7.36×10-4 (T[K]-200)1.5. The spectral width of the diamond Raman line increases with temperature (Fig. 4.32). The line broadens with temperature almost linearly within a range from 300 to
4.2 Raman Scattering 101
700 K (Bachmann and Wiechert 1992). At a temperature of 14.9 K the width of the line reduces by 0.17 cm-1 as compared to its width at RT (Badzian et al. 1988). The temperature dependence of the line width in CVD diamond films strongly depends on the orientation and atmosphere during heating (Li-Chyong Chen et al. 1995). In CVD diamond films grown on Mo substrates the line width shows a minimum at about 200°C when cooling after the growth down to RT (Bernardez and McCarty 1993).
15
FWHM, cm
-1
12
9
6
3
0 0
500
1000
1500
2000
TEMPERATURE, K
Fig. 4.32. Temperature variation of FWHM of the diamond Raman line in natural diamond. The curve is an extrapolation of the experimental data presented by Herchen et al. (1992)
Isotopic Effects. In pure 13C diamond the diamond Raman line is found at about 1280 to 1288.7 cm -1 (McNamara et al. 1992; Chrenko 1988) (Fig. 4.27). In CVD epitaxial films of content 12C0.5:13C0.5 the line lies at a wavenumber of 1313 cm-1 (FWHM of 8.6 cm -1) (Behr et al. 1993). The shift of the diamond Raman line with change of the 12C:13C isotope composition is not linear with respect to the average atomic mass, but departs from linearity by about 5 cm-1 at the middle of the range (Davies 1994a; Hass et al. 1991; Bahnholzer and Anthony 1992; Hass et al. 1992; Anthony and Banholzer 1992). In contrast, no nonlinearity has been observed by Collins et al. (1994b). The position of the diamond Raman line in diamonds of different isotope content 12C1-x13Cx can be calculated using the following expression: ω[cm-1] = 1332.82 – 34.77 x – 16.98 x2 (Vogelgesang et al. 1996; Nazare and Neves 2001). The largest FWHM of the line (about 8.5 cm-1) is observed at an isotopic content of 60 to 70 atomic % of 13C isotope (the broadening of the Raman line is larger when light atoms are placed in a heavy-atom lattice, than when heavy atoms are placed in a light-atom lattice (Hass et al. 1991; Chrenko 1988; Anthony and Banholzer 1992).
102
4 Scattering
Stimulated Emission and Resonance Enhancement. A resonance enhancement of the diamond Raman line by two to three orders of magnitude is observed in N+ ion implanted type IIa diamonds annealed at temperatures above 500°C (at 530 nm laser excitation) (Zaitsev and Varichenko 1985a) (Fig. 4.33, 4.34). Stimulated Raman emission (Stokes line) can be observed in 2 mm thick type IIa natural diamond (both sides parallel polished with wedge angle below 10') under ruby laser excitation at a power of 530 MW/cm2. By increasing the laser power from 530 to 800 MW/cm2 the Raman emission may increase by 106 times. The stimulated line excited with 20 MW/cm2 ruby laser irradiation at RT may shift to 1325 cm-1 from its original position at 1332 cm-1 (Eckhardt et al. 1963). The width of the stimulated line is an order of magnitude narrower than the normal line width, for instance of 0.1 to 0.2 cm-1. When excited with a ruby laser the stimulated line may be split into two components separated by about 0.35 cm-1 (McQuillan et al. 1970). 1310 to 1450 cm-1 (usually at around 1350 cm-1); the D-band (also referred to as the disorder peak). FWHM of this Lorentzian-shaped band may vary within the range 80 to 400 cm-1. The D-band is observed in CVD diamond films, diamond-like materials, ion implanted diamonds, in cracked regions of natural diamonds which have undergone a pressure of 300 GPa, after rough thermochemical treatment on hot transition metals (Sato et al. 1991; Bou and Vandenbulcke 1991; Sato et al. 1992; Khomich et al. 1995b; Vohra and McCauley 1993; Rats et al. 1995; Olson et al. 1993; von Kaenel et al. 1996; Bou et al. 1992; Zaitsev et al. 1998b; Gogotsi et al. 1998). However, the D-band is not produced by neutron irradiation (Popovici et al. 1996). The band is a dominating feature in the SERS spectra taken from flame grown PCCVD diamond films and in nanocrystalline (grain size in the range of several nanometers) CVD diamond films deposited by the bias flame technique (Hogmark et al. 1996). The D-band can have a Gaussian shape (Bou and Vandenbulcke 1991). The feature may be especially intense in isolated crystallites of CVD diamond films (Sanchez et al. 1996). The relative intensity of the D-band reduces with quantum energy of the exciting light: at a wavelength of 228.9 nm the band is almost not detectable (Bormett et al. 1995). The spectral position of the band shifts linearly from about 1440 down to 1310 cm-1 with the change of the excitation wavelength from 244 to 780 nm (Leeds et al. 1997). The band shifts to about 1260 cm-1 in 13C diamonds (Zaitsev et al. 1996a). The shape of the band in irradiated synthetic diamonds does not depend on the growth sector (Zaitsev et al. 1996a). The nature of the D-band is attributed to nondiamond carbon phases: disordered or nanocrystalline graphite, disordered glassy carbon, sp2-hybridized carbon phases, microcrystalline defective graphite, inclusions of amorphous diamond-like carbon, or a diamond precursor phase ( Knight and White 1989a; Bachmann and Wiechert 1991; Gerber et al. 1994; Bachman et al. 1994a; McNamara et al. 1992; Sato et al. 1991; Bachmann and Wiechert 1992; Badzian et al. 1988; Chalker et al. 1991; Nistor et al. 1997; Sharma et al. 1985; Mermoux 1992; Beckman et al. 1994; Bachmann and Wiechert 1991; Gerber et al. 1994; Sanchez et al. 1996). It is believed that the D-band results from scattering at the zone boundary modes, which
4.2 Raman Scattering 103
DIAMOND RAMAN LINE INTENSITY, arb. units
are active as a result of breakdown of the selection rule k = 0 for Raman scattering. It is also attributed to the forbidden A1g mode of disordered microcrystalline graphite (scattering from the edge of the Brillouin zone due to a surface effect; size effect of hexagonal planes in the graphite micro-inclusions) (Okada et al. 1992; Tuinstra and Koening 1970; Badzian et al. 1988). The D-band is thought to relate mainly to grain boundary defects (von Kaenel et al. 1996) (Fig. 4.44, 4.35). The relative intensity of the Raman D-band can be used for the evaluation of the grain boundary density in PCCVD diamond films (von Kaenel et al. 1996). However, this criterion is applicable mostly for poor diamond films of low crystalline perfection. The graphitic component of CVD diamond films (detected as the D-band at 1400 cm-1) has been found to be clustered rather than being uniformly distributed (Williams et al. 1994).
1
0,1
0,01
0
200
400
600
800
1000
1200
TEMPERATURE, °C
Fig. 4.33. Intensity change of the diamond Raman line with annealing temperature in a natural type IIa diamond previously implanted with 300 keV N+ ions at a dose of 1016 cm-2 and annealed at 1600°C under 60 kbar (Varichenko 1986)
1390 to 1420 cm-1, FWHM of 100 cm-1; a broad band observed after high dose ion implantation and subsequent annealing at 800°C (Fontaine et al. 1994). The feature is attributed to microcrystalline graphite (Gheeraert et al. 1992a; Fontaine et al. 1993). This band may be a modified D-band. 1400 cm-1, FWHM of 25 cm-1; a line observed in PCCVD and epitaxial CVD diamond films, in nanocrystalline highly defective diamond formed at the initial stage of CVD diamond growth. The intensity of the peak correlates with that of the diamond line. This feature may be formed in regions of disordered graphite (see the
104
4 Scattering
D-band) within nano-crystalline diamond grains (Hayward et al. 1995; Janssen et al. 1992). CL INTENSITY, RAMAN INTENSITY, arb. units
350
a
300 250 200 150 100 50 0 0
10
20
30
40
50
60
PL INTENSITY, RAMAN INTENSITY, arb. units
DEPTH, µm
6
b 4
2
0 0
10
20
30
40
50
DEPTH, µm
Fig. 4.34. (a) Depth distribution of CL intensity of the 575 nm center (m) and intensity change of the diamond Raman line with removal of ion irradiated layer (l) in a type IIa low nitrogen natural diamond implanted with 60 MeV N+ ions at a dose of 1015 cm-2 and subsequently annealed at 500°C (excitation with 532 nm Ar laser line) (Varichenko 1986). (b) Change of PL intensities of the 575 nm center (m), the 638 nm center (¨) and the diamond Raman line (l) in a type IIa low nitrogen natural diamond implanted with 60 MeV N+ ions at a dose of 1015 cm-2 and subsequently annealed at 1400°C with removal the ion irradiated layer (excitation with 532 nm Ar-laser line) (Varichenko 1986)
4.2 Raman Scattering 105
1332.8
1580 1350 1620
1200
1300
1400
1500
1600
1700
RAMAN SHIFT, cm-1
Fig. 4.35. Raman spectrum (excitation with 514.5 nm Ar laser line) of a good-quality CVD diamond film after rough thermochemical lapping on a low carbon steel plate at a temperature of 600°C. The main broad features of the spectrum are the D- and G-bands at wavenumbers of 1350 and 1580 cm-1 respectively (Zaitsev et al. 1998b)
1576
1332 1406
1000
1200
1400
1600
1800
2000
-1
WAVENUMBER, cm
Fig. 4.36. RT Raman spectrum of a dark CVD diamond film (mechanical grade quality) thermochemically treated by low carbon steel at a temperature of 1000°C
1406 cm-1; a band appearing in CVD diamond films thermochemically treated by hot transition metals (Fig. 4.36).
106
4 Scattering
1420 to 1440 cm-1, FWHM of 100 cm-1; a band typical of CVD diamond grown with oxygen (or CO2) addition to the growth gas mixture. The band is very pronounced in CVD diamond films grown at low temperatures (below 500°C). The band appears in the spectra of diamond coatings deposited by the laser assisted method. The feature is thought to originate from microtwin regions (Badzian et al. 1997a; Badzian A. and Badzian T. 1997b; Stiegler et al. 1996). This band may relate to the D-band (Fig. 4.37). 1422 cm-1, FWHM ∼ 15 cm-1; a line observed in diamonds implanted with ions in the MeV energy range (Prawer et al. 1998). 1447 cm-1, FWHM ∼ 15 cm-1; a line appearing in Raman spectra of diamonds of a different nature implanted with ions of energy in the MeV energy range. This feature probably relates to intrinsic radiation point defects (Prawer et al. 1998; *) (Fig. 4.38).
1337
1000
1100
1200
1300
1440
1400
1500
1600
1700
-1
RAMAN SHIFT, cm
Fig. 4.37. Raman spectrum of a CVD diamond film grown with oxygen addition to the growth gas mixture (Badzian et al. 1997a)
1450 to 1480 cm-1, FWHM of 20 to 150 cm-1; a band observed in PCCVD diamond films. The band is enhanced in the films deposited at low temperatures. The band may be composed of three bands peaked at around 1430, 1470 and 1540 cm-1. A similar band is observed in diamonds after indentations with diamond tips (Gogotsi et al. 1998). The feature is attributed to vibrations of trans-polyacetilene molecules (amorphous polyacetylene) (Muranaka et al. 1991a; Beckman et al. 1994; Nemanich et al. 1988; Bou and Vandenbulcke 1991; Loh and Cappelli 1993; Rats et al. 1995; Jackman et al. 1995; Olson et al. 1993; Muranaka et al. 1991b). This band is
4.2 Raman Scattering 107
possibly also observed in heavily boron-doped CVD diamond films. An alternative model of the feature is diamond precursors (Zhang et al. 1996) (Fig. 4.16, 4.18, 4.20).
RAMAN SCATTERING, counts
splitting region of diamond Raman line
10
4
1450 790
1
1500 1640
1055
1687 1755 1814 2
3
10
3
400
600
800
1000
1200
1400
1600
WAVENUMBER, cm
1800
2000
2200
-1
Fig. 4.38. Raman spectra of diamonds implanted with Xe+ ions of energy of a few hundred keV at a dose of 2.5×1014 cm-2. (1) PCCVD diamond film. The spectrum was taken from the surface irradiated with "in-coming" ions. (2) HPHT synthetic single crystal. The spectrum was taken from the surface irradiated with "out-going" ions. (3) The Raman spectrum of the nonirradiated area of the synthetic single crystal is shown for comparison. Note the logarithmic scale on the intensity axis
1467 cm-1, FWHM of 10 cm-1; a weak feature observed in diamonds implanted with ions of energy in the MeV range (Prawer et al. 1998). 1469 cm-1, FWHM of 15 cm-1; a line observed in diamonds implanted with ions of energy in the MeV range (Prawer et al. 1998). 1492 cm-1, FWHM of 60 cm-1; a band observed in single-crystal diamonds under high pressure (200 to 300 GPa). The feature is attributed to a new metastable carbon phase consisting of an amorphous mixture of four-fold coordinated diamond and three-fold coordinated graphitic carbon. This phase is quenched by decompression (Vohra and McCauley 1993) (Fig. 4.39). 1475 to 1564 cm-1, FWHM of 80 to 220 cm-1; a band observed in CVD diamond films, in diamond-like carbon films, in CVD nanocrystalline diamond films, in heavy ion implanted type IIa natural diamonds and CVD diamond films (Gheeraert et al. 1992a; Sato et al. 1991; Bou and Vandenbulcke 1991; Knight et al. 1991; Lai
108
4 Scattering
et al. 1995; Harper et al. 1991). The band remains stable in ion implanted CVD diamond films after RTA treatment (Harper et al. 1991). The feature is predominantly localized at the (111) facets of the diamond crystallites (Bou and Vandenbulcke 1991). The band is destroyed (removed) by 100 eV Ar+ ion exposure (Bou and Vandenbulcke 1991). The spectrum of the band does not depend on the excitation direction of the laser beam (Bou and Vandenbulcke 1991). The feature is attributed to inclusions of disordered sp2-bonded carbon, intermediate carbon defects in diamond crystallites controlling the confinement length of diamond phonons, or amorphous diamond-like carbon (disordered sp3 carbon) (Gheeraert et al. 1992a; 39, 40, 41, Bonnot 1990; Sharma et al. 1985; Mermoux 1992; Knight et al. 1991; Fontaine et al. 1993; Nistor et al. 1997). The band relates possibly to the G-band. This structure is assumed to be responsible for the electrical conductivity of ion implanted diamond (Sato et al. 1991). However, no direct correlation has been found between the band intensity and the electrical conductivity of PCCVD diamond films implanted with 100 keV Ga+ ions with doses in the range from 1014 to 5×1016 cm-2 (Dobrinets et al. 2000) (Fig. 4.40).
1332
D-band 1491
1000
1200
1400
G-band
1600
WAVENUMBER, cm
1800
2000
-1
Fig. 4.39. Micro-Raman spectrum taken from the cracked area of a type Ia diamond anvil after applying a pressure of 300 GPa on it. Excitation is with the 514.5 nm Ar laser line (Vohra and McCauley 1993)
1485 to 1500 cm-1, FWHM of 15 cm-1; a peak observed in ion implanted diamonds of different origin. The feature is associated with certain intrinsic point defects (Weiser et al. 1996; *), or with a sp2 hybridized carbon phase (Fig. 4.38, 4.41). 1500 cm-1, FWHM of 40 cm-1; a band observed in fine-grained diamond films. However, this band is usually absent from the spectra of microsize PCCVD diamond
4.2 Raman Scattering 109
films (Khomich et al. 1995b). A similar weak band may appear in CVD diamond films after ablation in air with 193 nm laser (Chan et al. 1996).
1497 520 1332
1135
400
600
800
1000
1200
1400
WAVENUMBER, cm
1600
1800
-1
Fig. 4.40. Raman spectrum of a thick CVD diamond film prepared on a Si substrate by the hot filament method (Knight et al. 1991)
one-phonon DOS of diamond lattice
b
1625
1487
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
-1
WAVENUMBER, cm
Fig. 4.41. Raman spectrum of a PCCVD diamond film irradiated with 1 MeV protons at a dose of 1017 cm-2. In the range from 200 to 1350 cm-1 the spectrum closely reproduces the one-phonon DOS of diamond. The features at 1487 and 1625 cm-1 are, possibly, due to sp2 hybridized carbon phases (Mejer et al. 2000)
110
4 Scattering
1500 cm-1, FWHM of 350 to 500 cm-1; the I-band. The I-band is a broad asymmetrical feature emerging in a range from 1000 to 1700 cm-1 in any diamonds after high dose implantation of light ions (Niwase et al. 1994; Fontaine et al. 1994; Denisenko 1995; Dobrinets et al. 2000). 320 keV Xe ion irradiation with a dose of 1014 cm-2 does not result in this band, whereas a dose of 2×1016 cm-2 does (a sort of phase transition should occur, that is the implantation must exceed a certain critical dose) (Uzan-Saguy et al. 1995). The I-band may also be observed in some lowquality as-grown CVD diamond films. The spectrum of the band coincides well with that observed in a-C films (Yoshikawa et al. 1989). The band may dominate the SERS spectra of CVD diamond films deposited onto Ag substrates at early growth stages (Lopez-Rios 1996). A possible origin of the I-band is amorphous carbon (McNamara et al. 1992), or sp2 carbon clusters of various sizes (Yoshikawa et al. 1989). In contrast to the broad nondiamond Raman bands observed in as-grown CVD diamond films, the I-band is excited more intensively with greater quanta (Dobrinets 2000) (Fig. 4.42, 4.43). 1510 to 1550 cm-1, FWHM may vary from 30 to 120 cm-1; a band frequently observed in CVD diamond films. The band is particularly strong in films grown at low temperatures (below 600°C). A similar band appears in diamonds after indentations with diamond tips (Gogotsi et al. 1998). The feature is attributed to the "diamite" or "bridged graphite" structure (Huong 1991; Huong et al. 1992; Stiegler et al. 1996) (Fig. 4.18).
12000 10000 2 8000
3
6000 4000 1 2000 0
1000
1200
1400
1600
1800
RAMAN SHFT, cm-1 Fig. 4.42. Raman spectra of natural type IIa diamond: 1 - pristine; 2 - after multi-energy B+ ion implantation in a range from 15 to 150 keV with a total dose of 2×1016 cm-2 (the values of energies and corresponding doses have been taken to obtain an even boron distribution over the implantation depth); 3 - after said implantation and subsequent annealing at 1500°C (Denisenko 1995)
4.2 Raman Scattering 111
12000
INTENSITY, arb. units
9000
excitation at 638 nm
diamond line
6000
3000 excitation at 514 nm
I-band
0 800 1000 1200 1400 1600 1800 -1
RAMAN SHIFT, cm
Fig. 4.43. Raman spectra of a good-quality PCCVD diamond film as-implanted with 100 keV Ga+ ions at a dose of 2×1016 cm-2. The upper spectrum was excited with an Ar laser (514 nm line); the lower spectrum, with a He-Ne laser (638 nm line). The luminescence background was subtracted from the spectra. The intensities of the spectra were reduced to an equal laser power. Note that the relative intensity of the I-band is much lower when excited with the He-Ne laser (Dobrinets 2000)
1520 to 1610 cm-1, FWHM of 40 to 200 cm-1; the G-band. The G-band is a Gaussian-shaped band observed in as-grown CVD diamond films and diamond-like materials, in oxidized CVD diamond films, in cracked regions of natural diamonds, and in diamonds which have undergone a pressure in the range of 300 GPa (Bou and Vandenbulcke 1991; 622, Gogotsi et al. 1998). (Fig. 4.35, 4.44). The intensity of the G-band observed in HFCVD diamond films correlates with the CH3 content in the gas mixture (Harris et al. 1996). The G-band is a relatively sensitive feature with respect to volume defects present in PCCVD diamond films. The band is readily detected in CVD diamond films deposited onto FeSi2 substrates (Chen and Narayan 1993). In CVD diamond films deposited onto Si substrates the G-band intensity decreases with film thickness (Dollinger et al. 1995). The G-band is a dominating feature in SERS spectra taken from flame-grown PCCVD diamond films (Okada et al. 1992) and in nanocrystalline CVD diamond films grown by the bias flame technique (Hogmark et al. 1996). A similar band appears in CVD
112
4 Scattering
diamond films after 193 nm laser ablation in vacuum (Chan et al. 1996). In PCCVD diamond films the band is preferentially observed at the grain boundaries (Hayward et al. 1995). The G-band may also be produced by ion implantation, however, it does not appear in neutron irradiated diamonds (Popovici et al. 1996). The spectral position of the G-band varies with change of the quantum energy of the exciting laser light. This variation is explained by the spectral dependence of the excitation efficiency of the band and, consequently, by the change of the band shape at different excitation wavelengths (Wagner et al. 1989). The G-band is composed of two bands peaked at about 1470 and 1600 cm-1 (the mentioned change of spectral position of the band may result from the change of relative intensities of these bands) (von Kaenel et al. 1996). The existence of the two components is clearly seen in some cases: for instance, in highly phosphorous-doped CVD diamond films the G-band splits into two bands peaked at 1480 and 1555 cm-1 (Bohr et al. 1995). The spectral position of the 1600 cm-1 component does not change with the change of the excitation wavelength from 244 to 780 nm (Wagner et al. 1989; Leeds et al. 1997). Nature: The G-band is attributed to the nonhomogeneously broadened G-line (the main Raman line of graphite) (Bachmann and Wiechert 1992; Nistor et al. 1997). The vibrational origin of the band is the doubly degenerate deformation vibrations of the hexagonal carbon rings (Okada et al. 1992). In CVD diamond films the G-band results from graphite-like inclusions consisting of sp2-hybridized carbon atoms (Lobashi et al. 1988; Sato and Kamo 1989; Bachmann and Wiechert 1991; Gerber et al. 1994). The feature reveals the presence of defective or microcrystalline graphite structures, or nanocrystalline (glassy) carbon (Gheeraert et al. 1992a; Knight and White 1989a; Fayette et al. 1994; McNamara et al. 1992; Shroder et al. 1990; Sato et al. 1991; Nakamizo et al. 1978; Wei Zhu et al. 1991; Sato et al. 1992; Khomich et al. 1995b; Fontaine et al. 1994; Fontaine et al. 1993; Rats et al. 1995). The variations of the spectral position of the G-band is attributed to bond angle distortions in graphitic planes (Bou and Vandenbulcke 1991). It is likely that the Raman G-band and the paramagnetic dangling bonds observed in PCCVD diamond films originate from the same defects (von Kaenel et al. 1996). 1530 cm-1; a band especially pronounced in phosphorous-doped CVD diamond films (Bohr et al. 1995). 1540 cm-1, FWHM of 10 cm-1; a weak feature observed in diamonds implanted with ions of energy in the MeV range (Prawer et al. 1998). The feature may appear in diamonds deformed under high mechanical stress (Gogotsi et al. 1998) (Fig. 4.18). 1560 cm-1, FWHM of 250 cm-1; a broad band observed in type Ib synthetic diamonds after high dose implantation with D+ ions. The feature is attributed to disordered diamond-like carbon (Wagner et al. 1991; Chen and Narayan 1993). It is assumed that this carbon phase may strongly involve C-H or C-D bonds (Niwase et al. 1994). 1563 cm-1, FWHM of 15 cm-1; a weak feature observed in diamond implanted with light ions of energy in the MeV range (Prawer et al. 1998).
4.2 Raman Scattering 113
1600
a
1350
1160
1000
1200
1400
1600
1800
-1
WAVENUMBER, cm
b
1000
1580
1360
1200
1400
1600
1800
-1
WAVENUMBER, cm
Fig. 4.44. (a) Raman spectrum of a nanocrystalline (average grain size of 8 nm) CVD diamond film deposited using a bias hot flame technique (Hogmark et al. 1996). (b) Raman spectrum of a single-crystal diamond surface graphitized by laser irradiation in air (Fayette et al. 1994). The main features of the spectra are the D- and G-bands. No trace of the diamond Raman line is seen in either spectrum
1568 to 1577 cm-1; FWHM of 23 to 80 cm-1; a band observed in meteoritic carbon and CVD diamond films (Beckman et al. 1994; Knight et al. 1991). This is an intense feature of the Raman spectra of low-quality CVD diamond films grown by the cyclic deposition technique (Cline et al. 1992). The band is strongly present in the spectra of diamonds treated by hot transition metals (Choi et al. 1996; *). The band is attributed to crystalline graphite or polyacetylene (Beckman et al. 1994).
114
4 Scattering
1585 cm-1, FWHM of 14 to 30 cm-1; the G-line. The G-line is the carbon-carbon first-order doubly degenerate stretching mode in the hexagonal layers of single crystal (or polycrystalline) graphite: the E2g mode of graphite phonons with the D46h space group (three-fold coordinated sp2 bonded carbon structure) (Tuinstra and Koening 1970). The G-line is accompanied by a weak peak at a wavenumber of 42 cm-1 (Bachmann and Wiechert 1991; McNamara et al. 1992; Jackman et al. 1995; Chalker et al. 1991; Harris et al. 1996). The Raman efficiency of the graphite phonon is 306(1±0.25) 10-7 cm-1sr-1, that is 50 times higher than that for diamond (Bou and Vandenbulcke 1991; Grimsditch and Ramdas 1975). The G-line may vary its position from 1565 to 1585 cm-1 depending on the quality of graphite. It can be detected in poor-quality CVD diamond films (Olson et al. 1993) (Fig. 4.6, 4.45, 4.46). 1600 to 1620 cm-1, FWHM of 20 to 30 cm-1; a line observed in as-grown CVD diamond films. The feature is especially intense in isolated crystallites of diamond films (Sanchez et al. 1996). A similar band appears in CVD diamond films after laser ablation in vacuum (193 nm laser line) (Chan et al. 1996). A weak feature at the same spectral position is observed in diamonds treated by hot transition metals. The line correlates with the peak at 3234 cm-1. A similar line is observed in carbines. This feature also appears in diamonds after neutron irradiation with doses in the range of 1021 cm-2. In the irradiated diamonds the band is strongly reduced in intensity upon annealing at temperatures above 650°C (activation of motion of vacancies?) (Blank et al. 1999). This feature is attributed to amorphous carbon or microcrystalline graphite (Knight and White 1989a; Beckman et al. 1994; Jackman et al. 1995). Alternatively the feature is assigned to hydrogen bound to sp2 bonded carbon (*). 1620 to 1640 cm-1, FWHM of 20 to 60 cm-1; a band observed in ion implanted type IIa natural diamonds and in neutron irradiated CVD diamond films. The band is very prominent in diamonds implanted with ions of the MeV energy range. In neutron irradiated diamonds the band anneals out at temperatures below 1000°C (Popovici et al. 1996). The band is attributed to radiation point defects (or amorphous carbon with sp2 bonding?), possibly so-called "dumb-bell" defects (Lai et al. 1995; Popovici et al. 1996; Morelli et al. 1993b; Weiser et al. 1996; Prawer et al. 1998) (Fig. 4.38, 4.41, 4.47, 4.48). 1687 cm-1, FWHM of 20 cm-1; a peak observed in ion implanted diamonds of different origin. The feature is associated with some intrinsic point defects (Fig. 4.38). 1696 cm-1, FWHM of 15 cm-1; a weak narrow band observed in some MPCVD diamond films (Bou et al. 1992). 1730 cm-1, FWHM of 70 cm-1; a band observed in spectra of diamonds after indentations with diamond tips. The feature is attributed to C=O stretching vibrations (carbonil groups on the surface of carbon). This band appears as a result
4.2 Raman Scattering 115
of mechanochemical reaction between diamond and air under high pressure (Gogotsi et al. 1998) (Fig. 4.18).
4
1579
10 RAMAN SCATTERING, counts
2683 2637 2455
1614 1493
3
10
2
10
0
1000
2000
3000
4000
-1
WAVENUMBER, cm
Fig. 4.45. RT Raman spectrum of single crystal graphite taken at RT with He-Ne laser excitation at a wavelength of 632.8 nm. The main feature at 1579 cm-1 is believed to be the G-line. Note the logarithmic scale of the intensity axis.
2735 2450
488 nm
3248 1582 (G-band)
228.9 nm N2
1000
2000
3177
3000
WAVENUMBER, cm
3250
4000
-1
Fig. 4.46. RT Raman spectra of highly ordered pyrolytic graphite excited at wavelengths of 228.9 and 488 nm at RT (Bormett et al. 1995)
116
4 Scattering
1631
1496
1649
1447 1422
1683
1467 1540
1400
1563
1500
1600
1700
-1
WAVENUMBER, cm
Fig. 4.47. Raman spectrum of a diamond implanted at RT with 3.5 MeV He + ions at a dose of 1017 cm-2. The spectrum has been taken for the polarization in which the selection rules forbid the observation of the diamond Raman line (Prawer et al. 1998)
1320
800
1000
1200
1400
1629
1600
WAVENUMBER, cm
1800
2000
-1
Fig. 4.48. Raman spectrum of a CVD diamond film irradiated with reactor neutrons (2.7×1020 cm-2 of thermal neutrons and 3.1×1020 cm-2 of neutrons with energy above 100 keV) (Popovici et al. 1996)
1755 cm-1, FWHM of 25 cm-1; a peak observed in ion implanted diamonds of different origin. The feature is associated with some intrinsic point defects (Fig. 4.38).
4.2 Raman Scattering 117
1814 cm-1, FWHM of 25 cm-1; a peak observed in ion implanted diamonds of different origin. The feature is associated with some intrinsic point defects (Fig. 4.38). 2100 cm-1; a band expected to result from carbine inclusions imbedded into the diamond lattice (Gogotsi et al. 1998). 2177 cm-1, FWHM of 15 cm-1; a peak observed in the spectral region of secondorder Raman scattering. The peak is particularly strong in diamond powder synthesized by spontaneous crystallization. This feature is attributed to activated electric-dipole transitions, or a two-phonon combination vibration located at defects (Semchinova et al. 1997) (Fig. 4.49). 2313 cm-1; a relatively broad feature observed in type I diamonds. The band is tentatively attributed to nitrogen-related vibration (Solin and Ramdas 1970) (Fig. 4.50). 2331 cm-1; a relatively broad weak band observed in type I diamonds. The feature is ascribed tentatively to nitrogen-related vibration (Solin and Ramdas 1970) (Fig. 4.50). 2450 cm-1; a band observed in CVD diamond films. The band is excited particularly effectively with quanta of low energies. The feature is attributed to a combination band related to the D-band (Bormett et al. 1995). Possibly this feature is also observed in pure highly ordered graphite (Bormett et al. 1995) (Fig. 4.46). 2459 cm-1; the most intensive band of the two-phonon Raman band of an ideal diamond lattice. The band spreads in a spectral region from 2130 to 2690 cm-1. Typically, the strongest two-phonon Raman spectrum of an undisturbed diamond lattice is spread from 2176 to 2668 cm-1 (Bienemann-Kuespert et al. 1967). There is a line at 2015 cm-1 which is thought to be related to the two-phonon spectrum (Bienemann-Kuespert et al. 1967). The most prominent features are relatively broad bands at 2177 (see above), 2330, 2457, 2490 cm -1 and a narrow line at 2668 cm-1 (FWHM of 2 cm-1). The maximum intensity of the band at 2459 cm -1 corresponds to the doubled energy of the densest optical phonon region (around 1220 cm-1). In synthetic diamonds containing 89% of 13C atoms these features are located at 2106, 2230, 2376, 2415 and 2578.5 cm-1 (Eckhardt et al. 1963; Chrenko 1988). In diamonds containing 99% of 13C isotopes the features of the spectrum have been reported to be at wavenumbers of 2372 (the main peak), 2415 (a shoulder), and 2575 cm-1 (2O phonon) (Field 1992). The relative intensities of the features do not change with temperature from 4.2 to 300 K in diamonds with 13C isotope content from 1.1 to 89 % (Chrenko 1988; Washington and Cummins 1977). Second-order scattering from PCCVD diamond films is identical to that of natural diamond except for the effects of finite grain size (Wagner et al. 1989). The rates of the pressure induced shifts of the two-phonon bands are given in Table 4.4 (Davies
118
4 Scattering
1994a; Parsons 1976). The highest energy line at 2668 cm-1 shows stimulated emission under 20 MW/cm2 ruby laser irradiation (Eckhardt et al. 1963) (Fig. 4.3). 2460 to 2470 cm-1, FWHM of 50 cm-1; a weak feature observed in diamonds treated by hot transition metals (*) and in CVD diamond films grown in the presence of oxygen. The feature is well resolved in Raman spectra excited in the UV spectral region (Bormett et al. 1995). This is a graphite-related feature (Fig. 4.6, 4.51, 4.52).
2177
2000
2200
2400
2600
2800
-1
WAVENUMBER, cm
Fig. 4.49. Raman spectrum of diamond powder synthesized by spontaneous crystallization (Semchinova et al. 1997)
2654 cm-1, FWHM of 70 cm-1; a strong band observed in diamonds treated by hot transition metals. The band correlates with the 1329 cm-1 line (second order replica of the 1329 cm-1 line?) (Fig. 4.8). 2669 cm-1; a strong band characteristic of pure graphite (Fig. 4.6). 2725 cm-1; a band observed in CVD diamond films. The band is excited particularly effectively with quanta of low energies. The feature is attributed to the overtone of the D-band (Bormett et al. 1995). Possibly this line is also observed in highly ordered graphite (Bormett et al. 1995) (Fig. 4.46). 2930 cm-1; a weak relatively narrow line observed in diamonds treated by hot transition metals and in CVD diamond films. The line may vary its spectral position in a range from 2918 to 2945 cm-1. A similar feature is also excited in pure graphite.
4.2 Raman Scattering 119
The feature is relatively strong when excited with quanta of energies about Eg. The line is attributed to C-H stretching vibrations of nondiamond carbon. The intensity of this band is thought to be a measure of the hydrogen content in diamond. The detection limit of hydrogen using the intensity of the 2930 cm-1 line is evaluated to be 0.001atom% (for excitation at a wavelength of 228.9 nm) (Bormett et al. 1995; *) (Fig. 4.6, 4.8, 4.52).
2331 2313
2000
2200
2400 WAVENUMBER, cm
2600 -1
WAVE NUMBER, cm
2800
-1
Fig. 4.50. Second-order Raman scattering in type I natural diamond. The features at 2313 and 2331 cm-1 probably originate from nitrogen-related defects (Solin and Ramdas 1970)
Table 4.4. Pressure induced shifts of two-phonon optical bands (Davies 1994a; Parsons 1976) Spectral position [cm-1] 1864 2178 2256 2333 2360 2370 2460 2467 2491 2501 2519 2667
Assignment TA(X3)+TO(X4) LO(W2)+TO(W1) LO(X1)+TO(X4)
2LO(X1) LO(L2-)+TO(L3-) order 2O(Σ1)
2LO(L2-) 2O(Γ25+)
Shift rate [cm-1/GPa] 5.6±0.8 8.7±0.5 8.3±0.5 8.1±0.5 6.2±0.8 6.9±0.8 7.4±0.5 8.5±0.5 6.6±0.6 7.2±0.5 8.6±0.6 7.1±0.4
120
4 Scattering
10
5
2659 RAMAN SCATTERING, counts
2693 2612
10
4
2460 2920
3
10 2200
2400
2600
2800
3000
3200
-1
WAVENUMBER, cm
Fig. 4.51. RT Raman spectrum of a dark CVD diamond film (mechanical grade quality) thermochemically treated by low carbon steel in anAr+H2 atmosphere at a temperature of 1000°C. Note the logarithmic scale of the intensity axis
2938 3160
N2
2000
2470
2500
3000
3500
4000
4500
-1
WAVENUMBER, cm
Fig. 4.52. Raman spectrum of a CVD diamond film grown with addition of oxygen to the gas mixture. Excitation with the 228.9 nm laser line. The band at 2938 is believed to be due to C-H vibrations (Bormett et al. 1995)
2950 to 3160 cm-1; a band observed in some CVD diamond films. The feature may be relatively strong in CVD diamond films grown with a small addition of oxygen to
4.2 Raman Scattering 121
the gas mixture. The band is excited particularly effectively with quanta of low energies. The band is attributed to be related to the graphite band at 1585 cm-1, or to a combination band related to the D-band (a second-order scattering?) (Bormett et al. 1995; *) (Fig. 4.46, 4.52). 3234 cm-1, FWHM of 50 cm-1; a band observed in diamonds treated by hot transition metals. The band is especially strong after treatment in a hydrogen-containing atmosphere. The band correlates with the 1617 cm-1 feature (two-phonon replica of the 1617 cm-1 line?). The feature is tentatively attributed to C-H vibrations of hydrogen atoms bound to graphite inclusions (Bormett et al. 1995; *) (Fig. 4.8, 4.11). 3238 cm-1; a feature characteristic of pure graphite (Fig. 4.6). 3300 cm-1, a band observed in natural diamonds. This feature is most pronounced in high-quality natural diamonds. The band is attributed to the third-order Raman scattering of diamond (Bormett et al. 1995) (Fig. 4.1). 3825 cm-1, FWHM of 350 cm-1; a broad band observed in natural diamonds. This is the main band of the third-order Raman scattering of diamond. In natural diamonds the band reduces its intensity and it shifts to lower wavenumbers with reduction in the crystal size (in natural diamond powder of 40 to 60 µm size the band intensity reduces by an order of magnitude and it shifts down to 3650 cm -1 as compared to the first-order line) (Bormett et al. 1995) (Fig. 4.1).
4.3
Miscallaneous
In Raman spectra of natural diamonds up to six naturally occurring weak bands can be found in the spectral range from 1158 to 1585 cm-1 in addition to the main line at 1332 cm-1 (Bienemann-Kuespert et al. 1967; Bhagavantam 1930b; Bhagavantam 1930c). CVD diamond Raman spectra have very high signal/noise ratios when excited with a 206.5 nm CW laser (frequency doubling the 413 nm line of a Kr-ion laser) and no interfering luminescence background (Holtz et al. 1996). Stimulated Brillouin scattering in type IIa natural diamond cannot be excited by excitation with a ruby laser (694 nm wavelength) at powers up to 70 MW/cm2 (McQuillan et al. 1970). Resonant enhancement of Raman scattering at nondiamond inclusions is observed for two bands: the A-component (the main line at 1510 cm-1) and the B-component (the main line at 1430 cm-1). The B-component is strongly excited with quanta of wavelengths from 476.5 to 514.5 nm (2.41-2.6 eV). There is a sharp splash of intensity of both bands after annealing at 1300°C. Both centers anneal out at 2200°C (Clark and Dickerson 1992b; Clark and Dickerson 1994). The resonant
122
4 Scattering
Raman scattering effect in CVD diamond films of a different origin is presented by Wagner et al. (1991). The resonance enhancement mechanism is believed to be responsible for especially efficient Raman excitation of nondiamond carbon inclusions in CVD diamond films at low quantum energy, for instance, at 1.16 eV. An example of the resonance enhancement effect is given by Woerner et al. (1996), where IR Raman scattering (excitation with a 1.064 µm Nd+-YAG laser) from the nondiamond carbon phase (in the range from 1100 to 1600 cm-1) has been found to be much stronger with respect to the diamond Raman line as compared to those taken in the visible spectral region. In contrast, the diamond Raman line (1332 cm-1) is better excited for high quantum energy excitation, for instance, at 4.82 eV. The ratio of the diamond peak intensity to that of graphite increases by about 100 times with change of the excitation wavelength from 780 to 244 nm (Bou and Vandenbulcke 1991; Wagner et al. 1991; Yoshikawa et al. 1989; Williams K. et al. 1994; Wagner et al. 1989; Sails et al. 1996; Wagner et al. 1992; Leeds et al. 1997). A possible reason for the Raman scattering enhancement on nondiamond carbon at low laser quantum energy is the proximity of the exciting quanta to the absorption band Eg of the amorphous carbon, which can vary from 0.4 to 3 eV (Bou and Vandenbulcke 1991). Another explanation is a selective absorption effect (Clark and Dickerson 1992b; Wagner et al. 1989). Resonance enhancement of the diamond Raman line is expected when the quantum energy of the exciting light approaches the diamond bandgap. However, a noticeable enhancement can also be observed for quanta within the bandgap spectral range (Kulisch et al. 1996; Calleja et al. 1978). Raman spectra of good-quality CVD diamond films (grown with a reduced content of methane, below 0.5%) excited in the IR spectral range (for instance at 780 nm) exhibit a number of relatively narrow lines at about 1620, 1570, 1520, 1430, 1390, 1370, 1350, 1310, 1270, 1230, 1200, 1160, 1140, 1090, 1030, 1010 cm-1, the FWHM of each line ranging from 10 to 20 cm-1. These features may appear due to the resonance enhancement of the Raman scattering (Leeds et al. 1997) (Fig. 4.53). The average size of the highly absorbing nondiamond regions is smaller in textured than in textured diamond films. This causes enhanced Raman scattering from the nondiamond carbon in the textured films (Wagner et al. 1992). A shoulder on the high-frequency side of the diamond Raman line (spreading to about 800 cm-1) observed in CVD diamond films is related to planar defects (intrusion of graphite planes between (111) diamond planes). The shoulder is more pronounced in disordered materials (Badzian et al. 1988). The defects of a diamond lattice composed of sp3 bonded carbon atoms show up in Raman scattering features at frequencies below 1332 cm-1. In contrast, the sp2 (graphitic) bonded species reveal Raman features at frequencies above 1332 cm-1. The reason for that is that the force constants of the sp2 carbon bonds are stronger than those of the sp3 carbon bonds (McNamara et al. 1992; Shroder et al. 1990; Nemanich et al. 1991). The relative Raman cross-section for diamond to graphite is 1:50 (Wada and Solin 1981). The relative intensity for diamond (sp3) to graphite (sp2) bonded carbon
4.3 Miscellaneous 123
in CVD diamond films as measured with Ar laser excitation is 1:75, that is the scattering due to graphite is 75 times stronger than that due to diamond (Shroder et al. 1990). Moreover, the penetration depth of visible light in graphite is of the order of 50 nm, which also reduces the Raman signal from regions containing graphite or shielded by graphite (Hayward et al. 1995). The Raman cross-section of amorphous carbon inclusions in comparison with the diamond matrix in CVD diamond films has been found to be greater by a factor of 233 for the 514.5 nm laser excitation. Based on these two models, quality factors Q can be introduced to evaluate the content of amorphous carbon in CVD diamond films (Sails et al. 1996; Silva et al. 1996): Q514.5 =
ID I ID + C 233
QAr − laser =
× 100
ID I ID + C 75
× 100
where ID is the integrated intensity of the diamond Raman peak, and ID + IC is the total intensity of the whole Raman spectrum. The lower limit of validity of the QAr-laser factor is estimated to be as high as 80 (Silva et al. 1996). A simple evaluation of CVD diamond film quality can be given as the intensity ratio between the G-band and the diamond band (Shroder et al. 1990; Bergman et al. 1993; von Kaenel et al. 1995; Robins et al. 1992), or the G-band + D-band and the diamond Raman line (von Kaenel et al. 1996). In PCCVD diamond films the relative intensities of the G- and D-bands correlate with the average grain boundary density. This correlation is especially pronounced for the D-band being almost linear (von Kaenel et al. 1996).
1162 1017
1279
1236 1139 1198
1040 1089
1308 1341 1367 1389 1426
1517
1000
1100
1200
1300
1400
WAVENUMBER, cm
1500
1600
1700
-1
Fig. 4.53. Raman spectrum of a CVD diamond film grown from a mixture with 0.36% of CH4. Excitation with a laser line at 780 nm (Leeds et al. 1997)
124
4 Scattering
A more reliable evaluation of structural quality of CVD diamond films (so-called Raman quality) is drawn from the measurements of FWHM of the diamond Raman peak (the narrower line the better quality), relative intensities of the graphitic bands and the PL background (the lower the intensities of both features the better the quality) (Harris et al. 1996). Using this method it has been shown that the Raman quality of HFCVD diamond films is not affected by the C2H2 content in the gas mixture (Harris et al. 1996). High-temperature (at about 700°C, for about 10 min) treatment of CVD diamond films in oxygen strongly removes from their Raman spectra any features related to graphitic phases and narrows the diamond line (Bachmann et al. 1993). The following approximate relations between the thermal conductivity of CVD diamond films Λ and sp2-carbon content (the ratio of the diamond line intensity I1332 to that of the 1500 cm-1 graphitic G-band I1500), or FWHM of the diamond line ∆ω1332 can be derived from the data of (Morelli 1994): Λ[W cm-1 K-1] ≈ 0.4 (I1332/I1500) + 2.5. Λ[W cm-1 K-1] ≈ -1.33 ∆ω1332[cm-1] + 20. The dependence of the thermal conductivity of neutron irradiated diamond single crystals versus the change of FWHM of the diamond Raman line δω1332 is found by Morelli (1994). Roughly this dependence can be given as follows: Λ[W cm-1 K-1] ≈ -2.4 lg(δω1332[cm-1]) + 12.2. The following approximate correlation between the thermal conductivity and the Raman D-band intensity can be found for PCCVD diamond films* (von Kaenel et al. 1996): Λ[W cm-1 K-1] ≈ -0.23(ID-band/Idiam) + 7.5. Ion sputtering of CVD diamond films with Ar +, O+ or N+ ions of energy 1 keV does not noticeably change their Raman spectra (Ilias et al. 1996). For instance, ion sputtering with 1 keV Ar + ions does not cause nondiamond Raman features in goodquality PCCVD diamond films (Ilias et al. 1996). However, the absence of nondiamond features in Raman spectra taken from ion-sputtered diamond surfaces may be due to the very small thickness of the ion-damaged layer and, consequently, the very low Raman efficiency. Almost all Raman features in the spectral range from 300 to 1700 cm-1 (broad bands at wavenumbers below 1332 cm-1 and narrow bands at wavenumbers above 1332 cm-1) observed in ion implanted diamond are observed as "forbidden" spectra, indicating that the defects associated with these features are not aligned with the cubic diamond lattice (Prawer et al. 1998). Nondiamond Raman features are reduced significantly in CVD diamond films grown at elevated pressures (to 60 Torr) of the reaction gas mixture and, consequently, at elevated microwave power (to 75 W/cm3). This reduction can exceed by an order of magnitude that observed in films grown at a pressure of 10 Torr and a power of 3 W/cm3 (Sharda et al. 1989). Raman spectra of heavily boron-doped CVD diamond films resemble those of amorphous diamond (Zhang et al. 1996) (Fig. 4.16).
5
Optical Electronic Transitions
5.1
Optical Bands
In this chapter the Moessbauer-type optical centers are listed after the energies of their zero-phonon transitions. The broad bands are given after the spectral positions of their maxima or by the spectral position of their onset at longer wavelengths. All spectral positions are given for liquid nitrogen temperature, unless otherwise is indicated. 0.30 eV (4130 nm), 0.351 eV (3534 nm), 0.507 eV (2448 nm), A; features observed in synthetic diamonds grown with As impurity. These bands are possibly electronic transitions at an As-related donor center (see also 0.524 eV transition) (Bokii et al. 1986; Kluev et al. 1974a; Kluev et al. 1972c). The line at 0.507 eV is possibly a onephonon replica of the 0.351 eV transition coupling with the TO phonon of energy 0.159 eV (Kluev et al. 1972c). No noticeable growth sector anisotropy has been found for the capture of As in synthetic diamonds during growth (Laptev et al. 1996). 0.3307 eV (3748 nm; 2668.2 cm-1); A; ZPL; the H1d center. The H1d center can be produced by irradiation and subsequent annealing at temperatures above 600°C only in type IaA diamonds. ZPL width at LNT is of 60 cm-1. At room temperature ZPL shifts to a wavenumber of 2678.7 cm -1. In 13C diamond at LNT ZPL of the H1d center shifts down by –9.2 cm-1. No isotopic shift of the H1d, H1e, H1f and H1g centers is observed when replacing 14N isotopes by 15N. The intensity of the center attains its maximum after annealing at 1000°C. The H1d and H1g centers are formed during annealing in a complimentary fashion with vanishing of the GR1, ND1, 2.068 eV and TH5 centers and in a similar fashion with the formation of the nitrogen-related vacancy centers H3, H4, 1.945 eV. The H1d center anneals out at temperatures above 1400°C. The H1d and H1g centers are similar to the H1b center. The H1d center interacts with vibrations of energy 63.9 meV. In 13C diamonds this vibration has an energy of 62.0 meV. The centers H1d, H1e, H1f and H1g are tentatively attributed to multivacancy defects (Kiflawi et al. 1999) (see the H1e, H1f, H1g centers) (Fig. 5.1). 0.347 eV (3572 nm, 2800 cm-1); A, EA, PC. In some papers this center is called the B center. The 0.372 eV line is the most intensive feature of the center in absorption
126
5 Optical Electronic Transitions
at LNT. The center is observed in any boron-doped semiconducting diamonds (Smith and Taylor 1962; Mort et al. 1991).
4
a
ABSORPTION COEFFICIENT, cm
-1
H1b (2024 nm) H1c (1933 nm)
3
2
1
0 1850
1900
1950
2000
2050
2100
WAVELENGTH, nm
45
b
2668.2 (H1d)
ABSORPTION COEFFICIENT, cm
-1
40 35
3107 (hydrogen)
30 25
4442.4 (H1g)
3184
20
3393
15 4396 (H1f)
2912.8 (H1e)
10
3237.4
5
4915
3437
0 2500
3000
3500
4000
WAVENUMBER, cm
4500
5000
5500
-1
Fig. 5.1. (a) Absorption spectrum of the H1c and H1b centers at 77 K in a natural diamond after irradiation and annealing at 1000°C (Collins 1997). (b) Spectra of the H1d, H1e, H1f and H1g centers in electron irradiated type Ia diamonds taken at LNT (the intrinsic absorption of the diamond lattice is subtracted from the spectra) (Kiflawi et al. 1999)
The absorption of the B center is a continuum starting at an energy about EA = 0.370 eV (boron acceptor activation energy) and extending to about 2.2 eV (Johnson et al. 1964). There are related maxima at 0.304, 0.336, 0.342, 0.350, 0.363 (the 0.363 eV peak can be shifted to 0.368 eV in boron ion implanted type IIa diamonds (Sandhu et al. 1989)), 0.466 (possibly the 0.305 eV transition +0.161 eV
5.1 Optical Bands 127
LO-phonon), 0.508 (possibly the 0.347 eV transition +0.161 eV LO-phonon), 0.528, 0.625, 0.670 and 0.830 eV. All these features can be very broad and poorly resolved in CVD diamond films (Fig. 5.2). This broadening is especially strong in highly doped films (Mort et al. 1991). At high boron concentration (higher than 1019 cm-3) the fine structure of the lines is broadened out into the photoionization continuum (in PCCVD diamond films this effect is clearly seen at boron concentrations above 100 ppm) (Teremetskaya et al. 1995; Ertz et al. 1995). Synthetic HPHT diamonds grown from a Fe-Mg-Zn-C growth medium (intentionally boron-undoped) show intense absorption bands at 1290, 2460 and 2800 cm-1 characteristic of semiconducting boron-doped diamonds, indicating unintentional contamination with boron (Bakul et al. 1975). The bands at 0.304, 0.347 and 0.363 eV exhibit fine structures at low temperature. All the bands in the range from 0.300 to 0.365 eV exhibit doublet structures with an equal splitting (probably due to spin-orbital splitting) of 2.1 meV, the components of these doublets being thermolized. This fine doublet structure is not observed in synthetic diamonds because of nonhomogeneous broadening of the lines (Collins 1993a; Crowther et al. 1967b; Charette 1961a) (see Table 5.1).
Table 5.1. Optical transitions from the ground to excited states of boron acceptor showing spin-orbital splitting (Smith and Taylor 1962; Crowther et al. 1967b) Electronic transitions
Energy [eV] (Crowther et Energy [eV] (Smith and al. 1967b) Taylor 1962) 0.3046 0.3042±2
II*) - 2[Γ8(g) - Γ8(4)] and I* ) - 2[Γ7(g) - Γ6(4)] II - 3[Γ7(g) - Γ8(3)] I - 3[Γ8(g) - Γ8(3)] II - 4[Γ7(g) - Γ8(5)] I - 4[Γ8(g) - Γ8(5)] II - 5[Γ8(g) - Γ6(1)] I-5 II - 6 I-6 II - 7 I-7 II - 8
0.3356±1 0.3373±1 0.3404 0.34148 0.34210 0.34353 0.3451 0.34637 0.34710 0.34913 0.3524 0.35456 0.35579 0.35789 0.36273
0.3360 0.3377 0.3408 0.3418 0.3425 0.3439 0.3456 0.3473 0.3497 0.3528 0.3552 0.3562 0.3585 0.3628 0.3653
I-8
0.3646
-
*) I and II denote the ground states and the figures 2 - 8 denote the excited states.
