Fundamentals of Phosphors

FUNDAMENTALS OF PHOSPHORS Edited by Willian1. M. Yen Shigeo Shionoya (Deceased) Hajime Yamamoto o ~,~~F;'~:~~O"P Bo...

Author: William M. Yen | Shigeo(decease) Shionoya | Hajime Yamamoto

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FUNDAMENTALS OF PHOSPHORS

Edited by

Willian1. M. Yen Shigeo Shionoya (Deceased) Hajime Yamamoto

o ~,~~F;'~:~~O"P

Boca Raton l ondon New York

CRC Press is an imprint of th e Taylor & Fran cis Group, an informa business

This mat erial was previously published in Phosphor Handbook, Second Edition © 2007 by Taylor and Fran cis Gro up, LLC.

CRC Press Taylor & Fran cis Group 6000 Broken Sound Parkway NW. Suite 30 0 Boca Rato n, FL 33487-2742 © 200 7 by Taylor & Fra ncis Group, LLC

CRC Press is an imprint of Taylor & Fran cis Group, an lnforma business No claim to original U.S. Govern me nt wor ks Printed in th e United Stat es of America on acid-free paper 10 9 8 7 65 4 3 21 Int ern ational Standard Book Nu mber-IO : 1-4200 -43 67-6 (Hard cover) Int ernat ional Sta nda rd Book Nu mber -13: 978- 1-4200- 4367-9 (Hardcover) This book contains information obtained from auth enti c and highl y regarded so urces. Reprinted materi al is qu ot ed with permission . and sou rces ar e ind icated. A wide variety of referen ces are list ed . Reason able effort s have been made to publi sh reliabl e data a nd in form ati on , but th e author and th e publishe r ca n not ass ume responsibility for the validity of all mat erial s or for th e co nseque nces of th eir use. No part of th is book may be repr int ed . repr oduced. transmitted, or utilized in any form by a ny elect ronic. mech anic al. or other mean s, now known or hereaft er invent ed. including photocopying. microfilming, and recording, or in any information storage or retrieval system. without written permission from th e publishers. For permi ssion to phot ocop y or use material electron ica lly from th is work , pleas e access www.copyright. com (http:// www.copyrighr.com /) or co ntac t th e Copyr ight C learance Cen ter , Inc. (CCC) 222 Rosewood Dr ive, Danvers. MA 01923. 978-750-8400. CCC is a not-for -p rofit organ ization th at pro vides licen ses and registration for a varie ty of users. For organ iza t ion s th at have been gra nted a photocopy licen se by th e CCC, a sep arate syst em of payment has been arran ged. Trademark Notice: Product or corporate n ame s may be tr adem ark s or registered tr adem arks, and are used on ly for identification an d expl an ation with out int ent to in fr inge. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Dedication

Dr. Shigeo Shionoya 1923-2001

This volume is a testament to the many contributions Dr. Shionoua made to phosphor art and is dedicated to his menlOry.

In Memoriam Shigeo Shionoya Formerly of the University of Tokyo The Institute for Solid State Phy sics Tokyo, Japan

Shosaku Tanaka Tottori University Department of Electrical & Electronic Engineering Tottori , Japan

The Editors William M. Yen obtained his BS. degree from the University of Redlands, Redlands, California in 1956 and his Ph .D. (physics) from Washington University in St. Louis in 1962. He served from 1962-65 as a Research Associate at Stanford University under the tutelage of Professor A.L. Schawlow, following which he accepted an assistant professorship at the University of Wisconsin -Madison. He was promoted to full professorship in 1972 and retired from this position in 1990 to assume the Graham Perdue Chair in Physics at the University of Georgia-Athens. Dr. Yen has been the recipient of a J.5. Guggenheim Fellowship (1979-80), of an A. von Humboldt Senior US. Scientist Award (1985, 1990), and of a Senior Fulbright to Australia (1995). He was recently awarded the Lamar Dodd Creative Research Award by the University of Georgia Research Foundation. He is the recipient of the ICL Prize for Luminescence Research awarded in Beijing in August 2005. He has been appointed to visiting professorships at numerous institutions including the University of Tokyo, the University of Paris (Orsay), and the Australian National University. He was named the first Edwin T. Jaynes Visiting Professor by Washington University in 2004 and has been appointed to an affiliated research professorship at the Uni versity of Hawaii (Manoa). He is also an honorary professor at the University San Antonio de Abad in Cusco, Peru and of the Northern Jiatong University, Beijing, China. He has been on the technical staff of Bell Labs (1966) and of the Livermore Laser Fusion Effort (1974-76). Dr. Yen has been elected to fellowship in the American Physical Society, the Optical Society of America, the American Association for the Advancement of Science and by the U.S. Electrochemical Society.

Professor Shionoya was born on April 30, 1923, in the Hongo area of Tokyo, Japan and passed away in October 2001. He received his baccalaureate in applied chemistry from the faculty of engineering, University of Tokyo, in 1945. He served as a research associate at the University of Tokyo until he moved to the department of electrochemistry, Yokohama National University as an associate professor in 1951. From 1957 to 1959, he was appointed to a visiting position in Professor H.P. Kallman's group in the physics department of New York University. While there, he was awarded a doctorate in engineering from the University of Tokyo in 1958 for work related to the industrial development of solid-state inorganic phosphor materials. In 1959, he joined the Institute for Solid State Physics (lSSP, Busseiken) of the University of Tokyo as an associate professor; he was promoted to full professorship in the Optical Properties Division of the ISSP in 1967. Following a reorganization of ISSP in 1980, he was named head of the High Power Laser Group of the Division of Solid State under Extreme Conditions. He retired from the post in 1984 with the title of emeritus professor. He helped in the establishment of the Tokyo Engineering University in 1986 and served in the administration and as a professor of Physics. On his retirement from the Tokyo Engineering University in 1994, he was also named emeritus professor in that institution.

During his career, he published more than two hundred scientific papers and authored or edited a number of books-the Handbook on Optical Properties of Solids (in Japanese, 1984) and the Phosphor Handbook (1998). Professor Shionoya has been recognized for his many contributions to phosphor art. In 1977, he won the Nishina Award for his research on high-density excitation effects in semicond uctors using picosecond spectroscopy. He was recognized by the Electrochemical Society in 1979 for his contributions to advances in phosphor research. Finally, in 1984 he was the first recipient of the ICL Prize for Luminescence Research. Hajime Yamamoto received his B.s. and PhD. degrees in applied chemistry from the University of Tokyo in 1962 and 1967. His Ph.D . work was p erformed at the Institute for Solid State Physics under late Professors Shohji Makishima and Shigeo Shionoya on spectroscopy of rare earth ions in solids. Soon after graduation he joined Central Research Laboratory, Hitachi Ltd., where he worked mainly on phosphors and p-type ZnSe thin films. From 1971 to 1972, he was a visiting fellow at Professor Donald S. McClure 's laboratory, Department of Chemistry, Princeton University. In 1991, he retired from Hitachi Ltd . and moved to Tokyo University of Technology as a professor of the faculty of engineering. Since 2003, he has been a professor at the School of Bionics of the same university. Dr. Yamamoto serves as a chairperson of the Phosphor Research Society and is an organizing committee member of the Workshop on EL Displa ys, LEOs and Phosphors, International Display Workshops. He was one of the recipients of Tanahashi Memorial Award of the Japanese Electrochemical Society in 1988, and the Phosphor Award of the Phosphor Research Society in 2000 and 2005.

Preface This volume originated from the Phosphor Handbook which has enjoyed a moderate amount of sale success as part of the CRC Laser and Optical Science and Technology Series and which recently went into its second edition. The original Handbook was published in Japanese in 1987 through an effort of the Phosphor Research Society of Japan. 111e late professor Shionoya was largely instrumental in getting us involved in the translation and publication of the English version. Since the English publication in 1998, the Handbook has gained wide acceptance by the technical community as a central reference on the basic properties as well as the applied and practical aspects of phosphor materials. As we had expected, advances in the display and information technologies continue to consume and demand phosphor materials which are more efficient and more targeted to specific uses. These continuing changes in the demand necessitated an update and revision of the Handbook and resulted in the publication of the second edition which incorporates almost all additional topics, especially those of current interest such as quantum cutting and LED white lighting phosphor materials. At the same time, it has also become apparent to some of us that the evolution of recent technologies will continue to place demands on the phosphor art and that research activity in the understanding and development of new phosphor materials will continue to experience increases. For this reason, it has been decided by CRC Press that a series of titles dedicated to Phosphor Properties be inaugurated through the publication of correlated sections of the Phosphor Handbook into three separate volumes. Volume I deals with the fundamental properties of luminescence as applied to solid state phosphor materials; the second volume includes the description of the synthesis and optical properties of phosphors used in different applications while the third addresses experimental methods for phosphor evaluation. The division of the Handbook into these sections, will allow us as editors to maintain the currency and timeliness of the volumes by updating only the section(s) which necessitate it. We hope that this new organization of a technical series continues to serve the purpose of serving as a general reference to all aspects of phosphor properties and applications and as a starting point for further advances and developments in the phosphor art. William M. Yen Athens, GA, USA October, 2006 Hajime Yamamoto Tokyo, Japan October, 2006

Contributors Chihaya Adachi Kyushu University Fukuoka, Japan

Hiroyuki Matsunami Kyoto University Kyoto, Japan

Pieter Dorenbos Delft University of Technology Delft, The Netherlands

Mamoru Mitomo National Institute of Materials Science Tsukuba, Japan

Gen-ichi Hatakoshi Toshiba Research Consulting Corp. Kawasaki, Japan

Noboru Miura Meiji University Kawasaki, Japan

Naoto Hirosaki National Institute of Materials Science Tsukuba, Japan Sumiaki Ibuki Formerly of Mitsubishi Electric Corp. Amagasaki, Japan Kenichi Iga Formerly of Tokyo Institute of Technology Yokohama, Japan Tsuyoshi Kano Formerly of Hitachi, Ltd ., Tokyo, Japan Hiroshi Kobayashi Tokushima Bunri University Kagawa, Japan

Makoto Morita Formerly of Seikei University Tokyo, Japan Shuji Nakamura University of California Santa Barbara, California Eiichiro Nakazawa Formerly of Kogakuin University Tokyo, Japan Shigetoshi Nara Hiroshima University Hiroshima, Japan Hiroshi Sasakura Formerly of Tottori University Tottori, Japan

Hiroshi Kukimoto Toppan Printing Co., Ltd . Tokyo, Japan

Masaaki Tamatani Toshiba Research Consulting Corporation Kawasaki, Japan

Yasuaki Masumoto University of Tsukuba Ibaraki, Japan

Shinkichi Tanimizu Formerly of Hitachi, Ltd . Tokyo, Japan

Tetsuo Tsutsui Kyushu University Fukuoka, Japan

Hajime Yamamoto Tokyo University of Technology Tokyo, Japan

Rong-jun Xie Advanced Materials Laboratory, National Institute of Materials Science Tsukuba, Japan

Toshiya Yokogawa Matsushita Electric Ind. Co., Ltd. Kyoto, Japan

Contents Chapter 1

Chapter 2

Index

Fundamentals of luminescence 1.1 Ab sorption and em iss ion of light... 1.2 Electronic sta tes an d optical transition of solid crystals 1.3 Luminescence of a localized center 1.4 Impurities and luminescence in semiconductors 1.5 Luminescence of organic comp ound s 1.6 Luminescence of low-dimensional systems 1.7 Transient characteristics of luminescence 1.8 Excitation energy transfer and cooperative optical phenomena 1.9 Excitation mechanism of luminescence by cathode-ray and ionizing radiation 1.10 Inorganic electroluminescence 1.11 Lanthanide level locations and its impact on phosphor performance Principal phosphor materials and their optical properties 2.1 Luminescence centers of ns --type ions 2.2 Luminescence centers of transition metal ions 2.3 Luminescence centers of rare-earth ions 2.4 Luminescence cen ters of complex ions 2.5 Ia-VlIb compounds 2.6 IIa-VIb compounds 2.7 IIb-VIb compounds 2.8 ZnSe and related luminescent materials 2.9 IIIb-Vb comp oun ds 2.10 (Al,Ga ,In)(P,A s) alloys emitting visible luminescence 2.11 (AI,Ga,In)(P,As) alloys emitting infrared luminescence 2.12 GaN and related luminescence materials 2.13 Silicon carbide (SiC) as a luminescence material... 2.14 Oxynitride phosphors

1 1 11 25 39 51 61 73 89 101 111 129 145 145 157 181 205 217 221 237 265 273 283 291 299 313 321 329

chapter one - section one

Fundamentals of luminescence Eiichiro Naka za wa Contents 1.1 Absorp tion an d emission of ligh t... 1.1.1 Abso rp tion and reflection of light in cry stals 1.1.1.1 Optical consta n t and com p lex d ielectric cons tan t.. 1.1.1.2 Absorp tion coefficient 1.1.1.3 Reflec tivity and tran sm issi vity 1.1.2 Absorp tion and emission of light by impu rity a toms 1.1.2.1 Cla ssical harmonic oscilla tor model of op tica l cen ters 1.1.2.2 Elect ro ni c tran sition in an a tom 1.1.2.3 Electric dipole tran sition p robabili ty 1.1.2.4 Intensit y of light emission an d absorption 1.1.2.5 Os cilla tor strength 1.1.2.6 Impurity atom s in cry stals 1.1.2.7 For bidden transition 1.1.2.8 Selection ru le Referen ce

1.1

1 2 2 .3 3 4 .4

5 6 7 8 9 9 9 10

A bsorption and emission of light

Most phosphors are comp osed of a transparent microcrystallin e host (or a matrix) and an activa tor, i.e., a small amo un t of in ten tion ally added im p ur ity atom s dis tri bu ted in the host crys tal. Th eref ore, th e lu min escen ce processes ofa phosp h or can be divided into two parts: the processes main ly related to th e h ost, and th ose that occu r around an d within the activa tor. Processes rela ted to optical absorption, reflec tion, and tra ns m ission by the host crys tal are d iscussed, from a macroscopic p oint of view, in 1.1.1. O ther h ost processes (e.g., excitation by electron bombardment an d th e migration and transfer of the exc ita tion en ergy in the host) are di scu ssed in a later sect ion. 1.1.2 d eals wi th ph enomena rela ted to the activa tor atom ba sed on th e theory of atomi c spectra. The interaction betw een the h ost and th e activa tor is not explicitl y discussed in thi s section; in th is sense, the ho st is treat ed onl y as a m edi um for the acti va tor. The interaction processes such as the transfer of the h ost exci ta tion energy to the activat or w ill be discussed in detail for eac h ph osphor elsewhere.'

1

2

1.1.1

Fundamentals of Phosphors

Absorption and reflection of light in crystals

Since a la rge number of phosphor host mater ials are tran sparent and nonmagnetic, their optical p rope rtie s can be represented by the optical con st ants or by a complex dielectric constant.

1.1.1.1 Optical constant and complex dielectric constant The electric and magnet ic field s of a light wa ve, propagating in a uniform matrix with an ang ular frequency to (= 2n:v, v:frequency) and velo city v = wl k are:

E = Eo expH k.r - wt)]

(1)

(2)

-

where r is the position vector and k is the complex w av e vector. E and H in a nonmagn etic di electric material, w ith a ma gn et ic permeability that is nearly equal to th at in a vacuum (u = ~o) and with uniform dielectr ic cons tan t £ and electric conductivit y 0 , sati sfy the next two equations derived from Maxwell 's equations.

(3)

aH + £~o -a H 22

2

V' H

= 0~o -

at

(4)

at

-

-

In order that Eqs. 1 an d 2 satisfy Eqs. 3 and 4, the k -vector and its length k , whi ch is a complex number, should sa tisfy th e following relation:

(5) where £ is the complex dielectric const ant d efin ed by: _

£

,

•

= £ + 1£

1/

cr == E + 1 •

(6)

W

Therefore, the refractive ind ex, which is a real number defined as n == clv = ck no in a transparent media, is also a com p lex number :

11

.

-

l

£

= 11 + IK == ck/w = ~

Jl/2

(7)

where c is the velocity of light in vac u um and is equal to (£ Oflot1/ 2 from Eq. 5. The last term in Eq. 7 is also derived fro m Eq. 5. The real and im aginary parts of the comp lex refr acti ve index, i.e.. the real refractive index n and the extinction index K, are call ed optical constants, and are the rep resentat ive

Chapter one: Fundamentals of luminescence

3

constants of the macroscop ic optical properties of the m at erial. The op tical constan ts in a nonmagneti c material are related to each other using Eqs. 6 and 7,

(8)

(9)

Both of the optical cons tan ts, n and K, are functions of ang ular frequency wand, hen ce, are referr ed to as dispersion relations. The di spersion rel ati on s for a mat erial are obtained by mea suring and analyzing the refle ction or transmission spectrum of the material ov er a wid e spectra l region.

1.1.1.2 Absorption coefficient The inten sity of the light pr opagating in a med ia a d istance x from the incident surface havin g been decreased by the optical absorp tion is given by Lambert's law. 1= 10 exp( -ax)

(10)

wh ere 10 is the incid ent light intensity minus reflection losses at the su rface, and « (cm') is the absorp tion coefficient of the media. Using Eqs. 5 and 7, Eq, 1 may be rew ritt en as:

E = Eo exp( -soxx] c) exp[ - iw(t + nx]c)]

(11)

and, since the intensity of light is p roportional to the square o f its elec tric field s treng th E, the absorp tion coeffic ien t may be ide n tified as :

a = 2WK/ C

(12)

Therefore, K is a factor that represents the extinction of light due to the ab sorpti on b y the medi a. There are sev eral wa ys to rep resent the absorption of light by a medium, as d escribed below. 1. Absorption coefficien t, a(cm - 1) : l/ In = rOO< 2. Absorption cros s-section, a l N (cm-). Here, N is the number o f ab sorption center s pe r unit volume. 3. Optical d ensit y, abs orbance, 0 = -loglQ(1lIo) 4. Absorptivi ty, (10 - 1)110 x 100, (%) 5. Mola r extin ction coefficient, t = a loglQclC. Here, C(mol/ I) is th e molar con centration of absorption centers in a so lu tion or gas .

1.1 .1.3 Reflectivity and transmissivity When a light beam is incident normall y on an optically smooth crys tal surface, the ratio of the intensities of the reflected light to the incident light, i.e.. normal surface reflectivity Ro' can be written in terms of the optical cons tan ts, n and K, by


4

(13)

Th en , for a sa mp le w ith an absorp tion coefficie n t a and th ickness d th at is lar ge enough to negl ect interference effects, th e overall normal reflec tivity an d transmissivit y, i.e., th e ra tio of th e transmitted light to the inciden t, a re; resp ectively:

R = Ro(1+ f

exp( -ad) )

(14)

(15)

If ab so rption is ze ro (a = 0), then,

(16)

1.1.2

Absorption and emission of light by impurity atoms

Th e emission of lig ht fro m a ma terial orig ina tes from two typ es of m echan isms: thermal emission and luminescence. Whi le all th e a toms composing th e solid participa te in the light em iss ion in th e thermal process, in the luminescen ce process a very small n um ber of a toms (impuri ties in m ost cases or crystal defects) are exci ted and take p art in the emission of light. The impuri ty ato m or defect and its surro u nding a toms form a lumin escent or an emi tting cen ter. In m ost phosphors, the lumin escence center is forme d by intentionall y incorpora ted impurity a tom s called activators. This secti on treat s the absorp tio n and emission of light by these impuri ty a toms o r local defects.

1.1.2.1

Classical harmonic oscillator model of optical centers

The absorption and emission of light by an a tom can be described in the mos t si mplified sch em e by a linea r h arm onic osci lla tor, as shown in Figure 1, composed of a posi tive charg e (+e) fixed a t z = 0 and an electron bo u nd and osc illa ting around it a long the z-axis. Th e elec tr ic dipole moment of th e osci lla tor w ith a cha racteristic angular freq uency W o is given by: M

= ez = M o exp(iwJ)

(17)

a nd its energy, th e sum of th e kinetic an d potential en er gies, is (mew;/2e2 )M;, where me is the m ass of th e electron. Such a vibra ting electric dipole transfer s energy to electromagneti c radiati on a t an average ra te of (w;/121rt oc3 )M5per second, and therefore has a tot al ene rgy decay rat e given by:

(18)

Chapter one:

Fundamentals of luminescence

5

B= 0

Z

Figure 1 Electrom agn etic rad iation from an electric dipole osc illato r. The len gth of the arrow gives the intensity of the rad iation to the direction .

When the cha nge of the energy of this oscillator is expresse d as an exponential function e'! », its time constant T" is equal to A o-l, which is th e radiat ive lifetime of the oscillator, i.e., the time it tak es for the oscillator to lose its energy to r 1 of the initi al en ergy. From Eq. 8, the radi ative lifetime of an oscillator with a 600-nm (CDo = 3 X 1015 S- I) w avelen gth is To = 10-8 s. The int ensity of the emission from an electric dipole oscilla tor dep ends on the direction of the propagation, as shown in Figure 1. A more detailed an alysi s of absorption and emission processes of light by an atom will be d iscussed usin g quantum mechanics in the following subsection.

1.1.2.2 Electronic transition in an atom In quantum mech an ics, the energy of the electrons localized in an at om or a molecul e have discrete valu es as sho wn in Figure 2. The absorption and emission of light by an

- .....----m

(a)

(b)

(c)

-...I-----n Figure 2 Absorp tion (a), spontaneo us emission (b), an d induced emission (c) of a photon by a two level system .


6

at om, th erefore, is not a gradual and continuous process as discussed in the abo ve sectio n usin g a classical dipole oscillator, but is an instantaneous transition betw een two discrete ene rgy levels (stat es), m an d n in Figure 2, and should be treated statistically. The ene rgy of the photon absorbed or emitted at the tran siti on m H n is: (19)

w here E" and £"/ are th e ene rgies of the initial and final sta tes of the transiti on , resp ectively, and CO/l1I1 ( =2 1tV m ,,) is th e ang ular freque ncy of light. Th ere are tw o possibl e emission processes, as shown in Figure 2; one is called spo ntaneous emission (b), and th e othe r is stim ula ted emis sion (c). The stimulated emission is ind uced by an inciden t photon, as is the case with the absorption process (a). Laser action is based on this typ e of emission p rocess. The in tensi ty of th e absorp tion and em ission of photons can be enumerated by a transition p rob abil ity per a tom per second . Th e probability for an atom in a radiation field of ene rgy d en sity p(com,,) to absorb a photon , m aking the transition from n to m, is given by

Wmil -- B

fl -;l N

p(co "1/1 )

(20)

w he re BII_ is the transiti on probability or Eins tein's B-coefficient of optical absorption, and p(co) is eq ua l to l(co)/ c in which 1(co) is th e light intensity, i.e., the energy per second per unit area perp en d icular to the direction of light. On the othe r hand, th e p rob ab ility of th e em ission of light is the sum of the spontaneous emi ssion p robability A m->" (Einst ein 's A-coefficient) and the stimulated emission probability BII1 ->IIP(col/I,J Th e stimula ted emission probability coefficien t Bm _ is equal to B'Hm' The equilib rium of op tica l absorp tion an d emission between the atoms in the states m and n is expressed by th e followin g equa tion. , Hr

H /

(21)

where N mand N" ar e the number of at om s in th e sta tes m and n, resp ectively. Takin g into account the Boltzmann d istribution of the sys tem and Plank's equa tion of radi at ion in thermodynamic equilibrium, th e follow ing eq ua tion is obtaine d from Eq. 21 for the spontaneous mission probability. (22) Therefore, the probabilities of optical absorp tion, and the spo n taneo us and ind uced emissions between m and n are related to one another.

1.1.2.3

Electric dipole transition probability

In a quantum mechanical treatment, op tical tran siti on s of an atom are ind uced by per-turbing the energy of th e system by L.,(-erJ E, in wh ich Yj is the pos ition vec tor of the electron from the atom cen ter and, th erefore, L.,(-erj ) is the electric d ip ole moment of the atom (see Eq. 17). In thi s electric d ip ole ap proxima tion, the tran sit ion probability of optical absorption is gi ven by:

W

-

_

1t_ 2

"''' - 3£ octl

2

)IM

l(co "'"

1

"'"

(23)

Chapter one:


7

Here, the dipole moment, M",n is defined by:

(24)

where \jim and \jill are the wavefunctions of the states m and n, respectively. The direction of this dipole moment determines the polarization of the light absorbed or emitted. In Eq.23, however, it is assumed that the optical center is isotropic and then (M l1m ) z 2 = 2 I M mn 1 / 3 for light polarized in the z-direction. Equating the right-hand side of Eq. 23 to that of Eq. 20, the absorption transition probability coefficient BII--;m and then, from Eq. 22, the spontaneous emission probability coefficient A m can be obtained as follows: 1

1

_ ) 11

(25)

1.1.2.4

Intensity of light emission and absorption

The intensity of light is generally defined as the energy transmitted per second through a unit area perpendicular to the direction of light. The spontaneous emission intensity of an atom is proportional to the energy of the emitted photon, multiplied by the transition probability per second given by Eq. 25.

(26)

Likewise, the amount of light with intensity I o(w l1rll ) to be absorbed by an atom per second is equal to the photon energy wmll multiplied by the absorption probability coefficient and the energy density la/C. It is more convenient, however, to use a radiative lifetime and absorption cross-section to express the ability of an atom to make an optical transition than to use the amount of light energy absorbed or emitted by the transition. The radiative lifetime 'nm is defined as the inverse of the spontaneous emission probability A I1H

rr

•

-1 "t11/11

-

A

(27)

m-:,n

If there are several terminal states of the transition and the relaxation is controlled only by spontaneous emission processes, the decay rate of the emitting level is determined by the sum of the transition probabilities to all final states: Am

= L... ~ A m-"m

(28)

rr

and the number of the excited atoms decreases exponentially, exp(-t/'t), with time a constant, = A",-I, called the natural lifetime. In general, however, the real lifetime of the DC


8

excited state m is controlled not only by radiative processes , but also by nonradiative ones (see 1.7). The absorption cross-section G represents the probabili ty of an atom to absorb a ph oton incident on a unit area. (If there are N absorptive atoms per unit volume, the absorption coefficient a in Eq. 10 is equal to GN. Therefore, since the intensity of the light with a photon per second per unit area is 10 = O)mn in Eq. 23, th e absorption cross-section is given by:

G l/J1l

1.1.2.5

[2

ItO) mil 1

3E

Cn M

a

(29)

11m

Oscillator strength

The oscillator streng th of an optical center is often used in order to represent the streng th of light absorption and emission of the center. It is defined by the following equation as a dimensionless quantity.

-

J,,,,, -

2m e0) nm

ne2

IM (

2 - 2me0) nm M

1

mn). -

3ne2

I m"1,

(30)

The third term of this equation is giv en by assuming that the tr an sition is isot ropic, as it . is the case w ith Eq. 24. The radiative lifetime and absorption cross-section are expressed by usin g the oscillator strength as:

(31)

(32)

Now on e can estimate the oscill ato r strength of a harmonic oscillator with the electric dipole moment M = - er in a quantum mechanical manner. The result is that onl y one electric dipole transition between the ground state (n = 0) and the first excited state (m = 1) is allowed, and the oscillat or s treng th of this transition is flO = 1. Therefore, the su m mation of all the oscillator strengths of the tran sition from the state n = 0 is also 'In!;110= 1 (m =/=- 0). This relation is true for anyone electron system; for N-electron systems, the following fsum rul e should be sati sfied ; that is,

(33) m-.r. n

At the beginning of this section, th e em ission rat e of a line ar harmonic oscillator wa s classically obtained as A o in Eq. 18. Th en , the total tran sition pr obability given by Eq. 32 with f = 1 in a quantum mechanical schem e coincid es with the em ission rat e of the classical linear oscillator A o, multiplied by a factor of 3, corresponding to the three degrees of freedom of th e motion of the electron in the present system.

Chapter one:


9

1.1.2.6 Impurity atoms in crystals Since the electric field actin g on an impurity atom or optical cen ter in a cryst al is different from th at in vacuum d ue to the effect of the p ola riza tio n of the su rro und ing a toms, and the light ve locity is reduced to c/ 11 (see Eq. 7), the radiat ive lifet im e an d th e ab sorption cross-section ar e changed from those in vac uu m . In a cubic cryst al, for exampl e, Eqs, 31 and 32 are chan ged, by the intern al local field, to:

(34)

(35)

1.1.2.7 Forb idden transition In the case that the electric d ip ole moment of a transition M ll m of Eq. 25 becom es zero, the probability of the electric d ip ole (E1) tran sition in Eq, 25 and 26 is als o zero. Sinc e th e electric d ipole tran sition generally ha s the largest tra nsi tion proba bility, this situation is usually expressed by the ter m forbidd en transition . Since th e electric di p ole m oment operat or in the in tegral of Eg. 24 is an odd func tion (od d p arity), th e electr ic d ipole mom ent is zero if the initial and final s tates of the tran sition h av e the sa me parity; th at is, both of the w av efunction s of these sta tes are either an even or od d function , and the transit ion is said to b e parity forbid d en . Likewise, sinc e the electric dipole moment op erator in the integral of Eg. 24 has no spin op era tor, transition s be tw een ini tia l and final states w ith different sp in m u ltip licities ar e spi n forb idden . In Eq . 24 for the dipole moment, the effects of the high e r-ord er pertur ba tio ns are neglected. If th e neglected ter ms are in cluded, the transition mome n t is w ritten as follows: 2

I ., -I( ., M

- er

2

2

2

c 37tffi 2 + -- r x + __11/_ " er· r ( 2mc P}"" 40c2 I( )11/, 1

I

(36)

1

where the firs t term on the rig h t-hand side is the con trib ution of the electric dipole (E1) term previously given in Eg. 24; the second term, in w h ich p d enotes th e momen tum of an electron, is that of magnetic d ip ole (M1); and th e th ird term is that of an elec tric quadrupole transition (E2). Provided th at (r)"," is abou t th e ra d ius of a hydrogen a tom (0.5 A) and ffi mll is 1015 rad / s for visible light, rad iati ve lifetimes es tima ted from Eq, 26 an d 36 are - 10-s s for El, - 10-3 s for M1, an d _ 10-1 S for E2. El-tr'a nsitions ar e forbidden (par ity forbidden ) for f -f and d-d tran sit ion s of free rar eearth ions and transition- metal ions because the ele ctron con figur ations, and hence the parities of the initial and final sta tes, are the same. In crystals, however, the E1 tran sition is pa rtially allowed by the od d com pone n t of the crystal field, an d thi s pa rtially allow ed or forced E1 tran sition h as the rad iat ive lifeti me of - 10-3 s. (See 2.2).

1.1.2.8 Selection rule The se lection ru le governi ng w h ethe r a d ipole tra ns ition is allo w ed be tw een th e s ta tes m and n is determin ed by the tran sition matrix elemen ts (er)1I/1l and (r x P)'WI in Eg. 36. How ever, a group theoretic al inspection of the sy m me tries of the wavefun ctions o f these states and the opera tors er an d r x P enables th e d et ermination of the selec tion rules w ithout calculating the ma trix eleme n ts.


10

When an a tom is free or in a sph erica l symmetry field , its elec troni c states are denot ed by a se t of th e quantum numbers S, L, an d J in the LS-coupling scheme. Here, S, L, and J denote the quantum number of the spin, orbital, and total angu la r momentum, resp ectivel y, a nd L'iS, for exa m p le, denotes the difference in S between th e states m and n. Then the sele ction rules for E1 an d M1 transit ions in the LS-coupling scheme are giv en by: .15=0,

L'iL = O or

±1

L'if = 0 or ± 1 (J = 0 ~ J = 0, not allowed)

(37) (38)

If th e sp in -orbi t in teraction is too large to use the L5-coupling scheme, the JJ-coupling scheme might be used, in which many (5, L)-terms a re mix ed int o a J-state. In the JJcoupling schem e, therefore, the L'iS and L'iL selection rules in Eqs. 37 ad 38 are less strict, and only the L'iJ selection rule applies. While th e E1 transitions between the sta tes with th e same parity are forbid den, as in th e case of th e f-f transitions of free rare-earth ions, th ey become part ially allow ed for ions in cryst als due to the effe cts of crys tal fields of odd pari ty. The se lection rule for th e p artially a llow ed E1f-f tr ansi tion is 1.1 JI :::; 6 U= 0 - 0,1 , 3,5 ar e forbidden). M1 transitions are always parity allowed because of the even parity of the magnetic dipole operator r x p in Eq. 36 .

Reference 1. Practical Applications of Phosphors, Yen, WM. , Shionoya, 5., and Ya mamoto, H., Eds., CRC Press, Boca Raton, 2006.

chapter one - section two

Fundamentals of luminescence Shigetoshi Nara and Sumiaki Ibuki

Cont ents 1.2 Electronic states and optical transiti on of so lid cr ystals 1.2.1 Outline of band theory 1.2.2 Fundamental absorption, d ire ct transition, and indirect transition 1.2.3 Exciton References

1.2 1.2.1

11 11 18 22 24

Electronic states and optical transition of solid crystals Outline of band theory

First, a brief d escription of crystal properties is given. As is w ell known, a crystal consists of a periodic con figura tion of a tom s, which is called a crystal lattice. There are man y di fferent kinds of crystal lattices and th ey are classified, in general, according to their symmetries, which specify in variant properties for translational and rotational operation s. Figure 3 shows a few, typical examples of crystal structu re s, i.e., a rock-salt (belonging to on e of the cubic gro u ps ) s tructu re, a zinc-blende (also a cubic group) structure, and a wurtzeite (a he xagonal group) structure, respectivel y. Second, con side r the electronic st at es in these crystals. In an isolated state, ea ch atom has electrons that exist in d is crete electronic energy levels, and the states of these bound electrons are characterized b y a to m ic wavefunctions . Their di screte en erg y levels, however, will have finite spectral width in th e condensed state because of the o ve rlaps between electron ic w avefunctions belonging to different atoms . This is because electro ns can be come itinerant between atoms, until finally they fall into delocalized ele ctron ic states call ed electronic energy bands, w hi ch also obey th e sy m metries of cr ystals. In these energy bands, the states with lower en ergies a re occupied by ele ctro n s origin a ting from bound electrons of a toms and a re called valence bands. The energy bands having higher energies are not occupied by e lectro ns and a re called conduction bands. Usuall y, in materials having crystal sym m etrie s such as rock-salt, zinc-blende, or wurtzeite structures, there is no electronic s ta te b etween the top of the val ence b and (the highest sta te of occupi ed bands) and the bottom of th e conduction band (the low est state of un occupied bands); thi s region is called the bandgap. The reason why unoccupied s ta tes a re called

11

12


.-- -or..:-...-,- - -- : : _-; ....... ... ...

-

/"'T

.,/

./

I

... ~ ::

~

,

./

,..~

-'..; ' .:::=....---IH:;ec

Q

Q

-

./ .,/

I

:;>

./ I

./

V

- - ~ - ....... ::.~

.,/ ,/

-

_ _----::~F-

./

./'

.......

---

- ---;- ' ........

V

./

• : Na

O :CI

O :s

. : Zn

. : Zn

O:s

Figure 3 The confi g uration of th e a to ms in thre e impor tan t kin ds of crysta l s tru ctures. (a) rock -sa lt type, (b) zinc-blen de type, and (c) wur tze ite typ e, respectively.

con duction bands is due to th e fac t th at an elec tron in a cond uc tion ban d is almost freely m ob ile if it is excited from a val ence band by so me me thod: for exa mp le, by abso rp tion of light quanta. In con tras t, electrons in va len ce bands cannot be mob ile becau se of a fundamen ta l p rop erty o f elec tro ns ; as [ermion s, only two elec trons (sp in up and do w n ) ca n occupy an electronic sta te. Th us, it is necessary for electrons in the va lence ba nd to have emp ty states in order for them to m ove freely w hen an elec tric field is ap plied . After an elec tro n is exci ted to th e cond uc tion ba nd, a hole that remains in the va lence band behaves as if it w ere a m obile pa rticle wi th a p ositiv e ch arge. Th is hyp othetical particle is ca lled a positive hole. The schema tic d esc ripti on of the se exci tatio ns are show n in Figure 4. As noted above, ba nd gaps are strongly rela ted to the op tical p roper ties and the electric cond u ctivity of crystals. A m ethod to evaluate th ese electronic band struc tu res in a qu antitative way using q uan tu m m ech ani cs is briefly d escribed. The m otion of elect-rons under th e in fluence of electric field s ge ner a ted b y a toms th at ta ke so me d efin ite space config ura tion spe cified by th e sy mmetry of the crys tal lattice, can be descri be d by the foll owing Schrodinger equa tion .

(39)

whe re VCr) is an effective potential applied to each electron and ha s the p ro pert y of: (40)

due to th e translati on al sy mme try of a give n crysta l lattice. R" is a latt ice vec tor indica ting the nth positi on of atoms in the lattice. In th e Fourier rep resentation, the potential VC r) can be writt en as :

V(r) =

I 1I

V" eiG"r

(41)


13

E

E

Conduction Band

...

Forbidden Band

----- - --- - --- ----

~~

Valence Band

---il--K Figure 4 The typic al band dispe rsion near the mini mum band gap in a se micond uc tor or an insulator wi th a direct bandgap in the Brillouin zon e.

where Gil is a reciprocal lattice vector. (See any element ary book of solid -state physics for the defin ition o f G,J It is d ifficult to solve Eq. 39 in ge ne ra l, but w ith the help of the translation al and rotational symme tries inh erent in the eq uation, it is possible to p red ict a general fun ctional form of solutions. The solu tion wa s first found b y Bloch and is ca lled Bloch's theorem. The solution \jI(r) should be of the form : (42) and is called a Bloch f unction. k is the wave vector and uk(r) is th e period ic fun ction of lattice translations, such as : (43)

As one can see in Eq. 40, uk(r) can also be expanded in a Fourier se ries as: (44)

where C,,(k) is a Fourier coefficient. Th e form of the solution represented by Eq . 42 shows that the wave vectors k are well-defined quantu m n umbers of the electronic sta tes in a given crys tal. Putting Eq. 44 into Eq. 42 an d usin g Eq. 41, on e can rew rite Eq . 39 in the following form :


14

(45)

w here E eigenvalues d etermined b y :

I {~(k + G,)2- E}8 2rn

C G I

"

+ Vc - G I

II

1=0

(46)

Henceforth, th e k-dep endence of the Fourier compon ents CII(k) are negle cted . These formulas are in the form of in fini te dimensional determinant equa tions . For finite dimensions by considering amplitudes of v c,_c" in a given crystal, one can solve Eq. 46 approximately. Then the energy eige nv alues E(k) (energy band) m ay be obtaine d as a function of wave vector k and th e Fourier coefficients CII" In order to ob tain qu alit ative interpretation of ene rg y band an d properties of a wa vefuncti on , one can s ta rt wi th th e O th order approximati on of Eq . 46 by taking (47)

in Eq. 44 or 45; thi s is eq uiva len t to taking VII = 0 for all n (a va nis hing or constant crystal potential m od el). Then, Eq. 46 give s:

(48)

This corresp onds to the free electron model. As th e ne xt ap proxim ation, consider the case th at the nonvanishing compon ents of VII are only for n = 0, 1. Eq . 46 becomes:

Vc I 2

~(k+G )2_ E 2m

=0

(49)

1

Thi s me ans th at, in k- space, th e tw o free electrons ha ving E(k) and E(k + G) ar e in indep endent sta tes in the absence of the crystal potential even when Ilk I = Ilk + Gil; this energy degener acy is lifted under the existence of nonvanishing VG ' In the above case, the eigenvalue equation can be solved ea sily an d th e solution gives

E= ~ {E(k) + E(k + G)}± .J { E(k) - ~(k + G) r +V~

(50)

Fig ure 5 shows the glob al profile of E as a fun ction of k in one dimension. One can see the exis tence of ene rgy ga p at the wave vec tor th at sa tisfies: (51)

Chapter one:


15

E (k) Eo (k-G , )

_

_

- l -<e::::...