There is a boron-related absorption in the one-phonon spectral region (below 0.165 eV), the integrated absorption of which is about 2% of the integrated absorption of the 0.347 eV peak (Davies 1994a). There are also weak peaks at 0.266,
128
5 Optical Electronic Transitions
0.270 and 0.290 (or 0.293) eV assigned to transitions at the excited states of the boron center (Davies 1994a; Pereira E. and Monteiro 1991; Struzhkin and Eremets 1988; Klimenkova et al. 1975c). The EA value increases with hydrostatic pressure P at a rate of dEA/dP ∼ 3×10-6 eV/bar (Pel et al. 1996). The absorption intensity of the B center is relatively strong. This is conditioned by a relatively low electronic transition energy (of only twice the Raman energy) resulting in considerable vibronic mixing (Davies 1994b). The lines at 0.304, 0.347, 0.363, 0.462 and 0.508 eV are especially strong when detected by the EA technique, simultaneously showing the Stark effect on the 0.304 eV line (Vavilov et al. 1985; Karatygina and Konorova 1976). The absorption intensity increases with temperature drop at a rate of 0.013 cm-1/K excluding the 0.347 eV peak, the intensity of which increases at a rate of 0.038 cm-1/K (Charette 1959). A much stronger temperature dependence has been found for the integrated absorption intensity of the 0.347 eV line by BienemannKuespert et al. (1967): 3 cm-1/K. The bands at 0.304 and 0.347 eV are suppressed by neutron irradiation (Malogolovets et al. 1978c). Electron irradiation with energy above 0.4 MeV also reduces the B center intensity considerably (an effect of the electrical compensation of boron acceptors with radiation-induced donors) (Collins 1977) (Fig. 5.3). The removal rate of the B center by 2 MeV electrons is about six times greater at a temperature of 100 K than at RT (Collins 1977). The B center is attributed to boron acceptors. The features at 0.304, 0.347 and 0.363 eV are caused by transitions at the first, second and third excited states, respectively. The features of energies less than 0.37 eV are due to transitions of the bound holes between the ground state and various excited states of the acceptor center. The more energetic peaks are transitions of the bound holes with emission of one or more lattice phonons of energy 159 meV (Ertz et al. 1995). The 0.830 eV peak is a three-phonon replica. The Huang-Rhys factor of electron-phonon coupling at the center is S = 0.18±0.02 (Davies 1994a; Pereira and Santos 1990a; Pereira and Santos 1987b). The step observed at 0.530 eV is believed to be due to transitions assisted by the LOk=0 phonon (Davies 1994a; Pereira and Santos 1990a). More than 30 peaks in an interval from 0.330 to 0.371 eV are interpreted as transitions to excited states, some of the lines having a stress-induced origin (Dean 1965; Pereira and Monteiro 1990b). The symmetry of the transitions are: 0.347 eV - Γ1; 0.342 eV Γ′25; 0.305 eV - Γ′25; 0.334 eV - Γ12 (Crowther et al. 1967b; Struzhkin and Eremets 1988). The concentration of uncompensated boron acceptors can be calculated by the following formulas (Bokii et al. 1986; Collins and Williams 1971; Chrenko 1973; Lightowlers and Collins 1976a; Enckevort 1994; Malogolovets 1981; Novikov 1968; Nachalnaja et al. 1980): 0.360 meV
(NA-ND)[ppm] = 4.45×10-3
∫ A( E ) dE
[meV/cm],
0.325 meV
(NA-ND)[cm-3] = 0.63×1016 I0.348eV = 0.54×1014µ0.348eV,
5.1 Optical Bands 129
(NA-ND)[cm-3] = 0.7×1016 µ2810cm-1 ≈ 1.6×1017 µ1290cm-1, (NA-ND)[cm-3] = 0.54×1014 µ2810cm-1, where I is the integrated line intensity and ND is the compensating donor concentration (mostly nitrogen). Using these relations the NA-ND value can be measured from the optical absorption with an accuracy of 20% (Kurdumov et al. 1994). The acceptor content in boron-doped synthetic diamonds is strongly anisotropic: NA-ND < 0 for the {100} and {110} growth sectors and NA-ND > 0 for the {111} and {211} growth sectors. 0.3617 eV (3427 nm; 2918.2 cm-1); A; ZPL; the H1e center. ZPL width of the H1e center measured at LNT is 77 cm-1. The relatively large widths of ZPLs of the H1d and H1e centers are explained by the proximity of the transitions to the edge of the diamond intrinsic two-phonon absorption at 2664 cm-1. At RT the ZPL of the H1e center shifts negligibly to 2917.0 cm-1. In 13C diamond at LNT the ZPL of the center shifts down by –20.3 cm-1. The center can be produced by irradiation and annealing at temperatures above 1200°C only in type IaB diamonds. The maximum intensity of the center is attained after annealing at temperatures of 1500-1600°C. The H1e center anneals out at 1800°C. The spectrum of the H1e center consists of at least three components at 3081.0 (very weak), 3237.4 and 3437.0 cm-1. The 3437 cm -1 band is possibly a phonon replica of the ZPL (Kiflawi et al. 1999). The center interacts with vibrations of energy 64.9 meV. The temperature dependencies of the intensities of the centers H1d, e, f, g are well described by the relation I = exp[-S (1+2n)], where n is the Bose-Einstein population term and S is the HuangRhys factor (Fig. 5.1). 0.3836 eV (3231 nm, 3095 cm-1); A; a band produced in type Ia (both IaA and IaB) diamonds by heavy electron irradiation. The band appears immediately after irradiation at RT. The spectral position of the band maximum may vary from 3194 to 3110 cm -1 for different samples. The band anneals out at a temperature of 1000°C (Kiflawi et al. 1999) (Fig. 5.4). 0.438 eV (2830 nm), see 0.524 eV. 0.524 eV (2365 nm); A; a band accompanied by two lines at 0.578 and 0.438 eV. The center is observed in synthetic diamonds doped with As. It is attributed to a multicharged As-related donor center (the 0.30 eV center is possibly another charge state of the 0.524 eV center). The 0.524 eV center is stable to a temperature of 1500°C. The energy 0.524 eV is the optical ionization threshold of the acceptor, whereas the 0.578 eV and 0.438 eV lines are attributed to transitions involving excited states. Preliminary models of the center are: (As+V), (As+divacancy), (V+As+V) (Rotner et al. 1983) (Fig. 5.5). 0.5448 eV (2275 nm; 4396 cm-1); A; the H1f center. ZPL width at LNT is 13 cm-1. At RT the ZPL of the H1f center lies at 4400.0 cm-1. In 13C diamond at LNT the ZPL
130
5 Optical Electronic Transitions
shifts down by –7.6 cm-1. The H1f center can be produced by irradiation and subsequent annealing at temperatures above 1200°C in type IaA and IaB diamonds. The maximum intensity of the center is attained after annealing at 1500-1600°C. The H1f center anneals out at a temperature of 1800°C. The center shows very similar annealing behavior to that of the H1e center. The H1f center interacts with vibrations of energy 64.3 meV (Kiflawi et al. 1999) (see the H1d, H1e, H1g centers) (Fig. 5.1).
ABSORPTION COEFFICIENT, cm
-1
35
85 K
30
a
0.347
25 20
0.342 0.350 0.336
15 0.305 0.363
10
0.508 0.462 0.625 0.670
Raman energy 0.165
5 0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
QUANTUM ENERGY, eV
1200 2837 (0.351 eV)
b
ABSORPTION, cm
-1
1000 800
2467 (0.305 eV)
600 400 200 0 2000
phonon assisted bands
2500
3000
3500
4000
4500
5000
-1 cm-1
WAVE NUMBER,cm WAVENUMBER,
Fig. 5.2. Absorption spectra related to boron acceptors: (a) Spectrum taken at LNT from a natural type IIb diamond; (b) spectrum taken at a temperature of 20 K from a weakly boron doped (about 50 ppm) CVD diamond film (Smith and Taylor 1962; Ertz et al. 1995)
INTEGRATED ABSORPTION, eV/m
5.1 Optical Bands 131
1
10
0
10
-1
10
0
17
1x10
2x10
17
ELECTRON IRRADIATION DOSE, cm
3x10
17
-2
Fig. 5.3. Integrated absorption (taken at RT) of the boron-related B center versus dose of irradiation with 2 MeV electrons (the irradiation temperature is 300 K) (Collins 1977)
0.5506 eV (2251 nm; 4442.4 cm-1); A; the H1g center. ZPL width of the H1g center at LNT is 25 cm-1. At RT the ZPL shifts down to 4437 cm -1. In 13C diamonds at LNT the ZPL shifts up by +9.6 cm-1. The H1g center can be produced by irradiation and subsequent annealing at temperatures above 600°C only in type IaA diamonds. The maximum intensity of the center is attained after annealing at a temperature of 1000°C. The center anneals out at temperatures above 1400°C (Kiflawi et al. 1999) (see the H1d, H1e, H1f centers) (Fig. 5.1). 0.578 eV (2144 nm), see 0.524 eV. 0.612 eV (2024 nm; 4941 cm-1); A; the H1b center; a line appearing in nitrogencontaining diamonds after irradiation and annealing at temperatures above 650°C. The intensity of the H1b center attains a maximum after annealing at about 1000 to 1100°C. The center anneals out at a temperature of 1400°C. The H1b center has monoclinic-I symmetry. It is characterized by a very weak vibronic side-band (S ∼ 0.1). The ZPL of the H1b center does not exhibit an isotopic shift in 13C diamonds (Davies and Collins 1999). The H1b center is believed to be formed by trapping the 2.086 eV center on the A-aggregate of nitrogen (Field 1992; Collins and Stanley 1985b; Davies 1994a; Clark et al. 1956c; Collins et al. 1986a; Collins 1997; Lawson et al. 1992b; Buerki et al. 1999) (Fig. 5.1). 0.6408 eV (1934 nm; 5171 cm-1); A; ZPL; the H1c center; a narrow line showing behavior similar to that of the H1b center. The H1c center is believed to be formed by the 2.086 eV center trapped at the B-aggregates of nitrogen (Field 1992; Collins
132
5 Optical Electronic Transitions
and Stanley 1985b; Collins et al. 1986a; Collins 1997; Lowson et al. 1992b; Buerki et al. 1999) (Fig. 5.1).
3095
2400
2600
2800
3000
3107 (hydrogen)
3200
3400
3600
3800
4000
-1
WAVENUMBER, cm
Fig. 5.4. Absorption spectrum of the 3095 cm-1 (0.3836 eV) band produced in type Ia diamond by heavy electron irradiation (Kiflawi et al. 1999)
ABSORPTION COEFFICIENT, cm
-1
4
3 2.37 2.14
2.83
2
1
0 1
2
3
4
5
WAVELENGTH, µm
Fig. 5.5. Absorption spectrum of a synthetic diamond doped with As. The concentration of the As-related donors is 4.5×1016 cm-3 (Rotner et al. 1983)
5.1 Optical Bands 133
0.6906 eV (1795 nm, 5571 cm-1); A; FWHM is 50 cm-1; a Gaussian-shaped line observed in homoepitaxial CVD diamond films. The line shifts by about 10 cm-1 to lower energies and decreases about 3 times in intensity with temperature increase from LNT to RT (Fuchs et al. 1995b). No shift is observed for the line in 13C diamonds. In deuterated 12C:D diamond films the line shifts to 0.6754 eV (5449 cm-1). In 12C:50%H:50%D diamonds the line splits into three components at 0.6796, 0.6835 and 0.6871 eV (5483, 5514 and 5544 cm-1). The center is attributed to an electronic transition at a complicated defect incorporating several hydrogen atoms (Fuchs et al. 1995a; Fuchs et al. 1995b) (Fig. 5.6). 0.77 eV (1610 nm); A; the amber center; a center most readily occuring in type Ib diamonds. The amber center shows continuous absorption up to at least 3.5 eV superimposed by two bands at 2.2 and 3.3 eV, which results in the characteristic amber coloration of diamond. The center also shows a peak at 0.52 eV. The amber center interacts with vibrations of energy 83 and 89 meV (Walker 1979).
7346
5544 5514
7238
5483
7191
6877 6833
5000
5500
6000
6500
7000
7500
8000
-1
WAVENUMBER, cm
Fig. 5.6. Absorption spectrum of a homoepitaxial CVD diamond film deuterated to 50% (Fuchs et al. 1995b)
0.8515 eV (1456 nm, 6870 cm-1); A, FWHM is around 50 cm-1; a Gaussian-shaped line observed in homoepitaxial CVD diamond films. The line shifts by about -10 cm-1 to lower energies and decreases about 3 times in intensity with temperature increase from LNT to RT (Fuchs et al. 1995b). In deuterated 12C:[50%H:50%D] diamonds the line splits into two components locating at 0.8470 and 0.8524 eV (6833 and 6877 cm-1). In 13C diamonds the line shifts to 0.8517 eV (6872 cm-1). The center is attributed to an electronic transition at a defect incorporating one hydrogen atom (Fuchs et al. 1995a; Fuchs et al. 1995b) (Fig. 5.6).
134
5 Optical Electronic Transitions
0.8972 eV (1382 nm, 7238 cm-1); A; FWHM is around 50 cm-1; a Gaussian-shaped line observed in homoepitaxial CVD diamond films. The line shifts by about -10 cm-1 to lower energies and decreases about 3 times in intensity with temperature increase from LNT to RT (Fuchs et al. 1995b). In 13C diamond the spectral position of the line is at 0.8974 eV (7240 cm -1). In deuterated 12C:D films the spectral position of the line is at 0.8913 eV (7191 cm-1). In partially deuterated 12 C:[50%H:50%D] diamonds the line splits into two components locating at 0.8913 and 0.8972 eV (7191 and 7238 cm-1). The center is attributed to an electronic transition at a defect incorporating one hydrogen atom (Fuchs et al. 1995a; Fuchs et al. 1995b) (Fig. 5.6). 0.9130 eV (1358 nm, 7366 cm-1); A; FWHM is around 50 cm-1; a Gaussian-shaped line observed in homoepitaxial CVD diamond films. The line shifts by about -10 cm-1 to lower energies and decreases about 3 times in intensity with temperature increase from LNT to RT (Fuchs et al. 1995b). In deuterated 12C:D diamonds the spectral position is at 0.9078 eV (7324 cm-1). In 13C diamonds the line shifts to 0.9145 eV (7378 cm -1). In partially deuterated 12C:[50%H:50%D] diamonds the spectral position is at 0.9105 eV (7346 cm-1). The center is attributed to an electronic transition at a defect incorporating one hydrogen atom (Fuchs et al. 1995a; Fuchs et al. 1995b). (Fig. 3.6, 5.6). 1.053 eV (1177 nm); PL; ZPL; a center observed at low temperatures in type Ia diamonds (Ruoff et al. 1991a). 1.118 eV (1108.4 nm); CL; ZPL; a center observed in type IIa and Ia natural diamonds after ion implantation with various species. The intensity of the center decreases with increase in nitrogen content. The center, however, does not relate to nitrogen. The center attains maximum intensity upon annealing at a temperature of 800°C. The feature anneals out at temperatures above 900°C. The center relates possibly to a multivacancy complex (Gippius and Kazarian 1995). 1.149 eV (1077.9 nm); CL; ZPL; a center observed in type IIa and Ia natural diamonds after ion implantation with various species. The center intensity decreases with increase in nitrogen content. The center, however, does not relate to nitrogen. The center shows a complicated annealing behavior, attaining maxima upon annealing at temperatures of 400, 700 and 1000°C. It anneals out at temperatures above 1000°C. The center relates possibly to a multivacancy complex (Gippius and Kazarian 1995). 1.177 eV (1046 nm); PL; ZPL; a center observed in synthetic Ib diamonds after neutron irradiation with doses ranging from 1017 to 1019 cm-2 and subsequent annealing at 800°C (Vins 1988; Vins et al. 1988). 1.22 eV (1016 nm); A; ZPL; a center observed in synthetic diamonds grown by the temperature-gradient method using a nickel catalyst and nitrogen getter. The ZPL of the center consists of up to eight overlapping components at: 1.21148 eV (FWHM
5.1 Optical Bands 135
1.46 meV), 1.21327 eV (FWHM 1.9 meV), 1.21554 eV (FWHM 1.13 meV), 1.21753 eV (FWHM 3 meV), 1.2192 eV (FWHM 2.67 meV, the main line), 1.2255 eV (FWHM 3.82 meV), 1.2280 eV (FWHM 5.22 meV) and 1.23018 eV (FWHM 1.11 meV). Electron-phonon coupling at the center is very complicated, exhibiting interaction with vibrations of energies 64, 113, 152 and 164 meV. The center is dichroic. The unique peculiarity is two mutually perpendicular dipole moments existing at the center. There is a correlation between the decay of the 1.40 eV nickel-related center and strengthening of the 1.22 eV center. The 1.22 eV center is exclusively confined to the {111} growth sectors. The center is thought to be a different charge state of the 1.40 eV nickel center (Collins and Spear 1982a; Collins and Spear 1983b; Davies 1994a; Lawson et al. 1993c) (Fig. 5.7, 5.165). 1.245 eV (996.0 nm); A, ZPL; a center observed in nickel- and nitrogen-containing synthetic diamonds grown by the temperature gradient method and subsequently annealed at temperatures above 2000 K (Yelisseyev and Nadolinny 1995a).
ABSORPTION COEFFICIENT, cm
-1
7 6 5 4 3 2 1 0 1.20
1.21
1.22
1.23
1.24
1.25
QUANTUM ENERGY, eV
Fig. 5.7. Absorption spectrum of ZPL of the 1.22 eV center taken at LNT from a brown diamond (Lawson et al. 1993c)
1.249 eV (991.8 nm); CL; ZPL; a center observed in natural diamonds (both Ia and IIa types) implanted with Ti+ ions. The center is also excited in Ti-containing diamonds after ion irradiation with other species (in type IIa diamonds it appears just after irradiation, in type Ia diamonds it requires annealing at 1500°C). The center intensity strongly increases after annealing; maximum intensity is attained after annealing at about 1200 to 1300°C. The center remains stable at a temperature of 1600°C. The ZPL width (measured at 77 K) increases with the annealing temperature in a range from 400 to 1300°C. The ZPL consists of at least three components separated from each other by ≈ 1 meV (this fine structure is sample
136
5 Optical Electronic Transitions
dependent). The center intensity does not correlate with the nitrogen content. Two broad bands at 1.13 and 1.16 eV are possibly due to electron-phonon interaction with acoustic short-wavelength phonons propagating along the direction (the Σ branch at the K point). The relatively narrow line (FWHM of 5 meV) at 1.217 eV is a replica due to quasilocal vibration of energy 32 meV involving two Ti atoms (calculated values ωR = 32.8 meV, ∆ωR = 11 meV). The center is attributed to some loose but geometrically definite complexes with other impurities or intrinsic defects. These complexes may change with annealing causing the variable ZPL broadening (Gippius and Collins 1993a; Kazarian and Gippius 1993; Gippius 1995; Gippius 1993; Gippius et al. 1994). A tentative model of the center comprises two Ti atoms locating in the neighboring tetrahedral (or hexagonal) interstitial positions along the axis (*) (Fig. 5.8). 1.25 eV (992 nm); CL, PL; FWHM of 0.3 eV; a broad band observed in type IaA diamonds containing platelets and characterized by the secondary absorption edge at 3.7 eV. The luminescence of the band is strongly polarized with the electric vector lying in the plane of the platelets (E||/E⊥ ∼ 40). The intensity of the band correlates with the intensities of the N3 center and the platelet-related 7.3 µm absorption band. The N3 and N2 centers are possibly involved in PL excitation of the 1.25 eV band. The band intensity increases almost linearly with exciting quantum energy in the range from 2.4 to 2.7 eV. The relative intensity of the 1.25 eV band to that of the A-band may considerably increase with temperature increase from LNT to RT. The 1.25 eV band possibly relates to the giant B' platelets precipitated in (100) planes. The band is thought to be attributed to the radiative recombination of electrons from the platelet-related energy band (spreading from EV + 0.7 eV to EV + 1.8 eV) to the valence band (Davies 1977c; Field 1992; Davies 1994a; Wight et al. 1971; Ruoff et al. 1991a; Desgreniers et al. 1989; Kiflawi and Lang 1977; Wight et al. 1971) (Fig. 5.9). 1.256 eV (986.3 nm); A, PL, TSL, not excited in CL (Collins 1991a); ZPL; the H2 center. The H2 center appears in synthetic type Ib diamonds after annealing at 1700°C (Yelisseyev and Nadolinny 1995a), or after radiation damage and 1500 to 1800°C annealing (Satoh 1992). It can also be observed in natural type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). Lower annealing temperatures do not result in formation of the center. In natural diamonds the H2 center is stable at least at 900°C. The modulation of intensities of the H2 and H3 centers is achieved by illumination at wavelengths of 600 nm or shorter (Mita et al. 1990). The symmetry of the H2 center is rhombic-I. In absorption the center interacts with local vibrational mode of an energy 167.1 meV giving rise a line at 1.424 eV (Lawson et al. 1992b). This vibration is attributed to a pure carbon-carbon vibration (Lawson et al. 1992b; Lawson et al. 1991). The ZPL of the H2 center in 13C diamonds shifts by +0.93 meV (Davies 1994a; Davies and Collins 1999). The center interacts with an acoustic vibration of energy 70 meV. The center has a relatively large electron-phonon coupling: S ~ 4 (Davies 1977c). The center is possibly active in EPR (Mita et al. 1993; Nishida et al. 1993). A possible model of the H2 center is two nitrogen atoms sandwiching a vacancy along the
5.1 Optical Bands 137
direction (the negative charge state of the H3 center) (Davies 1994a; Mainwood et al. 1994; Mita et al. 1990; Lawson et al. 1992b). It is believed that annealing irradiated diamonds at 900°C results in the electron transfer from single substitutional nitrogen atoms to H3 centers creating the H2 centers (Buerki et al. 1999) (Fig. 5.10).
119 meV 85 meV 32 meV
1.249 1.217
1.16 1.13 x6
1.10
1.15
1.20
1.25
QUANTUM ENERGY, eV
1.249 eV Ti-related center
substitutional C d
interstitial Ti
c b a
λ/2
Fig. 5.8. CL spectrum (at LHeT) of a natural diamond implanted with Ti + ions and subsequently annealed at 1400°C. The ZPL of the center shows fine structure (Gippius and Collins 1993a). The center interacts predominantly with quasilocal vibration of energy 32 meV (two Ti atoms) and acoustic Σ phonons at the K point (energies of 89 and 119 meV). A possible model of the Ti-related center is a pair of interstitial Ti atoms bound along the direction. The amplitudes of the atoms participating in the acoustic Σ (at the K point, in the (110) plane) vibrations are shown with letters a, b, c, d. Note that this model assumes the Ti-Ti electronic bond to be about half of the Σ-phonon wavelength at the K point and, consequently, undergo a strong deformation induced by this vibrational mode
138
5 Optical Electronic Transitions
1.25
0.8
1.0
1.2
1.4
1.6
QUANTUM ENERGY, eV
Fig. 5.9. PL spectrum of a diamond of type Ia taken at RT; excitation with Ar-laser line at 438 nm (Ruoff et al. 1991a)
ABSORPTION COEFFICIENT, cm
-1
1.0
0.8
0.6
0.4
0.2
0.0 0.7
0.8
0.9
1.0
1.1
1.2
1.3
QUANTUM ENERGY, eV
Fig. 5.10. PL spectrum (corrected) of the H2 center taken from a synthetic diamond (Lawson et al. 1992b)
1.264 eV (980.6 nm); CL; ZPL; a center observed in some natural diamonds. The center interacts with 60 meV vibrations. The electron-phonon coupling at the center is relatively low: S ~ 1 (Davies 1977c).
5.1 Optical Bands 139
1.277 eV (970.5 nm); CL, ZPL; a center observed in natural diamonds (of both types Ia and IIa) implanted with Ti+ ions and subsequently annealed at temperatures above 800°C. It is also created in Ti-containing diamonds after ion irradiation with other species. In type Ia diamonds annealing at 1500°C is required to create the center. The center is stable at temperatures over 1600°C. The center intensity does not correlate with nitrogen content (Kazarian and Gippius 1993; Gippius 1993; Gippius et al. 1994). Possibly this center is observed in nickel- and nitrogencontaining synthetic diamonds grown by the temperature gradient method and subsequently annealed at about 1500°C (Kupriyanov et al. 1999). (Fig. 5.13a). 1.31 eV (946 nm); possibly ZPL; a sharp line observed in some CVD diamond films (Collins 1997; Allers and Collins 1995). 1.328 eV (933.4 nm, at 17 K); PL; ZPL; FWHM of ZPL is about 40 meV at 17 K; a center observed at low temperatures in type Ia diamonds (Ruoff et al. 1991a; Desgreniers et al. 1989) (Fig. 5.11). 1.357 eV (913.9 nm); A; ZPL; a center observed in nickel- and nitrogen-containing synthetic diamonds grown by the temperature gradient method and subsequently annealed at about 1700°C (Yelisseyev and Nadolinny 1995a). 1.360 eV (911.4 nm, at 17 K); PL; FWHM of ZPL is about 40 meV at 17 K; a center observed at low temperatures in type Ia diamonds (Ruoff et al. 1991a; Desgreniers et al. 1989) (Fig. 5.11).
1.328
1.360
1.574
1.77
1.2
1.3
1.4
1.5
1.6
1.7
1.8
QUANTUM ENERGY, eV
Fig. 5.11. PL spectrum (corrected for spectral response) of a type Ia diamond. The spectrum was taken at a temperature of 17 K using excitation with the 488 nm Ar laser line (Ruoff et al. 1991a)
140
5 Optical Electronic Transitions
1.4 eV (890 nm); A; FWHM of 0.5 eV; a broad band observed in synthetic diamonds grown in the presence of nickel and adding nitrogen getters. The band shows the maximum intensity for nitrogen concentrations in the range from 10 to 50 ppm (Collins et al. 1990c; Collins and Spear 1982a; Collins and Spear 1983b; Davies 1994a; Lawson et al. 1993c; Collins 1990b). 1.4008, 1.4035 eV (884.85, 883.15 nm); A, PL, CL, XL (Davies 1994a; Vins 1988; Yelisseyev et al. 1987); ZPL doublet; the 1.40 eV center. (Fig. 5.12, 5.165). The 1.40 center is readily observed in synthetic diamonds grown with a nickel catalyst. It is rarely observed in some type Ib natural diamonds. The center can be artificially created in natural type IIa diamonds implanted with Ni+ ions (Gippius et al. 1983; Dean 1965). The center is enhanced in synthetic diamonds by neutron irradiation and annealing at 800°C (Vins 1988). The center is suppressed by HPHT treatment (6 GPa, 1650 K) (Yelisseyev et al. 1987). It is not observed in highly doped type IIb diamonds (Kurdumov et al. 1994). It is a very temperature stable center: it does not anneal out completely at a temperature of 2500°C (Kupriyanov et al. 1999). However, annealing of the center in some diamonds may occur at 2150°C (Nadolinny and Yelisseyev 1993). In CL at a temperature of 100 K the phonon-assisted band of the center shows features at the following quantum energies: 1.340, 1300, 1.271, 1.236, 1.160 eV. In absorption there is a strong weakly structured band ranging from 1.7 to 2.1 eV (the band peaks at around 1.8 eV) related to the center (Lawson and Kanda 1993a). There is a fine structure of ZPL: each line of the doublet consists of at least four equispaced components associated with stable Ni isotopes (1.40376, 1.40096 eV 58 Ni; 1.40359, 1.40079 eV - 60Ni; 1.40343, 1.40063 eV - 61Ni+62Ni; 1.40327 eV 64 Ni) (Nazare et al. 1991b; Davies et al. 1989). The relative intensities of the components well coincide with the natural abundance of Ni isotopes within a 15% error. In ion implanted diamonds the doublet narrows strongly after annealing at temperatures above 1300°C (Gippius 1993). In CL the 1.40 eV center interacts predominantly with 60 meV quasilocal vibrations involving Ni atoms (the peak is at 1.34 eV, FWHM of 50 meV; calculated values ωR = 44 meV, ∆ωR = 20 meV) and phonons of energies 95, 125 and 165 meV. The electron-phonon coupling is relatively low: S ~ 1.6. The PL excitation spectrum of the 1.40 eV center in synthetic type Ib diamonds consists of the bandgap band (< 220 nm) and three broad bands peaked at about 260, 310 and 650 nm, being the most intensive within a range from 250 to 400 nm (see the 484 nm nickel-related center) (Vins 1988; Yelisseyev et al. 1986). In 13C diamonds the ZPL of the center shifts by -0.5 meV (Collins et al. 1988c; Davies 1994a; Collins and Davies 1988b; Davies and Collins 1999). The PL intensity of the center may not change with temperature increase to 200°C (Kurdumov et al. 1994). In synthetic diamonds the PL of the center may be strongly quenched under a temperature increase from 150 to 300 K according to Mott's law with parameters Eq = 0.085 eV, τA = 150 when being excited within a spectral range from 200 to 400 nm (Vins 1988). The center is the most prominent in samples with the dominating 484 nm nickel-related center (the CL intensity of the 484 nm center follows the CL intensity of the 1.4 eV center squared?) (Collins et al. 1990c; Collins and Spear 1983b; Malogolovets 1979). However, there is no
5.1 Optical Bands 141
correlation between the intensities of these centers in PL (Collins and Spear 1983b; Vins 1988). A high content of Ni in the growth media (above 60%) suppresses the PL intensity of the center (Vins 1988). The center shows very strong luminescence in synthetic diamonds grown in Al or Ti-containing media (Vins 1988). Nitrogen suppresses the center intensity; however in low-nitrogen synthetic diamonds the 1.4 eV center disappears in absorption after annealing at 1800°C (Lawson and Kanda 1993b; Kupriyanov et al. 1999). The 1.40 eV center is the predominant nickel-related feature in synthetic diamonds with nitrogen content less than 10 ppm (these diamonds have a green color) (Lawson and Kanda 1993a). In synthetic diamonds the intensity of the center increases with concentration of the paramagnetic Ni-related center according to a power law with a factor of 1 to 2 (Vins 1988). In synthetic diamonds the center is confined to the {111} growth sectors, being undetectable in the {100}, {110}, and {113} sectors. The emission of the center is strongly polarized in synthetic diamonds with the E vector preferentially directed along (111) growth planes (Field 1992; Collins et al. 1990c; Collins 1989; Lang 1980). The decay time of the center in CL is 20 to 33 ns (Davies 1977c; Davies 1994a; Nazare et al. 1991b). The common model of the 1.40 eV center is interstitial Nii+ ion relaxed along the direction. The electronic configuration of the Ni-ion is 3d9 (S=1/2, local symmetry C3v). The 1.40 eV center is an electronic transition from a 2E ground state to a 2A excited state (a nondegenerate orbital state). The ZPL doublet is formed by electronic transitions between Γ4−Γ5 and Γ4−Γ4 states (Nazare et al. 1990; Nazare et al. 1991b; Davies et al. 1989; Paslovsky and Lowther 1992). The doublet structure of the ZPL is a result of the spin-orbital interaction splitting the ground state of the center by 2.59 meV (Collins and Spear 1982a; Collins and Spear 1983b). The center is tentatively associated with the NIRIM-2 paramagnetic center (Isoya et al. 1990a). According to this model the concentration of the Ni atoms NNi(1.40) incorporated in the 1.40 eV centers can be found from the absorption coefficient µ1.40 integrated over the whole ZPL: NNi(1.40)[cm-3] = 1.7×1014µ1.40[cm-1] (Nazare et al. 1990). The 1.4 eV center is also tentatively attributed to substitutional Ni +3 ions (Vins 1988). 1.404 eV (883.2 nm); A; ZPL; a center observed in nickel- and nitrogen-containing synthetic diamonds grown by the temperature gradient method and subsequently annealed at about 1700°C (Yelisseyev and Nadolinny 1995a). 1.413 eV (877.2 nm); PL; ZPL; a center observed in nickel- and nitrogen-containing synthetic diamonds grown by the temperature gradient method and subsequently annealed at temperatures above 1500°C. The center vanishes after annealing at temperatures above 1950°C. The center interacts predominantly with 59 meV quasilocal vibrations. The PLE spectrum of the 1.413 eV center has a maximum at wavelengths of 560 to 580 nm (Kupriyanov et al. 1999; Yelisseyev et al. 1999) (Fig. 5.13a). 1.423 eV (868 nm); possibly ZPL; a sharp line observed in CVD diamond films (Collins 1997; Allers and Collins 1995).
142
5 Optical Electronic Transitions
1.401, 1.404
165 meV
a
127 meV 99 meV 70 meV 50 meV
1.16
0.9
1.0
1.1
1.2
1.3
1.4
1.5
QUANTUM ENERGY, eV 1.404
b 1.401
1.395
1.400
1.405
1.410
QUANTUM ENERGY, eV 58
c
60 61
62
?
64
Ni
Ni
Ni
Ni
Ni
1.4028 1.4030 1.4032 1.4034
1.4036
1.4038
1.4040
1.4042
QUANTUM ENERGY, eV
Fig. 5.12. (a) and (b) CL (taken at LNT) of a synthetic HPHT diamond grown with a Ni catalyst: general spectrum of the 1.4 eV center and its ZPL (Collins and Spear 1983b). The energies of the vibrations dominating the vibrational side-band are marked. The strongest vibronic band, lying 60 meV away from the ZPL, is thought to be a superposition of a 50 meV quasilocal vibration involving one Ni atom and the low-energy tail of the diamond phonon density. (c) Absorption spectrum of the 1.404 eV line measured at 11.5 K. The fine structure of the line corresponds to the centers containing different Ni isotopes (Davies et al. 1989)
5.1 Optical Bands 143
1.428 eV (868.0 nm); CL; ZPL; a center observed in type Ib synthetic diamonds grown using nickel. The center is activated after annealing at 1700°C. The center anneals out when heated to 2200°C (Field 1992; Collins and Stanley 1985b). The center is formed only in {111} growth sectors of synthetic diamonds. This center is attributed to a nickel-related defect (Collins and Woad 1993a). 1.441 eV (860.2 nm); A; ZPL; a center observed in nickel- and nitrogen-containing synthetic diamonds grown by the temperature gradient method and subsequently annealed at about 2000 K (Yelisseyev and Nadolinny 1995a). 1.446 eV (857 nm); possibly ZPL; a sharp line observed in some CVD diamond films (Collins 1997; Allers and Collins 1995). 1.448 eV (856.0 nm); CL; ZPL; a center observed in type Ib synthetic diamonds grown using nickel. The center is activated after annealing to 1700°C. The center anneals out when heated to 2200°C (Field 1992; Collins and Stanley 1985b). It is formed in synthetic diamonds only in the {111} growth sectors. The center is attributed to a nickel-related defect (Collins and Woad 1993a). 1.466 eV (845 nm); CL; ZPL; a center created in oxygen ion-implanted natural diamonds. The center appears only after annealing at temperatures above 1500°C. The center is stimulated by 4 MeV electron irradiation and subsequent annealing at 1650°C. It is attributed to an oxygen-containing defect. The center does not contain nitrogen (Gippius and Kazarian 1993b; Gippius 1992; Gippius 1993). The 1.466 eV center could not be excited in PL of synthetic type Ib diamonds with high oxygen content (Neves et al. 1999). 1.472 eV (842 nm); PL, CL; a relatively broad line (FWHM of 20 meV) observed in HPHT synthetic diamonds grown by the temperature gradient method using a Co catalyst. The center is observed in regions with low concentration of aggregated nitrogen. It is ascribed tentatively to a Co-related defect. Nitrogen, possibly, does not take part in the formation of the center (Kanda and Watanabe 1999) (Fig. 5.13b). 1.474 eV (841 nm); PL; ZPL; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Co and Si (Sittas et al. 1995; Sittas et al. 1996). 1.482 eV (836 nm); CL; ZPL; a center created in oxygen ion-implanted natural diamonds. The center appears only after annealing at temperatures above 1500°C. It is stimulated by 4 MeV electron irradiation and subsequent annealing at 1650°C. The center is attributed to an oxygen-containing defect, which does not contain nitrogen (Gippius and Kazarian 1993b; Gippius 1992; Gippius 1993). The 1.482 eV center could not be excited in PL of synthetic type Ib diamonds with high oxygen content (Neves et al. 1999).
144
5 Optical Electronic Transitions
1.489 eV (832.4 nm); PL; ZPL; a center observed in nickel- and nitrogen-containing synthetic diamonds grown by the temperature gradient method and subsequently annealed at about 1500°C (Kupriyanov et al. 1999) (Fig. 5.13a). 1.493 eV (830.2 nm); possibly ZPL; a sharp line observed in some CVD diamond films (Collins 1997; Allers and Collins 1995).
PL
a
1.4
1.563 46 meV
1.413
1.648
59 meV
II
1.276
1.489
I
1.2
1.3
1.4
1.5
1.6
1.7
QUANTUM ENERGY, eV
b
Co-related centers 544 842 623 580
500
600
700
800
900
WAVELENGTH, nm
Fig. 5.13. (a) PL spectra (LNT, excitation at a wavelength of 560 nm) of a nitrogen- and nickel-containing synthetic diamond grown by the temperature gradient method after annealing at 1500°C (I) and 2200°C (II) (Kupriyanov et al. 1999); (b) PL spectrum (taken at LNT) of Co-related luminescence centers in a synthetic diamond grown with a Co solvent catalyst by the temperature gradient method. The measurement was performed in a microscopic region free from nitrogen-related luminescence (Kanda and Watanabe 1999)
5.1 Optical Bands 145
1.50 eV (826 nm); A; the N1 center (N stands for Naturally occurring); a center naturally occurring in type Ia diamonds. The center shows a weak temperature dependence. The phonon side-band of the N1 center is dominated by 60 meV vibrations. The center has a similar appearance to the N2 center (Davies 1977c; Walker J. 1979; Clark et al. 1956a). The N1 center is attributed to a forbidden transition on the N3V defect (the N3 center) (Sobolev and Yurjeva 1990) (Fig. 5.14). 1.521 eV (814.9 nm); A; ZPL; a center observed in type IIb diamonds after radiation damage. The center is strongly photochromic when illuminated with light of quantum energy above 2 eV (Bokii et al. 1986; Field 1992). 1.525 eV (813 nm); PL at about 7 K; ZPL; a center observed in type IaB natural diamonds irradiated by neutrons of energy above 1 MeV with a dose of 1019 cm-2 and subsequently annealed at a temperature of 950°C. The center can be created in high-nitrogen synthetic diamonds annealed at high temperatures (e. g. 2000°C) and irradiated with neutrons (Osvet et al. 1997). The ZPL of the center is characterized by very broad nonhomogeneous widths (about 6 nm). Because of this the line is detected at RT without noticeable broadening. Spectral holes of width 22 cm-1 can be burnt in the ZPL at temperatures as high as 300 K. The homogenous width of the ZPL at LHeT is 5 GHz (Bauer et al. 1993; Sildos and Osvet 1994a; Sildos and Osvet 1994b; Sildos et al. 1995). The center tentatively ascribed to a defect related to the B-aggregates of nitrogen (Fig. 5.15).
ABSORPTION COEFFICIENT, cm
-1
0.20 2.596 (N2)
0.15
0.10 1.5 (N1)
0.05
0.00 1.2
1.6
2.0
2.4
2.8
QUANTUM ENERGY, eV
Fig. 5.14. Absorption spectrum of a natural type I diamond taken at LNT (Clark et al. 1956a)
1.527 eV (811.5 nm); CL, PL; the most intensive ZPL of a center formed in diamonds of any type by Xe+ ion implantation and subsequent annealing. The center
146
5 Optical Electronic Transitions
can be effectively excited after annealing at a temperatures as low as 700°C. In the PL spectrum there is a related line at 793.5 nm (at RT), the intensity of which is proportional to that of the ZPL at 811.5 nm. The 793.5 nm line is tentatively attributed to a second ZPL of the center. The center interacts predominantly with vibrations of energy 69 meV. The intensity of the center strongly increases upon annealing at temperatures to 1500°C. The center is attributed to a Xe-containing defect (Zaitsev 1992a; Zaitsev 1992b) (Fig. 5.16).
774
813
731 734
700
720
740
760
780
800
820
840
WAVELENGTH, nm
Fig. 5.15. PL spectrum of a natural type Ia diamond (nitrogen content: 4×1019 cm-3 of the A-aggregates, 8×1019 cm-3 of the B1-aggregates and 2×1015 cm-3 of the platelets) irradiated with neutrons of energy above 1 MeV at a dose of 1019 cm-2 and subsequently annealed at 950°C. PL is excited with an Ar laser at a temperature of 7 K (Sildos et al. 1995)
1.528 eV (811 nm); PL; ZPL; a center observed in synthetic type Ib diamonds after neutron irradiation with doses of 1017 to 1019 cm-2 and subsequent annealing at 800°C (Vins 1988; Vins et al. 1988). 1.530 eV (810 nm); PL; a broad band observed in compressed type Ia diamonds. 1.530 eV is the spectral position of the band maximum measured at RT at a pressure of 400 GPa. The band shows a feature positioning at around 840 nm at a pressure of 405 GPa. The band shifts towards shorter wavelengths with pressure increase. The band is induced by pressure over 2.5 Mbar. This feature is thought to be caused by tetragonal distortion of the diamond lattice (Ruoff et al. 1991b) (Fig. 5.17).
5.1 Optical Bands 147 6373 (NV)
a 8109 (Xe)
1.3K
LNT
RT 6000
6500
7000
7500
8000
8500
9000
WAVELENGTH, A°
8116
b
~ 100 meV 28 meV
8075
77 K
1.3 K
7800
8000
8200
8400
8600
8800
9000
WAVELENGTH, A°
813
c
793.5
Xe (RT)
780
800
820
840
860
880
WAVELENGTH, nm
Fig. 5.16. (a) PL of the nitrogen-related center 638 nm and the Xe-related center with ZPL at 811 nm in a natural diamond implanted with 500 keV Xe2+ ions at a dose of 5×1014 cm-2 and subsequently annealed at 1400°C. The spectra have been taken at 1.3 K, LNT and RT. (b) Detailed spectra in the spectral region from 7800 to 9000 Å reveal a weak broad band at around 8670 Å, which is possibly a phonon replica of the 811 nm Xe center (Gorokhovsky and Zaitsev 1998). (c) RT spectrum in the ZPL region of the Xe-related center (taken on a sample implanted with a dose of 1013 Xe/cm-2 and annealed at 1200°C) exhibits a weak line at 793.5 nm, which could be a second ZPL of the center
148
5 Optical Electronic Transitions
1.534 eV (808 nm); PL; ZPL; a "multiple-lines" band. This is the most intensive line of a group (1.443, 1.469, 1.482, 1.508, 1.534 and 1.549 eV) of ZPLs observed in some HPHT synthetic diamonds grown in the presence of Ni, Zr and Si. All these lines appear with correlated intensities (Sittas et al. 1995; Sittas et al. 1996). Possibly this very center is observed in high-nitrogen synthetic diamonds grown with Ni-containing catalyst after annealing at temperature above 1700°C. In these diamonds the center is destroyed by annealing at temperature above 1950°C (Yelisseyev et al. 1999) (Fig. 5.18).
810
840
700
750
800
850
900
WAVELENGTH, nm
Fig. 5.17. Stress-induced PL taken from a type Ia natural diamond under a pressure of 405 GPa (Ruoff et al. 1991b)
1.541 eV (804.3 nm); PL; ZPL; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Ni and Si (Sittas et al. 1995; Sittas et al. 1996). 1.50 eV (800 nm); a broad band observed in EL of synthetic diamonds at temperatures to 200°C (Levinson and Halperin 1979). 1.559 eV (795 nm); PL; a center naturally occurring in some low-nitrogen type Ib natural diamonds (Bokii et al. 1986). 1.563 eV (793.5 nm); A, PL, CL; ZPL; a naturally occurring center in diamonds containing the S2 and S3 centers. The 1.563 eV center is a feature of the PLE spectra of the S2 and S3 centers (Kupriyanov et al. 1999). The intensity of the 1.563 eV center correlates well with the content of the S2 center. The 1.563 eV
5.1 Optical Bands 149
center is particularly intensive in type IaB diamonds. It is also observed in as-grown synthetic diamonds grown by the temperature gradient method with nickelcontaining catalysts. The center is especially strong in deep-yellow nitrogencontaining sectors of synthetic diamonds (Yelisseyev et al. 1996; Palyanov et al. 1997b). The center is induced in high-nitrogen nickel-containing synthetic diamonds by heating at temperatures above 1600°C (Yelisseyev et al. 1999). It is effectively created in nitrogen- and nickel-containing synthetic diamonds by electron irradiation and subsequent annealing at 900°C (Osvet et al. 1997). The center is present only in the {111} growth sectors of synthetic diamonds (Nadolinny et al. 1999). The PL of the center may dominate in diamonds annealed at temperatures above 2200°C. The 1.563 eV center is stable at temperatures over 2500°C (Bokii et al. 1986; Field 1992; Collins and Stanley 1985b; Kluev et al. 1982; Lawson and Kanda 1993a; Nadolinny and Yelisseyev 1993; Yelisseyev and Nadolinny 1995a; Lawson and Kanda 1993b; Yelisseyev and Nadolinny 1993; Martynovich and Sapozhnikov 1980; Yelisseyev et al. 1996; Solin 1972; Kupriyanov et al. 1999). The center gives a greenish color to synthetic diamonds (Yelisseyev et al. 1996). The PLE spectrum of the 1.563 eV center consists of its own absorption spectrum, a number of broad bands in the range from 2.2 to 4 eV, the 1.929 and 2.750 eV centers as well as a narrow ZPL at 3.22 eV (385.2 nm). The most intensive PLE broad bands have maxima at 3.35 eV (370 nm) and 2.88 eV (430 nm); the former band is believed to relate directly to the 1.563 eV center (Kupriyanov et al. 1999, Nadolinny et al. 1999). In some diamonds the PLE spectrum of the center consists of two broad bands centered at 2.88 eV and 3.65 eV (Bokii et al. 1986; Field 1992; Plotnikova et al. 1980). In PL the center interacts with vibrations of energies 17, 33(40), 70 and 153 meV, which produce corresponding maxima in PLE spectrum of the center. The 3.22 eV transition of the PLE spectrum interacts with 99 meV vibrations (Yelisseyev et al. 1999). The 70 meV mode is the predominant one in PL and probably belongs to the Λ3(A) vibrational branch of the diamond lattice (Martynovich and Sapozhnikov 1980; Nadolinny et al. 1999). The electron-phonon coupling of the center has a moderate value: S = 3.5, α = 0.03 (calculated for the 70 meV mode) (Martynovich and Sapozhnikov 1980). This is a dipole electronic transition (dipole moment d = 4×10-30 Cm). The lifetime of the center in PL is (2.7 to 9.7)×10-8 s (Martynovich and Sapozhnikov 1980). The PL intensity and lifetime of the center decrease slightly with heating to RT (Martynovich and Sapozhnikov 1980). A possible EPR analog of the 793.5 nm center is the NE8 center This center is attributed to a nickel-related defect (Collins and Woad 1993a; Yelisseyev et al. 1992b; Yelisseyev et al. 1999). Possibly it is a defect containing nickel bound to the A-aggregates of nitrogen (Lawson and Kanda 1993a). An alternative model of the center is a defect composed of only nickel atoms (Yelisseyev et al. 1996). A complex atomic structure of the center containing one Ni atom, two vacancies, and four N atoms was suggested in (Nadolinny et al. 1999) basing on the ESR data of the NE8 center (Fig. 5.19). 1.573 eV (788 nm, at 17 K); PL; ZPL; a center observed in some diamonds. FWHM of ZPL may be as broad as 40 meV at 17 K and as narrow as 19 meV at RT (Solin 1972). It is a naturally occurring center in type Ia diamonds. The center is frequently present in type Ia yellow diamonds and diamonds of a mixed cubo-octahedral habit.
150
5 Optical Electronic Transitions
The intensity of the center correlates with that of the S2 center. The center interacts with vibrations of energy 38, 47 and 56 meV. The PL excitation spectrum of the center consists of two broad bands centered at 2.88 eV and 3.65 eV. The center often coexists with the N2 and N3 centers. It is attributed to a defect formed of nitrogen aggregates (Bokii et al. 1986; Field 1992; Plotnikova et al. 1980; Vohra et al. 1989; Ruoff et al. 1991a; Desgreniers et al. 1989; Solin 1972) (Fig. 5.11).
1.534
1.508 1.469 1.443
1.40
1.45
1.549
1.482
1.50
1.55
1.60
QUANTUM ENERGY, eV
Fig. 5.18. PL spectrum of single-crystal synthetic diamonds grown using Ni-Si or Ni-Si-Zr catalysts. PL is excited at LNT with an Ar laser (Sittas et al. 1995)
1.593 eV (778 nm); PL; a center naturally occurring in lonsdaleite-containing diamonds and some type I diamonds (Bokii et al. 1986; Solin 1972). 1.594 eV (777.8 nm); CL; ZPL; a center observed in some CVD diamond films implanted with B+ ions and annealed at a temperature of 1000°C (Zaitsev et al. 1995). The center can also be observed in irradiated intentionally undoped CVD diamond films (Zaitsev et al. 1996b) (Fig. 5.20). 1.597 eV (776 nm); PL; ZPL; a center observed in synthetic Ib diamonds after neutron irradiation with doses from 1017 to 1019 cm-2 and subsequent annealing at 800°C (Vins 1988). 1.599 eV (775 nm, at 100-130 K); PL; a line observed in epitaxial CVD diamond films grown at very high temperatures of 1800-2100°C. The line appears to be a doublet with components peaking at wavelengths 776 and 782 nm. Several side-bands at 778 nm and higher wavelengths can also be detected. The PL of the
5.1 Optical Bands 151
center is preferentially excited in the green (∼500 nm) spectral region. The center is ascribed very tentatively to an extended defect (McCauley and Vohra 1995) (Fig. 5.21).
1.563
70 meV
35 meV
1.20
1.25
1.30
1.35
1.40
1.45
1.50
1.55
1.60
QUANTUM ENERGY, eV
PLE spectrum of the 1.563 eV center
1.929
3.3
1.563
2.75
1.5
2.0
2.5
3.0
3.5
4.0
QUANTUM ENERGY, eV
Fig. 5.19. PL and PLE spectra (taken at 77 K) of the 1.563 eV center excited in a synthetic diamond grown with a Ni-containing catalyst. The sample was annealed at 2500°C. The PL spectrum has been taken with excitation at a wavelength of 370 nm (Kupriyanov et al. 1999)
1.600 eV (775 nm); PL; ZPL; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Ni (Sittas et al. 1995; Sittas et al. 1996). The center appears after annealing at temperature above 1700°C and it is destroyed by annealing at temperatures above 1950°C (Yelisseyev et al. 1999).
152
5 Optical Electronic Transitions
7778.5
a
7650
7700
7750
7800
7850
7900
7950
8000
WAVELENGTH, A°
7372
b
7425
7200
7400
7778
7600
7800
8000
o
WAVELENGTH, A
Fig. 5.20. CL spectra (at LNT) taken from both sides of an intentionally undoped free standing CVD diamond film irradiated with 164.4 MeV 12C+5 ions at a dose of 1015 cm-2. The sample was grown on a Si substrate. The sample thickness is 0.4 mm (Zaitsev et al. 1996b)
1.602 eV (774 nm), PL at about 7 K; a line observed in type IaB natural diamonds irradiated with high-energy neutrons (energy > 1 MeV, dose of 1019 cm-2) and subsequently annealed at a temperature of 950°C. The center can also be observed in some natural type I diamonds (Solin 1972). It can be created in synthetic high-nitrogen diamonds; however it is ascent from the diamonds annealed at 2000°C (Osvet et al. 1997). The ZPL of the center is characterized by a very broad nonhomogeneous width ( ~ 6 nm); because of this, when heating from 7 K to RT the ZPL does not broaden noticeably. Spectral holes of width 22 cm-1 can be burnt in this line at temperatures up to RT. The homogenous width of the ZPL at LHeT is 5 GHz. The nitrogen aggregates are supposed to be involved in formation of the
5.1 Optical Bands 153
corresponding defect. The 1.602 eV center possibly relates to the 813 nm center (Bauer et al. 1993; Sildos and Osvet 1994a; Sildos et al. 1995). Tentatively the center is ascribed to a defect containing one or two nitrogen atoms (Fig. 5.15).
775 737
551
627 574
500
600
700
800
900
WAVELENGTH, nm
Fig. 5.21. PL of a homoepitaxial CVD diamond film grown at very high temperature (from 1800 to 2100°C). The spectrum has been measured at a temperature of 120 K (McCauley and Vohra 1995)
1.606 eV (771.8 nm); PL; possibly ZPL; a center occasionally observed in some natural type I diamonds (Solin 1972). 1.6 eV (770 nm); the radiation B-band (the name B-band is also used for the band at 1.8 eV); CL; a broad band of width 0.5 eV appearing in ion irradiated low-nitrogen diamonds after subsequent annealing at temperatures above 600°C. The band may be particularly strong in diamonds implanted with donor impurities. The center is stable at temperatures above 2200°C. The electronic transition of the center interacts with 65 meV vibration. The intensity of the band grows linearly with ion irradiation dose. Production efficiency of the B-band in ion irradiated diamonds correlates with nuclear stopping power of the ions. The B-band is strongly quenched by nitrogen impurity in any form. The luminescence of the B-band is rather temperature stable: at 400°C the band may retain 20% of its intensity at LNT (Gippius et al. 1982a; Filipp et al. 1992; Zaitsev 1992a; Varichenko et al. 1986c; Zaitsev 1999a, Zaitsev 1999b) (Fig. 5.22, 5.23, 5.24, 5.25). 1.619 eV (765.6 nm); PL; possibly ZPL; a center observed in some natural type I diamonds (Solin 1972).
154
5 Optical Electronic Transitions
575
a
H3
N3 +
N
+
+
Li +N B-band A-band 300
Li
+
non-implanted
400
500
600 700 800 WAVELENGTH, nm
900
1000
Fig. 5.22. CL spectra of a very low-nitrogen diamond sample, three areas of which have been implanted with N+, N++Li+ and Li + ions, respectively, and subsequently annealed at 1400°C. The N+ and Li + implantations were performed with energies of 200 and 100 keV, respectively, each at a dose of 5×1014 cm-2. All three spectra were taken at equal parameters of the CL setup. The B-band exhibits the highest intensity in the Li implanted area, whereas it is strongly suppressed by the presence of nitrogen. In contrast, the A-band is strongly suppressed by Li + ion implantation. However, the addition of nitrogen restores the A-band intensity
B-band
Li
389
400
b
+
H3
500
575
600
700
800
900
WAVELENGTH, nm
Fig. 5.23. CL spectrum of a very low-nitrogen natural diamond implanted with 100 keV Li + ions at a dose of 1015 cm-2. The B-band is pronounced feature of the spectrum
5.1 Optical Bands 155
1.0
CL INTENSITY, arb. units
H3 center 0.8 575 nm 0.6
0.4 B-band 0.2
0.0 -200
-100
0
100
200
300
400
TEMPERATURE, °C
CL INTENSITY, Sn, arb. units
Fig. 5.24. Temperature dependencies of CL intensity of the H3 center (l), 575 nm center (n), and B-band (m) in natural type IIa diamonds implanted with N+ (for the H3 and 575 nm centers) and Li + ions (for the B-band) and subsequently annealed at 1400°C
10
2
10
1
10
0
0
10
20
30
40
50
WAVENUMBER, cm
60
70
80
-1
Fig. 5.25. Depth distribution of CL intensity of the B-band in type IIa natural diamond irradiated with 82 MeV 12C ions at room temperature and subsequently annealed at 2200°C for 1 hour (squares). The B-band intensity follows the nuclear stopping power of the primary carbon ions (full line)
1.629 eV (760.9 nm); PL; possibly ZPL; a center observed in some natural type I diamonds (Solin 1972).
156
5 Optical Electronic Transitions
1.635 eV (758 nm); PL; a center naturally occurring in lonsdaleite-containing diamonds (Bokii et al. 1986). 1.637 eV (757 nm); PL; ZPL; a center observed in CVD diamond films grown at high temperature. The set of lines at 1.598, 1.586, 1.575, 1.568 and 1.540 eV is also attributed to this center. The center exhibits moderate electron-phonon coupling: S ~ 3.0 (McCauley and Vohra 1994a; McCauley and Vohra 1994b). 1.6390 eV (756.3 nm at 6 K); PL, CAS; ZPL; a center observed in MPCVD diamond films containing Si impurity. The 1.639 eV center relates to the 1.681 eV Si center. The 1.639 eV center possesses a 40 times stronger nonradiative recombination pathway as compared to the Si center. In some samples the ZPL of the center is very broad (Bilodeau et al. 1993; Sittas et al. 1995). Very tentatively the center is attributed to a vacancy-related center. 1.645 eV (753.5 nm); PL; ZPL; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Ni and Si (Sittas et al. 1995; Sittas et al. 1996). 1.647 eV (752.6 nm, at RT); PL; a line observed in some HFCVD diamond films. It appears to be stronger in the films deposited onto hard substrates like Mo and W. This line may be a vibronic side-band of the 1.681 eV Si center (Wang et al. 1996). 1.648 eV (752.8 nm); PL; ZPL; a center observed in nickel- and nitrogen-containing synthetic diamonds grown by the temperature gradient method and subsequently annealed at temperatures above 1700°C. The center may reduce its intensity after anneal at temperatures above 1950°C. The center interacts predominantly with quasilocal vibrations of energy 46 meV. The PLE spectrum of the 1.648 eV center is composed of absorption spectra of the 1.929 and 2.750 eV centers (Kupriyanov et al. 1999). The center is attributed to a nickel-containing defect (Nadolinny et al. 1999; Yelisseyev et al. 1999) (Fig. 5.13a, 5.26). 1.65 eV (750 nm); PL; a broad band with a maximum at 700 to 800 nm. The band is observed in natural polycrystalline diamonds containing lonsdaleite inclusions (Bokii et al. 1986). 1.652 eV (750.3 nm); PL; possibly ZPL; a center observed in some natural type I diamonds (Solin 1972). 1.658 eV (747.6 nm); PL; possibly ZPL; a center observed in some natural type I diamonds (Solin 1972). 1.660 eV (746.7 nm); PL, CL; ZPL; a center observed in synthetic diamonds containing aggregated nitrogen. The center is prominent after annealing at temperatures above 1600°C. Annealing at 2000°C does not affect intensity of the 1.660 eV center (Osvet et al. 1997). It anneals out on heating to 2200°C (the center
5.1 Optical Bands 157
disappears at temperatures providing the highest degree of defect transformation) (Kupriyanov et al. 1999). The center is observed only in diamonds grown using a nickel-containing catalyst. The center exhibits a relatively strong luminescence comparable with green luminescence of the S2 and S3 centers. The 1.660 eV center is present only in the {111} growth sectors. The center interacts predominantly with 50 meV vibrations; there is no evidence of interaction with lattice phonons (Yelisseyev et al. 1999). The strength of the electron-phonon coupling at the 1.660 eV center is relatively low: S ~ 1.3. The PL of the center is excited at wavelengths shorter than 515 nm (Field 1992; Collins and Stanley 1985b; Lawson et al. 1996). PLE spectrum of the 1.660 eV center exhibits a ZPL at 510.6 nm (Yelisseyev et al. 1999). The center is not active in absorption; neither is it excited at the wavelength of its ZPL (a possible explanation: the center is a transition between two excited electronic states) (Kupriyanov et al. 1999). The decay of the center intensity in PL can be approximated with a lifetime of 1.1 ms. The center possibly relates to a spin-forbidden transition (Osvet et al. 1997). The center is attributed to a nickel- and nitrogen-related defect (Collins and Woad 1993a; Lawson et al. 1996; Collins 1997; Kupriyanov et al. 1999; Yelisseyev et al. 1999) (Fig. 5.27, 5.49).
PLE spectrum of the 1.648 eV center
1.929
2.750
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
QUANTUM ENERGY, eV
Fig. 5.26. PLE spectrum of the 1.648 eV center. The 1.648 eV center dominates in diamonds annealed at high temperatures. The PLE spectrum of the 1.648 eV center does not contain its own absorption spectrum but is dominated by the 1.929 and 2.750 eV centers (Kupriyanov et al. 1999)
1.666 eV (744 nm); A, PL; ZPL; a center observed in type IaB natural diamonds after irradiation at a dose of 1019 neutron/cm2 and subsequent annealing at 950°C (Sildos and Osvet 1994b). The center probably interacts with 136 meV vibrations. 1.67 eV (741 nm); CL; ZPL; the most intensive line of a set of lines within the range 730 to 780 nm appearing in type IIa diamonds after implantation with Cr + ions and
158
5 Optical Electronic Transitions
subsequent annealing at temperatures above 1000°C. The center possibly relates to a defect containing two Cr atoms. The lines at 755 and 770 nm (FWHM for both lines is 12 meV) are replicas due to a quasilocal 31 meV vibration involving two Cr atoms (calculated values ωR = 31.3 meV, ∆ωR = 10 meV) (Vavilov et al. 1982a; Zaitsev 1992a; Zaitsev 1999b) (Fig. 5.28).
PL
1.660
59 meV
1.704 1.50
1.55
1.60
1.65
1.70
1.75
QUANTUM ENERGY, eV
51 meV
PLE spectrum of the 1.660 eV center
2.427
28 meV 2.2
2.4
2.6
2.8
3.0
3.2
QUANTUM ENERGY, eV
Fig. 5.27. PL and PLE spectra (both at LNT) of the 1.660 eV center taken from a synthetic Ni-containing diamond annealed at 1500°C for 16 hours (Kupriyanov et al. 1999). The center exhibits dominating interaction with a 59 meV quasilocal vibrational mode. The center is excited exclusively in the absorption spectrum of the 2.427 eV center, showing the dominant electron-phonon interaction with 51 meV vibrations and a much weaker interaction with 28 meV vibrations
1.673 eV (741 nm); CL; 1.693 eV is the position of a weak ZPL of a broad structured band, called the C-band, naturally occurring in all natural and synthetic (except type Ib and IaA) diamonds. The ZPL may be relatively strong in diamonds
5.1 Optical Bands 159
of intermediate type. The center interacts with 53 meV vibration. The electronphonon coupling is strong: S ~ 7 to 8. The band structure broadens with temperature increase from 50 K to RT but the luminescence intensity remains constant. The band is tentatively attributed to a nitrogen-related defect (Walker J. 1979; Wight et al. 1971) (Fig. 5.29).
31 meV
Cr
741
754 770
720
740
760 WAVELENGTH, nm
780
800
Fig. 5.28. CL spectrum of a very low-nitrogen natural diamond implanted with 300 keV Cr + ions at a dose of 1014 cm-2 and subsequently annealed at 1400°C. Vertical lines show the structure due to interaction with quasilocal vibrations of two Cr atoms
B-band
1.673
C-band
1.2
1.4
1.6
1.8
2.0
2.2
QUANTUM ENERGY, eV
Fig. 5.29. CL spectrum of a natural type IIb diamond measured at 50 K. The spectrum is dominated by the B- and C-band luminescence (Wight et al. 1971)
160
5 Optical Electronic Transitions
1.673 and 1.665 eV (740.9 and 744.4 nm); ZPL doublet; the GR1 center (abbreviation of General Radiation); A, PL, CL, XL (Bienemann-Kuespert et al. 1967), reversibly observed in PC (no direct photoconductivity) (Vermeulen and Halperin 1981). The GR1 center is the most prominent electronic transition of the GR center. It is believed to be one of the most studied optical centers originated from point defects in solids (Fig. 5.30, 5.53).
GR1(ZPL)
2028 meV, two-phonon replica of the excited state
1.859 3H
1859.4 meV, one-phonon replica of the excited state
GR2
1690.8 meV, excited state
TR12
1.859 eV 1.685 eV 50 meV, paramagnetic, spin triplet
77K 1.5
2.0
2.5
6.2 meV 0 meV, ground state, spin-singlet
3.0
QUANTUM ENERGY, eV
Fig. 5.30. Absorption spectrum of a CVD diamond film as-irradiated with 2 MeV electrons at a dose of 1018 cm-2. The spectrum was recorded at LNT (Allers et al. 1998). The energy level scheme of the interstitial-related 1.685 eV center is shown. The allowed transitions are shown with arrows. All other transitions are forbidden (Davies et al. 2000)
The GR1 center is the main optical feature of irradiated diamond of any type. The GR1 center is created by almost any high-energy irradiation (electrons, γ-rays, ions). The impurity content may affect the intensity of the center strongly. Usually the GR1 center is not observed in high-nitrogen (in particular type Ib synthetic diamonds) after neutron irradiation: neither in as-irradiated samples nor after irradiation and subsequent annealing (Nishida et al. 1989). The GR1 center is strongly quenched in diamonds implanted with nitrogen ions (Gippius et al. 1982a; Zaitsev et al. 1979b; Zaitsev et al. 1991; Varichenko et al. 1986d). Also the GR1 center is not revealed in irradiated highly boron-doped type IIb diamonds. In lowboron-doped diamonds the center appears upon a certain irradiation dose compensating the acceptors (the GR1 center may even be observed in some natural type IIb diamonds without irradiation (Solin 1972)) (Bienemann-Kuespert et al. 1967). The GR1 center is suppressed in ion-implanted CVD diamond films by hydrogenation (Yagyu et al. 1995). A low concentration of the GR1 center (below 5×1012 cm-3) is always present in the top layer of the growth surface (a few micrometers deep) of CVD diamond films. The GR1 center is more easily produced in the vicinity of dislocations,
5.1 Optical Bands 161
strained regions (Phaal 1965), or nitrogen aggregates than in the regular lattice (Bradlow et al. 1981). The luminescence of the GR1 center is very pronounced in diamonds irradiated with high-energy ions (Zaitsev et al. 1987c). The depth distribution of the luminescence intensity of the center in diamonds irradiated with ions of energy above 1 MeV/a.m.u. clearly shows that vacancies in diamond are effectively created in the regions of dominating electronic stopping power, that is in strongly ionized lattice (Fig. 5.31, 5.32, 5.33). 10
3
CL INTENSITY, arb. units
a 10
2
10
1
10
0
0
20
40
60
80
100
DEPTH, µm
12
b 10
WIDTH, meV
8 6 4 2 0 0
20
40
60
80
100
DEPTH, µm
Fig. 5.31. Change of CL intensity (a) and FWHM of ZPL (b) of the GR1 center (s), the 575 nm center (l) and the 389 nm center (n) over depth in a synthetic type Ib diamond as-irradiated with 82 MeV C ions at a dose of 1015 cm-2 (Varichenko 1986). Note, that the intensity of the 575 nm and 389 nm centers reduces at the end of the ion range as the nuclear stopping power increases
162
5 Optical Electronic Transitions
10
2
C ions 82 MeV
CL INTENSITY, arb. units
a 10
1
10
0
10
-1
10
-2
0
20
40
60
80
DEPTH, µm
14
b
12
FWHM, meV
10 8 6 4 2 0 0
20
40
60
80
DEPTH, µm
Fig. 5.32. Change of intensity (a) and FWHM (b) of the GR1 center (l), the TR12 nm center (n), the 3H center (s) and 575 nm (m) over depth in a natural type IIa diamond as-irradiated with 82 MeV C ions at a dose of 3.3×1015 cm-2 (Varichenko 1986)
The ZPL of the GR1 center, and consequently the Debye-Waller factor, is relatively strong in PL taken from diamonds irradiated with high-energy ions. This is a common peculiarity for any ion species irrespective of the atomic mass (from carbon to xenon) (Fig. 5.34). The creation rate of the center by 2 MeV electrons at room temperature is 0.075 cm-1 in type IIa diamonds and 0.105 cm-1 in type IaA diamonds. Electrons of energy 1.9 to 1.5 MeV in diamonds containing 9 ppm of the C centers of nitrogen
5.1 Optical Bands 163
produce vacancies at a rate of NV[ppm] = 3×10-18 D[cm-2] (obtained from Lawson et al. 1998). The vacancy creation rate (measured on the intensity of the GR1 center) by 1.9 MeV electrons in synthetic single-crystal diamonds with 9 ppm of single substitutional nitrogen atoms is 0.6 vacancy/electron (Twitchen et al. 1999). Note that this value is seven times higher than that found by Davies et al. (1992).
CL INTENSITY, arb. units
100
10
1 0
2
4
6
8
10
DEPTH, µm
Fig. 5.33. Depth distribution of CL intensity of the GR1 center (l), TR12 center (n), and 575 nm center (m) in low-nitrogen type IIa natural diamond as-irradiated with 13.6 MeV B ions
In as-irradiated type IaA diamonds absorption of the GR1 center relates to that of the ND1 center as µND1/µGR1 = 0.53±0.03 (Davies et al. 1992). The GR1 and ND1 centers are usually of comparable intensity in diamonds with aggregated nitrogen (Davies 1994b). The GR1 center is very weak (in contrast to the ND1 center) in type Ib diamonds (Davies 1994b). In type Ia diamonds electron irradiation produces 10 times more neutral vacancies V0 (the GR center) than V- (the ND1 center) (Davies and Collins 1993). The GR1 center intensity is inversely proportional to the V+ EPR center (positively charged vacancy). There is a reversible reaction occurring under UV illumination (→ means ionization) and heating (← means deionization): 2VoGR1 ↔ V+EPR + V-ND1. Deionization occurs also under light illumination in the spectral region from 1.1 to 2.83 eV. The deionization optical spectra are strongly dependent on the type of diamond. There are common deionization bands at 2.5 and 2.8 eV. In high-nitrogen diamonds, deionization occurs in the spectral range from 2.5 to 4 eV with two bands at 2.8 and 3.4 eV. There is a trend: the lower the nitrogen content the stronger the ionization-deionization process (Sobolev and Aksenov 1979b; Sobolev et al. 1975a).
164
5 Optical Electronic Transitions
Table 5.2. Spectral features of the GR1 center in PL and A at LNT Spectral position in PL [nm] 741.0 757.8 773.6 783.5 798.0 803.0 803.8 811.5 817.1
Vibrational energy [cm-1] 297 547-552 730 982-987 1020-1035 1074-1078 1191-1195 1246-1252
823.5
1330-1333
Spectral position in A [nm] 741.0 723.2 710.7 700.1
Vibrational energy [cm-1] 333 550-552 811-803
691.5 686.9 685.3 681.6 678.1
984-987 1035-1037 1071-1078 1167-1176 1242-1252
672.8
1333-1342
Interpretation ZPL quasilocal vibr. hνVL1 TA(L) quasilocal vibr. hνVA2 T(W) LA(L) TO(X) L(X) LO(L), LO(Σ) O(Γ) Interpretation ZPL quasilocal vibr. hνVA1 TA(L) TA(X), possibly quasilocal vibration hνVA2 T(W) LA(L) TO(X) L(W) LO(L), LO(Σ) O(Γ)
Type of the critical point
P1 P0 P3 P0 P2 P2 P1 P3 Type of the critical point
P1 P2 P0 P3 P0 P0 P2 P1 P3
The spectral structure of the electron-vibrational band of the GR1 center is presented in Table 5.2 (Nedzvetskii and Gaisin 1972; Dolling and Couley 1963). The spectral features of the GR1 center are very sharp after irradiation at low temperatures. However, in type IaA diamonds the spectrum of the center is strongly broadened even after low-temperature irradiation (the effect of strong interaction of vacancies with aggregated nitrogen). Low-energy (a few keV) electrons produce in some type IIa and Ia (nitrogen concentration to 1019 cm-3) natural diamonds a broadened GR1 center (FWHM of the ZPL is of 12 meV at LNT) exhibiting a noticeably lower α than that of common GR1 centers (Zaitsev et al. 1980; Gippius et al. 1981) (Fig. 5.34). Annealing at a temperature above 275°C noticeably sharpens the ZPLs of the GR centers; for instance, heating to 400°C causes a very pronounced sharpening of the features of the GR1 center (Collins et al. 1988a; Collins 1997; Dobrinets 1986). However the ZPL of the GR1 center (in CL) may broaden after annealing in the temperature range from 300 to 500°C in diamonds that have experienced strong nuclear stopping of high-energy ion irradiation. A possible reason of this broadening
5.1 Optical Bands 165
is an increase in nonhomogeneous lattice distortion around vacancies due to interaction with interstitial atoms (Fig. 5.35). Strong broadening of the ZPL in areas damaged via high nuclear stopping power of energetic ions can also be observed in as-irradiated diamonds (Fig. 5.32). This broadening anneals out at temperatures above 500°C (Fig. 5.36). 100
742.4 (GR1, ZPL)
PL, arb. units; ABSORPTION, %
a
anti-Stokes Stokes
80
60
A
506.3 (3H)
40
PL 20
0 300
400
500
600
700
800
900
WAVELENGTH, nm
b
732
GR1 (LNT)
735
738
741
744
747
750
753
WAVELENGTH, nm
Fig. 5.34. (a) PL and A spectra taken at RT from a good-quality PCCVD diamond film irradiated with 144 MeV 13C ions at a dose of 2.5×1015 cm-2 (PL) and a natural diamond irradiated with 210 MeV Kr ions at a dose of 2×1013 cm-2 (A). PL is excited with the 514 nm Ar laser line. The absorption spectrum is dominated by the GR1 and 3H centers. The ZPL of the GR1 center in PL is very pronounced, whereas it is almost absent from the A spectrum. One can recognize the first quasilocal vibration of the center in the anti-Stokes region. The spectrum is not corrected for the detector response of the set-up. (b) CL spectra (at LNT) of ZPL of the GR1 center created in a natural type IIa diamond by 1.5 MeV electrons at a dose of 1017 cm-2 (dashed line) and subthreshold electrons of energy 10 keV at a dose of 1020 cm-2 (full line)
166
5 Optical Electronic Transitions
12
GR1
at maximum defect production 10
FWHM, meV
8
6
4 at surface 2
0 0
100
200
300
400
500
600
700
ANNEALING TEMPERATURE, °C
Fig. 5.35. Change of FWHM of ZPL of the GR1 center with annealing in type IIa natural diamond irradiated with 82 MeV 12C ions at a dose of 1015 cm-2. The spectra of the center have been taken in CL from the irradiated surface and from the lateral side of the sample. In the latter case the GR1 center is excited predominantly from the highly damaged regions that have experienced strong nuclear stopping
10
WIDTH, meV
8
6
4
2
0 0
100
200
300
400
500
600
700
TEMPERATURE, °C
Fig. 5.36. Change of FWHM of the GR1 center ZPL with annealing as measured in PL at LNT from the rear (l) and front (m) sides of a diamond sample, the front side of which was implanted with 60 MeV N ions at a dose of 2.8×1015 cm-2. PL was excited with the 514 nm Ar laser line. It is seen that the ZPL is much broader in the buried layer at the depth of the projected ion range Rp excited through the transparent nonirradiated rear side. Obviously the intense damage via nuclear stopping favors the strong nonhomogeneous distortion of vacancies as compared with those created by electronic stopping from the front side
5.1 Optical Bands 167
The ZPL width of the GR1 center may be very different, when measured in A, PL or CL. This effect could originate from different excitation mechanisms by energetic electrons and subbandgap-energy photons. The former may cause strong distortion of optical centers via excitation of their surroundings. The broad ZPL of the GR1 center excited in the CL of diamonds irradiated with high-energy ions may result from distortion of single vacancies locating in proximity to the ion tracks (5.37).
12
CL
GR1
FWHM, meV
8
PL 4
0 0
10
20
30
40
50
60
DEPTH, µm
Fig. 5.37. Change of spectral width of ZPL of the GR1 center over depth in natural type IIa diamond irradiated with 60 MeV N ions as detected in CL and PL
The phonon side-band structure of the GR1 center sharpens with temperature drop from RT down to LNT and there is no further narrowing with temperature fall down to 4.2 K (Nedzvetskii and Gaisin 1972). In heavily damaged diamonds the contribution of temperature-related broadening is considerably suppressed due to strong nonhomogeneous broadening (Fig. 5.38). The main component of the ZPL taken from as-grown PCCVD diamond films is often split by at least two more components (Allers and Mainwood 1997). In CL the ZPL of the GR1 has a set of broader low-energy satellites (temperature dependent) resulting from perturbed vacancies. These satellites are not found in PL when excited at any wavelength (Collins 1978b; Mazzaschi et al. 1981; Collins et al. 1988a). In nitrogen-containing diamonds the ZPL splits due to stress with preferential direction along caused by the nitrogen defects locating beyond the second coordination sphere from the vacancy (Bradlow et al. 1981). The GR1 ZPL in 13C diamonds shifts by +2.9 meV (Collins et al. 1988c; Davies 1994a; Collins and Davies 1988b; Davies and Collins 1999). At RT the ZPL of the GR1 center shifts in 13C diamonds by +2.0 meV (Fig. 5.39).
168
5 Optical Electronic Transitions
20 18 16
WIDTH, SHIFT, meV
14
1
12 10 8
2
6 3
4 2 0 -2
4
-4 -6 50
100
150
200
250
300
350
TEMPERATURE, K
Fig. 5.38. Temperature broadening (1, 2, 3) and spectral shift (4) of ZPL of the GR1 center in natural type IIa diamonds irradiated with 60 MeV N ions (taken from the irradiated surface) with different doses: 1, 4 - at a dose of 5.3×1015 cm-2, 2 - at a dose of 1.7×1014 cm-2, 3 - at a dose of 1.7×1014 cm-2 and subsequent annealing at 500°C (Varichenko 1986). Note, that the temperature-induced broadening is relatively low for centers with high nonhomogeneous broadening
13
C
1.65
1.66
1.67
12
C
GR1 (at LNT)
1.68
1.69
1.70
PHOTON ENERGY, eV
Fig. 5.39. CL spectra at LNT of ZPL of the GR1 center in isotopically pure synthetic diamonds
12
C and
13
C
Transitions GR2-8. In absorption the GR center is accompanied by a broad absorption band extending to about 2.4 eV and a series of sharp lines GR2-GR8 (in
5.1 Optical Bands 169
the spectral range from 2.881 to 3.00 eV) superimposed on a broad continuum extending into the UV region. All the features GR2-GR8 are present in the PL excitation spectrum of the GR1 center (Mohammed et al. 1982a; Collins et al. 1988a) (Fig. 5.40).
GR3
GR2 GR6a GR6b GR7b
GR5
GR8 GR4
GR7a
410
415
420
425
430
435
WAVELENGTH, nm
Fig. 5.40. PLE spectrum of the GR1 center excited at LNT with a tunable dye laser (Collins et al. 1988a)
The exact spectral positions, FWHM and excited state g-values of the GR2-8 transitions are: GR2 2.8806 eV, 0.73 meV, -0.50±0.10; GR3 2.8868 eV, 0.92 meV, -0.40±0.10; GR4 2.902 eV, |g| < 0.1; GR5 2.938 eV, 1.4 meV, |g| < 0.1; GR6a 2.9577 eV, 0.91 meV, -0.60±0.10, effective g value is -2.0; GR6b 2.9600 eV, 0.86 meV, -0.95±0.10, effective g value is of -2.0; GR7a 2.9760 eV, 0.78 meV, -0.55±0.10; GR7b 2.9817 eV, 0.84, -0.60±0.10; GR8a 2.9963 eV, 0.69, -0.65±0.10, effective g value is -1.8; GR8b 2.9975 eV, 0.69, -0.05±0.05, effective g value is 1.8; GR8c 3.0015 eV, 0.68 meV, -0.30±0.10; GR8d 3.0044 eV, 0.63, -1.00±0.10; GR8e 3.0063 eV, 0.63 meV, |g| < 0.2 (Collins 1978d; Manson et al. 1980). The GR2-8 lines show positive (holes) photoconductivity (Farrer and Vermeulen 1972; Clark et al. 1979b). The GR2-8 transitions are not excited in luminescence, because of their ground states lying in the valence band (Zaitsev 1992a). The behavior of the GR2-8 centers is similar to that of the ND1 center (Lowther 1994). Isotope shifts for the GR2-8 lines in 13C diamond are (in meV): GR2 +7, GR3 +7.2, GR6a +7.2, GR6b +7, GR7a +7.1, GR7b +7.4, GR8a +6.9, GR8b +6.9, GR8c +6.9 (Davies and Collins 1999). The GR centers anneal out normally at temperatures above 600°C. They are not produced by irradiation at temperatures over 600°C (Anderson G. et al. 1993; Zaitsev et al. 2000). However, in diamonds irradiated with heavy high-energy ions
170
5 Optical Electronic Transitions
the GR1 center may survive to temperatures as high as 1000°C, although after annealing at temperatures above 600°C its spectrum broadens strongly (Dobrinets 1986) (Fig. 5.41).
10
2
10
1
10
0
CL INTENSITY, arb. units
a
10
-1
10
-2
10
-3
0
200
400
600
800
1000
1200
ANNEALING TEMPERATURE, °C
10
2
10
1
10
0
CL INTENSITY, arb. units
b
10
-1
10
-2
10
-3
0
200
400
600
800
1000
1200
ANNEALING TEMPERATURE, °C
Fig. 5.41. (a) Change of CL intensity of the GR1 (l), TR12 (n), and 3H (s) centers with annealing temperature in a type IIa diamond irradiated with 82 MeV C ions at a dose of 1015 cm-2 (Varichenko 1986). (b) Annealing behavior of the GR1 center (l), 563 nm center (u) and nitrogen-related centers 575 nm (m) and 638 nm (o) excited in CL in synthetic type Ib diamonds irradiated with 62 MeV Co ions at a dose of 5×1014 cm-2. The GR1 center exhibits a local minimum in intensity after annealing at 500°C. The nitrogen-related 638 nm center is observed only after annealing at 500°C, whereas the 575 nm center shows its usual annealing behavior characteristic of ion implanted diamonds (Dobrinets 1986)
5.1 Optical Bands 171
In some heavily irradiated diamonds (for instance after ion or neutron irradiation) the intensity of the GR1 center may strongly increase after annealing at temperatures above 400°C (Zaitsev and Zaitsev 1989) (Fig. 5.42).
1.00
LNT CL INTENSITY, arb. units
0.75
0.50
0.25
0.00 0
100
200
300
400
500
600
700
800
ANNEALING TEMPERATURE, °C
Fig. 5.42. CL intensity (taken at LNT) of centers 575 nm (l), GR1 (m), TR12 (? ) and 656 nm (u) in a natural type Ia diamond irradiated with neutrons at a dose of 1017 cm-2 versus annealing temperature (Zaitsev and Zaitsev 1989). The intensity of the vacancy-related GR1 and TR12 centers strongly increases after annealing at a temperature of 550°C (Zaitsev 1988)
The destruction of the GR1 center by annealing correlates with the intensity increase of the TH5 (2.543 eV) center implying the formation of the latter from the neutral vacancies. The annealing kinetics of the GR1 center in type IIa diamond is described by a combination of first and second order processes (due to low concentration of vacancy traps). In type I diamonds the GR1 annealing kinetics is of first order due to high concentration of the trapping nitrogen (Allers et al. 1998). The activation energy of V0 migration in natural type I diamonds is 2.3 to 2.45 eV (Davies et al. 1992; Davies and Collins 1993; *). The activation energy of the GR1 center migration in CVD diamond films can be much lower, ranging from 1.5 to 2.5 eV (Collins et al. 1994a; Allers et al. 1998). This variation in the activation energy may result from different degrees of crystal lattice perfection and, in particular, from the different content of interstitial-related defects (*). Theory predicts the migration energy of V0 in ideal diamond lattice to be 2.8 eV (Breuer ref Briddon 1995). Annealing of the GR1 center in type IIa natural diamonds has two stages: a rapid stage (due to annihilation with interstitials) and a slow stage (actually the migration of the vacancies themselves) (Allers et al. 1998). Electron-Phonon Coupling. The GR1 center interacts with vibrations of 41 meV (e-mode) and 93 meV (t-mode) in absorption (Davies 1982) but 36 meV in luminescence (Walker 1979; Wight et al. 1971; Nedzvetskii and Gaisin 1973b). In PL at RT the bands resulting from vibrations of energies 48 meV and 80 meV
172
5 Optical Electronic Transitions
(78 meV in 13C diamond) can be resolved. The 36 meV mode is also seen in the anti-Stokes spectral region in PL taken at RT as a weak band at 1.707 eV (*) (Fig. 5.34). The higher energies of the phonons interacting with the GR1 center in absorption than those in luminescence imply a strengthening of the C-C vacancy bonds with excitation (a similar effect is known for the C2 molecule). The shape of the vibrational band (both in luminescence and absorption) except the quasilocal peaks are well described by the phonon density function of the diamond lattice. The interaction with quasilocal vibrations is shown to result in a reduction of the relative intensities of the lattice phonons (Nedzvetskii and Gaisin 1972). Theoretically calculated energies of the quasilocal vibrations of the nonexcited vacancy are: 297 cm-1 (possibly hωV1) and 473, 595, 705 cm-1 (possibly hωV2) (McCombie et al. 1963). Coupling of the excited T2 state to the t-mode is not larger than that to the emode (Muramatsu 1983). In absorption the Huang-Rhys factor of the GR1 center is S ≈ 3.7, 5.7 and 9.2 when derived from the ZPL intensity, the Stokes shift and the bandwidth respectively (Walker 1979). In luminescence SE×e ∼ 2.5, ST×e ∼ 0.15, ST×t ∼ 0.3 (Davies 1982). The Debye-Waller factor of the GR1 center in A is 0.047 (the same as for the ND1 center) (Davies et al. 1992). In diamonds irradiated with high-energy ions the electron-phonon coupling at the GR1 center is considerably lower due to strong nonhomogeneous broadening resulting in the Debye-Waller factor ranging from 0.1 to 0.2. The Debye-Waller factor in these strongly depends on temperature (Fig. 5.43).
DEBYE-WALLER FACTOR
0.28
0.24
0.20
0.16
0.12
0.08 0
50
100
150
200
250
300
350
TEMPERATURE, K
Fig. 5.43. Temperature change of the Debye-Waller factor of the GR1 center taken in CL at LNT in natural diamonds irradiated with 60 MeV N ions at doses of 5.3×1015 cm-2 (l) and 1.7×1014 cm-2 (m) (Varichenko 1986)
The GR1 center shows a linear Stark effect implying a lack of inversion symmetry (Kaplyanskii et al. 1971; Davies and Manson 1980a). The GR1 center has a negligible response to totally symmetrical distortion. This means that the ground E
5.1 Optical Bands 173
state of the center is negligibly coupled to A1-phonons and strongly with E-phonons (Davies and Penchina 1973b; Davies 1982). Luminescence of the GR1 center is unpolarized (even when excited with polarized light), which is a consequence of its tetrahedral symmetry (Clark and Norris 1971b). The lifetime of the GR center in PL is 2.5 ns at temperatures about 15 K. It is 1.4 ns at RT. In CL at 77 K the lifetime is 3.1 ns and its value falls to 0.4 ns at 473 K (Davies et al. 1987b; Collins 1987). The luminescence efficiency of the GR1 center is φ = 0.014 (Davies et al. 1987b). The GR1 center has a strong nonradiative decay channel (Bilodeau et al. 1993). The radiative lifetime is evaluated to be 182 ns. The activation energy of the nonradiative processes at the GR1 center is 0.15 eV (Davies et al. 1987b; Collins 1987; Lin et al. 1995). De-excitation of the GR1 center from the higher excited levels to the lowest excited level occurs in a time less than 1 ns (Davies 1994a; Davies et al. 1987b). Hole burning in ZPL of the GR1 center has been shown. The nonhomogeneous line width is 1000 GHz. The hole width is 195 MHz, and the lifetime is 3 ns. The hole may live for several minutes (Harley et al. 1984). The nature of the GR center is unambiguously attributed to the neutral single vacancy V0. The GR1 electronic transition occurs between a 1E doubly degenerate ground orbital state and the 1T2(C3V) triply degenerate orbital excited state in the defect of Td point symmetry. The ground state of the center probably lies at an energy of 2.86 eV below the conduction band (Yelisseyev 1977). The GR1 and GR2-8 centers are believed to be one and the same defect, namely V0, having the same ground state. However, there is an opinion that the GR2-8 centers relate rather to V- (negatively charged vacancy) than to V0 (Lowther 1994). Interaction with bound hole excitons is responsible for the GR2-8 transitions (Lowther 1994). The splitting of the ZPL of the GR1 center (8 meV) results from a strong dynamic JahnTeller distortion of the ground state (Lowther 1976; Collins 1978d; Coulson and Larkins 1971; Stoneham 1977; Lowther 1975; Lannoo and Stoneham 1968). Both the ground and excited states undergo dynamic Jahn-Teller distortion. In the excited state E/hω = 0.834 (Lowther 1978; Davies 1979a). The GR2-8 lines arise from 1E to 1 T1(C3v) electronic transitions in the defect of Td point symmetry. The sharpness of the GR2-8 lines implies the localization of the corresponding ground states very close to the valence band edge (Zaitsev 1992a; Mainwood 1978). Upon excitation the excited GR2-8 centers relax nonradiatively to the first exited level, afterwards producing GR1 luminescence (Collins 1981a). The oscillator strength of the negatively charged vacancy V- (the ND1 center) is four times greater than that of V0 (the GR1 center) (Davies et al. 1992; Davies and Collins 1993). The g-value of the excited state of the GR1 center is very low g = 1.68×10-2 as a result of a magnetic moment delocalization due to the transfer of the orbital moment over the four dangling bonds of the vacancy (Lowther and Stoneham 1978; Coulson and Kearsley 1957; Coulson and Larkins 1971) and it is temperature independent, indicating zero spin of this state (Douglas and Runciman 1977b). There is a model of the GR center as a donor defect (Davies 1994a; Davies et al. 1992; Halperin and Vermeulen 1983; Clark et al. 1979a; Clark and Mitchell 1977; Walker et al. 1974; Clark and Walker 1973; Vermeulen et al. 1975b; Bourgoin and Lannoo 1981, 1983; Davies and Penchina 1973b; Collins and Williams 1971; Collins and Lightowlers 1979a; Collins
174
5 Optical Electronic Transitions
1993a). However in (Collins 1977) the donor nature of the GR1 center was refused. The concentration of neutral vacancies can be evaluated on the integrated intensity of the ZPL of the GR1 center in absorption at LNT as (Lawson et al. 1998; Twitchen et al. 1999): NV0[cm-3] = (0.8 to 1.02)×1016 µGR1 ZPL[meV cm-1].
1.679 eV (738.2 nm); A, PL; ZPL; FWHM of 7 meV at a temperature of 80 K; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Ni and Si (Sittas et al. 1995; Sittas et al. 1996; Kiflawi et al. 1997). The center is created in synthetic Ib diamonds by heating at temperatures above 1900°C (Kluev et al. 1982). In electron irradiated silicon-doped synthetic diamonds the center strongly increases after annealing at 900°C. The center anneals out at temperatures above 1100°C (Kiflawi et al. 1997). 1.681 eV (737.5 nm); ZPL; A, PL, CL, EL (Melnikov et al. 1994), CAS; the Si center. The Si center with ZPL at 1.681 eV is a very common feature in the spectra of CVD diamond films (Zaitsev et al. 1979c) (Fig. 5.44). The plasma interaction with the Si substrate is believed to be the main reason for the formation of the Si center in MPCVD diamond films (Bergman et al. 1993). The center does not form in films grown from hydrogen-poor systems (Joeris et al. 1996). The center is preferentially formed in {111} growth sectors of homoepitaxial CVD diamond films (Yacobi et al. 1993). However no growth sector dependence has been found by Sittas et al. (1995) or Sittas et al. (1996). The center is easily formed in epitaxial CVD diamond films grown on (13-1) and (141) facets of diamond single-crystals (Katsumata 1992). In PCCVD diamond films the Si center is distributed relatively evenly over the grains and grain boundaries (Bachmann et al. 1993). However a preferential distribution over the grain boundaries has been found by Hayward et al. (1995). The Si center is readily observed in CVD diamond films grown from gas mixtures containing CO and CF4, whereas it is very weak or even absent from the spectra of films grown from a pure CH4 mixture (Graham et al. 1991a). Commonly the Si center is very strong in CVD diamond films grown on silicon substrates, being particularly prominent in the films deposited on n-type Si substrates (unintentional doping of the growing diamond film with the donors from the Si substrate and, due to this, promoting Si center luminescence?) (Wang et al. 1996), and when measured from the substrate side (Sauer 1999). It is particularly strong in MWCVD diamond films grown on Si substrates at elevated gas pressure (to 60 Torr) and elevated MW power (to 75 W/cm3) (Sharda et al. 1989). In CVD diamond films deposited onto Si substrates the center intensity decreases with the films thickness (Dollinger et al. 1995). In CVD diamond films the Si center intensity correlates with the graphiterelated Raman feature (the G-band): the center is strongest in the regions exhibiting a strong G-band (Hayward et al. 1995; Dollinger et al. 1995). In CVD diamond films the PL intensity (not CL) of the Si center correlates also with the intensity of the diamond Raman line (Celii et al. 1991; Plano and Ravi 1989). Formation of the Si center leads to a significant reduction in the overall CL intensity of the CVD diamond film (the center intensity can serve as an indicator of inferior quality of
5.1 Optical Bands 175
CVD diamond films) (Yacobi et al. 1991; *). The PL intensity of the Si center is high in CVD diamond films exhibiting a weak PL band at 1.87 eV (possibly the 1.945 eV nitrogen center) and vice versa (Celii et al. 1991). In CVD diamond films the center is strongly suppressed by boron impurities (it almost disappears at a boron content above 200 ppm), and oxygen (Zaitsev et al. 1981; Davies 1994a; Wang X. et al.1993; Bilodeau et al. 1993; Ruan et al. 1992b; Wang et al. 1992; Davies 1994b). The center is more intense in high-nitrogen type Ib diamonds (Bachmann and Wiechert 1992). Absorption of the Si center in CVD diamond films can be enhanced by an order of magnitude by UV illumination, the enhancement being reversible with a decay time up to several hours (Iakoubovskii and Adriaenssens 1999b).
155 meV 125 meV 90 meV 64 meV 40 meV
15 K
77 K 700
750
800
850
900
WAVELENGTH, nm
Fig. 5.44. PL spectra of Si center taken from a CVD diamond film at temperature 77 K (excitation with the 514.5 nm Ar laser line). The vibrational structure of the center is shown for a temperature of 15 K (excitation at a wavelength of 737 nm) (Gorokhovsky et al. 1995; Goss et al. 1996)
Occasionally the Si center is naturally occurring in some type I natural diamonds (Solin 1972). However the center is created in any natural diamonds by Si+ ion implantation (Fig. 5.45). The center is not observed in type IIb natural diamonds implanted with Si+ ions (Collins et al. 1990a). This effect is possibly due to the low position of the Fermi level in p-type diamonds making the ground state of the center empty (*). The center is created in low-nitrogen HPHT synthetic diamonds (grown with Ti or Zr nitrogen getters) doped with Si, being most intense in diamonds grown with a Fe catalyst (Sittas et al. 1996). In HPHT diamonds the center can be predominantly incorporated in {111} growth sectors (Clark et al. 1993; Zaitsev 1992a). PL of the Si center in synthetic diamonds is never observed in areas exhibiting nitrogen-related luminescence (Sittas et al. 1996). Nevertheless a much stronger appearance of the Si
176
5 Optical Electronic Transitions
center is observed after Si+ ion implantation in high-nitrogen HPHT diamonds than in low-nitrogen ones (Collins et al. 1990a). This effect is possibly due to the n-type character of the high-nitrogen synthetic diamonds, rather than due to incorporation of nitrogen into the Si center (*).
H4 H3 439.2 428 491 423.5
575 738 (Si) 578
N3 389
300
400
457.7
718
500
600
700
800
900
WAVELENGTH, nm
Fig. 5.45. CL spectrum (at LNT) of a natural type IaB diamond implanted with 200 keV Si + ions at a dose of 1014 cm-2 and subsequently annealed at 1400°C
The PL intensity of the Si center in CVD diamond films is almost independent of temperature from LHeT up to 100 K. At higher temperatures it falls down drastically. Usually the PL intensity of the Si center at RT amounts to only a few percent of that observed at low temperatures. However in some diamonds the PL intensity of the Si center at RT may be only slightly lower than that at 100 K. The activation energy of the temperature quenching of the Si center is 90 meV (Bergman et al. 1994b). The absorption of the center is depleted by a thermally activated process with ET ∼ 45 meV (Davies 1994b). The Si center is quenched by electron irradiation (due to annihilation of the vacancy component of the center with radiation-induced interstitials, or due to change of the Fermi level position?) (Steeds et al. 1999a). The luminescence efficiency of the Si center is very high (> 80%) (Bilodeau et al. 1993). The spectral structure of the Si center is rather complicated. The PL spectrum of the center consists of the following lines: 1.6810 (ZPL), 1.6390 (ZPL, probably is not related directly to the 1.681 eV center), 1.7241, 1.6164, 1.7484, 1.5557, 1.8096, 1.5274, 1.8381, 1.7686, 1.7849, 1.8590 and 1.8784 eV (Feng and Schwartz 1993; Bilodeau et al. 1993). Only the 1.6810 and 1.6390 eV lines are active in CAS (Bilodeau et al. 1993). Several weak narrow lines are observed in the CL spectrum of the Si center between the 1.681 eV and 1.62 eV lines (Collins et al. 1994b). Narrow lines at 1.517, 1.524, 1.525, 1.567 and 1.595 eV observed at a temperature of 5 K are possibly ZPLs related to the Si center (Sittas et al. 1996). There is a
5.1 Optical Bands 177
tendency for the spectral position of ZPL to shift towards lower energies as the energy of the diamond Raman line increases: in CVD diamond films with Raman lines at 1335 and 1333cm-1 the Si center ZPL may lie at LNT at 1.678 and 1.682 eV respectively (Sternschulte et al. 1994). The FWHM of the unresolved ZPL of the Si center in CVD diamond films may range from 0.8 to 35 meV and be shifted down to 1.68 eV (Collins et al. 1994a; Sternschulte et al. 1994; Clark et al. 1995). In some low-quality PCCVD diamond films the ZPL of the Si center may be strongly split due to nonhomogeneity of mechanical stress developing between crystallites (see Fig. 5.46).
733 738 743.5
1
2
700
725
750
775
800
WAVELENGTH, nm
Fig. 5.46. PL spectra of ZPL of the 738 nm Si center in a CVD diamond film at RT. 1 - spectrum taken from the tip of a small single crystallite of pentagonal shape (about 2 µm size) imbedded between larger crystallites of cubic shape. 2 - spectrum taken from one of the adjacent large crystallites
When taken with good spectral resolution the ZPL of the Si center is a doublet with components at 1.6820 and 1.6828 eV. The origin of this doublet is a splitting of the excited state of the center (Collins et al. 1994a; Sternschulte et al. 1994; Clark et al. 1995). At temperatures below 60 K each of the ZPL components split further at least into doublets (Sittas et al. 1996). However at lower temperatures up to 12 subcomponents can be detected. At a temperature of 30 K these subcomponents have spectral positions at 1.6813, 1.6815, 1.6817, 1.6819, 1.6820, 1.6822 (the main line), 1.6824, 1.6826, 1.6828, 1.6830, 1.6831 (the main line) and 1.6833 eV (ZPL spectrum can be found in Clark et al. (1995)). The main components at 1.6822 and 1.6831 meV are thermolized in luminescence with an activation energy of 0.8 to 1.07 eV meV (it corresponds to the separation energy between the components) implying splitting in the excited state (Sternschulte et al. 1994; Clark et al. 1995). The most intensive pairs of subcomponents at 1.6833, 1.6831 eV and 1.6822, 1.6820 eV are thermolized in absorption with an activation energy of 0.2 meV (this
178
5 Optical Electronic Transitions
corresponds to the energy of their separation) implying splitting in the ground state (Clark et al. 1995) (Fig. 5.47).
28
28
28
28
29
1.6823
Si or Si+ Si
29
Si or Si+ Si
30
28
30
Si or Si+ Si
1.6828
1.6819 1.6816
1.6825
a 1.681
1.6832
b
c
1.682
d
1.683
1.684
QUANTUM ENERGY, eV
upper state
a
b
c
d
lower state
Fig. 5.47. Fine structure of ZPL of the 738 nm Si center taken in PL (632.8 nm excitation light) at a temperature of 10 K from a synthetic diamond. Three groups of lines originating from the centers involving different Si isotopes are shown with arrows (the isotope content is shown for the one-Si-atom-model and the two-Si-atom-model). The level scheme of the ZPL transitions shows two-fold splitting of the excited and ground states, which is believed to originate from a local uniaxial deformation of the diamond lattice surrounding the Si center. The lines corresponding to the transitions on the splitting diagram are shown in the spectrum with the letters a, b, c, d (Sternschulte et al. 1995)
PL excitation of the Si center occurs over the whole visible spectral region. The PLE spectrum shows features at 2.07, 2.15, 2.26 and 2.4 eV (McCauley and Vohra
5.1 Optical Bands 179
1995; Iakoubovskii and Adriaenssens 1999b). However, the most effective the center is excited via the bandgap transitions (>5.5 eV) and zero-phonon transitions (over three orders of magnitude more intensive than the excitation in a spectral region from 2.0 to 2.5 eV (Gorokhovsky et al. 1995; Iakoubovskii and Adriaenssens 1999b) (Fig. 5.48).
b
Si-center A
PLE (at RT)
PLE (at LNT)
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Quantum Energy, eV
Fig. 5.48. Absorption and PLE spectra of the Si center taken in a CVD diamond film at LNT and RT. Both A and PLE spectra show an identical weak structure in a spectral range from 1.8 to 1.9 eV. The shape of the UV PLE spectrum in the bandgap excitation region is sample dependent (Iakoubovskii and Adriaenssens 1999a)
Electron-phonon coupling of the Si center in absorption is characterized mainly by 33 meV vibrations. The Huang-Rhys factor of the electron-phonon coupling in absorption is 0.24±0.02 (Collins et al. 1994a). Further vibrational modes of energies 80, 135, 154 meV (as well as a 215 meV mode, which could be a local vibration of the Si center) can be seen in A and PLE spectra (Iakoubovskii and Adriaenssens 1999b). In PL the center interacts with 40 (local mode), 64 (TA lattice mode, or local mode), 83 (a local mode observed in the anti-Stokes region as a vibration of energy 87.6 meV; in the Stokes region the vibration energy may increase up to 95 meV), 125 (LA lattice mode), 148, 155 (LO lattice mode) and 163 meV vibrations giving corresponding features at 756 nm (1.645 eV, FWHM of 30 meV), 767 nm (1.618 eV, FWHM of 6 meV), 778 nm, 797 nm, and 814 nm (triplet) respectively (Feng and Schwartz 1993; Clark et al. 1993; Sittas et al. 1995; Sternschulte et al. 1994). The 40 and 64 meV modes are ascribed to quasilocal vibrations involving two and one Si atoms respectively (calculated values of the resonance peaks are ωR-twoSi = 45 meV, ∆ωR-twoSi = 21 meV and ωR-oneSi = 75 meV, ∆ωR-oneSi = 59 meV) (Fig. 5.44).