Eo (k)

----L

---""

k

a Figllre 5 The emergence of a band gap resulting from the int erferen ce between two plan e waves satisfying the Bragg condition, in a one-dimens ional model. This is called the Bragg condition. In the three-dimensional case, the wa ve vectors that satisfy Eq. 51 form clos ed polyhedrons in k space and are called the I st, 2nd, or 3rd, ..., nth Brillo uin zone . As sta ted so far, the electronic energy band structure is d et ermined by the sy m metry and Fourier amp litud es of the crystal potential VCr). Thus, on e need s to tak e a m ore realist ic model of them to get a more accurate description of the ele ctronic pr op erties. There are now many procedures that allow for the calculation of th e ene rgy band and to get the wavefunction of electrons in crystals. Two representative methods, the Pseudopotential method and the LCAO method (Linar Combination of Atomic Orb ital Method ), which are frequently app lied to outer-shell valence electrons in sem icond uc tors, are br iefly in troduced here. Firs t, consider the pseudopotential method. Eq. 46 is the fu n d am ental eq ua tion to get band struc tu res of electrons in crystals, but the size of the d et erminant eq ua tion will become very large if one wishes to solve the equation with suffic ien t acc uracy, because, in general, the Fourier components Vc do not decrease slo wly d ue to the Coul omb potenti al of each atom. This correspond's to the fact th at th e w av e fu nc tions of valen ce electrons are free-electron like (plane-wave like) in the intermed iat e reg io n between atoms and give rapid oscillations (atomic like) near the ion cor es. Therefore, to avoid this difficulty, one can take an effective potential in which the Coulomb potential is canceled by the rapid oscillations of wavefunctio ns . Th e rapid osciUation of w avefunctions originates from the orthogonalizati on between atom ic-like propert ies of wavefu nctions near ion core s. It means th at one in tro d uces new wavefu nclions and a wea k effective potential instead of pl an e wa ves and a Co u lombic po ten tia l to represent the elec tro nic states . Thi s effective p otenti al gives a sm all n umber of reciprocal wave vectors (G) that can reproduce band s truc tures wi th a corres po ndi ng sma ll number of Four ier compo nen ts. Thi s potential is called th e pscudopotential. TIle pseudopo ten tial method necessaril y results in some ar bitra riness wi th respect to th e choice of th ese effective potenti als, depe nd ing on th e- selec tion of effective wav efunction s . It is even p ossible to parametri ze a sma ll number of co mp one n ts in Vc " and to de ter mine th em em pirically.


16

For example, taking severa l Vc va lues in high sy m me try p oints in the Brillouin zone and, aft er adjus ting them so as to reprod uce the bandgaps ob taine d w ith expe rimen tal measur emen ts, on e calculates the band dispersion E(k ) over th e en tire region. In contrast, the LCAO m ethod ap p rox ima tes the Bloch sta tes of val ence electrons by using a lin ear combination of bound a tomic w ave fun ction s. For examp le, (52)

sa tisfies th e Bloch condition s tated in Eq, 42, w here

>Q)

>.

>.

Ol

OJ

~ -2 ....

-2

OJ

"Q)

-4

LU

llJ

-6

c

c

GaAs

-8 -10 L 1 -12

-

L

XU, K

k

--

XU, K

k

Figure 6 Calculated band structures of (a) Si and (b) GaAs using a combined pseudopotential and LCAO method. (From Chadi, D.J., Phys. Rev., B16, 3572, 1977. With permission.)

calculate band structures approximately at or near specific points in the Brillouin zone (e.g., the top of the valence band or a conduction band minimum). In particular, such procedures are quite useful when the bands are degenerate at some point in the Brillouin zone of interest. Now, assume that the Bloch function is known at k = k o and is expressed as 'Vll k o (r) . Define a new wavefunction as: (57)

and expand the Bloch function in terms of 1lIlk(r) as: (58) II'

Introducing these wavefunctions into Eq. 39 obtains the energy dispersion E(ko + k) in the vicinity of k., In particular, near the high symmetry points of the Brillouin zone, the energy dispersion takes the following form:

(59)

where (l/m\ is called the effective mass tensor. From Eq. 59, the effective mass tensor is given as:


18

(i, } = x, y, z)

(60)

For th e iso tro pi c case, Eq . 60 give s the scalar effective m ass m' as:

1 m

(61)

Eq. 61 indicat es th at m' is proportional to the inverse of curva ture near th e extr emal poin ts of the di sp ersion relation, E vs. k. Furthermore, Figure 5 illus tra tes the tw o typica l cases that occu r near th e bandgap, th at is, a positive effectiv e mass at the bottom of the con duction band and a ne gative effectiv e m ass at the top of the va len ce band, d ep ending on the sign o f d 2E / dk 2 a t each ex trema l p oint. Hence, und er an applied electric field E, the specific cha rge el m' of an elec tro n becomes negative, w hile it becomes p ositi ve for a hole . This is the reason why a hole look s like a p articl e w ith a p ositi ve charge. In th e ac tua l calcu la tion of physical p roperties, the followi ng quantity is also im portant: (62) Th is is ca lled the density of states a nd represents th e number of states bet w een E an d E + dE. We ass u me in Eq. 62 th at space is isotropic and m' can be used . Th e band stru ctu res of se m ico nd uctors h a ve been in tensively investi gat ed expe rimentally using optical ab sorption and/ or refl ection spec tra. As shown in Figure 7, in many co mpou n d semiconductor s (most of III-V an d II-VI combinati on in th e periodic table), cond uction bands consist main ly of s-orbital s of the cation, and vale nce bands consist p r in cip all y of p-orbi ta ls of th e anion. Man y comp oun d semicon d uctors have a direct bandgap, w h ich means th at the con d uc tion ban d m inim u m an d th e valenc e band m a ximum a re both a t th e I p oint (k = 0). It sho uld be n ot ed that the sta tes jus t n ear th e m a ximum o f the vale nce band in zi nc-blen de typ e se micon d uc tors consis t of tw o orbitals, n amely 1 8 which is tw ofold d egenerate an d 1 7 w ithout d egener acy; th ese originate from th e spin-orbit interaction. It is known th a t the twofold d egen er acy of I S is lifted in th e k =f. 0 region correspond ing to a light and a h eavy hole, resp ectively. On the other h an d , in wurzite-type cryst als, the valence b and top is split b y b oth th e spin-orb it in terac tio n an d th e crys ta lline fie ld effect; th e b and maximum then consists of three orbi tals: 1 9, 1 9, and 1 7 w ith o ut d egen era cy. In Ga P, th e con d u ction band m inimum is at th e X p oint (k = [100]), a n d thi s com po und h as an in direct band gap , as described in th e n ext sec tio n .

1.2.2 Fundamental absorption, direct transition, and indirect transition When so lid crystals are irrad iat ed by light, various op tica l phenomen a occur : for example, tr an smi ssion , reflection, an d absorp tion . In p articul ar, absorp tion is the annihila tion of light (photon) res u lting fro m th e crea tion of an elec tro nic excitation or lattice excitation in crys ta ls. O nce electrons ob tai n energy from light, th e electrons are exci ted to higher sta tes . In s uch quantum m ech an ical phenomena, one can on ly calculate the probability of exc ita tion . The probability d ep ends on the d istribution of mi croscop ic ene rgy level s of

Chapter aile: Fundamentals of luminescence

19

\

\

E

r g(Al

r , (B)

r , (C)

r , (B)

10001

[0001

[000]

(a)

(b)

(C)

[100]

Figure 7 The typical band dispersion near r-point (k = 0) for II-VI or III-V semiconductor compounds. (a) a direct type in zinc-blende s tructure; (b) a direct type in wurzeite structure; and (c) an indirect type in zin c-bJende structure (GaP) .

electrons in that system. The excited electrons will come back to their initial states after they release the excitation energy in the form of light emission or through lattice vibrations. Absorption of light by electrons from valence bands to conduction bands results in the fundamental absorption of the crystal. Crystals are transparent when the energy of the incident light is below the energy gaps of crystals; excitation of electrons to the conduction band becomes possible at a light energy equal to, or larger than the bandgap. The intensity of absorption can be calculated using the absorption coefficient a(hv) given by the following formula : (63) where n, and nr are the number density of electronic s ta tes in an initial state (occupied byelectron) and in a final state (unoccupied by electron), respectively, and Pi[ is the transition probability between them. In the calculation of Eq. 63, quantum mechanics requires that two conditions are satisfied. The first is energy conservation and the second is momentum conservation. Theformer means that the energy difference between the i.nitial state and the final state should be equal to the energy of the incident photon, and the latter means that the momentum difference between the two states should be equal to the momentum of the incident light. It is quite important to note that the momentum of light is three or four orders of magnitude smaller than that of the electrons. These conditions can be written as (t? 12m' )k; = (tl 212m' )k;2 + hv (energy conservation); Jik.r = n( k, + q) (momentum

20


- -- - Ef

hv

L----------

k

Figure 8 The op tica l absorption due to a direct transiti on from a valence band s tate to a conduction band s ta te.

conser vat ion); and v = cq if one assumes a free-ele ctron -like dispersion for band structure E(k), w here kfa nd k, are the fina l and initial wave vec tors, respectively, c is the light velocity, and q is th e ph oton momentu m . One can neglect th e momentum of abso rbed ph otons compared to those of electrons or la ttice vibration s. It res ults in op tical tran sitions occurring alm os t ver tically on the energy dispersion curve in the Brillouin zo ne . Thi s ru le is called the momentum selection rule or k-selection rule. As shown in Fig ure 8, consi der first the case th at th e minimum ban d gap occurs at the top of valen ce band and at the bo tto m of conduc tio n band; in such a case , the electrons of the va lence band are exci ted to the cond uc tion band wi th th e same momentum. This case is called a direct transition, and the m at er ials having th is type of band struc ture are called direct gap materials. The absor p tion coefficient, Eq. 63, is written as: ' a(hv)=A , (hV- Eg )1/2

(64)

wi th the use of Eqs . 63 and 64. A ' is a co ns tan t rela ted to the effective m asses of elect rons and holes. Th us, one can experi men tally measure the ban d ga p Eg, beca use th e absorp tion coe fficient increases steeply from the edge of the bandgap . In actua l measurem ents, the abs orption inc reases exp onenti ally because of th e existence of impuri ties near Eg . In some materi als, it can occur th at the transition at k = a is forbidden by some selection ru le; the transiti on probability is then propo r tion al to (hv - EJ in the k "'" a region and the absorption coefficien t becomes: '

a(hv) = A' ( hv - Eg )

3/ 2

(65)


21

E

k

Figure 9 The op tical abso rp tion du e to an ind irect tran sition from a va lence band state to a conduction band sta te. The mom entum of electron changes due to a sim ultaneous absorp tion or emission of a ph on on .

In contrast to the direct transition , in the case sh own in Fig ur e 9, both the energy and the momentum of elec trons are cha nged in the process; exci tation of thi s typ e is called an indirect transition. This tran siti on corre sp onds to cases in which the minimum bandgap occurs bet ween two sta tes with d iffere n t k-valu es in the Brillouin zo ne . In this case, conservat ion of m om entum cann o t be provided by th e photon, an d th e tran sition necessarily mu st be associated with the exci ta tion or ab sorption o f phonon s (la ttice vib ra tions) . This leads to a decrease in transiti on probability due to a hi gher-order stoc has tic p rocess. The materials having such band str uc tur e are called indirect gap materials. An expression for the abso rption coefficien t accom panied by phonon absorption is:

(66)

while the coefficien t accompanied by phonon emission is:

(67)

where, in both formulas, Ep is the phonon energy. In closin g this section, the light emission process is briefly di scussed . The in tens ity of light em ission R can be writt en as: (68)


22

where n il is the number density of electro ns exis ting in upper energy states and n, is the number density of empty states with lower energy. The large difference from absorption is in the fact that, usually speaking, at a given temper ature electrons are found only in the vicinity of conduction band minimum and light emission is observed only from these electrons. Then, Eq. 68 can be written as: Conduction Band

/11/IIJ/////II/ /IJ/I/ lI!It/

n=,

'""-- n =; >.

OJ .... CD

C

W

Valence Band Figur e 10

Energ y levels of a free exciton.

(hVk-TE J

L = B' ( hv - Eg + Ep ) 1/2 exp -

(69)

g

B

confirming that em ission is only observed in the vicinity of Eg . In the case of ind irect transiti on s, light emission occurs from electronic transitions accompanied by ph onon emission (cold band); light em ission at higher energy corresponding to phonon absorption (hot band ) ha s a relati vely small probability since it requires th e presence of thermal phonon s, Hot-band emission vanishes completely at low temperatures.

1.2.3 Exciton Although all ele ctrons in crystals are specified by the energy band states they occupy, a character istic excited s ta te ca lled the exciton, which is not derived from th e band theo ry, exist s in almost all se m icond uc tors or ionic crystals. Consid er the case where one electron is excited in th e cond uction band an d a hole is left in the valence band. An att rac tive Coulomb pot ential exists between them and can result in a bound state analogous to a h ydrogen atom . This config ura tion is called an exciton. The binding energy of an exciton is calculated , by ana logy, to a hydrogen atom as:

(70)

where n (= 1, 2, 3, ... ) is a quantum number specifying the states, E is the dielectr ic cons tan t of cr yst als, and m'. is the reduced m ass of an exciton. An exci ton can mov e freely th rough the crystal. The energy levels of the free exciton are shown in Figur e 10. Th e s ta te corresponding to the limit of n ---7 is the minimum of 00

Chapter one:

23


the conducti on band, as shown in the figure. The energy of the lowest exciton s tate obtained by putting n = 1 is: (71)

Two or thr ee kinds of excitons can be gene rated, depending on th e splittin g of the valence band, as was shown in Figure 7. Th ey are named, from th e top of the valence band, as A- and B-excitons in zinc-blende typ e crys tals; and A, B, and C-excitons in wurzite-type crys tals. The re are two kinds of A-excitons in zinc-blende m aterials originating from the existence of a light and heavy hole, as ha s already been n oted . Wavelength [A] 4600

4800

4900

B1

,-, I , , ,

12

J

/

I

I

I

A1

I

I I I

10

I

I

I

Jell

I I I

I

,-

8

"'-, Ql E

0

~

I

> ~

uJ

2

>-

e'

Q'

S

Ql

c w

I"-

1:::-

uT

(')

N II

S

Ol

lU

o

x k= ~ (100)

(000)

k

Figure 21 Energy level and wavefun ction of N isoelectron ic trap in GaP in the k-space. (From Holon yak. N ., Campbell, J.e., Lee, M.H, et al.. ]. Appl. Phys" 44,5517, 1973. With permi ssion .)

distance between the d ono r and acce p to r in th e pair. Th erefore , the recombinati on energy E, is given by :

t; == E cf j -

(98) == 1':: -

(ED + E'I) + e2

4rrcr

In this formul a, r takes d iscrete valu es. For sma ller r va lue s, ea ch D-A pair emission line sho u ld be se pa ra ted , so that a se ries of sha rp em ission lin es should be obser ved . For larg er r values, on the other hand, interval s among eac h em issio n line are small, so that they will not be resolved an d a broad em ission band will be obser ve d . The transition probabilit y should be p roportional to the squa re of th e overl ap of th e electro n and h ole wavefu nctions . Usually, the wa vefunction of a donor electron is more w ide ly sp rea d than that of an acceptor hole. Th e elect ron wavefunc tion of a h ydrogenlike donor is assumed to d ecrease expo ne ntially with r. Therefore, the tran sition p robability W(r ) is exp ressed as:

Figure 22 Luminescence spectrum of GaP:N (4.2K). (From Thomas, D.G. and Hopfield, J.J., Phys. Rev., 150, 680, 1966. With permission.)

o ----..--

~

E(r)

Figure 23 Energy levels of a donor-acceptor pair. (99)

where r s is the Bah!' radius of the donor electron and Wo is a con stant related to the D-A pairs. As a typical exa mp le of D-A pair luminescence, a spectrum of S donor and Si acceptor pairs in GaP is sh own in Figure 24.11 Both Sand Si substitute for P. The P site, in other words the site of on e of the tw o elements cons titu ting GaP, composes a face-centered cubic lattice. In this lattice, r is given by l(l /2)mp /2a, where m is the shell number and a is th e lattice constant. For the shell number s m = 1, 2, ' " 12, 13, 15, 16, ..., there exis ts atoms;


46

100

c 80

C 60

r

'iii

C')

c

Q)

C 40

B A

Rb

20 0 2.18

2.2 0

2.2 2

2. 24

2.26

2 .28

2.30

2.32

Photon Energy (eV)

Figure 24 Lwnine scence spectr um of D-A pai rs in GaP:Si,S (1.6K). (From Tho mas, D.G., Gershenzan, M., an d Trum bore, F.A., Phys. Rev., 133, A269, 1964. With permission .)

but for m = 14,30,46, ... , ato ms d o not exist. Assuming th at th e p osition of each em ission lin e is given by Eq . 98 with r given in thi s way and ED + EA = 0.14 eV (E g = 2.35 eV), it is poss ible to determ ine th e sh ell nu mber for each lin e, as shown in Figure 24. As expecte d, lines for m = 14,30, ... d o no t appear as seen in th e figure. Agreeme n t be tw een ex perime n t and th eor y is surprisingly good . As underst ood from Eqs . 98 an d 99, th e sm alle r the r value is, the sh or ter the lu minesc ence w avelength em itte d and the hi g her the transition p rob ability beco mes; in o ther words, the shorte r the d ecay time. The refore, if one observes a time-resol ved e mission spectrum fo r a bro ad ba nd co m posed of many unresolved pair lin es, th e em ission p ea k of th e broad b and shou ld sh ift to longer w av elen gths with the lapse of time. The broad band peaking a t 2.21 eV in Figure 24 is the en semble of m any unresolv ed p air lin es. Figure 25 12 shows tim e-resolved luminescence sp ec tra of thi s ba nd . It is clearl y seen th at th e p eak sh ifts to longer wavelengths wi th time, as expec ted . Simi lar tim e shifts in D-A p a ir lumin escence h ave been observ ed in II-VI comp ounds su ch as Zn Se and CdS. (See 2.7.)

1.4.5

Deep levels

As th e final s tage of thi s sec tion, lu m in escence and related phen omena cau sed by de ep lev els in semiconductors are discussed . Certain d efects and impurities crea te de ep localized levels with la rge ioni za tion ener gi es. In th ese d eep levels, elec tron- la ttice in teractio ns ar e ge ne ra lly stro ng, so th a t th e n onradiative recom bination takes pl ace via the se levels, thus low ering the lumin escen ce efficien cies of emittin g cen ters . Fu rther, th ese deep centers sometim es m ove an d multip ly by th ems elves in crystals, and ca us e th e d eteriora tio n of luminescence d evices because of th e local he ating by multiphon on emission . Cha nges of th e states of d eep levels ca used by photoexcit ati on are stu died from measurements of conductivity, capacitance, and magn etic prop er ties. In this w ay, the struc ture, den sity, position of energy levels, and ca p tu re and release proba bilities for carrier s have been determine for various deep lev els. Calculat ion s of binding ene rgies of d eep levels usin g w avefunc tions of th e con d uc tion and va lence ban ds h ave also been p erformed . In th is way, bin d in g energies of in GaP and GaAs an d th ose of Ca and As vacancies in GaAs are obtained. Calculat ions are fu r th er made for complex defects in cludin g 0, for exampl e, a compl ex of and Si o r Ge vaca ncy, and a toms occupying antisites.P

°

°

47

Chapter one: Fundamentals of Luminescence 5

C Crystal M78200K

5

o 5

10'

5 ~

'00

c

100 usee

(j)

:f

10

5

10- 1

5

10- 2 5

10- 3 5 2.12

2.16

2.20

2.24

2 .28

2 .32

Photon Energy (eV)

Figure 25 Time-resolved luminescence spectra of D-A pa irs in GaP:Si,S (20K). (From Th omas, D.G., Hopfield, J.J., and Au gu stniak, W.M., Phys. Reo., 140, A202, 1965. With perm ission.)

Transition metal s incorporat ed in semicon d uc tors usu all y creat e d eep levels and exhibit luminescence. Since elec tron- la ttice in teractions are s tro ng, broad-band spectra with rela tively weak zero-ph onon lin es are usu all y observed. Figure 26 shows luminescence spectra of Cr 3+ in GaA s14 as an exa m ple. Co u pling with ph on ons results in the phonon side ba nds shown in Figure 26. As for the nonradi ative recombina tio n th rough defects, no t only the Auger reco mbination p rocess but also m an y phonon emission process are ob served. The tran sition probabilities of the latter increase wh en related levels are d eep and crys tal te mp eratures are hig h. In cer tain cases, the en ergy level of a locali zed trap is sha llow before trapping

48

Fundamentals of Phosphors Wavelength (nm)

1700

1650

1600

1550 LO

LA

I I

1500 TA IAr

1450

I

LPE M 228

OJ

U

C

OJ

-11-

U CfJ

OJ

c

Resolution

'E ::l

--l

.840

.860

.880

5900 6000 6100 6200 6300 6400 6500 6600 6700 6800 6900 7000 7100 Wavenumber (cm' ) Figure 26 Lum inescen ce spectrum of Cr3+ in GaAs (4.2K ). (From Stocker, H.J. an d Schmidt, M., App!. Phys., 47, 2450, 1976. With permi ssion .)

J.

E

L-

_ _.....I_

.........;::..-"--~

Q

Figure 27

Config ur ationa l coordinate model of d eep defect level. (C: cond uction band, V: val ence band, D: d eep d efect.)

an electron; but after trapping, lattice relaxation and th e rea rrangement of surrounding a toms tak e place and the energy level is m ade d eep, as shown by the config ur ationa l coord inate model (1.3.2) in Figure 27. In th is state, the diffe ren ce betwe en the op tical

Chapter one:


49

activation en ergy an d th ermal activation energy is large, and nonrad iat ive recombin at ion through the emiss ion of ma ny phonons occurs w ith high probab ility."

References 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11.

12. 13. 14. 15.

Litton , C.W, Reynold s, D.C., Co llins, r.c. and Par k, YS., Phys. Rev. u u.. 25, 1619, 1970. . Thom as, D.G. and Hopfield, ].J., Phys. Reu., 175, 1021, 1968. Henry, c.H. and Nassau, K., Phys. Reo., 81, 1628, 1970. Nels on, D.F., Cu th bert, ].D., Dean , PJ ., and Thomas, D.G., Phys. Rev. Leti., 17, 1262, 1966. Kukimoto, H ., Shionoya. S., Toyotomi, S., an d Morigaki. K., f. Phys. Soc. [pn., 28, no, 1970. Han amur a, E., J. Phys. Soc. [p n., 28, 120, 1970. Holonyak, [r., N., Campbell, I.C; Lee, M.H ., Verdeyen. J.T., Johnson, WL., Cr aford, M.G., and Finn, D., I. App!. Phus., 5517, 1973. Thomas, D.G. and Hopfi eld , J.J., Phys. Reo., 150, 680, 1966. Aften, A.c. and Haaus tra, J.H., Phys. u «. 11, 97, 1964. Merz, J.L., Phys. Rev., 176, 961, 1968. Thomas, D.G., Ce rshenzon. M., and Tru mb ore, F.A., Phys. Rev., 133, A269, 1964. Thom as, D.G., Ho pfield . J.J., and Augustyn iak, WM., Phys. Rev., 140, A202, 1965. AIt, H.Ch ., Materials Science Forum , 143-147, 283, 1994. Stocker, B .]. and Schm id t, M., f. Appl. Phys., 47, 2450, 1976. Kuki mo to, H ., Solid State Phys., 17, 79, 1982 (in Jap an ese).

chapter one - section five

Fundamentals of luminescence Chihaya A dachi and Tetsuo Tsutsui

Contents 1.5 Luminesc ence of orga ni c compounds 1.5.1 Origin of luminescence in org anic compounds 1.5.2 Electronically excited st ates of or gan ic molecules and their p hotolu m inesce nce 1.5.3 Fluo rescence of organic molecu les in a solid state 1.5.4 Q uan tum yield of fluo rescence 1.5.5 Organic fluorescen t and phosphorescence compou nds with hi gh gua n tu m yield s References

51 51 52 54

56 56 59

1.5 Luminescence of organic compounds 1.5.1 Origin of luminescence in organic compounds The luminescen ce of orga nic compoun d s is essentially based on localized rt-electron systems wit hin ind ivid ual organic m olecules' . Th is is in clear con trast to inorgan ic phosphors where luminescen ce is de termined by the ir latti ce structures, and thu s their luminescence isaltered or d isappears altogether when the crys ta ls me lt or d ecom p ose. In organic lumi nescent compo unds, in con tras t, it is the rr-electron systems of individ ua l m olecu les tha t are respo nsible for luminescence. Therefore, even when organ ic crystals melt in to amorphous agg regates, lu minescence still persists. Further, when molecules are in vapor p hase or in solu tion, they basically demonstrate sim ilar lu minescence sp ectru m as in soli d films . Lum ines cence from organic compoun ds can be class ified into two ca tegories: luminescence fro m electronica lly exci ted sing let (51) or triplet (T )) s ta tes . Em ission from sing le t exci ted sta tes, called " fluorescence," is commonly observed in conventiona l organic compound s. Em ission from triplet excited sta tes, called "p hos phorescence," is rarely observed in conventional organic com pounds at ambien t tem peratures due to the small radiative decay ra te of phosphorescence. Electronically exci ted s ta tes of organic compou nd s are ea sily prod uc ed not only via photoexci ta tion but also by other exci ta tion methods (such as chemical rea ction s, electrochemical reactions, mec ha ni ca l forces , hea t, and electric charge recomb ina tio n) capable of prod ucing electronically excited s ta tes in organic mo lec ules, as d epicted in Fig ure 28.

51

52

Electroluminescence Triboluminescence


Chemilumine scence

Ground state

Figure 28 Th e various excitation methods, i.e., light absorption, thermal, chemical and chargedparticle, and d ecay process es, i.e., photoluminescence, thermal deactivation, and energy transfer and migration, in organic molecules. It should be emphasized that any kind of luminescence in organic compounds is due to

well-defined electronically neutral singlet or triplet excited states in the organic molecules, ev en though luminescence can be produced by a variety of excitation methods having different names Like photoluminescence, chemiluminescence, electrochemiluminescence, triboluminescence, th ermoluminescence, and electroluminescence. In add ition to radiative deca y, the excited molecules also decay nonradiatively through thermal deactivation and energy transfer and migra tion.

1.5.2 Electronically excited states of organic molecules and their photoluminescence Electronic transiti ons in organic molecules ar e described by the molecular orbitals of 0 ele ctrons and It-electrons. Each molecular orbital can accept two electrons with antiparal1el spins accord ing to Pauli's exclusion principle, and both a and It-electrons participate in chemical b onding. He re, It-electrons demonstrate a variety of photo- and electronic activities compared with a-electrons, sin ce a- electrons become located a t deeper energy levels compared with those of It-electrons (Figure 29). The ground state is characterized by the nelectrons in the highest occupied mol ecul ar orbital (HOMO) . In ord er to produce an electronically excited sta te, a mol ecul e must absorb energy equal to or greater than the en ergy difference between th e HOMO and the lowest unoccupied molecular orbital (lUMO) levels (100)

With ab sorption of en ergy, an electron is promoted from HOMO to lUMO, and this constitutes an electronic transition from the ground state (So) to an electronically excited state (S1)' Here, the energy level diagrams (Jablonski dia gram) for molecular orbitals for the ground


53

LUMO

1 ~o" I tJE

n-electrons {

:,J,.,

I/J n- l

-H-- "

a-electrons

--r-r-

¢2

---t-t-I/Jl Ground state

~O

1

. .,

¢M1

tJE

l'

l'

HOMO

~ ¢n

-t--T-H-- ¢i ¢n-l

--r-r---t-t-

1J2

¢,

Lowest excited state

Figure 29 Energy leve l diagrams of molec u lar orbitals and electro n con fig ura tio ns for gro und and singlet excited states.

and excited sta tes are commonly used for the description of electronic transitions in organic molecules (Figu re 30). In Figure 30, the transition of an electron from HOMO to LUMO is expressed in terms of its spin s tates. Th e electronic tra nsitions are expressed in terms of the difference in energy bet w een the gro lmd and excited s tates in the electron ic-sta te diagram. The spin multipl icity of the states is implicitly ind icated by the notations of 5 (sing let) or T (triplet). The gro un d state, 50' and lowest singlet and triplet sta tes, 51and T" are com posed of multiple vibra tional states, d ue to vibron ic and rota tion ene rgy levels of the molecules. When an energ y larger th an the HOMO-LUMO ene rgy differen ce is abs orbed by a molecu le, either higher vibronic sta tes within th e 51 sta tes or higher singlet excited states 52 and 53 are pro duce d . Th e highe r vibro nic states of 5, rela x to the low est vibron ic state of 51 w ithin a timescale of - 10- 12 s. The high er energy sing le t s tates s uch as 52 and 53 rela x to the 5, sta te via nonradi at ive, in ternal conversion (IC) p ro cesses. Trip le t exci ted states are usually prod uced via an in tersys tem crossi ng (I5C) p rocess from 5, ~Tl' since th e transiti on probability of di rect exci ta tion from 50 int o T] is ve ry sma ll. Also, the higher energy triplet states such as T2 and T3 relax to the T 1 sta te via nonr ad iat ive processes. Thus, radi ativ e tran sitions take pl ace as all. electronic tran sition from the lowest excited states of 51 or T, to the gr ound sta te 50' The radiative tran sition from 51 ~50 is classified as a spin-a llowed tra nsition and the refore the tim escale of the transition is of the order of - 10-9 S . On the other hand, the tim escale of the T 1~ So tran sition is m uch lon ger, ranging from micro- to milliseco n ds because the pro cess is intri ns ically spinforbidden . The emission sp ectr a of org anic m olecul es o ften exhibi t a vibron ic structure beca use the ground state also con tains vibronic and rota tiona l fine structures. Figur e 31 sho ws schematically the relation between a bsorp tion an d emission spectra . An emission spec trum looks like the mirror image of the electronic absorpti on sp ectru m of a molecule d ue to the pre sence of vibra tional levels in each energy leve l. The emission wa velength for the rad iative transition from the lowest 5, state to the lowest 50 state, the 0-0


54

(2)

(6) (3) Q)

ro

(2)

81

(4)

a,', a,'). In this cas e, the center-of-mass motion of excitons is quantized. Nanometer-size CuCl mi crocrystallites are typical examples of th e w eak confinement regime; th e gro un d -state en ergy is written as: (119)

where M = m ,' + m hois th e translational m ass of the exciton. Typical experimental d at a for three categories are shown in Figure 40.6 CdS, CuBr, and CuCl mi crocrystallites belong to strong, intermediate, and weak confinement regimes, res pectively. With a decrease in mi crocrystallite size, the continu ou s band ch anges into a se ries of di screte levels in CdS, alth ough th e levels ar e broadened becau se of the size di stribution. In the case of Cu Cl, the excit on absor p tion b ands show blue-shifts with a decrease in siz e. The lumines cence of semiconductor microcr ystallites not only depend s on the mi crocrys ta llites themselve s, but a lso on their s urfaces a nd th eir surroundings since the sur face:v olume ratio in these syste ms is large. Th e luminescen ce sp ectrum then depends on the preparation co nd itions of mi cro crystallites . Thus it is th at some samples sh ow donorac cep tor pair re combination, but other samples do not; in others, the edge luminescence a t low temperature consists of exciton and bound exciton lum inescence. The exciton luminescence spec trum of m any sa m p les sh ows Stokes shift fro m the ab sorption spectru m, indicating th e pres ence of localized e xcitons. Typical examples of the luminescence spectra of CdSe mi crocr ystall ite s an d CuCI m icrocryst all ites are show n in Figures 41 and 42.7,8 Impurities or d efects in insulating crystals oft en dominat e their luminescen ce spectra; thi s is also th e case w ith semiconductor microcrystallites, but ad dition al effect s occur in the latter. Nanometer-size semiconductor microcrystall ites can be composed of as few as lOL 106 a toms; if the con centration of cen ters is less than ppm, considerabl e amoun ts of th e microcrysta llites are free from impurities or defects. Thi s conjecture is ve ri fied in AgBr microcrystallites, w h ich are indirect tr an sition m at erials." Figure 43 sho w s luminescen ce spectra of AgBr microcryst allites with averag e radii of 11.9, 9.4, 6.8, and 4.2 nm. The h igh er-energy band observ ed at 2.7 eV is the in direct exciton luminescence, and the lower-en ergy band observ ed at 2.5 eV is the bound exciton luminescen ce of iodine im p ur ities . In contrast to AgBr bulk crys ta ls, the indire ct exci ton luminescen ce is strong comp ared w ith the bound exciton luminescence a t iodine impuritie s . The rati o of the ind irect exciton lumin escence to th e bound exciton lu minescence a t iodine impurit ies increases w ith the decrease in size of AgBr microcrystallites. This increase in ratio shows that the number of impurity-free mi crocryst all ites increases with the decrease in size. Simultaneously, th e dec ay of the indirect exc iton luminescence ap p roaches single exp onential d ecay approxim ating the rad iative lifetime of the free in d irec t exciton. Th e blue-shift of th e indirect excito n luminescen ce shown in Figure 43 is due to th e exciton quantum confinement effect, as di scussed previou sly. Nano m eter-size se mi con d ucto r microcrystallites can be us ed as a laser medium.'? Figure 44 shows th e lasing spectrum of CuCI microcr ystallites. When the micro cryst allites embedded in a N aCl cry sta l are pl aced in a cavi ty and excited by a nitrogen laser, lasing occurs a t a certain threshold . Th e em iss ion sp ec tru m bel ow th e thresh old , sho w n in Figure 44, arises from ex citon ic molecule (biexciton ) luminescen ce. Above th e th reshold, th e broad excitonic m olecu le em ission band is con ver ted to a sharp emission spe ctru m ha v ing a maximum p eak at 391.4 nm. In this cas e, the lasing spectrum is composed of a few longitudinal m odes of the laser cav ity consi sting of mi rro rs se pa ra ted by 0.07 run . The optical gain of th e CuCl m icrocryst allites com pa red with th at in a bulk Cu CI sa mple is found to be much large r. The h igh optical ga in of CuCI m icrocrystallite s comes from th e s pa tia l confinement of exc itons, resu ltin g in th e enhanced formation efficiency of excit onic molecules.

Chapter one:

69


~

Z3

" ,,

;-

Z W

" f 1 t:

0

J

if)

"" ""

I

-J

"", ,

I I

< U

CuCl

f f

i= CL

a

2 3.4

~ z

:

.....

Z3 ....

W

I

f-

-z C /)

UJ

~

Z 0

C /) C/)

UV

UV+IR-

(b)

~ UJ

Tl M E Figure 56 Photostimulation and photoquenching simulation for the case (b) under a constant excitation and (a) in the after-glow process after the termination of excitation.

or photoquenching of luminescence; that is, an increase or decrease of the emission intensity as schematically shown in Figure 56(a). Under a stationary excitation shown in Figure 56(b), the stimulation enhances and the quenching reduces the emission intensity temporarily. These phenomena are utilized for IR detection and radiographic imaging, in which the intensity of the stimulated emission is used to measure the intensity of IR light or Xvravs.!' Photostimulation is caused by the radiative recombination of the electrons (holes) released by photoactivation from deep trap levels, as shown in Figure 57(a). On the other hand, photoquenching is caused by the nonradiative recombination of holes (electrons) photoactivated from luminescent centers as shown in Figure 57(b). Figure 58 depicts the configuration coordinate model of photostimulation. The activation energy Eo of photostimulation is not generally equal to the thermal activation energy e, of trapped electrons discussed before with reference to Figure 49. Since the optical absorption process takes place in a very short time period without changing the configuration of the atoms in the center at that moment, the process is represented by the straight vertical transition in Figure 58 from state III (a trap or the quasi-stable state of emitting centers) to state II (the conduction band or emitting centers). On the other hand, the thermal activation needs energy t l to overcome at least the lowest barrier between states II and III in Figure 58; hence, the activation energy 101 is generally smaller than £0. Photostimulation spectra (i.e., excitation spectra for IR stimulation) can be used to measure the optical activation energy to.

Chapter one:


87

...

C.B .

RADIATIVE RECOMBINATION

(a)

(b)

NONRADIATIVE RECOMBINATION

V.B. Figure 57

Photostim u lation process (a), an d photoq uenc hing process (b) in an en e rgy ba n d sch eme. C.B. and VB. indica te the co nduction ba n d and the val ence ban d of th e hos t crysta l, respective ly.

II

Figure 58

Pho tostim ula tion in configurational coordina te m odels.

References 1. 2. 3. 4.

5. 6. 7. 8. 9. 10.

11.

Nakazawa, E., J. Lum inesc., 18/19, 272, 1979. Weber, M.L Phys. Reo., B8, 54, 1973. Hoogenstraaten, v«, Philips Res. Rept., 13,515, 1958. Ran dall, J.T. an d Wilkins, M.H. F., Proc. Roy. Soc., Al84, 366, 1945. Cur ie, D., Luminescence in Crystals, John Wiley & Son s, 1963, ch a p. VI. Avouris. P. an d Morgan , T.N., J. Chern. Phys., 74, 4347, 1981. Kivits, P. and Hagebe u k, H.J.L., J. LII11Iinesc., 15, 1, 1977. Bube, R.H., Phys. Rev., 80, 655, 1950. Nakazaw a, E., [pn. J. App!. Phys., 23(9), L755, 1984. Na kazawa , E., Oyo Buturi, 55(2), 145, 1986 (in Jap a nese). Practical Applications of Phosphors, Yen, W.M., Sh ion oya, S., an d Yam a moto, H , Ed s.. CRC Press , Boca Raton, 2006.

chapter one - section eight

Fundamentals of luminescence Eiichiro Nakazawa

1.8 Excitat ion en ergy tran sfer and coop er ati ve op tica l phenomena 1.8.1 Excitatio n energy transfer 1.8.1.1 Theory of resonan t en ergy transfer 1.8.1.2 Diffusion of excitation 1.8.1.3 Sens itiza tio n of luminescence 1.8.1.4 Concentra tion quenching of luminescen ce 1.8.2 Cooperat ive optical phenomena Referen ces

1.8

89 89 90 94 95 96 97 100

Excitation energy transfer and cooperative optical phenomena

1.8.1

Excitation energy transfer

The process of excitation en ergy transfer from an excited point in a cryst al to a luminescent center can be classified into the followin g two types. 1. Migration of free electrons (holes), electron -hole pairs, or quasi-p articles such as excitons and plasmons conveys the excitati on energy to luminescent cent ers. Thi s typ e of transfer seems to be active especially in s uch semicond uctor-like hosts as ZnS and CdS, which are w id ely used as cathode-ray tube (CRT) phosphors. In th e ini tial excitation process of CRT phosphors, the local excitation b y a high-energy electro n pr oduces several hund reds of these particles, and th ey are di sp ersed in th e crystal by this typ e of transfer. (See 1.9.) 2. Excitation energy is tran sferred from an excited cen ter (ene rgy donor) to an unexcited center (energy accep tor) by means of quantum mechan ical resonan ce.l? Thi s typ e of tran sfer is practi cally utilized for the sensitiza tion of lu minescence in lamp phosphors, w hich are mostl y oxid es or oxoacid salts w ith less- mobile electrons and holes than in CRT phosphor mat erials. In this section, the res on ant energy transfer process and related phenomena, such as the se nsitization and quenching of luminescen ce, w ill be discus sed . More recent experimental studies on this topi c are referred to in the referen ce list. '

89


90

e

~------R -----~~

D Figure 59

A

Coulomb in teraction in a resonant en erg y transfer process.

~--R ------:~

D

A

Figure 60 Resonant energ y trans fer by the exchange interaction, in wh ich the ove rla pping of the wavefu nctions of D and A (sha d ed region) i.s necessa ry.

1.8.1.1

Theory of resonant energy transfer

De xter 's theory of resona nt energy transfer- has elucidated that two optical centers w ithin a certain di stance may be in resonance and tr an sfer th e excit ation en ergy from one (donor) to the other (acceptor). The close proximity of th e center s en ables th em to be connected by the electrostatic interaction shown in Figure 59 or by the quantum mechanical exchange interacti on shown in Figure 60. Th e energy donor, which is called a sensitizer in p ractical usa ge, is d enoted hereafter by D and the acceptor by A. For resonant energy transfer to take place, it is necessary that the tran sition energies of D and A be equal. (a) Multipolar Interaction. The me chanisms of resonant en er gy transfer can be classified into sev eral typ es based on the character of th e transitions in 0 and A. Wh en both transiti ons in 0 and A are of electric dipole character (dipole-d ipole interaction), the probability pe r second of en er gy transfer from D to A is given by :

(146)

Chapter one: Fundamentals of lumin escence

91

Here, R is th e sep ara tion b etween 0 and A, n is the re fractive index of the crystal, 0 A is the absorption cross-sec tion of A, an d is the rad iati ve lifetime of D . Likewi se, the tran sfer probability du e to th e dipole-quadrupole interact ion is:

'0

(a = 1.266)

(147)

In these equa tions , f o(E) and FII (E) re p rese n t th e sh ape of the 0 emission an d A absorption spec tra, res pectively, which ar e normalized (i.e., ff d E)dE = 1 an d fFA E)dE = 1. The integrals in Eqs. 146 and 147 are, therefore, th e ov erl apping ra tios of these two spectra, which is a measure o f the resonance con dition. The tr ansfer p robabilities du e to all multipolar interact ion s-i .e., dipole-d ipole (d-d) in Eq. 146, d ip ole-qu adrupole (d- q) in Eq. 147, an d quadrupole-quadrupole (q-q), ar e sum ma rized in Eq. 148 with thei r R d epe n den ce being noticed .