180
5 Optical Electronic Transitions
A weak Si center may be observed in diamonds implanted with Si + ions prior to annealing, however subsequent annealing at temperatures above 900°C increases the intensity of the center drastically. The PL intensity of the Si center in irradiated CVD diamond films and HPHT synthetic diamonds shows nonmonotonic behavior with annealing: the intensity shows two maxima at temperatures of 700-800°C and 1100-1300°C. In contrast, absorption of the center in irradiated diamonds exhibits a common increase with annealing temperature attaining a saturation at a temperature of about 900°C. The increase of the center intensity with annealing in irradiated diamonds is possibly a consequence of migration of vacancies, which can be involved into the center. The Si center is stable above 2200°C. The FWHM of the ZPL does not change with annealing temperature until 1100°C; with further temperature increase the ZPL width falls and after annealing at a temperature of 2200°C may be as narrow as 40 cm-1 (Collins et al. 1994a; Clark and Dickerson 1991; Clark and Dickerson 1994; Ruan et al. 1991a; Pereira and Pereira 1992; Kiflawi et al. 1997; Clark et al. 1995). The Si center is characterized by two radiative decay times of 2.3 ns and 100 µs, the long time decay resulting from the interaction with a reservoir (Khong et al. 1994; Davies 1994b). At LNT the short decay time may vary from 2.3 to 4 ns for homoepitaxial CVD diamond films; in PCCVD diamond films it may reduce down to 1 ns at a temperature of 150 K (Sternschulte et al. 1994). There is a nonradiative decay channel of the Si center (Bilodeau et al. 1993). There are the following models of the Si center: (i) a substitutional silicon atom bound to a vacancy, the complex being in the neutral charge state (Collins et al. 1994a; Clark and Dickerson 1991; Iakoubovskii and Adriaenssens 1999b); (ii) a center containing silicon and nitrogen atoms (Collins et al. 1990a; Clark and Dickerson 1992b); (iii) two Si atoms located along the axis and bound with vacancies, as well as a two-silicon atom model (Zaitsev et al. 1981; Gorokhovsky et al. 1995; Graham et al. 1991b). An ab initio simulated model of the center as a Si atom centered in a divacancy gives the following parameters: D3d symmetry; the S=1 spin configuration is more stable, than that with S = 0 by 0.25 eV; the optical transition of energy 1.86 eV between the 1E' and 1E'' electronic states; doublet splitting is most likely due to the Jahn-Teller effect; the radiative lifetime is 3 ns (Goss et al. 1996). The fine structure of the ZPL is believed to originate from an adjacent large-size defect producing a 0.06 GPa internal stress of the crystal lattice surrounding the Si center. This stress results in a two-fold splitting of both the excited and ground states (Collins et al. 1994a; Collins 1994; Sternschulte et al. 1995). At LHeT in perfect diamonds the ZPL of the Si center splits into 12 lines grouped into three groups because of the three stable isotopes of Si, the transitions occurring between an orbitally two-fold degenerate ground state split by 0.02 meV and a doublet excited state split by 1.07 eV meV (Fig. 5.47). The relative intensities of the different groups of lines are proportional to the natural abundances of the stable 28Si, 29Si and 30Si isotopes within a 120% error for the one-Si-atom-model and within a 9% error for the two-Si-atom-model, strongly supporting the two-Si-atommodel (Clark et al. 1995; *). The feature in the vibronic side-band resulting from the 64 meV mode is attributed to Si-Si quasilocal vibration. The 125, 148, 155 and 163 meV replicas are attributed to the LA, TO, LO phonons at the L point and the O
5.1 Optical Bands 181
phonon in the center of the Brillouin zone respectively (Gorokhovsky et al. 1995; Graham et al. 1991b). Zeeman splitting of the ZPL points to a preferential symmetry axis of the defect: for instance it may be a defect of tetragonal symmetry with perturbation towards monoclinic symmetry (Sternschulte et al. 1995). The data on optical bleaching suggest that the Si center is a defect in a neutral charge state (Iakoubovskii and Adriaenssens 1999b). The ground state of the Si center is found to be 2.05 eV below the conduction band (Iakoubovskii and Adriaenssens 1999b). 1.685 eV (736 nm); A, no CL; ZPL; a center observed in all types of irradiated diamonds (Fig. 5.30). The 1.685 eV center relates to the 1.859 eV feature, which overlaps usually with much stronger vibronic band of the GR1 center (Coomer et al. 1999). However the relation between the 1.685 and 1.859 eV lines has been questioned (Collins 1999). The 1.685 eV line freezes out on lowering temperature at the same rate as the 1.665 eV component of the GR1 center does and it is not observed below 80 K. In contrast, the 1.859 eV line is observable down to a temperature of 2 K (Davies and Collins 1999). Intensity change of the 1.859 eV line with temperature I1.858(T) follows a relation (Walker 1977b): I1.858(T) = I0 exp[-S(1+6.58 T2/TD2)][1 + dg exp(6[meV]/kBT)]-1. These peculiarities suggest that the 1.685 eV center is a transition from a level lying by 6 meV above the ground state (Davies 1977c; Walker 1979; Sobolev and Yurjeva 1982; Davies 1974a; Walker 1977b; Davies 1994b). An energy diagram of the 1.685 eV center is thought to contain five levels with the following energies: I - 0 eV, II - 0.0062 eV, III - 0.050 (0.033) eV (spin 1), IV - 1.6908 eV, V - 1.8594 eV and VI – 2.028 eV, where the II→IV transition is the 1.685 eV line, and the I→V transition is the 1.859 eV line (Fig. 5.30). An alternative interpretation considers the 1.859 eV line as a vibrational replica of the forbidden I-IV transition resiling from the interaction with 168.6 meV phonons (Davies and Collins 1999; Coomer et al. 1999; Davies et al. 2000). These are the phonons forming the sharp peak at the high energy cutoff of the diamond phonon density curve (Fig. 3.2). The energy reduction ratio of these phonons by changing from 12C to 13C diamond is 0.960 ≈ (12/13)0.5 confirming their intrinsic nature. The 1.685 eV center anneals out at temperatures about 400°C. Similarly the 1.859 eV line is destroyed by heating at temperatures of 400-500°C, the intensity reduction being described by first-order kinetics. Activation energy of the annealing of the 1.859 eV line is from 1.56 to 168 eV (Davies 1977c; Davies 1974a; Walker 1977b; Allers et al. 1998; *). Note that the thermal activation energy of migration of the split self-interstitial in diamond lattice is estimated to be of 1.6 to 1.7 eV (Hunt et al. 2000; Allers et al. 1998; Breuer and Briddon 1995). The 1.859 eV line anneals out completely during rapid annealing stage of the GR1 center (the process is explaned by annihilation of vacancies and interstitials) (Allers et al. 1998). During electron irradiation the intensity of both lines (1.685 and 1.859 eV) attains a maximum at a dose of 1018 cm-2 and then it falls at higher doses. At low doses the dose dependence can be approximately described by a linear law (Walker 1977b). Both the 1.685 and 1.859 eV lines correlate with the R2 paramagnetic center, which is thought to result from a split interstitial (Faulkner and Lomer
182
5 Optical Electronic Transitions
1962; Coomer et al. 1999; Walker 1979; Collins 1999). There is also a good correlation between the intensities of the 1.859 eV line and the R11 center (active in absorption and EPR) (Allers et al. 1998). The 1.685 eV center is an A−A transition at a tetragonal defect (Walker 1979; Walker 1977b). Isotope shifts of the 1.685 and 1.859 eV lines on going from 12C to 13C diamond are +(1.3-1.7) and –(5.2-5.4) meV respectively (Davies and Collins 1999; Davies et al. 2000). The center has D2 symmetry (Davies et al. 2000). The 1.685 eV center is attributed to a self-interstitial defect (Bokii et al. 1986; Walker J. 1979; Sobolev and Yurjeva 1982; Allers et al. 1998). In particular, it is thought to be a distorted vacancy, possibly a carbon atom in a -split interstitial state trapped by a neutral vacancy V0 along that axis (Hunt et al. 1999). Theory supports the idea that the -split self-interstitial is a stable defect when in charge states from –1 to +2 (Mainwood et al. 1993; Mainwood 1999). However, a role of an impurity in the formation of the center is not excluded. 1.689 eV (734 nm); PL at about 7 K; ZPL; a center observed in type IaB natural diamonds irradiated by 1019 neutron/cm2 and subsequently annealed at a temperature of 950°C (Sildos and Osvet 1994a; Sildos et al. 1995). The center can be detected in some natural type I diamonds at LNT (Solin 1972) (Fig. 5.15). 1.691 eV (733.0 nm); A; a narrow ZPL (FWHM of 1.5 meV at 80 K) observed in Si-doped electron irradiated low-nitrogen synthetic diamonds. The center intensity strongly increases after annealing at a temperature of 900°C. The center anneals out at temperatures above 1100°C (Kiflawi et al. 1997). 1.693 eV (732.1 nm) and a complex series of sharp lines between 2.2 and 2.7 eV (560-460 nm); A, CL. The 1.693 eV line is one of the strongest features observed in synthetic type Ib nickel-containing diamonds both in as-grown samples and after annealing to a temperature of 1600°C (Lawson and Kanda 1993a; Yelisseyev and Nadolinny 1995a; Lawson and Kanda 1993b). The center may be seen in diamonds grown at high temperatures (Field 1992; Collins and Stanley 1985b). The 1.693 eV center anneals out on heating to 1800°C. It is observed only in diamonds grown using a nickel catalyst (Field 1992). The center is present only in the {111} growth sectors (Collins and Woad 1993a; Lawson and Kanda 1993b; Yelisseyev et al. 1996). The center attains maximum intensity after complete annealing of the 1.883 and 2.51 eV nickel-related centers and conversion of about 70% of dispersed nitrogen into the A-aggregates (Lawson and Kanda 1993a). The maximized integrated intensity of the 1.693 eV center (attained after annealing) is proportional to the initial intensity of the 1.883 eV center (Lawson and Kanda 1993a). ZPL of the center is actually a doublet with the components at 1.963 eV (α, main) and 1.956 eV (β), intensities of which are thermolized with an activation energy of 35 meV (Nazare et al. 1999). The 1.693 eV center interacts predominantly with vibrations of energy 50-52 meV (Collins and Stanley 1985b; Lawson and Kanda 1993a; Nadolinny and Yelisseyev 1993; Yelisseyev et al. 1992b; Nazare et al. 1999). The electron-phonon coupling is weak: S = 1.3 (Nazare et al. 1999). The 1.963 eV center is an electronic transition between A1 and B1 states at a defect of rhombic-I (C2v)
5.1 Optical Bands 183
symmetry (Nazare et al. 1999). The center is very "soft": SP ∼ 8.5 meV/GPa. It has been suggested that the 1.693 eV center originates from a nickel-related defect (Collins and Woad 1993a). A possible model of the center involves two substitutional Ni atoms in different charge states (Nis--Nis+) (Goss et al. 1995). An alternative model is that involving one nickel and single nitrogen atoms (Lawson and Kanda 1993a; Lawson and Kanda 1993b; Yelisseyev et al. 1996; Yelisseyev et al. 1999). 1.694 eV (731.7 nm); A, PL at around 7 K; ZPL; a center observed in type IaB natural diamonds irradiated by high-energy (>1 MeV) neutrons at doses of 1019 neutron/cm2 and subsequently annealed at a temperature of 950°C. The center possibly relates to the 813 nm center (Sildos and Osvet 1994a; Sildos et al. 1995). 1.696 eV (730.7 nm); A, PL; a ZPL accompanied by a set of narrow lines at 739, 734, 726, 728 and 724 nm. The 1.696 eV center is observed in type IaB natural diamonds irradiated by neutrons with doses around 1019 neutron/cm2 and subsequently annealed at a temperature of 950°C. Spectral holes can be burnt in the line at temperatures up to 200 K (Sildos and Osvet 1994a; Sildos and Osvet 1994b; Sildos et al. 1995). 1.700 eV (729 nm); PL; possibly ZPL; a center observed in some natural type I diamonds (Solin 1972). 1.7 eV (730 nm); PL, XL; a broad band with a maximum in the spectral range from 700 to 730 nm observed in brown-smoky diamonds subjected to plastic deformation and in natural polycrystalline diamonds containing lonsdaleite inclusions. The band is observed also in ballas. The band originates possible from dislocations (Bokii et al. 1986; Plotnikova et al. 1980). 1.702 eV (728.3 nm at RT); possibly ZPL; PL; a center observed in some CVD diamond coatings deposited by the laser assisted method (Badzian et al. 1997a). 1.705 eV (727.0 nm); PL, CL; ZPL; a center observed in pristine synthetic diamonds grown in Ni-containing environment and in type Ib synthetic diamonds after annealing at temperatures above 1500°C (Lawson et al. 1996; Osvet et al. 1997). Probably this center was observed in nitrogen-containing synthetic diamonds grown in the presence of Ni and Si (Sittas et al. 1995; Sittas et al. 1996). The 1.705 eV center appears only in diamonds grown using a nickel catalyst and it forms only in {111} growth sectors. Annealing at 2000°C does not affect intensity of the 1.705 eV center (Osvet et al. 1997), however the center anneals out on heating at 2200°C (the center disappears at temperatures providing the highest degree of defect transformation) (Kupriyanov et al. 1999). The 1.705 eV center appears to be more temperature stable than the 1.660 eV center (Kupriyanov et al. 1999). The center exhibits a relatively strong luminescence comparable with the green luminescence due to the S2- and S3 centers. The center interacts predominantly with vibrations of energies 59 (the main mode) and 34 meV. The electron-phonon interaction is very
184
5 Optical Electronic Transitions
weak: S ~ 0.4 (Kupriyanov et al. 1999). The center is not active in absorption; neither is it excited in luminescence at the wavelength of its ZPL (possible explanation: a transition between two excited electronic states) (Kupriyanov et al. 1999). The PL of the center is excited at wavelengths below 515 nm (Field 1992; Collins and Stanley 1985b; Lawson et al. 1996). PLE spectrum of the center exhibits a ZPL at 515.5 nm (Yelisseyev et al. 1999). Intensity decay of the center in PL can be approximated with a lifetime of 0.49 ms. The center possibly relates to a spinforbidden transition (Osvet et al. 1997). The 1.705 eV center appears to possesses triclinic symmetry (Johnston et al. 1999). The center is attributed to a nickel-related defect (Collins and Woad 1993a; Collins 1997; Lawson et al. 1996; Yelisseyev et al. 1999) (Fig. 5.49).
PL
1.704
59 meV
34 meV
1.660
1.50
1.55
1.60
1.65
1.70
1.75
QUANTUM ENERGY, eV
PLE spectrum of the 1.704 eV center
34 meV
2.401
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
QUANTUM ENERGY, eV
Fig. 5.49. PL and PLE spectra (both at LNT) and of the 1.704 eV center taken from a synthetic Ni-containing diamond annealed at 1700°C for 12 hours (Kupriyanov et al. 1999). The center exhibits dominating interaction with a 59 meV quasilocal vibrational mode and a weak interaction with 34 meV vibrations. The center is excited exclusively in the absorption spectrum of the 2.401 eV center, showing dominant electron-phonon interaction with 34 meV vibrations
5.1 Optical Bands 185
1.707 eV (726 nm); PL; a broad band of FWHM 0.17 eV with weak structure at 1.73, 1.707 and 1.686 eV observed in HFCVD diamond films grown using tungsten filament. The center intensity increases with reduction of the film quality. Tentatively the center is attributed to a W-containing defect (Harris et al. 1996). 1.707 eV (726.1 nm); PL; ZPL; a center observed in some brown diamonds which do not exhibit yellow luminescence when excited at a wavelength of 365 nm. The center can be observed also in some natural type I diamonds. The vibrational side-band of the center is characterized by predominant interaction with vibrations of energy 51 meV. The strength of the electron-vibrational coupling at the center is moderate: S = 3.6. The 1.707 eV center is a slow luminescence one: decay time of the center ranges from 1 to 15 ms (Field 1992; Nazare et al. 1985a; Solin 1972). The low-temperature lifetime is of 5.7 ms after excitation at temperatures of 50 to 220 K (Davies 1994a). 1.711 eV (724.4 nm); A; a broad ZPL (FWHM of 8 meV at 80 K) observed in electron and neutron irradiated low-nitrogen synthetic diamonds (attention to Si-doped synthetic diamonds). The center intensity strongly increases after annealing at temperatures of 800 to 900°C. The 1.711 eV center anneals out at temperatures above 1100°C (Kiflawi et al. 1997; Vins et al. 1988). 1.712 eV (724.0 nm at RT); PL; a band with the typical shape of a vibration assisted wing spreading from about 1.55 to 1.8 eV. The band is observed in laser assisted PCCVD diamond coatings. On (111) facets of the crystallites the band may split into a doublet with components at 1.715 and 1.682 eV (Badzian et al. 1997a). The band is possibly the electron-vibrational side-band of a center with ZPL at about 1.8 eV (*). 1.714 eV (723 nm); A, PL at about 7 K; a center observed in type IaB natural diamonds and synthetic type Ib diamonds irradiated by high-energy (>1 MeV) neutrons at doses of 1019 neutron/cm2 and subsequently annealed at temperatures of 800 to 950°C. The electric dipole moment of the electronic transition is oriented along the axis. Spectral holes can be burnt in the line (Sildos and Osvet 1994a; Sildos et al. 1995; Vins 1988). 1.715 and 1.720 eV (722.6 and 720.6 nm); PL; ZPL doublet; a weak feature observed in spectra of synthetic diamonds grown with Ni-containing catalysts. This center is effectively created in nitrogen- and nickel-containing synthetic diamonds by electron irradiation and subsequent annealing at temperatures above 900°C (Osvet et al. 1997). The center anneals out at temperatures above 1950°C. The center interacts with vibrations of energy 36 meV (Yelisseyev et al. 1998). The doublet structure of its ZPL is caused by a 4.7 meV splitting in the excited state (Osvet et al. 1997). The center is ascribed to a nickel-containing defect (Nadolinny et al. 1999; Yelisseyev et al. 1999).
186
5 Optical Electronic Transitions
1.722 eV (720 nm); A; a band observed in some synthetic diamonds doped with Ti and In (Klimenkova et al. 1975b; Butuzov et al. 1976). 1.72 eV (719 nm); CL; ZPL; a center observed in type Ia natural diamonds after ion implantation and subsequent annealing at temperatures above 800°C (Gippius et al. 1982a). This center is possibly observed in as-grown MWCVD diamond films as a result of irradiation with the plasma ions (Heiderhoff 1997; *) (Fig. 5.45). 1.723 and 1.731 eV (716 and 719.5 nm); the NG center; CL; ZPL doublet. The NG center is created in low-nitrogen IIa diamonds by Ne+ ion implantation. The center is strongly quenched by annealing at a temperature of 700°C. However the NG center remains stable at temperatures above 1400°C. The electronic transition of the center interacts predominantly with vibrations of energy 25 meV. The electron-vibrational interaction at the NG center is strong: S ~ 5. The NG center is tentatively attributed to a vacancy-related defect trapping one Ne atom (Zaitsev 1992a; Tkachev et al. 1985b; Tkachev et al. 1985c).
1.737 eV (713.4 nm); PL; a center observed in high-nitrogen synthetic diamonds grown in Ni-containing melts. The center is activated by annealing at temperatures above 1700°C. The center is stable at temperatures above 1950°C. It is attributed to a Ni-containing defect (Yelisseyev et al. 1999). 1.735 and 1.750 eV (713 and 709 nm); CL, PL; ZPLs; the W5 center and W3 center. Both centers are observed in some flame grown and hot filament grown CVD diamond films. Both centers interact with a 24 meV vibration, which is believed to be a quasilocal mode involving one W atom (spectral width of the vibronic replicas is 5.5 meV; calculated parameters of the vibration are ωR = 23 meV, ∆ωR = 6 meV). The electron-vibrational coupling at the centers has low strength: S ~ 1.5. The centers are strongly suppressed with increasing electron excitation intensity (intensity saturation due to relatively long lifetime of the centers?). Tentatively the W centers are attributed to defects containing W atoms (Steeds et al. 1995; Burton et al. 1995b; Anderson et al. 1997) (Fig. 5.50). 1.74 eV (714 nm); PL; a center naturally occurring in some natural diamonds containing lonsdaleite (Bokii et al. 1986). 1.74 eV (712 nm); PL at about 7 K; ZPL; a center observed in type IaB natural diamonds and synthetic type Ib diamonds irradiated by high-energy neutrons (>1 MeV) at a fluence of 1019 neutron/cm2 and subsequently annealed at temperatures of 800 to 950°C (Sildos et al. 1995; Vins 1988; Vins et al. 1988). 1.742 eV (711.5 nm); PL; ZPL; 1.743 eV (711.0 nm); A, PL; ZPL; a center observed in synthetic diamonds grown by the temperature gradient method in Fe-Ni-C systems. The center is especially strong in the deep-yellow nitrogen-containing sectors of these diamonds. Probably
5.1 Optical Bands 187
this center was observed by Sittas et al. (1995) and Sittas et al. (1996) in some nitrogen-containing synthetic diamonds grown in the presence of Ni and Si.The center relates to a nickel-containing defect, which may also incorporate nitrogen (Yelisseyev et al. 1996). 1.746 eV (710.2 nm); A; ZPL; a center observed in type Ib synthetic diamonds grown by the temperature gradient method using a nickel catalyst. The center is immune to annealing at a temperature of 1700°C (Yelisseyev et al. 1992b; Yelisseyev and Nadolinny 1995a). 1.753 eV (706.9 nm); PL; ZPL; a center observed in high-nitrogen synthetic diamonds grown with Ni-containing catalysts. The center appears after annealing at temperature above 1700°C and it is destroyed by annealing at temperatures above 1950°C (Yelisseyev et al. 1999). 1.760 eV (726.6 nm); PL; possibly ZPL; a center observed in some natural type I diamonds (Solin 1972).
1.735 (W5) 24 meV 1.741 (W4)
1.750 (W3) 1.754 (W2) 1.759 (W1)
PL, 20 K exit. 2.41 eV 127 meV
CL, LNT
1.55
1.60
1.65
1.70
1.75
1.80
1.85
QUANTUM ENERGY, eV
Fig. 5.50. PL and CL spectra (at 20 K and LNT) of a HFCVD diamond film grown using tungsten filament. The marked lines W1 through W5 are believed to be ZPLs of W-related centers. The 24 meV vibration is a quasilocal mode of W atom (Steeds et al. 1995). It is interesting that the vibronic side-band of the center in CL extends to about 165 meV (Raman frequency) giving a hint that the feature at 1.61 eV is a phonon replica due to interaction with optical phonons of energy 127 meV (Steeds et al. 1995; Anderson et al. 1997)
1.762 eV (703.6 nm); CL; ZPL; a center observed in some type Ib diamonds. The center interacts with 33 meV vibrations (Davies 1977c). Possibly this very feature is
188
5 Optical Electronic Transitions
observed in spectra of synthetic diamonds grown in Ni-containing environment. It is ascribed to a nickel-containing defect (Nadolinny et al. 1999). 1.767 eV (701.5 nm); PL; a small peak (possibly ZPL) observed in some microwave plasma deposited CVD diamond films (Wei Zhu et al. 1993).
1.770 eV (700.1 nm); PL; a center observed in high-nitrogen synthetic diamonds grown in Ni-containing melts after annealing at temperatures above 1700°C. The center is stable at temperatures above 1950°C. The dominant electron-phonon coupling at the center occurs with a 38 meV mode. The center is attributed to Nicontaining defects (Yelisseyev et al. 1999). Probably this center is mentioned as the 1.767 eV center (see above). 1.770 eV (700.3 nm); PL; ZPL; FWHM of ZPL is about 40 meV at 17 K; a naturally occurring center observed in type Ia yellow diamonds and diamonds of mixed cubo-octahedral shape. The center interacts with vibrations of energy 63 meV. The 1.770 eV center coexists with the N2- and N3 centers. The center is attributed to a defect containing aggregated nitrogen (Field 1992; Plotnikova et al. 1980; Vohra et al. 1989; Ruoff et al. 1991a; Desgreniers et al. 1989). Possibly this feature is also observed in spectra of synthetic diamonds grown in Ni-containing environment. Alternative model of the center is a nickel-containing defect (Nadolinny et al. 1999) (Fig. 5.11). 1.77 eV (700 nm); A; a broad band (FWHM of 0.25 eV) observed in hydrogen-rich brown-grayish, yellow, gray-violet and chameleon diamonds (Fritsch et al. 1991a). 1.77 eV (700 nm); A; a weak relatively narrow band (FWHM of 50 meV) observed in neutron irradiated synthetic type Ib diamonds after annealing at temperatures over 300°C. The center is possibly formed prior to annealing in as-irradiated samples. The band anneals out at a temperature of 1000°C. Tentatively the center relates to an interstitial-type defect (Nishida et al. 1989). 1.77 eV (700 nm); PL; ZPL; a weak center occasionally occurring in type Ia gray diamonds. The intensity of the 1.77 eV center correlates with the intensity of the S2 center. The 1.77 eV center interacts predominantly with vibrations of energies 40 and 70 meV. The PL of the center is excited in two broad bands peaked at 2.88 eV and 3.65 eV. The ZPL of the center broadens to about 15 meV with temperature increase from LNT to RT (Bokii et al. 1986; Plotnikova et al. 1980; Solin 1972). 1.77 to 1.78 eV (697 to 701 nm); PL; a narrow band naturally occurring in lonsdaleite-containing diamonds (Bokii et al. 1986). 1.774 eV (698.7 nm); PL; the main line of a set of narrow lines at 1.807 (686), 1.783 (695) and 1.771 eV (700 nm) observed in HFCVD diamond films grown using a Ta filament. The relative intensity of the center is enhanced in films of low quality. The center shows very low electron-phonon coupling. However, a weak relatively broad
5.1 Optical Bands 189
band at 709 nm (FWHM is about 10 meV) can be recognized as a quasilocal vibration involving one Ta atom (calculated parameters of the vibration are: ωR = 23 meV, ∆ωR = 6 meV) in the vibrational side-band. The PL intensity of the center at RT is very weak (at least two orders of magnitude less that that at LNT). The 1.774 eV center is attributed to a defect containing one Ta atom (Harris et al. 1996; *) (Fig. 5.51). 1.78 eV (695 nm); A; a line observed in some synthetic diamonds doped with As (Klimenkova et al. 1975c). 1.787 eV (693.7 nm); PL; ZPL; a center observed in spectra of high-nitrogen synthetic diamonds grown in a Ni-containing environment. The center appears after annealing at temperatures above 1700°C. It is destroyed by annealing at temperatures above 1950°C. The center is ascribed to a nickel-containing defect (Nadolinny et al. 1999). 1.79 eV (693 nm); CL; possibly ZPL; a relatively broad line (FWHM of 30 meV) observed from the substrate side of some MWCVD diamond films deposited onto Si substrates. No obvious vibrational replicas assist the line (Zaitsev et al. 1998b). 1.785 eV (694.4 nm); PL; possibly ZPL; a center observed in some natural type I diamonds (Solin 1972).
Ta
698.7
709 26 meV
706.5
680
690
700
710
720
730
WAVELENGTH, nm
Fig. 5.51. PL spectrum of a CVD diamond film exhibiting emission of the Ta-related center. A weak broad band at 709 nm is believed to be a vibronic replica of the ZPL due to interaction with quasilocal vibration involving one Ta atom. The weak line at 706.5 nm probably does not belong to the 699 nm Ta-related center
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5 Optical Electronic Transitions
1.79 eV (690 nm); CL; a broad band of width 0.5 eV observed in microwave plasma deposited CVD diamond films and HPHT synthetic diamonds. The center is attributed to radiative recombination between remote (about 5th neighbor) donoracceptor pairs (Collins 1991b; Deneuville et al. 1992). 1.791 eV (692.1 nm); PL; possibly ZPL; a center observed in some natural type I diamonds (Solin 1972). 1.795 eV (690.5 nm); A, PL, XL; ZPL doublet; a center observed in type Ib synthetic diamonds grown by the temperature gradient method using a catalyst containing nickel. The center is stimulated by additions of boron in the growth media. The center stands annealing at 1700°C (Yelisseyev et al. 1992b; Yelisseyev and Nadolinny 1995a; Yelisseyev and Nadolinny 1993; Vins 1988). The center is tentatively attributed to a defect comprising Ni and B atoms (Vins 1988). 1.797 eV (689.9 nm); A; ZPL; a center observed in synthetic diamonds grown by the temperature gradient method in a Fe-Ni-C system. The center is especially strong in deep yellow nitrogen-containing sectors of these diamonds. The center relates to a nickel-containing defect, which may additionally contain nitrogen (Yelisseyev et al. 1996).
1.8 eV (690 nm); A, PL, CL; the B-band (not related to the 1.6 eV B-band); a broad band with FWHM of 0.35 eV observed in natural brown diamonds and type II diamonds. In CL at 50 K the band shows a fine structure, the components of which locate at: 2.138, 2.089, 2.077, 2.007, 1.967, 1.929, 1.899, 1.869, 1.830, 1.818, 1.795, 1.743 eV. The band possibly relates to dislocations (Orlov 1973; Bokii et al. 1986; Davies 1977c; Collins and Mohammed 1982b; Wight et al. 1971; Collins 1992a; Gomon 1966) (Fig. 5.29). 1.819 eV (681.4 nm); PL; ZPL; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Ni and Si (Sittas et al. 1995; Sittas et al. 1996). 1.819 eV (681.4 nm); PL; ZPL; a center observed in some brown diamonds which do not exhibit yellow luminescence when excited at a wavelength of 365 nm. The center interacts with vibrations of energy 49 meV, S = 3.8. This is a slow luminescent center. The lifetime is 17.5 ms after excitation at temperatures in the temperature range from 50 to 120 K (Field 1992; Nazare et al. 1985a; Gippius et al. 1982a; Davies 1994a). This center is possibly observed in some natural type I diamonds (Solin 1972). 1.821 eV (681 nm); A, PL at about 7 K; ZPL; a center observed in type IaB and IaA natural and synthetic diamonds irradiated by high-energy (> 1 MeV) neutrons with doses of 1019 neutron/cm2 and subsequently annealed at temperatures above 700°C (Osvet et al. 1997). The center is particularly strong when excited with the 514.5 nm Ar laser line (Nishida et al. 1989). In the PL the center interacts mainly with 60 meV
5.1 Optical Bands 191
vibrations and, possibly, with 30 meV vibrations. The strength of the electronphonon interaction is moderate: S ∼ 1.5 (Nishida et al. 1989; *). Spectral holes can be burnt up in ZPL at temperatures as high as 100 K. The homogenous width of the ZPL at LHeT is of 2 GHz. The lifetime of the excited state is 80 ps (Osvet et al. 1992; Sildos and Osvet 1994a; Sildos and Osvet 1994b; Sildos et al. 1995; Nishida et al. 1989). This center is possibly observed at LNT in some natural type I diamonds (Solin 1972). 1.829 eV (677.7 nm); PL; ZPL; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Ni and Si (Sittas et al. 1995; Sittas et al. 1996). The center appears after annealing at temperature above 1700°C and it is destroyed by annealing at temperature above 1950°C (Yelisseyev et al. 1999). Possibly this center is formed also in PCCVD diamond films, where it is distributed relatively evenly over the grains and grain boundaries (Bachmann et al. 1993). The center is attributed to a nitrogen-related defect (Bachmann et al. 1993). 1.834 eV (676 nm); PL at about 7 K; ZPL; a center observed in type IaB natural diamonds irradiated by neutrons with energy above 1 MeV at doses of 1019 neutron/cm2 and subsequently annealed at a temperature of 950°C (Sildos et al. 1995). 1.84 eV (675 nm); PL; a broad band observed in synthetic boron-doped diamonds. FWHM of the band is above 0.25 eV (Freitas et al. 1993a). The center is attributed to donor-acceptor recombination at distant pairs (boron+nitrogen complexes) (Freitas et al. 1994a; Freitas et al. 1994b). 1.85 to 1.86 eV (667 to 670 nm); PL; a narrow band naturally occurring in diamonds containing lonsdaleite (Bokii et al. 1986). 1.85 eV (670 nm); CL; called as the B-band; a band with FWHM of 0.3 eV naturally occurring in some type IIa diamonds. The band is obscured in type I crystals (Walker 1979; Wight et al. 1971). 1.852 eV (669.3 nm); A; no CL or PL; ZPL; a center observed in type Ib synthetic diamonds grown by the temperature gradient method after annealing at temperatures above 1500°C. The center interacts with vibrations of energy 44, 86 and 134 meV. This center is ascribed to a cobalt-nitrogen-related defect (Lawson et al. 1996; Collins 1997) (Fig. 5.52). 1.854 eV (668.6 nm); PL; ZPL; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Ni and Si (Sittas et al. 1995; Sittas et al. 1996). 1.859 eV (667 nm); see the 1.685 eV center.
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5 Optical Electronic Transitions
1.87 eV (663 nm); PL; a line observed in neutron irradiated type IaA and IaB natural and synthetic diamonds after annealing at temperatures above 700°C. The center is excited especially intensively by the 514.5 nm Ar laser line (Nishida et al. 1989). A persistent spectral hole burning has been shown in this line (Osvet et al. 1992; Osvet et al. 1997).
Co
1.852
44 meV
1.896
1.8
1.9
2.0
2.1
2.2
QUANTUM ENERGY, eV
Fig. 5.52. Absorption spectrum (at LNT) of the 1.852 eV Co-related center (Lawson et al. 1996)
1.878 eV (660 nm); A; a band observed in some synthetic diamonds doped with Ti and In (Klimenkova et al. 1975b; Butuzov et al. 1976). 1.88 eV (658 nm); PL; a narrow band naturally occurring in lonsdaleite-containing diamonds (Bokii et al. 1986). 1.880 and 1.884 eV (659 and 658 nm); CL; ZPL doublet of a center created in very low-nitrogen type IIa diamonds by Ne+ ion implantation. Electronic transition of the center interacts with vibrations of energy 32 meV. The electron-phonon coupling is weak: S ~ 1. The center anneals out at temperatures below 550°C. The center is tentatively attributed to an interstitial type defect containing Ne atoms (Zaitsev 1992a; Tkachev et al. 1985d). 1.883 eV (658.3 nm); A, PL; ZPL; a center observed in type Ib synthetic diamonds grown in a nickel-containing medium (also by the temperature gradient method) (Sittas et al. 1995; Yelisseyev and Nadolinny 1995a; Yelisseyev and Nadolinny 1993; Antsygin et al. 1996; Vins 1988; Vins and Yelisseyev 1987; Osvet et al. 1997). The center can be observed in CVD diamond films (Bachmann et al. 1993; Heiderhoff 1997). The center is mostly intensive in high-nitrogen diamonds. This is a predominant nickel-related feature in synthetic diamonds with nitrogen content
5.1 Optical Bands 193
greater than 50 ppm (these diamonds have a yellow color) (Lawson and Kanda 1993a). This center is also believed to be a radiation damage product (Davies 1977c). In the phonon side-band of the center there are features at 1.906, 1.914 and 1.943 eV. The 1.883 eV center anneals out at a temperature above 2000°C (Nadolinny and Yelisseyev 1993; Yelisseyev et al. 1999). Intensity of the center may strongly reduce upon annealing at temperatures above 1700°C (Yelisseyev et al. 1999). In absorption this center is much more pronounced than any other nickel-containing center (Vins 1988). The center is segregated in the {111} growth sectors of synthetic diamonds (Field 1992; Collins and Spear 1982a; Collins et al. 1990a; Collins and Spear 1983b; Lawson et al. 1993c; Yelisseyev et al. 1996). The 1.883 eV center strongly affects the thermal conductivity of synthetic diamonds. In 13 C diamonds the ZPL of the center shifts by +3.6 meV (Davies 1994a). The center interacts with 61 meV vibrations in absorption and 65 meV vibrations in luminescence (Nazare et al. 1993a). Interaction of the center with a 88 meV vibrational mode has been also reported (Yelisseyev et al. 1999). PLE spectrum of the center exhibits a ZPL at 511.1 nm (Yelisseyev et al. 1999). The electronic transition occurs between E (excited) and T2 (ground) states at a tetragonal defect. The center is attributed to a nickel-related defect (Collins 1990b). It is probably a substitutional Ni- ion (Nazare et al. 1993b), or a nickel-nitrogen defect involving one Ni atom (Yelisseyev et al. 1996; Yelisseyev et al. 1999). 1.888 eV (656.5 nm); PL; ZPL; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Ni and Si (Sittas et al. 1995; Sittas et al. 1996). 1.889 eV (656 nm); CL; ZPL; a center created in natural type Ia and common synthetic diamonds by neutron irradiation. The center has a relatively narrow ZPL and low-vibrational side-band. The relative intensity of the center increases strongly at doses at which the vacancy-related centers GR1, TR12 and 3H get suppressed. The center anneals gradually with temperature, increasing to 550°C (Zaitsev and Zaitsev 1989). (Fig. 5.42, 5.53). 1.893 eV (655 nm); A; a line observed in type IaB natural diamonds irradiated by high-energy (> 1 MeV) neutrons with doses of 1019 neutron/cm2 and subsequently annealed at a temperature of 950°C. Spectral holes can be burnt up in this line at temperatures as high as 200 K (Sildos and Osvet 1994a; Sildos and Osvet 1994b; Sildos et al. 1995). 1.9 eV (650 nm); PL; FWHM of 0.2 eV; a structureless band observed in singlecrystal diamonds under a pressure of 300 GPa (Vohra and McCauley 1993). 1.9 eV (650 nm); A; FWHM of 0.5 eV; a broad band provides a green color in some synthetic diamonds grown at temperatures close to the melting point of the solvent-catalyst. This band is thought to be the electron-phonon side-band of a Moessbauer-type center, the ZPL of which is expected to be at around 1 eV (Field 1992; Collins and Lawson 1989b).
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5 Optical Electronic Transitions
a
GR1
TR12
656
3H
640
450
500
550
600
650
700
750
800
WAVELENGTH, nm
656
b
590 486
400
500
600
700
800
900
1000
WAVELENGTH, nm
Fig. 5.53. CL spectra taken at LNT of natural type Ia diamonds with moderate nitrogen content after neutron irradiation at doses of 1.5×1018 cm-2 (a) and 6×1018 cm-2 (b). The temperature during the irradiation might attain 200°C (Zaitsev and Zaitsev 1989)
1.9 eV (650 nm); PL; a broad band observed in as-grown high-nitrogen synthetic diamonds grown with Ni-containing melt. The band anneals at temperatures above 1950 K (Yelisseyev et al. 1999). 1.9 eV (650 nm); a broad band observed in EL of synthetic diamonds at temperatures to 200°C (Levinson and Halperin 1979). 1.9 eV (650 nm); PL, CL; a broad band of width varying from 0.36 to 0.86 eV. The band is observed in CVD diamond films, for instance in films deposited at low
5.1 Optical Bands 195
temperatures without oxygen. The band might be related to the 2.156 eV center. The behavior of this band is very similar to that of the luminescence from hydrogenated SixC1-x alloys (Deneuville et al. 1993; Clark and Dickerson 1992b; Clark and Dickerson 1994). 1.9 eV (650 nm at RT); CL, no PL; FWHM of 0.2 eV; a broad band spreading from 1.5 to 2.1 eV. The band is observed in phosphorous-doped CVD diamond films. This band is different from the 1.84 eV band reported by Freitas et al. (1994a) and Freitas et al. (1994b). Tentatively the band is attributed to D-A radiative recombination on phosphorous donors (the energy level is assumed to be at Ec - 1.1 eV) and phosphorous-vacancy complexes expected to be acceptors with energy level at Ev + 2.8 eV (te Nijenhuis et al. 1997). An alternative model of the band is an intracenter Moessbauer-type luminescence on phosphorous-vacancy complexes with the ZPL at about 1.99 eV (similar to the luminescence of the NV 1.945 eV center *). 1.903 eV (651.5 nm); A; ZPL; a photoproduct of the phototransformation process in a group of lines around 650 nm observed in type IaB natural diamonds irradiated by high-energy (> 1 MeV) neutrons at a dose of 1019 neutron/cm2 and subsequently annealed at 950°C (Sildos and Osvet 1994a; Sildos et al. 1995) (Fig. 5.54). 1.908 eV (649.5 nm) and 1.925 eV (644 nm); A, PL, ZPL doublet, observed in type IaB natural diamonds and synthetic type Ib diamonds irradiated by high-energy (> 1 MeV) neutrons at doses of 1019 neutron/cm2 and subsequently annealed at temperatures of 800-950°C. Spectral holes can be burnt in the 1.908 eV line at temperatures up to 100 K. The homogenous width of this ZPL at LHeT is 0.7 GHz. The lifetime of the excited state of the center is 215 ps (Osvet et al. 1992; Sildos and Osvet 1994a; Sildos and Osvet 1994b; Sildos et al. 1995; Osvet et al. 1997; Vins 1988; Vins et al. 1988). The doublet is caused by splitting in the excited state. The 1.908 eV center relates to the ZPLs at 2.087, doublet 2.321 and 2.341 (caused by splitting in the excited state), 2.712 and 2.718 eV (Davies 1977c; Davies 1972a; Sildos et al. 1995) (Fig. 5.54). 1.92 eV (645 nm); A; a line observed in some synthetic diamonds exhibiting an absorption continuum from 300 to 200 nm (Klimenkova et al. 1975c). 1.923 eV (644.6 nm); PL; a line which is probably a one-phonon replica of the 1.967 eV center (Clark and Dickerson 1994). 1.924 eV (644.1 nm); A; ZPL; a photoproduct of the phototransformation process in a group of lines around 650 nm observed in type IaB natural diamonds irradiated by high-energy neutrons (> 1 MeV) at a dose of 1019 neutron/cm2 and subsequently annealed at 950°C (Sildos et al. 1995) (Fig. 5.54). 1.929 eV (642.6 nm); ZPL; a center observed in the PLE spectrum of the 1.648 eV center. The 1.929 eV center is induced in nitrogen- and nickel-containing synthetic
196
5 Optical Electronic Transitions
diamonds grown by the temperature gradient method and annealed at temperatures above 2200°C. The center interacts with 42 meV quasilocal vibrations, which, probably, are localized on one Ni atom (Kupriyanov et al. 1999). Possibly this center is mentioned as the 1.770 eV (700.1 nm) center observed in PL of high-nitrogen synthetic diamonds grown in Ni-containing melt after annealing at temperatures above 1700°C (Yelisseyev et al. 1999) (Fig. 5.26).
649.5 644.1
651.4
640
645
650
655
WAVELENGTH, nm
Fig. 5.54. Absorption spectrum of a natural diamond irradiated with neutrons. Three ZPLs at 644.1, 649.5 651.4 nm dominate the spectrum. Two holes have been burnt up with a CW laser at wavelengths of 648.3 and 649.2 nm (Sildos et al. 1995)
TR12
508.8
541.1
570.6
546.3
603.4 640
480
520
560
600
640
WAVELENGTH, nm
Fig. 5.55. CL spectrum (at LNT) of a good-quality low-nitrogen (almost no traces of the 389 and 575 nm centers) PCCVD diamond film as-irradiated with high-energy carbon ions at a dose of 3.5×1015 cm-2
5.1 Optical Bands 197
1.937 eV (640.0 nm); CL; ZPL; a weak center observed in some high-quality CVD diamond films irradiated with ions. The feature is also observed in type Ia diamonds irradiated with neutrons (Fig. 5.53, 5.55). 1.939 eV (639.2 nm); A, PL; ZPL; a center observed in some nitrogen-containing synthetic diamonds grown in the presence of Ni or Ni and Si (Lawson and Kanda 1993a; Sittas et al. 1995; Lawson and Kanda 1993b; Sittas et al. 1996; Kupriyanov et al. 1999; Yelisseyev et al. 1999). The center intensity increases after annealing at temperatures above 1500°C. The center is still stable at temperatures above 1900°C; however, it anneals out at 2200°C. In PL the center interacts with 59 meV vibrations. A possible model of the center is a defect containing a nickel atom bound to the A-aggregate of nitrogen (Lawson and Kanda 1993a) (Fig. 5.56, 5.69). 1.945 eV (638 nm); A, PL, PL excitation, CL observed only in nitrogen-containing diamonds irradiated with high-energy ions (Varichenko et al. 1985b; Varichenko et al. 1987a; Varichenko et al. 1986a; Zaitsev et al. 1987b), ODMR, Raman heterodyne detected EPR (Fisk et al. 1990), no XL (Vins 1988), no PL under UV excitation (Iakoubovskii and Adriaenssens 1999b); commonly referred to as the NV center (or the 638 nm center; recently the NV0 center) (Fig. 5.16, 5.57). The 638 nm center is a naturally occurring feature in nitrogen-containing diamonds of any origin. The center is especially intensive in type Ib diamonds, in which it is produced by any irradiation and annealing at temperatures above 550°C (Nishida et al. 1989, *). The 638 nm center results in intense purple coloration of synthetic diamonds subjected to heavy radiation damage and 800°C annealing. The NV center is produced in type IIa diamonds by N+ ion implantation and annealing at temperatures above 700°C. The center is intensively created in polycrystalline sintered diamond compacts by annealing at temperatures above 1300°C under a pressure of 9.5 GPa (Evans et al. 1984). In synthetic diamonds the 638 nm center can be created from the H3 center by heating at a temperature of 1200°C under a pressure of 50 kbar, the effect being stronger in {111} than {100} growth sectors (Jackson and Webb 1995). Collins (1979) observed the creation of the 638 nm center after annealing the H4 center in type IaB natural diamonds. In synthetic diamonds the center is distributed preferentially over the {111} growth sectors, and it is usually almost absent from the {100} and {113} sectors (Enckevort and Lochs 1988b). In CVD diamond films deposited onto Si substrates the center intensity increases with the film thickness (Dollinger et al. 1995; Freitas et al. 1990). The center intensity is suppressed by boron doping (the effect of recharging of the negatively charged NV- defects) (Freitas et al. 1994c; Srinivasan and Butler 1999). Formation of the 638 nm center (capture of a vacancy by an isolated nitrogen atom) is much more efficient process (by a factor of 50) than that for the H3 center (capture of vacancies by the A-aggregate) (Collins 1980). Electron irradiation greatly reduces the intensity of the 638 nm center due to its annihilation with radiation-induced interstitials (this effect is equally strong at LNT and LHeT) (Steeds et al. 1999a; Steeds et al. 1999a). Equally, the 638 nm center is suppressed by neutron irradiation. Simultaneously the 575 nm center intensity increases with neutron irradiation. This different behavior is
198
5 Optical Electronic Transitions
explained by the conversion of the 638 nm center (NV- model) into the 575 nm center (NV0 model) due to lowering of the Fermi level (Mita 1996), or due to transferring the substitutional nitrogen atoms (in the 638 nm center) into interstitial positions (in the 575 nm center) (*).
PL
59 meV
1.991
1.940 59 meV
1.65
1.70
1.75
1.80
1.85
1.90
1.95
2.00
2.05
QUANTUM ENERGY, eV
PLE spectrum of the 1.991 eV center 1.991
50 meV
Absorption 1.9
2.0
2.1
2.2
2.3
2.4
2.5
QUANTUM ENERGY, eV
Fig. 5.56. PL and PLE spectra (at LNT) of the 1.991 and 1.940 eV centers taken from a nitrogen- and nickel-containing synthetic diamond annealed at 1500°C. The PL spectrum has been excited at a wavelength of 580 nm. The PLE spectrum is compared with the absorption spectrum taken with better spectral resolution (Kupriyanov et al. 1999; *)
The concentration of the 638 nm center in natural diamonds does not usually exceed a value of 1017 cm-3 (Kurdumov et al. 1994). The concentration of the center can be evaluated from the integrated intensity of its ZPL in absorption at LNT as: NNV-[cm-3] = 8.8×1015 µ638nm[meV cm-1] (Lawson et al. 1998).
5.1 Optical Bands 199
RT
Raman
638
575.5
500
550
600
650
700
750
800
850
900
WAVELENGTH, nm
Fig. 5.57. PL spectrum of a high-quality single-crystal free-standing CVD diamond film excited at RT with the 514 nm Ar laser line. The 638 nm nitrogen-related center exhibits a pronounced ZPL accompanied by six phonon replicas. Both the 575 and 638 nm centers exhibit strong anti-Stokes luminescence
The annealing behavior of the 638 nm center depends strongly on the irradiation procedure. In nonirradiated diamonds the center remains stable at 1500°C (Collins 1980). In electron-irradiated and ion-implanted diamonds the 638 nm center may anneal out at about 1500°C (Zaitsev 1992a). In electron irradiated synthetic diamonds the center gradually anneals out at 1500°C, the fraction of the remaining centers N638-rem/N638-total after the annealing being dependent on the electron dose D: N638-rem/N638-total ≈ -0.15 log(10-17D[cm-2]) + 0.38 (Collins 1981b). In nitrogencontaining diamonds irradiated with heavy high-energy ions the 638 nm center may appear after annealing in a narrow temperature range of 400 to 600°, whereas the 575 nm center shows its usual increase in intensity with annealing temperature (Dobrinets 1986) (Fig. 5.41, 5.58). The temperature dependence of luminescence intensity of the 638 nm center is not strong. The PL intensity of the center in synthetic diamonds irradiated with neutrons and annealed at 800°C falls by about a factor of 2 when heating from LNT to RT, the parameters of the temperature quenching being ET = 0.06±0.02 eV and τA ≈ 10 in (Vins 1988). The PL excitation spectrum of the 638 nm center coincides with that of its absorption (the center is not excited through the bandgap electronic transitions) (Vins 1988) (Fig. 5.59). In 13C diamonds the ZPL of the 638 nm center shifts by +2.1 meV (Collins et al. 1988c; Davies 1994a; Collins and Davies 1988b; Davies and Collins 1999). The center exhibits a linear Stark effect, which indicates the absence of inversion symmetry. The Stark-shift constants are given by Davies and Manson (1980a) and
200
5 Optical Electronic Transitions
van Oort and Glasbeek (1990): R3D = 0.35±0.02 Hz cm/V, R2E = 17±2.5 Hz cm/V, R15 4.6 eV due to the C-defects in type Ib diamonds; a structured band at hν > 5.2 eV due to the N9 center. The maximum of the nitrogen-related A-band (?) excited in CL shifts from 2.33 eV (532 nm) to 2.45 eV (506 nm) with increasing density of exciting electron from 1 to 10 A/cm2 ? (Bezrukov et al. 1979). The temperature dependence of the A-band depends on diamond type and type of excitation. In most natural diamonds (these diamonds usually are not active in TSL) the A-band intensity falls almost to zero by a temperature above 400 K (Dean and Male 1964c; Collins 1990a; Dean et al. 1960; Vilutis and Krongauz 1963; Kurdumov et al. 1994). The dislocation A-band excited in EL can still be detected at temperatures as high as 200°C. The PL of the A-band is temperature independent at temperatures up to 250 K (Levinson and Halperin 1979; Melnikov et al. 1998). The CL intensity of the dislocation A-band may decrease by 2 to 10 times (depending of the sample and its type) with temperature increase from LNT to RT, the decrease being most prominent in type IIb diamonds (Yamamoto et al. 1984). The temperature dependence of the A-band intensity in CL shows a hysteresis by cooling and heating, the effect being most pronounced (the temperature difference for equal intensities attains 80 K) in type IIb diamonds (Yamamoto et al. 1984). In CVD diamond films the A-band intensity attains a maximum at a temperature about 170 K and then falls rapidly with a temperature increase above 200 K (Khong and Collins 1993; Kawarada et al. 1990b). The activation energy of the thermal quenching of the dislocation-related A-band in type IIb natural and CVD diamonds is 0.3 eV (Iakoubovskii and Adriaenssens 1999a). The CL intensity of the dislocation A-band decreases (as compared to the exciton emission) with irradiation time (Won et al. 1994). The band is quenched by ion implantation but recovers after subsequent annealing (Gheeraert et al. 1994a) (Fig.
5.1 Optical Bands 313
5.136). The CL intensity of the dislocation A-band in natural diamonds with moderate nitrogen content is not reduced after ion implantation with light ions (N+ and ions of lower masses) at a dose as high as 2×1013 cm-2. The CL intensity of the A-band is strongly suppressed by implantation of donor species (e.g. Li), however is restored after subsequent N+ ion implantation (Zaitsev 1999b). The A-band remains stable after 0.8 MeV electron irradiation (0.8 MeV electrons can create in diamond only point defects), but it is destroyed by 20 MeV irradiation (20 MeV electrons can create defect clusters in diamond) (Yokota et al. 1992). Irradiation with 20 keV electrons at doses above 1017 cm-2 may change noticeably the shape of the A-band via induction of some additional luminescence centers overlapping with the A-band (Heiderhoff 1997). Luminescence of the dislocation A-band is extinguished by 200 keV electrons (Yamamoto et al. 1984). In CVD diamond films the A-band is quenched by intense excimer laser irradiation (Cremades and Piqueras 1995).