(148)

Here, s in the third term is 6, 8, an d 10 for (d d) , (dq), and (qq), respect ively. If the d ip ole tran sition is allo wed for b oth D an d A, the m agnitudes of a , are a dd > a dq > a qq, an d the di pole-dipole interacti on has the h ighest transfer prob ability. How ever, if the dipole tran sition is not complet ely all owed fo r 0 andl or A , as is the case with the f-f transition of ra re-ear th ions, it is p robable that the higher-ord er interaction, d-q or q-q, may h ave the larger transfer probab ilit y for sma ll d ist ance p airs due to the higher-order expone n t of R in Eq . 148.4•5 Since th e emission in tens ity an d the radiat ive lifetime of 0 are d ecreased by en ergy transfer, the me ch anism of th e transfer can be an alyzed using the d ependence of the transfer p robability on th e pair d istance give n by Eq . 148, an d hence the domina n t mechanism among (dd ), (dq ), an d (qg) can be determined . When the acceptors ar e randomly distributed w ith various di st ances from a donor 0 in a crys tal, the em iss ion decay curv e of D is not an expone n tial on e. It is giv en by th e following equa tion for the multipolar in teractio ns."

~(t) = exp [ - _t _ r(l- ~) ~ [_t J Co

3/

1:0

5

' ],

(5 = 6, 8, 10)

(149)

1:0

ro

is the ga m ma func tion, a nd C and Co ar e, respectively, th e con centration of A Here, and its crit ical concen tra tion a t which the tran sfer probability is eq ua l to the ra d iat ive probability (1/1:0 ) of D. Thus, the emission efficiency 11 and th e em issio n d ecay time cons tan t 1:111 can be estim ated using Eq. 149 an d th e foll ow ing eq ua tions :

-2l = 11 1J

L~(t)dt 1:0

(150)

Funda mentals of Phosphors

':JL

1.00

>f:-< 0.50

>-
C)

95

,-, ,,, ,,,

A

I..

\

.I \ "

0::

n: .. II""\-" ~\ ::

W

Z

:,1'

W

A

W

, ,,,

>

.....

:5w

,,

,.

I

\.

"

E

II'"

,

"

.\ ' " "1.:\'

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ J J~

25000

\

, \ \' " , ..\\,

I

-»

a:::

, ,,

-, \ \

-- -,

23000 21000 19000 17000 1 WAVE NUMBER [cm- ]

,,

',\

,

\.,

.\.'

-

",,~~,

" ......

""

15000

Figure 64 Emis sion spe ctra of a lamp ph osp hor, Ca s(P0 4 MF,Cl):Sb3 +, Mn 2+, in which the Sb 3 + ions sensit ize the emission of Mn 2+ ions by an energy transfer process. The Sb3+ concentration is fixed to be 0.01 mollmol Ca. The Mn 2+ concentra tion is changed, AO , B:0.005, C O.OlD, D:0.020 and E:0.080 moll mol Ca . (From Baller, K.H. and Jerome, c.w.,;. Electrochem. Soc., 97, 265, 1950. With perm ission.)

1.8.1 .3 Sensitization of luminescence Energ y transfer p ro cess es ar e often us ed in practical phosphors in order to enha nce the emission efficiency. The process is called sensitization of lum inescence, and the en ergy donor is called a se ns itizer. The emission intensi ty of Mn -'-. ;....

9

1

# 0 5 LJE [eV]

4-

0

;.... (!)

,D

E

;:l

z

3

2 1 0

0

10

30 40 LJE [eV ]

20

50

60

Figure 73 Electron energy loss spectra of YV 0 4 : (a) peaks A to D origina te in the electronic transi-

tions of the V04- 3 comp lex; (b) peak E can be assigned to plasmon excitation. Peak G is du e to a transition from Y 4p orbital to the cond uction band and, peak H from V 3p to the conduction band. The origin of peak F is not identified. The strong peak at 0 eV indicat es the incident electrons with no energy loss. (From Tonomura, A., Endoh, L Yamamoto, H., and Usarni, K., f. Phys. Soc. Japan , 45, 1654, 1978. With permission.) Wh en Eois d ecrea sed at a fixed electron beam current, luminescence vanish es at a cer tain positive vo ltage, called the dead voltage. One of the explanations of the dead volt age is that, at sha llow R, th e primar y electron energy is dissipated within a dead lay er nea r th e surface, where nonradi ative processes dominate as a result of a high concentration of lattice d efects.'? It is also known, h owev er, that the dead voltage decreases with an in crea se in electrical con d uctivity, indicating that th e dead voltage is affected by electrical charging as we ll.

1.9.4 Ionization processes A charged p article, s uch as an elec tro n, loses its kinetic energy through various m odes of electrostatic interaction with constituent atoms when it passes through a so lid . Elemen tary processes leadin g to en ergy dissipation can be ob served exp erimen tally by the elec tron energy loss spec troscopy, which measures the energy lost by a p rim ar y elec tro n d ue to inelasti c sca ttering (corresp on di ng to the electrons (b) in Figure 70). Main loss p rocesses observed by this me tho d a re core-elec tron excita tion s and crea tion of pl asmons, w hic h are a collective excita tion mo de of th e va lenc e e lectro ns . Core- elect ron excita tion is observed in the range of 10 to 50 eV for m at er ials having elem en ts o f a la rge a to mic number, i.e., rare-earth compounds or hea vy me tal oxid es suc h as va na da tes or tungstates .P Th e p lasmo n energy is found in th e reg ion of 15 to 30 eV. Co m p ared wi th th ese excita tion m od es, the co ntrib ut ion of th e band-to-ba nd tran siti on is sma ll. As an example exhi b iting vario us mo de s of exci tation, th e electron ene rgy loss spec tr u m of YV04 is shown in Fig ure 73.14


106

Primary electron

Plama creation and other processes

Emission of optical phonon Ionizing process

c-,

bO .....

V

s:::

u..J

Threshold for ionization -

Ei

Secondary electrons

Carriers in thermal equilibrium Figure 74 A schem at ic illus tra tion of exci ta tion processes by a high -en ergy electron, whi ch p enetrat es in to a sol id . (From Robb ins, D.]., f. Electrochem. Soc., 127, 2694, 1980. With permission.)

Pla smons are converted to sing le-ele ctron excitation s in an extre mely short period of time, _10-15 s. As a con sequence, ele ctrons with energies of 10 to 50 eV are created eve ry tim e an energetic primary elect ron is scattered in a so lid as a result of core-electron exci ta tion or plasmon crea tion . This results in a series of ionization processes in a solid . Most of the electrons gen erat ed by the scattering events, or the seco ndary electrons, are still energetic enough to create other hot carr iers by Auger p rocesses. Seconda ry electron multiplication can last until th e ene rgy of the ele ctron falls below the threshold to crea te free carriers . All through thi s electro n energy loss process, sca tte ring is accompanied by phonon crea tion, as schematicall y shown in Figure 74 .' 0 Seco nd ary electro n multiplicati on is essen tially the same as the ph oto excitati on process in the va cu u m ultraviolet region. The ave rage en ergy required to crea te an electron-hole pair near the band edges, Ea" is gi ven by th e follo wing em pirical formula. " Em' = 2.67Eg + 0.87 reV]

(161)

wher e Eg is the bandgap energy ei the r for the direct or th e indi rec t ga p . This formula wa s origin ally obtained fo r eleme n ts or binary compound s with tetrahed ral bonding, but it is applied often to phosphors w ith more complex chem ica l co mpositions and cryst al stru ctur es. It is not , h owever, s tra igh tforward to define the bandgap energy for a materi al ha ving lo w-l yin g en ergy levels chara cte ris tic of a m olecular group, e.g.,

Chapter one:

Funda mentals of luminescence

107

vanada tes or tungst at es. The re fore, one m us t be care fu l in applyi ng th e above formul a to so me p hosphor s. As d escribed above, the average crea tion energy of an electron-hole pair is closely related to the catho d olu m inescence efficiency (see also Section 1.9.6). There is, h ow ever, another way to cons ider the luminescen ce efficiency; it foc uses on phon on emission," which compe tes with the electron-hole pa ir creation in th e ioniza tion processes. The phonon emission probability, denoted as R; here, is proportion al to th e in teraction of an electron wi th an op tical phonon, and is expressed as:

(162)

wh ere W w is the ene rgy of a lo ngi tudina l op tica l phon on in terac ting w ith an elec tron, an d an d £0 are high-frequency and sta tic d ielectric consta n ts, respectively. When multipl ied with the p ho non energy, the proba bility R; contributes to the pair creation energy EI1V as a term ind ependent of Eg, e.g., the second term 0.87 eV in Eq. 161. The lu minescence excited b y energetic particles is radi olum inescence.'? Th e exci ta tion mechan ism of ra d iolu m inescence h as its ow n characteri st ic processes, though it involves ion ization processes sim ilar to the ca tho dolu minescence processes. For exa mple, the ene rgy of y-rays can be d issipated b y three p rocesses: (1) th e Com pton effect, (2) the photo electric effect d irectly followed by X-ray emission and A uger effect, and (3) the creation of electro n-posi tron pair s. Su bse q ue n t to these processes, highl y energe tic seco nd ary electron s are created, follow ed by the exci ta tion of lum inescence centers, as is the case with ca tho do lumi nescence. A characteristic energy loss process of neutron s, which has no electr ic cha rges but much larger mass than an elec tron, is due to the recoil o f h ydrogen atoms. If the neutron energy is large enough, a recoiled hy d ro gen is ioni zed an d crea tes secondary electrons. It mu st be added, how ever, th at hydrogen atoms are n ot con ta ine d intentionally in inorganic ph osphors. £~

1.9.5

Energy transfer to luminescence centers

The fina l products of the seco ndary-electron m ultip lica tion are free electron s and free holes nea r the band edge, i.e., so-called ihermaliz ed electro ns and holes. Th ey reco mbine with each othe r, and a part of the recombination energy may be converted to luminescen ce light emission. The process in w hich eith er a thermalized elec tron-h ole pair or the en ergy released by their recom bin ati on is tran sferred to a luminescen ce cen ter is call ed host sensitization becau se the luminescen ce is se nsi tized by th e op tica l absorp tion of the h ost lattice. This p rocess is analogo us to the optica l excitation near the band edge. Det ailed studies were made on the optical exci tatio n of luminescen ce in IIb-VIb and lIIb-Vb com p ounds, as described in 2.7 and 2.8. Luminescence of rare-earth ions and Mn 2+ ion s arises becau se these ions capture electrons and holes b y actin g as isoe lec tro n ic traps.P-'" In inorganic compound s having com plex ions an d organic compounds, the excitation ene rgy is tran sferred to the luminescence cen ters through the m olecul ar energy levels.

1.9.6 Luminescence efficiency The catho dolu minescence energy efficiency 'Il, for all the p ro cesses d escribed above can be expressed b y 20 :

108

Fundamentals of Phosphors Table 2

Examp les of Ca tho do lu min escen ce Efficiency

Ch emical co m pos ition

WTDS designation

Zn 2SiO; :Mn2+ CaW04:Pb ZnS:Ag,Cl ZnS:Cu,A1 Y202S:Eu o, Y203:Eu 3 + Gd 20 2S:Tb3+ CsI:TI+ CaS:Ceo+ LaOBr:Tb3+

GJ BJ X X X RF GY

En ergy efficiency

Peak wavelength

(%)

(nm)

8 3.4 21 23, 17 13 8.7 15

525 425 450 530 626 611 544

11

22 20

544

Lu minescence colo r G reen BIlle Blue Gre en Red Red Yellowis h green Green Yellowis h green Yellow ish g reen

Note: The ph osphor screen designat ion by WTDS (Worldwide Phosphor Type Designa tion Sys tem) is presented. Man y da ta are collected in Alig, R.C. and Bloom,S., f. Electrochem. Soc., 124, 1136,1977.

(163) where 110 is the back-scattering factor given by Eq. 157,l1x th e me an ene rgy efficiency to create thermalized electro ns an d hole s by the primary electrons or Ex! Em" q the quantum efficiency of the luminescence excited by thermalized ele ctron-hole pairs, a nd E"III the mean energy of the emitted photon s. Thus, (164)

and also 11, < 1/3 acco rd ing to Eq. 161. TIle ene rg y efficienc y, luminescence peak wavelength and color are shown in Table 2 for some efficien t phosphors. For the commercial phosphors, ZnS:Ag,CI; ZnS:Cu,Al; Y20 25:Eu3+; an d Y zO,:Eu3+, w e find 11x = 1/3 from Eq. 163 by ass um ing that 110 = 0.1 and q = 0.9-1.0. Thi s va lue of 11, sugges ts that the energy efficiency is close to the limit predicted by Eq. 163 fo r these phosphors. It is to be emphasized , h ow ever, that this estimate does not exclu de a possibilit y for further improv ement in the efficiency of these phosphors, for example by 10 or 20%, si nce the calcu la ted values ar e based on a nu mber of approximations and simplifying ass u m p tions . It sh ou ld also be not ed that the band gap energy is not known acc ura tely for the phosphors gi ven in Table 2, except for Zn 5, CsI, and CaS. For the other phosph or s, the optical ab sorption edge must be us ed instead of the bandgap energy, leavin g the estimation of 11 approxim ate. For CaS, the indirect bandgap, 4.4 eV, gives 11, = 0.21, while the direct bandgap, 5.3 eV, g ives the value exceeding the lim it predicted by Eq . 163.

References 1. 2. 3. 4. 5. 6. 7.

Dekker, A.}., Solid State Physics, Prentice-Hall, Mar uzen, Tokyo , 1960, 418-420. Rud berg. E., Proc. Roy. Soc. (London), A127, 111 , 1930. Toml in, S.G., Proc. Roy. Soc. (London), 82, 465, 1963. Meyer, V C ., J. App l. Phys , 41, 4059, 1970. Kazan , B. and Kn oll, M ., Electron Image Storage, Academi c Press, New York, 1968, 22. Kan a, T. a nd Uchida, Y, [pn. J. Appl. Phys., 22, 1842, 1983. Ehrenberg, W. and Franks, L Proc. Phys. Soc., B66, 1057, 1953.

Chapter one: Fundamentals of luminescence 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

109

Ga rlick, G.F.J., Br. ]. Appl. Phys., 13, 541, 1962. Beth e, H.A., An ll. Physik, 13,541, 1930. Whidding ton, R., Proc. Roy. Soc. (London), A89, 554, 1914. Feldma n, C , Phys. Rev., 117, 455, 1960. Gerg ley, Gy., J. Phys. Chern. Solids, 17, 112, 1960. Yam am oto, H. an d Tonornura, A ., J. Luminesc., 12/13, 947, 1976. Tonomura. A., Endoh, J., Yamam ot o, H ., and Usarni, K. , J. Phys. Soc. Japa n, 45, 1654, 1978. Robb ins, D.J., J. Electrochem. Soc., 127, 2694, 1980. Klein , C A., J. A ppl. Phys., 39, 2029, 1968. For example, Brixner, L.H., Materials Chemistry and Physics, 14, 253, 1987; Derenzo, S.E., Moses, w.w., Ca hoon, J.L., Perera, R.L.C , and Litton, I.E., IEEE Trans. Nuc/. Sci., 37, 203, 1990. Robb ins, D.J. a nd Dean, P ]., Adv. Phys., 27, 499, 1978. Yama mo to, H . and Kano, T., J. Electrochem. Soc., 126, 305, 1979. Garlick, G .F.J., Cathode- and Radioluminescence in Luminescence of Inorganic Solids, Go ldberg, P., Ed., Academic Pre ss, New York, 1966, 385-417. Alig, R.C and Bloom, S., J. Electrochem. Soc., 124, 1136, 1977.

chapter one - section ten

Fundamentals of luminescence Shosaku Tanaka, Hiroshi Kobayashi, Hiroshi Sasakura, and Noboru Miura

Contents 1.10 Ino rgan ic elec trolumi nesc ence 1.10.1 Introdu ction 1.10.2 Injection EL 1.10.3 High-field EL 1.10.3.1 Injection of carriers 1.10.3.2 Electron energy distribution in high elec tr ic field 1.10.3.3 Excitation mechanism of luminescen ce centers Referenc es

111 111 112 113 114 118 l22 127

1.10 Inorganic electroluminescence 1.10.1

Introduction

Electro lum inescence (EL) is the generation of light by the application of an electric field to crys talline mat erials, or resulting from a current flow through semiconductors. Th e EL of ino rg an ic materi als is classified into the two groups: injection EL and high electric field EL. The high- field EL is further divided into two types: powder phosphor EL and th infilm EL. The classification of EL with regard to typical device applications is su m mar ize d as follows: EL

-r Injection EL L

Hi gh-fi eld EL

• Light-emitting diodes (LED), Laser di od es (LO)

-r Powder phosphor EL L

Thin-film EL

EL illumination p an els • EL d ispla y panels

Hi storically, the EL phen om enon was first observed by Destriau'-' in 1936, who observ ed luminescen ce produced from Zn S powder phosphors sus pend ed in cas tor oil wh en a strong electric field was a pp lied . Thi s type of EL is, today, classified as pow de r phosp hor EL. Lat er on, in the early 1960s, pol ycr ystalline ZnS thin film s were prep ared and use d as EL materials. Th is typ e of EL is typ ical of thin-film EL.

111


112

On the other hand, in 1952 Haynes and Briggs" reported infrared EL from forwardbiased p-n junctions in Ge and Si diodes. This type of EL is classified as injection EL. Visible EL is observed in diodes made of wide bandgap semiconductors, such as GaP. These diodes are called light-emitting diodes (LEOs) and have been widely used since the late 1960s. Semiconductor lasers, first demonstrated in 1962 using GaAs diodes, operate by stimulating injection EL light in an appropriate optical cavity. As will be described below, the mechanisms of light generation in injection EL and high-field EL are quite different from each other. In addition, the applications of these EL phenomena to electronic devices are different. Usually, the term EL is used, in a narrow sense, to mean high-field EL. In this section, therefore, the description will focus on the basic processes of the high-field EL, in particular on the excitation mechanisms in thin-film EL. The mechanisms of injection EL are described only briefly.

1.10.2 Injection EL The term "injection EL" is used to explain the phenomenon of luminescence produced by the injection of minority carriers. Energy band diagrams for p-n junction at thermal equilibrium and under forward biased conditions (p-type side.positive) are shown in Figures 75(a) and (b), respectively. At thermal equilibrium, a depletion layer is formed and a diffusion potential Vd across the junction is produced. When the p-n junction is forward-biased, the diffusion potential Vd decreases to (Vd - V), and electrons are injected from the n-region into the p-region while holes are injected from the p-region into the nregion; that is, minority carrier injection takes place. Subsequently, the minority carriers diffuse and recombine with majority carriers directly or through trapping at various kinds of recombination centers, producing injection EL. The total diffusion current on p-n junction is given by:

(165)

where Op and On are diffusion coefficients for holes and electrons, Pno and o"n are the concentrations of holes and electrons as minority carriers at thermal equilibrium, and L p and L, are diffusion lengths given by where '"C is the lifetime of the minority carriers. The LEOs that became commercially available in the late 1960s were the greenemitting GaPN and the red-emitting GaP:Zn,O diodes. GaP is a semiconductor having an indirect bandgap; the Nand (Zn,O) centers in GaP are isoelectronic traps that provide efficient recombination routes for electrons and holes to produce luminescence in this material (See 2.8.2). Very bright LEOs used for outdoor displays were developed using III-V coxipound alloys in the late 1980s to early 1990s; these alloys all have a direct bandgap. Green-, yellow-, orange-, and red-emitting LEOs with high brightness are fabricated using InGaAlP, GaAsP, or GaAlAs (See 2.8.3). In 1993 to 1994, GaJnN (another alloy with a direct bandgap) was developed, leading to very bright blue and green LEOs (See 2.8.5). Thus, LEOs covering the entire visible range with high brightness are now commercially available.

,ro:t ,


113

n-rype

p-type

Ec ----:O·········.·.···l·····

-qV d

••

Ef
Eo). When the electric field is extremely high, and the average electron energy, gEA, is larger than the optical phonon energy Eo, Eg. 171 can be rearranged into the following form:

f( e) ex: exp - -3£E -() ] [

(qEAl

(173)

121

Chapter one: Fundamentals of lumin escence

This function agrees with Wolff's distribution function derived using th e diffusion appro ximation "; Eq. 173 gives a threshold energy for the impa ct ionization th at is higher th an that for the lucky electron m odel, as shown in Figure 82. Recently, Bring u ier v'? investigated electron tran sport in ZnS-type, thin-film EL. Two basic tr ansport modes in th e lu cky-drift theory are considered. Firs t, the ballistic regime, which is defined in terms of the optical-phonon m ean fre e path A and th e electron-phonon colli sion rate 1h m • This regime implies a co llis ion -free (balli stic) mode. Second is the drift regime, which is characterized by th e length Ae and the rate 1h c of th e energy rela xati on. Thi s m ode predominates aft er the electro n has suffered one co llision sin ce, on ce it has collided , it is d efle cted and the probability of other colli sions is g rea tly increa sed. In the ballistic mod e, an electron tr avels with a group ve locity Vg(f), so th at A = V g'l:m; while in the drift mod e, th e motion is go verned by a field-d ependent drift velocity v d(f) an d A= v d'l:e' The lucky-drift m odel may be applied to the ca se w h ere r, s- 'l:m and A" ;» A, which should hold true for wide-gap semiconductors in the hi gh-field regime. When these two ineq ua lities are fulfilled , ea ch collisi on results in an appreciable momentum loss for the electro n, with little ene rgy loss. Over th e energy relaxa tio n len gth, an electro n drifting in the field los es its m omentum and direction , but con serves much of its en ergy. The en er gy exch ange between electron s and phonons is d escribed by th e electronphonon interaction Hamiltonian, where elec trons ca n em it or abs or b o ne phonon at a time. Because a phonon is a boson , th e p robability of th e phonon occupation number ch anging from n to (n+ 1) is proportional to (n+ 1), while a ch ang e from n to (n-1) is proporti on al to n . Therefore, the ratio of the phonon em ission re(n --7 n+ 1) to th e phonon ab so rption ra(n --7 n-1) rates is given by (n +1)/n . Because r, > r a, a n elect ro n experiences a net ene rg y loss to the lattice, tending to stabilize th e electron drift. Hot e lec tro ns in hi gh electric field lose energy mostl y to optical phonons an d also to zone-edge acous tic ph onons, though somewhat less effect ivel y. A t temperature T, th e phonon occu pa tion n um be r n(O)) is g iven as nuo) = l / (exp( O)/kT)- l) . For ZnS, the optical phone ener gy (I) is 44 m eV Thus, one obtains an occu p a tion number, n(O)) = 0.223 a t 300K. The analytica l expression fo r the saturated drift vel ocity V s in th e lucky-d rift th eory is given by : I/2

nO)

V

s

.

= ((2n +1)m' J

(174)

whi ch yields 1.38 x 107 em S-1 a t 300K for electrons in th e energy minimum r p oint a t k = (000) of the cond uc tio n band. In order to assess th e elect ro n- phonon coupling, the electron-phonon sca tterin g rate 1h (= r,_ + ra, re/ r. = (n + l) /n) need s to be d etermined. From th ese rates , th e a verage en er gy loss per un it time of an elec tro n ca n be deri ved; in the s teady sta te, thi s loss offsets the energy gained by drifting in the field, yielding: nO)(r - r ) = c

{/

flO)

(2n + 1)'I:

= qEv

.;

= 10 13 eV

S- I

(175)

By su bstituting n = 0.223 an d 0) = 44 meV into Eq . 175, one obtains 1h = 3.2 x 10 14 s-', or an elec tron m ean free tim e of 'I: = 3 fs. The co mpetition between heating by the fiel d and cooling by a lattice sca ttering deter mines not only the av erage energ y f a\' but a lso th e nonequilib rium ene rgy d istribution function. The en ergy b al ance cond ition is obtained by setting the following eq uation to zer o.


122

3 ,,-.,

>

(l)

'--'

;>

ro W

:>-.

2

•

on ~

(l)

I::

r..Ll (l)

on ro I-t

ES

s~

\

"'::!

4

6

2 X 10 V/cm

1.5

X

;::

4

(ZnS)

;:: !:>. ;:::,

/' 4E(4D)

'" :::. ;:::,

...e:

6

10 V/cm

2+

Mn

6

1 X 10 V/cm

3

3

3

»

00

_ 4T2 (40) -

(4A 1.4E) (40)

\

4T2 (40) 4T 1 (40)

1-0 ~

... ~

1j)

~ ~ ~

S·

'" Vl

C

UJ

g

2

o

«

I

o

!

100

I

I

200

«

I

300

[

I

400

Electron Energy Distribution

(a)

)Ir

2

2

1

1

0,

o

I

,

20

40

I

60

I

80

I

)t

o

"'";:: "'"

6A1(6S)

100

Excitation Cross Section

(b)

(c)

(a) Electron energy distribu tion f(£), (b) Mn 2+ impact excitation cross-secti on a(£) as a fun ction of ene rgy, an d (e) energy levels of Mn 2 +. (From Bhattacharyya, K., Coodnic k, S.M., and Wager, J.E, f. App l. Phys., 73, 3390, 1993. With permission.)

Figure 84

>-l

N

W


124

cen ters ; these ions are poten tial cand id ates for color EL. The excitation processes of these luminescent cen ters are described in this sec tion."

Electron-hole pair generation by hot electron impact ionization. In the ZnS host lattice, a high electric field of 2 x 106 V cm' is eno ug h to produce hot electrons. Consequently, th ese h ot electro ns ion ize the ZnS latti ce by collis ion, an d b y creating elec tron-hole pairs. This process is call ed impact ionizat ion of th e lattice. If imp ur ities, d on or and / or accep tor exist, they w ill als o be io nized . The electron-hole p airs are recaptured by the se ionized do nors and accep tors, an d luminescence is prod uced as a result of th e reco mbina tion of elec trons and holes. These p ro cesses are illustra ted in Figure 85(a). Th e ioniza tion rat e Pion of the la ttice is calc ulat ed us ing the follo wing eq uation:

~Vt1 l~( e)f( £ )d£ DC

(177)

g

where 0( £) is the ioni zat ion cross-section of the lat tice, 109 is the bandgap energy, and f(£) is the electron ene rgy distrib u tio n func tion. 0 (£) is p roportiona l to the produc t of the density of sta tes of th e va lence and cond uctio n bands. In cat hode-ray tubes, luminescence due to donor-accep tor pa ir reco m bination is very efficien t, an d ZnS:Ag,Cl and ZnS:Cu,Al(Cl) phosp hor s are widely and commonly used as blue and gr een phosp ho rs, resp ective ly. Zn S:Cu,Al(Cl ) p hosphors are also use d for powd er-typ e EL. However, these p hosphors are not efficien t w he n used in th in-film EL devices. Thi s is unders tood in terms of the reionization of the captured electrons and holes by the applied electric field p rior to their recombination .

Direct impact excitation of luminescent centers by hot electrons. If hot electrons in the host latti ce co llide di rec tly wi th localized lumin escent cen ters, the gro und -state elect rons of the cen ters are excited to higher levels, so th at lu m inescence is p rod uced, as illu strated in Fig ure 85(b). EL of ZnS:Mn 2+ is d ue to the impact excitation of the 3ds intra-sh ell configuration o f Mn 2+ cen ters . Sim ilarly, EL of trivalent rare-earth (RE)-do ped Zn S is ba sed on the im pact excita tion of the 4fn intra-she ll configura tio ns. This excitation mec hanis m is tho ugh t to be do mi nant in thirr-fil m EL d evi ce operation. Ass uming direct impact excitation, the excitation ra te P of cen ters can be exp ressed by:

(178)

w here 0(£,y) is the im pact excita tion cross-sec tion to the exci ted sta te y of the cen ters, f(£) is the energy d ist rib uti on of ho t elec tro ns discussed a bove, and Eo is the th reshold ene rgy for the excita tion . Alth oug h calculations of imp ac t exci ta tion and ion ization cross-sectio ns in free atoms or ions are very sop histicated and accu ra te, they are s till crude in solids. Allen 13 has pointed ou t tha t the proble ms lie in the form of the wavefu nc tions of the lu m in escen t cen ters to be used , especially w he n cova len t bonding wi th the host crystal is includ ed . The re is also a prob lem of die lectric screening. This screening sh ould be p roperly tak en as dependent on the energy and wave vector of carriers, or be taken appro ximatel y as a functio n of d ist ance r using the screened Coulomb po ten tia l exp resse d by are both used in d ispl ay and lighting ph osphors.' Th e 4£8 lin e emission of Tb3. is of ten res pons ible for th e green com po ne n t in tricolo r tube lightin g.' D y 3+ p lays an im po rtant rol e in the persistent luminescence phosphor SrAl z04:Eu z+;D y 3+,7.8 Er3' and Tm 3• a re, like Pr 3. , investigated for possible p hoton cas cade em ission p ho sp hor applica tio ns. 129

130


This bri ef and st ill incomplete summary illustrates the diver sity of applications invol ving the luminescen ce of lanthanide ion s. It also illustrates that w e can distinguish two types of lanthanide luminescent transition s. (1) Transitions between levels of the 4fn configuration . In thi s chap ter, the energy of ea ch 4f'I excited st at e rel ative to the lowest 4f" state will be reg arded as invariant w ith the type of compound . One may then use the Dieke diagram with the extension prov id ed by Wegh et aJ.9 to id entify the many po ssibl e luminescence em ission and optical ab sorption lines. (2) Transitions between the 4f"-1 Sd and the 4f" configurations. The ener gy of Sd levels, contrary to the 4f levels, depends very strongly on the type of compound. For example, the wavelen gth of the Sd--4f emission of Ce 3+ may range from the ultraviolet region in fluorides like that of KMgF 3 to the red region in sulfides lik e th at of Lu 253 . lo In all phosphor applications the color of emission and the quantum efficiency of the luminescence process are of crucial importance as is th e th ermal s tability of the emiss ion in some ap p lica tions . These three asp ects are related to the relative and absolute location of the lanthan ide energy lev els. For example, the position of the ho st-sensitive lowest Sd state relative to the host-inv ariant 4f s tates is important for the quenching beha vior of both Sd--4f an d 4f-4f emissions by multiphonon rel axat ion . The absolute position of the 4f and Sd stat es relative to val enc e band and conduction band states also affects lum inescence qu enching and charge-trappin g phenomena. Although it was realized lon g ago that absolute location is crucial for phosphor performance, the experimental and theoretical understanding of the placement of en erg y levels relativ e to the intrinsic bands of the host ha s been lacking. In th is sec tion, first, a su rvey is provided on ho w rela tiv e and absolute locati ons of lanthanide energy levels a ffect phosphor performance. Next, methods and m odels to determin e relative and ab solute locations are treat ed . After d iscu ssing the en ergy level s of th e free (or gaseous) lanthanide ions, the influen ce of the host compound on the location of th e Sd levels relative to the 4f levels is presented . Next, the influence of the host compound on the absolute location of the lowest 4fn s ta te above the top of the valence band is expl ained. This forms the basis for drawing sche m es for the absolute placement of both the 4f and Sd states of all the divalent and trivalent lanthanide ions.

1.11 .2 Level position and phosphor performance The importance of the relative and absolute po sition s of the energy levels of lanthanide ions is illustrated in Figure 86. We distinguish occupied states that can donate electrons and empty sta tes that can accept electrons. Let us start with the "occu pied states." Figure 86(a) illustrates the downward sh ift of the lowest-energy Sd level when a lanthan ide is brought from the gaseous state (free ion) into the crystalline environment of a com p ound (A). Due to the interaction with th e neighboring anion ligands (the crystal field interaction), the deg en erate Sd levels of th e free ion sp lit (cry stal field splitting), d epending on the site sym me try. In ad d ition, the whole Sd configuration shifts (centroid shift) toward lower ene rgy. The crystal field sp litting comb ined w ith the centroid shift lowers the lowest Sd lev el with an amount kn own as the redshift or d epression D. Clearly the value of 0 det ermines the color of e m issi on and wavelength of ab sorption of the 4f-Sd transitions. Figure 86(b) illustrates the im p or tan ce of lowest-energy Sd level location relati ve to 4£2 lev els in Pr3+ . With th e Sd level above the 150 level of Pr 3+, multiphon on relaxation from th e lowest Sd state to the lower lying 150 lev el takes place. A cascade emission of two photons may result, which lead s to quantum efficienc y larger than 100%. However, with the lowest Sd state bel ow ISO, broad-band Sd--4f emission is observed. Much research is devoted toward th e sear ch for Pr 3+ quantum-splitting phosphors and for finding efficient Sd--4f-emitting Pr 3+-doped mat erials for scin tilla tor applications. Dependin g on the precise

Chapter one:

~)

n 4f - - - - -


---........-

131

l~

CB

········T~f ·· ··········

lE~

l:lum.

........ L . VB

Ie)

(h)

(f)

¥:r' --l-. 3

2

5m + +8 = 5m

Eu2+ = Eu3 ++8

(i)

(k) VB

G!? +

.

~OE,;' Ce3+

(I) VB

Figure 86 Illustration of influe nce of level location on phosp hor properties: (a) the redshift 0 of the Sd state, (b) photon cascade emission in PrJ+, (c) Sd-4 f em ission quench ing by au toioniza tion, (d) anomalous Sd em ission, (e) thermal quenching by ionization, (f) qu enching b y int erval en ce charge transfer, (g) valence band cha rge tran sfer, (h) charge transfer lum inescenc e, (i) electron trap ping by Sm'", (j) ho le trapping by Ce 1+, (k) electro n tran sfer from Eu 2 + to Srn" , (l) lu min escence quenchi ng by lan thanide to lanthanide charge transfer.

locati on of the lowest Sd sta te in Nd 3+, Eu 2+, an d Sm 2+, either broad-band Sd-4f or narrowline 4f-4f emissions ca n be observ ed ." Figure 86(c), (d ), an d (e) sho w the in terp lay between th e localized Sd elect ron and the delocal ized con d uc tion band sta tes . If th e lowest Sd st at e is above the bottom of the cond uc tio n band as in Figure 86(c), a u toionization occu rs spon taneous ly an d no Sd-4f emission is observe d . This is th e case for LaAI0 3:Ce 3+, rare-earth sesquioxides Ln z0 3:Ce3 +, 1 and also for Eu 2+on trivalent rare-ear th sites in oxid e comp ounds. F Figure 86(d ) illustrates the situa tion wi th Sd ju st below th e con d u ction ban d . The Sd electron d elocalizes but remains in the vicini ty of the hol e left behind . Th e true nature of the s ta te, w hich is som etim es called an impurity trapped exciton sta te, is n ot precisely kn own . The recombinati on of th e electron w ith th e h ole lead s to the so-ca lled an omalous em ission cha racterized by a very large Stokes shift. 13•14 Finally, Fig ure 86(e) sh ows th e situa tion with th e Sd st ate we ll below the conduction band, leading to Sd-4f emission. Th e thermal q uenc hing of th is emission by means of ion izatio n to conduction b and states is con tro lled by th e energy EdC betw een the Sd s tate (d) and th e bottom of the con d u ction b and (C) .13.1 6 A revi ew on th e relationship between EdC for Eu 2 + and th ermal qu enching of its 5d-4f emissi on rec ently appeared ." Knowled ge on such relation ships is im p or tan t for de velopin g temperaturestable Eu -r-d oped light-emitting diod e (LED) phosphors or temperatu re-stable Ce 3+ -doped scin tilla tors. For elec tro lu minescence ap p lica tions, Ed C is an import ant paramete r to di scrim in ate the me chanism of im pac t ion ization agains t th e m echanism of field ionizati on ." Figure 86(f) sh ow s a typical situation for Pr 3 + in a transition m et al complex com p o und like CaTi 0 3 • Th e undesired blue emission from the Pr 3+ 3PO le vel is quenched b y

132


intervalence charge transfer (IVCT).18 The electron transfers from the 3Po level to the transition metal (Ti4 +). Th e electron is transferred back to the red emi tting Pr J+ 104 level. The position of the 3PO level relative to the transition metal-derived condu ction band controls the quenching process, and th ereby th e color of emission . So far we have di scu ssed examp les of ab solute location of "occupied states." However, a tri valent lanthanide ion may accep t an electro n to form a divalent lanthanide ion. The location of the occupied gro und -state level of a di valent lanthanide ion is therefore the sa me as the unoccupied electron-accepting st at e of the corresponding triv alent lanthanide ion . The accepted electron m ay originate from the va lence band, the con d uc tion band, or ano ther lanthanide ion. Figure 86(g) pertains to a Eu 3 +-doped compound . Eu 3+ introduces an unoccupied Eu ?" state in th e forbidden ga p . The excita tion of an elec tron from the valence band to the unoccupied s tate creates th e groun d state of Eu?". Th is is a dipoleallowed transition that is used, for exampl e, to se nsi tize Yz03:Eu 3+ phosphors to the 254 nrn Hg em ission in tube lighting.' Recombination of the electron with th e valence band hol e leaves the Eu 3+ion in th e 50 0 excited state res ulting in red 4f6-4f6 emi ssion. Figure 86(h) shows a similar situation for Yb3+. In the case of Yb3+ the recombination with the hole in the valence band produces a strong Stokes-shifted charge transfer (CT) luminescence. This type of luminescence gained cons ide rable interest for d eveloping scintilla tors for neutrino detection ." Clearly, the absolute location of the d ivalent lanthanide ground sta te is important for CT exc itation and CT luminescence en ergies. Figure 86(i) shows the trapping of an electron from the conduction band by Sm 3+ to form the gro un d sta te of Sm 2 +. Th e absolute location of an "unoccupied" div alent lanthanid e ground state determines the electron trapp in g depth provided by.the cor resp onding tri valent lanthanide ion . On the other hand, the abs olu te location of an "occup ied " lanthanide gro u nd state determines the valence b and hole trapping depth provided by that lanthanide ion. Figure 86(j) illus trates trapping of a hole from the va lence band by Ce 3+. Th is hole trapping is an important aspect of th e scin tillation mechanism in Ce 3+doped scin tilla tor s. Similarly, Eu-" is an efficient hole trap of importan ce for the X-ray s torage phosphor Baf'Br .Eu >. Phosphor properties become more complicated when we deal with "do uble lanthanid e-d oped systems." Figu re 86(k) shows the situation in Eu?" and Sm 3 +double-doped compounds lik e SrS and MgS that were studied for op tical data storage applicat ion s.v" The ultraviolet write pulse excites an electron from Eu z+ to the conduction band, which is then trapped by Sm 3+. Eu 3+ and Sm z+ are created in the process. An infrared read pulse liberates th e electron again from Srn?". resulting, eventuall y, in Eu?" Sd--4f emission. Similar mechanism s appl y for YzSiOs:Ce3+ ;Sm 3+ and LiYSi04 :Ce 3+;Sm 3+ compounds that were developed for X-ray and th ermal neutron storage phosphor applications, respectively.v' The true m ech anism in the p ersistent luminescence phosphor SrAl z04:Eu 2+;Oy3+ is still disputed. One ne eds to know th e absolu te level en ergy locations to arrive at plau sible mechanisms or to d iscard implausible ones." As a last example, Figure 86(1) sh ow s quenching of emi ssion in Ce 3+ and Eu 3+ co-d oped systems. Th e Ce 3+ electron excited to the lowest 5d state can jump to Eu 3 + when the unoccupied Eu?:' gro und state is locat ed at a lower ene rgy than the occupied lowest Ce3+ Sd excit ed s ta te. After the jump, Eu 2+ and Ce 4 + are formed . The Eu" electron can jump back to Ce 4 + if the unoccupied Ce-'+ ground state is locat ed below the occupied Eu?" ground state. The origi nal s ituation is rest ored without emi ssion of a photon. Similar quenchi ng routes p ertain to Ce 3+in Yb-based com poun d s, and with ap p rop riate level schemes, other "killin g" comb inations can be found as well. The a bove se t of examples shows the importance of ene rgy level locati ons for the performance of phosphors. Thi s im po rtan ce was realized lon g ago, but not until recentl y methods and m od els became av ailabl e that allow the determination of these abso lu te


133

p ositions. In the follow ing sec tions , the h istoric developm ents and current sta tus of absolute level pos ition ing are briefly review ed . Fo r d etail ed in form ation, origin al literature sho u ld be consulted .

1.11 .3 The free (gaseous) lanthanide ions The previous section illu st rat ed the import an ce of lanthan id e level locati ons for p hosph or per forman ce. To un derstan d an d p red ict th ese location s we firs t need to und er stand the properties of the free (gaseous) lanthani de ions. Fig ure 87 shows the d at a avai lab le on the energy (Efd ) needed to exci te an ele ctron from the low est le vel of the 4fnS d ll6s lll config uratio n to the low est level of the 4f"- I Sd 16s lll configura tion in the ga seous free lanthan id e ion s or atoms . The data are fro m Brewer" and M artin" togeth er wi th lat er up d at es." Data are most complete for th e n eutral atoms (m = 2, curve c), the mo n ovalen t lanthan id es (m = I , curve b), an d the div alent lanthani d es (m = 0, curve a). A universal curve, cur ve a in Figure 87, can be cons tr uc ted . By shifting the en er gy of thi s universal curve, the 4f-Sd energies as a fun ction of n can be reprod uce d irrespective of th e charge of th e lanthanide ion (0, +1, +2, or +3) or the number, m, of electrons in 6s (m = 0, I , or 2). This re marka ble phenome n on is d u e to the inner-shell nature of the 4f or bita l. Ap paren tly, the occu p ati on number of electrons in the 6s shell h as no influence on the universal be havior. The main features of th is un iversal va riation have been known for a long tim e and understood in terms of [orgensens spin pairin g th eory for th e bind in g of 4f ele ctron s.F The energy is large w he n the 4f configura tion is half - (11 = 7) or completely (11 = 14) fille d, and the energy is small when it is occu pie d by on e or eig h t electro ns . Figure 88 shows the binding ene rgy (or ioniza tion energy) of the 4f and 5d electrons in the free divalent an d free b-ivalent lanthanide ions wi th m = O. Whe n we ad d the corresponding energies, Efd , from Figure 87 to curv es b and d in Figure 88, w e ob tain the bind in g energies for the 5d electro n (see curves a and c). Th e stronger bindin g of the 4f and 5d electrons in the trivalent lanthani d es than in the d ivalent ones is due to a stron ger Coulomb attrac tion. Clearly, the binding of the 4f electron is resp onsible for th e un iversal behavior in the 4f-5d transitions. The bindin g energy of the Sd electron is ra the r cons tant wi th 11 w hic h indica tes that the na ture of the 5d sta te is relative ly invarian t wi th the typ e of lanthanide ion .

-

14

Pr"+

Lu3•

12

Y~

10

:>

8

p,': .' Ge'~'

6

Ce2' ....

(e)

2+

-

3 Eu ,

Yb"

~ :o "',~/:\ " ot(~ L " ~: ::: ::-' sm' \V~::~" :'''(~;\''"' L:' -:< " Q

-2

La

-4 -6

Ba'

-.,( " Ce'

'

(b)

""

(d)

L~ "" " " " " "

Eu'

· "· ....

'=--~-L_'-------'----L_'-------'----L_.l...-_'---'-_~--'----'---'

o

2

3

4

5

6

7

8

9

10

11

12

13

14

15

n Figure 87 Experimen tally obse rved energies Efd for the transition between the lowest 4f"5d o6s '" and the lowest 4f,,-15d J 6s'" s ta tes of free (gaseo us) lanth an id e ion s and a toms. A shift of the d ash ed curve (a) by -0.71 eV, - 1.09 eV, -5.42 eV, and +7.00 eV gives curves (b), (c), (d), and (e), respec tivel y.


134

(a) 5d- Ln 2 +

-20

5'

~

....

-25

,/

(b) 4f- Ln 2 +

>-

~

~

- 30

Q)

Ol C

'g

- 35

i:i5 -40

- 45

(d) 4f- Ln 3+

----..-...-.----.. LL_L.---'------'-_L.---'------'-_L.---'------'-_L.---'----'--'LJ 2

3

4

5

6

7

8

9

10

11

12

13

14

n Figure 88 The binding energy in e V of the Sd (curves a and c) and 4f electron (curves b and d) in the free di valent (cu rv es a and b) and free trivalent lant h a nide ions (curves c and d) .

1.11 .4 4f-Sd energy differences of lanthanide ions in compounds Figure 87 indica tes that the variation of Efd with n d oes not depen d on the cha rge of the lanthanide ion or on the number of electrons in the 6s orbital. It is also we ll establish ed that the Dieke diagram of 4f ene rgy levels is almost invarian t wi th the type of com po und. The situa tio n is com pletely differen t for the 5d sta tes . Th eir energies are in flue nce d 50 tim es stro ng er b y the ho st com pou nd than those of 4f s tates . Du e to crys tal field sp litt ing o f the 5d s ta tes and a shi ft (cen troi d shift) of th e average energy of the 5d con figuration, the lowest level of the 5d configur at ion decreases in energy as illustrat ed in Figure 89 for ee3+ in LiLu F4 (see also Figure 86(a)). The decrease is kn own as the re ds hift or depression O(n,Q,A ) = O(Q,A ) where n, Q, and A stand for the number of electron s in the 4f" ground sta te, the charge of the lanthanid e ion , and the name of the compo un d, respectivel y. The red shift depend s very strongly on A but ap pears, to good firs t approxim ation , independent of 11, i.e ., the typ e of lanthanide ion . Th is impl ies th at both th e crystal iield splitting and the cen troi d shift of th e 5d levels depen d on the typ e of com p ound but to a good first ap p ro xim ation are th e sam e for each lanthanide ion . Figure 90 shows this principle. It is an inverted Dieke diagram w here the zero of ene rgy is at th e lowes t Sd state of the free trivalent lanthanide ion . When the lanthanide ions are present in a comp ound, one simp ly need s to shif t the Sd leve ls down by the reds hi ft O(3+,A ) to find the ap propria te d iagr am for that com po und . Fig ure 90 illustrates th is for LiLuF 4 • Th e 4f-Sd tr an s ition ene rgy of each lan th anide ion can be read from the diagram . In eq uation form thi s is wri tten as:

Efd (n,3+,A)= Efd (n ,3+, fre e) - 0(3+, A)

(180)

w here Efd (n,3 +,free) is th e energy for th e first 4f"-4f"-1 Sd transitio n in the trivalent (3+) free lanthani d e ion." In addition to 4f-S d energies in LiLuF 4, the di agram also predicts that the lowest 5d sta te of Pr 3+ is below the 150 state, an d broad-band Sd-4f em ission and

Chapter one:


135

Crystal field splittin g

8 Free Ce

7 2

6

>' ~ c-, Ol

Q;

3

Centroid shift

+

Stoke s' shift

D

5 4

C

w

3

2 2 2

0

F 712 FSI2

The effect of the crystal field in teraction on the (d egen erate) free Ce 3+ ene rgy states in LiLuF4 • The comb ination of centroid shi ft and crystal field splitting d ecreases the lowest Sd sta te with a total ene rgy D. On the far right the Stokes shifted Sd--4f emission transitions are shown.

Figure 89

not narrow-band 150 line emission will be observed (see Figure 86(b)). The lowest-en ergy Nd 3+ Sd s tate in LiLuF4 is pred icted to be s table en ough ag ains t multiphonon relaxati on to the 2G 7/2 level. Indeed Nd 3+ Sd-4f emission has been observ ed . Red sh ift values are kno wn for man y hundreds of d ifferent com p ounds.P Figure 91 summarizes the redshift values O(3+,A) for the tri val ent lanthanide ion s. '? It is by definition zero for the free ions, and for the halides it increases from F to 1 in the seq uence F, Cl. Br, 1. For the chalcogenides, an increase in the sequ enc e 0, 5, 5e, and presumabl y Te is obse rved . This is directly connec ted with the properties of the anions th at affect the centroid shift. The origin of th e cen troid shift is ver y complicated and related with covalency and polarizability of the anion s in the compound.s' ' One may also interpret the absence or presence of vibronic structures in 5d excitation bands as indicative of 5d states contained within the conduction band." One- or two-step photoconductivity provides information on the location of 4f ground states relative to the bottom of the conduction band. 37--40 Another related technique is the microwave conductivity method developed by Joubert and coworkers that was applied to LU2SiOs:Ce3+41

1.11.6

Systematic variation in absolute level locations

The previous section provides an explanation on the techniques that have been used to obtain information on level positions. But often these techniques were applied to a specific lanthanide ion in a specific compound with the aim of understanding properties of that combination. Furthermore, each of these techniques provides its own source of unknown systema tic errors. These individual studies do not provide us with a broad overview on how level energies change with the type of lanthanide ion and the type of compound. Such an overview is needed to predict phosphor properties and to guide the researcher in the quest for new and better materials.

138


One of the first systematic approaches wa s by Ped rini et al. who undertook photoconductivity measurements to determine the location of the 4f ground state of divalent lanthanides in the fluorite compounds CaF2, SrF 2, and BaF 2 relative to the bottom of the conduction band.'? They also provide a model to exp lai n the observed variation in 4f gro und-state energy with n. Th e first sys tem atic approach to determine the levels of trivalent lanthanides was undertaken by Thiel and coworkers using XPS.42A1 They stu d ied the trivalent lanthanides in YJAl S0 12 an d determined the 4f ground -st ate energies relative to the valence band of the host cryst al. They also combined th eir find ings w ith the systematic in 4f-5d energy difference found in Ref. 23 to locate th e 5d s tates in the band gap. The absolute energy of the lowest 5d s tate ap pears relatively con stant with the typ e of lanthanide ion. Both XPS and photoconductivity experiments have drawbacks. The oscillator strength for th e transition of the localized 4f ground state to th e delocalized conduction band states is very sma ll and photoconductivity is rarely observed due to such direct transitions. Twostep phot oconductivity is observed more frequently. After a dipole-allowed excitation to the 5d state, it is either followed by autoionization (see Figure 86(c)) or thermally assisted ionization (see Figure 86(e)). For the XPS experiments, h igh Ln-t -concentrated samples are need ed ,42.44 and one has to deal with uncertain final state effects to obtain reliabl e d ata ." At thi s m om ent the amount of information obtained with these two methods is scarce. Although th ey provide us with very valuable id eas and insight on how level energies cha nge with the type of lanthanide ion, there is not eno ug h information to obtain detailed insigh ts int o th e effect of type of com pound . Another m ethod to obtain the systematic va ria tion in level positi on wi th the type of lanthanid e is CT spe ctroscopy. It appears that the energy of CT to Sm 3+ is always (at least in oxide compounds) a fixed amount higher than that for the CT to Eu 3+. The same applies for Tm 3+ and Yb3+. Thi s wa s noticed long ag022A6A7 and recon firm ed by more recent stud ies.4!>-5o An elaborate analysis of data on CT retrieved from th e literature revealed that the sys tem a tic behavior in CT energies holds for all lanthan ides in all typ es of different compounds.>' Figure 92 illustra tes the m ethod to construct diagrams with absolute level location of the di valent lanth anide in CaGa 2S4. The top of the valence band is defined as zero of energy. Th e arro ws numbered 1 through 6 show the observed ene rgies for CT to trivalent lanthanide ion s, and they prov id e us with the location of th e ground state of the corre sponding di valent lanthanides (see Figure 86(g)). Using th ese data we can cons truc t precisely the sa me universal curve, but in an inverted form, as found for the en ergy Efd of 4f-5d transition s in th e free lanthanide ions and atoms of Fig ure 87. Ar row 7 shows the energy of the first 4f-5d tr ansition in Eu 2+. Using Eq. 181, the abso lute locati on of the lowest 5d state for each divalent lanthanide ion can be d rawn in the scheme . It appears constant with n, The universal beh avior in th e energy of the lowest 4f state w ith 11 is determined by the binding of 4f electrons, similar to that depicted in Figure 88, but mod ified by the Madelung potential at the lanth anide si te in the comp ound . Th is Madelung potential incr eases w ith sma ller size of the lanthanide ion due to th e inw ard relaxation of the nei ghboring ne gati vely charge d an ions .14,3.1.39AJ The increase in 5d electron binding ene rgy by 1-2 eV, as observed fo r the free d iv alent lan thanides in Figure 88, is absen t in CaGa 2S4 where the binding of th e 5d electron is found independent of II. Thi s fortuitous situation for CaGa 2S4 , which is also expected for other sulfide compounds, does not apply to oxides and fluorid es. For th ese compoun ds it wa s found that from Eu 2+ to Yb2+ the binding of the levels grad ua lly decrease by abou t 0.5 eV.J.l.34 In other words, the 5d state of Yb2+ is found 0.5 eV closer to th e bottom of the conduction band than that of Eu 2+, w hich is


139

8

6

5

:

:

:

:

:

:

o

Ev

-1

-2

2

3

4

5

6

7

8

9

10 11 12 13 14

n +1

Figure 92 The location of the lowest 4f and lowest Sd states of the divalent lanthanide ion s in CaGa 2S4 , Arrows 1 through 6 show obs erved energ ies of charge tran sfer to Ln3 + . Arr ow 7 sh ow s the observed energy for the first 4f-Sd tran sition in Eu 2 + ,

consistent wi th the observ ation that Yb 2+ in oxides and fluorides is more suscep tible to anomalous emi ssion than Eu 2 + in these compounds." The unive rsal behavior in both 4f-5d energy d ifferences and CT energies form s the basis for a cons truction method of the d iagrams as seen in Figure 92. Only three ho stdep endent p aram eters, i.e., E CT (6,3+,A), D(2+,A), and the energy E y C (A) between the top of the valence band (V) and the bottom of the conduction band, are ne eded . These parameters are ava ilable for many different compounds.e! Figure 93 shows the energy ECf (6,3+,A ) of CT to Eu 3 + (with n = 6) in com pound (A), and Figure 94 shows the energy of the first excitonic absorption maximum. The mobility band gap , i.e. th e energy of the bottom of the conduction band at Ey C' is assumed to be 8% higher in energy.?' 9

8

7

:;:-

6

:?

5

.e-

+

M.

~ I-

o

iu

.~\

.,'" "0 ' ;(

:

"

.,

"' :g 0

:>

LL

.,

'" -e

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-o

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,&:

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. .

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1 0 -1 -2 -3 -4 0

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5

6

7

8

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10

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12

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n Figu re 95

The locat ion of the low est 4f (curves b a nd d) an d low est 5d s ta tes (curves a an d c) of the divalen t (curves a and b) and trivalent (cu rves c and d) lanthan id e ion s in YP0 4 • nand n + 1 are the n umber of electrons in the 4£ shell of the trivalen t a nd diva len t lanthan id e ion, respectively. Arrows indi cate sp ecific tran sitions tha t were also discussed in Fig ure 86. The hor izontal d ash ed lin e a t 8.55 eV is E'·'.


142

deepest for Ce 3+ followed by 1b3+. (4) The groun d-state energies for the divalent lanthanides are high above the top of th e valence band. In practice thi s means that even for Eu and Yb it is not po ssible to stabilize th e divalent state during syn thesis. (5) The trivalent lanthanides crea te st abl e electron traps becau se the ground sta tes of the corresponding d ivalent lanthan id es a re well below the cond uc tion band. (6) Th e gro und states of Sm 2+, Eu 2+, Tm 2+, and Yb2+ are below the 5d sta te of the trivalent lanthanides. This means th at Sm 3 +, Eu 3 +, Tm 3+, and Yb3+ can qu en ch the 5d emission of trivalent lanthanide ions.

1.11.7 Future prospects and pretailoring phosphor properties With th e m ethods described in th is sec tion one can construct level schemes for all the lanthanid e ions w ith few parameters. Th ese parameters are ava ilable for hundred s of d ifferent compounds. At this st age, th e sche mes still contain sys tema tic errors. Often the bottom of the conduction band is not we ll defined or known or levels ma y cha nge due to charge-compensating defects and lattice relaxation wh ich m ay result in (sys tematic) errors th at a re es timated at around 0.5 eY. Such errors are still very importan t for ph osphor performan ce because a few tenths of eV shift of absolute ene rgy level position may cha nge the performan ce of a phosphor from very good to us eless. Th e level schem es are, however, already very p ow erful in predictin g 4f-5d and CT tran sition energies. We may deduce trends in th e energy difference between the lowest 5d state and the bott om of the con d uc tion band, and then u se the se trends to guide the search for find ing better temperature s table phosphors." We ma y deduce tren ds in the absolute location of the lanthanid e gro u nd sta te th at determines its suscep tibili ty to oxid ization or red uc tion." For exa mp le, oxid ation of Eu 2 + is believed to play an important role in the degr ad ation of BaMgAI100 17:Eu 2+ phosphors/54 and kn owledge on level energ ies may p rovid e us ideas to further stab ilize Eu 2 +. The level schemes are p arti cularl y useful when more than one lanthan id e ion is present in th e sa m e compound. CT reactions and pathways from one lanthan id e to the other can be read from the lev el schem es. For perman ent informa tion stor age deep cha rge traps are re quired and for persistent luminescence shallow trap s are need ed . The level schemes provide very clear id eas on wha t combination of lanthanide ions a re need ed to obtain th e desired properties . Per haps eve n more importantl y at this stage is that the level sche mes p rovid e very clear ideas on w ha t combina tion not to choose for a specific applicat ion . Th is cha p ter has surveyed where we are tod ay with our kn owled ge and ex perimental techniques on the prediction and determination of abso lute locati on of lanthan ide ion energy levels in phosphors. Currently we have a basic model, but it need s to be more accura te . Asp ects like lattice relaxa tion, charge-com pe nsating defects, intrinsic defects, the n ature of th e bottom of the cond uc tion band, d yn amic properties involved in charge localization and delocali zati on processes, and th eoretical modeling all ne ed to be considered to improve o u r k now led ge fur the r. It will be th e nex t step on the route for the tailoring of phosphor properties be foreh and.

References 1. Blasse, G., and Gr abmaier, B.C., Luminescent Materials, Spr inger-Verlag, Berlin , 1994. 2. Weber, M.J., Inorgan ic scin tilla tors: Today and tomor row, ]. Lumin., 100/ 35, 2002. 3. van der Kolk, E., et al., Vacuum ultraviolet excitation and emi ssion properties of Pr-" an d Ce 3 '. in MS04 (M = Ba, Sr, and Ca) and pred icting quan tum splitting by Pr 3 + in oxides and fluorides, Phys. Rev., B64, 195129, 2001. 4. C hakraba rti, K ., Ma th ur, Y.K., Rhodes, J.F., and Abb un di, R.J., Stim u lated luminescence in rare -ea rth-doped MgS, ]. Appl. Phys., 64, 1362, 1988.

Chapter one:


143

5. Me ijerink, A., Schipper, w.J ., and Blasse, G., Ph ot ostimulated luminescence and thermall y s tim ulated luminescence of YzSiOs-Ce,Sm, J. Phys. 0 : A ppl. Phys., 24, 997, 1991. 6. Sido ren ko, A.Y.,et al., Storage effect in LiLnSiO. :Ce3 +,Sm 3+,Ln = Y,Lu phosphor, Nu cl. lnstrum . Methods, 537, 81, 2005. 7. Matsu zaw a, T., Aoki, Y, Takeuchi, N., and Murayarn a, Y, A new lon g ph osphorescent ph osphor with high brightness, SrAl z0 4:Eu 2 +,D y3+, f. Electrochem, Soc., 143,2670, 1996. 8. Dorenb os, P , Mechanism of persistent lum inescen ce in Eu> and Dy3+ co-d ope d alu mina te and silicate compounds, f. Elecirochem. Soc., 152, H107, 2005. 9. Wegh, R.T., Meijerink, A., Larnrn inrnak i, R.-J., an d Holsa, J., Extend ing Diek e's d iagr am , J. Lumi n., 87- 89, 1002, 2000. 10. Dorenb os, P., The 5d level po sit ion s of the trivalent lanthanides in inorganic compo un ds, J. Lumin., 91, 155, 2000. 11. Dorenbos, P., f ~ d tran sition energies of divalent lanthanides in inorganic compou nds, f. Phys.: Condens. Matter, 15, 575, 2003. 12. Dorenb os, P., Ene rgy of the first 4P~4f65d tran sition in Eu-'<doped compounds, f. Lumin., 104, 239, 2003. 13. McClur e, D.s . and Pedrini. c., Excitons trapped a t im p urity cen ters in highly ionic cryst als, Phys. Reu., 832, 8465, 1985. 14. Dorenb os, P., Anoma lo us luminescence of Eu 2+ and Yb 2+ in inorganic compounds, J. Phys.: Condens. Matt er, 15 2645, 2003. 15. Lyu, L.-J. and Ha mi lton, D.s., Radiative and nonrad iati ve relaxat ion mea surements in Ce 3+ doped crys tals, J. Lumin., 48&49,251, 1991. 16. Dorenbos. P., Th erm al q uenching of Eu> 5d--4f luminescen ce in inorganic compo unds, J. Phys.: Condens. Matt er, 17, 8103, 2005. 17. Bessiere , A., et al., Spec trosco py and lanthanide impuri ty level locati on s in CaGa zS4:Ln (Ln = Ce, Pr, Tb, Er, Srn), J. Eleci rochem. Soc., 151, H 254, 2004. 18. Boutinaud, P., et al., Making red emitting phosphors wi th PrJ+, Opt. Mater., 28, 9, 2006. 19. Gu erassim ova, N ., et al.. X-ray excited charge transf er lu minescence of ytterbium-containing aluminium ga rne ts. Chern. Phys. Lett., 339, 197,2001. 20. Brewer, L., Systematics and the Properties of the Lanthanides, edi ted by S.P Sinha, D. Reidel Publishing Com pany, Dord recht, The Netherlands, 1983, 17. 21. Martin, W.c., Energy d ifferences between two spectroscop ic sys tems in neutral, singly ion ized, and doubly ionized lanthanide atoms, J. Opt. Soc. A m., 61, 1682, 1971. 22. Jorgensen, C.K., Energy tran sfer spectra of lanthanide comp lexes, Mol. Phys., 5, 271, 1962. 23. Dorenbos. P., The 4f"B 4f"-15d transitions of the triv alen t lanthan ides in halo genides and chalcogenides, J. Lumin., 91, 91, 2000. 24. Andriessen, J., Dorenb os. P , and va n Eijk, C.W.E., Ab ini tio calculati on of the contribution from anion d ipol e po lari za tion and d ynam ic correlation to 4f- 5d exci ta tions of Ce J • in ionic compounds, Phys. Rev., B72, 045129, 2005. 25. Dorenbos, P., 5d- Ievel ene rgies of Ce 3+ and the crys talline environ me n t. 1. Fluoride compounds, Phys. Reu., B62, 15640,2000. 26. Dorenbos, P., 5d- leve l energies of Ce J+ and the crys talline environmen t. IV. Aluminates and simp le oxide s, J. Lumln ., 99, 283, 2002. 27. Dor enbos. P., 5d -level energies of Ce J ' and the cry st alline environmen t. II. Chloride, bromide, and iodid e com po unds , Phys. Reo., 862, 15650, 2000. 28. Dorenbos, P., Rela tion between Eu 2+ and Ce J + f-->d tran sition energ ies in inorganic compounds, f. Phys.: Condens. Matt er, 15, 4797, 2003. 29. van Pieterson , L., et al., 4fJl ~ 4 f" - l 5 d transitions of the light lanthan ides: Experiment an d theory, Phys. Reo. , 8 6, 045113, 2002. 30. van Pieterson, L., Reid, M.F., Burd ick, G.W., and Meijerink , A., 4 fJl ~4f"-15 d tran sitions of the heavy lanthanid es: Exp erim ent an d theor y, Phys. Rev., B65, 045114, 2002. 31. Dorenbos, P , Excha nge and crysta l field effects on the 4f,,-15d levels o f Tb 3+, J. P!lys.: Condens. Matter, 15, 6249, 2003.

144


32. Wong , We., McClure, OS ., Basun , S.A, and Kokta , M.R , Charge-exchange processes in titanium-doped sapphire crys tals. I. Charge-excha nge energies and titanium-bound excitons, Phys. Rev., B51, 5682, 1995. 33. H appek, U., Choi, J., an d Srivastava, A.M., Observation of cross-ionization in Gd3SczAI30 12:Ce3+, J. Lumin., 94-95, 7, 2001. 34. Do renbos, P., Systematic behaviour in triv alent lanthan die charge tran sfer energies, J. Phys.: Condens. Matter, 15,8417,2003. 35. Sato, S., Optical absorption and X-ray ph otoemission spe ctra of lanth an um and cerium halid es, J. Phys. Soc. [pn., 41, 913, 1976. 36. Lizzo. S., Meijerink, A, and Blasse, G., Luminescence of divalen t ytterbiu m in a lkaline ear th sulpha tes, J. Lumin., 59, 185, 1994. 37. [ia, D., Meltzer, RS. , and Yen, WM., Locat ion of the gro und state of Er3 +in dop ed YzOJ from two-step p ho toco nd uc tivity, Phys. Reo., B65, 235116, 2002. 38. van de r Kolk, E., et al., 5d elec tron de localiza tion of Ce3+ and PrJ+ in YzSiOs and LU zSiOs, Phys. Rev., B7l, 165120, 2005. 39. Ped rini, e., Rogemond, E, and McClure, Os. , Pho toio nization thresholds of rare -earth impurity ion s. Eu 2+ :CaFz, Ce3+;YAG, and SmJ+:CaFz, J. App!. Phys., 59, 1196, 1986. 40. Fuller, RL. an d McC lu re, OS ., Photoionization yields in th e dou bly d op ed SrF2:E u,Sm system, Phys. Rev., B43, 27, 1991. 41. Joubert, M.E, et al., A new microw ave reso nant technique for stu dying rare earth photoionization threshold s in d ielectri c cryst als u nd er laser irradia tion , Opt. Mater., 24, 137, 2003. 42. Thi el, e. W., Systematics of 4f electron energies relative to host ban ds by reson an t photoem ission of rare-ear th ions in aluminum garnets, Phys. Rev., B64, 085107, 2001. 43. Thiel, e.W, Sun, Y, an d Cone, RL., Progress in re lating rare-earth ion 4f and 5d energ y levels to hos t bands in op tical materials for hol e burning, quan tum informa tion and phosphors, J. Mod. Opt., 49, 2399, 2002. 44. Pid ol, L., Viana, B., Ga ltayries, A, and Dorenbos, P , Energy levels of lanth anide ions in a Lu 2Siz0 7:Ln 3 +host, Phys. Reo., B72, 125110, 2005. 45. Poo le, RT. , Leckey, R.e.G., Jenkin, J.G., and Liesegang, J., Electronic structure of the alkalineearth fluorides studied by photoelectron spectroscop y, Phys. Rev., B12, 5872, 1975. 46. Barnes, J.e. an d Pinco tt, H., Electron transfer spec tra of some lanth an ide (lU) comp lexes, J. Chem. Soc. (a), 842, 1966. 47. Blasse. G. and Bril, A , Broad-ba nd UV exci tation of Sm 3+-act iva ted phosph ors, Phys. Leti., 23, 440, 1966. 48. Krupa, [.C; Op tical excitations in lantha nide and actinide com po und s, J. of Alloys and Compounds, 225, 1, 1995. 49. Na kazawa, E., The lowest 4f-to-5d and cha rge-transfer transit ions of rar e ear th ions in YPO. hosts, J. Lumin., 100, 89, 2002. 50. Kru pa, j .C; H igh- energy op tical abso rp tion in f-compoun d s, J. Solid State Chem., 178, 483, 2005. 51. Do renbos, P, Th e Eu'" cha rge tran sfer energy and the relati on with the ba nd gap of compo unds , J. Lumin., 111, 89, 2004. 52. Jorgen sen , e.K., Modem Aspects of ligand Field Theory, No r th-Holland Pub lishing Company, Ams terdam, 1971. 53. Dorenbos. P , Valence s tability of lanthanide ions in inorganic compo unds, Chern . Mater., 17, 2005,6452. 54. H owe, B., and Diaz, A.L., Cha racterization of host-latti ce emission and energy transfer in BaMgA l JO0 17:Eu 2+, J. Lumin ., 109,51,2004.

chapter two - section one

Principal phosphor materials and their optical properties Shinkichi Tanimizu

Contents 2.1 Luminescen ce centers of ns--ryp e ions 2.1.1 Optical spectra of S2 ions in al kali halides 2.1.1.1 Absorpti on spectra 2.1.1.2 Structure of the A and C abso rption bands 2.1.1.3 Temperature dep end ence of the A, B, and C absor p tion bands 2.1.1.4 Emis sion spectra 2.1.2 S2 - Type ion centers in practical phosphors References

2.1

145 145 145 149 151 152 152 155

Luminescence centers of ns 2-typ e ions

Ions with the electronic configuration ns 2 for the grou nd state and nsnp for the first exci ted state (n = 4, 5, 6) are called ns--type ions. Table 1 shows 15 ions w ith the outer electro nic configuration S2. Luminescence from m ost of these ions incorporated in alkali halides and other crystals has been observed. Among these ions , luminescence and related optical p roperties of TI+ in KCI and othe r sim ilar crys tals have been most p recisely stu die d ." > so S2 ions are also called Tlr-like ions. As for powder phosphors, excitati on an d emiss ion spec tra of 5n2+, 5b3+, Tl' , Pb 2+, and Bp· ions int roduced int o various oxygen-d ominated ho st latti ces have been rep orted.v" though the an alys es of these spectra have not yet been completed due to s truc tureless broad-band spe ctra and unknown site symmetries . In this section, therefore, experimental and theoretical works on S2 ion s mainly in alkal i halides will be s umm ar ized .

2.1.1

Optical spectra of S2 ions in alkali halides 2.1.1.1 Absorption spectra

The in trinsic absorp tion edg e of a pure KCl cr yst al is located at about 7.51 eV (165 nm) at room temper ature. Wh en Tl' is in corporated as a substitutional imp u rity in th e crys tal with concen tra tions bel ow 0.01 mol %, four ab sorption bands appear below 7.51 eV, as sho w n in Figu re l (a). Th ey have be en lab eled A, B, C, and D bands in or der of increasing

145

146


ener gy. Similar bands are ob served by the inc orporation of Pb z+ or Ag- ions , as sho wn in Figures 1(b), (C).8-10 One or tw o D bands lying near the absorption edge are due to charge-transfer transition s from Cl to S2 ions or to perturbed excitons. and are not due to 52 ~ sp transitions. Th e following di scussion will, therefore, be restricted to the A, B, and C bands. First, a model based on free Tl' ions foll owing the original work of Seitz ! will be discu ssed. The 652 ground state is expressed by 15 0 , The 656p first exci ted s ta te con sists of a triplet 3p, and a singlet lP l . The order of these sta tes is 3PO' 3P I , 3P2, and IPI from th e lowenergy side . When a Tl' ion is introduced into an alkali halide h ost an d occupies a cation sit e, it is placed in an octa he d ral (0,,) crys ta l field . The energy levels of the Tl' ion a re lab eled b y the irredu cible representation of the 0 " point group. Th e labeling is made as foll ow s: for the ground sta te 150 ~ IAJ,~' and for th e excited state 3PO ~ 3A l", 3PJ ~ 3Tl l/' 3p z ~ 3E" + 3T zU' and IPI ~ lT l " . Th e lA lg ~ ITI " transition is dipole- and spin-all owed , while the lA lg ~ 3A I II transition is strictl y forbidden. The lA lg ~ IT l lI transition is partiall y allow ed by single t-trip let spi norbit m ixin g, and IA Jg ~ (JE" + 3T z,J is also allowed due to vib ron ic m ixin g of 3E" and 3T 2J1 with 3T l Then, the observed absorp tion bands sh own in Figure 1 can be assign ed as follows: l('

A band s : 1 A lg ~ 3Tl Jl B bands '. l A 19

~

3E + 3 T

C bands .' lA Ig

---7

IT

IJ

111

2u

CSo

~

3Pl )

CSo

~

3PZ )

(1 So

~

IP l )

Focu sin g on the characteri st ics of the A, B, and C abso rp tion bands, the centers of the gravity of th e ene rgies of these bands are given by " :

f A= F -

tj4 -

J(C'+(,/4)2+ (11.(,)2/ 2

~j = F- G + (,/ 2

Here, F an d G are the parameters of Coulomb a nd exchange en er gies as d efined by Condon and Sh ortley." (, is th e spi n -orb it cou pling cons ta n t. A for the A and C bands is called the Kin g-Van Vleck fact or.'? an d is a parameter expressin g the spa tial di fferenc e between the IT I II and 3T I " wavefunc tio ns . The v alues of r, an d A can be obtained fro m the va lues of f Aand f c ex trap ola ted to T = OK, as shown in Fig ure 2.B The oscillator streng th ratio of th e C to A bands is given by!':

wh ere

(La)

147

Chapter two: Principal phosphor materials and their optical properties Table 1 Ion s w ith the 1152 Config ura tion in th e Grou nd State Atomic No.

Elem en t

(ns)(np)

Ion sp ecies

29 30 31 32 33

Cu Zn Ga Ge As

(45)1 (45)2 (45)2(4p) I (45)2(4p)2 (45)2(4p)3

C uZno Ga+ Ge 2 + A s3+

47 48 49 50 51

Ag Cd In Sn Sb

(55)1 (55)2 (5s)2(5p) J (55)2(5p)2 (55)2(5p)3

AgCd o In+ 'Sn 2+ 'Sb3+

79 80 81 82 83

Au Hg

(65)1 (65)2 (6s)2(6p) I (6s)2(6p)2 (6s)2(6p)3

AuHgO i l+ 'P b2 + 'Bi3+

TI Pb Bi

. Luminescence is observed also in powd er phosphors. (See 2.1.2)

( a ) KCI : TI + co ---..... c

~

c-, '-
.

......

. (/J

----(/J ......

-:12K

c·(l) C

c:

------ : 300 K - - - : 80 K

;:l

.- >. C

.- e l-

131' iu*>

Q (a)

Q ( b)

Figure 7 Configu rational coo rd ina te model to accou nt for the AT and Ax emission bands: (a) without sp in-orbit int era ction, (b) with spi n- orbit in teraction. (From Farg e, Y and Fon tana , YP., Electronic and Vibratio nal Properties of Point Defects in Ionic Crystals, North -Holland Pub lish ing, Am sterdam , 1974,193; Ranfagni, A., Magnai, D., and Bacci, M., Adv. Phys., 32, 823, 1983; Jacobs, P W.M., J. Phys. Chem. Solids, 52, 35, 1991. With permission .)

from Sn 2+ , Sb 3 +, Tl", Pb 2 +, and Bj3+ are reported . These ions ar e m arked w ith ast er iks in the tabl e. Luminescence features of the above five io ns ar e as foll ows. 1. Th e luminescen ce is due to the A band tr an sition (JP , -7 150 ) , 2. Th e luminescenc e is usually associated w ith a large Stokes' sh ift, and the spectra are consid er ab ly broad, especially in case of Sn 2+ and Sb 3+ . 3. Th e luminescence decay is not ve ry fast and of the order of microseconds. This is because the luminescence transition is spin -for bid de n .


154

Spectral d ata-? and lie decay times of practical phosphors activated with 52-typ e ion s at room temper ature under 230- 260 nrn excitati on are given below. Sr ZPZ0 7:Sn z+ Excitation bands: Emission band:

(Ref. 21,22) 210, 233, and 250 nm. 464 nm with halfw idth 105 nm.

SrB 6010 :5 0 Z+ Excita tion bands: Emi ssion band: Decay time:

(Ref. 23) 260 and 325 nm. 420 nm with h alfwidth 68 nm. 5 us.

Cas(P04)3F:Sb3+

(Ref. 24, 25) • 175,26 202, 226, 235, 250, and 281 nm for 0 2-compensated samples. • 190,200, 225,246, and 267 nm for Na-compen sated samples. • 480 nm with halfwidth 140 nm. • 400 nm w ith half width 96 nm. • 7.7 us for 480 nm em issi on . • 1.95 us for 400 nm emission.

Excitation b ands:

Emission bands: Decay tim es :

Th e behavior of Sb 3+ in fluorapatite [Ca s(P04hFj ho st lattice is not so sim ple, becau se of the existence of tw o differ ent Ca sit es an d charge com pe nsa tion . Th e low-lying exci ted st ates of Sb 3 + with and w ith ou t O 2 compensation w ere calcu lat ed by a molecular orbital model.> However, the rea son why the d ecay times for 480 and 400 nrn emission bands differ n oticeably ha s not yet been elucid at ed . YP0 4 :Sb 3 + Exci ta tion bands: Emission bands: Decay tim e:

(Ref. 27, 28) 155 nm, 177-202 nm, 230 n rn, and 244 nm. 295 nm with halfwidth 46 run , an d 395 nm w ith h alfwidth 143 nm . Below 1 us .

(Ca,Zn)3(P04)Z:TI+

(Ref. 29) 200 and 240 nm . 310 nm wi th h alfwidth 41 nm.

Excit ation bands: Em ission band :

The em ission p eaks vary w ith Zn contents. BaMgzAI160Z7:Tl+

Excit ation bands: Em ission bands:

Decay times: BaSi 20 S:Pb Z+

Excit ation bands: Emi ssion band :

(Ref. 30) • 200 nm and 245 nm for 1% Tl. • Unkn own for 3 an d 10% 11. • 1% TI: 295 nm with halfwidth 30 nm. • 3% TI: 420 nm with halfwidth 115 nm. • 10% TI: 460 nm with h alfw idth 115 nm. • 0.2 f.1S for 295 nm emission. • 0.6 us for 460 nm emission . (Ref. 31, 32) 187 and 238 nm. 350 nm with halfwidth 39 nm.

Chapter two: Principal phosphor materials and their optical properties

155

In BaO-Si0 2 sys tems, Ba 2Si04, BaSi03, and BaSi3 0 s, a re also kn own. Ba 2Si04:Pb 2+ rev eal s two em issions peaked at 317 an d 370 nm. The excitation bands lie a t 180,202, an d 260 nm . Pb Z+ in another host; SrAl12019:Pb z+ (Ref. 30) Excitat ion bands: Below 200 nm, an d 250 nm for 1% Pb . • Unknown for 25 and 75% Pb . Emission bands: • 1% Pb: 307 nrn w ith h alfwidth 40 nm. • 25% Pb : 307 nm wi th h al fwidth 46.nm, an d 385 nm w ith h al fwidth 75 nm, • 75% Pb : 405 nm with h alfwidth 80 n m , Decay time: • 0.4 us for 307 nm emission . As for the spec tra l d ata and d eca y times of Bi3+ activa ted phosphors, readers are referred to Referen ces 33, 34, 35, a nd 36. YP0 4:Bj3+ (Ref. 33,36) Excitation ban d s: 156, 169, 180, 220, 230, an d 325 nm (for a Bi-Bi p air) Em ission bands: 241 nm Decay time : 0.7 s

References 1. Sei tz, F, J. Chern. Phys., 6, 150, 1938. 2. Fowler, W.B., Electron ic Stat es and Optical Trans itions of Color Centers, in Physics of Color Centers, Fowl e r, WB., Ed ., Academ ic Press, New York, 1968, 133. 3. Farge, Y an d Fon tan a, M.P , Electronic and Vibratio nal Propert ies of Point Defects in Ionic Crystals, Nor th -H olland Pu blish ing Co ., Ams terdam, 1974, 193. 4. Ranfagni, A., Mag nai, D., and Bacc i, M ., Adv. Phys., 32, 823, 1983. 5. Jacobs, PWM., J. Phys. Chern. Solids, 52, 35, 1991. 6. Butl er, K.H ., Fluorescent Lamp Phosphors, Penn sylvania Sta te Unive rsi ty P ress, 1980, 16l. 7. Blasse, G. and Gra bma ie r, B.C., Luminescent Materials, Sp ringer Verlag , Berlin , 1994, 28. 8. Fuk uda, A., Science of Light (Ja pan), 13, 64, 1964. 9. Kleemann, W , Z. Physik, 234, 362, 1970. 10. Kojima, K., Shiman u ki, S., a nd Kojima, T , J. Phys. Soc. japan, 30, 1380, 1971. 11. Condon, E.U. and Shortley, G. H ., The Theory of Atom ic Spectra, Ca mb ridge University Press, Lond on , 1935. 12. King , G.W. and Van Vleck, J.H. , Phys. Reo., 56, 464, 1939. 13. H omma, A, Science of Light (japan), 17, 34, 1968. 14. Suga no, S., J. Chern. Phys., 36, 122, 1962. 15. Toyozawa, Y and moue, M., J. Phys. Soc. japan, 21, ] 663, ] 966; Toyozawa , Y, Optical Processes in Solids, Cambridge Univers ity Press, Lon d on , 53, 2003. ]6 . Fu kud a, A , J. Phys. Soc. japan, 27, 96, 1969. 17. Edgerton, R. a nd Teegard en, K., Phys. Rev., ] 29, 169, 1963. 18. Fukuda, A , Phys. Rev., Bl , 4161, ] 970. 19. H linka, J., Mi hokova , E., and Nikl, M., Phys. Stat. 501. , 166 (b), 503, 1991. 20. See Tabl e 10 and lOa in 5.6.2. 21. Ropp. R C. an d Mooney, RW, J. Electrochem. Soc., 107, 15 1960. 22. Ranby, P W , Mash, D.H., and Henderson, S.T, Br. J. Appl. Phys., Su pp l. 4, S18, 1955. 23. Leskela, M., Kos ken talo. I., a nd Blasse, G ., J. Solid State Chem., 59, 272, 1985. 24. Dav is, T S., Kreid ler, E.R, Parodi, J.A, an d Sou les, I.F , J. Lumi nesc., 4, 48, 1971. 25. Soules, I.E, Davis, I.S., and Kreid ler, E.R, J. Chern. Phys., 55, 1056, 1971; So ules , T F , Bateman, R.L., Hewes, R A., and Kreid ler, E.R., Phys. Reo., B7, 1657, 1973.