CL INTENSITY, arb. units
100
10
1
1E11
1E12
1E13
1E14
1E15
1E16
-2
ION DOSE, cm
Fig. 5.136. Reduction of CL intensity of the A-band in diamond as a result of ion implantation: (l) in type Ia diamond implanted with N+ ions; (n) in type IIa diamond implanted with N+ ions; (s) in type IIa diamond implanted with B+ ions. Radiation-caused reduction of the CL intensity of the GC2 center in cubic boron nitride single-crystals implanted with H+ ions is presented for comparison (m)
The A-band reveals no fine-line structure even at low temperature in very perfect diamonds. However, the dislocation-related A-band is thought to be a vibronic side-band of a center with ZPL at about 3.0 eV (the ZPL is not observed due to the very large Huang-Rhys factor of the center and the strong nonhomogeneous stress around dislocations) (Iakoubovskii and Adriaenssens 1999a) (Fig. 5.135). CL spectra taken from individual dislocations of different types are similar (Yamamoto et al. 1984). In natural diamonds the spectral position of the maximum of the dislocation-related A-band may range from 2.8 eV (445 nm) to 2.99 eV (415 nm). The A-band maximum measured in delayed CL on natural
314
5 Optical Electronic Transitions
diamonds exhibits a jump from 435 to 470 nm when the delay exceeds 27 µs (Heiderhoff 1997). In some CVD diamond films the A-band maximum can be found at 2.56 eV (485 nm). In some natural and synthetic diamonds the maximum of the nitrogen-related A-band may be shifted up to 530 nm? (Bezrukov et al. 1979). The shortest wavelength of the A-band maximum is found in very perfect CVD diamond films with very high thermal conductivity (∼2200 Wm-1K-1) (Fujimori and Nishibayashi 1993; Heiderhoff et al. 1995; Heiderhoff 1997; Zhang et al. 1994). In CVD diamond films the A-band widens as the deposition temperature decreases (Deneuville et al. 1993). The peak position and FWHM of the dislocation A-band excited in EL of diamond diodes do not change with the rate of charge carrier injection (Manfredotti et al. 1995). The A-band maximum shifts from 452 to 441 nm with decrease in the exciting electron energy from 20 to 8 keV (this effect is observed only at low electron current density of about 4×10-8 A/mm2). Decrease in the current density of the exciting electron beam may also cause a shift of the A-band maximum by a few nm towards longer wavelengths (Heiderhoff 1997). The internal quantum output of the A-band luminescence excited in natural diamonds with electrons and α-particles may attain a value of 0.01 photon per electron-hole pair. For EL in p-i-n diodes the quantum efficiency of the band may be four times higher (∼0.04) (Guseva et al. 1978; Dean and Male 1964e, *). The A-band luminescence exhibits a nonexponential decay. The decay time may change from less than 1 ms to greater than 50 ms as its intensity falls to 1/e of the initial value (Davies 1994a; Dean 1965). In CVD diamond films there are two decay times depending on the band. These two values are 45±10 ns/eV and 625±195 ns/eV (Khong et al. 1994). The decay time in CL depends also on the spectral position of the A-band maximum: the shorter the wavelength of the maximum the shorter the decay time (Heiderhoff 1997). The decay of the A-band with maximum at 485 nm (observed in type Ia diamonds) has three components: τ1 < 5×10-5 s (the spectrum of this component ranges in the UV region), τ2 ∼ 5×10-3 s (this component is the most intense at temperatures below 130 K; τ2 does not change with temperature down to 4.2 K), τ3 changes with temperature: τ3 ~ 9 ms at 300 K, τ3 ~ 20 ms at 130 K and τ3 ~ 720 ms at temperatures from 50 to 4.2 K. The second and third decay components have similar spectra (Sobolev and Dubov 1975b). The spectra of the 5×10-3 s component and the 720 ms component are broad bands with maxima at 415 and 470 nm and FWHMs of 0.7 and 0.8 eV respectively (Sobolev and Dubov 1979a). In CVD diamond films the decay of the A-band exhibits very fast components in the range 0.1 to 1 ns attributed to the intrinsic radiative decay of directly excited D-A pairs (Schneider et al. 1995). In some CVD diamond films (both undoped and nitrogen doped) the A-band shows a single exponential decay. This single decay time has the largest value at RT ranging from 4.8 to 5.3 ns. At LNT the decay time increases from 3.8 to 5.6 ns with decrease of the CL excitation electron current from 5×10-4 to 5×10-6 A/mm2 (Pereira and Pereira 1992; Heiderhoff et al. 1995). In high-purity CVD diamond films the A-band does not show any shift in time-resolved spectra. The CL decay time of the A-band in synthetic boron-doped diamonds excited at a temperature of 90 K is 10 µs when measured at a wavelength of 500 nm; it increases slightly (by a factor of 2) with a wavelength increase in the
5.1 Optical Bands 315
spectral range from 400 to 700 nm. In some MWCVD diamond films the CL of the A-band shows two decays with constants of 53 ns and 2.8 ns, whereas in some flame-grown CVD diamond films there are three decays: 60, 12 and 900 ns (Heiderhoff 1997). It appears that there are the A-bands of different nature. One of the A-band model is radiative recombination at dislocations. This model concerns a relatively narrow A-band peaked at 440 nm. This band is usually observed in low-nitrogen type II diamonds. There are different opinions about particular nature of the dislocation-related centers responsible for the optical transitions: they are donoracceptor pairs decorating dislocations (Yamamoto et al. 1984), and vacancies bound to dislocations (Prins 1995; Prins 1997). It is possible that the A-band originates only from dislocations decorated with D-A pairs, the nondecorated ones being nonluminescent (Bruley and Batson 1990). There is a dislocation-related model of the A-band considering luminescence on pure nondecorated dislocations (possibly 60°-edge dislocations) excluding D-A recombination. This model is used for the A-band with a maximum at 415 nm (Ruan et al. 1992a; Dean and Male 1964d; Sobolev and Dubov 1979a; Sobolev and Dubov 1975b). The pure dislocation model was strongly supported by EELS measurements (sensitivity of a few tens of atoms), which did not reveal any donors or acceptors (nitrogen or boron) at the dislocation core (Bruley and Batson 1990). The A-band is attributed to electronic transitions from deep-lying acceptor centers (may be electronic levels of dislocations) to the valence band (Prins 1994a). The second model of the A-band is intracenter transitions at the B1(N9) centers (platelets). This model relates to the broad A-band with a maximum at 480 nm observed in natural type I diamonds (and possibly synthetic diamonds?). The A-band is also considered as an electron-hole recombination at deep centers, the energy levels of which lie in the middle of the bandgap (Heiderhoff 1997; Manfredotti et al. 1995): EA = (Eg/2) ± 0.75 eV. The A-band emission is thought to occur through the formation of free excitons (Kawarada and Yamaguchi 1993a). The light emission of the A-band is thought to be a two-stage process (Kurdumov et al. 1994; Martynovich et al. 1977). The A-band is possibly related to the 2.75 eV (450 nm) PL band. The A-band is believed to be related to the 4 eV band (possibly a transition from the conduction band to the dislocation band localized at about 1.8 eV above the valence band (Jones and King 1983; *)). (Fig. 5.102). 2.88 eV (430 nm); the α-band; a broad band observed in the spectral range from 390 to 480 nm in PLE spectra of the S2 and S3 centers. At LNT the α-band is accompanied by ZPLs at 478.9, 477.6, 472.3, 467 nm (Bokii et al. 1986). 2.88 to 3.01 eV; the GR2 to GR8 centers, see 1.673 eV (the GR1 center). 2.889 eV (429.0 nm); CL; ZPL; a center observed in type Ib synthetic diamonds grown using pure cobalt as the solvent-catalyst (Field 1992).
316
5 Optical Electronic Transitions
2.896 eV (428 nm); ZPL; CL; a center observed in boron-doped CVD diamond films irradiated with neutrons. The center anneals out at temperatures below 1000°C (Popovici et al. 1996).
2.900 eV (427.5 nm); XL; possibly ZPL; a line observed in type IIb natural diamonds (Bienemann-Kuespert et al. 1967). 2.90 eV (427 nm); CL; ZPL; a center observed in some type Ia diamonds after ion implantation and subsequent annealing at 1400°C (Fig. 5.45). 2.91 eV (426 nm); A; a weak center observed in some gray natural diamonds (Reinitz et al. 1998) (Fig. 5.137).
384 394 426
741 300
400
500
600
700
800
900
WAVELENGTH, nm
Fig. 5.137. Absorption spectrum of a grayish yellow-green marquise natural diamond taken at a temperature of 72 K (Reinitz et al. 1998)
2.913 eV (425.5 nm); CL; ZPL; a center observed in some PCCVD diamond films. The center exhibits a relatively low electron-phonon coupling. It is attributed tentatively to a defect containing interstitial atoms (Melnikov et al. 1996). 2.916 eV (425.1 nm); A; ZPL; a center observed in type I diamonds. It is possibly an electronic transition to a higher excited state of the 595 nm center. The 2.916 eV center has D3d symmetry. The center shows a quadratic Stark effect implying no inversion symmetry of the corresponding defect (Field 1992; Davies and Nazare 1980b; Davies 1994a; Davies and Manson 1980a). 2.916 eV (425 nm); A; a narrow band observed in hydrogen-rich gray-violet and chameleon diamonds (Fritsch et al. 1991a).
5.1 Optical Bands 317
2.918 eV (424.9 nm); A; ZPL; a center observed in type Ib diamonds. It is activated after irradiation and annealing at about 600°C. The dominant electron-phonon interaction at the center occurs with 55 meV vibrations (Davies 1977c). 2.920 eV (424.6 nm); A; possibly ZPL; a line observed in type IaB diamonds after irradiation and annealing at a temperature of about 600°C. The center interacts with 12 meV vibrations. The center probably relates to the H4 center (Davies 1977c). 2.92 eV (424 nm); CL; a center observed in type IIa natural diamonds, undoped and boron-doped CVD diamond films. The center is easily destroyed by electron irradiation (Kawarada et al. 1990b; Yokota et al. 1992). 2.925 eV (423.8 nm); A; the H6 center (H stands for irradiated and Heated). The H6 center is observed in type I diamonds after irradiation and annealed at temperatures above 500°C (Clark et al. 1956c). 2.93 eV (423 nm); CL; ZPL; a center observed in some type Ia diamonds after ion implantation and subsequent annealing at 1400°C (Fig. 5.45, 5.131). 2.941 eV (421.5 nm); CL; ZPL; a weak line observed in some PCCVD diamond films (Melnikov et al. 1996). 2.95 eV (420 nm); CL, a band with FWHM of 0.1 eV observed in MWCVD diamond films grown on Si substrates (Yacobi et al. 1991). This band is possibly the phonon assisted band of the 415.2 nm N3CVD center (*). 2.95 eV (420 nm); PL, XL, GL; a band with FWHM of 50 to 90 meV observed at RT in synthetic diamonds. The band is not observed at LNT (Nedzvetskii and Gaisin 1973a; Vachidov et al. 1975a; Vilutis 1959; Sobolev et al. 1968b). The band is especially strong in diamonds doped with Al, B, N and Si (Vachidov et al. 1975a). The feature is ascribed tentatively to a distorted N3 center (Vachidov et al. 1975b), which seems to be doubtful (*). The band is not destroyed by neutron irradiation with a dose of 5×1018 cm-2 (Vachidov et al. 1975a). 2.964 eV (418.2 nm) and 2.974 eV (416.8 nm); PL; doublet of ZPLs; the 2.97 eV center. The 2.97 eV center is observed in brown natural diamonds and synthetic diamonds grown in the presence of Ti and Al. The doublet originates from electronic transitions between two excited states and the common ground state. The 2.97 eV center is excited in a broad band with a maximum at about 3.75 to 3.9 eV (Pereira and Santos 1988; Pereira and Santos 1993). This is a slow luminescent center. The low-temperature lifetime (at temperatures below 15 K) is 40 µs immediately after excitation and 130 µs upon thermalization of the spin sublevels in the temperature range of 30 to 150 K. The temperature range of nonexponential decay of the center is 20-40 K. The center is a triplet-singlet spin-forbidden electronic transition. The triplet level is not radiative but feeds the singlet emitting levels. No nonradiative processes are observed at the 2.97 eV center at temperatures up to RT. The
318
5 Optical Electronic Transitions
activation energy of the nonradiative decay is 0.155 eV. The ratio of the transition probabilities of the second and first ZPLs is 65 (Pereira and Santos 1993; Davies 1994a). The 2.974 eV transition interacts with vibrations of energies 92 (the most intensive one), 38 and 130 meV. The electron-phonon coupling at the 2.974 eV transition is quite strong: S = 4.0. The 2.964 eV transition interacts with vibrations of energies 79 (the most intensive one) and 38 meV. The strength of the electronphonon coupling of this transition is characterized by S = 3.7 (estimated from spectra by Pereira and Santos (1993)). No atomic model is yet available for this center (Fig. 5.138).
PL
PLE
2.964
2.974
2.6
2.8
3.0
3.2
3.4
3.6
3.8
QUANTUM ENERGY, eV
Fig. 5.138. PL and PLE spectra of the 2.97 eV center taken at a temperature of 90 K. The ZPL of the center is a doublet at 2.964 and 2.974 eV. The 2.974 eV line dominates at temperatures above 50 K. At lower temperatures the 2.964 eV line is dominant (Pereira and Santos 1993)
2.970 eV (417.5 nm); XL; possibly ZPL; a center observed in type I and IIb natural diamonds (Bienemann-Kuespert et al. 1967; Sobolev and Dubov 1979a). Possibly this feature is observed also in CL (Yamamoto et al. 1984). 2.971 eV (417.2 nm); PL, XL; ZPL; a center observed in some low-nitrogen type Ib synthetic diamonds grown using iron-nickel or cobalt solvent-catalysts. The center is accompanied by a line at 418.7 nm. The center is excited with light of wavelength shorter than 360 nm. In synthetic diamonds the center can be activated by heating to a temperature of 1400°C under a stabilizing pressure of 6 GPa (Vins 1988; Yelisseyev et al. 1987). The center is readily formed in synthetic diamonds grown from Al-containing media. The PL center intensity of the center decreases strongly in synthetic diamonds after annealing at 1800°C for 20 h (Kanda and Watanabe 1998). The center interacts predominantly with vibrations of energy 110 to 120 meV. The 2.971 eV center is always accompanied by the 484 nm nickel-related center. The spectrum of the 2.971 eV center is very similar to the N3 center. The
5.1 Optical Bands 319
radiative decay of the center is also similar to that of the N3 center (Field 1992; Lawson et al. 1996; Vins 1988). The center is excited particularly intensively in synthetic diamonds of moderate nitrogen content (high nitrogen concentration quenches the center) (Kanda and Watanabe 1998). The 2.971 eV center is tentatively attributed to a defect, which can contain an Al atom and a Ni+ ion (Vins 1988; Yelisseyev et al. 1987). Alternatively the center is attributed to a defect containing Ni and N atoms (Kanda and Watanabe 1998) (Fig. 5.139).
RT 484
LNT
418 400
450
500
550
600
650
WAVELENGTH, nm
Fig. 5.139. PL spectra of a pre-annealed synthetic diamond grown using a Ni-catalyst with nitrogen getter. The spectra were taken at RT and a temperature of 113 K (LNT) (Kanda and Watanabe 1998). The band with a maximum at 460 nm is believed to be the electron-phonon side-band of the 417.2 nm center
2.975 eV (416.8 nm); A; the H7 center (H stands for irradiated and Heated). The H7 center is observed in type I diamonds after irradiation and annealing at temperatures above 500°C (Clark et al. 1956c). 2.985 eV (415.2 nm); CL; ZPL; the N3CVD center. The N3CVD center is observed in some virgin CVD diamond films (Collins 1992b; Gheeraert et al. 1994a; Collins et al. 1989c; Yacobi et al. 1991; Yelisseyev et al. 1988). The electronic transition of the center interacts predominantly with 70 meV vibrations. The ZPL width of the center is very sensitive to mechanical stress. The N3CVD center seems to be not related to the N3 center. The N3CVD center is rather resistant against radiation: it survives after high-energy carbon ion irradiation with doses to 5×1015 cm-2 (Fig. 5.134).
320
5 Optical Electronic Transitions
2.985 eV (415.2 nm); A, CL, EL, XL (Pologrudov et al. 1964), PL, PLE, IL; ZPL; the N3 center (in some early publications the B3 center (Sobolev et al. 1969b)). The abbreviation N was taken for Naturally occurring. The N3 center is a very common optical feature in most type Ia natural diamonds containing the B-aggregates of nitrogen. About 95% of natural diamonds revealing N3 center photoluminescence belong to the Ia type (Yelisseyev 1977). Very often the defects responsible for the N3 center are distributed in natural type Ia diamonds as small clusters giving rise to a bright blue CL (Lang 1977). The N3 center is observed in some synthetic diamonds. The center is particularly strong in HPHT samples doped with cyan compounds (Nikitin 1971). Probably the N3 center is also observed in some CVD diamond films (Collins et al. 1990a). Normally the N3 center absorption is proportional to the absorption intensity of the platelet peak and the peak of the B-aggregates of nitrogen (Field 1992; Woods 1986). The N3 center is created in type IIa diamonds by nitrogen ion implantation and subsequent annealing at temperatures above 1200°C. It can also be created in synthetic type Ib diamonds by heating at temperatures above 1700°C (Collins and Stanley 1985b; Kluev et al. 1982; Chrenko et al. 1977; Lawson et al. 1996; Evans and Qi 1982a). Often the N3 center arises in brown diamonds within stacking faults in (111) planes (Graham and Buseck 1994). The N3 center may be highly localized at cracks or regions of plastic deformation (Kanda and Watanabe 1999; Kanda and Watanabe 1997). Some transformation of the B-aggregates of nitrogen into the N3 center in natural type IaB diamonds occurs at temperatures above 1960°C. No formation of the N3 center at high temperatures occurs in type IaA diamonds, whereas it is formed in type IaA+Ib diamonds (Brozel et al. 1978). PL of the N3 center can be excited by A-band luminescence (Collins 1974). The CL intensity of the N3 center is strongly suppressed by implantation of donor species (e.g. Li) (Zaitsev 1999b). (Fig. 5.22, 5.75, 5.76). The vibrational band of the N3 center in luminescence at RT extends out to 1.4 eV. The intensity of this low-energy tail falls at low temperatures (Solin 1972). The ZPL of the N3 center is split by 0.59 meV (the splitting occurs in the excited state). The nonhomogeneous width of the N3 center ZPL can be as narrow as 350 GHz (Davies 1994a; Harley et al. 1984). The PLE spectrum is characterized by bands at 3.2 (the N3 absorption spectrum), 3.6, 4.6 and 5.4 eV (the latter may show fine structure with peaks at 5.25 (the N9 center ?), 5.34 and 5.39 eV). In some natural diamonds PL of the N3 center can be excited at quantum energies above 5.2 eV and within a range from 3 to 3.5 eV (Yelisseyev 1977; Iakoubovskii and Adriaenssens 1999a) (Fig. 5.140). The luminescence efficiency of the N3 center is φ = 0.25÷0.29 (Davies 1994a). The oscillator strength of the N3 center is within the range of 0.16 to 0.36 (Sobolev et al. 1969b). The concentration of the defects responsible for the N3 center in natural diamonds can attain a value of 4×1016 cm-3 (Bokii et al. 1986). Usually absorption of the N3 center in natural diamonds does not exceed a value of 3 cm-1 (Clark et al. 1956a). A noticeable reduction in IL intensity of the N3 center excited by 3 MeV protons occurs at doses above 5×1014 cm-2 (Bettiol et al. 1994). Emission of the N3 center in type I diamonds is quenched by energy transfer to the A-aggregates; the probability of this transfer for one N3 center to one A-aggregate at
5.1 Optical Bands 321
one lattice spacing in unit time is 0.3×1016 s-1 (Davies 1994a; Crossfield et al. 1974; Thomaz and Davies 1978). However, no quenching of the N3 center luminescence was observed in high-nitrogen {111} growth sectors of synthetic diamonds grown by the temperature gradient method (Kanda and Jia 2000).
PLE of the N3 center
2.5
3.0
3.5 4.0 4.5 Quantum Energy, eV
5.0
5.5
6.0
Fig. 5.140. PLE spectrum of the N3 center in a natural type Ia diamond taken at LNT. There are two primary excitation regions of the center: the intracenter excitation from 3 to 3.5 eV and the interband excitation at energies above 5.2 eV (Iakoubovskii and Adriaenssens 1999a)
By its nature the N3 center is a high-temperature center. However the temperature dependence of its luminescence intensity depends on the perfection of the diamond and the type of excitation. The PL of the N3 center excited in the 5.4 eV band is extinguished at temperatures above 120°C, whereas, when excited in the 3.2 eV band, the N3 center is extinguished only at temperatures over 400°C (a similar temperature quenching is also observed for X-ray excitation) (Vilutis and Penzina 1965; Bienemann-Kuespert et al. 1967). In some natural diamonds exhibiting TSL the N3 center luminescence intensity is almost unchanged in a temperature range from LNT to 250°C. At higher temperatures the intensity falls but is still detectable at temperatures up to 500°C. In some low-nitrogen diamonds CL of the N3 center can be observed at temperatures as high as 600°C (Bokii et al. 1986; Vilutis and Krongauz 1963; Gomon 1960a; Bienemann-Kuespert et al. 1967; *). In some low-nitrogen natural diamonds the PL intensity of the center increases by about 170% with temperature decrease from RT to LNT (Bienemann-Kuespert et al. 1967). The N3 center is an electronic transition between A and E(C3v) states at a defect of trigonal C3v symmetry. The symmetry axis of the center is along the direction. The excited state of the center lies about 0.4 eV below the conduction band. The lower excited level is about 0.5 eV below the conduction band (Sobolev and Yeliseev 1976). The N3 center is a σ electronic transition. The N3 center shows a very strong linear Stark effect implying the absence of inversion symmetry
322
5 Optical Electronic Transitions
of the corresponding defect (Kaplyanskii et al. 1971; Kaplyanskii et al. 1970a; Clark et al. 1962; Yelisseyev 1977). Isotope shift of the N3 center ZPL is +4.5 meV (Davies and Collins 1999). The N3 center reveals no Jahn-Teller effect. The excited electronic state of the N3 center is coupled predominantly to totally symmetrical vibrational modes. The g-value of the excited state is positive (Douglas and Runciman 1977a). In absorption the center shows a dominant interaction with 93 and 165 meV vibrations. The electron-phonon coupling strength is moderate: S = 3.45. In luminescence the center interacts preferentially with 80 meV vibrations (Davies 1977c; Gomon 1960a; Englman 1965). An interpretation of the vibrational side-band of the N3 center in luminescence is given in Table 5.8 (Sobolev et al. 1969c; Kurdumov et al. 1994; *).
Table 5.8. Spectral structure of the N3 center in luminescence Spectral positions of the peaks [eV] 2.985 2.949 2.942 2.938 2.933 2.921 2.897 2.889 2.872 2.864 2.853 2.841 2.836 2.834 2.827 2.823 2.805 2.744 2.678 2.591
Interpretation (coupling with vibrations at the mentioned symmetry points) ZPL local vibration at vacancy (hνvacancy) ? ? ? L3 K3 X3 W2 K1, W1 X4 X1 L3' L2' K3+L3, hνvacancy +K3 Γ25' K2 L2'+K3, L3'+K3, X1+K3 2L2', 2L3', K3+L2'+L3' K3+L2'+X1, K3+2X1, K3+2L3', K3+2L2'
Polarization of the PL of the N3 center is reduced when it is excited with quanta of energy above 3.6 eV. The electron-phonon interaction reduces the polarization of the center luminescence: the polarization of the phonon-assisted band is considerably lower than that for ZPL (Clark and Norris 1970). The intrinsic decay time of the N3 center in PL is about 40 ns in a temperature range from 77 to 400 K. The decay time is reduced by interaction with the A-aggregates of nitrogen: it is below 20 ns in diamonds, showing an absorption strength above 30 cm-1 at a wavenumber of 1282 cm-1. The radiative decay time of the N3 center is 150 ns. Reversible population of the quartet levels of the center
5.1 Optical Bands 323
leads to a delayed emission at temperatures above 90 K with a decay time of 7 ms; this decay time decreases with temperature (Davies 1994a; Thomaz and Davies 1978; Pereira and Monteiro 1990b). In some natural diamonds (regardless of the type) the N3 center may show DL with a time constant up to 5 min (BienemannKuespert et al. 1967; Yelisseyev 1977). The activation energy of the nonradiative relaxation processes of the N3 center is 0.566 eV (Thomaz and Davies 1978). Hole burning in ZPL has been demonstrated by Davies (1994a) and Harley et al. (1984). The atomic model of the N3 center is a trio of the nearest substitutional nitrogen atoms in the (111) plane bonded to a common vacancy, the nitrogen atoms relaxing away from the vacancy by about 1/8 of the normal C-C distance (N3V-defect) (Scherbakova et al. 1978; Davies 1994a; Clark et al. 1956a; Davies and Summersgill 1973c; Collins and Woods 1982c; Woods 1986; Sobolev et al. 1969c; Nedzvetskii and Dymke 1970; Nedzvetskii and Gaisin 1974; Kaplyanskii et al. 1970a; Davies 1974b; Thomaz and Braga 1972; Crowther and Dean 1967a; van Wyk 1982; Lowther 1984; Davies et al. 1978; Douglas and Runciman 1977a; Davies 1981; Fritsch et al. 1991a; Mainwood 1994). Ab initio simulation of the N3V defect gives the following parameters: N-C bond length is 0.143-0.144 nm; C-C bond lengths of the unique C atom is 0.146 nm; the 2A1 to 2E transition has an energy of 2.8 eV; the radiative lifetime is 10 ns (Goss et al. 1996). The N3 center is an allowed transition on the N3V-defect, whereas the N1 (1.5 eV), N2 (2.596 eV) and N4 (3.603 eV) centers are thought to be forbidden transitions on the same defect (Sobolev and Yurjeva 1990). The N3 center is paramagnetic: the paramagnetic P2 center is attributed to the N3 center (Davies et al. 1978; Loubser and van Wyk 1978). 2.99 eV (415 nm); A; an intense band observed in some synthetic diamonds doped with Al (Klimenkova et al. 1975c). 2.99 eV (414.5 nm); CL; ZPL; a center observed in some CVD diamond films. The center interacts with vibrations of energy 70 meV. The ZPL of the center is a doublet split by about 6 meV. The decay time of the center is of 63±5 ns (Dischler et al. 1994; Khong et al. 1994). The center intensity is almost unchanged up to a temperature of 120 K and then it falls down considerably at temperatures above 130 K (Khong and Collins 1993). 3.004 eV (412.7 nm); A; the H8 center; a center observed in natural type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). 3.04 eV (407.8 nm); A, PC; ZPL; the R9 center; a radiation-induced center observed in type Ia and IIa diamonds. There is a line at 3.09 eV which possibly relates to the R9 center (Davies 1977c; Clark et al. 1956a; Farrer and Vermeulen 1972). 3.053 eV (406.0 nm); XL; ZPL; a center observed in some natural diamonds (Sobolev and Dubov 1979a).
324
5 Optical Electronic Transitions
3.062 eV (405.0 nm); A; the H9 center; a center observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). 3.064 eV (404.6 nm); A; the low-energy line of a set of lines observed in type Ib low-nitrogen synthetic diamonds grown using a nickel catalyst. Further lines are at 3.076, 3.090, 3.102, 3.117, 3.128 eV. All the centers are segregated in the {111} growth sectors (Field 1992). 3.065 and 3.076 eV (404.4 and 403.0 nm); A, L; ZPL doublet; the 3.1 eV center; a center observed in synthetic diamonds grown in the presence of nickel. The center is confined to the {111} growth sectors (Field 1992; Davies 1994a; Collins and Spear 1982a; Collins and Spear 1983b; Lawson et al. 1993c). The center interacts predominantly with a 26 meV quasilocal vibration attributed to two Ni atoms (calculated parameters of the quasilocal vibration are: ωR = 29.2 meV, ∆ωR = 8.9 meV). The line at 3.09 eV (FWHM of 7 meV) is the first vibrational replica of the main ZPL due to interaction with the quasilocal vibration. The phonon-assisted side-band of the center exhibits a predominant interaction with long-wave optical phonons of an energy below 165 meV. The center is tentatively attributed to a defect containing two Ni atoms (Fig. 5.141, 5.165).
60
ABSORPTION COEFFICIENT, cm
-1
157 meV
50 26 meV
40 30
3.076 3.065
20 10 0 3.00
3.05
3.10
3.15
3.20
3.25
3.30
QUANTUM ENERGY, eV
Fig. 5.141. Absorption spectrum of the 3.1 eV center taken from octahedral growth sectors of a synthetic diamond grown using a nickel catalyst with 2.5 atom% Ti added (Collins et al. 1990c). A 26 meV quasilocal vibration (tentatively ascribed to two Ni atoms) form a periodic structure in the spectral range from 3.06 to 3.12 eV. The 3.1 eV center clearly exhibits an interaction with optical phonons of energy 157 meV (compare with the maximum of the diamond phonon density in Fig. 3.1)
5.1 Optical Bands 325
3.07 eV (404 nm); A; a narrow line observed in some high-hydrogen gray-violet diamonds (Fritsch et al. 1991a). 3.09 eV (398.5 nm); CL; ZPL; the most intense line of a set of lines within a spectral range from 385 to 430 nm observed in type IIa diamonds after Ag+ ion implantation and subsequent annealing at temperatures above 1000°C. In the spectral region of ZPL five components can be resolved at 397.7, 398.5, 399.4, 400.5 and 401.2 nm. The line at 401.2 nm (FWHM of 4 meV) is tentatively attributed to a 21 meV quasilocal vibration involving two Ag atoms (calculated parameters of the quasilocal vibration are: ωR = 21 meV, ∆ωR = 4.4 meV). A set of lines at 417 nm (the main lines at 414.8, 417.6 and 420.5 nm) are possibly phonon replicas of ZPL due to interaction with optical phonons of energy 122, 142 and 162 meV (the latter is the Raman phonon). The spectral structure of the center resembles that of the 484 nm Ni-related center. The center is attributed to a defect containing two Ag atoms (Field 1992; Vavilov et al. 1982a; Zaitsev 1992a; Zaitsev 1999a) (Fig. 5.142). 3.092 eV (400.9 nm); CL; ZPL; a center observed at LNT in some CVD diamond films, for instance, in epitaxial undoped CVD diamond films grown on (100) surfaces of natural diamonds. The center interacts with a quasilocal vibration of energy 154 meV and with a local vibration (related to carbon atoms) of energy 216 meV (Dischler et al. 1994; Field 1992; Collins et al. 1989c). 3.1 eV (400 nm); A; ZPL; a center observed in low-nitrogen synthetic diamonds grown using a nickel-containing catalyst (Collins et al. 1990c; Lawson and Kanda 1993b). 3.1 eV (400 nm); PL, IL, EL (Zhang et al. 1996a); FWHM of 0.4 eV; the N-band. The N-band is observed in intentionally undoped CVD diamond films as-grown or annealed at a temperature of 600°C. The decay time of luminescence of the N-band is 0.13 ms. The spectral position, width and decay time of the band are independent of temperature in the range from 10 K to RT. The N-band is preferentially excited in the spectral range from 230 to 300 nm (Pereira and Pereira 1992). The band is tentatively attributed to interstitial nitrogen atoms. It is thought to be 5p(1/2, 3/2)→3(1/2, 3/2) electronic transitions at nitrogen atoms (this interpretation seems very doubtful*) (Zhang et al. 1996a). 3.12 eV (397 nm); CL; a broad band with FWHM of 0.67 eV observed only in boron-doped HPHT synthetic diamonds containing very small concentrations of uncompensated nitrogen donors. Dispersed nitrogen and uncompensated boron quench the band. The band is tentatively attributed to compensated boron (Lawson et al. 1994b; Lawson et al. 1995). 3.13 eV (396 nm); A; a line observed in some pink natural diamonds (Collins 1997).
326
5 Optical Electronic Transitions
398.5
Ag
417.5
B-band
400
500
600
700
800
900
WAVELENGTH, nm
398.5
Ag 165 meV 162 meV 142 meV
122 meV
414.8 417.5 420.5
400
440
480
WAVELENGTH, nm
21 meV 398.5
401.2 397.7
400.5
21 meV
396
398
400
402
404
406
WAVELENGTH, nm
Fig. 5.142. CL spectra of a low-nitrogen natural diamond implanted with 350 keV Ag+ ions at a dose of 1014 cm-2 and subsequently annealed at 1400°C. The general spectrum of the 399 nm Ag-related center and the structure of its ZPL are presented. The line at 401.2 nm is the first vibrational replica of the main ZPL due to interaction with the quasilocal vibration localizing at two Ag atoms. The lines at 414.8, 417.5 and 420.5 nm are possibly replicas due to interaction with short-wavelength optical phonons of energy 122, 142 and 162 meV
5.1 Optical Bands 327
3.150 eV (393.6 nm); A, PC, no luminescence; ZPL; the ND1 center (or the R10 center (Clark et al. 1956a)). The ND1 center was reported For the first time by Dyer and Du Preez (1965b). The ND1 center is the common radiation center of diamond. It is observed in type Ia, Ib, IIa diamonds (possibly in some type IIb diamonds (Kurdumov et al. 1994)) as a result of radiation damage at room temperature. The ND1 center anneals out at temperatures above 500°C. The ND1 center is very weak (in contrast to the GR1 center) in type IIa diamonds (Davies 1994b). The GR1 and ND1 centers are of comparable intensity in diamonds with aggregated nitrogen (Davies 1994b). In diamonds containing the C centers of nitrogen in concentrations above a few ppm the ND1 center dominates over the GR1 center (Lawson et al. 1998). The ND1 center is strongly activated by heating at temperatures above 300°C. In photoconductivity the ND1 center acts as a donor (Davies et al. 1992). However ptype PC related to the ND1 center was reported by Tatarinov (1986) implying an acceptor nature of the center. There is photochromic coupling between the ND1 and the GR1 centers: light illumination in the absorption range of the ND1 center reduces its strength and increases the intensity of the GR1 center. After illumination the intensities of both centers recover after several hours in the dark or after heating to 500°C (Dyer and Du Preez 1965b). The ND1 center intensity is proportional to the V+ EPR center (positively charged vacancy, see the GR1 center) (Sobolev and Aksenov 1979b; Sobolev et al. 1975a). The ND1 center is paramagnetic: the S1 center is its ESR analog (Baldwin 1963; Isoya et al. 1992). The oscillator strength of the ND1 center relative to that of the GR1 transition is fND1/fGR1 = 4.0 (Davies et al. 1992). The ND1 center shows a linear Stark effect implying no inversion symmetry of the corresponding defect (Kaplyanskii et al. 1971). The predominant electron-phonon interaction at the ND1 center occurs with vibrations of energy 76 to 80 meV. The strength of the electron-phonon coupling is moderate: S = 3.18 (Davies 1977c). The Jahn-Teller coupling in the excited state of the ND1 center (primarily with t-symmetry modes, EJT/hω = 0.477) is similar to that of the GR1 center (Lowther 1978). Isotope shift of the ND1 center ZPL is +4 meV (Davies and Collins 1999). The ND1 center is attributed to a negatively charged single vacancy V- (Davies 1977c; Walker 1979; Davies 1994a; Davies et al. 1992; Clark et al. 1979a; Davies 1977b (the V- model was proposed for the first time); Clark and Mitchell 1971a; Douglas and Runciman 1977a; Davies and Lightowlers 1970; Dyer and Du Preez 1965b; Zaitsev and Tkachev V. 1988a; Sobolev 1972; Sobolev 1975). The ND1 center is an electronic transition between the 4A2 (S = 3/2, ground) to 4T1 (excited) levels at a defect of Td symmetry. Theoretical calculations give the same multiplets for the ground and excited states and satisfactorily predict the energy of the electronic transition (Breuer ref Briddon 1995). The g-value of the excited state is positive (Douglas and Runciman 1977a). The ground state is 2.6 eV below the conduction band (Yelisseyev 1977). The migration energy of V- is predicted to be 3.4 eV (Breuer ref Briddon 1995). The concentration of negatively charged vacancies can be evaluated from the integrated intensity of the ND1 ZPL in absorption at LNT as: NV-[cm-3] = 2.6×1015 µND1 ZPL[meV cm-1] (Lawson et al. 1998). A corrected relation is: NV-[cm-3] = 2.1×1015 µND1 ZPL[meV cm-1] (Twitchen et
328
5 Optical Electronic Transitions
al. 1999). Based on the absorption intensity of the ND1 center it has been found that the vacancy creation rate in diamond by 1.9 MeV electron irradiation is 0.5 vacancies per electron. Based on the p-type PC the ND1 center was also ascribed to a positively charged vacancy V+ with the ground level lying within the valence band (Tatarinov 1986). 3.150 eV (393.5 nm); A, CL; ZPL; a center observed in natural brown diamonds exhibiting yellow PL. The center is created in nitrogen-containing diamonds by electron irradiation (Bienemann-Kuespert et al. 1967). The decay time of the center in CL at LNT is 45 ns. The 3.150 eV center shows behavior similar to that of the 3.204 and 3.224 eV centers. Electronic transition of the 3.150 eV center interacts predominantly with 54 meV vibrations. The 3.150 eV center is not related to the ND1 center (Field 1992; Jorge et al. 1983) (Fig. 5.137). 3.170 eV (391.0 nm); CL; ZPL; a center observed in natural brown diamonds exhibiting yellow PL (Field 1992; Jorge et al. 1983). 3.18 eV (390 nm); A; a line observed in natural diamonds of pink color (Collins 1997).
3.170 eV (391 nm); CL; ZPL; a center observed in natural type IIb diamonds and boron-doped CVD diamond films irradiated with electrons and neutrons. The center possesses a very complicated ZPL revealing at least seven components (the most intense line is labeled the α-line) at temperatures below 60 K, and at least four components at temperatures above 60 K. The ZPL components shift strongly towards higher energies with temperature increase. The spectral positions of the main components at different temperatures are: the α-line is at 3.1565 eV (4 K), 3.157 eV (20 K), 3.171 eV (60 K), 3.184 eV (90 K); the β-line is at 3.132 (4 K), 3.135 eV (20 K), 3.149 eV (60 K); the γ-line is at 3.20 eV (90 K), the δ-line is at 3.225 eV (90 K). The electron energy scheme of the center is thought to comprise a two level ground state Ig (symmetry 4A2) and IIg and a two-level excited state Ie (symmetry 4T1) and IIe (symmetry 4A2) giving rise to the following transitions: α - Ie→Ig; β - Ie→IIg; γ - IIe→IIg; δ - IIe→Ig (Mazzaschi et al. 1980; Popovici et al. 1996). 3.188 eV (389 nm); A, CL, PL; ZPL; the 3.188 eV center or the 389 nm center (Fig. 5.143). The 3.188 eV center is the radiation damage product characteristic of all types of diamonds. It is also the common feature of CVD diamond films. Absorption of the 3.188 eV center is very weak and is observed only in type Ib diamonds. The 3.188 eV center is readily formed in any diamonds by N+ ion implantation (subsequent annealing is not required, the ion doses can be as low as 109 cm-2 for the center to be detectable in CL). The center is produced in CVD diamond films by ion implantation with any species (including N+) and subsequent plasma hydrogenation (the hydrogenation can be performed at 900°C) (Mori et al. 1992b; Yagyu et al. 1995). The 3.188 eV center reveals the highest intensity in type Ib diamonds. In
5.1 Optical Bands 329
contrast the center is weak in low-nitrogen type IIa diamonds. Usually the center is much more intense in synthetic HPHT diamonds than in natural type Ia diamonds irradiated at equal conditions. The center readily forms in {111} and {100} growth sectors of synthetic nitrogen-doped diamonds. There is, however, a preferential accumulation of the center in {111} growth sectors of single crystallites of intentionally undoped PCCVD diamond films (Yokota et al. 1992; Yokota et al. 1990b) (Fig. 5.31).
389
360
380
400
420
440
460
480
WAVELENGTH, nm
Fig. 5.143. Comparison of LNT and RT (bold line) CL spectra of the 389 nm center taken at identical conditions from a CVD diamond film as-irradiated with 164.4 MeV 12C+5 ions at a dose of 3.5×1015 cm-2. The 389 nm center is the only feature of the spectrum. Both spectra are plotted against the same intensity scale (Zaitsev et al. 1996b)
The 3.188 eV center is activated in pristine and irradiated diamonds with an electron beam of a few keV energy at LNT and RT; this effect is especially pronounced in diamonds irradiated with ions and neutrons. The rate of electron activation of the 3.188 eV center correlates linearly with concentration of the B center. Qualitatively the action of the electron activation corresponds to thermal annealing at a temperature of 500°C (Robins et al. 1989a; Zaitsev et al. 1980; Malogolovets 1979; Malogolovets 1986b; Gippius et al. 1981). The annealing behavior of the 3.188 eV center may strongly depend on the type of diamond and irradiation conditions, indicating that in some cases the creation and destruction of the center is completely determined by the surrounding defects (Fig. 5.144). The intensity of the 3.188 eV center strongly increases in electron, neutron and ion irradiated diamonds after 600 to 800°C annealing. The CL intensity of the center in some type IIa diamonds can increase by two orders of magnitude after implantation of transition metals (e.g. Ni) at moderate doses (about 1014 cm-2) and subsequent annealing at 1400°C. The 389 nm center is readily created in HPHT synthetic diamonds during growth at a temperature 1500°C. However the center
330
5 Optical Electronic Transitions
does not survive in synthetic diamonds after annealing at 1800°C (Kanda and Jia 2000).
4
CL INTENSITY, arb. units
10
3
10
2
10
1
10
0
10
0
200
400
600
800
1000
1200
1400
1600
ANNEALING TEMPERATURE, °C
Fig. 5.144. Annealing curves of the 3.188 eV nitrogen-related center in: (l) - an as-grown CVD diamond film; (o) - a very low-nitrogen type IIa natural diamond implanted with 300 keV N+ ion at a dose of 1014 cm-2; (n) – a moderate nitrogen type Ia natural diamond implanted with 350 keV N+ ion at a dose of 1014 cm-2; (u) – a low-nitrogen type IIa natural diamond implanted with 335 MeV N ions; (m) – a low-nitrogen type IIa natural diamond implanted with 80 keV N+ ions at a dose of 1016 cm-2
The sharp structure between 2.99 and 3.02 eV in the vibronic side-band of the center is associated with electronic transitions involving local vibrational modes. The center shows predominant interaction with 75 meV acoustic phonons and the 165 meV zone center LO phonons. The feature at 2.863 eV is associated with 2LO electron-phonon transitions (Fig. 5.145). The strength of the electron-phonon coupling at the center in CL is relatively weak: S = 1.82 (Davies 1977c; Collins and Davies 1988b). All the features of the vibrational side-band occur due to carbon vibrations except the mode of energy 178.8 meV attributed to C-N vibration (Collins and Woods 1987b; Collins and Lawson 1989a; Davies 1994a; Collins and Davies 1988b). There is a line at 2.803 eV, which is believed to be a feature related to the 3.188 eV center (Kurdumov et al. 1994). The spectral shape of the vibrational sideband of the center created by neutron irradiation and subsequent annealing is suspected to be very similar to the A-band (Yokota et al. 1992). The spectral position of ZPL of the 3.188 eV center in PL in some CVD diamond films can be shifted to 3.20 eV (Dischler et al. 1994). In 13C diamonds the ZPL of the center shifts by +3.3 meV (Collins et al. 1988c; Davies 1994a; Collins and Davies 1988b; Davies and Collins 1999). The 3.188 eV center is a temperature hard center: its ZPL exhibits relatively weak broadening and shift with temperature increase from LNT to RT. In high-nitrogen type Ib diamonds irradiated with high-
5.1 Optical Bands 331
energy ions the ZPL of the center has very pronounced nonhomogeneous broadening, which is almost independent of temperature (Fig. 5.146).
161 meV 138 meV 77 meV
a
180 meV 161 meV
TR12
138 meV 77 meV
4415
77 meV 161 meV 181 meV
179 meV
3800
4000
4200
4400
4600
4800
WAVELENGTH, A°
b
161
3.188 eV center
165 157
77 72
152 146
58
98
138 115 129 190.2 124
0
50
100
178.5
150
200
PHONON ENERGY, meV
Fig. 5.145. (a) CL spectrum (at LNT) taken from an intentionally undoped CVD diamond film irradiated with 164.4 MeV 12C+5 ions at a dose of 3.5×1015 cm-2 (Zaitsev et al. 1996b). The vibronic side-band exhibits all the features characteristic of the phonon density of states of the diamond lattice. The second and even third orders of vibrational replicas are seen. The bands at 4195 ? and 4320 ? are vibronic replicas of the 4094 ? band. The 4369 ? band is the second replica of the 4135 ? band. (b) First-order vibrational spectrum of the 3.188 eV center. The quasilocal and local vibrations involving nitrogen atom are marked with bold figures. All the marked intrinsic vibrational features can be recognized as the maxima in the phonon density of states of diamond (see Fig. 3.1 and Fig. 3.2)
The CL decay time of the 3.188 eV center in boron-doped synthetic diamonds at a temperature of 90 K is probably 0.5 µs (Bezrukov et al. 1979).
332
5 Optical Electronic Transitions
10 9 8
WIDTH, SHIFT, meV
7 6 5 4 3 2 1 0 -1 -2 -3 0
50
100
150
200
250
300
350
TEMPERATURE, K
Fig. 5.146. Temperature broadening (m, l) and spectral shift (n) of ZPL of the 389 nm center in diamonds irradiated with 82 MeV C ions: in a natural type IIa diamond subsequently annealed at 1000°C (l, n); in a synthetic type Ib diamond subsequently annealed at 1600°C (m) (Varichenko 1986)
Possible models of the 3.188 eV center are: (i) a defect containing interstitial nitrogen atoms (Zaitsev et al. 1982a; Collins et al. 1989c); (ii) a defect containing a substitutional nitrogen atom bound to interstitial carbon atoms (Collins and Woods 1987b; Collins et al. 1993b), in particular, it might be a single substitutional nitrogen atom bound to the nearest carbon atom in the tetrahedral interstitial position (Zaitsev 2000) (Fig. 5.147); (iii) a single substitutional nitrogen shifted along the axis forming an elongated C−N pseudomolecule (Dischler et al. 1994; Field 1992; Collins and Woods 1987b; Collins et al. 1988c; Collins et al. 1993b; Briddon and Jones 1993; Collins and Davies 1988b). 3.204 eV (386.9 nm); CL; ZPL; a center observed in natural brown diamonds exhibiting yellow PL. The center shows behavior similar to that of the 3.105 and 3.224 eV centers. The predominant electron-phonon interaction at the center occurs with vibrations of energy 54 meV (Field 1992; Jorge et al. 1983; Mohammed et al. 1982a). 3.216 eV (385.4 nm); CL; ZPL; a center observed in natural brown diamonds exhibiting yellow PL when excited with 365 nm light (Field 1992; Jorge et al. 1983; Mohammed et al. 1982a). 3.224 eV (384.5 nm); A, CL; ZPL; a center observed in natural brown diamonds exhibiting yellow PL when excited with 365 nm light. The center shows a behavior similar to that of the 3.150 and 3.204 eV lines. The predominant electron-phonon interaction at the center occurs with vibrations of energy 54 meV (Field 1992; Jorge et al. 1983; Mohammed et al. 1982a) (Fig. 5.137).
5.1 Optical Bands 333
389 nm center f
e
h d i
b
a
c
g k
substitutional carbon, C interstitial carbon, C (I) substitutional nitrogen, N
Fig. 5.147. Atomic model of the 389 nm nitrogen-related center: a substitutional nitrogen atom bound to a carbon atom in the nearest tetrahedral interstitial position. The bold arrows show the atomic movements at the vibrations: abc - transverse optical (polarized normal to (110) plane) Σ2 phonon at the K point; ed and ik - transverse acoustic Λ3 phonon at the L point; h – local vibration of the nitrogen atom (178.5 meV); edg - quasilocal vibration of the C-N-C(I) fragment (suggested energy of 58 meV)
3.26 eV (380 nm); A; a narrow line observed in high-hydrogen gray-violet diamonds (Fritsch et al. 1991a). 3.26 eV (380 nm at RT); EL; a broad band with FWHM ∼ 0.1 eV observed in CVD diamond films. The feature is tentatively attributed to isolated interstitial nitrogen atoms localized at interfaces of the electroluminescence structures formed by amorphous diamond and undoped CVD diamond, or undoped CVD diamond and boron-doped CVD diamond. The band is thought to be 3p(1/2, 3/2)→3(1/2, 3/2) transitions at nitrogen atoms (this interpretation seems to be very doubtful*) (Zhang et al. 1996a). 3.270 eV (379.2 nm); A, PL excitation of the H3 center; the H10 center (H stands for irradiated and Heated); a narrow line observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). The feature is attributed to an electronic transition to an excited state of the H3 center (Collins 1983). 3.272 eV (378.8 nm); CL; ZPL; a center observed in some CVD diamond films. This center is also observed in ion-irradiated CVD diamond films (Field 1992; Collins et al. 1989c; Zaitsev et al. 1996b) (Fig. 5.145).
334
5 Optical Electronic Transitions
3.3 eV (380 nm); A; a broad band with FWHM 0.2 eV observed in type Ib diamonds. Absorption of the band is polarized with the E-vector parallel to the axis. The feature is attributed to electronic transitions from the ground to the first excited state of a trigonally distorted substitutional nitrogen donor (Koppitz et al. 1986). 3.311 eV (374.4 nm); A, PL excitation of the H3 center; the H11 center (H stands for irradiated and Heated); a narrow line observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). The feature is attributed to an electronic transition to an excited state of the H3 center (Collins 1983). 3.341 eV (371.1 nm); A, PL excitation of the 2.535 eV luminescence center; a center observed in type Ib synthetic diamonds grown by the temperature gradient method. The center is activated by high-temperature treatment at about 1700°C under stabilizing pressure (Yelisseyev and Nadolinny 1995a; Lawson and Kanda 1993b). The center is strong in nitrogen-poor sectors of synthetic diamonds (Yelisseyev et al. 1996). 3.343 eV (371.0 nm); A, PL excitation of the H3 center; the H12 center (H stands for irradiated and Heated); a narrow line observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). The feature is attributed to an electronic transition to an excited state of the H3 center (Collins 1983). 3.361 eV (368.9 nm); A, PL excitation of the H3 center; the H13 center (H stands for irradiated and Heated); a narrow line observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). The feature is attributed to an electronic transition to an excited state of the H3 center (Collins 1983). 3.380 eV (366.7 nm) A, PL excitation of the S2 and 2.535 eV luminescence centers; a narrow line observed in type Ib synthetic diamonds grown by the temperature gradient method and in common synthetic Ib diamonds heated at temperatures above 2000 K. It is possibly the 367.4 nm center observed in synthetic diamonds grown by the temperature gradient method at 1750°C (Antsygin et al. 1996). The center can be observed also in natural diamonds. In as-grown synthetic diamonds the center is often observed near seeds, inclusions and interfaces between {111} growth sectors. The center is strong in nitrogen-poor sectors of synthetic diamonds (Yelisseyev et al. 1996). The center interacts with quasilocal vibrations of energy 32±2 meV. The electron-phonon coupling at the center is very strong: S ∼ 5 to 10 (Field 1992; Collins and Stanley 1985b; Yelisseyev et al. 1992b; Yelisseyev and Nadolinny 1995a). The 3.380 eV center possibly corresponds to the NE2 paramagnetic center related to the Ni+ ion. The concentration of the center can be measured using the following relation NNE2[cm-3] = 0.4×1017 µ366.7 (Yelisseyev et al. 1996).
5.1 Optical Bands 335
3.40 eV (365 nm); A; a line observed in some synthetic diamonds exhibiting an absorption continuum from 300 to 200 nm (Klimenkova et al. 1975c). 3.403 eV (364.3 nm); A, PL excitation of the H3 center; the H14 center (H stands for irradiated and Heated); a narrow line observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). The feature is interpreted as an electronic transition to an excited state of the H3 center (Collins 1983). 3.412 eV (363.4 nm); A, PL excitation of the S2 and 2.535 eV luminescence centers; a narrow line observed in natural and in type Ib synthetic diamonds grown by the temperature gradient method and in common synthetic diamonds after hightemperature treatment at a temperature of 1700°C under stabilizing pressure. The center can be observed in as-grown samples near seeds, inclusions and interfaces between {111} growth sectors (Collins and Stanley 1985b; Yelisseyev et al. 1992b; Yelisseyev and Nadolinny 1995a; Antsygin et al. 1996). The feature may be particularly strong in nitrogen-poor sectors of synthetic diamonds (Yelisseyev et al. 1996). 3.443 eV (360.1 nm); A, PL excitation of the S2 and 2.535 eV luminescence centers; a narrow line observed in natural and in type Ib synthetic diamonds grown by the temperature gradient method and in synthetic diamonds after hightemperature treatment at a temperature of 1700°C under stabilizing pressure. The center can be observed in as-grown samples near seeds, inclusions and interfaces between the {111} growth sectors (Collins and Stanley 1985b; Yelisseyev et al. 1992b; Yelisseyev and Nadolinny 1995a; Antsygin et al. 1996). The feature may be particularly strong in low-nitrogen sectors of synthetic diamonds (Yelisseyev et al. 1996). 3.45 eV (359 nm); CL; ZPL; a center observed in boron-doped CVD diamond films irradiated with neutrons. The center anneals out at temperatures below 1000°C (Popovici et al. 1996). 3.462 eV (358.0 nm); CL; a narrow line observed in CVD diamond films after annealing at temperatures of 1200°C (Ruan et al. 1991b). 3.463 eV (358.0 nm); A, PL excitation of the H3 center; the H15 center (H stands for irradiated and Heated); a narrow line observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). The feature is attributed to an electronic transition to an excited state of the H3 center (Collins 1983). 3.47 eV (357.2 nm); CL; a band of width 0.35 to 0.6 eV produced in type IIb diamonds by ion implantation at low temperatures and subsequent rapid annealing at a temperature of 1200°C. The band is readily generated by boron ion implantation. It is also observed in as-grown boron-doped synthetic HPHT and in boron-doped and
336
5 Optical Electronic Transitions
undoped CVD diamond films (Graham et al. 1991b). The band has a low intensity in type IIa diamonds. The spectral position of the band may change from 3.5 to 3.35 eV (Lawson et al. 1994c; Buhaenko et al. 1994; Graham et al. 1991b; Lawson et al. 1995). The band correlates in intensity with the 4.6 eV band. Possible origin of the 3.47 eV band is uncompensated boron acceptors (Prins 1994b; Prins 1994c; Prins 1994d).