156 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

Fundamentals of Phosphors Tan imizu , S. and Suzu ki, T., Elecirochem. Soc., Extended Abstr., 74-1, No . 96, 236,1 974. Grafmeyer, J., Bourcet , r.c, and [anin, J., J. Luminesc., 11, 369, 1976. Omen, E.W.].L., Srnit , WM .A., an d Blasse, G., Phys. Rev., B37, 18, 1988. Nagy, R , Wollentin, RW, and Lui, CK. J. Electrochem . Soc., 97, 29, 1950. Sornmerdijk, ].L., Verstegen, ].M.P.]., and Bril, A., Philips Res. Repts., 29, 517, 1974. Clapp, R H . and Ginther, R]., f. Opt. Soc. A m., 37, 355, 1947. Butler, K.H ., Trans. Elecirochem. Soc., 91, 265, 1947. Blasse, G. and Bril, A., f. Chern . Phys., 48, 217, 1968. Boulon, G., f. Physique, 32, 333, 1971. Blass e. G ., Prog. Solid State Chem., 18, 79, 1988. ]-Ste!, T., Huppert z, P., Ma yr, W , Wiechert, D.U. J. Lumin., 106,225, 2004.

chapter two - section two

Principal phosphor materials and their optical properties Ma saaki Tamatani

Contents 2.2 Lum inescence cen ters of transition me tal ions 2.2.1 Cr yst al field theory 2.2.1.1 The simples t case: 3d1 elect ron config ura tion 2.2.1.2 Th e cases of m ore than on e d elec tron 2.2.1.3 Tanabe-Sugano diagrams 2.2.1.4 Spin-orb it interaction 2.2.1.5 Int en sities of em ission and abso rp tion band s 2.2.2 Effects of elec tron clou d expans ion 2.2.2.1 Nephelau xeti c effect 2.2.2.2 Ch arge-transfer band 2.2.3 Cr 3+ Phosphors (3d3 ) 2.2.4 Mn4+ Phos phors (3d3 ) : 2.2.5 Mn 2+ Phosphors (3dS) 2.2.5.1 Crystal field 2.2.5.2 Different Mn 2+ sites in cry st als 2.2.5.3 UV absorp tion 2.2.5.4 Lum inescen ce decay tim e 2.2.6 Fe 3+ Phosp hors (3dS) References

. 2.2 2.2.1

157 157 158 161 163 164 164 167 167 168 168 172 173 173 175 176 177 177 178

Luminescence centers of transition metal ions Crystal field iheoru':'

The 3d tran sition me tal ions ut ilized in commercia l powder p hosphors have th ree electro ns (in the case of Cr3 + and Mn 4+) or five elec trons (Mn 2+ and Fe 3 +) occupying th e ou ter most 3d electron orbi tals of the ions. When the 3d ions are in corp orated in to liguids or so lids, spectroscopic prope rties (such as spectral p osition s, wid th s, an d int ensities of lum inescence and absorption bands) are considerably changed from those of gaseous free ion s.

157


158

These cha nges a re explained in term s of crystalf ield theory, whi ch assumes an ions (ligands) sur ro undi ng the m etal ion as point electric cha rges. When th e theory is exten de d to take into consid eration the overlap of elect ron orbi ta ls of the metal ion and ligands, it is called ligand f ield theoru. In the followin g, th ese theor ies w ill be de scribe d briefly. For m ore d etails, the read er is ref erred to Refer ence 1.

2.2.1.1 The simplest case: 3d J electron conf iguration Firs t, take the case of an ion th at h as the 3d! electron conf igura tion, such as Ti3 + , Table 4 sh ows th e w a vefunctions for the five 3d ele ctron orbitals, and Figure 8 the electron di stributions for these orb itals. For a free ion , the energies of the five 3d orb itals are id entical, a nd are d et ermined by an elec tron kin eti c ener gy and a cen tra l field potential cau sed by th e inner electron she ll." In cases where different orbitals h ave the sa me energy, the orbitals are said to be degen erate. When thi s ion is incorpora ted in a crys ta l, s ur ro un di ng an ions affect it. Consider the case where there are six anions (negative p oint ch arge s) at a d ist ance R from a cen tral ca tion nucleus locat ed a t ±x, ±y, and ±z as show n by ope n circles in Figu re 8. Th is ligand a rr an gement is call ed the oc tahed ra l coord ina tio n. These anio ns in d uce an elec tros tatic p ot ential V on a 3d elec tron of the central cati on , which is exp ressed by ;= 6

Ze2

i

IR,-rl

V=I, -

(4)

Here, R; rep resents a po si tion of the i th ani on, r a position of the 3d electron (coordinates x, y, z), Z a va len cy of an anion, and e an electron ch a rge. When IRi ! ~ Ir , the followin g equa tion is ob ta ine d from Eq. 4 by the expansion on r up to 4 th o rde r.

(5)

Th e effec t of the potential Von the 3d elec tro n or bi tal ene rgy is expressed by th e follow in g int egra tion.

f

",(3d)V", (3d)d, = ( 3dIVI3d)

(6)

The first ter m of Eq. 5 increases the ene rgy of all five orbi tals by th e same amo un t. It ma y be ne gl ected in the field of optical spec tro scopy, where on ly energy differences among ' ele ctron states are meanin gful. From the second term in Eq. 5, the following orbi ta l ene rgies ar e ob tained . (7)

(uIVlu) = (vIVlv) = 6Dq * H ere, the spin-orb it interaction of an elect ron is negl ected.

(8)

159

Chapter two: Principal phosphor materials and their optical properties Table 4

Wave func tions for a 3d Electro n

<Jl" = ) 5/ 16rr R3d (r)(1/ r1)(3 z2

_1' 2 )

l(l ,.

= J5/16rr R3d(r)( 1/r2 )(x2 - !/)

l(l ~

= \(l5j4n H1.,(r)(1/ r2 )yz

l(l 'l

= J15/4rr R3A r) (lj r2 )zx

brings about luminescence wi th an in tere sting lin e stru ctu re in the 680- to nO-nm spectra l region in various ho st materials . In particular, the optical spectra of ruby (A 1203 :C r h ) were fu lly expl ain ed for the first tim e by a p plying crys tal field theory (1958) 13; ruby was ut ilized for the first solid state laser (1960).I'J Figures 17 a nd 18 sh ow the luminescence' > and absorption ! spec tra of ruby crys tals, respectively. The two strong lum inescen ce lin es a t 694.3 nm (= 14399 cm " ) and 692.9 , See 2.4. For rar e-e a rth phosphor s, the e ffect of th e cha rge-tran sfe r bands is inv estigated in co ns ide rable d etail w ith resp ect to the fluorescence proper ties of Fr transit ions ."

Chapter two:

Principal phosphor materials and their optical properties

169

'A z U,"p4) 2A I

10

2

3

Dq/B

Figure 15 Energy level diagram for the d7 configuration. (From Kamirnura, H., Sugano, S., and Tanabe, Y., Ligand Field Theory and its Applications, Syokabo, Tokyo, 1969 (in Japanese). With permission.)

nm (= 14428 em:") with width of ~10 crrr' and decay time of 3.4 ms at room temperature are called R j and R 2 lines. They lie at the same wavelengths as lines observed in the absorption spectrum (zero-phonon lines). These lines correspond to the transition from 2£(t23) ---7 4A2(t23) in Figure 11. The 2£ level splits into two levels due to a combination of the spin-orbit interaction and symmetry reduction in the crystal field from cubic to trigonal.' Two strong absorption bands at ~ 18000 cm' and -25000 cm' correspond to the spin-allowed transitions from the ground level (4A2(t} )) to the "T2(t/ c) and 4T1(t} e) levels, respectively. The spectral band shape differs, depending on the electric field direction of the incident light due to the axial symmetry in the crystal field (dichroism). Many spin doublets originate from the t/e configuration of Cr3+ in addition to the above two spin quartets." Transitions from the ground level (4A2 ) to those spin doublets are spinforbidden, the corresponding absorption bands being very weak to observe." Strong spinallowed absorption bands to those spin doublets, however, are observable from 2£(t23), when a number of Cr 3+ ions are produced by an intense light excitation into this excited state (excited-state absorption)." For 11 multiplet levels, including those obtained through excited-state absorption studies, all the properties of the absorption bands-such as spectral position, absorption intensity, and dependence on the polarized light-have been found to agree very well with those predicted from crystal (ligand) field theory.l-" As shown in Figure 17, with the increase in Cr3+ concentration, additional luminescence lines begin to appear at the longer wavelength side of the R lines, and grow up to be broad bands that become stronger than R lines; this is accompanied by the reduction * In Figure 11, positions for these doublets are not shown clearly. *' In a strong crystal field, two-electron transitions such as I,' --'> 12c' are forbidden.


170

r=4.71 B=1030

60 IS

50 40 3

l

_~---;>,,-

1

2

E

3

Dq,/B Figure 16 Energy level dia gram for the dB con fig ura tion. (Fro m Kamimura, H ., Sugan o. S., and Tanabe, Y, Ligand Field Theory and its Applications, Syo ka bo, Tokyo, 1969 (in Jap an ese). With perrnission .) Table 6 Oscill at or Strength and Luminescence Decay Time La port e's ru le allow ed

Sp in-a llowed

f t

Spin-forbidd en

f r

Electric dipole

Magnetic dipole

-1 -5 ns 10-2-1 0-3 0.5-5 us

-10-" -1 ms 10-8-10- 9 . 102- 103 ms

Laporte 's rul e forbidde n Electri c di pole Latt ice vibration v" u allowed allowe d _10--1 - 10--1 -50 IlS - 50 us 10-6-10-7 10-6- 10-7 5-50 ms 5-50 ms

Note: 1.fvalu es for the case of sp in-allowed are estimated in Reference 1.fvaJues fo r the case of spin-fo rbidden are assu med to be 10-2-10-3 of those fo r sp in-a llowed. 2. Decay times are calculated from Eqs. 22 and 23, ass uming EJ Eri , = (1/2 + 2)/3 (Lo renz field), /1 = 1.6, and A" = 500 nm.

in the luminescence decay time of R lin es, in the case of Figu re 17, from 3.5 ms to 0.8 ms at room ternp e rature.t '' Additional lin es are attributed to magnetic ally coupled Cr3+-Cr3+ pairs and clu sters. Luminescence lin es are as signed to suc h pair s up to the fourth nearest neighbor; for examp le, the N ] lin e is assig ned to pairin g to the third nearest neighbor, and N 2 to the fourth near est ." In compounds suc h as va rious ga lliu m ga m ets in w hich Cr 3+ ions are locat ed in weak crysta l fields, 4T 2(4F), instead of 2EeG), is th e em itting lev el. 18 As exp ected from Fig ure 11, the luminescen ce spec tru m consists of a broad band in the near-infrared region, i.e., at a longer wavelen gth region than that in th e 2E case. The de cay tim e is as short as - 0.1 ms because the transiti on is spin-allowed . These properties ma ke them promising candi d ates

171


100 ::0-

C r 20 ~ =

~ u

0.055 %

'"

lI-

o

U

o

700

720

700

740

720

760

800

780

820

A [nm ]

Figure 17 Luminescence spectra in rubies (at 77K). (Figure 1 in the source shows lum inescence spectra and deca y times for rubies containing 0.4, 0.86, 1.5, and 8% concentrations of Cr 203, in add ition to the above two examples.) (From Tolst oi, N.A., Liu, S., and Lapidus, M.E., Opt. Spectrosc., 13, 133, 1962. With p ermission.)

9

8 7 6

5

,, I

3 2 1

o

I

,

I

I

\

I

I

:

I I I

4

E -L C 3 ( 0") E II C 3 ( 7[)

, I I I

(0.28 wt % C r 2 0 3 room temperature)

' I

I I I I

, ,

I

I

I I

\ \

,

'

. 35

40

45

Wave number [x 10 3 em-I] Figure 18 Absorption spectra of a ru by. (Courtesy of A. Misu, unpublish ed. ) E rep resen ts the electric field directio n of an incident ligh t, and C3 do es a three-fold axis di rection of the crystal. Spec trum at higher energies than 35000 crrr ' is for natural light. Absorption lines arou nd 15000 and 20000 cm-1 are shown only in the case of the 0 spectrum, qualit atively w ith respec t to intensi ty and linewid th. (From Karnim ura, H., Suga no, S., and Tanabe, Y., Ligand Field Theory and its Applications, Syokabo, Tokyo, 1969 (in Japa nese) . With p ermis sion .)

for tu nab le solid-sta te laser materials.P'" Th e change of the emitti ng sta te depending on the host materials is a good example of th e importance of the crys tal field in de ter mining the opti cal pro perties of the transition-metal-doped compo unds. Table 7 shows the crystal field parameters ob tained from absorption spectra and luminescence decay times for Cr 3+ in several hosts. Most lumin escen ce bands in 3d ions are caused by electric dipole transi tions. In such materi als as MgA l z0 4 and MgO, in which a metal ion lies in the crystal field wi th the inversion sy m me try, how ever, the R lines occur via a magne tic dipole process-l-" : conseq uen tly, the decay tim es are lon g.

172

Fundamentals of Phosphors Table 7 Cryst al Field Par amet er s for Host

A

Dq

B

(nm)

(em ")

(crrr" )

1630 1630 1825 1660 1750 1725 1471 1508

640 780 700 650 800 640 645 656

1720

765 918

692.9 H

a- A120 3 (ruby) Be3AJ2Si6018 (Emerald) MgA1P 4 MgO LiAlsOs a Y1A1s012 Gd 3GasO '2 Y3Ga50 12

694.3 682.1 679.226 682.2 681 .9 698 27 715.8 701.6 688.7 687.7 745 (broad )" 730 (b roa d)"

Cr (H 2O lo3+ Fre e ion

684.2

Abs. (lG)25

r-h

C (cm" ) 3300 2960 3200 3200 2900 3200

T

(ms) 3

(R)

36.5 (N) 12 (N) 3.7 1.5 0.16 0.24

Ref. 23 23 21 22 24 28 18 18 3 3

Note: A: peak waveleng th of Iuminescence.t: l ie d ecay time ; (R), room tem per ature; (N), 77K . ., Ord e red ty pe b 4T l -4

' A 2 tran si tion, othe rw ise ' F.

-4

.IA, transition.

2.2.4 Mn 4+ phosphors (3d 3) Only 3.5MgO ·0.5Mg F2· Ge0 2 :Mn 4 + is now in practical use amo ng the Mn4+ phosphors, though 6MgO 'As 2 0 s:M n 4 +, which has a performan ce a lmos t e q ua l to that of 3.5MgO ·0.5MgF 2 ·Ge02 :Mn 4+, was used previously." and a number of titanate phosphors were d eveloped between 1940 and 1950.30 Luminescence bands due to Mn4+ exist at 620 to 700 nrn in most ho st mat erials. The spectrum has a structure cons isting of several broad lines orig ina ting from transitions aided by lattice vibra tion . In Al2 0 , and Mg 2Ti04, it rese mbles the R lin es of Cr-" . and is assigned to the 2[ (t / ) -1 4A 2(t 23) transition. Figure 19 shows the luminescence spectra for 3.5Mg O·0.5Mg F2· Ge0 2:Mn 4+. It consists of more than s ix lines a t room temperature; the inten sity of the lin es at the shorter wa velength side d ecreases a t low temperatures. This beh avi or is exp laine d by assuming that thermal eq uilibrium exis ts between two lev els in th e emi tting sta te, and that there ar e more than tw o levels in the gro und state." As for the origin of the emi tting and ground states, diff erent assign me n ts hav e been proposed. Kem en y and Haake assigned the bands to the 4T2(t/ c) -1 4A 2(t 23) tran sition in Figure 11, assuming the Mn4+ site has octahed ral coordination." They propose that the 4T2 level splits into two levels due to the low symmetry field, and that mo re than two vib ronic leve ls accompany the gro un d state. Butler insisted that a (Mn04 )4- complex rep laced (GeO)I-, which is tetrahedrally coo rdina ted .F In this case, the appropriate energy diagr am is Figure 15 instead of Figure 11, and the luminescen ce origin ates from the 2E(e3) -1 4T1(e2t2) tran sition ." The 2[ and 4T1 levels spli t into tw o and three due to the low sym metry field , resp ectively. These proposals, however, could not accoun t for s uch facts as the luminescen ce has a decay time of the order of millis econds; in ad di tion, no visible luminescence has been obse rved due to Mn 4+ in so lid -state materials in which the metal ion s are tetrah edrally coordina ted. Ibuki's group ass igned the lines to tran sitions from tw o excited levels of 2EUl) and 2T 1U?) to the gro u nd s tate 4A 2U/ ) in Figure 11, ass uming Mn 4+ has an octahe d ral coord ination.P Th e main pea k struc tu re in the range 640 to 680 nm at room temperature origina tes from the lattice vibra tion asso ciat ed w ith the 2£ -1 4A2 zero-phonon transition at 640 nm . Blasse explained the s pectral charac teristics by assuming only on e electron ic tran siti on of 2£ -1 4A2 in octahe d ra lly coo rdin ated Mn 4 ' . 34 Both the ground an d excited sta tes are * See 2.2.1.3. The trans ition correspon ds to 2EU / C2 CZh Oil

Td

175

Mn 2+Sites and Lu minescen ce Properties Site

Co ord in at ion number

Inv ersion sym me try

Ie (nm)

Ca Zn Mg (A site) (A site) 2Zn 2Zn 2Ca 2Sr 3Ca 64 2M g 65 Ca Zn

8 6 6 (4) (4) 4 4 663 6 6 6 6 4

g g g

495 587 60242 506 513 525 537 570' 558 550 620 660 740 588 591

u u u u

u u u U

g u

r

(rns)

8346 100 10462 4 5 12 10 1466 30 2.2-4. 867 0.25

Note: 1. 2Ca in the site co lumn means exis tence of two different Ca sites. (A site) mea ns larger probability for existence in A sites than for octahed ral B sites. 2. Excep t for those referr ed , crystal symmetries follow those in Reference 61, and luminescence wavel eng ths and de cay times in Reference 5l. 3. In the inversion symme try column, g and u corresp ond to exis tence and nonexistence of a center of sy mmetry, resp ectively. a A valu e ob tained in an Sb-Mn co-doped sample.

In CaF 2:Mn2+, th ou gh Mn 2+ occupies a cubic site w ith hi gh coordination number, Dq is not so large because th e ani on valency of F- is smaller than th at of 0 2- . In additi on, B is large because o f th e s ma lle r nephelauxetic eff ect ." C onsequ ently, thi s com p ou n d yields the shortest luminescen ce wave leng th (-495 nm) observed a mon g Mn -t-doped phosphors ." Since every excited level of d5 is either a spin quartet or a d oublet , all transitions from the ground se xtet to them are sp in-fo rb id d en . Optical ab sorption inten sity is weak, and the phosphors are not colored (i.e., the powder bod y co lo r is white). Th e 4A ) and 4E(4G) levels ha ve th e sa m e energy an d are parallel to the ground level GA l in Fig ure 13. The absorption band corres p ond ing to GAl -7 4A l ,4E(4G) therefore h as a n arrow bandwidth, lying at - 425 nrn , irrespectiv e of th e kind of host material. w' ? One n otices that this band sp lits into more than one lin e when carefu lly investigated. Th e sp litt ing is consid ered to reflect the reduction of the crysta l field symmetry.w" Table 10 shows the cr ystal fiel d parameters for Mn 2+ in rep resentati ve phosphors . Note that Oq / B for th e te trahedral coordination is smaller «1) th an th at (>1) for the octahedral one.

2.2.5.2

Different Mn 2+ sites in crystals

Since the luminescen ce w avelen gth due to Mn2+ is se nsitive to the m agnitud e of the crystal field, several em ission bands are obs erv ed w hen different typ es of Mn2+sites ex is t in a host cr ystal. In SrAI I2 0 19 , th e band s at 515,560, and 590 nm are co nsidere d to orig in a te from Mn 2+ ion s replacing tetrahedrall y coo rd in a ted A J3+, fivefold co o rd ina ted A J3+, and 12-fold coord in a ted Sr 2+, respect ivel y." In lanthanum aluminate, whi ch ha s a la ye r structure of s pinel blocks, a 680-nm band is ob served due to Mn 2+ in oc ta hed ra l coordination, in ad d ition to a green -emi tting band due to tetrahedral coordinati on .v Tw o e m iss ion • The other shortest peak wavelength is at 460 to 470 rim. observed in 5rSb ,O,.,;7 in which Mn> is consid ered to be located in an extraordi nary weak crystal field (5r-0 distance is as large as 2.5 A).


176 Table 10

Host

Crystal Field Parameters for Mn2 +

A

Dq

(nm)

(crrr")

B (crn')

C (crrr")

Coordination

Ref.

MgGa204 LaAlu018 Zn2Si0 4 Cas (P° 4hF Mg4Ta 209 CaF2 hex·ZnS

504 517 525 572 659 495 59}51

520 543 540 760 425 (2375) 520

624 572 (624) 691 (698) 770 630

3468 3455 (3468) 3841 (3678) 3449 3040

(4) 4 4 6 6 8 4

48 41 48 68 55 46 69

Mn(H2O )62 + Free ion

Abs. 372.5 ClG)l5

1230

860 860

3850 3850

6

3 3

Note: Band C values in parenthesis, which were obtained from other phosphors, are used for calculating Oq values.

bands separated by about 50 nm were recognized long ago in Mn-r-doped alkaline earth siliea tes. 51 Even in the case of the same coordination number, different luminescence bands may come from Mn> ions occupying crystallographically different sites. In CaS(P04)3F, there are principally Ca(I) and Ca(II) sites having different crystallographic symmetries; several additional sites accompany these two main calcium sites. The correspondence between the luminescence bands and the various sites has been investigated by means of polarized light,sZ ESR,s3 and excitation'? spectral studies. In the case of the commercially available phosphor Ca 5(p04h(F,Cl):Sb3+,Mnz+ (for Cool White fluorescent lamps), the Mn?:' band consists of three bands at 585, 584, and 596 nm, originating from Mn> ions replacing Ca(I), Ca(II), and Cl, respectively." Figure 21 shows the spectra in Zn zSi0 4:Mnz+, where two zero-phonon lines are observed at very low temperatures (504.6 and 515.3 nm at 4.2K).5S These lines are assigned to two types of Mn-" differing in their distance to the nearest oxygen; one is 1.90 A and the other is 1.93 A. Since the Oq value depends on the fifth power of the distance (Eqs. 9 and B), a 7% difference in the Oq value is expected between the two types of Mn> sites; this is consistent with the difference estimated by crystal field theory from the observed line positions (2% difference)." The polarization of the luminescence light observed in a single crystal is also related to the site symmetry of Mn 2 ' .% The zero-phonon lines are accompanied by broad bands in the longer wavelength side; these originate from latticeelectron interactions and are known as vibronic sidebands (See Section 1.3.) Multi zerophonon lines resulting from different Mn?" sites are also observed in Mg 4Ta z0 9 ) O and LiAl sOs·37 In ZnS doped with high concentrations of Mn?", although there is only one cation site crystallographically, two zero-phonon lines appear at 558.9 and 562.8 nm at low temperatures. These are ascribed to a single Mn?" ion (1: = 1.65 ms) and a Mn-t-Mn?" pair (1: = 0.33 ms)." In this material, the luminescence band shifts to longer wavelength and is accompanied by a decrease in decay time with increasing Mn> concentration; this is also observed in such hosts as Zn ZSi04/s1 MgGa Z04,ss ZnAl z04,sl CdSi03,sl and ZnF z.Sl Most of these effects are attributed to Mn-t-Mn-" interactions.

2.2.5.3

UVabsorption

Lamp phosphors must absorb the mercury ultraviolet (UV) line at 254 nm. In most cases, Mn> does not have strong absorption bands in this region. To counter the problem, energy-


177

Wavelength (nm)

560

540

520

500

>-.

+-'

C/l

c:

'"

o

o

I:~C~?{

Ca F,

:

S rF,

:: : : ~()

_- ~ .-

~~

HaF

~ I: ("(\Y1

([ .aA l e e! P .

voc :

I

LaOe( Ca r RO , LaO Rr

I

Oxyhalides

II I I

I

ScBO,

: ~~

r HO ,

:III

LaBO , C. UO,

Fluorides

Ii

r

I

Borates

I I I

: I

II('\; Y, AI, O..

C." MSi, O,

Y, S iO, C. , S iJl; )TO,

LaPO, (el'OI

CdPO ,

~

Aluminates

: III i I I I II

, ;~ ~:" ,/I

Silicates

: . I J II I III I ::1 I III

: I I ( I I: I I

I

"

I

l Phos Phates I

Figure 27 Ene rgies o f Sd excited levels of CeJ + in various hos t crystals. (From Na rita, K. and Taya, A., Tech. Digest, Phosphor Res. Soc. 147th Meeting, 1979 (in Jap an ese). With permission .)

on Eu 2+ described below) and varies from near-ultraviolet to the green region . Typ ical luminescence spectra of some Ce ' t-activated phosphors are shown in Figure 28.28 The two emiss ion peaks are due to the two terminating levels, 2Fs/2 and 2F7I2, of the 4/confi guration of Ce 3.... The d ecay tim e of the Ce 3+emission is 10- 7 to 10-8 s, the shortest in obs erve d lanthanide ions. Th is is due to tw o reasons: the d --7 / tran sition is both parity-allowed and sp inallow ed since 5d1 and 4j1 states are spin doubl ets." By virtue of the short d ecay time, Y2SiO s:Ce3+ and YAl0 3 :Ce3+are us ed for flying spo t scanners or bea m-in d ex typ e cathoderay tubes. Also, Ce 3+ is often used for the sens itizatio n of Tb 3+ luminescence in su ch hosts as CeMgAI I1019 .3o

2.3.3.2 Pr3+ Lum inescence of Pr 3+ cons ists of man y multiplets, as follows: - 515 nm e pa --7 3HJ , - 670 nm CPo --7 3F2) , - 770 nm e po --7 3F4 ) , - 630 n m (lD z --7 3H6 ) , - 410 nm (lSo --7 116 ) , and ultraviolet (5d --7 4f) tran sition s. Th e relativ e in tensities of the p ea ks depend on the host crys tals. As an exa mple, the emissio n sp ectru m of YzOzS:Pr3 + is sho w n in Figure 29. The ra d iative de cay tim e of the 3PO --7 3HJ or 3FJ em issio n is - 10--5 s, w h ich is the sho rtes t lifetime observe d in 4/ --74/ transitions. For example, in YzOzS ho st, d ecay times until 1/10 init ial in tensity are 6.7 us for Pr 3+, 2.7 ms for Th3+, and 0.86 m s for Eu 1 +. 2 The short d ecay time of Pr 3+ is ascribe d to the spin-allowed cha rac ter of the tran sition. Since th e sh or t decay time is fit for fast in forma tion p rocessing, Gd z0 2S(F):Pr3 +,Ce 3+ceramic has been developed for an xray d etecto r in X-ra y comp u ted tomography."


192

f\' f\

0

I

J\ '1[\

E

c

F

/\ f\

300

400 300

HM

Figure 28 Emission spectra and excitation wavelengths of Ce 3+ in var io us hos ts. (A) YPO" 254-nm exci tation; (B) YPO" 324-nm exci ta tion; (C) GdPO" 280-nm excitati on ; (D) LaPO" 254-nm excitati on; (E)LaPO v 280-run exci ta tion ; (F) YBOJ , 254-nm exci tatio n. (From Bu tler, K.H ., Fluorescent Lamp Phosphors, Technologyand Theory, The Pennsylvan ia State University Press , 1980, 261. With permission.)

>-. .......

Y20 2S : 0.3%Pr RT

(/)

c::

-e-

-.. ......

5

Do - . 7 F

195

I

(fJ

c

(\) ......

Saz (C d, E u)

c

N a (L u ,

s,» o,

N bO s

(\) C,)

c

(\)

o: (\)

c E :::l

(\)

> ro

'';::; (\)

~

580

60

620 A [nm ]

580

Wavelength

600

620

A [nrn]

Figure 32 Emission sp ectr a of Eu" from the sites h aving the inversio n sy nmetry. (From Blass e, G. and BriJ , A., Philips Tech. Rev., 31, 304, 1970. With perm ission. )

phosphor s.v Thi s proposal was dramatically ful filled for th e firs t time in 1964 b y newly develop ed YV0 4: Eu 3+ .43 Since then, Eu 3+ phosphors have co mple tely replaced b road-band em itting Mn2+phosph ors or (Zn,Cd)S;Ag, which were p redominantly in use a t th at time. Just aft er th e introduction of YV04:Eu 3+, an o th er Eu r'-activated p hosp hor, Y202S;Eu 3+, was de velopedv' an d is in current use due to its better energy efficiency as well as its stability during recyclin g in th e screen ing process of CRT prod uction . The possibili ty of further improveme nt can occu r in m ater ials wi th single-line emission, as in Y2(W04)3:Eu 3+.45 Use of narrow-b and luminescence is also advan tageous in three-band flu orescent lamp applications, where both brig h tness and color reproducibility are req uired . For h igh color ren derin g lam ps, YzO}:Eu 3+has been used as the red-emitting co mpon en t. Th e seque nce of excita tion, relaxation , and em ission processes in YzOzS;Eu 3+is exp lained by the config ur atio na l coordi na te m odel shown in Fig ur e 33 .46 Th e excita tion of Eu 3+ tak es place from the bottom of the 7Fo curve, rising along the stra igh t ve r tica l lin e, until it crosses the charge-transfer sta te (CTS). Relaxation occurs alon g th e CTS curve. Near th e bottom of the CTS curve, the exci ta tion is tran sferred to 50, states. Rela xati on to th e bottom of th e sO, states is followed by light emission down ward to 7F, states. Thi s model can exp lain th e following exp eri me n tal findings. (1) No luminescenc e is fou nd fro m 50 3 in YzOzS:Eu 3+ . (2) The lu minescen ce efficiency is higher for p hosp hors wi th higher CTS ene rgy." (3) Th e quenching temperat ur e of th e luminescence from 50, is higher as J (0,1,2,3) d ecreases. The excited 4f states m ay d issocia te in to an electron-hole p air. This model is supp or ted by the obse rva tion that the excita tion through th e 7Fo ~ 50 Z transiti on of La20 2S:Eu 3+ ca uses en ergy storage th at can be conver ted to luminescen ce by h ea ting. Th e luminescence is th e result of the recomb ina tion of a th erm ally released hol e with an Eu ?" ion.-l8,49 By taking a mod el where CTS is a combin at ion of 4[7 elec tro ns plus a h ole, one finds that th e res ulting sp in m ultip licities sh ou ld b e 7 an d 9. It is the former s tate that affec ts op tical pro perties re lated to th e 7F, s ta te by sp in -restricted covalency." The in ten sity ratio of the lum ine scen ce from 50 0 ~ 7F2 and from SOD ~ 7F1 decreases wi th in creasing CTS energy sequentia lly as ScV0 4, YV0 4, ScP0 4, and YP0 4, a ll of w h ich h ave th e sa me type of zircon stru ctu re .f The above intensity ra tio is small in YF3:Eu3+, ev en th ough Eu 3+ occupies a site without inversion sy m m e try" It is to be n oted that CTSs in flu or ides h ave


196

45 ,.----.-----r----------,.....-r.r-rJ'l

40

35

'::'

30

I

E u

0

25

~

5Dl 5D,-

>-. 01)

lIj)

20 -

C

.i]

15

'F; 7

r, "'-..

7F," .. 10 - 7FJ'

-r.

7FI -

5

7Fo /

0'--- - - - :::0;::;= =--- - - - - - ----'

Configurational coordinate Figure 33 Configurational coordina te model of Y202S:Eu3+, (From Stru ck, C.W. an d Fong er, W.H., f. Luminesc., 1/2, 456, 1970. With perrnission.) h igher energies th an th ose in oxides. Th ese results s ugges t th at hi gh er CTS energies reduce th e strength of th e elec tric dipole tran sition 'Do -7 7F2 in Eu 3+.

2.3.3.8

Eu2+

The elec tro ni c configu ration of Eu 2+ is 4f and is id entical to that of Cd 3+. The lowest excited state of 4f levels is locat ed at abo u t 28 x 103 crrr' and is higher than th e 4f65d1 level in m ost crystals, so th at Eu 2+ usually gives broad- ban d emi ssion due to f -d transitions The w avelength posi tio ns of the emission band s depen d very much on hosts, changin g from the nearUV to th e red. Thi s depen d en ce is interpre ted as due to the crystal field sp litting of the 5d level, as sh own schem ati cally in figure 34.53 With inc reas ing crystal field stre ng th, the emission bands shift to longer waveleng th. The lu minescenc e peak ene rgy of the 5d-4f transit ion s of Eu 2+ and Ce3+ are affec ted most by crystal parameters de n oting electro nelect ro n rep u lsion; on this basis, a good fit of the energies can be obtained." The near-U 'V lu min escence of Eu 2+ in (Sr,MghP20 7 is used for lamps in copying machines us ing photosensitiv e diazo d yes. Th e blue lu mi nescence in BaMgAllO0 17 is used for th ree- band fluores cent lamps. (See Fig ure 35 .)54 Ba(f,Br): Eu 2+ showing vio le t lu m inescence is used for X-ray d et ect ion th rough phot ost tmul ation.v Red lu minescen ce is observed in Eu- t -activat ed CaSJ6; th e crystal field is stronger in sulfides th an in flu or id es an d oxi des . The lifetime of th e Eu 2+ luminescen ce is 10-5-10-6 s, w h ich is relatively long for an allowed tra nsiti on . This can be explained as follows. The groun d sta te of 4f is 85, an d the multiplicity o f the exci ted s ta te 4j65d1 is 6 or 8; th e se xte t portion of th e excited sta te contributes to the sp in -forbidden character of th e transition." Sha rp-line lumin escen ce a t ~360 n m du e to an f -f transition and havin g a lifetime of milliseconds is ob served when the crystal field is w eak so that the low est excited state of


hf7-

-

-

-

--r-

-

-

....:::,.,c---

-

-

-

-

-

6p

197

J

u.v.

blu e

- --

ye l l

IJ

--4) 6

Figure 34 Schema tic diagram of th e energies of 4f and 'if5d l levels in Eu 2+ infl uenced by crysta l field ~. (From Blasse . G., Materi al science o f the luminescen ce of inorganic solids , in Luminescence of Inorganic Solids, Diba rtolo, B., Plenum Press, 1978,457. Wi th permission.)

i

,I

.I

I I ttf r.· 'Ii

~:

..

400

500

600

Wavel ength ----- A.

700 (nrn)

Emission spec tra of Eu 2+ in BaMgAl lO0 17 and rela ted com pou nd s usin g 254-nm exci ta tion a t 300K . --- -: Bao.9sEu o o5MgA l lo017 ' _ ._ . -: Ba O.S2, E u o osMgo sAI IOSOI 7.12S' - - -: Ban.75Euo.Il,Mgo.2Allo.sO,7.2' - - : Bao7oEuoll5 AIIl0l7.25' (From Smets. B.M,J. and Verlijsdon k. J.G., Mater. Res. Bull., 21, 1305, 1986. With permission.)

Figure 35

4f(6PJ) is lower than the 4j65d1 sta te, as illus tra ted in Fig ure 34. Th e host crys tals rep orted to produce UV lumin escen ce a re BaAlFs' SrAIF 5 56 (see Fig u re 36), BaM g (S0 4)2P SrBe2Si20 7, 58 and Sr(F,Cl).s9

2.3.3 .9 Cd J + The low es t excited 4f level of C d 3+ (6P 7 / 2 ) gives rise to sharp-line lumin escen ce at -315 nm 60 an d can sensitize the lu minescen ce of other rare-earth ions>' Th e ene rgy levels of the CTS and the 4f65d1 sta tes are th e high est amo ng rare-earth ion s, so tha t C d 3+ causes no quenching in other rare-earth ions . As a consequence, C d 3 + serves, as Y" does, as a good cons tituen t ca tion in host crys tals to be su bs titu ted by luminescent rare-ea r th ion s. For X-ray phosphors, Cd 3+ is be tter sui ted as a constituen t than Y 3+ si nce it has a hi gher absorp tion cross-section due to its larger atomic number.


198

7

.-c

6

.

'"-'

sr:

2.3.3.11

Dy3+ 61,65

The luminescen ce lines of Oy3+ are in the 470 to SOO-nm region du e to th e 4F9 / 2 ~ 6H15 / 2 transition, and in the 570 to 600-nm reg ion due to the 6F15 /2 -7 6F ll /2 tran sition. Th e color

200


0.1 % T b

RT

400

70 0

Figure 38 Emission spectra of Lnp 2S:Th3+ (0.1%) (Ln = La, Cd, Y, and Lu) at room temperature. (From Hoshina, T., Luminescence of Rnre Earth Ions, Sony Resear ch Center Rep.. 1983 (in Japanese). With perrni ssion .)

of the luminescence is close to white. In Y(P,V)04' the rel ative intensity of the latter d ecreases with increasing P concentration. Thi s can be understood if one considers that the 6.J = 2 transition probability decreases with a decrease in the p olarity of the neighboring ions as in the case of the 50 0 ~ 7F2 transition of Eu 3+ . The energy of the CTS and 4f5d 1 is relatively large so that direct UV excitation of D y3+ is not effective. The excitation via host complex ions by en ergy transfer can however be effective. The quantum efficiency of UV-excited (250-270 run) luminescence of YV04:Dy3+ has been reported to be as high as 65%.

2.3.3.12

Dy2+ 6(,

Luminescence of Dy2+ has been reported to consist of line spec tra at 2.3-2.7 urn a t 77K and 4.2K in CaF 2, SrF 2, and BaF2 . D y 2+ in these hosts w as prepared by the reduction of D y 3+ th rough y-ray irradiation.

2.3.3.13

Dy4+ 67

Luminescence lin es of Cs 3DyF 7 :Dy4+ a t 525 nm due to 50. j due to 50 4 --? 7F3 transition have be en reported.

2.3.3.14

--?

7F5 transition and a t 630 nm

H 0 3+

Efficient luminescence of H 0 3+ h as rar ely been found due to the cro wded energy level diagram of thi s ion. In LaCI 3, cross-re lax a tion between (552 --? 514) H (518 --? 517) at an


201

int erioninc dista nce of 7.5 A has been reported. " A g reen luminescence due to the 5F4 , 552 ---j 5[8 transition has been reported in an infrared -to-visible up-conversion phosphor, LiYF4:Yb3+,H o1+.UQ

2.3.3.15

H 0 2+

Infra red luminescen ce of H 0 2+in C aF 2 ap p ea ring aroun d 1.8 urn a t 77K h as been rep or ted.T

2.3.3.16

Er3 +

Green lum inescen ce due to th e 45 3/ 2 ---j 4[15/2 transition of Er 3+h as been reported in infraredto-visibl e up-conver sion phosphors, su ch as LaF 3:Yb3+,Er3+,7! an d NaYF{:Yb3+,Er 3+.72 This luminescen ce was a lso reported in ZnS ,73 Y20 3,7{ and Y202S .75 Th e emiss ion color is a w ell sa turated gre en. Er3+ ions embedded in an optical fiber (sev eral hundreds ppm) function as an optical am p lifier for 1.55 -~m semiconductor laser light. Popul at ion in ver sion is realized between low er sublev els of 4[13/2 and upper sublev els of 4115/2' This technology h as been dev eloped for op tical am p lifica tion in the lon g-d istance op tical fibe r communication systems."

2.3.3.1 7

Tm 3 +

The blue lu minescence of Tm 3+due to the IG4 ---j 3H6 transition h as be en reported in ZnS / 7 as well as in in frared-to-vi sible up-conversion phosphors sensitized by Yb3+ such as YF3:Tm 3+, Yb3+.78 Electrolumin escent ZnS:TmF3 has also been in vest igated as the blue com ponent of multicolor di splays."? Th e efficiency of the blue lumin escence of Tm3+ is low, and is limited by th e com pe titive infrar ed luminescence, which has a hi gh efficien cy.