3.474 eV (356.9 nm) A, PL excitation of the S2 and 2.535 eV luminescence centers; a narrow line observed in natural and in type Ib synthetic diamonds grown by the temperature gradient method and in common synthetic diamonds after hightemperature treatment at a temperature of 1700°C under stabilizing pressure. Possibly this very center was observed in synthetic diamonds grown by the temperature gradient method at 1750°C as the 357.1 nm center (Antsygin et al. 1996). In as-grown samples the 3.474 eV center can be observed near seeds, inclusions and interfaces between {111} growth sectors (Collins and Stanley 1985b; Yelisseyev et al. 1992b; Yelisseyev and Nadolinny 1995a). The center can be particularly strong in nitrogen-poor sectors of synthetic diamonds (Yelisseyev et al. 1996). 3.487 eV (355.5 nm); CL; ZPL; a weak line observed in some PCCVD diamond films (Melnikov et al. 1996). 3.502 eV (354.0 nm); A, PL excitation of the 2.535 eV luminescence center; a narrow line observed in type Ib nickel-containing synthetic diamonds grown by the temperature gradient method. The feature is activated after high-temperature treatment at a temperature of 1700°C under stabilizing (Yelisseyev and Nadolinny 1995a). 3.504 eV (353.8 nm); A, PL excitation of the H3 center; the H16 center (H stands for irradiated and Heated); a narrow line observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). The feature is attributed to an electronic transition to an excited state of the H3 center (Collins 1983). 3.54 eV (350 nm); A; a line observed in some synthetic diamonds exhibiting an absorption continuum from 300 to 200 nm (Klimenkova et al. 1975c). 3.543 eV (349.9 nm); A, PL excitation of the H3 center; the H17 center (H stands for irradiated and Heated); a narrow line observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al. 1956c). The feature is attributed to an electronic transition to an excited state of the H3 center (Collins 1983). 3.560 eV (348.2 nm); A, PL excitation of the H3 center; the H18 center (H stands for irradiated and Heated); a narrow line observed in type I diamonds after irradiation and subsequent annealing at temperatures above 500°C (Clark et al.
5.1 Optical Bands 337
1956c). The feature is an electronic transition to an excited state of the H3 center (Collins 1983). 3.560 eV (348.2 nm); A, PL excitation of the S2 and 2.535 eV luminescence centers; a narrow line observed in type Ib nickel-containing synthetic diamonds grown by the temperature gradient method after high-temperature treatment at a temperature of 1700°C under stabilizing pressure (Collins and Stanley 1985b; Yelisseyev and Nadolinny 1995a; Antsygin et al. 1996). 3.57 eV (347 nm); CL; a narrow line appearing in CVD diamond films after annealing at 1200°C (Ruan et al. 1991b). 3.570 eV (347.2 nm); A, PL excitation of the S2 center; a narrow line observed in type Ib synthetic diamonds grown by the temperature gradient method and in common synthetic diamonds after high-temperature treatment at a temperature of 1700°C under stabilizing pressure (Collins and Stanley 1985b; Yelisseyev et al. 1992b; Yelisseyev and Nadolinny 1995a). 3.593 eV (345.0 nm); A; ZPL; a center observed in type I natural diamonds. The center interacts predominantly with vibrations of energy 80 meV. 3.6 eV (340 nm); A; a broad band of FWHM 0.35 eV appearing in nitrogencontaining synthetic diamonds after high PT treatment (~1900°C, 5.5 GPa) (Nadolinny and Yelisseyev 1993; Yelisseyev et al. 1992b; Yelisseyev and Nadolinny 1993). 3.603 eV (344.2 nm); A, PL excitation; the N4 center (N stands for Naturally occurring). The N4 center is a naturally occurring center in type I natural diamonds. It is attributed to a vibronic transition at the N3 center between the ground state and a nondegenerate excited state, where pure electronic excitations are forbidden. There is a line at 3.58 eV related to the N4 center. The N4 center predominantly interacts with vibrations of energy 79 meV. The N4 center is a π electronic transition oriented perpendicular to the N3 transition (Bokii et al. 1986; Davies 1994a; Clark et al. 1956a; Clark and Norris 1970; Davies et al. 1978; Clark and Norris 1971b; Sobolev and Yurjeva 1990). 3.607 eV (343.6 nm) or 3.612 eV; A, PL excitation of the S2 center; a narrow line observed in type Ib synthetic diamonds grown by the temperature gradient method and in common synthetic diamonds after high-temperature treatment at a temperature of 1700°C (Collins and Stanley 1985b; Yelisseyev et al. 1992b; Yelisseyev and Nadolinny 1995a; Antsygin et al. 1996). 3.65 eV (340 nm); PL excitation spectra of the S2 and S3 centers; the β-band; a broad band spreading from 310 to 370 nm. The band is assisted with ZPLs at 367.0, 363.6, 360.4 and 356.7 nm (Bokii et al. 1986).
338
5 Optical Electronic Transitions
3.65 eV (340 nm); EL; a band of FWHM 0.15 eV observed at RT in intentionally undoped CVD diamond films. There is an optimal strength of electrical field (about 104 V/cm) exciting the maximum intensity of the band (Manfredotti et al. 1995). Tentatively the band is attributed to electronic transitions from the conduction band to a dislocation band energy level, which localizes at about 1.76 eV above the valence band (*).
3.689 eV (336.1 nm) or 3.612 eV; A, PL excitation of the S2 center; a narrow line observed in type Ib synthetic diamonds grown by the temperature gradient method. The feature is activated after high-temperature high-pressure treatment at about 1700°C (Yelisseyev and Nadolinny 1995a). 3.75 eV (330 nm); a very broad band of FWHM 0.7 eV observed in CVD diamond films. The band is believed to appear due to oxygen diffusion into the superficial diamond layer after treatment in saturated CrO3+H2SO4 solution at 200°C followed by H2-plasma treatment. The feature is attributed to oxygen-related defects (Mori et al. 1992a). 3.762 eV (329.6 nm); A; ZPL; the N5 center (N stands for Naturally occurring); a naturally occurring center in type IaA diamonds. The N5 center is an electronic transition between A1 and A2 states at a defect of trigonal symmetry (Walker 1979). The center interacts with vibrations of energies 113 and 159 meV. It is attributed to internal transitions at the A-aggregates of nitrogen (Davies 1994a; Clark et al. 1956a; Davies 1976). 3.81 eV (325 nm); A; a line observed in some synthetic diamonds exhibiting an absorption continuum from 200 to 300 nm (Klimenkova et al. 1975c). 3.85 eV (322 nm); EL; a band of FWHM 0.15 eV observed at RT in intentionally undoped CVD diamond films. There is an optimal strength of electrical field (about 104 V/cm) exciting the maximum intensity of the band (Manfredotti et al. 1995). 3.853 eV (321.7 nm); CL; a sharp line observed in some CVD diamond films (Field 1992). 3.877 eV (319.7 nm); A; a narrow line observed in some natural diamonds. This feature does not participate in the excitation process of the S2 center (Clark et al. 1956a; Robertson et al. 1934; Ilyin et al. 1971b). 3.9 eV (320 nm); A; a broad band of FWHM 0.25 eV observed in type Ib diamonds. Absorption of the band is polarized with the E-vector being perpendicular to the axis. The feature is attributed to electronic transitions from the ground to the first excited state of a trigonally distorted substitutional nitrogen donor (Koppitz et al. 1986).
5.1 Optical Bands 339
3.901 eV (317.7 nm); A; ZPL; a center observed in type IaA diamonds. This is a naturally occurring center. The center is attributed to an electronic transition between A and E states at a defect of trigonal symmetry (Walker 1979). The center interacts predominantly with vibrations of energy 113 meV. It does not participate in excitation of the S2 center. The 3.901 eV center is interpreted as an internal transition at the A-aggregate of nitrogen (Davies 1994a; Clark et al. 1956a; Davies 1976; Ilyin et al. 1971b). 3.928 eV (315.6 nm); A; ZPL; the N6 center (N stands for Naturally occurring); a naturally occurring center in type IaA diamonds. This is an electronic transition from the A to E state at a defect of trigonal symmetry (Walker 1979). The center interacts predominantly with vibrations of energies 113 and 159 meV. The vibronic transition with quantum energy 4.042 eV (306.5 nm) resulting from coupling with the 113 meV vibrations is labeled the N7 center (Bokii et al. 1986; Davies 1977c; Walker 1979). The N6 center is attributed to an internal transitions at the A-aggregate of nitrogen (Davies 1994a; Clark et al. 1956a; Davies 1976). The concentration of the A-aggregates of nitrogen can be evaluated from the absorption strength of the N7 center using the relation: NA = 11.6×1018 µ306.5. The N6 center does not participate in excitation of the S2 center (Bokii et al. 1986; Davies 1977c; Kaiser and Bond 1959; Kurdumov et al. 1994; Ilyin et al. 1971b). 3.988 eV (310.9 nm); A; ZPL; the R11 center; a center observed in type IIa diamonds. This is a radiation-induced center. The lines at 4.04, 4.11, 4.33, 4.63 eV possibly relate to the R11 center (Davies 1977c; Clark et al. 1956a; Allers et al. 1998). The R11 center anneals out completely during the rapid annealing stage of the GR1 center (considerable annealing occurs at 400°C). Annealing of the center is described by first-order kinetics. The activation energy of the annealing is 1.74 eV. The R11 center is attributed to an interstitial-related defect (Allers et al. 1998). The R11 center is possibly a transition at excited states of the 1.858 eV center (Allers et al. 1998) (Fig. 5.148). 4 eV (310 nm); CL, EL, IL; the L-band; a band of FWHM ~ 0.7 eV produced in natural type IIa diamonds at LNT by C+ ion implantation and subsequent rapid annealing at a temperature of 1200°C. The band is observed in emission of p-n diodes made on type IIb natural diamonds by C+ or P+ ion implantation and subsequent annealing at temperatures above 1100°C (annealing at temperatures above 1300°C quenches the band) (Naidoo and Prins 1998). The band is observed also in CVD diamond films. The band anneals out at 1600°C. The emission intensity of the L-band rapidly falls with temperature increase to 335 K. The L-band is absent from the spectra of the samples containing boron acceptors. The band is quenched by B+ and P+ ion implantation (Prins 1995). It is believed that the L-band relates to the A-band. The L-band is assigned tentatively to electron transitions from the conduction band to deep-lying donor centers related to vacancy-type defects (Prins 1994a; Prins 1993b; Prins 1995; Prins 1997; Taguchi et al. 1987) (Fig. 5.102).
340
5 Optical Electronic Transitions
4.022 eV (308.2 nm); A; a narrow line observed in some natural diamonds. The center does not participate in excitation of the S2 center (Clark et al. 1956a; Robertson et al. 1934; Ilyin et al. 1971b).
77K
R11
5RL
3.8
4.0
4.2
4.4
4.6
4.8
QUANTUM ENERGY, eV
Fig. 5.148. Absorption spectrum of a CVD diamond film as-irradiated with 2 MeV electrons at a dose of 1018 cm-2. The spectrum was recorded at LNT. The "ultraviolet continuum" has been subtracted from the spectrum (Allers et al. 1998)
4.042 eV (306.5 nm); the N7 center; see 3.928 eV (the N6 center). 4.05 and 4.12 eV (306 and 301 nm); A; two of the most intense features of a complicated band in the spectral region from 290 to 310 nm observed in synthetic diamonds grown by the temperature gradient method. The absorption strength of the band is 5 to 8 cm-1. This band differs by its spectral structure from the band reported by Dyer et al. (1965a). The feature is tentatively attributed to an intrinsic nitrogenfree defect (Vins 1988). 4.059 eV (305.4 nm); A; ZPL; a center observed in type Ib natural and synthetic diamonds. This is a naturally occurring center. The center interacts predominantly with vibrations of energies 61 and 120 meV. The electron-phonon coupling is relatively strong: S = 6. The center is an electronic transition at a defect of trigonal symmetry with A1 ground state. The center is attributed to single substitutional nitrogen atoms. The center does not participate in excitation of the S2 center (Field 1992; Nazare and Neves 1987; Davies 1994a; Ilyin et al. 1971b). 4.088 eV (303.2 nm); A; a narrow line observed in some natural diamonds. The center does not participate in excitation of the S2 center (Clark et al. 1956a; Robertson et al. 1934; Ilyin et al. 1971b).
5.1 Optical Bands 341
4.13 eV (300 nm); IL; a sharp line attributed to the atomic emission line of C+ ions used for the IL excitation (Taguchi et al. 1987). 4.13 eV (300 nm); a very broad band extending from 230 to 450 nm. This band is the typical absorption feature of ion-implanted diamonds. The band is not produced at temperatures above 650°C. An enhanced production rate of the band is observed at 200°C by N+ ion irradiation at a dose of 1014 cm-2 (probably due to incorporation of substitutional nitrogen in the diamond lattice) (Anderson G. et al. 1993). This band is possibly a strongly distorted ND1 center (3.149 eV) (*). 4.137, 4.27, 4.316, 4.355 eV (299.7, 290, 287.2, 284.7 nm); A; narrow lines naturally occurring in type Ib diamonds attributed to electronic transitions at isolated substitutional nitrogen atoms (Davies 1977c). 4.184 eV (296.2 nm); A; the first line of three naturally occurring ZPLs (the other two are at 4.191 and 4.197 eV) observed in natural type Ib diamonds. The lines are not observed in synthetic type Ib diamonds. All these centers interact with vibrations of energy 36 meV. The features are attributed to isolated substitutional nitrogen atoms (Davies 1977c; Field 1992; Collins et al. 1990a; Nazare and Neves 1987; Davies 1994a). 4.19 eV (296 nm); A; the N8 center (N for Naturally occurring); a narrow line naturally occurring in type IaA diamonds. The feature is attributed to an intracenter electronic transition at the A-aggregate of nitrogen (Bokii et al. 1986; Davies 1977c; Walker J. 1979; Clark et al. 1956a). 4.301 eV (288.2 nm); IL; a sharp (like in atomic spectra) line from Si + ions observed in spectra of natural diamonds and CVD diamond films (Taguchi et al. 1987). 4.328 eV (286.4 nm); A; ZPL; a center observed in type Ib diamonds after radiation damage and subsequent annealing at 800°C. The intensity of the center and its stress splitting parameters correlate with those of the 1.945 eV center. The 4.328 eV center is an electronic A−E transition at a defect of trigonal symmetry (Field 1992). 4.374 eV (283.4 nm); A, PL excitation of a broad band in the green-yellow spectral region; a weak narrow peak tentatively attributed to the B' center (Bokii et al. 1986; Sobolev and Dubov 1979a; Sobolev et al. 1968c). 4.38 eV (283 nm); A; a line observed in some neutron-irradiated type I diamonds (Bienemann-Kuespert et al. 1967). 4.43 eV (280 nm); A, PL; a weak band observed in some nitrogen-containing natural diamonds. This feature possibly relates to the B' center (Bokii et al. 1986; Davies 1994a; Sobolev et al. 1968d).
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5 Optical Electronic Transitions
4.436 eV (279.5 nm); CL; ZPL; a radiation-induced center observed in type IIa and IIb diamonds. The center interacts predominantly with vibrations of energy 237 meV. The 4.436 eV center is not related to the 5RL center (Davies 1977c). 4.45 eV (279 nm); EL; a band with FWHM of 0.5 eV observed in emission spectra of p-n diodes made on natural type IIb diamonds by P+ ion implantation (n-type area) and subsequent annealing at temperatures above 1300°C (Naidoo and Prins 1998). A tentative model of the band is recombination on D-A pairs (phosphorousrelated donors with ED ~ Eg – 0.5 eV and boron-related acceptors) (*) (Fig. 5.102). 4.468 eV (277.4 nm); A; a naturally occurring center observed in some type Ib diamonds (Field 1992; Nazare and Neves 1987). 4.5 eV (270 nm); CL; a band with FWHM of 0.4 eV observed in synthetic HPHT and CVD boron-doped diamonds. The band is produced in type IIb diamonds by cold ion implantation and subsequent rapid annealing at 1200°C. The band can be observed also in as-grown synthetic type IIb diamonds. The band has a weak intensity in type IIa diamonds. It is created by boron ion implantation. The spectral position of the band maximum may possibly vary from 4.4 to 4.6 eV (Sternschulte et al. 1996a). At temperatures of 15 K the band shows a poorly resolved fine structure at 4.35, 4.5 and 4.65 eV (Sternschulte et al. 1996a). The maximum intensity of the band is attained at a temperature of 130 K. The band is completely quenched at temperatures above 200 K. The band is strongly quenched by nitrogen defects and particularly by dispersed nitrogen atoms (Lawson et al. 1994a; Lawson et al. 1994c; Lawson et al. 1995). The 4.5 eV band is attributed to boron-related defects, in particular, to uncompensated boron acceptors (Prins 1994b; Prins 1994c; Prins 1994d) (Fig. 5.149). 4.56 eV (272 nm); A; a band in a spectral region from 265 to 276 nm observed in synthetic diamonds grown by the temperature gradient method. The absorption strength of the band may attain a value of 5 to 8 cm -1 The band differs by its spectral structure from the band reported by Dyer et al. (1965a). A very similar band is observed at RT in CL spectra of p-type synthetic diamonds (Kerimov et al. 1975). The feature is tentatively attributed to an intrinsic defect containing no nitrogen (Vins 1988). 4.567 eV (271.4 nm); ZPL; A; a naturally occurring center observed in type Ib natural and synthetic diamonds. The center interacts predominantly with vibrations of energies 59 meV and 106 meV. The center shows relatively weak electronphonon coupling: S = 1.8. The 4.567 eV center is an electronic transition at a defect of trigonal symmetry with the A1 level. The center is attributed to single substitutional nitrogen atoms (Davies 1977c; Field 1992; Nazare and Neves 1987). 4.582 eV (270.5 nm); A, CL; ZPL; the 5RL center; a common radiation damage center observed in type IIa, IIb and Ib diamonds of various origin. Two weak lines at 4.407 and 4.390 eV (spectral positions at 4 K) are also ZPLs of the 5RL center
5.1 Optical Bands 343
(Mazzaschi et al. 1980). The 5RL center is observed in absorption only in type II diamonds. The 5RL center is readily formed by irradiation in {100} growth sectors of synthetic diamonds (Yokota et al. 1992; Yokota et al. 1990b). The center can be observed in as-grown CVD diamond films. The presence of oxygen in the growth gas mixture suppresses the 5RL center in spectra of CVD diamond films (Partlow et al. 1990; Heiderhoff 1997). Luminescence of the center is observed only at low temperatures (Yokota et al. 1992). At RT the 5RL center almost vanishes. Annealing at a temperature of 500°C optimizes the absorption intensity of the 5RL center. In CVD diamond films the center gradually anneals within a temperature range from 800 to 1200°C (Ruan et al. 1991b; Field 1992). In CL the 5RL center interacts with local vibrational modes of energy 237 (the predominant mode; in 13C diamond the vibration energy is reduced down to 228.7 meV (Collins and Davies 1988b)), 193 and 175 meV (Fig. 5.150). In absorption the predominant interaction occurs with vibrations of energies 202 (the most intense) and 167 meV. The HuangRhys factor for the predominant CL mode in perfect diamonds is 1.6 (1.3 ± 0.2 at a temperature of 4K (Mazzaschi et al. 1980)). In PCCVD diamond films the HuangRys factor may attain a value of 2.5. In luminescence the 5RL center interacts also with a quasilocal vibration of energy 58 meV. The decay time of the 5RL center in CL at temperatures {100}>{113} (Davies 1994a). A view parallel to the direction shows within the {001} growth sectors of nitrogen-containing synthetic diamonds four-fold symmetrical sectored patterns of deeper yellow coloration pointing radially in the diagonal ±[110] and ±[1-10] directions ("Maltese Cross") (Field 1992). Intensively colored yellow diamonds with strong green luminescence to visible light (so-called "green transmitters") are rare in nature and very expensive. Treatedcolor "green transmitters" exhibit an intense H2 center (1.256 eV), whereas the natural ones rarely possess the H2 center. The "green transmitters" show a strong greenish-yellow afterglow for at least 30 s when illuminated in the UV range. Most the natural "green transmitters" are of type IaB (Buerki et al. 1999).
6.3
Green
The light apple-green color of nitrogen-containing diamonds can be caused by a few hour high-temperature treatment under pressure resulting in the formation of the A-aggregates of nitrogen (Nadolinny and Yelisseyev 1993). The yellow color of type Ib diamonds deepens to green at nitrogen concentration much above 1000 ppm (Nassau 1993). The greenish color of synthetic Ib diamonds after treatment at temperatures above 2000 K is a result of absorption of the 1.562 eV center (Yelisseyev and Nadolinny 1995a; Yelisseyev et al. 1996). Some natural diamonds are coated with a green "skin" resulting from α-particle damage producing GR1 absorption (Field 1992; Welbourn 1994) (Fig. 6.4). Most green natural diamonds owe their color to thin (about 20 µm thick) transparent green coats. Natural diamonds with green color of the whole body are extremely rare (Field 1992). Often green coated natural diamonds have shells which are translucent but not transparent. These coats have a multilayer structure in the submicrometer scale, changing in color and opacity from layer to layer (Field 1992). These shells can also be of green-gray color. Natural colorless diamonds get a green or grayish-green color after irradiation with light ions like protons, deutrons, α-particles, neutrons (the most effective are slow neutrons) or electrons of energy above 1 MeV. High-energy ions are very
382
6 Coloration of Diamond
effective in dark green coloration of diamond. A high-dose irradiation gives a nontransparent dark green color (Zaitsev and Varichenko 1988b; BienemannKuespert et al. 1967). The green color produced by light ions is more brilliant than that after neutron irradiation (Bienemann-Kuespert et al. 1967; *) (Fig. 6.4b). 10
ABSORPTION COEFFICIENT, cm
-1
a 8
visible light
6
4
N3
2 H3 0 200
300
400
500
GR1
600
700
800
WAVELENGTH, nm
100
b ABSORPTION, %
90 80 70 60 50 40 300
400
500
600
700
800
900
WAVELENGTH, nm
Fig. 6.4. Absorption spectra of green diamonds: (a) naturally irradiated green-skinned diamond of type IaA (Welbourn 1994); (b) natural type IIa diamond artificially colored dark green by 210 MeV Kr ion irradiation at a dose of 2×1013 cm-2 (courtesy of A. Melnikov).
The blue-green coloration of natural diamonds induced by 3 MeV electron irradiation converts to a yellow-green color after subsequent annealing at temperatures above 700°C (Bienemann-Kuespert et al. 1967).
6.3 Green 383
The light green color of some natural diamonds can be caused by neutron irradiation and subsequent annealing at 450°C (Bienemann-Kuespert et al. 1967). Nickel impurities can make nitrogen-containing synthetic diamonds (nitrogen content below 10 ppm) of green color. The color is provided by the 1.4 eV nickelrelated center, which gives a broad absorption band at around 1.8 eV. These diamonds exhibit in absorption also a abroad band with a maximum near to 1.4 eV (900 nm) and a complex multipeak structure in a spectral range from 600 to 800 nm (Collins et al. 1990c; Lawson et al. 1993c; Lawson and Kanda 1993a; Lawson and Kanda 1993b; Kanda and Watanabe 1998). The slight greenish color of synthetic diamonds annealed at about 1800°C can be caused by the NE centers giving rise to the characteristic absorption in the visible and UV regions (Yelisseyev and Nadolinny 1993). Some diamonds possess a bottle green color, which is not related to a particular optical center (Collins 1982a).
6.4
Blue
Natural blue diamonds are only of type IIb (Bienemann-Kuespert et al. 1967). The blue color of type IIb diamonds is due to absorption of boron acceptors (Collins and Spear 1982a; Lawson et al. 1993c). Boron is a very effective color center in diamond. Boron concentrations as low as 1 ppm cause a considerable blue coloration of diamond (Nassau 1993). Synthetic diamonds grown from Fe-Mg-Zn-C media exhibit a blue coloration due to low nitrogen content and occasional boron contamination (Bakul et al. 1975). The blue coloration of synthetic diamonds is growth-sector dependent. The {110} and {113} growth sectors of synthetic diamonds are colored blue with only a slight addition of boron, whereas it does not occur for {111} sectors. Large amounts of boron make the {111} sectors deeper blue than the {110} and {113} sectors (Davies 1994a). The dark-blue color of natural diamonds may also be caused by absorption of the GR1 center (Fritsch and Shigley 1991b). Blue color diamonds can be obtained artificially by 0.5 MeV electron irradiation, though this radiation-induced blue color is noticeably different from the boron-related color of type IIb natural diamonds. Nevertheless the coloration of natural blue diamonds can be improved by electron-, neutron- or γ-irradiation (Bienemann-Kuespert et al. 1967). Yellow diamonds can get a blue-green color after proton irradiation of a few MeV energy (Bienemann-Kuespert et al. 1967). Some diamonds irradiated with α-particles get a slight violet coloration after subsequent UV irradiation (Bienemann-Kuespert et al. 1967). A lilac coloration of synthetic type Ib diamonds with nitrogen content of 1019 cm-3 is observed after neutron irradiation at doses above 1018 cm-2 and subsequent annealing at 800°C. This color is a result of an absorption continuum spreading at wavelength below 700 nm (Vins 1988).
384
6 Coloration of Diamond
6.5
Brown
The brown coloration of natural diamonds is very common. There is a grayishbrown color due to absorption over the whole visible spectral range monotonically rising to shorter wavelengths. A slight absorption of this type is a common feature of type IIa diamonds (Clark et al. 1956a; Welbourn 1994; Prins 1993a) (Fig. 6.5). The brown coloration of many natural diamonds is caused by a broad absorption band in the blue spectral region (maximum at 480 nm). These diamonds show usually show a very complex luminescence (Pereira et al. 1986). The reddish-brown color of some natural diamonds originates from absorption over the whole visible spectral range and two additional broad bands located at about 550 and 380 nm (Welbourn 1994). The unevenness of color in many reddishbrown and purplish-brown natural diamonds is likely to be caused by plastic deformation and, consequently, by dislocations (Field 1992). Dislocations with a density of 1010 cm-2 cause the smoky-brown color of natural diamonds (Bokii et al. 1986; Samsonenko et al. 1974; Samsonenko et al. 1978). Nickel impurities makes low-nitrogen synthetic diamonds of a brown color. The deep brown color of synthetic diamonds grown by the temperature-gradient method using a nickel catalyst and nitrogen getter is caused by continuous absorption with a threshold at around 1.7 eV. These diamonds usually show no fine structure in the absorption continuum over the whole visible region, though there is an IR absorption band consisting of several peaks at around 1000 nm (Collins et al. 1990c; Lawson et al. 1993c; Yelisseyev et al. 1988; Kanda and Watanabe 1998) (Fig. 5.165). Poor-nitrogen synthetic diamonds containing nickel have a green color, which converts into a dull gray-brown color after annealing at temperatures above 1800°C. This change originates from a continuous absorption linearly increasing from about 1.4 eV to the UV region (Lawson and Kanda 1993a; Lawson and Kanda 1993b). Yellow high-nitrogen synthetic diamonds get their deep brown color after annealing at temperatures above 1500°C (Kanda and Watanabe 1998). A dark brown color can be obtained by heavy irradiation with light high-energy ions (Bienemann-Kuespert et al. 1967; *). The H3 and H4 centers may cause the brown color of diamonds subjected to irradiation and subsequent annealing at 800°C. The intensity of this coloration depends on the amount and type of irradiation (Collins 1997). The brownish color of some thick CVD diamond films is due to the true color of individual crystal grains, which is accounted for by finely dispersed nondiamond (probably carbonaceous) particles. These particle can be segregated at dislocations (Sussman et al. 1994a). The brownish color of some flame-grown epitaxial CVD diamond films is believed to be caused by light scattering on (001)-oriented hydrogen platelets (Janssen et al. 1992). The enhanced hydrogen concentration may stimulate the brownish color of some MPCVD diamond films (Jubber and Milne 1996). The
6.5 Brown 385
brownish color of some CVD diamond films can also be caused by absorption of intense 2.156 eV nitrogen-related center (Kania and Oelhafen 1995). Homoepitaxial CVD diamond films grown on diamond substrates can have a brown color with intensity increasing in the order of the substrate orientation: (100) ≤ (110) < (111) (Davies 1994a).
30 ABSORPTION COEFFICIENT, cm
-1
a 25
visible light
20 15 10 5 0 200
300
400
500
600
700
800
WAVELENGTH, nm 20
ABSORPTION COEFFICIENT, cm
-1
b visible light 15
10
5
0 200
300
400
500
600
700
800
WAVELENGTH, nm
Fig. 6.5. Absorption spectra of gray-brown natural diamonds of type IIa (a) and type IaA (b) (Welbourn 1994)
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6 Coloration of Diamond
6.6
White
White diamonds are usually hydrogen-rich diamonds. They are generally pure type IaB diamonds (Fritsch et al. 1991a). The white-yellowish color of CVD diamond films can be caused by absorption on the 2.156 eV center (Lopez-Rios 1996). Mildly graphitized microdiamonds have mat-white surfaces (Moore 1979). Synthetic diamonds grown from media with high Ti content often have a whitemilky color (Malogolovets et al. 1975b). Some natural gem-quality colorless diamonds have a bluish-white tint, which is actually caused by fluorescence induced by the UV component of daylight (Bakon and Szymanski 1993). Fluorescence can change the color of diamonds when observing at different illuminations (Wilks and Wilks 1991).
6.7
Dark and Black
The main reason for the black coloration of diamond is the high content of nondiamond inclusions and sp2 bonded microdefects. Some black natural diamonds have their color due to shiny black coats of graphitized material (Moore 1979). The dark color of poor-quality CVD diamond films is due to absorption by amorphous carbon (Robins et al. 1991b). Partial carbonization of diamonds resulting in their black color occurs after heavy irradiation with energetic particles. Ion and fast neutron irradiation at sufficient doses convert colorless diamonds into black ones (Bienemann-Kuespert et al. 1967; *). Neutron irradiation with a dose of 1018 cm-2 makes synthetic type Ib diamonds black and nontransparent (Vins et al. 1988).
6.8
Miscellaneous
Chameleon diamonds typically change their coloration reversibly from gray-green to bright-yellow when heated in a flame (Raal 1982; Raal 1969). These diamonds are usually hydrogen-rich diamonds of pure IaA type (Fritsch et al. 1991a). The coloration of some natural diamonds can be caused by inclusions containing iron, titanium, silicon and aluminum oxides, as well as pyrope, olivine and other minerals (Bakon and Szymanski 1993). Most colorless type Ia natural diamonds do not change their color after heating to 2550 K (Evans 1979). Irradiation-induced coloration can be changed from blue-green to gold-brown depending on the irradiation intensity. This effect is believed to be caused by heating
6.8 Miscellaneous 387
during irradiation (Bienemann-Kuespert et al. 1967). The irradiation-induced coloration is usually stable to temperatures as high as 700°C. In some cases, however, irradiation-induced coloration is changed after heating to temperatures as low as 300°C (e.g. electron irradiated diamonds or high-energy ion irradiated diamonds) (Bienemann-Kuespert et al. 1967; *). All yellow and brown type I natural diamonds contain Fe impurities in concentrations up to 24 ppm. Many of them contain additionally Ti and Cu impurities (Bienemann-Kuespert et al. 1967). Natural diamonds treated by irradiation and/or annealing with the aim of artificial coloration very often show absorption centers with ZPLs at 595, 1936 and 2024 nm. The absence of these centers is a good proof of the virgin state of natural diamonds (Gumilevskii 1952). Synthetic diamonds grown by the temperature gradient method in the Fe-Ni-C growth system change their color from green-yellow to orange-yellow and further to pale yellow with a temperature change from 1350 to 1600°C (Palyanov et al. 1991). The hydrogen-related absorption line at 3235 cm-1 is usually intense in natural diamonds of a gray to violet color (Fritsch et al. 1991a).
7
Physical Classification of Diamond
The physical classification of diamond is based on the optical absorption of nitrogen, boron and hydrogen-related defects and paramagnetic absorption of single substitutional nitrogen. The first classification of diamonds into types I and II based on the peculiarities of their optical absorption was given by Robertson et al. (1934). The first definite classification of the modern type was given by Dyer et al. (1965a). A different classification of diamonds based on their physical properties, morphology and mineralogy is given by Orlov (1973); Kurdumov et al. (1994); and Beskrovanov (1985).
7.1
Type I
Type I comprises diamonds in which impurity-related optical and paramagnetic absorption are dominated by nitrogen defects. Historically type I diamonds were those which were transparent to 300 nm (Robertson et al. 1934). In general, the impurity content of natural type I diamonds is more varied compared to that of type II diamonds (Bienemann-Kuespert et al. 1967). The most evident difference between type I and II diamonds comes from IR absorption spectra, which are considered to be the main criterion for this differentiation (Robertson et al. 1934). About 98% of natural diamonds contain nitrogen with concentrations detectable in optical absorption (Field 1992). 74% of them have a nitrogen content high enough to be definitely classified as type I (Bienemann-Kuespert et al. 1967). Nitrogen is regularly present in natural diamonds at levels as high as 200 to 4000 ppm (Kaiser and Bond 1959; Lang 1977; Palyanov et al. 1997a). In extreme type I diamonds the nitrogen concentration attains a value of 0.25 atom% (Kaiser and Bond 1959; Kurdumov et al. 1994). Usually nitrogen in type I natural diamonds is not distributed uniformly. It forms areas of increased concentration, or may even gather in clusters of about 10 atoms (Berman et al. 1975). 7.1.1
Type Ia
Type Ia comprises type I diamonds, which do not show absorption due to single substitutional nitrogen atoms. Nitrogen impurities are present in type Ia diamonds as nonparamagnetic aggregates. The typical nitrogen concentration in natural type Ia diamonds is about 500 ppm (Field 1992). In high-nitrogen type Ia diamonds the
390
7 Physical Classification of Diamond
nitrogen concentration may be as high as 3000 ppm (Nazare and Neves 2001). To be of pure type Ia diamond should contain single substitutional nitrogen atoms in concentrations below 1016 cm-3 (Brozel et al. 1978). Type IaA (type Ia in some publications) comprises type Ia diamonds containing predominantly the A-aggregates of nitrogen. The discovery of the A, B and B′ defects in infrared absorption was reported for the first time by Sutherland et al. (1954). Typically type IaA natural diamonds contain 500 ppm of nitrogen in the form of A-aggregates and 0.01 to 0.1 ppm of dispersed nitrogen atoms (Field 1992; Evans and Qi 1982b). Characteristic optical features of type IaA diamonds are the UV absorption continuum at wavelengths below 330 nm and the IR absorption peaks at wavelengths of 7.8, 8.3, 9.1 and 20.8 µm. The H3 center is commonly observed in type IaA diamonds (Kurdumov et al. 1994; Plotnikova 1990). Most gem-quality natural diamonds contain nitrogen predominantly in the form of A-aggregates and are classified as type IaA (Davies 1977c) (Fig. 3.22). Type IaB (type III or intermediate type in some publications) comprises type Ia diamonds containing nitrogen predominantly in the form of B-aggregates (Clark et al. 1956a; Clark et al. 1956c). Very small amounts of type Ia diamonds are of true type IaB (Field 1992). Characteristic optical features of type IaB diamonds are the IR absorption peaks at 7.5, 8.5, 9.1 and 12.8 µm and UV optical centers N9 and N10. The absorption coefficient of the B-aggregates of nitrogen in type IaB diamonds attains a value of 30 cm-1 (Kluev et al. 1982). The H4 and S2 centers are commonly observed in type IaB diamonds (Kurdumov et al. 1994; Plotnikova 1990; Ilyin et al. 1970). Type III is characterized by an additional criterion: the nitrogen concentration in type III diamonds should not exceed 1019 cm-3 (Kluev et al. 1972a) (Fig. 3.27). Type IaB' comprises diamonds containing B'-defects (platelets). The concentration the B'-defects in type IaB' diamonds may attain a value up to 1015 cm-3 (Kluev et al. 1982). Characteristic optical features of type IaB' diamonds are the absorption continuum at wavelengths below 330 nm, the peaks at 0.267, 0.283, 7.3, 7.5, 7.8, 8.5 and 9.1 µm as well as the N3 center (Kurdumov et al. 1994; Plotnikova 1990). The B'-defects are a notable feature in most natural type Ia diamonds (Lang 1977). Type IaB' regular comprises diamonds with a relatively high content of B'defects, the absorption intensity of which correlates well with that of the B-aggregates of nitrogen (Collins 1997). Type IaB' irregular comprises diamonds with a low content of B'-aggregates, the absorption intensity of which is considerably weaker relative to the absorption of the B-aggregates of nitrogen (Collins 1997). Type IaB without B' has been proposed as a separate type for those type IaB diamonds which contain no platelets (Beckman et al. 1994; Woods et al. 1993b). 7.1.2
Type Ib
Type Ib comprises type I diamonds, which contain paramagnetic single substitutional nitrogen atoms as the dominating defects. Type Ib diamonds were
7.1 Type I 391
discovered using IR absorption by Dyer et al. (1965a). Most synthetic nitrogencontaining diamonds are of type Ib, but only 1 in 1000 natural diamonds are of this type. The nitrogen concentration in synthetic diamonds is usually an order of magnitude higher than that in natural ones (Palyanov et al. 1997a). Typically natural type Ib diamonds contain about 40 to 100 ppm of nitrogen, which is mostly (about 80%) in the dispersed form, the rest (20%) of the nitrogen is in aggregated forms (Field 1992; Evans and Qi 1982b; Nazare and Neves 2001). The utmost concentration of dispersed nitrogen in natural type Ib diamonds attains a value of 5×1019 cm-3 (Brozel et al. 1978). Common synthetic HPHT diamonds contain 1018 to 1019 cm-3 dispersed nitrogen (Kluev et al. 1982). Synthetic diamonds grown with a pure nickel catalyst usually contain about 300 ppm of single substitutional nitrogen atoms (Kanda and Watanabe 1998). The characteristic features of type Ib diamonds are the absorption continuum at wavelengths below 500 nm and the absorption peaks at 0.275, 0.370, 7.4. 7.8, 8.8 and 9.1 µm. A common feature of type Ib diamonds is also the presence of S1, 638 and 575 nm centers (Kurdumov et al. 1994; Kluev et al. 1973; *). The criterion of pure type Ib diamonds is given by Novikov (1968) as µ1135/µ1290 = 2.9±0.1. This criterion is valid for diamonds with µ1135 values up to 50 cm-1 (Fig. 3.28). 7.1.3
Type Ic
Type Ic comprises diamonds containing high concentrations of dislocations. Though named as Ic this type of diamond does not relate to nitrogen. The characteristic features of type Ic diamonds are the absorption continuum at wavelengths below 900 nm, a peak at 560 nm and IR absorption peaks characteristic of type Ia diamonds. A common feature of type Ic diamonds is the PL band with a maximum at 730 nm (Kurdumov et al. 1994; Samsonenko et al. 1974).
7.2
Type II
Type II comprises diamonds showing no optical and paramagnetic absorption due to nitrogen-related defects. The nitrogen content in true type II diamonds is below 1017 cm-3. However, very often diamonds with nitrogen content below 1018 cm-3 are also referred to as type II (Sumida and Lang 1981). Historically type II diamonds were those which exhibited optical transparency up to 230 nm (Robertson et al. 1934). Type II diamonds are rare in nature. Only about 1 to 2% of natural diamonds do not show any traces of nitrogen-related defects in absorption (BienemannKuespert et al. 1967; Kurdumov et al. 1994). 7.2.1
Type IIa
Type IIa comprises type II diamonds which do not show specific IR optical absorption due to boron and hydrogen impurities in the one-phonon region. Usually
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7 Physical Classification of Diamond
diamonds with nitrogen content less than 1018 cm-3 are classified as type IIa. Often type IIa diamonds with nitrogen content above 1017 cm-3 may also contain boron in concentrations typical of type IIb diamonds. However, all the boron acceptors in such type IIa diamonds are compensated with nitrogen. The electrical specific conductivity of type IIa diamonds is of 1016 Ωcm (Nazare and Neves 2001). Type IIa diamonds are the most optically transparent diamonds. The characteristic feature of type IIa diamonds is the fundamental absorption continuum at wavelengths below 220 nm. Intentionally undoped high-quality CVD diamond films always fall into the type IIa category. 7.2.2
Type IIb
Type IIb comprises type II diamonds showing optical absorption due to boron impurities. Type IIb diamonds were first discovered and classified by Custers (1952). In some publications type IIb is used for any semiconducting diamonds containing uncompensated acceptors or donors. The characteristic difference between type IIa and type IIb diamonds is the weak absorption tail (of strength of a few cm-1) spreading from about 1 eV up to the fundamental band-gap energy of 5.5 eV: in type IIa diamonds the intensity of this absorption continuum increases towards shorter wavelengths, whereas in type IIb it increases towards greater wavelengths (this absorption results in the characteristic blue color of type IIb diamonds) (Bienemann-Kuespert et al. 1967). The boron concentration in natural type IIb diamonds usually does not exceed 1017 cm-3, whereas synthetic diamonds can be doped with boron in concentrations up to 1020 cm-3 (Kurdumov et al. 1994; Nazare and Neves 2001). The electrical specific conductivity of natural type IIb diamonds may vary in the range from 5 to 105 Ωcm (Nazare and Neves 2001). The concentration of compensating donors (mostly nitrogen) in natural type IIb diamonds is typically 5×1015 cm-3 (Collins 1997). The most characteristic features of type IIb diamonds are the fundamental absorption continuum at wavelengths below 220 nm, a continuum at wavelengths above 300 nm and a peak at 1282 cm-1, the intensity of which increases considerably with temperature drop from RT down to LNT. 7.2.3
Type IIc
Type IIc comprises type II diamonds with dominating hydrogen-related absorption at around 2900 cm-1 (Janssen et al. 1991).
7.3
Miscellaneous
There is a mixed type of diamond characterized by PL with dominating H3, H4, N3, S2, S3, 603.8 nm, 700 nm, 788 nm, 703 nm and S1 centers in addition to the features
7.3 Miscellaneous 393
mentioned above for pure IaA, IaB, Ib and IaB' types. It was proposed to classify these diamonds as type I(A+B+C+B') (Kurdumov et al. 1994; Kluev et al. 1973). The type of diamond can be changed during HPHT treatment. These transformations are especially evident for synthetic diamonds. Synthetic diamonds grown at relatively low temperatures (≤1400°C) are always of type Ib. With increase in the growth temperature to 1750°C the type of diamonds changes as: Ib → Ib+IaA → IaA+Ib → IaA (Palyanov et al. 1997a; Paljanov et al. 1997). The dominating form of nitrogen in synthetic diamonds strongly depends on their growth rate vg. In diamonds grown by the temperature gradient method at a temperature of 1500°C dispersed nitrogen (the C-defects) dominate at vg > 25 mg/mm2s, whereas the A-aggregates of nitrogen dominate at vg > 15 mg/mm2s (Paljanov 1997). Thus, fast growing diamonds are of type Ib. The slow growth promotes the formation of type IaA diamonds.
8
Interaction with Energetic Light Beams
Diamond is damaged when irradiated with energetic light beams. High light energy can be delivered either by a high flux of quanta of moderate energy (usually of a few electron-volts) or by moderate fluxes of high-energy quanta. The processes developing in the diamond matrix exposed to light beams may be significantly different depending on the quantum energy: (i) mere thermal heating; (ii) valence excitation of carbon atoms; (iii) deep shell excitation of carbon atoms and their ionization. As a result the exposed diamond area may be converted into graphite or amorphous carbon, may sublimate or be converted into carbon compounds when radiated in certain atmospheres.
8.1
Laser Treatment
The crystal structure of diamond is changed by powerful laser fluxes. When irradiated with pulsed lasers, the interaction of diamond with the first laser pulse appears to be the same as for other dielectrics. The interaction with the following pulses may be strongly affected by the nondiamond phases formed during the first pulse. If the electrical field of the light exceeds 106 V/m the electrical breakdown of diamond occurs due to multiphonon absorption (Davies 1994a). The velocity of the graphitization process of diamond during laser exposure is of the order of 80m/s (Davies 1994a). Laser-induced damage of diamond by a series of subthreshold pulses is characterized by the formation of carbon clusters at the initial stage. There is a general rule: the less perfect the diamond the lower laser energy required for ablation of the diamond surface (Chmel 1997; Kondyrev et al. 1992; Ralchenko et al. 1995c). Etching of diamond with laser beams of quantum energies below the bandgap starts at defect regions and nondiamond inclusions. The etching process proceeds in two stages: first, conversion of diamond into graphitic carbon and, second, sublimation and/or oxidation of the carbon at elevated temperatures (Rothschild et al. 1986). Such a mechanism based on direct evaporation of diamond implies the physical nature of the effect. Thus the ambient atmosphere does not influence the process.
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8 Interaction with Energetic Light Beams
The thresholds of different types of destruction of diamond by irradiation with laser pulses are given as follows (Sussman et al. 1994a): (i) bulk dielectric breakdown: 200 to 60 MW/mm2 for pulse duration from 100 to 1000 ns; (ii) enhanced dielectric breakdown: 130 to 12 MW/mm2 for pulse duration from 10 to 1000 ns; (iii) melting: 1 to 0.2 MW/mm2 for pulse duration from 20 to 100 ns; (iv) graphitization: 0.4 to 0.1 MW/mm2 for pulse duration from 10 to 40 ns.
8.1.1
ArF Laser (6.42 eV, 193 nm)
The energy threshold of ablation by 193 nm laser irradiation is 3 J/cm2 for type Ib diamond and 4 J/cm2 for type IIa diamond (Harano et al. 1994). The damage threshold value increases by an order of magnitude when the concentration of the 13 C isotope decreases from 1.07% to 0.07%. In the former case a laser energy flux of 300 MJ/cm² is sufficient to damage the diamond. In isotopic pure diamond even 3000 MJ/cm² flux cannot cause damage (Bahnholzer and Anthony 1992; Bray and Anthony 1991a). This effect is not associated with the bandgap shift, but with the higher absorption coefficient of isotopic-enriched diamonds. The broadened phonon lines of isotopic-enriched crystals make indirect phonon-assisted electronic transitions more likely (Bray and Anthony 1991a; Bray and Anthony 1991b; Anthony and Banholzer 1992). Etch rates in air of 80 µm thick free-standing MP PCCVD diamond film (crystallite size of 30 µm) by 12 J/cm2 20 ns pulses at 20 Hz repetition are 22 nm/pulse and 31 nm/pulse for the growth and substrate sides respectively. The angle of the walls of the resulting V-shaped groove increases from 25 to 60° with reduction in the laser power from 12 to 6 J/cm2. In vacuum the etch rate of the growth side at equal laser irradiation conditions is reduced to 0.6 nm/pulse (Chan et al. 1996). 20 ns pulses of energy 2-3 J/cm2 evaporate the diamond surface with a velocity of 1 m/s (Boegli et al. 1992). Treatment of PCCVD diamond films (Si-substrate, film thickness 5 µm, surface roughness of 0.5 µm) with 20 ns pulses (repetition rate 20 Hz, energy of 12 J/cm2) in He and H2 atmospheres (pressure of 105 Pa) causes a black dust on the surface. This effect is more pronounced for He. No such effect is observed in air, N2, O2 and vacuum. The highest ablation rate is observed in vacuum (about 6 nm/pulse); the lowest – for N2 (about 1.5 nm/pulse) (Gloor et al. 1997). 15 ns pulses with an energy of 24 J/cm2 remove a layer 30 to 40 µm thick at incident angles varying from 60 to 30° from the normal (Chein et al. 1995). The ablation of diamond with 0.1% of 13C isotope content occurs under irradiation with 15 ns pulses at a fluence of 30 MJ/cm2. The ablation rate of diamond of natural isotope content by 15 ns pulses at a fluence of 2 J/cm2 is 0.8 µm per pulse (Deshmukh et al. 1994; Ralchenko and Pimenov 1998). Irradiation with a ArF laser operating at a wavelength of 193 nm in a pulse regime at 50 Hz results in a smooth, glassy surface of treated diamond, the etch rate attaining a value of 12 µm/min (Johnston et al. 1993; Chalker and Johnston 1996). 12 ns pulses make a CVD diamond film surface (composed of grains of size 40 to
8.1 Laser Treatment 397
50 µm) smooth after about 500 repetitions with an energy of 20 J/cm2 and 30 to 50 repetitions with an energy of 37 J/cm2 (Cremades et al. 1996). The 193 nm laser radiation may form a terraced structure on the diamond surface implying that the laser-induced evaporation proceeds via a step mechanism related to growth defects (Boegli et al. 1992). 15 ns pulses of a ArF laser blackens the surface of synthetic type IIa diamonds of natural isotope content at a fluence of 2 J/cm2. In contrast this laser cannot blacken the surface of a diamond with 0.1% of 13C isotope even at a fluence of 30 MJ/cm2 (possibly due to high thermal diffusivity) (Ralchenko and Pimenov 1998). The effective absorption length of 193 nm irradiation by an ArF laser with respect to optically caused damage is evaluated for virgin diamond as 10 µm. In a graphitized diamond this value reduces down to 0.1 µm (McEntee 1995). The typical thickness of the laser-modified surface layer is about 150 to 200 nm (Cremades and Piqueras 1995; Ageev et al. 1988; Boegli et al. 1992; Boudina et al. 1993; Johnston et al. 1993; Pimenov et al. 1993). 20 ns pulses of 193 nm laser radiation with energies above 0.5 J/cm2 convert a 40 to 60 nm thick surface layer of (100)- and (110)-oriented diamond in a graphite-like carbon phase, which is highly conductive and stable at temperatures of 1800°C (Geis et al. 1989). For 20 ns pulses the thermal diffusion length in PCCVD diamond films is estimated as 3.3 µm (Boegli et al. 1992). The Raman spectra reveal that the diamond surface treated by pulsed irradiation by an ArF laser in any atmosphere is converted into glassy carbon (nanocrystalline graphite) (Gloor et al. 1997).
8.1.2
KrF Laser (5.0 eV, 248 nm)
The ablation threshold of type Ib diamond by a KrF laser irradiation at a wavelength of 248 nm is 3 J/cm2. For type IIa diamonds this value is much higher, attaining 25 J/cm2 (Harano et al. 1994). For CVD diamond films set at an angle of 45° to the laser beam in an O2 atmosphere the damage threshold is 1.7 J/cm2 (Dong-Gu Lee et al. 1994). Effective etching of CVD diamond films by 20 ns pulses of KrF laser irradiation may start at an energy density of 3 J/cm2. The removal rate R increases approximately as the square of the energy density E: R[nm/pulse] ≈ 0.6(E[J/cm2])1.7. Shorter pulses reveal a lower threshold energy for diamond removal: 0.8 ns pulses start to etch CVD diamond films at a density as low as 0.5 J/cm2, the removal rate increasing almost linearly with energy density: R[nm/pulse] ≈ 13(E[J/cm2])0.77. For 15 ns pulses of energy 0.2 J with repetition rate 50 Hz the R value for diamond materials can be evaluated as: R[nm/pulse] ∼ 25(E-Ethr)[J/cm2], where the threshold value Ethr increases from 1 to 2 J/cm2 and further to 7 J/cm2 for nanocrystalline, polycrystalline and type Ia single-crystal natural diamonds, respectively (Ralchenko et al. 1995c; *). 0.8 to 40 ns laser pulses of energy from 1 to 10 J/cm2 cause graphitization of PCCVD diamond films. This effect is used for polishing rough CVD diamond films
398
8 Interaction with Energetic Light Beams
with a final roughness down to 0.1 µm (Pimenov et al. 1993; Blatter et al. 1991; Ageev et al. 1993; Singh and Lee 1996; Ralchenko and Pimenov 1998). 248 nm laser irradiation with a pulse energy of 1.6 J/cm2 strongly diminishes the initial roughness of PCCVD diamond films and increases their IR transmittance in the spectral range 400 to 4000 cm-1 up to 30 to 60% (Boudina et al. 1993). 20 ns pulses of energy 3.5 J/cm2 with 20 Hz repetition rate form a "ripple" structure on the surface of CVD diamond films when at normal incidence. A pulse energy of 1.5 J/cm2 is not enough for this effect (Dong-Gu Lee et al. 1994). 248 nm laser irradiation with a pulse energy of 1.6 J/cm2 produces a conductive layer on insulating CVD diamond films after exposure to about 60 pulses. The specific electrical resistivity of this was evaluated to be 0.2 Ωcm (Boudina et al. 1993). This value is far too high to be assigned to graphite. More likely the conductive layer is formed of glassy amorphous carbon. KrF laser pulses of energy density 1.6 J/cm2 and duration in the femtosecond range evaporate diamond. The removal rate and etch rate of this evaporation process are 3×10-6 mm3/pulse and 23.6 nm/pulse respectively. The absolute rate of diamond removal by fs-laser pulses is 13.8 nm/(J cm2 pulse). For ns-laser pulses the absolute removal rate is considerably lower: 4 nm/(J cm2 pulse) (Molian et al. 1995).
8.1.3
Frequency Quadrupled Nd:YAG Laser (4.66 eV, 266 nm)
The damage threshold of PCCVD diamond films deposited on a Si substrate for 266 nm pulsed laser irradiation (4-5 ns pulses, beam focus of 2 to 10 µm) is 4 to 5 GW/cm2 (John et al. 1993; Jubber and Milne 1996).
8.1.4
XeCl Laser (4.02 eV, 308 nm)
14 to 40 ns pulses of a XeCl laser with energies in the range from 0.1 to 200 (J/cm 2)/pulse can be used for graphitization of CVD diamond films and natural type IIa diamonds. Such treatment can be used for polishing of CVD diamond films with final roughness of the treated surface down to 27 nm (Rothschild et al. 1986; Ageev et al. 1993; Tokarev et al. 1995; Ralchenko and Pimenov 1998). The graphitization threshold of CVD diamond films irradiated by XeCl laser pulses does not exceed 0.02 J/cm 2. This value is two orders of magnitude lower than that of the etching threshold of CVD diamond (Ageev et al. 1988). The etching threshold energy required for PCCVD diamond films by XeCl laser pulses of duration from 14 to 40 ns is 1 J/cm2 (Ageev et al. 1993). The removal rate R308nm for one laser pulse can be given as a function of the pulse energy E308nm by the expression: R308nm[nm] ≈ 43(E[J/cm2] – 1). This relation remains the same for xenon and air atmosphere. The removal rate for 40 ns pulses of energy 8.5 J/cm2 is as high as 0.3 µm/pulse (Ageev et al. 1993; *).
8.1 Laser Treatment 399
8.1.5
Copper Vapor Laser (2.43 eV, 510 nm)
Irradiation of CVD diamond films immersed in water with 10 ns pulses of energy 18 (J/cm2)/pulse with repetition rate of 8 kHz results in removal of the irradiated layer at a rate of 50 µm/s. When immersed in dimethylsulphoxide these films are etched at a rate as high as 0.1 µm/pulse (Shafeev et al. 1997a).
8.1.6
Ar+-Ion Laser (2.99 eV, 415 nm and 2.54 eV, 488 nm)
A CW beam of power above 1 W may cause graphitization of CVD diamond films (Ralchenko et al. 1995b).
8.1.7
Frequency Doubled Nd:YAG Laser (2.33 eV, 532 nm)
Frequency doubled Nd:YAG laser irradiation operating in the pulse regime may effectively damage CVD diamond films. If a thick PCCVD diamond film with optical absorption of 11.3% is irradiated with 10 ns laser pulses a visible damage spot appears on the unpolished surface by a pulse energy of 1 to 3 J/cm2 (1 to 3 MW/mm2); the damage on the polished surface occurs at much higher energies of 8 to 14 J/cm2 (8 to 11 MW/mm2). A film with 6.6% absorption at a wavelength of 532 nm is damaged by 10 to 14 J/cm2 (10 to 14 MW/mm2) laser pulses (Sussman et al. 1994a; Sussmann et al. 1994b; Ralchenko and Pimenov 1998). Accumulative damage has been observed in diamond under 30 ps laser pulses of a power of 17 to 90 kW after exposure to several hundred pulses. The effect is explained by two-phonon absorption at color centers induced in diamond by intense light. The generation rate of these color centers may attain a value of 8×1018 cm-3 per pulse. The energy transmission coefficient of diamond for 532 nm light is practically constant for intensities up to 3.6 GW/cm2. The calculated threshold for the twophonon absorption of this light is found to be of 2.6×10-4 cm/MW (Liu et al. 1978). The removal rate of thick CVD diamond films (over 0.7 mm) by irradiation with 15 ns pulses exhibits saturating behavior, the saturation occurring at energies above 100 J/cm2 (Ozkan et al. 1997). Annealing of diamond implanted with 2.8 MeV C+ ions can be performed with a pulsed Nd-silicate glass laser (pulse duration of 15 to 40 ns) (Prawer et al. 1992). The results of the laser action depend on the pulse energy: at 10 to 23 J/cm2 there is nearly complete regrowth of the ion-implanted layer for implantation doses up to 2×1015 cm-2; at 23 to 28 J/cm2 a buried graphitized layer is formed for implantation doses greater than 2×1015 cm-2; at energies above 30 J/cm2 the ion-implanted layer is completely graphitized. A single 15 to 40 ns pulse of Q-switched focused laser radiation may well restore the buried layer implanted by 4 MeV P+ ions at a dose of 1015 cm-2. In contrast, a shallower implanted layer produced by 640 keV ions at a dose of 1015 cm-2 is fully graphitized by a single laser pulse. The temperature and
400
8 Interaction with Energetic Light Beams
pressure developing in the buried ion-implanted layer during pulsed laser irradiation has been estimated to be about 3000 K and 5-10 GPa respectively (Allen et al. 1993). The critical powers for self-focusing Pf and catastrophic self-focusing Pcf in diamond for a 532 nm laser beam are Pf = 47 kW and Pcf = 174 kW (Liu et al. 1978).
8.1.8
Ruby Laser (1.786 eV, 694 nm)
Damage of type IIa natural diamond is observed under irradiation with 30 ns pulses of a ruby laser at a power density of 1.5 GW/cm2 (McQuillan et al. 1970).
8.1.9
Nd:YAG Laser (1.17 eV, 1.06 µm)
The damage threshold of CVD diamond films for irradiation with 12 ns pulses from a Nd:YAG laser is in the range 21 to 31 J/cm2 (Sussmann et al. 1994b; Ralchenko and Pimenov 1998). Single pulse damage thresholds for natural diamond are 1600, 320 and 200 MW/mm2 for the bulk, natural face and polished surface, respectively (Chmel 1997). A visible damage spot appears on a thick PCCVD diamond film by applying a 16 to 31 J/cm2 (13 to 25 MW/mm2) Nd:YAG laser pulse, provided the absorption of the film at a wavelength of 1.06 µm is about 2% and the thermal conductivity of the film is 1880 to 1950 W/mK (Sussman et al. 1994a). A 30 ns single pulse of intensity providing an electrical field strength of 6 MV/cm causes damage of pristine face of natural diamond at RT. If the intensity is slightly reduced so that the electrical field strength is 5.5 MV/cm it may need 104 such pulses to cause damage at RT. At a temperature of 200°C the corresponding values of the electrical field strength are 4.5 and 4 MV/cm respectively (Chmel 1997; Kondyrev et al. 1992). The etching of CVD diamond films put onto a metal substrate and immersed into 10% KOH solution can be performed with a pulsed Nd:YAG laser operating at 5 kHz frequency with an average power of 1.6 W and focused to a spot of 50 to 100 µm (Mermoux 1992). The destruction of the diamond surface during a single scan of the laser beam is observed at a linear velocity of 20 mm/min. The intrinsic breakdown threshold of diamond for 30 ps pulses of Nd:YAG laser is 21.5 MV/cm (Liu et al. 1978). The sharpest focusing and the highest peak intensity of a 1.06 µm single mode laser beam achieved on a diamond surface is 5 µm and 1000 MW/mm2 respectively (Davies 1994a). The critical power of self-focusing Pf in diamond for 1.06 µm laser light is Pf = 148 kW (Liu et al. 1978).
8.1 Laser Treatment 401
The maximum allowable power density of a CW laser operating at a wavelength of 1 µm for the diamond optical window is 20 MW/cm2 for an aperture of diameter 1 cm (Klein 1993).
8.1.10 CO2 Laser (0.117 eV, 10.6 µm) Damage thresholds for 50 ns pulses of 10.6 µm irradiation of a CO2 laser are 5.6 to 12.7 MW/mm2 (29-66 J/cm2) and 12.5-18.5 MW/mm2 (above 93 J/cm2) for bulk high-quality CVD diamond films and for natural type IIa diamond respectively (Sussmann et al. 1994b). For 100 ns pulses the damage threshold of diamond attains a value of 40 MW/mm2 (Wood 1986); in particular, it is 22-40 MW/mm2 for natural type IIa diamond and 17 MW/mm2 for synthetic type Ib diamond (Sussmann 1993). The damage threshold of CVD diamond films for 150 ns pulses is 50 J/cm2 (Ralchenko and Pimenov 1998). The damage threshold value strongly correlates with the strength of optical absorption in the visible spectral range (for instance, at a wavelength of 532 nm) and with the surface roughness. In contrast, there is poor correlation of the damage threshold with the content of nondiamond (carbonaceous) inclusions (Sussmann et al. 1994b). A visible damage spot appears on a thick high-quality PCCVD diamond film and type IIa natural diamond by applying a 29 to 93 J/cm 2 (5.6 to 18.5 MW/mm2) pulse of a CO2 laser. In order to stand such high laser power the optical absorption of the film at a wavelength of 10.6 µm must be below 0.5% and its thermal conductivity must be at least 2000 W/mK (Sussman et al. 1994a). 20 ns pulses of a power 1 GW/cm2 result in graphitization of PCCVD diamond films. A laser operating at such parameters can be used for polishing of rough CVD diamond films (Ravi and Zarifis 1993). Optical breakdown of diamond exposed to 1 s pulses of 10.6 µm laser irradiation occurs at a power density of 1 MW/cm2 (Davies 1994a). The smallest focusing of a 10.6 µm laser beam achieved on diamond is 100 µm (Davies 1994a).
8.2
Synchrotron Irradiation
SR with a critical quantum energy of 425 eV results in a significant etching effect of the surface of any diamond material (PCCVD diamond films, HPHT synthetic diamonds and natural diamonds) in O2 or an oxygen-containing atmosphere. Temperature in the range from -140 to 370°C does not influence the etching effect considerably, though the etching rate is a bit higher at elevated temperatures. There is almost a linear dependence of the removal rate of diamond on the pressure of oxygen in a range to 0.2 Torr. The removal rate of diamond at 370°C in an O2 atmosphere at a pressure of 0.1 Torr is as high as 1 nm A-1 s-1. The SR etching effect does not relate to mere heating of the diamond because of its relatively low flux
402
8 Interaction with Energetic Light Beams
power. More probably it relates to ionization stimulated oxidation of the diamond surface with formation of volatile compounds like CO or CO2. Strong ionization of the diamond surface under 425 eV quanta appears to be a realistic process. Indeed such quanta are absorbed in a layer of 0.1 µm thick mostly due to K-shell ionization. The K-shell holes in carbon atoms recombine predominantly over the Auger process (the probability of Auger recombination is 0.9), resulting in strong ionization of the valence electrons (Ishiguro et al. 1996; Garcia et al. 1973; *).
8.3
Miscellaneous
Laser treatment of diamond with fs-pulses, in contrast to ns-pulses, is capable of preventing the formation of nondiamond phases during diamond etching (Molian et al. 1995). Fine polishing increases the laser damage threshold of PCCVD diamond films by 3 to 4 times (Sussman et al. 1994a). The highest experimentally measured laser damaged threshold for type IIa natural diamond is 40 MW/mm2, which is about three times higher than that for PCCVD diamond films (Sussman et al. 1994a; Wood 1986). Black nondiamond inclusions do not noticeably affect the laser damage threshold of CVD diamond films (Sussman et al. 1994a). This especially concerns films with high thermal conductivity. An increase in the thermal conductivity of diamond material by 50% may cause a 10-fold increase in its laser damage threshold (Anthony and Banholzer 1992). The mechanism of interaction of excimer laser radiation with diamond is believed to be graphitization of a surface layer and subsequent sublimation of this graphite (Ageev et al. 1988; Boegli et al. 1992). Laser polishing of PCCVD diamond films is limited by the ratio of vertical to lateral roughness (the asperity sharpness), which correlates with the average size of crystallite (Boegli et al. 1992). Laser ablation of diamond immersed in a liquid (water, dimethylsulphoxide) leaves little graphite on the treated surface. Raman spectra reveal only weak traces of nondiamond phases on the treated surfaces. The diamond surface ablated in liquid by laser irradiation is not electrically conductive. In contrast, analogous laser treatment in air produces a strongly graphitized surface; the Raman spectra in this case reveal only broadened G- and D-bands and do not exhibit any narrow diamond line (Fig. 4.44). After ablation in air the graphite layer is several times thicker than that formed in liquid, and its electrical surface conductivity is considerably higher ((Shafeev et al. 1997a; Shafeev et al. 1997b; Jamil et al. 1995). The threshold energy of laser irradiation required for ablation in a liquid is several times higher than that in air. However once the ablation has started, its rate does not depend significantly on the surrounding medium, though the etching rate of diamond in liquid is slightly higher than that in air (Shafeev et al. 1997a).
8.3 Miscellaneous 403
When the laser beam is inclined above a critical angle (above 80°) with respect to the treated diamond surface, nonperiodic corrugations of the surface (called "ripples") are developed on the ablated surface preventing further reduction of surface roughness. The ripple pattern can be irregular. The surface ripples are formed readily at elevated laser energies: for instance, 20 ns pulses from a KrF laser must have an energy above 1.5 J/cm2 to provide ripple formation (Singh and Lee 1996). In order to eliminate the ripple-effect and make polishing smoother one can rotate the sample during irradiation (Tosin et al. 1995). The ripple structure on the diamond surface can be formed by intense laser irradiation (above ablation threshold) even at normal incidence (Dong-Gu Lee et al. 1994). The mechanism of ripple formation is explained by a shadowing effect and has no direct connection with optical interference (no dependence on the laser wavelength). However, spontaneous periodic surface structures on the diamond surface can be produced by highly coherent laser light as a result of optical interference (Davies 1994a). Self-focusing in type IIa natural diamond is not achieved for ruby laser pulses (wavelength of 694 nm) with energy 70 MW/cm2 (McQuillan et al. 1970). Laser irradiation can effectively remove the nucleation centers from the substrates intended for CVD diamond deposition. This effect is used for negative image patterning of the substrates for selective deposition of diamond (Chapliev et al. 1991). There is a limitation to the use of intense laser beams in air connected with air breakdown, which occurs when the laser power exceeds 6000 MW/cm2 (for the visible spectral range). The breakdown strongly ionizes the air and forms a large plume of carbon plasma at the treated diamond surface. This plume may absorb and scatter up to 90% of the energy of the laser beam. Because of this effect there is an optimum laser power to achieve maximum removal rate by laser ablation (Johnson C. et al. 1964; Fowler and Smith 1975).
9
Thermostimulated Luminescence and Tunnel Luminescence
A very characteristic feature of XL of natural and synthetic diamonds is delayed light emission with time constants of tenths of minutes. This delayed luminescence results from TL and TSL. The contribution of TL in the delayed luminescence of synthetic diamonds is 90% (Vins 1988; Yelisseyev and Sobolev 1979b; Yelisseyev 1977; Vins and Yelisseyev 1989; Yelisseyev et al. 1988). Most natural diamonds are capable of storing light-sums during external excitation, in this way being active in TSL or TL (Bobrevich et al. 1959). TL of semiconducting type IIb diamond can be excited via injection of electrons through a thin-layered M-i-M structure deposited onto the diamond surface. If the upper metal layer M1 of the resulting M1-i-M2-p structure is negatively biased the electrons may tunnel from M1 into the p-substrate through the i-M2 layers causing TL (Lepek et al. 1976).
9.1
TSL and TL Features
TSL and TL centers of diamond are listed below after their temperature maxima: 105 K; observed in natural diamonds exhibiting strong PL of the N3 center. The parameters of the center are: width δ1/δ2 = 7/6, E1 = 0.15 eV, E2 = 0.21 eV, E3 = 0.13 eV, f02 = 4×108 Hz, f03 = 4×106 Hz (Yelisseyev 1977). 132 K; observed in natural type Ib diamonds with dominating PL of the S1 center and in synthetic diamonds grown by the temperature gradient method in Fe-Ni-C media. The parameters of the center are: ET ≈ 0.13-0.15 eV, f0 = 3.6×103 Hz (Yelisseyev 1977; Nadolinny and Yelisseyev 1994). 132 K; observed in natural type Ib diamonds exhibiting strong PL of the S1 center. The parameters of the center are: width δ1/δ2 = 8/12, E1 = 0.13 eV, E2 = 0.14 eV, E3 = 0.15 eV, f02 = 4×103 Hz, f03 = 1×104 Hz (Yelisseyev 1977).
406
9 Thermostimulated Luminescence and Tunnel Luminescence
140 K; observed in natural diamonds exhibiting strong PL of the N3 center. The parameters of the center are: width δ1/δ2 = 9/9, E1 = 0.24 eV, E2 = 0.26 eV, E3 = 0.19 eV, f02 = 5×107 Hz, f03 = 1×106 Hz (Yelisseyev 1977). 150 K; observed in common synthetic diamonds after excitation at LNT with γ-rays at a dose of 106 Roentgen, with X-rays, or with UV light at wavelengths below 300 nm. The heating rate during observation is 8 K/min. The peak is stimulated by nitrogen and boron impurities and very strongly (three orders of magnitude) by doping with a mixture of impurities Ti+In+As. ET = 0.09 eV (Vachidov et al. 1975a). 159 K; observed in natural type Ib diamonds exhibiting strong PL of the S1 center. The parameters of the center are: width δ1/δ2 = 9/11, E1 = 0.41 eV, E2 = 0.45 eV, E3 = 0.22 eV, f02 = 6×1012 Hz, f03 = 5×106 Hz (Yelisseyev 1977). 160 K; observed in boron-doped synthetic diamonds, in undoped synthetic diamonds grown by the temperature gradient method, and in natural type Ib diamonds with dominating PL of the S1 center. The feature is excited at LNT by X-rays, or by UV light at wavelength below 300 nm. The parameters of the center are: ET ≈ 0.38-0.47 eV; f0 = (6-6.4)×1013 Hz (Vins 1988; Yelisseyev 1977). In some synthetic diamonds grown by the temperature gradient method in a Fe-Ni-C medium the 160 K peak shows the following parameters: ET ≈ 0.22 eV; f0 = 2.5×103 Hz, kinetics order II (Nadolinny and Yelisseyev 1994). The traps responsible for this peak are also involved in TL of most diamonds grown using the common synthesis method. These traps are electron traps. The spectral content of the peak is mostly the 484 nm center (Vins and Yelisseyev 1989). 185 K; observed in natural type Ib diamonds exhibiting strong PL of the S1 center. The parameters of the center are: width δ1/δ2 = 11/16, E1 = 0.21 eV, E3 = 0.21 eV, f03 = 8×1013 Hz (Yelisseyev 1977; Yelisseyev 1977). 200-230 K; observed in natural diamonds containing platelets and exhibiting strong PL of the N3 center. This feature is characterized by yellow light emission. The feature has the following parameters: width δ1/δ2 = 85/60, E1 = 0.11-0.14 eV (Yelisseyev 1977). 210 K; observed in natural diamonds exhibiting strong PL of the N3 center. The parameters of the center are: width δ1/δ2 = 11/11, E1 = 0.35 eV, E2 = 0.38 eV, E3 = 0.35 eV, f02 = 2×107 Hz, f03 = 5×106 Hz (Yelisseyev 1977). 230 K; observed as the dominating peak in boron-doped synthetic diamonds. The feature is weak in undoped synthetic diamonds grown by the temperature gradient method. The center shows first order-kinetics when excited at LNT with X-rays or UV light at wavelengths below 240 nm. The parameters of the center are: ET ≈ 0.36-0.37 eV, f0 = 2×107 Hz. The traps related to this peak are also responsible
9.1 TSL and TL Features 407
for TL of synthetic diamonds grown by the temperature gradient method and semiconducting synthetic diamonds. The spectral content of the peak is the boronrelated band peaked at 500 nm (Vins 1988; Vins and Yelisseyev 1989). A possible origin of the feature is a hole trap center localized at ionized substitutional boron atoms (Nahum and Halperin 1963). 240-245 K; observed in common synthetic diamonds. The peak is suppressed by nitrogen. It is stimulated by boron and very strongly (three orders of magnitude) by doping with a mixture of Ti+In+As. The feature is not observed in natural diamonds. The center can be excited at LNT by γ-rays at a dose of 106 Roentgen. The heating rate during observation is 8 K/min. It is a center of second order kinetics; ET = 0.23 eV (Vachidov et al. 1975a). 250 K; observed in common synthetic diamonds after excitation at LNT by X-rays or UV light at wavelengths below 300 nm. The feature is strongly suppressed by nitrogen impurities (both the C- and A-defects). The center has the parameters: ET ≈ 0.64-0.67 eV, f0 = 4×1012 Hz. The traps related to this peak are also responsible for tunnel luminescence in synthetic diamonds grown by the temperature gradient method. The spectral content of the peak is the 500 nm boron-related band (Vins 1988; Vins and Yelisseyev 1989). 260-270 K; observed in synthetic diamonds grown by the temperature gradient method. It is the dominating TSL peak observed after excitation at LNT with X-rays or with UV light at wavelengths below 300 nm. The feature is not observed in natural diamonds. The center has second-order kinetics. The parameters of the center are: ET ≈ 0.61-0.64 eV, f0 = 2×1011 Hz. The traps related to this TSL center do not contribute to the TL of synthetic diamonds (Vins 1988). 291-295 K; the most intense peak observed in natural type Ib diamonds exhibiting in PL dominating the S1 center. The parameters of the center are: width δ1/δ2 = 8/12, E1 = 0.50-0.52 eV, E2 = 0.52 eV, E3 = 0.51 eV, f02 = 3×107 Hz, f03 = 2×107 Hz. The traps related to this TSL feature also give the main contribution to the TL of the corresponding diamonds (Vins 1988; Yelisseyev 1977; Yelisseyev 1977). 310 K; observed in common synthetic diamonds after excitation at LNT with γ-rays at a dose of 106 Roentgen, X-rays, or with UV light at a wavelength below 300 nm. The feature can also be observed in synthetic diamonds grown by the temperature gradient method (Vins 1988). The heating rate during observation is 8 K/min. This feature is stimulated by boron doping and very strongly (up to three orders of magnitude) by doping with a mixture Ti+In+As. The parameters of the center are: ET = 0.39-0.47 eV, f0 = 3×106 Hz (Vins 1988; Vachidov et al. 1975a). 320 K; observed in natural diamonds containing platelets and exhibiting strong PL of the N3 center. The parameters of the center are: width δ1/δ2 = 25/30,
408
9 Thermostimulated Luminescence and Tunnel Luminescence
E1 = 0.33-0.40 eV, E3 = 0.28 eV, f03 = 3×103 Hz. The center is characterized by the emission of yellow light (Yelisseyev 1977). 340 K; observed in nitrogen-doped synthetic diamonds after excitation at LNT with γ-rays at a dose of 106 Roentgen. The heating rate during observation is of 8 K/min. The intensity of this TSL peak increases with nitrogen content. The temperature activation energy of the center is ET = 0.53 eV (Vachidov et al. 1975a). 360 K; observed in synthetic diamonds grown by the temperature gradient method in a Fe-Ni-C system after UV excitation at LNT. The parameters of the center are: ET ≈ 0.38 eV, f0 = 4.5×102 Hz. This is a center of second-order kinetics (Nadolinny and Yelisseyev 1994). 360-375 K; observed in common synthetic diamonds and in undoped synthetic diamonds grown by the temperature gradient method. The center is excited at RT by X rays. The feature is weak in boron-doped synthetic diamonds. It is also observed in natural type Ib diamonds with dominating S1 center in PL. The parameters of the center are: ET ≈ 0.70-0.71 eV, f0 = 9×107 Hz. The traps related to this TSL feature do not contribute to the TL of natural diamonds (Vins 1988; Yelisseyev 1977). 380 K; observed in common synthetic diamonds and in natural type Ib diamonds exhibiting strong PL of the S1 center. The center is excited at RT by γ-rays at a dose of 106 Roentgen. The heating rate during observation is 8 K/min. The parameters of the center are: ET ≈ 0.67-0.72 eV, f0 = 2.4×107 Hz (Vins 1988; Vachidov et al. 1975a; Yelisseyev 1977). 380 K; observed in natural diamonds containing platelets and exhibiting strong PL of the N3 center. The parameters of the center are: width δ1/δ2 = 40/42, E1 = 0.50-0.54 eV, E2 = 0.43 eV, E3 = 0.28 eV, f02 = 103 Hz, f03 = 3×103 Hz. The luminescence of the center is characterized by blue light emission (Yelisseyev 1977). 380 K; observed in natural diamonds exhibiting strong PL of the N3 center. The parameters of the center are: width δ1/δ2 = 21/19, E1 = 0.85 eV, E2 = 0.79 eV, E3 = 0.55 eV, f02 = 3×108 Hz, f03 = 2×107 Hz (Yelisseyev 1977). 388 K; a peak observed in synthetic diamonds after UV or X-ray excitation. The feature is tentatively attributed to the N9 center (Nikitin et al. 1968). 390 K; observed in synthetic nitrogen-containing diamonds after γ-ray excitation at LNT at a dose of 106 Roentgen. The heating rate during observation is 8 K/min. The intensity of the TSL peak increases with nitrogen content (Vachidov et al. 1975a).
9.1 TSL and TL Features 409
420 K; observed in common synthetic diamonds after excitation at 330 K by γ-rays at a dose of 106 Roentgen. The heating rate during observation is 8 K/min (Vachidov et al. 1975a). 480 K; blue light TSL emission peak observed in natural diamonds containing platelets and exhibiting strong PL of the N3 center. The parameters of the center are: E1 = 0.70-0.75 eV, E2 = 0.75 eV, f02 = 2×105 Hz (Yelisseyev 1977). 510 K; observed in good-quality CVD diamond films of different origin after β-irradiation at RT. This feature originates from three centers with the parameters: ET1 = 1.04 eV, ET2 = 0.99 eV, ET3 = 1.1 eV, f01 = 1.7×108 Hz, f01 = 2.6×108 Hz, f03 = 4.6×1010 Hz. These centers are interpreted as hole traps (Vittone et al. 1999). 530 K; a weak TSL peak observed in natural type Ib diamonds with dominating S1 center in PL (Yelisseyev 1977). 530 K; observed in natural diamonds exhibiting strong PL of the N3 center. The parameters of the center are: width δ1/δ2 = 22/23, E1 = 1.35-1.45 eV, E2 = 1.32 eV, E3 = 1.18 eV, f02 = 1×1011 Hz, f03 = 1×108 Hz (Yelisseyev 1977). 540 K; observed in synthetic diamonds doped with nitrogen and in natural type Ib diamonds exhibiting strong PL of the S1 center. The center is excited at LNT with γ-rays at a dose of 106 Roentgen. The heating rate during observation is 8 K/min. The intensity of this TSL peak increases with nitrogen content. The parameters of the center are: ET = 0.7-1.43 eV, f0 = 1×109 Hz (Vins 1988; Vachidov et al. 1975a). 552 K; a TSL peak of blue light emission observed in natural diamonds containing platelets and exhibiting strong PL of the N3 center. The parameters of the center are: width δ1/δ2 = 25/23, E1 = 1.25-1.35 eV, E2 = 1.45 eV, E3 = 1.29 eV, f02 = 3×1012 Hz, f03 = 2×109 Hz (Yelisseyev 1977). 600 K; observed in synthetic diamonds grown by the temperature gradient method in a Fe-Ni-C system. The feature is excited with UV light at LNT. The parameters of the center are: ET ≈ 0.53-0.63 eV, f0 = 3×102 Hz. This is a center of first-order kinetics (Nadolinny and Yelisseyev 1994).
9.2
Optical Centers in TSL
A yellow luminescence band with its main contribution from the H3 center shows in natural diamond re-emission at temperatures of 80 and 280°C when excited with X-rays at RT (Vilutis and Krongauz 1963; Bobrevich et al. 1959).
410
9 Thermostimulated Luminescence and Tunnel Luminescence
The S1 center in natural diamonds shows strong TSL peaks at temperatures of 290 and 370 K as well as weak peaks at 110, 130, 140, 175 and 520 K after excitation at a wavelength of 365 nm (Sobolev and Yeliseev 1976). The 484 nm center; see the 160 K TSL peak above. The A-band (possibly that related to the B1 center) in natural diamonds exhibits strong TSL peaks at 110, 140 and 200 K and weak broad peaks at 300, 400 and 520 K after excitation at a wavelength of 365 nm (Sobolev and Yeliseev 1976). The N3 center in natural diamonds often exhibits, after excitation at a wavelength of 365 nm, a continuous TSL background in a temperature range from LHeT to 600 K superimposed on weak broad peaks at 160, 310 and 540 K (Sobolev and Yeliseev 1976). The N3 center can be observed in TSL of natural diamonds of any type (Ia, IIa, intermediate). In contrast, the N3b center is observed only in diamonds with B'-aggregates (Yelisseyev 1977). A blue luminescence band (most likely the N3 center) shows TSL at temperatures of 360 and 510 K when excited with X-rays at RT (Vilutis and Krongauz 1963; Bobrevich et al. 1959).
9.3
Miscellaneous
Doping with silicon reduces the TL intensity of synthetic diamonds (Vachidov et al. 1975a). When excited through the N3 center, the traps active in the TSL of natural diamonds containing B' centers can be filled up to 70% of the light-sum maximum. In type IIa diamonds or diamonds of intermediate type this filling attains only 0.5% of the light-sum maximum (Yelisseyev 1977). To fill all the traps (deep and shallow) active in TL the diamond should be illuminated first at high temperatures and then at low temperatures (Vachidov et al. 1975a). Nitrogen impurity may considerably stimulate TL in synthetic diamonds (Vachidov et al. 1975a; Nikitin et al. 1968). Yakutian diamonds exhibiting no TSL have predominantly the shape of flattened octahedra (spinel-low twins) (Vilutis and Krongauz 1963). Most deep traps responsible for TSL in diamond are believed to be radicals localized on point and extended defects (Sobolev 1984). Synthetic diamonds doped with Al, Ga, In and P exhibit TSL peaks at 160, 240 and 320 K after γ-excitation. Doping with B and with a mixture of B+Ti results in additional peaks at 360 K and 420 K respectively (Vachidov et al. 1975a). The light-sums accumulated in synthetic type Ib diamonds are reduced by at least an order of magnitude after neutron irradiation with doses above 1017 cm-2. Subsequent annealing at 800°C does not restore the light-sums characteristic of nonirradiated diamonds. This effect is attributed to a reduction in the trap concentration but not to a reduction in the radiative recombination efficiency (Vins 1988; Vins et al. 1988).
9.3 Miscellaneous 411
The TL of natural diamonds containing the B' centers is connected with yellow TSL peaks observed at temperatures below 400 K. The TL decay is characterized by the parameter χ < 1 (Yelisseyev 1977). The TL of natural diamonds of type IIa and diamonds of intermediate type is connected with TSL peaks observed at temperatures 110, 145 and 210 K. The TL decay of these features is characterized by χ < 1 when excited over the A centers (λ < 300 nm), and with χ > 1 when excited over the N3 centers (Yelisseyev 1977). TL in diamonds is observed at temperatures below 400 K. The TL contribution to DL may increase up to 95 % at LHeT (Yelisseyev 1977). The interaction between defects leads to a broadening of the TSL peaks. This broadening can be very strong in natural diamonds containing platelets. Small-size platelets cause the highest broadening resulting in a continuous TSL background in a temperature range from 4 to 80 K (Yelisseyev 1977). In natural diamonds of different types (Ia, IIa and Ib) excitation of TSL and DL as well as recombination of the accumulated charge occur via the same centers. The charge carriers responsible for these processes are believed to be electrons (Yelisseyev 1977).
10
Photoconductivity
10.1
Thresholds and Peaks
PC features are listed below after the quantum energies of their maxima (P) or lowenergy thresholds (TH) measured in the dc current regime. The centers suppressing PC are also given as "PC killers". 0.347, 0.349, 0.356, 0.360, 0.363 and 0.365 eV (3.57, 3.55, 3.48, 3.44, 3.41 and 3.40 µm); P; p-type features observed at LNT in p-type diamonds doped with boron. These bands correspond to excitation to the excited states of boron acceptors. These peaks are superimposed on a complex broad PC band starting from 0.37 eV and extending towards the visible region. In synthetic single crystals with moderate boron concentration (of 1017 cm-3) the broad PC band is superimposed also on many (over 15) minima appearing due to charge carrier capture by the excited states of boron acceptors: e.g. minima at 1.180, 1.015, 0.850, 0.686 and 0.520 eV due to capture by the excited states at 0.348 and 0.363 eV, minima at 0.953, 0.790, 0.629 and 0.466 eV due to capture by the excited state at 0.304 eV, minima at 0.612 and 0.450 eV due to capture by the excited state at 0.288 eV, and a minimum at 0.499 eV due to capture by the excited state at 0.335 eV (Fig. 10.1). The spectral positions of these minima En correspond to the energies of the acceptor excited states Ei plus one or several TO-phonon energies hω = 165 meV: En = Ei + 165n - 0.75n (the last term appears due to anharmonism of the phononphonon interaction). At higher photon energies the TO-phonon energy becomes smaller than 165 meV due to peculiarities of the band structure of diamond: deeper holes have to dissipate more momentum (Collins et al. 1969; Rohrer et al. 1998). The 0.520 eV minimum may exhibit fine structure corresponding to the fine structure of the excited states; the separation between the minima of this fine structure ranges from 10 to 20 meV (Rohrer et al. 1997a). These PC minima appear as a result of reduction in the lifetime of the holes down to 10-12 s after they have been captured on the excited states. Hole capture occurs mostly on the 0.304 and 0.348 eV levels; the latter level is dominant giving up to 11 phonon replicas. The minimum at 0.406 meV corresponds to hole capture by the 0.24 eV excited state with emission of one TO-phonon; electronic transition to this state from the ground sate is forbidden and it is not seen in absorption. The minimum at 0.47 eV shows fine structure (Vishnevskii et al. 1978; Collins et al. 1969).
414
10 Photoconductivity
The PC spectrum of synthetic IIb diamond varies weakly with temperature within a range from 77 to 150 K (Rohrer et al. 1997a). In CVD diamond films with boron concentration of 1019 cm-3 the minima due to the 0.348 and 0.363 eV levels are absent at a temperature of 130 K. At 77 K CVD diamond films exhibit sharper and more complex PC structure than that of synthetic single crystals. In PC spectra of CVD diamond films the minima in a range from 0.55 to 0.72 eV transform into maxima indicating that the holes captured by the 0.304 and 0.335 eV states become mobile. The mobility mechanism is believed to be tunneling of the holes to neighboring ionized centers (Collins and Lightowlers 1968; Rohrer et al. 1998). The broad features at energies below 0.373 eV arising due to perturbed centers are mostly prominent in the spectra of CVD diamond films. In CVD diamond films with boron content of 1019 cm-3 the PC intensity at around 0.35 eV is of the same value as that at energies above 0.373 eV occurring over the valence band; this effect of increased PC is due to high mobility of holes in the excited states of boron acceptors (Rohrer et al. 1998). The boron acceptor related PC threshold at a quantum energy of 0.4 eV is usually observed at LNT in type Ia natural diamonds. This feature may be particularly strong in samples containing high concentrations of the B-aggregates of nitrogen (Tatarinov 1986).
PHOTOCONDUCTIVITY, arb. units
50
40
30
20
10
0 0.2
0.4
0.6
0.8
1.0
1.2
1.4
QUANTUM ENERGY, eV
Fig. 10.1. Oscillatory photoconductivity of boron-doped CVD diamond films measured at LNT (Rohrer et al. 1998)
0.62 eV (2.0 µm); TH; observed in synthetic diamonds doped with a mixture of impurities Ti+In+P (Butuzov et al. 1976). 0.77-0.69 eV (1.6–1.8 µm); TH; observed in synthetic diamonds doped with a mixture of impurities Ti+In+As (Butuzov et al. 1976).
10.1 Thresholds and Peaks 415
0.8 eV (1.55 µm); TH; n-type PC feature observed in some natural diamonds with very low concentration of the A-aggregates of nitrogen. The PC spectra of such diamonds can be quite uniform up to 5 eV. This peculiarity is explained by very short lifetime of the excited holes, which cannot contribute noticeably to the photocurrent. This PC feature can be stimulated by X-ray irradiation (Tatarinov 1986). 0.9 eV (1.38 µm); TH; p-type PC induced in type Ia natural diamonds exposed to X-ray irradiation (Tatarinov 1986). 1.0 eV (1.24 µm); TH; observed in (100)-oriented nominally undoped (however, can be about 10 ppm of nitrogen) CVD diamond films grown on Si substrates. The feature is attributed to a set of electronic transitions to π*-states of sp2-bounded carbon atoms located at grain boundaries. Another explanation of the 1 eV threshold is a broad energy distribution of nitrogen donor levels in stressed areas of CVD diamond films (Rohrer et al. 1997a) (Fig. 10.2).
3
10
Eg
PHOTOCONDUCTIVITY, arb. units
2
10
4.2 eV band
1
3 eV band
10
2.3 eV band 0
10
-1
10
1 eV band
-2
10
-3
10
-4
10
1
2
3
4
5
6
QUANTUM ENERGY, eV
Fig. 10.2. PC spectrum of a CVD diamond film containing nitrogen impurity. The spectrum was recorded at RT (Rohrer et al. 1997a). The spectrum is a continuum intensity of which increases with quantum energy. The features of the continuum are PC thresholds at about 1, 2.3, 3, 4.2 eV and Eg
1.03 eV (1.2 µm); TH; P; observed in synthetic diamonds doped with mixtures of impurities Ti+In+As and Ti+In+P (Butuzov et al. 1976). 1.08 eV (1.15 µm); TH; a PC continuum in the spectral region from 1.08 to 2.5 eV enhanced in irradiated semiconducting diamonds by illumination with light of quantum energy above 1.55 eV. This enhancement is especially strong (two orders of magnitude) under illumination with quanta above 2.5 eV. The spectrum of the
416
10 Photoconductivity
enhanced PC has a maximum at 1.6 eV. The photo-enhanced PC band relates to the TSC peak at 500 K (activation energy of 0.5 eV). Photo-enhanced PC is bleached thermally by heating at temperatures below 600 K. The effect of enhancement and bleaching is due to internal photo-emission of holes from an acceptor level lying at 0.5 eV above the valence band (Vermeulen and Halperin 1981) (Fig. 10.3).
PHOTOCONDUCTIVITY, arb. units
1.0
0.5
N3
0.0 200
300
400
500
600
700
WAVELENGTH, nm
Fig. 10.3. PC spectrum of a type IaAB natural diamond taken at RT (Yelisseyev et al. 1995b)
1.2 eV (1000 nm); TH; low-energy threshold of a PC band ranging up to 3 eV. The band is observed at RT in p-type synthetic diamonds. The maximum of the band is usually at about 2.8 eV. The band maximum shifts towards greater wavelengths with increasing boron concentration. The intensity of the band falls with decreasing temperature. The spectral position of the band maximum differs for different growth sectors of synthetic diamonds (Vishnevskii et al. 1978). 1.2 to 2.0 eV (1000 to 620 nm); HT; threshold of a continuum overlapping with peaks at 330, 353, 365, 374, 383, 393 (the ND1 center), 405 (the R9B center), 413420 (the GR4-GR8 centers), 427-430 (the GR2, 3 centers), 437-440 (the TR14TR17 centers), 452, 463, 468, 482, 494, 505, 595 and 830 nm (Fig. 10.4). This complicated PC spectrum is a common feature of brown diamonds and highdislocation CVD diamond films. Illumination with light of wavelength 300-400 nm at LNT reduces the ND1 center-related band and simultaneously increases the 450 nm band and the long wavelength part (at wavelengths above 500 nm) of the spectrum; the intensity increase of the long wavelength part is unstable and decays in about 10 min after cutoff of the excitation (Fig. 10.5). The primary origin of this PC feature is believed to be dislocations (Samsonenko et al. 1978; Yelisseyev et al. 1995b; Samsonenko et al. 1997).
10.1 Thresholds and Peaks 417
PHOTOCONDUCTIVITY, arb. units
10
0
ND1 GR2, 3
10
-1
GR1
10
-2
10
-3
200
400
600
800
1000
WAVELENGTH, nm
Fig. 10.4. PC spectrum (at RT) of a natural diamond rich with inclusions and dislocations. The spectrum is corrected for the set-up response (Yelisseyev et al. 1995b)
PHOTOCONDUCTIVITY, arb. units
1.0
0.5
0.0 200
300
400
500
600
WAVELENGTH, nm
Fig. 10.5. PC spectrum (at LNT) of a natural diamond with moderate nitrogen content after prior illumination with light in the spectral region 300 to 400 nm (Yelisseyev et al. 1995b)
1.38 and 1.46 eV; TH; a feature observed in CVD diamond films exhibiting absorption bands in the spectral range from 1.3 to 1.5 eV. This PC feature can be strong enough to observe fine structure (Collins 1997; Allers and Collins 1995) (Fig. 10.6).
418
10 Photoconductivity
100
PHOTOCURRENT, arb. units
1.53 80
60 1.46 40 1.38 20
0 1.0
1.5
2.0
2.5
3.0
QUANTUM ENERGY, eV
Fig. 10.6. Photoconductivity spectrum taken at LNT from a CVD diamond film (Collins 1997)
1.39 eV (890 nm); TH; P; observed in synthetic diamonds doped with a mixture of species Ti+In+As and Ti+In+P, as well as doped with boron (Butuzov et al. 1976). 1.4 to 1.7 eV (880 to 730 nm); TH; a feature of n-type photoconductivity readily observed in type Ib diamonds (Rohrer et al. 1997a). The threshold may vary from 1.4 to 1.7 eV in type Ia diamonds with high concentration of the A-aggregates of nitrogen (above 1018 cm-3). In type Ia diamonds with low concentration of the A-aggregates it may range from 0.8 to 2.3 eV. The feature can be stimulated by Xray irradiation (Tatarinov 1986). 1.5 eV (830 nm); TH; a PC continuum observed in electron-irradiated semiconducting diamond. The continuum extends from the near IR to the UV spectral range. The 1.5 eV threshold of the continuum shifts to 1.25 eV with temperature increase from LNT to 500 K. 1.67 eV (742 nm); PC killer; ZPL of the GR1 center. Suppression of PC due to the GR1 center is usually observed in irradiated semiconducting diamonds (Vermeulen and Halperin 1981). 1.7 eV (730 nm); TH; low-energy threshold of a band with maximum at about 2.4 eV observed in type Ib diamonds. This band is attributed to substitutional nitrogen donors. The band intensity increases after annealing at temperatures to 400°C (Farrer 1969a).
10.1 Thresholds and Peaks 419
1.7 eV (730 nm); a PC continuum at energies below 1.7 eV observed in type Ib diamonds due to some trapping levels. This PC feature can be quenched by illumination with light of quantum energy below 2.5 eV (Farrer 1969a). 1.7 eV (730 nm); TH; a broad PC threshold observed in CVD diamond films. The dominating charge carriers responsible for this PC feature are holes. The spectrum of the feature is interpreted as two electronic transitions with energies of 2.5 and 3 eV, which occur between the conduction and valence bands through a level lying at 3 eV above the valence band (Manfredotti et al. 1995). 1.9 to 2.3 eV (650 to 540 nm); TH; a feature of p-type photoconductivity observed in all natural type Ia diamonds irrespective of their impurity content. This feature is thought to be induced by an intrinsic defect, for instance, dislocations (Tatarinov 1986). 1.94 to 2.0 eV (620 to 640 nm); TH; a feature observed in synthetic diamonds doped with mixtures of species Ti+In+As, Ti+In+P, or Ti+Zr (Butuzov et al. 1976). 2 eV (640 nm); TH; several thresholds with energies below 2 eV observed in natural diamonds. The nitrogen content does not noticeably influence these features. The features are observed in PC and PSC spectra (Tatarinov 1986). 2 eV (640 nm); P; a peak observed in PC spectra of natural semiconducting diamonds. The feature results from internal photo-emission of holes into the diamond from a metal contact (the illuminated metal contact must be biased positive; the peak does not appear with nonmetal electrodes). The 2 eV peak is accompanied by peaks at 2.5 (495), 3.3 (375), 4.3 (288), 5.1 (243) and 5.6 eV (221 nm), which correspond to some energy levels in the forbidden gap of semiconducting diamond (these peaks are also seen in tunnel spectroscopy) (Lepek et al. 1976; Lepek et al. 1979). 2.07 eV (600 nm); P; a broad band readily observed in high-dislocation diamonds. The feature is attributed to dislocations (Samsonenko et al. 1974; Samsonenko et al. 1978; Samsonenko and Timchenko 1986). 2.07 to 2.14 eV (580 to 600 nm); TH; observed in synthetic diamonds doped with mixtures of species Ti+In+As, Ti+In+P, or Ti+Zr (Butuzov et al. 1976). 2.25 eV (550 nm); TH; observed in synthetic diamonds doped with mixtures of species Ti+In+As, Ti+In+P, or Ti+Zr (Butuzov et al. 1976). 2.3 eV (540 nm); TH; observed in (100)-oriented nominally undoped CVD diamond films grown on a Si substrate (the films can contain about 10 ppm of nitrogen). The feature is attributed to a set of trap levels in the bandgap. The feature may be a result of either ionization of nitrogen donors, or photo-ionization of the amber center (the 0.77 eV center) (Rohrer et al. 1997a). (Fig. 10.2).
420
10 Photoconductivity
2.38 eV (520 nm); TH; observed in synthetic diamonds doped with mixtures of species Ti+In+As, Ti+In+P, or Ti+Zr (Butuzov et al. 1976). 3.0 eV; TH; observed in (100)-oriented nominally undoped CVD diamond films grown on Si substrates; the films can contain about 10 ppm of nitrogen. The feature is attributed to a set of energy levels in the bandgap (Rohrer et al. 1997a). Note that the ND1center may also exhibit photoconductivity in this spectral region in type I diamonds after electron irradiation with a dose of 3×1017 cm-2 (Farrer and Vermeulen 1972; Sobolev and Yeliseev 1976) (Fig. 10.2). 3.1 to 3.5 eV (400 to 350 nm); P; the ND1 center; a set of peaks of p-type conductivity readily observed in type Ia natural diamonds with n-type conductivity. The ND1 center exhibits no PC in p-type natural diamonds (Tatarinov 1986). The spectral structure of this feature can be observed at RT and it is not frozen out even at LHeT (Farrer and Vermeulen 1972) (Fig. 10.4). 3.3 to 3.6 eV (375 to 345 nm); HT; a common PC feature of n-type photoconductivity observed in type Ia natural diamonds (Tatarinov 1986). 4.0 eV (310 nm); TH; a feature always observed in type IaA diamonds with concentration of A-aggregates of nitrogen above 1018 cm-3. It may be absent from spectra of diamonds with low concentrations of the A-aggregates (Tatarinov 1986). The feature was found to be of n-type photoconductivity (Denham et al. 1967; Konorova and Shevchenko 1967). However, p-type photoconductivity was ascribed to the feature based on photo-Hall effect measurements (Tatarinov 1986). The feature is attributed to the A-aggregates of nitrogen (Denham et al. 1967; Collins 1997; Konorova and Shevchenko 1967). 4.05 eV (306 nm); TH; observed in {100} growth sectors of nominally undoped synthetic diamonds. It can also be detected in low-boron doped diamonds. The feature correlates with the absorption intensity of the nitrogen donors. The feature is believed to relate to uncompensated nitrogen donors (Denham et al. 1967; Konorova et al. 1965; Vishnevskii et al. 1978). 4.2 eV (295 nm); TH; observed in (100)-oriented nominally undoped CVD diamond films grown on Si substrates; the films may contain about 10 ppm of nitrogen. The feature is bleached by UV illumination and cannot be restored afterwards by annealing at temperatures up to 800 K. Tentatively the feature is attributed to a set of electron levels associated with surface defects (Rohrer et al. 1997a; Rohrer et al. 1998) (Fig. 10.2). 4.6 eV (270 nm); TH; observed in {100} growth sectors of nominally undoped synthetic diamonds. It is also observed in boron-doped diamonds with low boron content. The feature is ascribed to uncompensated nitrogen donors. In low-nitrogen diamonds the intensity of the feature correlates with the intensity of the
10.1 Thresholds and Peaks 421
PHOTOCURRENT PER PHOTON, arb. units
corresponding nitrogen-related absorption band (Denham et al. 1967; Chrenko et al. 1971; Konorova et al. 1965; Vishnevskii et al. 1978) (Fig. 10.7).
10
2
10
1
10
0
-1
10
-2
10
2
3
4
5
6
QUANTUM ENERGY, eV
Fig. 10.7. Photoconductivity spectrum of type Ib diamond. The main feature of the spectrum is the 4.6 eV threshold ascribed to uncompensated nitrogen donors. The spectrum is corrected for spectral response of the set-up
4.86 eV (255 nm); P; a PC peak observed in type Ia diamonds containing platelets. The low-energy threshold of this peak is at 4.05-4.1 eV. The peak is not usually observed in type Ib diamonds. This feature is possibly related to platelets (Konorova et al. 1965; Vermeulen and Nabarro 1967). 5.1 eV (245 nm); P; the N9 center; a feature observed at RT. At LNT the N9 center is absent from PC spectra; the freezing out of the N9 center is explained by the location of the excited state of the N9 center very close to the bottom of the conduction band (Nahum and Halperin 1062; Sobolev and Yeliseev 1976). In diamonds with high concentration of the B-aggregates of nitrogen the N9 centers exhibits intrinsic photoconductivity, that is the N9 center suppresses PC in both p- and n-type samples (Tatarinov 1986). 5.250 eV (236 nm); TH; a feature is attributed to the exciton ionization occurring in type IIa natural and synthetic boron-doped diamonds. The feature is well observed in high-quality CVD diamond films. The energy of the feature is Eex - hωcTO (Eex = 5.400 eV at RT; TO phonon energy 141 meV, at k = kc). This PC feature appears due to thermally dissociating excitons (Collins 1993b; Vishnevskii et al. 1978; Rohrer et al. 1998; Hiscock and Collins 1999) (Fig. A.2, A.3).
422
10 Photoconductivity
5.322 eV (232.9 nm); TH; observed in synthetic semiconducting boron-doped diamonds. The feature is ascribed to ionization of excitons interacting with 87 meV TA phonons at k = kc. The energy of the feature is Eex - hωcTA (Vishnevskii et al. 1978) (Fig. A.2, A.3). 5.353 eV (231.6 nm); TH; observed in synthetic boron-doped diamonds. The feature is ascribed to ionization of indirect excitons bound to neutral acceptors. The energy of the feature is Eg - Eax, where Eax = 56 meV is the binding energy of the excitons (Vishnevskii et al. 1978). 5.489 eV (225.8 nm); TH; observed in synthetic boron-doped diamonds and highquality natural and CVD diamonds. The feature is ascribed to ionization of excitons of energy Eg + hωcTA (hωcTA = 87 meV, at k = kc) (Vishnevskii et al. 1978; Hiscock and Collins 1999) (Fig. A.2, A.3). 5.550 eV (223.3 nm); TH; readily observed in synthetic boron-doped diamonds, high-quality type IIa diamonds and high-quality CVD diamond films. The feature is ascribed to ionization of excitons with absorption of TO phonons. Energy of the transition is Eex + hωcTO; hωcTO = 141 meV, at k = kc (Vishnevskii et al. 1978; Hiscock and Collins 1999) (Fig. 10.8). This threshold can be recognized in the spectra in Fig. A.2, A.3.
QUANTUM EFFICIENCY, %
30
20
10
0 120
140
160
180
200
220
240
WAVELENGTH, nm
Fig. 10.8. Quantum efficiency (at RT) of a photoconductive detector made on natural type IIa diamond (Binari et al. 1993). The main feature of the spectrum is the threshold due to ionization of excitons with absorption of TO phonons
10.2 Microwave Photoconductivity 423
10.2
Microwave Photoconductivity
MWPC can be effectively used for characterization and selection of diamonds for electronic applications (Zaitsev et al. 1992), for instance, for fabrication of p-i-p and p-i-n injecting structures by boron and lithium ion implantation. The electronic quality of diamond can be assessed from the intensity and the spectral position of the MWPC maximum (Fig. 10.9).