2.3.3.1 8

Yb3 +

The in frare d absorp tio n band of Yb3+ a t abo u t 1 urn du e to th e 5Fs/2 ---j sF7J2 tr ansition is util ized fo r Er3+-doped in frar ed-to-vis ible up- conversion phosphors as a sensitizer.Fl-" The CTS ene rg y of Yb3+ ions is low next to th e low est of Eu 1+ a mong the tr ivalent lanthanide ion s (see Figure 25). Yb3+has no 4f ene rg y levels inter acting wi th CTS, so that lum inescence due to th e direct transition from CTS to the 4f levels can occur. This luminescence has been observed in phosphates? an d ox ysulfide ho st s." Fig ure 39 shows the excita tion and emi ssion spe ctr a of Y20 2S :Yb3+ and La 202S:Yb3+.81 As se en in Figure 33, CTS is ch ar acterize d by a fa irly la rge Frank-Condon sh ift. As a re sult, the emission sp ectra are com p osed of two fairly bro ad bands terminati ng in 2Fs/2 and 2F7 / 2, as shown in Figure 39.

2.3.3.19

Yb2+

The em ission an d ab sorption of Yb2+ d u e to th e 4f 4 H 4f 35d l transiti on ha ve been report ed. F Em ission p eak s ar e a t 432 nrn in Sr 3(P0 4 h (see Fig ure 40), 505 run in Ca 2P0 4 CI, 560 nm in Srs(P04)3CI, an d 624 nm in BaS(P04)3CI. Th e lifetimes of th e em issio ns are be tw een 1-6 x 10-5 s.

References 1. Blasse, c., Handbook on the Physics and Chemistry of Rare Earths, ed . by Csc hn eid ne r, Jr., K.A. and Eyring, L., Vol. 4, North-Hollan d Pub. 1979, 237. 2. Hoshina, T, Luminescence of Rare Earth Ions, Sony Research Center Rep. (Suppl.) 1983 (in Japanese). 3. Ad achi, C ., Rare Earths-Their Properties and Applications, ed . by Kan o, T and Yan ag ida , H., Ci hodo Pu b. 1980, 1 (in Jap an ese). Kana, T, ibid, 173. 4. Ofelt, c.s, f. Chem. Phys., 38, 2171,1963. 5. Dieke, G.H ., Spectra and Energy Levels of Rare Earth Ions in Crystals, Int erscience, 1968; American Institute of Physics Handbook, 3rd ed ition, McGraw-H ill, 1972, 7-25. 6. Riseber g, L.A. an d Moos, H.W., Phys. Reo., 174, 429, 1968.

202


Wavelength (nm)

900800700 600 500

::-.

300

250

-

: 80 K --- : 300 K

~

~

- .... -

400

Y 2 0 2 5 : Yb 3 +

~

Ll

, ~

Ll

15

10

20

35

30

25

Wave number [ X I0 c m 3

40

l]

Figure 39 Excitation an d emission sp ectra of YZ02S:Yb3+ and La zOzS: Yb3+. (From Naka zawa, E., J. Luminesc., 18/1 9, 272, 1979. With pe rm ission.) (b) Exc itatio n spectrum

D

100

2c:

80

(a) Emiss io n spectrum

'en ' an d abso rp tion exp eriments in CS2U0 2Cl4_xBrx mixed crystals" con firm the (auoJ state as th e lowest exc ite d sta te . The sta tes arising fro m the (rr} oJ configuration must be taken into acc oun t to consid er the higher ele ctronic exc ited states. Until th e nature of the excited elec tron ic state of Q = 1 eng, auo u) wa s fin all y clarified in 1976, the od d pa ri ty state Jl u was thought to be the lowest excited sta te. Th er efore, reports on urany l ions published befo re 1976 must be read w ith this rese rv a tio n in mind . Figure 44 sh ows assign m en ts and positions (in units of crrr") of electronic levels of uranyl ions as d eterm in ed from the absorp tion spectra of Cs 2 U0 2 Cl,. 2.1

2.4.3.2 Luminescence spectra A luminescen ce sp ectr um from a Cs 2U0 2Cl4 single crystal at 13K, accompanied by vib ronic structure du e to Morita and Shoki." is shown in Fig ure 45. The Frank-Condon p attern shows v ibron ic progressions of th e fund amental vibrations, V s = 837 cm' and v." = 916 crrr', of th e UO/+ ion. By applying th e con fig u ration al coord ina te m odel to Cs 2U0 2Cl4, th e nuclear d ispl acement i1Q is es timated to be 0.094 A for th e two p otential minima of the IEgc ng) exc ited sta te and the IA ,g(l L g+) grou nd state in 0 411 (0_,,) symmetry." Emi ssion peaks w ith sy m bo l * in the figure are due to traps, and these peaks disappear above 20K. The fin e s truc tu res se en in th e vi bronic progressions are electri c dipole-allowed tr an sitions due to co u p ling with odd-p arity lattice vibrations.


I 27700 cm- 1 39 crn "

1 60b c

Q =3

22600

Q =2

340 22000

6 000 cm- J

Q=3

50

,

Q=4

!

26200 50

.dq

211

20350

Q =2

904 20 100

ll)=*=~~=:::::::::::= 1.6

Q =

1

Q =

0

liv 500nm

.--...---~

_ _ _...1..-

Figure 44 Ene rg y levels and their assig nmen ts of UOl ' ion in O'olt symmet ry. Emission is d ue to the magnetic dip ole-allowed J n ~ ---7 l ~g+ (O_J,) transition . (From Denning, R.G., Snellgrove, T.R., and Wood wark, D.R., Molec. Pilys., 32, 419, 1976. With p er rnission .)

Flin t and Tann er" ha ve in vest igated the luminescence of various other uranyl complexes, of the series A zUOzCl4 ·nH 20 (A = Rb' , Cs", K+, (CH3) lN ') They found good agreement between the molecular vibr ations observed in th e luminescenc e spec tra and tho se reported in infr ared and Ram an spe ctra. Dynamic aspects of luminescen ce of [U0 2Cl4 ]2phosphors have also proved to be of interest. Krol" has investigat ed the decay of the luminescen ce of Cs zUOzCl4 at l.5K under strong laser irradiation and ob tain ed nonexp onential decays; these decays are thou ght to be due to the presence of bie xcitons associated with inter ionic inte ractions . Localization of excitons ha s al so been reported in CsUO Z(N03 ) 3 .Z9 Excitation energy transfer to trap s has been studied in CszUOzBr} Oin the temperature range between 1.5 and 2SK and compa red with a diffusion-limited transfer model. There are additional spectral features in ur anyl compounds. For example, opticall y active single crystals of N aUO z(CH 3COOh exhibit" a series of complicated vibronic Jines due to the p resen ce of two emi ssion centers, which are resolved by the difference of the degree of circul ar polarization in luminescence. Decay times of the luminescence of uranyl ~-diketonato complexes" in liquid solven ts ha ve been found to be in th e 1 to SOO-ns range; the drastic va riations are understood in terms of changes in the nonradiative rate cons ta nts correlated to the energy position of the zero-phonon emis sion lin e.

2.4.4 Platinum complex ion centers Platinum(II) and mi xed-valence platinum(ll, IV) com plex ions have also been investigated exten sively. The best known platinum(II) complex is a yellow -green comp oun d , ba rium tetracynoplatinate (II) Ba[Pt(CN) 4] ·4H zO (abbrev ia ted BCP), w h ich p ossesses a linear chain


212

* Trap >-.. ......

'(jj

c

, Sr2+ , Ba2+; B = Mgz+, Ca z+, Sr z+, Ba2 +). Wolf and Karnml erSack" reported infrared emission of rare-earth ion s incorporated into a ve ry com p licated com po und 18R-Ba6BizW3018' In this case, there are three W06&- ion sites in the compound w ith an hexag onal closed-packed p olymorphic struc tu re . The emission spectra consist of two bands at 21700 and 17000 crrr? due to two 6c sites and one 3a site, respectively. The corresp on d ing excita tion bands a re at 36000 cm' (6c) an d 29000 crrr-' (3a), respectively. The luminescence of W0 6&- ion s can also be seen in othe r materials su ch as Li6W0 6, 12RBazLazMgWzOlz, a nd Ca 3La zWZ0 12•

2.4.5.2 Perspective of other interesting centers The above-mentioned W06&-lumin escence center is on e of the closed-sh ell transition metal co mp lex ion s, ge ne rally expressed as (Mo6 ]n- (where M = Ti, Mo. Nb, Zr, Ta, an d W). Two papers-w on th e luminescence p roperties of MoO/- and MoO,,"- complexes have been published. Rec ently, luminescence from a e ur op ium octamolybd at e pol ymer, Euz(HzO)dMosOd 6Hz047 and the p icos econd de cay of the transient ab so rbance of (WlOOd 4- in ace toni trile" ha ve been reported. The lumin escence of ur anat e (U06&-) cent ers in solid s ha ve been review ed by Bleijenberg ,"? Thus far, thi s disc uss ion of luminescence centers of com p lex ions focuse d on practical phosphors. H owever, under the ca teg or y of complex ion s, a more general survey is p ossible. Compl ex com p o un d s consis t of a central met al ion and surround in g an ions or organic ligands . In th ese compounds, there are-in princi pl e-four possible luminescence processes that origi nat e from the cen tra l metal ion, from th e ligand, from ligand-to-metal cha rge-transfer (LMCT), and from metal-to-ligand ch ar ge-tran sfer (MLCT) tr an siti ons. Due to the se different tran sition processes, the luminescen ce fro m complex ions can eith er be sharp or broad, a nd can occur in a bro ad sp ectral region . Ur anyl complexes luminescing of green-yellow color are examples of cen tr al metal ion transitions. Eu(III) ~-diketonato complex, a typical NMR shift reagent, also shows bright an d sh arp red lumine scence due to the central Eu (III) ion. For more th an half a century, the luminescence of the Zinc( II) 8hydroxyquinolinat o com p lex ha s been sh own to be due to the aroma tic organic ligands. Emission transition s due to the LMCT sche m e is found in sche elite compounds. Phosphorescence due to MLCT tran sitions is predominant in com p lexes su ch as ruthenium(II) trisb ip yridyl ((Ru (bpYhF+), metal-phthal ocy anines (e.g., Cu-Pc, a fam ous pigment), and metallo p orphyrins (e.g., Mg-TPP). The latter two complexes are usually considered as organic phosphors because of Io -memberedn-ring structures.

Chapter two:


215

In the future, one will be able to design new phosphors of complex ion types that can be excited by va rious excitation sources such as hi gh electron beams, X-ray lasers, and NIR-Iaser di odes. Phosphors of complex ions will con tin ue to play a useful role in luminescence applications.

References 1. Morita, M., MoO/ -, WO/ - compounds, and on e-dimen sional com po un ds, in Hikaribussei Handbook (Handbook of Optical Properties of Solids), Shionoya. S., Toyoza w a, Y, Kod a, 1., and Kukirnoto. H. , Eds., Asa kura Shoten, Tokyo, 1984, ch ap . 2. 12. 6 an d 2. 19. 2. (in Jap anese) . 2. Blasse, G., Structure and Bonding, 42, 1, 1980. 3. Ballhau sen , C.]. a nd Liehr, A.D., J. Mol. Spectrosc., 4, 190, 1960. 4. Ziegler, 1., Rank, A, a nd Baerends, E.]., Chem. Pliys., 16,209, 1976. 5. Keb ab cioglu, R and Mueller, A., Chern. Phys. Leii., 8, 59, 1971. 6. Koepke', C , Wojtowica, A]., and Le rnpicki. A., f. Luminesc., 54, 345, 1993. 7. Blasse, G., Radi ati on less processes in luminescent materials, in Radiationless Processes, DiBartolo , B., Ed ., Plenum Pre ss, Ne w York, 1980, 287. 8. Bernh ardt, H.J., Phys. Stat. Sol.ta), 91, 643, 1985. 9. Rent, E.G., Opt. Spectrosc. (USS R), 57, 90, 1985. 10. Cr oen ink, I.A., H akfoort, C, and Blasse, G., Phys. Stat. Sol.ia), 54, 329, 1979. 11. Bohm , M ., Erb , 0 ., a nd Scharrnan , A. , f. Luminesc., 33, 315, 1985. 12. Herren, M. and Mor ita, M .,]. Luminesc., 66/67, 268, 1996. 13. Blasse, G. and Bokkers, G.,]. Solid. State. Chem., 49, 126, 1983. 14. Shi rak awa, Y, Tak ah ar a. T , and Ni sh imura, T , Tech. Digest, Phosphor Res. Soc. Meeting, 206, 15,1985. 15. Tew s, W , Herzog, G., and Roth , 1., Z. Phys. Chern . Leipzig, 266, 989, 1985. 16. Blasse, G., Verhaar, H.CG., Lammers, M.J.L Win gelfeld, G., H oppe, R, an d De Maayer, P., f. Luminesc., 29,497, 1984. 17. Koep ke, c., Wojtow icz, A .]., a nd Lernpicki, A , IEEE f. Quant . u«. 31, 1554, 1995. 18. H az enkarnp, M .F., Strijbosch, AW P.M., an d Blasse, G., f. Solid State Chent., 97, 115, 1992. 19. H erren, M., Ni shiuchi, H., and Morita, M., ]. Chem. Phys., 101,4461, 1 1)

'--'

o

L.

L'.3

>.. -2 L.

en ......

1)

C

LU

- 4

-6

L.

- 8 - 10

L6

- 12 A

r

X

U,K

Wave vector Figure 54 Band struc tur e of ZnSe. (From Chelikowsky, 1976. With permi ssion.)

J.R.

and Co he n, M.L., Phys. Rev., B14, 556,

As shown in Fig ur e 7(a) of 1.2, the valence band in th e Z8 struc ture is sp lit by th e spin-orbit interaction into a hi gher ly ing r s(A) s ta te (in w hi ch the orbital s tate is doubly degenerate) and a nondegenerat e r 7 (8) s ta te. In th e IN struc ture as shown in Figure 7(b), on the other h and, all the orbi ta l d egeneracy is lift ed by the spi n-orbi t in teraction and the aniosotrop y of crystal field , and the split st at es are r 9(A), r 7(8), an d r 7 (C) in descending order of en er gy. Th e case of ZnO is an exception: L9(A) and 1 7(8 ) ar e reversed, so that the order is L7(A), 19(8 ), and r 7(C) instead . Th is origina tes from th e fact that in ZnO the sp litting b y the spin-orbi t in teraction is negative an d sma lle r th an th at due to the crys tal field an isot rop y, unlike o the r IIb-VIb compounds . Th e negati ve spin-orbit splittin g ari ses be cau se of mixin g of th e d orbitals of Zn with th e va lence band . In MX-type compound se micon du ctors, the bandgap ene rgy Eg usually increa ses if M or X is replaced by a hea vier element. Looking at E~ values in Tabl e 20, it can be noted that this general rule is us uall y obse rved, except in the case of ZnO, where the Eg value is smaller than that of ZnS . Th is is also caused by the mixing of the Zn d orbital w ith the valence band. It is seen in th e band s tr uc ture of ZnSe, sh ow n in Figure 54, that in the cond uction band there are two minima in u pper en ergy regions at the L [k = (111)] and the X [k = (100)] points with energies o f 1.2 and 1.8 eV ab ove th e bottom of th e con d uc tion b and , res pec tiv ely. The co nd uc tio n band s truc ture of ZnS is very sim ilar, having th e tw o upper minima at the sa me p oints. Th e exis ten ce of these tw o upper m in ima plays an important role in th e exci ta tio n process of high-field, thin-film electro lumi nesc en ce in ZnS (See 1.10). The fact th at IIb -VIb compou nd s are direct-gap semiconductors means that they are appropriate ho st m at erials for phosphors. If one compares the radiati ve recombina tion coefficient of electrons an d h oles for direct and indirect transitions, the value for the form er is four orders of magn itude lar ger. In practical phosphors, the radiat ive emission is not caused by direct recombinati on , but by transitions taking place via energy levels of activators introduced as impurities. For imp ur ities as donors or acceptors, their ene rgy levels

Chapter two:


241

4.2K

~

Ci3

z

~

E-
-

90

I" \ MERCURY

80

, I

o 70 w

,

w

,

ct:

z 60

w

ARC

, I I I , I I I

I

, I I , I ,

50

> 40 ~

--' 30 w ct: 20

I I

I

I I I I

I

I

I

I

I I

I

I

I

I I

1

10

I

o'-__

..c......l_

_

-----L _ _

- L- l -_

---L_

-----...I.- L------=""'--'=="'--_

50005100 5200 530 0 540 0

.-J

5 500 5600 5700

WAVELENGTH (A) Figure 56 Spectrum of the edge emission of CdS at 4K; dashed line rep rese nts spectrometer window. (From Klick, c.c. J. Opt. Soc. AIIl ., 37, 939, 1947. With perrn ission .)

an 13 line d ue to excitons bound to io nized d onors is obse rve d at waveleng th a little shorter than 12, With increasing tem pera ture from 4.2K, bound excitons are released from impurities, so tha t only the lu min escen ce line due to free excitons is observed as can be seen in the figure . The binding energy of the exciton in ZB-type ZnS is 36 meV, so that exciton lu minescence is obs er ved up to room temperature. The exci ton binding energy in ZnO is as large as 59 meV and is the largest among IIb-VIb comp ound s. Lu m inescence of free exciton s is observed a t 385 nm at room temp erat ur e in p ure ZnO. Th is ult raviolet luminescence was fou nd as early as the 1940s} but it was not recogn ized a t that tim e that th is lu m in escen ce origi na tes from excitons . Th is lu minescen ce persists up to fairly high temperatures; it is still obse rved at temperatures as high as 770K ?

2.7.2.5

Type of conductivity and its con trol

As-grow n single crysta ls of ZnO, ZnS, ZnSL', and Cd S are usua lly n-type in conductiv ity, while th ose of ZnTe are p-ty pe. The cond u ctiv ity co ntrol of Ilb -VIb compou nd s, es peciall y for ZnSe, has made remarkable progress recen tly. This progress is due to the demand to develo p blue and bl ue-g reen light-emitting diodes and semicond uc tor lasers. Th e p rep aration of p-ty pe ZnSe wi th high conductivi ty has been a fund amental problem , w hich was solved rece n tly b y in trod uci ng nitrogen accep tors usin g nitrogen plasma (See 2.7.6). Prese n tly, it is possible to contro l the type of conductivity in mos t llb -Vlb co mpounds.

2.7.3 Luminescence of shallow donors and acceptors Sin ce th e 1940s, it has been kn ow n that pure Cd S crys tals show luminesce nce at low room tem perat ur e wi th a characteris tic sp ectra l structure on the low-en ergy side of the fundam en tal absorption ed ge. Th is luminescence was called edge emission. Its spectru m is shown in Fig ur e 56.8 It has been es tablished th at the characteristic edge emi ssion is ob served in all IIb-Vlb compounds except for ZnO.


243

The lines in Figure 56 are equally spaced, with an interval of about 40 meV, which is equal to the energy of longitudinal optical (LO) phonons in CdS. The halfwidth of the lines is approximately 5 meV. The relative intensities of the lines in the figure (numbered n = 0, 1, 2, ... from the short wavelength side) decrease toward longer wavelengths with ratios of 1.00:0.87:0.38:0.12:0.030:0.015. This ratio exactly obeys the Poisson distribution In = e-sSn/n' with S = 0.87. The n = 0 line is known as the zero-phonon line, while lines of n = 1, 2, ... are caused by simultaneous emission of 1, 2, ... LO phonons. It has been established that the characteristics of the edge emission are satisfactorily interpreted in terms of donor(D)-acceptor(A) pair luminescence (see 1.4.4). The transition energy E of this luminescence is a function of the distance r between 0 and A in a pair, and is given by: (37) where ED and E A are the ionization energies of a neutral donor and acceptor, respectively, and £. is the static dielectric constant. The transition probability W also depends on rand is expressed by (38) where r B is Bohr radius of the donor electron and W o is a constant related to the D-A pair. The mechanism for donor-acceptor pair luminescence was first verified in the edge emission in GaP doped with S donors and Si acceptors (see 1.4.4 and 2.8).9 The intra-pair distance r is distributed discretely, so that a spectrum consisting of discrete lines is expected. In GaP:Si,S, a great number of sharp lines were observed adjacent to the highenergy tail of the n = 0 line, and the value of r for each line was determined. On this basis, the main part of the n = 0 line is thought to be composed of a large number of unresolved pair lines for pairs with relatively large r values. A great number of sharp lines were also observed in the edge emission of CdS 10 ,11 and ZnSe. 12,n These facts present clear evidence as to the origin of the edge emission. In ZnSe, the identification of each line has been made in analogy to the GaP case; in CdS, the analysis is not easy to make since the spectra are much more complicated because of the W structure. Eqs. 37 and 38 indicate that the pair emission energy shifts to lower energies and the decay time becomes longer with increasing r values. Then one expects that in the timeresolved spectra of the edge emission, the peaks of the lines composed of unresolved pair lines should shift to lower energies as a function of time after pulse excitation. This has been observed in CdS,14 and presents further evidence for the pair emission mechanism in the edge emission. The fact that the relative intensity ratio of the edge emission lines obeys a Poisson distribution indicates that the configurational coordinate model (see 1.3.2) is applicable to each pair center with a different r value. The donors and acceptors participating in the edge emission are shallow. In these cases, the constant S appearing in the Poisson distribution, which is called the Huang-Rhys-Pekar factor and a measure of the strength of the electron-phonon interaction, is small, of the order of 1 or less; the phonon coupled to the center is the LO phonon of the entire lattice, but not a local mode phonon. The depths of donor and acceptor levels (ED and E A ) in IIb-VIb compounds are determined from bound exciton emission lines, edge emission spectra, or absorption spectra between a donor or acceptor level and the band. The available data on levels are


244 Table 21

Depths of Donor and Acceptor Levels, ED and E A (meV) in IIb-VIb Compounds Lia (a) Donor Br B Al Ga In F Cl Eo, calc

Zns

110 29±2

ZnSe ZnTe CdS CdSe a

25,6

100 25,6 18.5

33,9 20±2

27.2

28,2

28,2 35,1

26,2 20,1 32.7

33,1

33,8

32,5

32,1

21 28

Interstitial Li.

(b) Acceptor ZnS ZnSe ZnTe CdS CdSe

EA, cd ," 108 62

Li

Na

Cu

Ag

Au

N

P

As

150 114 60.5 165 109

190 102 62,8 169

1250 650 148 1100

720 430 121 260

1200 550 277

110

85,500 63.5 120,600

110 79 750

Note: Calculated values by the effective mass approximation' S, ED, . "k and E", cok' are also shown,

shown in Table 21. Calculated values of ED E A by the effective mass approximation are also shown."

2.7.4 ZnS-type phosphors 2.7.4.1

Luminescence of deep donors and acceptors

ZnS type phosphors such as the green-emitting ZnS:Cu,Al and the blue-emitting ZnS:Ag,Cl are very important from a practical point of view, especially as phosphors for cathode-ray tubes. Luminescence centers in these phosphors are formed from deep donors or deep acceptors, or by their association at the nearest-neighbor sites. In this subsection, a brief history of the development of these phosphors will be given first, and then the characteristics and the mechanisms of their luminescence will be explained,

(a) History, After the research by Sidot described in 2.7,1, it became gradually clear that when ZnS powders are fired with the addition of a small amount of metallic salt, luminescence characteristic of that metal is produced. In the 1920s, it was established that a small amount of copper produces green luminescence, while silver produces blue luminescence. In this sense, copper and silver were called activators of luminescence, The firing is made at 900 to 1200°C with the addition of halides (such as NaCl) with low melting points as fluxes, It was found that if the firing is made without the addition of activators but with a halide flux, blue luminescence is produced, Thus, this type of blue luminescence was called self-activated luminescence, In the 1930s and 1940s, research on ZnS-type phosphors was very active, Results of the research are described in detail in a book by Leverenz." published in 1950, In this book, the emission spectra of a great number of phosphors in ZnS, (Zn,Cd)S, or Zn(S,Se) hosts activated with Cu or Ag are shown. The spectral data shown in the book are still ver y useful. It should be noted that this book was written before the concept of the co-activator was conceived, so that chemical formulas of some phosphors given in the book are always not appropriate, and care must be exercised, For example, a phosphor written as ZnS:Ag[NaCl] should be written as ZnS:Ag,Cl according to the rule in use today The ZnS self-activate phosphor is shown as ZnS:[Zn] in the book, but should be ZnS:Cl(orAl) instead, as will be explained.

Chapter two:


245

In the lat e 1940s, Krog er and co-workers v" demonstrated that halide flu x ad ded in the fir ing p rocess to ZnS phosphors not only p romote s cryst al grow th, but introduces ha lid e ion s (VIlli gro up ani ons) into the ZnS latti ce, and that th ese hal ide ion s participate in the formation of luminescen ce cen ters, Kroger et al, ass u med th at the copper or silver activators are in the m on ovalen t state and subs titu te for Zn 2+ ion s, and that charge com pens ation for the m onova len t activators is accom pl ishe d by introducin g VIlli gro up anio ns subs titu ting for 51- ions, It w as sup po sed that charge com p ens ati on should occu r not only w ith VII grou p an ion s, b ut also with IIIb gro up ca tions, such as AP+, substituting for Zn 2 + ions, Kroger's group 19 clearly sho we d that if AP+ions are introdu ced w ithou t using halid e flu xes, simi lar kinds of lumi nescenc e are produced, and thus ev ide nce d the above ass u mption , The VIIb or Illb ions were called co-activators , These ions are ind isp ensabl e for th e formatio n of luminescen ce centers, but the luminescen ce spec tr um is d eter mined only by the kin d of lb ion activ ators and is almos t indepe nd ent of th e kind o f co-ac tiva tors. Thi s is the reason for the naming of co-ac tiva tors . In those d ays, the natur e of the electroni c tran sitions respo ns ible for th e lu m in escen ce in ZnS p hospho rs wa s active ly di scussed. The so-called Scho n- Klase ns m odel, first proposed by Schon and then dis cussed in detail b y Klasen s." ga ined ge ne ra l acceptance. Thi s mode l assu mes th at th e luminescen ce is caus ed by the recombination of an electron in th e cond uction band, wi th a hol e located in a level a little above the val en ce band . Prencer and Willia ms ?' pointed ou t th at Ib gro up activator s and VlI b or IIIb gro up co-ac tivators should be recogni zed , res pectively, as the accep tor s an d th e d on ors. It wa s assumed that donors and acceptors are sp ati all y associa ted in some way; then it was p roposed that the luminescence tak es place in cen ters of p airs of d onors (co-ac tiva tors) and accepto rs (ac tiva tors) associa ted at the secon d and th ird n eare st-neighbor site, and that the luminescence tra nsit ion occ urs from the excited sta te of d onors to the ground state of accep tors. This w as the first proposal for the d on or-acceptor pair luminescen ce concep t, w hic h was later recognized as a basis for underst and ing se m icond uc tor lu m inescence as mention ed in 2.7.3. The above narrati on tou ches upon the esse n tia l point s of the progr ess in resea rch in this area up until the 1950s. This resea rch was ac tive ly p ursued in the 1960s. As a result, the lumine scen ce mech an ism of ZnS-typ e phosphors using activ ators of Ib eleme n ts has been elucida ted qu ite thorou ghly. Th is will be d escrib ed below.

(b) Ctassifica tion and emission spectra. The luminescence of ZnS- ty pe phosphor s us ing Ib grou p activators (Cu, Ag ) and lIIb (AI, Ga, In ) or VIlli (CI, Br, I) group co-ac tiva tors can be classi fied in to five kin ds, de pending on th e relative ra tio of th e concentration s of activators (X) and co-ac tiva tors (Y). This condi tion is shown in Figure 57.22 The rang e of conc entrations for X and Y is 10-6 to 10~ mol /mol. Th e labe ls of th e lu m ine scen ce in th e figure or igin ate from the emission color in the case that th e activa tor is Cu; that is, G = green, B = blue, and R = red. R-Cu,In ap pears onl y when the co-act iva tor is a IIIb gro up element. SA me an s the self-activated blue luminescen ce. Fig ure 58 de picts the em ission spectra of these five kinds of phosph or s at room temperature and at 4.2K.23 As sh ow n in Tabl e 20, the bandgap ene rgy E ,~ of Zn S is 0.08 eV larger for the W structu re tha n for the ZB structure. Corresponding to 'this, the emiss ion peaks of ph osphors wi th the W s tructur e are sh ifte d by almos t this amo u nt toward shorter wavelength. In ge neral, ph osphors p rep ared b y firing above 1000°C have the W struc tu re, whil e those below this ha ve the ZB structure. Emissio n peaks at room temper ature ar e located at lon ger wa velengths than th ose a t 4.2K, excep t in the case of th e SA luminescen ce. The lon g waveleng th shift is almost p roportional to that of Eg . The SA luminescen ce shows the in verse beh avior; that is, the peak at room temperature is locat ed at shorter


246

eo

o

t ,;/

/

/

/

Equal cone . line

- - - log [ YJ

Figure 57 Five kinds of luminescence in ZnS phosphors classified from the point of view of the relative ratio of the conce n trations of activa tors (X) and co-activators (Y). G-Cu: gre en Cu. B-Cu: blue Cu . R-Cu : red Cu, R-Cu, In: red Cu . In, SA: self -acti vated blue. (Fro m van Goal, w., Philips Res. Repi. Suppl., 3, 1, 1961. With permission. )

wavelengths. These emission spectra a re almost independent of the kind of co-activators, except for the case of the SA luminescence. The G-Cu emission spe ctra of ZnS :Cu ,AI shown in Figure 58 are almost the sa m e as th ose of ZnS:Cu,Cl . In the case of the SA luminescenc e, th e spectra of ZnS:CI an d ZnS:AI are a little different. The spectrum of ZnS:AI is slightly shifted to longer wa velengths. If the activator is changed from Cu to Ag, the emi ss ion peaks a re shifted by 0.4 to 0.5 eV to sh orter w avelengths. Th e blue luminescence of ZnS :Ag,C1 (peak at 45 nrn , W typ e) corres pond s to the G-Cu luminescence. Au , a Ib group element, also acts as an acti vator. The luminescence of ZnS:Au,Al corresponding to the G-Cu luminescence has its p eak a t 550 run in the ZB structure, which is shifted slightly to longer wavelengths relative to ZnS:Cu,AI. ZnS an d CdS, and also ZnS and ZnSe, form binar y allo ys (solid solutions) with relatively sim p le properties . Eg cha nge s almost in proportion to the compositi on . For example, E, in (Zn,Cd)S (W) changes from 3.91 eV for ZnS to 2.58 eV in CdS, almost in proport ion to the ratio of Cd. The five kinds of luminescence discussed above also appear in the allo ye d materials an d have similar properties. In Zn,Cd 1_xS:Ag,C1 (W), the emi ssion peak ch anges almost p roportionally to Eg , i.e., change s from 435 nm for x = 1 to 635 nm for x = 0.4 continuously. It is possible to obtain a d esired luminescen ce color from blue to red by sim p ly adjusting the composition . In ZnS ,Se1_,:Ag ,CI, the si tua tion is similar, but th e change of the em ission peak is n ot alw ays proportional to Eg and sometimes a we ak subband appears. Among the five kinds of luminescen ce discussed abo ve, the important on e for practi cal use is the G-Cu luminescence, which is produced when the conc en tra tions of the activat or and co-activator are ne arl y equal; in thi s case, charge compensation is readily and simply attained. ZnS:Cu,Al (green-emitting) an d ZnS:Ag,Cl (blue-emittin g) phospho rs are

Chapter two:


247

Wavel en gth (nm) 800 900 1 000

Figure 58 Spe ctra of the five kinds of lu m inescence in znS phosp hors a t room tem perature (so lid line) an d 4.2K (do tted line). Activ at ors and co-activ at ors of the se ph osphors and th e crystal struc ture are shown below. (Shion oya, S., Kod a, T., Era, K , an d Iujiwara, H ., I. Pilys. Soc. Japan , 19, 1157, 1964. With pe rmission .) Luminesce nce G-CLI B-CLI SA R-Cu R-Cu,ln

Activa tor

Co -activator

Crystal s truc ture

Cu Cu

AI

W ZB ZB W ZB

I

Cl Cu C ll

In

extrem ely im p o rt ant in CRT app lica tions. ZnS:C u,Au,AI (g reen-e mi tting) phosph or s in wh ich both Cu and A u are used as ac tiva tors a lso fin d usage in th is area . The excitation sp ectra for these five kinds of luminescen ce consis t of tw o band s in all cases. The fir st one, h aving th e peak a t 325 to 340 rim, correspon d s to the fund am ental absorp tion edge (or th e exc iton posi tion ) of th e ZnS host crystal . and is called th e host exci ta tion ban d . Th e se con d , h avin g th e p eak at 360 to 400 nm in the longer w avelen gth region , is characteris tic of the luminescence cen ter, an d is ca lled the ch aracteristic exci tation ba nd . This band is p rod uced by th e transition fro m th e g ro un d s ta te of th e ce n ter (corresp on d in g to th e acce p tor level ) to the excited sta te of the ce n ter (corresponding to th e donor level) or to the conducti on b and. As an example, the excita tio n sp ectra for th e SA lu minescence in a ZnS:CI single crysta l (Z8) are show n in Figure 59(a) .I«.24 An absorp tion spec trum of the crys ta l and the absorp tio n band of the SA center a re show n in Figure 59(b) for com parison. (c) A tomic structure of luminescence centers and luminescence transitions. T he a to m ic s tr uc tur e of the luminescence centers and the nature of luminescence tran sitions for th e five kin ds of luminescen ce mention ed above were eluci da ted in 1960s, mostly by th e resea rch of Shio n oy a and co-workers, as wi ll b e describ ed below . Experim ental tools th at pl ayed important roles in clarifying these s ubjects in clu ded measurements of th e polariza tion of luminescence light using phosphor si ng le crysta ls grown by melti ng p owder p hospho rs under high argon p ress u re" an d m easurem ents of time-resol ved e m ission spectra . The essen tial ch aracteristics deri ved fro m the results of these m easu rements, as well as th e


248 Wavelength (nm)

300

320

340

360 380 400

10 -

440 ( a)

C

' Vi

c

"'"sy mmetry. In ZnSe, the cen ter cor res pond in g to the B-Cu center in ZnS sho ws gr een luminescence. Measurements of op tically detected electron spin resonance indicates that the center has th e stru cture of a pair composed of a s ubstitu tional Cu" and an interst itial CU+.36 It is clear th at the B-Cu center in ZnS h as th e sa m e typ e of structure and ha s C31' sym me try. Th e B-Cu luminescen ce shows no sp ectra l shift, indicating th at it is du e to an in tra-cen ter transitions. H ow ever, th e nature of the initial state of the transition is not clear.

(d) Other luminescence characteristics (i) Stimulation and quenching. It has be en known sin ce th e beginning of this cen tu ry that Cu-activated gree n ZnS phosphors show di stinct s tim ulation or quenching of the lum ines cenc e if irradiated b y red or near-infrared (NIR) light while under ultraviolet excitation or during the luminescent decay following excitation. Whether stimulation or quenching occurs depends on the conditions of ob servation, i.e., the ratio of the intensity of the excitati on light to the red or NIR light and on the temperature. Exposure with red or NIR light during de cay usually results in stimulation first, changing to quenching as time

Chapter two:


257

progresses. The spec tra of the light p ro ducing stim ula tion and qu en chin g h ave t w o p eak s at 0.6- 0.8 and 1.3 11m. It is seen from Figures 64 an d 65 that these two pe aks corresp ond to th e ind uced absorption bands C and Of indica ting th at absorp tion b y excited Cu activa tors (Cu 2+ ion s) is res po ns ible for these phenomena. By measuring time-resol ved emission spec tra an d phot oconductivity under irr adiation with red to NIR light, the m echanisms for s tim u la tion and quenching ha ve been es tab lishe d ." H oles are cre a ted in th e va lence band by in d uce d absorp tion of th e C and D bands (in th e case of the C ba n d , onl y at room temper ature). These holes m ove in the va lence band and a re tra p ped b y unexcited Cu activa tors . If Al (or CI) co-activ ators exis t in the vi cinity an d h ave tr apped electrons, g reen luminescence is produced im med iat ely. In O-A type lum in escence, th e tran sition probab ility is larg er when th e in tra pair di stance is smaller, so th at this imme d ia te luminescen ce p rocess occ urs. Und er the s tim ula tion irr ad iat ion of red to NIR ligh t with ultravi olet excitat ion, th e average value of th e intra-pair di st ance for excited Cu-AI pa irs becom es sho rte r, increasing the average tra ns ition p roba b ility and producing stim ula ted luminesce nce . On the ot her han d, qu enc hing is cau sed by a p rocess w here holes crea ted in th e va lence ba nd are trapped by vario us nonrad iative recomb in ation cen ters, thus de creasin g the effective number of hol es .

(ii) Kille r effect. It h as also been kn own since the 1920s th at the lumine scen ce in tensity of ZnS p hosphors is greatl y reduce d by con tamination with very sm all amounts of th e iro n gro up elemen ts Fe, Ni , and Co . Beca use of it, the iron gro up eleme n ts were called killers of lu m inescen ce. It follows that it is very important to re move iro n gro up eleme n ts in th e manu facturing processes of Zn S phosph ors. Figu re 67 sho ws how con tamina tion by Fe 2+, N i2+, and C 0 2+ reduces the in tens ity of the green lumine scen ce in Zn S:Cu ,Ap a The res ult s for Mn 2+, w hich also bel ongs to the iron gro up , are also sho wn . In th is case, an or ange luminescen ce of Mn 2 + is p roduced , as will be de scribed in 2.7.4.2; the Tv1n2+ in tens ity is also shown in the figure. The iron gro up ions have absorp tion band s in the visibl e region, an d their spectra ove rla p the G-Cu luminescen ce spectru m. It is th ou ght th at resonance en ergy tr ansfer from excited G-Cu centers to iron grou p ion s takes place, redu cing the G-Cu lumine scence intensity and caus ing the killer effect. The overlap betw een the lu minescen ce an d th e absorp tion spectra of iron gr oup ion s ob tained us ing sing le crys tals, an d the d ecrease in the green lu minescen ce intensity ca used by energy tran sfer w ere calcul at ed. The results are sh own in Figure 67(b) by th e d otted lin es. The de crea se of the luminescen ce inten sity in Mn 2+ is we ll de scribed b y this en ergy transfer effec t. In the case of Fe 2+, Ni2+, an d C 0 2+, the actual in ten sity decrease is considerably more th an th at w hic h was calc ula ted . As the reas on for th is disagree men t, it is sugges ted th at iron gro up ion s create d eep levels, and electrons and hol es recombine nonradiatively via the se levels. (iii) Concentration quenching and luminescence saturation. In ZnS :Cu,AI phosphors, if the concen tra tion of activat ors and co-acti va tors is increased to obtain br igh ter phosphors, concen tration qu enching results. Th e op tim um con centration is Cu : 1.2 x 10-4 mol/mol and AI: 2 x 10-4. The se phosphors sh ow, w hen used in CRTs, th e phenomenon of lu minescence intensity satura tion when the curren t d en sit y is raised, as show n in Figure 68.39 A very sim ilar ph enomen on occurs in ZnS:Ag,AI.4o Luminescen ce satu rati on phenomena p resen t serious p robl ems in the practical use of these p hosphors for CRT p urposes. The cause of the concen tration qu enching and luminescence saturation is not we ll understood , b u t the n onradiat ive A uger effec t is thou ght to pl ay an importan t ro le." In th is effect, excited Cu-AI pa irs are annihilated n onradiatively, th eir en ergy is tr an sfe rred

258


1.0.,.----.--------....,..,.-....,,..---------------,

~

~

conventional DMZn

CJ)

z

UJ ~

Z

...J

0W

>

Ex

~

...J

ur

a:

2.76

high purity DMZn

2.78 PHOTON

ENERGY

2.80 (eV)

Figure 71 Pho toluminescence spectra at 10K of ZnSe films grown using a conventi ona l DMZn source in whi ch about 15 ppm chlorine impurities are involved (upper trace), and using a purified source for which the ch lorine content is below the detection limit of 5 ppm. The em ission line label ed Ex is du e to free excitons, and emission lines 12 and I, are due to exciton s bound to neutral accep tors . (From Kukirnoto, H., J. Crystal Growth, 101, 953, 1990. With perrnission.)

grow th . A (quadrupole) m ass spectrometer is essential for monitoring the gas composition in the MBE gro w th cha mber. Early ZnSe-based laser diodes show room-tempera hire and con tin uous wave (RTCW) lifetimes of the order of a minute because of degradation cau sed by extende d crystalline defects such as s tacking faults . Transmission electron microscopy (TEM) im aging indicates that the degradation or iginates from di slocation networks that dev elop ed in the quantum well region during lasing. The dislocation networks w ere produ ced by th e stacking faults nucleated at the II-VI/GaAs interface and extending into th e II-VI lay er. To reduce the stacking fault den sity, incorporation of GaAs and ZnS e buffer lay ers and Zn treatment of the II-VI/GaAs interface were employed." The low est defect density film s were reported to be obtained wh en the (2X4) As-stabilized GaAs su rface was exposed to a Zn flu x, which resulted in (2X4) to (l X4) surface reconstructions. Thi s was then followed by the epita xial gro w th of ZnSe. Stacking fault densities of 103 crrr? or less were ach ieved und er th is grow th condition.

2.8.3 n-Type doping Group III atoms such as Al and Ga substituting in Zn sites and G roup VII atoms such as CI and I in Se sites are typical impurities producin g n-ty pe carr iers in ZnSe crys tals.

268


n-Type doping in ZnSe during MBE growth h as been extensively stu di ed . Ga impurities ha ve often been used as a donor dopant altho ugh ma ximum ca rrie r concentrations are limited to approximatel y 1017 crrr-'. The photoluminescence (PL) properties of Al- or Gadoped la yers shows a remarkable degradation of the band-edge em ission when the carrier co ncen tra tion exceeds 10 17 crrr-'.? Cl impurities w er e also studied as a d onor dopant at the Se site.'? In Cl doping, n-type carrier concentra.tion increases with th e temperature of th e Zn Cl 2 cell. The maximum ca rrier concentration has been estimated to be 10 19 crrr' , resulting in a sm all resistivity of 10 °' Dcm. Lately, Cl im p u ri ties have been used to fabricate bluegreen laser diodes and light-emitting diodes because of advantages presented by controllab ilit y and crystallin e quality.

2.8.4

p- Type doping

Group I atoms such as Li and Na at Zn sites and gr oup V atoms such as N, P, and As at Se sites are typical impurities us ed to produce p-type ca rr iers in ZnSe crystals , Net acceptor concentrations (N,\-N o) of 10 1( cm-' have be en ac hiev ed using Li d oping at a growth temperature of 300°C. 1l Capacitan ce-voltage (C- V) profil ing is usually used to measure N A-N o, which implies uncompensated acceptor concen tra tion. When th e Li impurity concentration (N A) exceeds 10 17 crrr', N ,\-N o decreases due to increased com pensa tion . This com p ensa tion is thought to origin a te from in creased concentration (N o) of Li interstitial donors in heavily doped ZnSe. Lithium doping is also problematic in that lithium atoms ca n ea sily diffuse within th e epitaxial layer. Hi ghly resistive ZnSe films h av e been grown w ith As an d P doping . A first principles total energy calculation suggests th at two neutral accep tors co mbi ne to form a new deep state that results in the high resist ivity of As- and P-doped ZnSe.12 Exp erimental results, which show p-type conduction is difficult in As- o r P-d oped ZnSe, are con sist ent with this proposed model. An important breakthrough came with the deve lopment of a N 2 plasma so ur ce for MBE.13·14 Th is technique empl oy s a small helical-coil RF plasma chamber re p lacing the Knudsen cell in the MBE chamber, The active nitrogen species is thought to be eith er neutral, mon oatomic N free radicals, or neutral, excit ed N 2 mol ecules. Th is techn ique ha s been u sed to ach ieve N A-N o = 3.4 x 1017 crrr' and blue emission in LEOs. Thi s ad va nce was rapidly followed by the first Z nSe-based laser. Ma ximum net acceptor con centration ha s been limited to around 1 x 10 18 em>' in ZnSe. At present, ho w ever, the nitrogen -p Jasma doping is the best wa y available to a. chieve p-type ZnSe an d has been most frequently used to grow p-n junctions by MBE and to fabricate ZnSe-b ased laser diodes. N incorporation d ep ends on the growth temperature and the plasma power. Increased N incorporation is found with low growth temper ature and hi gh RF p ower. Photoluminescence (PL) spectra in lightly N-doped ZnSI' la yer s w ith co nce n tra tions less than 10 17 crrr-' show a neutral acc ep to r bound-exciton em iss ion and a weak em ission due to donor-a cceptor pair (OA P) recombination. With in creasing N con centration , up to 10 1 ~ crrr', OAP em iss ion became dominant in the PL spectrum. This highly N-dop ed ZnSe sho ws a p-type con d uction as confirm ed by capacitance-voltage and Van de r Pau measurements. From PL an a lys es of th e exci ton ic and DAP e miss ion s, th e N- acceptor ionization en ergy w as estimated to b e ab out 100 meV, which is in good agreemen t w ith the result calculated with an effective ma ss approximation .

2.8.5 ZnS e-based blue-green laser diodes ZnSe-based blue-green laser diodes have been s tud ied int en sively to be applied in nextgenera tion, high-d ensity optical di sk m emories and la ser printers. Since the first demon-

Chapter two:


269

stration of II-VI blue-green laser diodes ." further improvemen ts in mat erials qua lity coupled with the use of w id e ba ndgap ZnMgSSe qua ternary alloys for im p ro ved elec trical as well as optical confinement and th e developmen t of oh m ic con tac ts to p-type laye rs have led to room-tem p era tu re (RT) CW opera tion of ZnSe-ba sed laser di odes with very reduced thr esh old currents and voltages has bee n ach ieved ." Th e firs t elec trica lly injected ZnSe-based laser was obtained using ZnSSe cladd in g layer s latt ice-m a tched to the GaAs subs tra te an d a ZnCdSe single quantum we ll surro un de d by Z nSe w aveg uide layers . The b and structure in the strain ed-l a ye r ZI1n H1CdolsSe/ZnSe sys tem wa s th ou ght to be a type I quantum well s truct ure wi th cond uction an d va len ce ban d offse ts of L\E c = 230 meV and L\E v = 50 meV, resp ectively. Acco rd ing to a com mon anio n ru le, th e con duc tion band offset is relatively la rger than that of the va lence band in th is sys tem. Op tical an d electrical confinement in th is pro totyp ical laser structure is quite w eak d ue to th e cons train t in the de vice d esign by the la rge lattice m ism a tch between ZnSe and CdSe. The use of the latti ce-m at ch ed qua ternary ZnMgSSe allow s grea ter refrac tive index and ba n dgap differ en ces to be realized . Th e inco rporation of Mg in to th e cladding layer impro ves th e confi nemen t factor, resulting in the RTCW op era tion of the II-VI lasers. Shor ter-wavelength lasers with a ZnSe ac tive layer have also been mad e possible. A typical s truct ure of the ZnCdSe /ZnSSe /ZnMgSSe sepa ra te-con fine me n t he terostructure (SCH) lase r is sho w n schema tically in Figure 72.16 Th e incorpora tion of GaA s :Si and ZnSe:Cl buffer layers and the Zn beam ex pos ure on a n As -s tabi lized surface of the GaA;' b uffer laye r were employed to red uce s tack ing fault d en sity. The s tack ing fau lt d en sit y of the laser s tructure was es tima ted to be 3 x 103 cm-2 . For th e p- and t l-: Zn 1_, Mg,SySe 1_y cladd ing layers, d esigned for optical confi nemen t, the Mg concen tra tio n "vas nominally x = 0.1 and the sul fur concen tra tion y = 0.15. The Cd comp osi tion of 0.35 in th e 'ZnCdSe active layer results in lasing w ave length A = 514.7 nm. Low -re sista nce quasi-ohmi c con tac t to p-ZnSe :N is usually achieved usin g heavily p-doped ZnTe:N and ZnSe /ZnTe m u ltiquantum w ells as an intermediate layer. The threshold current under CW operation wa s found to be 32 m A, corresponding to a th reshold cur ren t densi ty of 533 A crrr-', for a laser d iode with a stripe area of 600 11m x 10 urn an d 70 /95°ft) h igh reflectiv e coa tin g. The th reshold voltage was 11 V. Currently, th e life time of laser diodes op er ating at a tem p era tu re of 20°C has been reported to be 101.5 hours, the longest for ZnSe -based Jaser diodes .'? The spectacu lar progress in edge-emitting laser s h as stimula ted exploration of more ad vanced designs such as the ve rtica l-cav ity sur face -emitting lasers (VCSELs) opera ting in the blue-gre en region. VCSELs ha ve recentl y attracted m u ch atten tion becau se of thei r sur face-norma l op eration, potential for extremely low thr eshold curren ts, and th e eas e with which they ma y be fabricated in closely spaced and tw o-d imension al arra ys. Th ese lasers ar e ideal for integration wi th othe r devices suc h as transist ors for ph ot on ic sw itch ing app lications. Outpu t character istics s uch as n a rrow diver genc e beams and opera tion in a single longitud inal mode, d ue to th e large mod e spacing of a short cavity, are ad di tion al adva ntages . Blue-green VCSELs have experience d significan t progress recently. For example, electrical pumped ope ra tion has bee n demo ns tra ted a t 77K.ISThe VCSEL s truc tu res used we re con sistent w ith a CdZnSe /ZnSe multiqu antu m- w ell (MQW) active layer, 11- and p-ZnSe cladding layers, and two Si0 2 / Ti0 2 d istribu ted Bragg re flec tors (DBRs), as shown in Figur e 73. The reflec tivi ty of the Si0 2 / Ti0 2 dielectric mirrors was grea ter th an 99%. Th e VCSEL devices were cha racterized at 77K under pulsed operation. A very low th reshold current of 3 rnA was obtained in th e VCSEL. Sin gle lon gi tu d inal mode opera tio n was obtained a t the lasin g wavelength of 484 nm. Above th e thr eshold , the far-fie ld ra d iation angle was as narrow as 7°, which indicated the spa tial coherence exp ected for VCSEL

270

Fundamentals of Phosphors Pd I Pti A u ele ct rode insulator r-/'''----7I'-------"""L-

Z nT e :N

tL2000

Bandgap' (eV)

D

7.2 3.8

ID ID

2.0 1.6

Effecti ve m as s" Electro n

1.2

3.26 2.40

1525

D ID

6.20 2,45

0.29

-

5.662

-

3.60

1740

ID

2.15

ZB

6.136

-

4.26

1080

10

1.63

GaN

W

3.189

D

3.39

0.5 1.56 (11), 0.19 (.i) 0.39 1.64 (II), 0.23 (.i) 0.22

GaP

ZB

5.451

-

4.13

1465

10

2.27

0.25

GaAs

ZB

5.653

-

5.32

1238

D

1.43

0.0665

GaS b

ZB

6.096

-

5.61

712

D

0.70

0.042

InN In P

W ZB

3.533 5.869

InAs

ZB

6.058

4.980

5.185

5.693

6.10

'J

Pro p erties o f HIb-Vb Compounds

6.88 4.79

- 1200 1070

D D

0.6-0.7 1.34

0.04 0.079

5.67

943

D

0.35

0.023

H ole

>j::,.

Mobili ty (ern? V-I S-I) Elect ron

H o le

Die lec tri c constan t Sta tic £0

Optica l e;

7.1 6.85 (Lc), 5.09 (lie) 11

4.5 4.95 (Lc), 4.10 (llc) 8.2 10.2

0.51 (h), 0.2 (I)

0.63 (h), 0.20 (I) 0.5 (h ), 0.26 (I) 0.5 (h) , 0.11 (I) 0.8 0.67 (h) , 0.17 (1 ) 0.475 (h), 0.087 (I) 0.32 (h), 0.045 (I) 0.45 (h ), 0.12 (I) 0.41 (h), 0.025 (1 )

80

Refractive index" 2.12 (0.589) 2.20 (0.05) 3.0 - 3.5

8.5 9.8

4.8 7.5

2.25 (0.4) 2.99 (0.5)

180

290

10.1

8.2

3.2 (0.56)

200

400

12.0

10.2

3.45 (1.1)

5.4

2.00 (0.58)

300

100

9.5 (ole), 10.4 (lIe) 11.0

9.1

5.19 (0.344)

8500

400

12.9

10.9

3.66 (0.8)

4000

1400

15.7

14.4

3.82 (1.8)

1200

"'i'"j

:::

5.

:::. ~

~

S-

4000 4600

650

15.0 12.6

6.3 9.6

3.33 (1.0)

33000

460

15.2

12.3

3.52 (3.74)

Vi' ~ 'V

~

0

VI

"'l::l

5"

Vi

Tabl e 23 La tti ce ca ns t. (A) C rys ta l Ma te ria l struct u re" a c InSb

ZB

6.479

-

Den si ty (g crrr') 5.78

Melting p oint (OC ) 525

Band Bandgap ' structure" (eY) D

0.18

, Z B: zin c-bien de , H: hexagonal , W: wurtzite . 0: direct type, 10: ind irect ty p e. , At 300K.

b

d

II, .1: pa ra llel and

perpend icular to th e principa l ax is; h and I: heavy an d light holes .

" Wave lengt h urn in paren thesis.

n

Prop er ties of Illb-Vb Com pou nds (continue d) Effec tive mass:' Elec tron 0.014

Mobi lity (cm2 Y-1 S-l)

i5 -;:;. Di electric co nsta nt

H ole

Electron

H ole

Static

0.40 (h) , 0.016 (I)

78000

750

16.8

Eo

Optica l E••

Re frac tive index"

15.7

4.00 (7.87)

'"-e

~ ~

"0

"" ' 3 " -B' ;'2,.

""1:::!

;:,

N

Z C

.:

/"'-

r 4." f-

c

(J

0

17

10

•

• -

f--

(J (J

CI>

(J

c 0

gJ :J

.... CI> C

0

c

N

0

'.'./ '

f-

....

CI>

0/./

o/~/:/

()

e-

0

0 0 ...ro ...c. .......c ro

Nzn

/A

>>
5 kA cnr? urrr" ) or a high -outpu t power d en sity (> 1 m W urrr") or for p ho todiodes used for low- n oise d etection of very weak op tica l sig nals. The bandgap of a specific qua ternary crys tal can be varied widely while com pletely maintaining the lattic e match to a binary crys tal used as a subs tra te, as show n in Figure 83. An example is Ga) nl_xAsyPt_y, wh ich uti lizes InP (a = 5.8696 A) as a substra te; the ban d gap can be changed in the region of 0.7 :S Eg:S 1.35 eV when the composi tion is adj usted along the vertical line. The correspondi ng emission wavelength ra nges from 0.92 to 1.67 urn . The ternar y ma teria ls lattice-ma tche d to the InP substrate are Alo.47Ino53As and Gao47Il\Js3As. 291

292

Fundamentals of Phosphors 2.5,-----------------------,

AlAs

?-----,

2.0

,

s~ UJl:n

1.5

>. Cl

:;;

c:

W

a.

'" c: CD '"

Cl "0

1.0

~,

GaNAs

GainNAs

"

GalnAs

0.5 InAs

a 5.5

5.6

5.7

5.8

5.9

6.0

6.1

Lattice Constant a (Al

Figure 83

Diagram of lattice constant vs. bandgap for several compound semiconductors.

Possible compound crystals corresponding to light emission of 0.8 to 211mare as follows: 1. 2. 3. 4. 5. 6.

Ga)n1_xAsyP1_y (InP): (Ga 1_xAIJ)n1_yAs(lnP): Gal_xAlxAsySb1_y(GaSb): Ga)nJ_xAsySb1_/lnAs): Ga)nl_xAsySbJ_y(GaSb): Ga)nl_xNxAsl_x(GaAs):

0.92 < Ag < 01.67 (11m) 0.83 < Ag < 1.55 (11m) 0.8 < Ag < 1.7 (11m) 1.68 < Ag < 2 (11m) 1.8 < Ag < 2 (11m) 1.1 < Ag < 1.6 (urn)

The binaries in the parentheses indicate the substrates to be used. Crystal growth of these materials is possible with a lattice mismatch ±0.1 % or less. Among these, the heterostructure composed of Ga)nl_xAsyPl_x and InP has been widely employed as a material for semiconductor lasers or photodiodes for lightwave systems. The relationship between x, y, and the bandgap energy associated with Ga)nl_xAsyPl_Yf which are lattice-matched to InP, can be expressed as follows.

x=

0.466y (0 :s; x :s; 1) 1.03 - 0.03y

(41)

(42) which was phenomenologically obtained by Nahory et al.' The values of x and yare no longer independent of one another, since the lattice constant must be adjusted so as to be matched to that of the InP substrate, 5.86875 A. Consequently, the bandgap energy can be expressed by specifying the Ga or As contents. The band-structure parameters of GalnAsP IInP are summarized in Table 27.2

Longer-wavelength materials. Fluoride glass fibers have found use in long-distance optical communication in the 2- to 4-l1m wavelength range. Signal loss in fluoride glass fibers is predicted to be one or two orders of magnitude lower than that for silica fibers. Also, this spectral band is important for LIDAR (Light Detection and Ranging) and optical


293

Table 27 The Band Structure Parameters of Ga)n1 _,AsrP1 _yll nP

Parameter

Dependence on the mole fractions x and y

Energy gap at zero dopin g Heavy-hole mass Light-hole mass Dielectric constant Spin-orbit splitting Condu ction-band mass

Eg leV] = 1.35 - O.72y + 0.12y2 m hJ1I m o = (1-y)[0.79x + 0.45(l -x)] + y[0.45x + O.4(l -x)] m;h I m o = (1-y)[0.14x + 0.12(1- x)] + y[0.082x + 0.0261(1- x)] e = (l -y)[8.4x + 9.6(l- x)] + y[13.1x + 12.2(l-x)] 6 leV] = 0.11- 0.31y + 0.09x2

rn, I rna = 0.080 - 0.039y

From Ag raw al, G.P. and Dutta, N .K., Long-uxnielengt): Semiconductor La, crs, Van Nostran d Reinhold, Ne w York , 1986, 85. With permi ssion .

sensing. A potential material system to cover the wavelength range from 1.7 to 5 urn is GalnAsSb l AlGaAsSb.

2.11.2

Determination of GalnAsP/InP solid compositions

First, a review of the general concep ts of crystal preparation for GalnAsP latticematched to InP, which ha s been co m m o n ly u s ed in light-emitting devices. Ga)nl_xAsyPJ_r contains two controllable parameters, enabling independent adjustment of the lattice constant and the bandgap energy. The lattice constant a(x,y) of Ga)nl_xAsll_y is g iven as follows:

a(x, y) = a(GaAs)xy + a(GaP)x(l- y) + a(InAs)(l- x )y + a(InP)(l- x)(l- y)

(43)

According to measurements by Nahory et al..' the binary lattice constants are: a(GaAs) = 5.653 A, a(GaP) = 5.4512 A, a(InAs) = 6.0590 A, and a(InP) = 5.8696 A. The following equation is obtained by inserting this data into Eq. 43:

a(x, y) = 0.1894y - 0.4184x + O.013xy + 5.8696

(A)

(44)

The relation between x and y, therefore, is given by the followin g equation, when th e a(x,y ) coincides with the lattice constant of InP :

0.1894y - O.4184x + 0.0130xy

=0

(45)

Usually, Eq. 45 is approximated as:

x = 0.467y

(46)

According to the theory by Moon et aJ.3 an d experimental res u lts, the relation between the bandgap energy and compositions x an d y is given by:

Eg (x, y) = 1.35 + 0.672x -1.091y + 0.758x 2 + 0.101y 2

(47)

-O.157xy - 0.312x 2y + O.109xy 2 The bandgap energy calcu la ted in terms of x an d y using Eq . 47 agrees with the phenomenological results of N ahory et al.'


294 3.0 Indirect Gap Surface

Direct Gap Surfac e

-:

2.0

1.0

2.0

- x____ >

~

Indirect Gap Region

\ , /Lattice

OJ

W

" InAs

Lattice Match to GaAs

Match to InP GaAs

Figure 84 Bandgap energy V S, compos ition s x and y in Ga)nl_xAsI'PI _y' (From Casey, H.C. and Panish , M.B., Heterostructure Lasers, Part B, Academic Press, New York, 1978. With permission .)

The bandgap energy vs . composi tions x and y is illus tra ted in Figure 84.4 With the aid of this fig ure , one can obtain th e band s truc tur e of Ga inAsP la ttice-matched to InP for the en tire set of allowed compositions of y. The bandgap of GainAsP in the vicinity of GaP is seen to be ind irect in th e figure.

2.11.3 Crystal growth Liquid phase epitaxy (LPE). In the case of liquid phase epi taxy, on e has to determine the liqu id com position of an In-rich melt in thermal eq uilib rium wi th the solid phase of th e desired x an d y com position s for Ga)nl _xAsyPl_jr The As comp osition y in th e Ga)n J_xASyPl_y solid of the desired bandgap energy is given by Eq. 42 when its lattic e cons tan t is eq ual to that of In P. The Ga composition x is ob tained by Eq. 46. In this way, the atomic frac tions of Ga, As, and P in the In-r ich m elt that exis ts in equi librium with the desired Gaxlnl_,AsyP I_y solid can be obtained . The actua l weights of InP, In As, and GaAs per gram of In can be es tima ted . The degree of la ttice m isma tching !Lia/aI can be examined by X-ray diffraction and shou ld be less th an 0.05%. Metal-organic chemical vapor deposi tion (MOCV D). In th e meta l-o rg anic chemical va po r dep osit ion (MOCY D) method , gas so urces are used for growth of the structu res.' To sa tisfy the latt ice-ma tch cond ition, the flow rates of trim ethylin d ium and ars ine (AsH 3 ) are fixed and the triethylgallium flow ra te is adjus ted . The phosphin e (PH 3) flow rate is varied to obtai n different compositions . Grow th rat es of InP and qua ternary ma terials are abo ut 2 urn Zh, differing sligh tly for d ifferent alloy composi tion s. The compositions are calcu lated from the wave leng th of the pho tolu minescence spectral peak int ensities. Chemical beam epitaxy (CBE). Trimeth ylindium and triethylgalliu m with H 2 carrie r gas are used as Gro up III sources in chemical bea m epitaxy (CBE) de posi tion." Group Y sou rces are pure AsH 3 and PH3, w hich are precracked at 1000°C by a high -tem perature

Chapter two:


295

crac king cell. Solid Si and Be are used as n-type an d p-typ e d op ants, resp ectively. The typi cal grow th temperature is 500°C, which m us t be calibrat ed, for example, using the melting point of InSb (525°C). Typical growth ra tes for InP, Ga lnAsP (J"g = 1.3 urn) , and GaInAsP (A g = 1.55 urn) are 1.5, 3.8, and 4.2 um / h, respectively. Impurity d oping control ov er w id e ranges is one of the m ost important issu es in the fabrica tion of op toe lectronic devices. The adv an tages of using Be ar e that it is a well-be have d accep tor p roducing a shallow level above the va lence band, an d it can be incorporat ed into GalnAsP at a relati vely high level (on the order of 1019 em >'). The impurity lev els of GalnAs grown by var ious ep itaxial techn iques are 3 x 1015 crrr? by MBE, 8 x 1015 crrr' by MOCVD, an d 5 x 1014 crrr? by CBE.

2.11.4

Applied devices

Semiconductor lasers emitting 1 to 1.6-J.1n1 wavelength. The op tical fiber made of silica glass exhib its a very low transmission loss, i.e.. 0.154 dB/km a t 1.55 urn. Th e ma terial dis pe rsion of retractiv e index is minimum at the w avelength of 1.3 urn. These are ad va n tageou s for long-distance optical comm un ica tions . Semiconductor las er s emitting l .3-~m w avelen gth using lattice-matched Ga lnAs P I InP h ave been de veloped having low th resholds of ab out 10 rnA and very long d evice lifetimes. The l .3-~m waveleng th sy st em h as been used since 1980 in public telephon e networks a nd undersea cable syste ms. In th e 1990s, the 1.55-flm system wa s realized b y tak in g the advantage of the min imum tran smission loss . In th is case, the linewidth of th e light sour ce must be very small, since the d ispersion of th e silica fiber is rela tively lar ge com pa red to that at 1.3 urn . Figure 85 exh ibit an exa mp le of a single-mode laser struc ture that p rovides n ar row lin ew idth ev en w hen modulated a t high speed -signa ls." High -p ow er semicon d uc tor lasers em ittin g at 1.48 urn are employed as a pumping so urce for Er-dop ed op tical fiber amplifier (EDFA). A su rface-em itt ing laser operatin g a t thi s w avelength is sho w n in Figure 86 an d is expected to be us ed in long-w avel en gth netw orks and optical int erconnects." For th e purpose of subs tan tially improv ing laser performance, qua n tu m wells h ave been consi de red for use as the active region of se micon d uc tor las er s. Figure 87 giv es an exa m p le of quantum wire lasers employ in g a GaInAs / GaInAsP sys tem that em its a t 1.55 um .? Other optoelectronic devices. The coun terp ar t of se micond u ctor laser s is a photod etector that receives the tran smitted op tical signa l. Photodiodes h avin g high qu an tum efficiencies in wavelength 1.3 to 1.6 urn band employ th e GalnAs ternary se micon duc tors latti ce-m atched to InP as w ell. Th is sys tem p rovid es low-noise and high-sp eed photodiodes, i.e., PIN di odes and avalan che photodiodes (APDs). Infra red (IR) detectors an d CCDs are important for infrared imaging. Illuminat ion by IR LEDs a re useful for imagi ng as w ell. Eye-safe radiation in th e 1.3- to 1.5 5-~m ra nge is another im po rtant issue in IR imaging.

References 1. Nahory, R.E., Pollack, M.A., Johnst one, W.O., and Barn es, RL., Appl. Phys. Leit., 33, 659, 1978. 2. Agra wal, G.P. an d Dutta, N.K., Long-Wavelength Semiconductor Lasers, Van Nostran d Rein hold , New York, 1986, 85. 3. Moon, R L., An typas, G.A., and Jam es, t. w, J. Electron. Mater., 3, 635, 1974. 4. Casey, H.C. and Pani sh, M.B., Heterostructure Lasers, Part B, Acad emic Press, New York, 1978. 5. Man asevit, H.M., Appl. Phys. Lett., 12, 156, 1968. 6. Tsang, w.I., IEEE ]. Quant. Electron., QE-23, 936, 1987.

N '-0

.>

0'\

Si0 2 p-InP n-GalnAsP (Blocking) p-InP p-InP elee trode As ymmetric Gratings

Waveguide Structure n-lnP

-120nm

Ga O.dnO.5 3As

active tc 1 '" 30 crn'

passive tc 2"'" 200 cm- 1

(A~:l~t; ~ m)

=I~::m =*= 8nm

- - f,@//P// //%/.1

,

."'=

'J)

::

....:: Q)

50

..J ~

350

400

450

500

550

600

650

Wavelength (nm) Figure 93 Electrolumi nesce nce sp ectra of a GaInN / AIGaN double-heterostructure blu e LED. (From Nakamura, S., Jpn . J. Opi., 23, 701, 1994. With permission .)

GuInNgreen SQW LEDs Single-Quantum-Well Structure (SQW) p-Alo.2Gao.8N

•

p-electrode p-GaN p-Alo.2Gao.8N Gao.ssIno.4sN n·GaN -

6 Gao.5sIno.4sN

"-G'N~,

n-elec~ode

Energy

GaN buffer layer Sapphire substrate -

Figure 94 The struc tur e of green SQW LED. (From N ak amu ra, S., Senoh , M ., Iw asa, N., Nagah ama, S., Yamad a, T., and Mukai, T., [pn . J. App l. Phys. Lett ., 34, L1332, 1995. With perrnission.)

306


(a) Blue ,.-.,

100

(b) Green

(c) Yellow

.... Vl

·2 ::s

.0 s.. co=

'-"

....>. c ....e Vl

50

Q,l

...:l ~

0

400

450

500

550

600

650

700

Wavelength (nm) Figure 95

Electroluminescence of (a) blue, (b) green, and (c) yellow SQW LEDs at a forward current of 20 mA. (From Nakamura,S., Senoh. M., Iwasa, N., and Nagahama, 5., [pn. J. App!. Phys., 34, L797, 1995. With permission.)

peak wavelength and the FWHM of the typical blue SQW LEOs are 450 and 20 nm, respectively; of the green 525 and 30 nm; and of the yellow 600 and 50 rim, respectively. When th e peak wavelength becomes longer, the FWHM of the EL spectra increases, probably due to the inhomogeneities in the GaInN layer or due to strain between well and barrier layers of the SQW caused by lattice mismatch and differences in the thermal expansion coefficients. At 20 mA, the output power and the external quantum efficienc y of the blue SQW LEOs are 5 mWand 9.1%, respectively. Those of the green SQW LEOs are 3 m Wand 6.3%, respectively. A typical on-a xis luminous intensity of the green SQW LEOs with a 10° cone viewing angle is 10 cd at 20 rnA. These values of output power, external quantum efficiency, and luminous intensity of blue and green SQW LEOs are more than 100 times higher than those of conven tional blue SiC and green GaP LEOs. By combining these highpower and high-brightness blue GaInN SQW, green GaInN SQW, and red AlGaA s LEOs, many kinds of applications such as LED full -color displays and LED white lamps for use in place of light bulbs or fluorescent lamps are now possible. These devices have the characteristics of high reliability, high durability, and low energy consumption. Figure 96 is a chromaticity diagram in which the positions of the blue and green GaInN SQW LEOs are shown. The chromaticity coordinates of commercially available green GaP LEOs, green AIGalnP LEOs, and red AlGaAs LEOs are also shown. The color range of light emitted by a full-color LED lamp in the chromaticity diagram is shown as the region inside each triangle, which is drawn by connecting the positions of three primary color LED lamps. Three color ranges (triangles) are shown for differences only in the green LED (green GalnN SQW, green GaP, and green AIGalnP LEOs). In this figure, the color range of lamps composed of a blue GaInN SQW LED, a green GaInN SQW LED, and a red AlGa As LED is the widest. This means that the GaInN blue and green SQW LEOs show much better color an d color purity in comparison with other blue and green LEOs. Using these blue and green SQW LEOs together with LEOs made of AlGaAs, more realistic LED full color displays have been demonstrated.

Chapter two:


307

0.9 , - - - - - - - - - - - - - - - - - - - ,

0.8

0.7 0.6 500

0.5 0.4

0.2 0.1 Blue GaInN

LED

470 450

0.1

0.2

0.3

0.4

0.5

0.6

0.7

x Figure 96 Chromaticity diagram in which blue GalnN SQW LED, green GaInN SQW LED, green GaP LED, green AIGalnP LED, and red AIGaAs LED are shown. (From N akamura, S., Senoh, M., Iwa sa, N., Nagaharna. S., Yamad a, T., and Mukai, T., [pn . ]. App/. Phys. Leu., 34, L1332, 1995. With permission.)

2.12.7

GalnN multiquantum-well (MQW) LDs

The structure of the GalnN MQW LOs is shown in Figure 97. The GalnN MQW LD device consists of a 300-A-thick GaN buffer layer grown at a low temperature of 550°C, a 3-llmthick layer of n-type GaN:Si, a O.l-l1m-thick layer of n-type Gao9SlnOosN:Si, a O.5-l1m-thick layer of n-type AloosGao92N :Si, and a O.l-l1m-thick layer of n-type GaN:Si. At this point, the MQW structure consists of four 35-A-th ick undoped GaOSSInalSN well layers by 70-Athick undoped Ga a.9sIn0.Q2N barrier layers. The four well layers form the gain medium. The het erostructure is then capped with a 200-A-thick layer of p-type AlozGa osN :Mg, a (Ll-um-thick layer of p-type GaN:Mg, a O.5-llm-thick layer of p-type AloosGao92N:Mg, and a O.5-llm-thick layer of p-type GaN:Mg. The n-type and p-type GaN layers are used for light-guiding, while the n-type and p-type AlaosGa on N layers act as cladding for confinement of the carriers and the light from the active region. Figure 98 shows typical voltage-current (V-I) characteristics and the light output power (L) per coated facet of the LD as a function of the forward OC current at RT. No stimulated emission was observed up to a threshold current of 80 rnA, corresponding to a current density of 3.6 kA crrr-', as shown in Figure 98. The operating voltage at the threshold wa s 5.5 V.


308

Ridge-waveguide purplish-blue InGaN MQW LDs Mult-Quantum-Well Structure (MQW) p-Alo.osGao.nN

p-electrode p-GaN p-Alo.osGao.nN p-GaN p-Alo.2Gao.sN -=~iiiiii~ GaInNMQWn-GaN n-Alo.osGao.92N n-GaN n-Gao.9sIno.osN

x=O.02 .--_...... x=O. IS

n-GaN

n.AI"."GaM'~ x

n-electrode

Energy

GaN buffer layer F=============~ (0001) sapphire substrate

Figure 97 The structure of the Ga lnN MQ W LOs. (Fro m Na kam ura,S., Seno h, M., Naga hama, 5., Iwasa, N., Yamada, T , Ma tsus hi ta, T, Sugimo to, Y., an d Kiyo ku, H ., Presen ted at the 9th Annual

Meeting of IEEE Lasers and Electro-Optics Society, Boston , POU, Nov. 18-21, 1996. With permission.)

10

3

CW 20°C

8

~

E 2

'--' l~

6

>

4

-

'--' ~

~

OJ)

0

~

l::

La

TA

1000

a

:;j

..0 l-< C(j

>.

+->

100

~

Ul

l:: (l)

+->

l::

~

+->

.c ,,..,bD

10

....J

2.40

2.30

Photon energy (eV) Figure 104 Pho toluminescence spec tru m of excitons bo und at N don ors in 3C-SiC. Ecx ind icates the exci ton bandgap. (0: zero p honon; TA: tra nsverse aco ustic; LA: longitu d inal acou stic; TO: tran sverse optic; and LO: lon gitu dinal optic). (From Choyke. w.y., Mater. Res. Bull., 4, S141-S152, 1969. With permission .)

corresponding to th e zero-pho non line, th e exciton b indi ng energy for N donors is estima ted to be 10 m eV. Since the resolution of peak energ ies is much bet ter than that in the absorption spectra, th e exac t value of phonon energies can be obtai ned from the pho tolumines cence spectra . In the pho toluminescence spectr um of 6H-SiC, there exis ts a zero-p honon pea k du e to the recom bination of exci tons bound at N do nors su bstituted in to hexa gon al C sites and tw o zero-p honon p eaks due to those located in cub ic C sites.' Since the energy levels of N do nors in inequivalent (hexagonal, cubic) sites are different, the pho tolumi nescence peak s have differe n t energies.

2.13.3.2 Luminescencefrom donor-acceptor pairs In SiC, N atoms belongi ng to the fifth colum n of the period ic table work as do nors, and B, AI, and Ga in the th ird column work as acceptors. When donors an d acceptors are sim ultaneously inco rporated in a crystal, electrons bound at donors and holes at accep tors can create a pair due to the Coulombic force between electrons and ho les. Th is in terac tion leads to strong p ho toluminescence through recombina tion and is known as don or-acceptor pair luminescen ce. Fig ure 105 shows th e photoluminescence spec trum from N- Al donor-acceptor pa ir recombination in 3C-SiC at 1.8K.4 This gives a pec uliar structure showing the recombination of ele ctro ns and holes in d on or-acceptor pairs of type 2 w ith N donors replaci ng C and Al rep lacing Si. From a de tailed ana lysis of th is pec uliar structure, the va lue of 310 meV is ob tained for the sum of ED(N) and EA(AI), where ED(N ) is the N-don or level

Chapter two: Fundamentals of luminescence

317

Wavelength (nm) 540

550

560

<J ''';

c

;:l

19 17 20 18

.......

3C-SiC:N.Al ~1

16

10

1.8K

8

~ 15

12 13

<J

..c::: be

j 2.20

2.24

2.28

2.32

Photon energy (eV) Figure 105 Ph otoluminescence s pectru m of N donor-Al acceptor pair recombination in 3C-SiC. The number for each peak ind ica tes the order of di stance between donor and acceptor. (From Choyke, W.I. and Patrick, L., Phys Rev., B2, 4959-4965,1970. With permission.)

and EA(AI) the AI-acceptor level. At 77K or higher, the spectrum changes to that due to the recombination of free electrons and holes bound at Al-acceptors (free-to-accep tor recombination) because of thermal excitation of electrons bound at N-donors to the conduction band. From the spectrum, the value of EA (AI) can be determined precisely. Based on the se studies, the values of EA(AI) := 257 meV and ED(N) := 53 m eV were obt ained." From a sim ilar analysis, the B- and Ga-acceptor level s can also be determined. In most SiC polytypes, except for 3C-SiC and 2H-SiC, th ere are inequivalent sites, and impurities substituting into those sites give rise to different energy levels. Thu s, spectra of d on or-ac ceptor pair recombination and free-to-acceptor recombination can become complicated . As examples, donor-acceptor pair recombination spectra in 6H-SiC at 4.2K are sh own in Figure 106(a) and free-toacceptor recombination sp ectra at 77K in Figure 106(b).s Although the energy levels are different for different acceptors (B, AI, and Ga), the shap es of spe ctra are quite sim ila r when the abscissa is sh ifted by an energy of the order of 0.05 eV, as shown in the figure . The B series (peaks denoted as B) in the sp ectra show donor-acceptor pair luminescence for N donors in hexagonal C sites and Al acceptors, and the C series (pe aks denoted as C) arising from N donors in cub ic C sites and Al acceptors. Here, the en ergy level s of Al acceptors are thought to be very sim ilar, whether they are in hexagonal or cubic Si site s. The subscripts in the figure are defined as follows: 0 im plies a zero-phonon peak and LO implies peaks involving longitudinal optical phonons. Peaks A indicate free-toacceptor recombination : Nand Ab are due to accep tor s substituting in to he xagonal and cubic Si sites , respectively. Since there are three different sites for donors and acceptors, respectively, in 6H-SiC, anal ysis for the p eculiar s tructure observed in the sp ect ra bec omes ve ry difficult. The photon energ y, hv(R) , from donor-a cceptor pair lumin escence is gi ven b y hv(R) -R6exp(-4ncR3/3), where R is the distance between a donor and an acceptor and c is larger of the don or or acceptor conc entrations. By curve fitting of the ab ove relati on to the spectra, the value of ED+ EAcan be obtained," Since the value of EA is calculated from free-to-acceptor recombination as in Figure 106(b), EDcan also be determined. Although


318

;:r------------::--=------, CLO BLO §

6H-SiC

B2LO C2LO

4.2K

•

• •

~

.gL.._...L-__...L---...L.--...I...---..Jo-..::-----' .,.., ...J

2.0

1.9

2.1

2.2

Photon energy (eV)

6H-SiC

17K

~

.g ......__

---1._ _---''--_ _''--_ _......._ _-&-......

.,.., 1. 9 ...J

2.0

2.1

2.3

2.4

Photon energy (eV) Pho tolum inescence spec tra of (a) d on or-acceptor pair recombination at 4.2K and (b) freeto-acceptor reco mbination at 77K in 6H-SiC d op ed with 8, AI, an d Ga. A o: free-te-Al accep tor peak, (b) 8 0 : N- d on or(hexagonal site)- Al acceptor, (c) Co: N- d on or(cubic site)-Al accep tor. LO ind icates longitudinal phonon. (From Ikeda , M., Mat sunami, H., and Tana ka, T., Phys. Reo., 822, 2842-2854, 1980. With p erm ission. )

Figu re 106

on e hexagon al site and tw o cu bic sites exis t in 6H- SiC, the difference between the energies for the tw o cubic sit es seems to be very sm all. Curv e fitting wa s car rie d out b y ass uming that the luminescence in tens ity related to cubic sites is two tim es larger than that related to hexagonal sites. The calcul at ed en ergy levels of impurities are given in Table 29. In the tabl e, the results of d ifferen t poly ty pes are a lso shown ." In each polytyp e, the ra tio between th e accepto r ene rgy levels for cu bic and hexagon al sites is very sma ll, whereas tha t of donor energy levels is large.

2.13.3.3

Other lu minescence centers

In ad d ition to the above lu minescen ce cen ters, lu minescen ce due to d efects p roduce d by ion implan tat ion ? and due to th e localized cen ters such as Ti2 have been reported.

Chapter two: Fundamentals of luminescence Table 29

Polytype 3C-SiC 6H-SiC 4H-SiC

319

Energy Levels of Donor and Acceptors

Site

Don or N

C C H C H

56.5 155 100 124 66

Energy level (meV) Acceptor Al Ca

B

254 249 239

343 333 317

735 723 698

191

267

647

From Ikeda , M., et al., Phys. R£'V., 822,2842,1980. With permission .

2.13.4 Crystal growth and doping Crystals of SiC have been grown by the so-called Acheson method, in which a mixture of Si02 and C is heated to ab out 2000°C. To grow pure single crystals, the powdered SiC crystal mixture is sublimed in a specially designed crucible by the Lely method . Recent large-diameter (approximately 2-inch diameter) single crystal boules have been produced by a modified Lely method ut ilizing a SiC seed in the sublimation growth. On those single crys tals, epitaxial growth has been carried out by either liquid phase epitaxy (LPE) or vapor phase epitaxy (VPE). In LPE, molten Si in a graphite crucible is used as a melt in which a SiC substrate is dipped into.s In VPE, chemical vapor deposit ion (CVD) with SiH4 and C3H8 has been widely used. To get a high-quality epitaxial layer at low temperatures, step-con trolled epitaxy is used, which utilizes step-flow growth on offoriented SiC substrates'? Doping with third column elements as donors or fifth column elements as acceptors can be done easily through both in LPE and VPE.