WAVELENGTH, nm
Fig. 10.9. MWPC diagram for selection of diamonds of electronic quality: (I) - diamonds suitable for fabrication of stable semiconducting areas by B+ and Li + ion implantation and p-i and n-i junctions with low-energy barrier; (II) - diamonds suitable for fabrication of semiconducting areas by ion implantation and p-i and n-i junctions with high-energy barrier; (III) - diamonds unsuitable for fabrication of semiconductive areas by ion implantation. As an example a change of electronic quality of low-nitrogen type IIa natural diamond after irradiation and annealing treatments is shown by the curves: (a) – as-irradiated; (b) - after irradiation and subsequent annealing at 900°C; (c) - after surface graphitization in low vacuum at a temperature of 1200°C (Zaitsev et al. 1997b)
The first area of the plot in Fig. 10.9 comprises diamonds exhibiting intense photoconductivity with maxima at energies above Eg. These diamonds exhibit highly conductive p-type boron implanted areas and effective hole injection in p-i-p structures at electrical fields of about 104 V/cm. The second area comprises diamonds still having high MWPC intensity with maxima close to Eg and diamonds showing MWPC of moderate intensity and maxima well above Eg. The diamonds from the second area also support high p-type conductivity after boron ion implantation. However, hole injection in p-i-p structures on diamonds of the second area occurs only at fields above 105 V/cm. The diamonds covered by the third area have low-intensity MWPC with maxima below Eg. These diamonds exhibit strong
424
10 Photoconductivity and Photostimulated Current
compensation of implanted boron acceptors (the conductivity of boron implanted areas may vary over eight decades at equal implantation conditions), and hole injection in p-i-p structures cannot be achieved even in fields above 106 V/cm. There is a tendency that the MWPC intensity of diamond increases with decrease in the nitrogen content (Fig. 10.10). It is seen that nitrogen only restricts the possible MWPC intensity from above. The birefringence measurements on lownitrogen crystals (the A-aggregate concentration below 1.5×1018 cm-3) show that internal mechanical strains is another main factor reducing MWPC. Actually the internal mechanical stress is the primary reason affecting MWPC of natural diamonds with concentration of the A-aggregates of nitrogen up to 5×1019 cm-3. MWPC of low-nitrogen natural diamonds exponentially depends on their internal structural perfection measured by birefringence (POM method) (Zakharov 1997) (Fig. 10.11). 10 5
10 4
10 3
10 2
10 1
10 17
10 18
10 19
10 20
A-AGGREGATE NITROGEN CONCENTRATION, 1/cm³
Fig. 10.10. MWPC intensity of natural diamonds versus concentration of the A-aggregates of nitrogen. The broken line indicates the highest value of possible MWPC intensity limited by nitrogen impurity
Annealing in vacuum drastically changes the shape and intensity of MWPC spectra of natural type IIa diamonds. The MWPC intensity may increase by several times after heating at temperatures of 800 to 900°C and then it falls down by a several orders of magnitude after further annealing at temperatures above 1000°C. This strong decrease in PC intensity is believed to be caused by desorption of hydrogen from the diamond surface at high temperatures (Zakharov 1997) (Fig. 10.12, 10.13). CVD diamond films may show a MWPC continuum of moderate intensity over the whole UV and visible spectral regions (Zakharov 1997) (Fig. 10.14). MWPC of nitrogen-doped CVD diamond films is lower by a few orders of magnitude than that of intentionally undoped films (Dresselhaus and Kalish 1992). PCCVD diamond
10.2 Microwave Photoconductivity 425
films of low-quality (smoky-brown coloration) show noticeable MWPC in the spectral range 200 to 700 nm (Zakharov 1997). 5000
4000
3000
2000
1000
0 0
100
200
300
400
BIREFRINGENCE, arb. units
Fig. 10.11. MWPC intensity of low-nitrogen natural diamonds versus value of internal lattice strain measured by birefringence (Zakharov 1997, Zaitsev et al. 1997b)
Mechanical polishing may increase the MWPC intensity of natural diamonds by an order of magnitude. A factor affecting the MWPC of nontreated natural diamonds is silicate and carbonate films covering their surface (especially (111) facets) as well as micro-inclusions locating near the surface (Zakharov 1997).
MW PC INTENSITY, arb. units
10
5
Eex + TA
10
4
10
3
10
2
10
1
10
0
210
Eex + 116 meV
215
220
225
Eex + 43 meV
230
235
240
WAVELENGTH, nm
Fig. 10.12. Typical change of shape of MWPC spectrum (taken at RT) of type IIa natural diamond with annealing: before annealing (continuous line), after annealing at 900°C (dashed line) and after annealing at 1200°C (doted line) (Zakharov 1997)
426
10 Photoconductivity and Photostimulated Current
100
b MWPC INTENSITY, arb. units
80
60
40
20
0 0
200
400
600
800
1000
1200
1400
1600
TEMPERATURE, °C
MW PC INTENSITY, arb. units
Fig. 10.13. Change of MWPC intensity with annealing temperature for two type IIa type natural diamond samples (Zakharov 1997)
100
1 10
2 1 200
300
400
500
600
700
WAVELENGTH, nm
Fig. 10.14. MWPC spectra of a good-quality CVD diamond film (1) and a natural type IIa diamond (Zakharov 1997, Zaitsev et al. 1997b)
10.3
Miscellaneous
The A-band and probably the 575 nm center are active in PC of natural p-type diamonds (Lepek et al. 1976). The N3, H3, H4 and S2 optical centers are not active in PC (Sobolev and Yeliseev 1976).
10.3 Miscellaneous 427
PC is a much more sensitive detection method for the radiation centers like R, TR, GR and ND. All of these centers can be relatively strong in PC, whereas in absorption they are beyond the detection limit (Yelisseyev et al. 1995b). Commonly natural diamonds with high concentration of the A-aggregates of nitrogen (above 1018 cm-3) exhibit p-type PC in a wide spectral range from 1.5 to 5.5 eV. In contrast, diamonds with low A-aggregate content show n-type PC over the whole spectral range from near IR to the band-gap energy (Tatarinov 1986). Commonly type Ib diamonds do not show PC at quantum energies above 4 eV (Farrer 1969a). Synthetic type Ib diamonds do not show remarkable PC in the spectral area 0.3 to 1 eV at RT (Vishnevskii et al. 1978). M-i-p diamond structures deposited by the CVD technique exhibit photosensitivity in a spectral region from about 850 nm to 300 nm (maximum at around 500-600 nm). The maximum of the integral sensitivity may attain a value of 1mA/lm (Alexenko and Spitsyn 1991). M-p-M photosensitive structures on diamond may exhibit a quantum efficiency above 10% at wavelengths of 130 to 220 nm (Binari et al. 1993). The collection distance of photo-excited charge carriers in UV PC detectors made on high-quality CVD diamond films may attain hundreds of microns (Collins 1997; McKeag et al. 1996). Diamond "solar-blind" photodetectors made of high-quality CVD diamond films may exhibit a large transient sensitivity to the visible light after exposure to the ultraviolet radiation. It is nteresting that simultaneous illumination with visible and UV light produces a photocurrent that is larger that the sum of the separate responces (Hiscock and Collins 1999).
11
Related Data
This paragraph summarizes miscellaneous data related to the optical properties of diamond.
11.1 Mechanical Properties Narrow nonluminescent (observed in CL) outcrops of thin sheets of the {110} growth sector usually have superior abrasion resistance (Field 1992). Brown diamonds exhibit a greater abrasion resistance and a rougher cleavage compared to those of colorless diamonds (Wilks J. and Wilks E. 1991). Plastic deformation occurring by sintering of diamond compacts produces in diamond grains intense nitrogen-related centers 575 nm (especially strong), 638 nm, and the H3 center. The reason for this is the production of vacancies by mobile dislocations and capturing these vacancies by nitrogen. This effect is particularly strong in small diamond grains (Evans et al. 1984). The spectra of optical centers taken from sintered diamond compacts are strongly broadened. This broadening corresponds to the internal stress comparable with that used for sintering (up to 6.5 GPa) (Evans et al. 1984). The internal nonhomogenous stress P in a diamond lattice can be evaluated from FWHM of ZPLs of optical centers measured at LNT (Collins and Robertson 1985a; Zaitsev et al. 1994; Evans et al. 1984) (e.g. Fig. 5.46): P[GPa] = 0.04 FWHM[meV] for the GR1 center; P[GPa] = (0.1 ÷ 0.15) FWHM[meV] for the 2.156 eV nitrogen-related center; P[GPa] = 0.32 FWHM[meV] for the 3.188 eV nitrogen-related center; P[GPa] = 0.28 FWHM[meV] for the 1.681 eV silicon-related center; P[GPa] = 0.1 FWHM[meV] for the 1.945 eV nitrogen-related center. The quantun energy of the optical gap of the diamond window under pressure above 300 GPa was found as: EL [eV] = 3.95 - 0.0051PS[GPa], where PS is the quasihydrostatic pressure experienced by the diamond window (Ruoff et al. 1991b). High-pressure anvils made of natural diamond exhibit a rapidly growing PL (possibly due to nitrogen) with pressure increasing above 100 GPa. At pressures above 280 GPa this luminescence vanishes. The intensity of this PL feature may exceed by two orders of magnitude the common PL of natural diamonds excited at normal pressure (Xu et al. 1986). The spectral range of this PL may range from 450 to 850 nm; the most effective excitation occurs at a wavelength of 480 nm. At a pressure of 270 GPa a band at 2.21 eV (FWHM about 0.1 eV) appears in high-purity
430
11 Related Data
synthetic diamond. By 370 GPa this band increases in intensity by three orders of magnitude and shifts down to 1.86 eV; at 370 GPa the band has an excitation threshold between 514 and 633 nm (Jun Liu and Vohra 1996). In contrast to natural diamonds, quasihydrostatic pressure up to 253 GPa does not produce in synthetic diamond any PL centers within a spectral range from 500 to 900 nm (Vohra et al. 1994). Nitrogen considerably influences the mechanical strength of diamond. Synthetic diamonds with increased concentration of C-defects (dispersed nitrogen) are characterized by lower mechanical strength (probably due to higher content of metal inclusions formed in such diamonds). High concentration of dispersed nitrogen (above 1000 ppm) may considerably weaken diamonds (Nassau 1993). The A- and B-aggregates of nitrogen appear not to affect the mechanical strength of diamond, whereas platelets (characteristic absorption at 7.3 µm) reduce it (Field 1992). It is known that platelets produce considerable strain in the diamond lattice. The luminescence of natural diamonds may correlate with their mechanical strength (Moore 1979; Kalinin et al. 1972). The areal distribution of intensity of visible PL in type IIa natural diamonds closely correlates with the distribution of internal mechanical stress measured by the birefringence method (Dean and Male 1964d; Bienemann-Kuespert et al. 1967). Natural diamonds active in PL are usually mechanically weaker than nonluminescent ones (Artsimovitch et al. 1965; Gumilevskii 1952). Diamonds revealing no PL under UV excitation exhibit less wear compared to luminescent diamonds (Artsimovich et al. 1969). However, diamonds with green-blue or yellow PL have the best wear properties (Wilks and Wilks 1991; Kalinin et al. 1972). Mechanical stress in diamond layers implanted with high-energy ions is distributed very nonhomogeneously. It is much higher in deeper layers of the implanted area causing strong broadening of ZPLs of optical centers (Fig. 5.75, 5.31, 5.32). Common ion implantation with energies of tens and a few hundreds of keV at high doses also causes considerable internal strain and corresponding nonhomogeneous broadening of ZPLs (Varichenko 1986) (Fig. 5.79). HPHT treatment of diamonds implanted with high-energy ions results in strong internal mechanical stress causing considerable broadening of spectra of optical centers. This stress can be removed by subsequent annealing in vacuum (Fig. 5.60). Especially pronounced broadening of ZPLs of optical centers in ion-implanted diamonds is observed after annealing at temperatures of defect transformations. Figure 5.61 shows the broadening of the ZPL of the 575 nm center at the temperature of its aggregation into the H3 center. Mechanical stress in deeply buried ion implanted layers can be detected by PL, provided the sample is transparent enough (Fig. 5.36). The shift of the diamond Raman line in CVD diamond films on nondiamond substrates due to mismatch of their thermal expansion coefficients implies good adhesion of the film (Fabisiak et al. 1992). The hardness of synthetic diamonds with nitrogen concentration above 1019 cm-3 increases with the intensity of boron-related optical centers (Novikov 1968).
11.2 Edge Electronic Transitions 431
11.2
Edge Electronic Transitions
Exciton CL (both free and bound to boron excitons) may be strong in isolated crystallites of CVD diamond films at initial growth stages. However the exciton luminescence vanishes when the crystallites coalesce and form a continuous closed film at later growth stages (Robins et al. 1993; Sternschulte et al. 1996a). This effect is explained by mechanical stress appearing due to intercrystallite interaction (see Fig. 5.46) (*), or by nonradiative recombination centers locating at the interfaces of the grain boundaries (Sternschulte et al. 1996a). The edge luminescence in diamond is strongly quenched by lattice deformation (Crowther and Dean 1964). This is the reason for the relatively low edge luminescence from PCCVD diamond films as compared to that excited in singlecrystal diamonds (Sharp and Collins 1995). A stress of 1 GPa quenches the edge emission by about 25% (Sharp and Collins 1995). Optical absorption of PCCVD diamond films at quantum energies of and above the fundamental absorption edge is higher than that of the bulk single crystals. This peculiarity is attributed to additional absorption by electronic states of a disorderinduced energy band (Khomich et al. 1995a; Robins et al. 1991b). In high-quality HPHT synthetic diamonds free exciton luminescence is stronger from {100} growth sectors than from {111} sectors. The reverse case takes place for bound exciton emission. However the most intense free exciton emission occurs from {110} sectors (Sharp and Collins 1995). Most of the deep traps controlling the working voltage of injecting electronic structures on diamond are also good recombination centers controlling τb. This means that the electronic quality of diamonds can also be characterized by the luminescence intensity of free exciton emission. Figure 11.1 shows that the stronger the exciton luminescence of diamond substrates the lower the onset voltage of p-i-p and p-i-n diodes made thereon. Emission of free and bound excitons in CVD diamond films is strongly suppressed by nitrogen. The films exhibiting strong nitrogen-related luminescence (e.g. the 2.156 and 3.188 eV centers) exhibit very weak exciton luminescence. This fact implies that the nitrogen-related defects are effective recombination centers (Sternschulte et al. 1996a). The exciton luminescence intensity is affected by ion irradiation (ion energy in the range to a few hundred keV) when carried out at doses above 1013 cm-2. Lower doses do not reduce the exciton luminescence. It is very strange that this dose threshold does not depend on the ion species (at least for B+, Na+, P+ and As+ ions) (Sternschulte et al. 1999a).
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10
17 19
A-aggregates of nitrogen, 1/cm³ 1018
17 1019
103 102 Theoretical limit
101 100 -1 10
100 101 102 FE intensity, arbit. units
103
Fig. 11.1. Onset voltage of p-i-p and p-i-n diodes versus CL intensity of free exciton measured on the corresponding diamond substrates. The free exciton CL intensity is scaled against concentration of the A-aggregates of nitrogen (Fahrner et al. 1998)
11.3
Thermal Properties
The intensity ratio of the H3 and 1.945 eV nitrogen-related centers measured in synthetic diamond can be used as an indicator of the highest temperature at which this diamond was treated (for the treatment temperatures above 1300°C). This method is especially sensitive for temperatures above 1600°C (Evans et al. 1984). The absorption strength µ1200 in the one-phonon spectral range measured, for instance, at a wavenumber of 1200 cm-1 can be used for evaluation of room temperature thermal resistivity of diamond γ (Woods et al. 1990a; Morelli and Uher 1993a; Burgemeister 1978): lg(γ[K cm W-1]) ≈ 0.02 lg(µ1200[cm-1]) + 0.006. The following relation between the absorption intensity of the GR1 center µGR1 and the thermal resistivity γ of electron irradiated type II diamonds can be derived from the experimental data in (Morelli 1994; Burgemeister et al. 1980): γ[cm K W-1] ≈ 1.6×10-3µGR1 [cm-1], that is the thermal resistivity of diamond is proportional to the vacancy concentration. The thermal conductivity of natural diamonds at 320 and 450 K is well described as functions of absorption coefficients of the A- and B-aggregates of nitrogen by the following empirical expressions: Λ320K[W/(m K)] = (5.18×10-4 + 2.6×10-5 µB[cm-1] + 2.1×10-5 µA[cm-1]0.8)-1, Λ450K[W/(m K)] = (8×10-4 + 2.5×10-5 µB[cm-1] + 2.7×10-5 µA[cm-1]0.8)-1,
11.3 Thermal Properties 433
where µA and µB are absorption coefficients for the principal bands of the A- and B-aggregates (Kluev et al. 1991). The following relation between the total concentration of the A- and B-aggregates of nitrogen NA+NB and thermal conductivity Λ of type Ia diamonds can be derived from the experimental data in (Burgemeister 1978; Morelli 1994): Λ[W/(cm K)] ≈ -8 lg{(NA+NB)[cm-3]} + 171. The thermal conductivity of natural diamonds depends on the content of the A-aggregates of nitrogen as follows (Kvaskov 1990): Λ[W/(cm K)] = [0.052 + 2.1×10-5(1.1µ1282 - 0.2µ1135)]-1. Values of the thermal conductivity parallel Λ|| and perpendicular Λ⊥ to the sample plane can be evaluated from the optical absorption coefficient µvis averaged over the visible spectral range according to the expression (Graebner 1995): Λ|| or ⊥ = A|| or ⊥ [µvis/B|| or ⊥ + 1]-1, where A|| = 20.21 W/(cm K), B|| = 136.7 cm-1, A⊥ = 22.05 W/(cm K), B⊥ = 209.0 cm-1. This expression is applicable to CVD diamond films made by various growth methods (Graebner 1995). CVD diamond films (irrespective of the growth technique) with an average absorption coefficient of less than 2 cm-1 show thermal conductivity of the best type IIa natural diamonds (20 to 25 W/(cm K)) (Graebner 1995). For thick PCCVD diamond films there is a trend: the more optically transparent the more thermally conductive (Sussman et al. 1994a; Graebner 1995). There is a good correlation between the in-plane thermal conductivity of CVD diamond films and the intensity of the diamond Raman line normalized to the strength of Raman scattering from nondiamond phases. Samples with Λ ≈ 20 W/(cm K) show the relative intensity of the diamond Raman line almost two orders of magnitude higher than those with Λ ≈ 7 W/(cm K) (Woerner et al. 1996). The thermal diffusivity of CVD diamond films falls exponentially from 10 to 0.3 cm2/s with increase in the content of the graphitic phase G from 0 to 3 % (G = IG/(50 ID), where ID and IG are the intensities of the diamond Raman line and the 1550 cm-1 band (Pryor et al. 1991; Plamann and Fournier 1996). A narrow diamond Raman line is a necessary, but not sufficient condition for high thermal conductivity of CVD diamond films (Plamann and Fournier 1996). The thermal conductivity of CVD diamond films correlates inversely with the intensity of the luminescence band at 2.0 eV, which is attributed to a nitrogencontaining defect and indirectly indicates the presence of sp2-bonded carbon (Bachmann et al. 1994b; Plamann and Fournier 1996). It has been noticed that the shorter the wavelength of the A-band maximum the higher the phase purity and the higher the thermal conductivity of CVD diamond films (Heiderhoff et al. 1995). The thermal conductivity of diamond can be evaluated by the PAS method (Obraztsov et al. 1996). Hydrogen-related defects detected in absorption within the IR range from 2800 to 3000 cm -1 strongly affect the thermal conductivity of CVD diamond films: Λ∼a[bCH+c]-1, where CH is the concentration of the hydrogen-related defects; a, b and c are parameters (McNamara et al. 1995). The thermal conductivity of CVD diamond films falls from about 1000 W/(m K) down to 150 W/(m K) when the hydrogen content increases from about 0.1 atom% to 1 atom% (Baba et al. 1991).
434
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Nickel impurity plays a dominant role in reduction of the thermal conductivity of synthetic diamonds (Palyanov et al. 1997a; Antsygin et al. 1995). For instance, there is a linear relation between the reduction in thermal diffusivity of nickel-containing diamonds and concentration of the 1.883 eV nickel-related center (Palyanov et al. 1997a; *): ΛT[cm2/s] = -1.71 µ1.883 eV[mm-1] + 10.4.
11.4
Lattice Structure and Defects
The lattice dilatation ∆a/a of type Ib diamonds is proportional to the absorption intensity of the nitrogen C-defects: ∆a/a = (2.95±0.27)×10-6 µ1130cm-1[cm-1] (Field 1992). The 1.647 eV PL line could possibly serve as a quantitative indication for the stress, defects and impurity content deteriorating the structural quality of diamond films (Wang et al. 1996). Local vibrational modes are observed as sharp lines only in very perfect almost defect-free diamonds (Fuchs et al. 1995b). Irradiation with slow neutrons does not noticeably change the intrinsic and nitrogen-related absorption in the range from 3.2 to 9.9 µm (Bienemann-Kuespert et al. 1967). The absorption coefficient of the graphitized buried layer produced in natural diamond by 350 keV He+ ion implantation with doses above 2.8×1016 cm-2 (2.8×1016 cm-2 is the critical dose of graphitization of diamond by 350 keV He+ ions) at a wavelength of 700 nm is 1.2×105 cm-1 (Khmelnitskiy et al. 1996). Platelets precipitates cause a green-yellow color of CL emission of plateletscontaining diamonds (assumed to be the H3 center). This emission is strongly polarized with the E vector in the platelet plane about 10 times stronger than that normal to the platelet plane (Lang 1977). Less than 1% of nondiamond carbon finely dispersed in diamond (submicrometer size particles) destroys its transparency (Field 1992). There is a tendency that the intensities of PL, Raman scattering from nondiamond phases, and EPR from dangling bonds in PCCVD diamond films, decrease with film thickness. This intensity reduction is caused by increase in the crystallite size and reduction in the defect density (Bergman et al. 1993; Bernardez and McCarty 1993; von Kaenel et al. 1996). The intensity of PL observed as the background in Raman spectra of CVD diamond films increases with reduction in the film growth temperature. The PL background attains a maximum at a growth temperature when noncrystalline (cauliflower type) structures appear (Stiegler et al. 1996). The absorption strength in the one-phonon spectral range at a wavenumber of 1200 cm-1 µ1200 cm-1 can be used for the evaluation of the total defect content in diamond and characterization of the crystal lattice perfection (Morelli and Uher 1993a).
11.4 Lattice Structure and Defects 435
In flame-grown PCCVD diamond films the PL intensity ratio of the 2.156 eV and 1.945 eV nitrogen-related centers follows the intensity ratio of the diamond Raman line and nondiamond G-band. In the most perfect CVD films only the 2.156 eV center is observed, in the films containing a high concentration of nondiamond phases only the 1.945 eV center is detected (Tzeng et al. 1991). The PL of CVD diamond films deposited by the combustion-flame method exhibits a high creation rate of nitrogen-related defects, when the films are grown under laminar flame conditions. In contrast, turbulent-flame growth conditions provide a low PL intensity of these defects (Freitas et al. 1992). The absorption coefficient of perfect single-crystal synthetic diamonds at a wavelength if 1 µm does not exceed 10-4 cm-1 (Klein 1993). There is a relation between the magnitude of lattice dilatation and the strength of infrared absorption due to substitutional nitrogen: ∆a/a = (2.75±0.14)×10-6 µC [cm-1], where µC is the absorption coefficient at a wavenumber of 1130 cm-1 (Lang et al. 1991). The intensity of the broad PL band in the visible spectral region excited in CVD diamond films with the 514.5 nm line of an Ar laser correlates with the 1525 cm-1 Raman band attributed to sp2-bonded carbon. This PL band is tentatively attributed to sp2-bonded carbon clusters (Robins et al. 1991a). Almost all CL in type II (including type IIb) natural diamonds results from dislocations, because it is not masked with stronger luminescence from other defects (mostly from nitrogen in natural diamonds, and from nitrogen and nickel in HPHT synthetic diamonds) (Wilks and Wilks 1991; *). Most, but not all, dislocations in natural diamonds are active in luminescence (Yamamoto et al. 1984; Pennycook et al. 1980). Intersecting stripe-like micro-CL is a characteristic of dislocation- and slip-band induced luminescence. The stripe-like CL pattern caused by the growth layers can not show intersections (Wilks and Wilks 1991). The slip bands are especially pronounced on polished surfaces of low-dislocation diamonds, both single crystals and polycrystals (Hanley et al. 1977; DeVries 1973). The dislocation kinks may be efficient nonradiative recombination centers in diamond (Yamamoto et al. 1984). The main nitrogen-related defects (the A-, C-, B-aggregates, the N3 center and the B' center) often show definite concentration ratios for diamonds from a particular deposit. Using these ratios the origin of natural diamonds can be established. For instance, the concentration ratio of the A-aggregates and C-defects in most diamonds from the Jakutian and Ulal deposits differs approximately by a factor of 2 (Sobolev 1978). Natural diamonds with N3 centers usually have an increased concentration of C centers, most of the nitrogen atoms being in the vicinity of each other (however not yet forming the closest pairs like the A- or B-aggregates). These nitrogen atoms, still revealing C center IR absorption, are no longer active any more in EPR. This way the nitrogen concentration in the C-form can be more than an order of magnitude higher when measured by IR absorption as compared to that measured by EPR. Such an aggregated-like distribution of the C centers is a characteristic feature of natural
436
11 Related Data
diamonds with yellow shells (Chrenko et al. 1971; Sobolev 1978; Samsonenko 1964). Nitrogen-related centers in most natural diamonds are not distributed uniformly but have distributed over local areas with very sharp boundaries. The concentration ratios of these centers in different areas may vary within three orders of magnitude, whereas the distribution of the centers within each area is rather uniform (Sobolev 1978). Mere neutron irradiation may not produce in A, CL, PL and XL spectra point radiation centers characteristic of irradiated diamonds (e.g. GR1, TR12, 3H, ND1) (Vins et al. 1988; *). However moderate simultaneous heating (or ionization) during neutron irradiation stimulates the formation of these centers (Zaitsev and Zaitsev 1989). (Fig. 5.42, 5.53). Neutron irradiation with a dose of 7×1017 cm-2 makes diamond completely dark and unsuitable for conventional absorption measurements. A 300°C annealing partially restores the transparency in the infrared region (wavelengths above 600 nm) so that absorption measurements are possible. In a temperature range from 600 to 750°C the absorption slightly increases again, due to, possibly, the formation of some vacancy-like defects (note that the common GR1 spectrum is not observed after annealing) (Nishida et al. 1989). In diamonds with high content of the A-aggregates of nitrogen no PL under UV excitation is observed (Davies and Summersgill 1973c; Brozel et al. 1978). Vacancies (the GR1 center as well) stabilize the diamond structure. For instance, there is an opinion that it is the presence of vacancies that makes CVD diamond growth possible (Allers and Mainwood 1997; Bar-Yam and Moustakis 1989). Elevated concentration of the Si center significantly reduces the overall CL intensity of the CVD diamond film. This way the Si center intensity can serve as an indicator of inferior quality of CVD diamond films (Yacobi et al. 1991; *).
11.5
Anisotropy and Polarization
A broad band observed in A and PL in the spectral range from 700 to 850 nm in diamonds irradiated with neutrons is polarized. It is nteresting that the polarization direction correlates with the neutron irradiation direction (Sildos and Osvet 1994b). Cuboid growth sectors of natural diamonds are rich in PL and CL optical centers observed in the visible and near-infrared ranges (Field 1992). The absorption and luminescence features attributed to Ni impurity in synthetic diamonds are segregated exclusively in the {111} growth sectors (Collins et al. 1990c; Lawson and Kanda 1993a). A distinctive feature of synthetic diamonds is a stronger CL in octahedral growth sectors than that in cubic sectors (Anthony and Banholzer 1992). Green CL from cubic growth sectors of synthetic diamonds is strongly polarized with the E vector parallel to the growth surface of the sectors (Field 1992).
11.5 Anisotropy and Polarization 437
In synthetic diamonds grown by the temperature gradient method the S2 centers (the 2.369 eV centers?) are suggested to be preferentially oriented (Johnston et al. 1999).
11.6
Electrical Properties
The intensity ratio of the D1 (5.21 eV) bound exciton to the B1 (5.27 eV) free exciton IBE/FE in boron-doped PCCVD diamond films is given as a function of the boron concentration NB and electrical conductivity σ (Kawarada et al. 1993b): IBE/FE ≈ 10 lg(NB[cm-3]),
IBE/FE ≈ 10 lg(σ[S/cm]+2).
The dependence is presented in Fig. 11.2.
CONDUCTIVITY, S/cm -2
25 CL INTENSITY RATIO BETO/FETO
10
10
-1
10
-0
20
15
10
5
0 10
17
18
10
19
20
10
BORON CONCENTRATION, cm
10 -3
Fig. 11.2. Intensity ratio of a boron bound exciton to a free exciton measured in CL at LNT in PCCVD diamond films versus boron concentration. Boron concentration was measured by SIMS (Kawarada et al. 1993b)
The intensity of the bound exciton luminescence is used to measure the efficiency of activation of ion implanted boron as acceptors. Only 5% of boron implanted with conventional energies and doses (e.g. energy of 170 keV at a dose of 2.4×1013 cm-2; boron concentration of 1018 cm-3) converts into acceptors after annealing at temperatures above 1200°C (Sternschulte et al. 1999a).
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11 Related Data
The electronic quality of diamond can be assessed by the free exciton luminescence intensity (Fahrner et al. 1998) (Fig. 11.1). The capacity of the p-M Shottky junction on diamond increases when illuminated with white light. This effect is due to the excitation of electrons from the valence band to the levels of ionized donors in the depleted layer (Lightowlers and Collins 1976b). The photocapacity process is characterized by time constants of the order of one hour. Capacity excitation starts at a quantum energy of 1 eV (Collins and Rafique 1978). The mobility of the charge carriers µeh excited by laser pulses (quantum energy of 6.26 eV, PC measurements) in CVD diamond films is reduced at concentrations neh above 1015 cm-3. This reduction can be approximated by the expression: µeh ∼ µ0[5.32 - 0.27lg(neh[cm-3])], where µ0 is the mobility value at the excitation levels below 1015 cm-3 (Pan et al. 1991). The higher the FWHM of the diamond Raman peak the lower the µτ product (Plano et al. 1993). There is a strong correlation between the FWHM of the diamond Raman peak and the threshold field Ve for cold electron emission from CVD diamond films: with a FWHM increase from 5 to 10 cm-1 the Ve value falls from 160 V/µm down to 30 V/µm (Zhu et al. 1995). Amorphous carbon giving rise to the 1500 cm-1 broad Raman band in ion implanted diamond is proposed as a measure of the electrical conductivity of the ion damaged layer (Sato et al. 1991). The GR1 center is not a donor. It is well detected in the absorption of irradiated p-type boron-doped diamonds (Collins 1977). The radius of the boron acceptor, as derived from spectroscopic data on bound exciton luminescence, is 0.36 nm (Sternschulte et al. 1996b). The Mott density for boron acceptors in diamond derived from the data on CL of the excitons bound to boron acceptors in CVD diamond films is 1.5×1020 cm-3 (Sauer 1999). The energy level of a phosphorous-related donor, as derived from the D-A recombination spectra observed in phosphorous doped CVD diamond films, was found to be ED = 0.65 eV (Sauer 1999; Sternschulte et al. 1998). The CL intensity of excitons bound to phosphorus-related donors (at 5.175 eV) can characterize the efficiency of n-type doping of diamond by P+ ion implantation (Sternschulte et al. 1999a). The luminescence intensity of the green band at 2.3 eV can be used for the characterization of the surface electrical conductivity of hydrogenated diamond (Iakoubovskii and Adriaenssens 1999c). The CL of the H3 center can be excited in synthetic boron-doped diamonds only when their electrical resistivity is as high as 1016 Ω. This means that full compensation of the boron acceptors is needed to activate the H3 centers in luminescence (Novikov 1968; *). Both the isolated substitutional nitrogen in type Ib diamonds and the A-aggregate in type Ia diamonds act as donors with ionization energies of 1.7 and 4.0 eV respectively (Nazare and Neves 2001). However, there is an opinion that the A-aggregates of nitrogen may reveal acceptor properties (Tatarinov 1986).
11.6 Electrical Properties 439
11.7
Luminescence Excitation
11.7.1 Cathodoluminescence The first data on the CL of diamond were reported by Crookes (1879). Synthetic diamonds heavily irradiated with neutrons do not exhibit any noticeable CL centers in the optical region 360 to 600 nm (Malogolovets 1979; *). Of all topographic techniques CL topography appears to have the most acute growth-sector discriminating capability. The CL intensity of CVD diamond films integrated over the spectral region from 1.5 to 3.5 eV approximately inversely correlates with the intensity of PL excited by an Ar-laser at a wavelength of 514.5 nm. This correlation is particularly valid for CL of the A-band (Robins et al. 1991a). The total CL intensity of CVD diamond films can increase after H-plasma treatment, which may occur due to hydrogen passivation of nonradiative recombination centers (Hayashi et al. 1996). The transversal and longitudinal distribution of electron-hole excitation by an electron beam is given by Heiderhoff (1997) and Wu and Wittry (1978). The average penetration depth of electrons L exciting CL in diamond can be found from the expressions: L[µm] = 0.018(E[keV])1.83 in the energy range E from 5 to 70 keV (Field 1992); L[µm] = 0.011(E[keV])1.71 in the energy range E from 30 to 180 keV (Joshi et al. 1993). Partlow et al. (1990) mention that the penetration depth of electrons of energy from 10 to 50 keV in diamond varies from 1.4 to 23 µm. The excitation depth versus electron energy is also given by Burton et al. (1995b) as: 10 keV – 0.9 µm; 20 keV - 2.9 µm; 30 keV - 5.6 µm. The maximum penetration depth Lm of electrons in diamond can also be given by the expression (Kanaya and Okayana 1972): Lm [µm] = 0.02(E[keV])1.67. The threshold energy of creation of an electron-hole pair by high-energy electron excitation (for instance, by β-particles) is 18.5±1.5 eV (BienemannKuespert et al. 1967; Dean and Male 1964e). Routine CL measurements of diamond at LNT can be successfully carried out for concentrations of optical centers as low as 1014 to 1016 cm-3 depending on the structural perfection of the samples (Novikov 1968; *). The CL intensity of all Moessbauer-type optical centers in synthetic diamonds is suppressed at concentrations of single nitrogen atoms above 2.5×1019 cm-3. In such high-nitrogen diamonds the CL spectra contain only broad bands (Novikov 1968). Electrons of energy of tens of keV may induce or suppress some optical centers in diamond via subthreshold defect production or ionization. For instance, such suppression is observed during CL measurements for features at 428 and 437 nm as well as for the A-band in the spectral region of 450 nm. In contrast, an induction is
440
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readily observed for the 575 nm center in as-grown CVD diamond films, or in irradiated diamonds by electrons with an energy of tens of keV; this effect can be very pronounced in N+ ion implanted type IIa diamonds (Heiderhoff 1997; *). The CL intensity ICL in diamond can be expressed as: ICL ∼ f(Ie) (Ee-Ed)m, where f(Ie) is a function of the electron beam current, Ee is the electron beam energy, Ed is the energy corresponding to the near-surface "dead layer" Ld, and m ranges from 1 to 3. At low densities of electron beam current CL intensity in natural diamonds is proportional to the excitation depth with Ld about 50 nm (Heiderhoff 1997). Low boron doping increases the overall intensity of CL (including exciton emission) in CVD diamond films due to improving their crystal quality and reducing the concentration of deep nonradiative recombination centers (Gheeraert et al. 1993; Won et al. 1996b; *). The internal quantum output of the CL of the A-band excited in natural diamonds with electrons and α-particles may attain a value of 0.01 photons per electron-hole pair (Dean and Male 1964e). The CL of most optical centers in n-type diamonds is low. For instance, the CL intensity of the nitrogen-related 575 nm, H3 and N3 centers, and the A-band is considerably suppressed by Li+ ion implantation. This effect is believed to be due to the change of the charge state of optical centers in n-type diamonds. (Fig. 5.22, 5.23, 5.76). The CL intensity of the Green Band (at 540 nm) taken from {111} growth sectors of synthetic diamonds may be strongly reduced when excited with fine focused electron beams. No such dependence on the electron beam density is observed in {110} growth sectors (Kanda and Jia 2000).
11.7.2 Photoluminescence The PL intensity of some nitrogen-related centers in flame-grown CVD diamond films is significantly reduced when (i) the films are grown from low air mixture feeding the torch, (ii) the flame is turbulent, and (iii) the films are doped (Davies 1994a). The PL of CVD diamond films excited with quanta of energy above Eg (e.g. a frequency-doubled CW Kr ion laser operating at a wavelength of 206.5 nm) exhibits features which could be seen only in CL but not by exitation with quanta of energy below Eg (Holtz et al. 1996). PL excited with UV light is generally much more intense (commonly by two orders of magnitude) in type I natural diamonds than that in type II diamonds. The weakest PL is characteristic of semiconducting type IIb natural diamonds (Bienemann-Kuespert et al. 1967). Homoepitaxial CVD diamond films, when grown on heavily ion implanted diamond substrates, exhibit more intense PL at a wavelength of 550 nm (2.25 eV) (excitation with the 514 nm Ar laser line) (Weiser et al. 1996). By PL and Raman measurements of CVD diamond films containing nondiamond inclusions it is necessary to know the penetration depth of the exciting light into
11.7 Luminescence Excitation 441
these inclusions. It has been found that Ar-laser light at a wavelength of 514.5 nm penetrates into amorphous carbon to a depth of 0.2 µm (Sails et al. 1996). The penetration depth of the Ar laser light (488 and 514.5 nm lines) into sintered diamond compounds may vary from 30 to 200 µm depending on the size of the grains constituting the compounds (Evans et al. 1984). A blue PL is a characteristic feature of type I natural diamonds (Vins 1988). Common synthetic diamonds do not exhibit PL when excited with UV light (e.g. with a mercury lamp) (Novikov 1968; Malogolovets 1979). In contrast, synthetic diamonds grown by the temperature gradient method show PL much more readily due to their higher perfection (Vins 1988; Yelisseyev et al. 1986). The PL spectra of synthetic diamonds (in contrast to natural diamonds?) excited at the bandgap are similar to that of XL. At LNT and RT this similarity is observed at the excitation wavelengths < 280 nm and 240 nm respectively (Vins 1988). Broad bands in PL spectra of synthetic diamonds are quenched by temperature according to Mott's law. The parameters of the quenching depend on the excitation wavelength. These are: quenching temperature range is 200 to 300 K, Eq varies from 0.19 to 0.25 eV, τA = 105; for excitation at wavelengths longer than 280 nm the quenching temperature range is 150 to 350 K, Eq varies from 0.08 to 0.12 eV, τA = 10 (Vins 1988). Intracenter Moessbauer-type PL in synthetic diamonds dominates for excitation at wavelengths longer than 280 nm (Vins 1988). Neutron irradiation at doses to 1016 cm-2 does not noticeably affect PL intensity of synthetic diamonds with a nitrogen content of 1019 cm-3. The irradiation doses above 1018 cm-2 quench PL of these diamonds almost completely (Vins 1988). HPHT annealing of type Ib nonluminescent synthetic diamonds at temperatures above 1900°C for at least 2 hours makes them active in PL under UV excitation (Kluev et al. 1982). The PL of natural diamonds (excitation at a wavelength of 365 nm) is strongly suppressed by nitrogen (predominantly by the A-aggregates). For instance, the N3 center is almost not seen in diamonds with a nitrogen content over 0.1 atom% (Clark et al. 1956a; Davies and Crossfield 1973a). Temperature quenching of the PL intensity of natural diamonds usually occurs in a range from 200 to 400°C, though for some PL centers full quenching is not attained at temperatures as high as 700°C. Some natural diamonds even exhibit an increase in PL intensity at about 500°C (Argunov et al. 1976).
11.7.3 X-Ray Luminescence Almost all natural diamonds show XL (Gomon 1966). Owing to this peculiarity XL is used as an important method of sorting diamonds from the ore (Suttill 1978). The spectral structure of Moessbauer-type centers observed in XL spectra of diamond is noticeably less resolved compared to the structure of the same centers excited in PL (*). Commonly the XL intensity of natural diamonds of types I and IIa does not differ so strongly as it does for the PL intensity of these diamonds (BienemannKuespert et al. 1967).
442
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No reports are known of the damage of diamonds by X-ryas with an energy of a few keV (Wei Zhu et al. 1993; *). This means that XL can be considered as a truly nondestructive characterization method. The penetration depth (1/α) of synchrotron radiation in diamond (Pan et al. 1991) depends on its energy: quantum energy from 0 to about 2.2 keV, peaked at about 1 keV 3.3 µm; quantum energy from 0.3 to about 2.2 keV, peaked at about 1.3 keV 4.3 µm; quantum energy from 0.5 to about 2.2 keV, peaked at about 1.6 keV 5.6 µm.
11.7.4 Electroluminescence One of the first reports on EL of diamond (on type IIb samples) was given by Wolfe and Woods (1957). The following electronic structures can be used for EL excitation of diamond: pi-p, p-i-n, M-i-M, p-M and αC-i-p (Zhang et al. 1996a; *), where p, i, n, M and αC stand for p-type diamond, intentionally undoped (intrinsic) diamond, n-type diamond, metal and amorphous carbon respectively. Impact EL can be excited in diamond at fields in the order of 105 V/cm (Franz 1961). Impact excitation developing in diamond-metal junctions is believed to be a reason for the EL of semiconducting diamonds in the vicinity of biased electrical contacts (Levinson and Halperin 1979). At DC bias the EL of natural type I diamonds is not observed in the ohmic range of the I-V curve. EL starts when a superlinear I-V dependence starts, indicating double injection or impact ionization regimes (Kolomoitsev et al. 1969, *). Annealing at temperatures of 1300°C may strongly stimulate the EL of insulating type I diamonds. This effect is believed to be due to surface graphitization (Kolomoitsev et al. 1969). The EL of the 520 nm band (possibly the H3 center *) and 575 nm center observed in synthetic diamonds is especially strongly excited in n-type samples grown from media containing Ti, As, P and Zr. The EL is excited in s-type regions of I-V characteristics (EL mechanism is possibly a double carrier injection in M-nM diodes *). The luminescence of most samples is distributed very nonuniformly over the volume (shining points and small areas). However some P- and As-doped crystals show spatially uniform excitation at electrical fields about 102 to 103 V/cm (Butuzov et al. 1976). Many synthetic diamonds show similar EL and PL spectra dominated by a broad band peaked at 520 nm, the H3 center and the 575 nm center. P-type diamonds also exhibit the A-band structured by some structure at around 420 (possibly the N3 center), 440 and 470 nm (Halperin and Nahum 1961; *). Many synthetic diamonds show at RT a broad EL band peaked at about 670 nm (Klimenkova et al. 1975b). The EL of intentionally undoped synthetic diamonds can be excited in electrical fields as low as 100 V/cm (Klimenkova et al. 1975b). The EL of type I natural diamonds at LNT usually exhibits the following centers: 575 nm, H3, H4, N3, possibly 389 nm, a feature at about 328 nm and the A-band (Bochkov et al. 1969; *). The EL of insulating type I diamonds can be
11.7 Luminescence Excitation 443
effectively excited by short pulses or high-frequency (>1MHz) voltage. The EL intensity shows a very definite exponential dependence on the applied voltage implying a subbreakthrough mechanism of excitation (Bochkov et al. 1969): IEL ∼ exp[-A/V1.5], where A is a constant. EL excited by alternating voltage (the Destriau effect) is observed in natural diamonds at electrical field above 103 V/cm. The EL brightness IEL and the electrical field E can be well described by the expression: IEL = k exp[-(a/E0.5)], where k and a are constants (Pologrudov et al. 1964). The intensity increases with frequency. At low electrical fields the frequency dependence has a maximum at 1 to 6 kHz. The EL intensity of the H3 and 575 nm centers in natural type I diamonds do not vary noticeably with temperature in a range from LNT to RT (Logie and Urlau 1959; *) (Fig. 5.110). EL and CL spectra taken from insulating diamonds are very similar, implying the same excitation mechanism by EL and CL (Kawarada et al. 1990b; Prior and Champion 1962; Fujimori et al. 1991; *); the main feature of this mechanism is excitation of electrons from the valence to the conduction band (*). The EL of M-i-M structures made from thick CVD diamond film (thickness of 400 µm) can be excited in electrical fields of 104 V/cm (Manfredotti et al. 1995). Nitrogen strongly suppresses the EL intensity of diamonds. The EL of nitrogencontaining diamonds is excited at much higher electrical fields (above 106 V/cm) compared to that of low-nitrogen diamonds. In some very low-nitrogen diamonds EL can start in electrical fields below 104 V/cm (Ivey 1963; Logie and Urlau 1959; *). The internal quantum output of the A-band luminescence excited in p-i-n diodes made on high-quality low-nitrogen natural diamonds may attain a value of 0.04 photons per electron-hole pair (Guseva et al. 1978; *). 11.7.5 Iono- and γ-Luminescence The threshold energy of creation of electron-hole pairs by high-energy light ion irradiation (for instance, by α-particles) is 24.5±1 eV (Bienemann-Kuespert et al. 1967; Kennedy 1959). A single 2 MeV proton generates in diamond about 1.5×105 electron-hole pairs in a region 24 µm deep and 1 to 2 µm wide. The vast majority of these pairs are concentrated at the end of the ion path (Manfredotti et al. 1997). The IL intensity of the A-band excited in CVD diamond films by 2 MeV protons is insensitive to the absorbed proton dose up to 1010 cm-2 (Manfredotti et al. 1997). Regions of high charge collection efficiency in CVD radiation detectors usually exhibit no luminescence under proton irradiation (with respect to the A-band? *) (Manfredotti et al. 1997). CVD diamond can be used as a scintillation detector (Manfredotti et al. 1997). The GL excitation threshold in synthetic diamonds for 60Co irradiation is 100 Roentgen/s (Vachidov et al. 1975a). The GL intensity of synthetic diamonds depends strongly on the total nitrogen content. The intensity exhibits a maximum for nitrogen concentrations of 0.6 to
444
11 Related Data
1 weight%. The GL intensity of diamonds with nitrogen concentrations below 0.05 weight% and above 2 weight% is an order of magnitude lower than that at the maximum (Vachidov et al. 1975a). The GL spectra of synthetic diamonds in the visible range do not depend on the content of B, Ti, Ga, Nb, Mn, N, Zr, P impurities. The amount of these impurities changes only The GL intensity (Vachidov et al. 1975a).
11.8
Impurities
The presence of oxygen in the growth gas mixture increases considerably the CL intensity of optical centers in the spectral region from 480 to 600 nm excited in CVD diamond films grown from this mixture (Heiderhoff 1997). Molecular oxygen, adsorbed on diamond surfaces, exhibits much weaker IR absorption compared with that of atomic oxygen (Novikov 1968). Traces of nitrogen detected as nitrogen-related centers are always revealed in any natural and synthetic diamonds by luminescence methods (Malogolovets 1983; Vins and Yelisseyev 1989). CL of the nitrogen related 575 nm, H3, and 389 nm centers is one the most sensitive techniques to detect nitrogen in irradiated diamonds; this method enables to routinely measure nitrogen concentrations as low as 1015 cm-3 in diamonds irradiated with protons or He+ ions of energy in the range 100 to 500 keV at doses from 1014 to 1015 cm-2 and subsequently annealed at temperatures above 800°C (Zaitsev et al. 1985c; Hatta et al. 1999). Synthetic diamonds grown with Fe catalysts and nitrogen getters are the most free from nitrogen-related optical centers (Sittas et al. 1996). In synthetic diamonds grown by the temperature gradient method an increase in content of optically active nitrogen from 160 to about 230 ppm is observed with increase in the growth temperature from 1400 to 1750°C. In contrast, the increase in the growth rate causes a decrease in the total nitrogen content. At a growth rate of 0.8 mg/h almost all nitrogen is bound into the A-aggregates. If the growth rate exceeds 3.5 mg/h, the nitrogen predominantly forms C-defects. The spatial distribution of the A- and C-defects in such diamonds can be described as a sequence of layers containing predominantly A- or C-defects (Antsygin et al. 1996). However, an increase in nitrogen concentration in the {001} growth sectors of HTHP synthetic diamonds grown at high growth rates was observed by Kiflawi and Kanda (1999). The luminescence efficiency of CVD diamond films falls with increase in nitrogen concentration. A 10fold decrease of overall luminescence intensity is observed in CVD films when the N/C ratio of their growth gas mixtures increases from 2 to 16% (Yokota et al. 1992). Most nitrogen in CVD diamond is believed to present in NV complexes (Srinivasan and Butler 1999). Single nitrogen atoms also strongly quench the PL and XL of synthetic diamonds (Vins 1988; Yelisseyev et al. 1987; Vins and Yelisseyev 1989). The decrease in the PL intensity of PCCVD diamond films after annealing at moderate temperatures (from 200 to 600°C) can be interpreted as a release of weakly bonded hydrogen (this hydrogen not active in IR absorption) (Bou and Vandenbulcke 1991). Hydrogen-induced vibrational optical bands in CVD diamond
11.8 Impurities 445
films correspond well to those observed in amorphous carbon, implying that hydrogen in diamond films is mostly incorporated in defective regions, grain boundaries or interfaces (Dischler et al. 1993). The intensity of the PL background always observed by Raman measurements of HFCVD diamond films strongly correlates with the CH3 content in the growth gas mixture (Harris et al. 1996). In synthetic diamonds grown and annealed at similar conditions the concentration of cobalt-nitrogen complexes detected as optical cobalt-related centers is much lower than that of nickel-nitrogen complexes detected as nickel-related centers (Lawson et al. 1996). Absorption intensity of the Ni-related centers is more than an order of magnitude weaker in natural diamonds than in synthetic ones (Nadolinny et al. 1999). High growth rate of synthetic diamonds increases concentration of Ni in their {111} sectors (Kiflawi and Kanda 1999).
11.9
Gem Diamonds
The gem quality of diamonds is determined by its unique primary optical parameters. The brilliance or "lustre" of diamond results from its high reflectivity (0.17 for normal incidence) (Collins 1997). The "life" or "sparkle" of shaped diamond is a consequence of the high refractive index resulting in a low internal reflection angle of only 24° (Collins 1997). The "fire" of diamonds is a result of its high dispersion (Collins 1997). The 595 nm center is usually used as an indicator of diamonds with artificial "fancy" coloration. However, this center can be annealed out at 1000°C without significant change in the color (Collins 1997; Collins 1978a). Normally natural "fancy"-colored diamonds do not contain any of the 595 nm, H1c or H1b centers (Collins 1997; Davies 1977a). In artificially colored diamonds ("fancy"-coloration) the ratio of the intensities of the H3 and H4 centers IH3/IH4 is the same as the ratio of the intensities of the A- and B-aggregates of nitrogen IA-aggr /IB-aggr. In contrast, in natural "fancy"-colored diamonds usually IH3/IH4 >> IA-aggr/IB-aggr. However, the IH3/IH4 ratio can be drastically reduced by prolonged annealing at 1400°C.
11.10
Miscellaneous
Many optical centers in diamond possess deep energy levels, that is they are characterized by highly localized ground and first excited states. This peculiarity is a consequence of the relatively small permittivity of diamond (ε = 5.66), which does not favor the formation of weakly bound (shallow) states (Davies 1994b). Some optical centers in diamond show a dynamic Jahn-Teller effect, the two main rules of which are: (i) orbitally degenerate ground states have large dynamic relaxations of energies comparable with phonon energies, and (ii) the relaxations are relatively unimportant in the excited electronic states (Davies 1994b).
446
11 Related Data
Yellow natural diamonds tend to be isotopically light. Colored natural diamonds can have a 0.05% enrichment in 13C isotope (Field 1992; Galimov 1984). Typically the quantum energies of ZPLs of optical centers related to point defects are 2 to 5 meV higher in 13C diamond than in 12C diamond. This is explained by softer modes in excited states of point defects in the 13C carbon lattice (Davies 1994b). The presence of the 1.829 eV luminescence center in spectra of PCCVD diamond films indicates their low etching resistance against hot oxygen (Bachmann et al. 1993). The generation of the second harmonic of 532 nm pulsed light in diamond is influenced by hydrogen absorbed on (111) surfaces and the surface dangling electron bonds (Buck and Schaich 1995). The probability of zero-phonon transitions of many optical centers in diamond is almost constant at temperatures below 100 K (Davies 1974b). Most ZPLs of optical centers in diamond exhibit a spectral shape well fitted by the expression (Davies 1970b): I(hν) ∝ [∆hν +(hν - hν0)2]-2, where ∆hν is a factor controlling the line width and hν0 is the spectral position of the maximum. The broadening of the ZPLs of optical centers in diamond is determined by two mechanisms: (i) quadratic electron-phonon coupling (for most centers this mechanism is negligible at temperatures below 80 K) (Davies 1974b; Maradudin 1966), and (ii) nonhomogeneous lattice strains, which is the main broadening mechanism working at any temperature. The luminescence of diamond can be excited by rubbing (triboluminescence) (Mellor 1988). Vacancy and nitrogen-containing optical centers in diamond are usually characterized by radiative decay times in the range from 10 to 100 ns and a radiative efficiency of about unity. This peculiarity makes it possible to obtain intense light emission from diamond. The energy transfer distance by excitation with UV light at 295 K attains in type IIb natural diamonds has a value of 50 µm. As a result the luminescing areas can be considerably larger than the areas of primary UV excitation (Dean and Male 1964d). This effect may strongly limit the spatial resolution of PL measurements with fine focused laser beams. The Verdet constant (Faraday effect), as measured in stress-free natural diamond in a magnetic field of 16700 Oe, is (Bienemann-Kuespert et al. 1967): 2.1 [s cm-1G-1] at 486 nm, 1.67 [s cm-1G-1] at 546 nm, 1.38 [s cm-1G-1] at 589 nm. The Brewster angle of diamond at a wavelength of 589.29 nm is 67.53° (Field 1992).
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