2.13.5 Light-emitting diodes Earlier, yellow light-emitting diodes (LEOs) of 6H-SiC utilizing N-B donor-acceptor pair luminescence wer e demonstrated; they were later replaced by GaAs 1_xPx:Nyellow LEOs. Blue LEOs of 6H-SiC p-n junction utilizing N-AI donor-acceptor pair luminescence are usually mad e by LPE6 or VPF methods. The mechanism for electro luminescence through injection of carriers was clarified by Ikeda et a1. 8 A typical spectrum of blue LEOs is show n in Figure 107.9 The spectral peak is located at 470 nrn with a width of 70 nm for a forward current IF of 20 rnA (0.3 x 0.3 rnm-). The diode consists of LPE-grown AJ-doped p-SiC/N-doped nSiC/n-6H-SiC substrate. LEOs are fabricated w ith a p-side down structure, and the light comes through the n-SiC. The maximum external quantum efficiency is 0.023% (IF = 5 rnA). Since the blue LEOs utilize N-Al donor-acceptor pair luminescence in n-type epila yers , the brightness increases with incorporation of AI, and it exceed s 20 mCd (IF = 20 rnA) .

References 1. Choyke, W.J., Mater. Res. Bull., 4, SI41-S152, 1969. 2. Choyke, w.J . an d Pat rick, L., Silicon Carbide-1973, Mar shall, RC., Faust, j.W., and Ryan, C.E., Eds., Unive rsity of South Carolina Press, 1974, 261-283. 3. Choyke, wj. and Pa trick, L., Phys. Rev., 127, 1868-1877, 1962. 4. Choyke, W.J. and Patrick, L., Phys. Rev., B2, 4959-4965, 1970. 5. Iked a, M., Matsunami , H., and Tanaka, T., Phys. Rev., B22, 2842-2854, 1980. 6. Matsunam i, H., Ikeda, M., Suzuki, A., and Tanak a, T., IEEE Trans. Elee. Devices, ED-24, 958961, 1977.


320

Room temperature

6H-SiC IF = 20 rnA

400

500 Wavelength (nm)

600

Figure 107 A typical spectrum of bright blue LEOs of 6H-SiC. (From Mat sushita, Y., Koga, K., Veda, Y, and Yamaguchi, T., Oyobuturi , 60, 159-162, 1991 (in Japanese).)

7. Shibahara, K., Kuroda, N ., Nishino, S., and Matsunami, H., Jpn . J. Appl. Phys., 26, U815U 817, 1987. 8. Ikeda, M., Ha yak awa, T., Yamagiwa, S., Matsunami, H., and Tan aka , T., J. Appl. Phus., 50, 8215-8225, 1979. 9. Matsushita, Y, Koga, K., Veda, Y, and Yamaguchi, T., Ouobuturi, 60, 159-162, 1991, (in Japanese).

chapter two - section fourteen

Fundamentals of luminescence Rong-Jun Xie, Naoto Hirosaki, and Mamoru Mitomo

Contents 2.14 Oxynitride phosphors 321 2.14.1 Introduction 321 2.14.2 Overview of oxynitride phosphors 322 323 2.14.3 Characteristics of typi cal oxyn itride phosphors 2.14.3.1 LaAl(Si6-zAlz)N lo-zO z:Ce3+ (z = 1) 323 2.14.3.2 ~-SiAION :Eu 2+ • • •• •• • •• ••• • •••••••• • • ••• •••• • • ••• • •• •• •• •• • • • • •• • • •• • • • • • • • • •• • • •• • • • •• • • •• •• •• • • 323 2.14.3.3 MSiP2N2:Eu2+ (M = Ca, Sr, Ba) 324 2.14.3.4 a -SiAION:Eu 2+ .• . .. •• ••. ••••.• ••.•• ••.• •• •. ••••. •••• •••• •••••. •. •.• .• .• .•.• . . . .• . . . . . . . . . . . . . . . . 325 2.14.3.5 M 2SisNs:Eu 2+ (M = Ca, Sr, Ba) 325 2.14.3.6 CaAl SiN 3:Eu 2+ . .. •. • .•• . . .• • •• ••. •. • • • • • • •• .• • . • • ••• • • . ••• .• • • • . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 2.13.4 Ap p lica tions of oxynitrid e phosphor s 327 Refer ences 328

2.14 Oxynitride phosphors 2.14.1 Introduction Inor ganic phosphors are composed of a host lattice d op ed w ith a sm all amo unt of im p ur ity ion s th at activate luminescence. Most of these materi als are oxid es, s ulfides, fluorid es, halides, and oxysulfides doped with transition metal ion s or ra re-earth ion s. Recentl y, with th e advent of solid-state lighting technologies as w ell as the d evel opment of plas ma an d field emission display panels, a great number of trad itional phosphor s can no t m eet the requirem ents for new applications, for example: (1) excita tion by near-ultraviolet (UV) or visible light; (2) efficient emission of appropriat e co lors; a nd (3) surviva l a t ad verse en viron m en ts. Therefore, novel phosphors with superior luminescent p ro pe rties are being sou gh t using new host materials. The integration of nitrogen (N) in sili cates or al u min osilica tes p roduces a wide ra ng e of com p lex stru ctur es with increased flexibility com pared to the oxos ilica tes . a nd thus a n ew class of materials, nitridosilicates, nit ridoaluminosilicat es, a nd s ialons, a re obtai n ed .' Th ese novel luminescent materials-the oxyn itride phosphors-ha ve been sy n th esized by d op in g w ith app ro pria te amounts of rare-earth ac tiva to rs .v -" The rare earths d op ed in the oxy n itrid e phosphors usually en ter into in ters titial sites a nd a re coor di na ted by (0 , N) 321

322


ions locat ed at various di stances. For tho se rare ea rths (i.e., Eu 2 + and Ce 3+) emi tting from their Sd excited state, w h ich is strongly affected by the crysta l-field envi ronme n t (e.g., covalency, coo rd ina tion, bond len gth, crys ta l-field stren gth), ap prop ria te e mission colors can be obta ined by carefully se lecting th e host lattice. Du e to a high er cha rge of N' compared w ith that of 0 2 - an d because of the nephelau xet ic effect (high cov alency), the crys tal-field sp li tting of th e Sd levels of rare ea rths is larger and the center of gravity of the Sd sta tes is shi fted to low en ergy (i.e., lon ger wav eleng th) in these oxynitride comp ounds. Furthermore, the Sto kes sh ift becomes smaller in a more rig id lattice, which results w he n more N 3- is inco rpo ra ted . This will result in more versa tile luminescent properties of oxyn itri de p hosp hors, incr easin g their range of applica tions. In this section, the characte ristic features and po ten tial applications of rare -earth-d oped nitrid e phosphors are d escribed .

2.14.2

Overview of oxynitride phosphors

Table 30 list s oxy n itr ide p hosp h ors reported in the literature in recen t yea rs. Th e host lattice of the se phosphors is based on nitridosilicates, oxonitridosilica tes, or oxo n itridoaluminosilicates, wh ich are derived from silica tes b y form al exc ha nges of a nd Si by N and AI, respectively. The struc ture of these host lattices is bui lt on highly conde nse d networks constructed from the co rner-sha ring (Si, Al)-(O, N) tet rah edra . Th e d egree of condensation of the net w or k s tr uc tures (i.e.. the molar rati o Si:X > 1:2, w ith X = 0 , N ) is higher than the max im u m va lue for oxosili cates (l :4:S; Si:O:S; 1:2).2\ Co nse que n tly, these highl y condensed materials e xh ibit high chem ica l and ther m al s tabilities . Moreov er, the str uctu ra l va riab ilities o f this class of mat er ials provide a significan t exte nsion of con ventional silica te che mistry, form in g a large famil y of Si-AI-O-N multiternary com poun d s.

°

Tabl e 30

Emissio n Co lor and Cr ys ta l Structure of Oxynitride Phosphors

Phosphor

Em iss ion color

Y-Si-O-N:Ce 3+

Blue

BaAI UO J6N:Eu z+

Blue

~-Alumi n a

[2,4J

JEM:Ce 3+

Blue

Ortho rho m bic

[1 9]

SrSiAl z03N 2:Eu +

Blue -green

Ortho rhomb ic

[1 4J

:Eu 2+

Blue-g reen

O r thorho m bic

[14]

BaSiz0 2 N 2 :Eu 2 +

Blue-g reen

Mo no clinic

[18]

a -SiA ION :Yb2+

G reen

H exagon a l

[15]

~-SiAI ON :Eu z+

Green

H exagona l

[17]

MYSi4N 7:Eu + (M = Sr, Ba)

Green

He xagonal

[12]

MSizOzN 2:Eu 2 + (M = Ca . Sr)

Gree n-yello w

Monoclinic

[18]

a -SiA ION:Eu 2+

Yello w- ora nge

Hexagonal

[7,8,10,11]

LaSi,No:Euz+

Red

Orthorhombic

[6]

LaEuSi 2N,Oz

Red

Orthorhombic

[6J

Ca zSisN s:Eu 2 •

Red

Monoclin ic

[5]

M 2Si sN s:Eu 2+ (M = Sr, Ba)

Red

O rthorhombic

[5]

Red

Ortho rho m bic

[20]

2

SrSisAI0 2N 7

2

Ca AISiN 3 :Eu

2+

Crystal structure

Refere nces

[3J

323


The most u sual ap proach es for synthesizin g oxynitr ide phosphors are so lid -s ta te reactions and gas-red uc tion-n itrid ation . The solid-sta te reac tion involves th e reac tion am ong ch emical com ponents including metals, n itride, and ox id e starting pow d er s a t high tem p era tures (1400-2000°C) under an N 2 atmosph ere. The nitridation reaction is ge nerally p erformed in an alumina boa t co n ta ining th e oxi de prec ursor powder load ed in side an alumina /quartz tu be th rough which N H ) or NH 3-CH 4 gas flows at appropri at e rates a t high temper atures (600- 1500°C) . The NH, or N H 3-CH 4 gas ac ts as both a redu cing and nit rid at ion agen t.

2.14.3

Characteristics of typical cximitride phosphors

2.14. 3.1 LaAI(Sit>_:Alz)NJO_zOz:Ce3+(z = 1) Crystal structure. The LaAI(Si6_zAlz)NIO_zOz (JEM) p hase was iden tified in th e prepara tio n of La-stabili zed a -SiAIO N materials.Alt h as an or thorho m bic structure (space g ro up Pbcn) with a = 9.4303, b = 9.7689, and c = 8.938 6 A. The Al a tom s and the (Si, AI) a to ms ar e tetrahed rall y coordi na ted by the (N, 0) a to ms, yield ing an AI(Si, AI)6(N, 0 )103- netwo rk. Th e La atoms a re locat ed in the tunnels ex te ndi ng alon g the [001] direction and are irreg ularly coord ina ted by seven (N , 0 ) atoms a t an average distance of 2.70 A. Luminescence characteristics. As show n in Figure 108, the emission sp ec tru m of JEM:C e 3 + disp lays a broad band w ith the p ea k located a t 475 nm und er 368-n m exci ta tio n ." The em ission effic iency (ex terna l q ua n tum efficien cy) is a bo u t 55% w hen exci ted at 368 nm. This blu e phosp ho r ha s a broad exc ita tion spec tru m , ex ten di ng from the U V to the visible ran ge. Wh en the concentration of Ce 3+ or th e 2 value increases, both th e exci ta tio n and emission spec tr a are red shifted. Preparation. The starting materials for JEM are Si3N 4 , AIN, AI20 y La 203, a nd Ce02 . The po w der phosph or is synthesized by heat ing th e powder mixtu re a t 1800-1 900°C for 2 h un d er 1.0 MPa N 2 .

2.14.3.2 !J SiAION:Eu 2+ Crystal structure. Th e struc ture of

~-S iAI ON is derived from ~-Si3N4 by su bstitution of AI-D by Si- N , and its che m ical composi tion ca n be w r itt en as Si(r.zAlzOzN : (2 re p resen ts

EX

EM

::i

.i C

'iii c Q)

.S --l Q..

350 400 450

500

550

600

650

Wavelength (nm)

Figure 108

Emiss ion and excit ati on of LaAl

700

200

250

300

350

Wavelength (nm) (Si (~ , A lJ N l o-: 0z : C e3+

(z = 1).

400

450


324

EX

EM

:i

~

z-

'(ij

c

QJ

.~ -'

Q..

450

500

550

600

650

700

200 250 300 350 400 450 500 550 600

Wave length (nm)

Figure 109

Emission and excitation of

Wavelength (nm) ~ -SiA1 0N : Eu 2+ .

the number of Al-O pairs substituting for Si- N pa irs and 0 < z ::; 4.2).Z3 ~-Si AI ON has a hexagonal crys tal struc tur e and the P63 space gr oup. In this structure, there are con tin uous channels parall el to the c direction. Luminescence characteristics. The ~-SiAION:Eu z+ phosphor gives intense green emission with th e peak located at 538 nm;" as seen in Figure 109. The broad emi ss ion spectrum has a fuU width of half maximum of 55 nrn . Two w ell-res olved broad bands cen tered at 303 and 400 nm are observ ed in the excitation spe ctru m. The broad excitation ran ge enables the ~-SiAION :Euz+ ph osphor to emit s trong ly under near UV (390-410 nm) or blu e-light excitation (450-470 nm) . This green phosphor has a chrom a ticity coord inates of x = 0.31 and y = 0.60. Th e ex tern al quantum efficien cy is abo u t 41% when excited at 405 nm. Preparation. Sta rting from Si3N4, AIN , Al z0 3, and Eu zOy the ~-SiAION:Euz+ ph osphor is synthesized at 1800-2000°C for 2 h under 1.0 MPa N z. An Eu concentration of 0) has been identified, s ugges ting that some modifications of MSizOzNz (M = Ca. Sr) exist depending on the syn thesis temperature. Luminescence characteristics. All MSizOzNz:Eu z+ph osphors have a broad-ba nd emission sp ec tr u m w ith d iffe rent full widths a t h alf maxi m u m : CaSiZ01N:Eu z+ , 97 n m ; SrSizOzN :Eu z+ , 82 nm; and BaSizOzN:Eu z+, 35 nm (see Figure 110). CaSi zOzN:6%Eu 2+ sh ow s a yellow ish em ission w ith a maximum at 562 nrn. SrSizOzN:6%Eu z+ emits gre en color with a ma ximum at 543 run , an d BaSi20 zN:6%Eu z+ yield s a blue-green emission with a peak at 491 nm. The excita tio n spectrum of CaSi zOzN:6%Eu z+ shows a flat broad band coverin g the 300-450 nm ran ge, wh ereas two well-resolved bro ad bands centered at 300 and 450 nm are seen in SrSizOzN:6%Eu z+ and BaSizOzN:6%Eu z+, resp ectively. Preparation. The MS izOzNz:Euz+ phosphors are synthesized by heating the powder mixture of Si3N4, sto, and alka line-ear th carbonates a t 1600°C under 0.5 MPa N z.

325


EM ,, , , , , , ,, ,, ,, ,, ,

EX

,r , r I I I I

, ,

I I

, ,

I

I

1

I

, , ,,

I I I

1

, ~ ,

I

,

.I I ','

CaSi2 0 2 N2 SrSi 2 0 2 N2 BaSi 202N 2 400

450

500

550

600

650

700

200

Wave length (nm)

300

350

400

450

500

550

Wavelength (nm)

Figure 110 Emission and excitation of MSiP 1N1:Eu 1+ (M

2.14.3.4

25 0

= Ca. Sr, Ba).

a-SiAION:Eu 2+

Crystal structure. ex-SiAlON is isostructural to ex-Si3N4 . It has a hexagonal crystal s tructure and the P31 c space group. The ex-SiAION unit cell con tent, consis ting of four "Si3N 4 " units, can be give n in a general formula MxSi t2_m_nAI/1I+"0,,N16_" (x is the solubility of the m etal M).2,.2(' In the ex-SiAION structure, 711+n (Si-N) bonds are rep laced by m (AI-N) bond s and n (AI- O) bon ds; the charge discrepancy cau sed by the subs titution is com pe nsa ted by the in trod uction of M cations includ in g Li'. M g 2+, Ca 2 ' , Y" . and lanthan id es. The M ca tions occupy the interstiti al sites in the ex-SiAION latti ce and are coordi na ted by seven (N, 0 ) an ions at thr ee different M-(N, 0 ) di stances. Luminescence characteristics. ex-SiAION: Eu 2 + phosphors give green-yello w, ye llow, or yellow -orange emissions w ith peaks located in the ran ge of 565-603 n m/ ,S,lO,!1 as sh own in Figure 111. The broad -band emi ssion spec tru m covers from 500 to 750 nm with the full wid th of half maximum of 94 nm . The excitation spectrum of Eu 2+ in ex-SiAlON has two broad bands wi th peaks a t 300 and 420 nm . respectively. The extern al quantu m efficiency of the ex-SiAION:Eu2+ ph osphor wi th optimal composition is ab ou t 58% when excited at 450 nm . By tailo ring the composition of the ho st latti ce and contro lling the concen tra tion of Eu 2+, the em ission color of ex-SiAION can be tuned th rou gh a w ide range. Preparation. The Ca-ex-SiAIO N:Eu 2 + phosphor is syn thesized by so lid -state reac tions. The p owder mixt ure of Si3 N 4 , AIN, CaC0 3, an d EU2 0 3 is fired at 1600- 1800°C for 2 h under 0.5 MPa N 2 . The gas-red uction- n itridation method is also used to prep are ex-SiAION :Eu 2+ phosphor." It is syn thesized from the CaO-AI 203-Si0 2 sys tem, by usin g an N H 3-CH 4 gas mixtu re as a reduction-nitridation agent. The Eu con centration in ex-SiAlON phosphors var ies from 0.5 to 10%.

2.14.3.5 M 2Si sN s:Eu2+ (M

= Cal

Sr, Ba)

Crystal structure. Ca 2S isN s has a monoclinic crystal sys tem w ith the space gro up of Cc, whereas both Sr2Si sNs and Ba 2SisNs have an orthorh ombic latt ice wi th the spa ce gro u p of Pmn2 1.27,2S The local coord ination in the str uctu res is qu ite similar for these ternary alka lineea rth silicon nit rid es, half of the nitrogen atom s connec ting two Si neighbors and th e o the r half ha ve three Si neighb ors. Each Ca atom in Ca 2SisN s is coordinated by sev en nitrogen

326


--c:J

~

?:'

' (jj

c

OJ

C

!

-l

0...

1

450

500

550

600

650

700

750

200

250

Wave length (nm)

Figu re 111

300

350

400

450

500

550

Wavelength (nm)

Emission and excita tio n of cx-SiA10 N :Eu 2 + .

atoms, whereas Sr in Sr 2Si sN 8 and Ba in Ba2Si sN s are coordinated by eig h t or nine ni trogen atoms. Luminescence characteristics. M 2Si sNs:Eu 2+ (M = Ca, Sr, Ba) phosp hors gi ve ora nge-red or red emission, as shown in Figure 112. A sing le, broad emi ssion ba nd is cen tered at 623, 640, and 650 nm for Ca 2SiSNg, Sr2SiSN g, and Ba2SisN 8, respectively. A red shif t in the emission waveleng th is observed with increasin g the ioni c size of alka line-ear th meta ls. Th e exci ta tion spectrum resembles eac h othe r, in dicat ing the chemical env ironmen t of Eu 2+ in these ma terials is very sim ilar. The exci tation spectrum ex tensively shifts to longer wavelengths, w ith the peak located at 450 nm for all samples . EM

:i ~

z(jj

c

,

.S

/~

OJ

-l

0...

.

-. _.- Ca2SisNs - - - - Sr2SisNs - - Ba2SisNs

550

600

650

700

750

Wavelength (nm)

Figu re 112

800

850

200 250 300 350 400 450 500 550 600 Wavelength (nm)

Emission a nd excita tion of M 2S isN H:Eu 2+ (M = Ca . Sf, Ba).

327


EX

EM

550

600

650

700

750

800

850

200 250 300 350 400 450 500 550 600

Wavelength (nm)

Figure 113

Wavelen gth (nm)

Emission and excita tion of CaA1SiN 3 :Eu 2+ .

Prepara tion. Th e tern ary alkaline-earth silicon nitr ides a re either sy n thesized by firing the powder mi xture of Si3Noj , M 3N 2, and Eu N a t 1600-1 800°C under 0.5 M Pa N 2 or p repared by the react ion s of metalli c alka line -ear ths wi th silicon d iimide at 1550-1 650°C un der nit rogen atmos phere.5.27.2R

2.14.3.6 CaAlS iN 3 : Eu2+ Crystal structure. Ca AJSiN 3 has an orthorhom bic crys tal s tru cture and th e space gro up of Cmcs. , the unit cell parameter being a = 9.8007, b = 5.6497, and c = 5.0627 A.20 Th e Ca atoms are found in the tunnels surrounded by s ix corner-sha ring tetrahedra of (AI, Si)N.j' Luminescence characteristics. CaAlSiN 3 :Eu 2 + is a red phosphor. The luminescen ce spectra are given in Fig ure 113. The excitation spectru m is ex tre me ly broad , rangi ng from 250 to 550 nm . Again a broa d emission band cen tere d at 650 nm is ob se rved w hen exci ted at 450 nm. The chro ma ticity coo rd ina tes of red p hos p hor a re x = 0.66 and y = 0.33. Th is p hos phor has an ex terna l qu antum efficien cy as high as 86% under 450 n m excita tion. The emission spec trum is red shifted with inc reasing Eu 2+concen trations. Preparation. Th e CaA lSiN 3 :Eu 2+ phosphor was syn thes ize d by firing a powder m ixture of Si3N.j, AIN, Ca 3N 2, and EuN at 1600-1800°C for 2 h und er 0.5 MPa N 2.

2.14.4 Applications of oxynitride phosphors As shown in the previous sec tion, oxynitride phosph or s emit efficien tly under UV and visible-light irrad iat ion . This co rrela tes well with the emission waveleng ths of th e UV ch ips or blue light-em itt ing di od e (LED) chips, m aking their use as d own-con version phosphors in w hi te LEOs feasible. We have prop osed that yellow a-SiAION p hosphors co uld be used to generat e warm w hi te light when com bined with a blue LED . The firs t w hi te LED lamp was rep orted b y Sakuma et al. usin g an ora nge- yellow a-SiAION: Eu 2+an d a blue LED chip." It emits w arm w hite light with the color temperature of 2800 K. To ob tain wh ite LED lamps with h igh color rendering index, ad di tiona l phosphors such as green and red phosphors are used . Sakuma et al. have rep ort ed white LEOs with va riou s co lor temper atures and a colo r rend ering index of >80 using ~-SiAION:Eu2+ (green) , Cf.-SiA ION:E u 2+ (ye llow), and

328


CaA1SiN3:Eu Z+ (red) phosphors.P Mueller-Mach et al. have used (Ca,Sr,Ba)SizOzNz:Euz+ (yellow-gre en) and (Ca,Sr,BahSisNs:Euz+ (orange-red) phosphors to fabricate highly efficient white LEDs.3\

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Index

Subject A Absorption, 2,18,22,36,52,54,65,86,93,97-98, 153, 218, 223, 258, 314 coefficient, in crystals, 3-4, 8, 19-21, 24, 185, 224, 248 cross-section, 3, 7-9, 91, 197 intensity of, 6, 19, 98, 151, 169, 171, 175 of light, 1,3-6,8,12, 19,40 spectrum, 23, 31, 41, 53, 55-56, 63-64, 68, 169, 227, 247-248, 255 Acceptors, 40-41,43-44,52,90, 92, 124,227,243, 247,285,287,295,300-301,317-318 Adiabatic approximation, 28 After-glow, 73-74, 76-81, 83-86, 84-86 (AI,Ga,In)(P,As) alloys emitting infrared luminescence applied devices, 295, 305 compound semiconductors based on lnP, 291-293 crystal growth, 284-285, 294-295 determination of GalnAsP /InP solid compositions, 293-294 emitting visible luminescence bandgap energy, 283-284 characteristics of InGaAIP crystals grown by MOCVD, 285-288 crystal growth, 284-285 light-emitting devices, 288-290 Anomalous emission, 131, 137-139, 139 Anthracene, 54-57

B Back-scattering factor, 103, 108 Bandgap, 11, 13, 15, 18-20, 18-21, 40, 43, 60, 62-63, 63, 108, 112, 118, 126, 208, 231, 287,289,291,294,309,314 energy, 43, 62, 106-108, 108, 124,240, 245, 278, 283,285-286,292-294,299,301-302 Band theory, 11-18, 11-19, 22 Bethe's formula, 104 Biexcitons, 24, 68 Bloch function, 13, 17, 126

Bloch's theorem, 13 Bohr radius, 40, 45, 64, 66-67, 92, 243, 278 Boltzmann distribution, 6, 31, 118 Born-Bethe treatment, 125-126 Bragg-condition, 14-15 Branching ratio, 75 Breathing mode and configurational coordinate model, 26-30

c Carbostyryls,59 Cathode-ray tubes (CRT), 102, 124, 229-230, 244 phosphors, 89, 229 Cathodoluminescence, excitation mechanism of, 101 Charge-transfer (CT), 168 state (CTS), 75-76, 165, 168, 188, 195, 199 Charge-transfer band, 168 Concentration-quenching processes, 56 Condon approximation, 30, 150 Conduction bands, 11-12, 18-19, 124 Configurational coordinates, 26 model, 26-30, 32, 34, 37, 48, 75, 78, 87, 149-150, 152, 195, 199, 207, 210, 243 Configuration interaction, 162 Cooperative optical phenomena, luminescence, 97-99 Correlation energy, 67 Coulomb attraction, 63, 133 Coulomb force, 24, 43 Coulomb interaction, in resonant energy transfer process, 90 Coulomb potential, 15, 22, 124 Coumarins, 56, 59 CRT, see Cathode-ray tubes Crystal lattice, 11-12 Crystal potential, 14-15 Crystal structures, type of rock-salt, 11-12,217 wurtzeite, 11-12 zinc-blende, 11-12, 18-19, 23, 222, 278, 284, 313

329


330

o Dead vo ltage, 105 Delta functi on , 63 Density of states, 18, 62-63 , 79, 124, 310 Dexte r mechanism, 56 Dexter 's theo ry of reso nan t energy tran sfer 90 Dieke dia gr am , 130, 134-135, 183 ' Dip ole mom ent, 4, 6-9, 7, 9 Dipole-quadrupole int eraction , 91 Dip ole transition , 8-9, 91, 185, 250 Direct ga p mater ial, 20, 24 Direct tran siti on, 18- 22, 24,41, 62,201, 223-224, 283 type semicond uc tors, 41, 43, 62, 238, 284 Distributed Bragg reflector s (DBR), 269, 279 Donor-acceptor pair (DAP), 43-46, 74, 122, 124, 227,243,268,278-280, 314, 316 Donors, 39-43, 94-95, 124,240-245,285, 300, 314, 316-317, 319

E Effective mass tensor, 17-18 Eigenvalue equation, 14 Einstein's B-coefficient of optical absorp tion 6 Electric dipole ' moment, 4, 6, 8-9 oscillator, electromagnetic radiation from , 5 transfers energy, 4 transition probability, 6-7, 30 transitions, 62, 98 Electro luminescence (EL), 52, 72, 111, 131, " 230-231, 240, 289,302,305,319 morgaruc, see In organic electrolum inescen ce qua ntum efficiency, 72 Electro mag ne tic rad iation from electric dipole osci llator, 5 Elec tronic ener gy bands, 11 Electro nic transitions, in organic molecules, 52-53 Electron orbital, spa tial distribution of, 27 Electro n-phonon int eraction, 31, 34, 37, 243, 252 Emission spec tra, 31, 53-55, 95, 145, 152, 194, 201, 212, 289, 309, 311, 323 Empty states, 12,22, 130 Ene rgy band , qualitati ve int erpretation of, 14 conse rva tion, 19 eigenva lues, 14, 162 Energy levels for electrons and holes, 61-62 of free exciton, 22 Excim ers , 54 Excitation energy transfer, 89-90 concen tration quenching of luminescence, 96-97 diffu sion of excitation, 94-95 reso na n t energy transfer, theory of, 90-9 3 exchange interaction, 92-93 multi polar int eraction, 90-92

ph on on-a ssisted energy tran sfer, 93-9 4 se ns itiza tion of luminescen ce, 95-96 Excitation migration , 94, 96 Excitonic molecul e, energy of, 24, 68 Excitons, 23-24, 41--43, 64-68, 89, 146, 213, 217-218, 224, 231, 241-242, 267, 278, 314,316

F Ferrn ion s. 12 First-order reaction type, chemical reaction kine tics, 84 Flu orescence , 51, 54-59, 73-76, 74, 81 lifetim e, of transition-metal ion, 36 mol ecules containing heteroatoms, 56 qu antum yield , 56-59 Forster mechanism, 55-56 Fourier coefficients, 14 Four ier ser ies, 13 Franck-Condo n coefficient, 36 Franck -Condo n factor, 30 Franck-Condo n prin cipl e, 27, 30 Free excitons, 40--41 , 241-242, 267 Frequ ency factor, 28, 75, 82

G GaAs qu antum wells, 64 GaN and rela ted lumin escence materials, 299-300 Ga lnN ,301-302 Ga ln N / AIGaN LED, 302-303 GaInN multiqu antum-well (MQW) LD, 307-311 GalnN sing le-qua ntu m-we ll (SQW) LEDs, 303-307 n-t yp e GaN, 300, 304, 307 p-type GaN, 300-301, 307 Ga ussian shape, 30-31, 34 Gian t oscillator stre ngt h effect, 41 Glow curve, 80-84 y-rays, ene rgy dissip ati on , 105

H H armon ic oscillatio n, 29 Harmonic osci llator, wave function of 30-3 1 H igh- en ergy electro n, excitation proc~sses by, 89, 106 Hi gh est occu pied m olecul ar orbita l (HO MO), 52-53, 206, 212 Host sensitization, 107 Ho t elec tro n, 119, 121, 124, 127 Hua ng-Rhys -Pekar factor, 31, 243, 252-253

IC p rocesses, see In ternal conversion pro cesses Image force, 114-115

Index: Subject

331

Impact ioni zation, 119- 121, 124-125, 131 Impurity trapped exciton state , 131 Indirect gap materials, 21 Indirect transition, 18-22, 24 type semico nd uctors , 41, 43 Inhomogeneous broadening, 36 Inorganic electroluminescence, 111 high-field EL, 111-11 4 electron ene rgy dis tribu tion in high electric field , 118- 122 excitation mechanism o f lum inescence centers, 122-1 27 injection of carriers, 114-118 injection EL, 111-112, 114 Intern al conve rsio n (IC) p rocesses, 53-54 Inte rsystem cross ing (ISC), 53-54 Inter valen ce cha rge tran sfer (!VCT), 131-132 Isoelectronic traps, 43, 107, 112, 220, 276, 278 Iv'Cl', see Inte rv alence cha rge transfer

J Jahn-Teller effect, 29, 34, 149, 219 IT-coupling scheme, 10 Jorgensen model of optical electronegativ ity, 140

K Killer effect, 257-259 Killer ions, 95 King-Van Vleck factor, 146

L Lagu erre's polyno mial fun ction s, 30 Lambert's law, 3 Lanthani de level locations and its impact on phosphor performance, 129- 130 absolute level locations, systematic variati on in,137-142 4f- 5d energy differences of lanthan ide ions in compounds, 134-136 free (gaseo us) lanthanide ions, 133 future prospects and pretailorin g ph osp hor pro perties, 142 level positio n and phosphor perform an ce, 130-133 me thods to determine abso lu te level locations, 137 Latt ice vector, 12- 13 LED, see Light-em itting diodes Ligand field theory, 158, 169, 171 Light , absorp tion and emission of, 1 in crysta ls absorptio n coefficient, 3 optical constan t and complex dielectric constan t, 2-3 reflectivity, 3-4 transmissivity, 3-4 by impuri ty atoms

classica l harmonic oscilla tor model of op tical cen ters , 4-5 electric d ipole transition probability, 6-7 electro nic tran sition in an atom, 5-6 forbidden tran siti on , 9 impur ity atoms in crys tals, 9 int ensity of light emission and absorption , 7-8 osci llator stre ng th, 8 selectio n ru le, 9-10, 20, 73-74 Light-emitting d iodes (LED), 72, 111-11 2 application for, 280-281 ph osphors, 131 Linear combina tion of ato mic or bital method (LCAO me tho d), 15-1 7 Local ized center, classifica tion of, 25-26 Low-dimension al sys tems, 61- 72 Lowest unoccu pi ed mo lecular orbital (LUMO), 52-53, 206, 212 LS-coupling schem e, 10 Lucky electron mod el, 120-121 Luminescence configurational coordina te mod el and clas sical mod el, 26-28 quantum mechani cs and, 28-30 D-A pair luminescence, 44-46 decay of, 73-76 fluorescen ce, 74-76 qu asistable state and phosphorescence, 76-77 trap s and phosphorescence, 77-80 of d onor-acceptor pairs and semicond uc tors, 43-46 exci tation mechanism of, by catho de-ray and ionizing radiation, 101 collision of primary electrons with solid surfaces, 101-103 energy transfer to luminescen ce centers, 107 ioni zation processes, 105-1 07 luminescence efficien cy, 107- 108 pe ne tra tion of primar y electrons in toa solid, 103-105 of exci tons bound to im purities and se mico nd uctors, 40-43 fun d am entals, electronic sta les an d op tical tra nsi tion of so lid crys tals absorp tion, d irect tran sition , and indirec t transition, 18-22 band theory, 11-19 exciton, 22-24 of isoelec lronic traps and semiconductors, 43 of localized center, 25-26 of low-d imensional systems, 61-72 nonr ad iati ve transitions, 36-37 of orga nic com pound s electronically excited states of organic molecules and their photoluminesce nce, 51-54 fluorescence of organic molecul es in a so lid sta le, 54-56

332 orga nic fluorescent and ph osphorescence comp ounds with high qu antum yields, 56--59 origin, 51- 52 quantum y ield of fluorescen ce, 56 ph otost irnul at ion and pho toq uenc hing, 85-87 polarization of, 33, 127, 248 of semiconductor microcrystal lites, 68 sensitization of, 89, 95-96 , 107 spectral shapes, 30-34 line broadening by tim e-d ep endent perturbation, 34-36 line broadening by time-indep end ent perturbation, 36 thermal quenching of, 26, 37 thermoluminescence and, 80-85 Luminescence centers of com plex ions , 205-206 comp lex ion centers perspective of oth er in teresting cen ters, 214-21 5 W06 &- ion , 214 plat inum complex ion centers, 211-212 oth er platinum comp lex ions, 213- 214 [Pt(CN )4F- com p lex ions, 212-213 Scheelite-typ e com po unds electronic struc tures of close d -shell mol ecul ar com plex cen ters, 206--207 general prop erties, 206 luminescen ce cent ers of MoO/ - ion type, 208-209 luminescen ce centers of V04 3-- ion type, 207-208 luminescenc e centers of WO/- ion type, 209-210 other closed- sh ell tran sition metal complex cen ters, 210 uranyl complex centers electronic struc ture , 210 luminescence s pec tra, 210-211 Lum inescen ce centers of ns--type ion s centers in practical phosphors, 152-1 55 op tica l spec tra of, in alka li halides absorption spec tra , 145-149 emiss ion spectra, 152 str ucture of the A an d C absorp tion ban d s, 149-1 51 temperatu re d ep end ence of the A, B, and C absorp tion bands, 151 Luminescen ce cen ters of rare-earth ions electronic config ura tion, 182-183 electro nic pr ocesses leading to luminescence d ivalent an d tet ravalent cations, 189 ene rgy tra nsfer, 189-190 4f ene rgy levels and relaxa tion, 183-1 88 4fo-l 5d 1 states and cha rge- trans fer states (CTS), 188 specific ion s Cel" 190-191 Dy2+, 200 Dyl" 199-200 Efl ', 201

Fundamentals oj Phosphors Eu2+, 196--197 Eu3" 194-196 Cd 3' , 197-198 H o z',201 Nd 3' , 193 N d 4+, 193 PrJ' , 191-1 93 Sm2+, 193-194 Sm3', 193 Tb3"198-1 99 Tm3+, 201 YbZ" 201 Yb3" 201 Luminescen ce cen ters of tran sition metal ions CrJ' phosphors (3d3 ) , 168- 172 crystal field theory, 157-164 cases of m ore than one d electron, 161-162 3d l electron configuration, 158-161 int ensities of emission and absorption band s, 164-167 spin-orbit int eraction, 164 Tanabe-Sugan o diagrams, 163-164 electron cloud expansio n, effect of charge-tra nsfer band , 168 n ephelau xetic effect, 167-168 Fe3, phosp hors (3d 5), 177- 178 Mn 2+ ph osph ors (3dS ) crysta l field, 173-1 75 different Mn> sites in crys tals, 175-1 76 luminescence decay time, 177 UV absorption, 176--177 Mn4+ phospho rs (3d3 ) , 172-173 Lum inescen ce material, silicon carbide (SiC) as band str ucture an d optical absorption, 314 crys tal grow th an d doping, 319 light- em itting d iodes, 319 lumin escence from d on or-acceptor pairs, 316--318 from excitons, 314-3 16 po lytypes, 313- 314 Lu minesce nce transi tions, atomi c struct ure of various cen ters and, 253-255 Lu minescen t tran siti on, ang ular freque ncy of, 29- 30

M Mesop ic visio n, 51 Metal orga nic vapor pha se ep itaxy (MOVPE), 265-266 Metal-t o-ligand cha rge transfer (MLCT), 59, 214 Molecular bea m epitaxy (MBE), 230, 238, 265-267, 284 Mome ntu m conserva tion , 19 selec tion ru le, 20 Mott transi tion, 24 Mult iplet, 162, 164-1 65 Multi qu antum-w ell (MQW) active laye r, 269 Mul tiqu antum-well (MQW) LD, Ca ln N, 307-311

Index:

333

Subject N

Nanometer-size semicond uc tor microcrys talli tes, 65 Naphthalimid es, 59 Naphtholylene benzimdazoles, 59 Natural lifetime, 7-8 Nephelau xetic effect, 167, 175, 177- 178, 322 Nonrad iative multiphonon transition pro bability, 76 No nr adia tive re laxation probability, 36-37 Nonrad iat ive tran siti on , 28, 54, 56, 74-76 , ] 99, 207,210 luminescence, 36-37 probabi lity by thermal activa tion, 75 ns2-type ions, luminescenc e cen ters of cent ers in practical phosphors, 152-]55 opti cal spec tra of, in alkali halides absorp tion spe ctra, 145-149 emission spec tra, 152 structure of the a and c absorption bands, 149-151 temperatu re dep endence of the A, B, and C abso rp tion bands, 151

o Octahedral coord ination, 158, 160, 172, 175, 178 Ia-VIIb compound s, 217-218 color cen ters, 218-219 intrinsic op tical properties band structure and exciton , 218 self-trappi ng of excitons and intrinsic lum inescence, 218 luminescence centers of ns--typ e ions, 219-2 20 lum inescence of isoe lectro nic traps, 220 Optica l abso rp tion spectrum, 63- 64 transition probability of, 6-7 Op tical centers classical harmonic oscillator model of, 4-5 oscillator strength of, 8 Op tical cons tant, 2-3 Organic compo u nds , luminescence of electron ically excited states of organic molecules and their pho tolumi nesce nce, 51-54 fluorescence of org anic molecules in a solid state, 54-5 6 organ ic fluo rescent and p hosphorescence com pounds with high qu ant um yield s, 56-59 origin, 51-52 qUil ntum yield of flu orescen ce, 56 Organic fluorescent mo lecules, classification, 56-57 Organ ic solids , fluorescence in, 54-56 Organic thin-film electro lumines cent devices, 59 Overlap integr al, 16, 30, 37, 126 Oxynitride phosphors, 321- 322 app lications of, 327- 328

characteristics of ~-SiAlON : E u 2 + , 323-324 LaAl (Si(,-zAl z)N ]o_p z:Ce3 +, 323 M2S isNs:Eu 2+, 325-327 MSi20 2N 2 :Eu 2+, 324 a- SiAlON :Eu 2+, 325 overview of, 322-323

p

Pauling electronegativity, 139-140 n-electron sys tem s, 51 Perrin 's mod el, 93 Pe rylene, 54, 57 Phon on (s), 31, 47, 76, 94, 106, 121 emission, 21 en ergy, 21 longitud inal op tical (LO), 42, 107 number, 32 number, and op tical transition, 32 Ram an scattering of, 36 Phosphor(s) applicati on s of, 129-130, 132, 327- 328 lanthanid e level locations and perf orm an ce of, see Lanthan id e level locati on s an d its impact on ph osphor per form ance localized lu minescen t cen ters, 25-26 luminescence centers of Cr3+, 168-1 72 Fe)' , 177-1 78 Mn 2+, 173- 177 Mn

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