Nucl. Tracks, Vol. 10, Nos. l/2, pp. 21.5424, 1985 0191-278x/85 $3.00 + .oO Printed in Great Britain Pergamon Press Ltd.

ELECTROLUMINESCENT MATERIALS

SHASHI BHUSHAN Department of Physics, Ravishanker University, Raipur, 492 010, India

(Received 9 February 1984; in revised form 1 June 1984)

Abstract-Electroluminescent (EL) materials are important because of their commercial applications as solid state indicators and display systems. In this review, various features of almost alI kinds of electroluminescent materials are described with a special emphasis on powder phosphors, the mechanism of which is comparatively less understood. Light emitting diodes (LED’s) from II-VI group compounds have been reviewed in addition to those from III-V group compounds. Injection Ittmineseence from structures like metal-semiconductor, met&insuIator-semicoiconductor and hetero- junctions is also discussed. The relative merits of light emitting thin film displays over other such displays are also reviewed.

1. INTRODUCTION

SUBSTANCES, which emit electromagnetic radiation (visible or near visible) due to the application of an electric field (AC or DC) are called electro- luminescence materials and the phenomenon involved in the emission of the light is termed as electroluminescence (EL). * This article reviews various features of such light emitting materials, which are now finding ever-increasing commercial applications (especially the light emitting diodes, the diode arrays and the phosphor displays). Although a number of articles have appeared on EL, (Destriau et al., 1955; Piper et al., 1958; Fischer et al., 1965; Fischer, 1966; Morehead, 1967; Cusano, 1967; Vechts et al., 1970; Pankove, 1977), they are all restricted to specific aspects of EL materials of different kinds. The present review therefore specifically aims to provide an overview of the different kinds of EL materials with a particular emphasis to powder phosphors, the mechanism of EL in which systems is relatively less understood.

2. MRCHANISM OF EL

The phenomenon of EL can be considered to comprise three sequential processes,

(i) excitation, (ii) transfer of energy from the site of excitation to

the site of emission and,

(iii) recombination.

2.1. Excitation The excitation of EL takes place by a supply of

energy (potential or kinetic or both) to the charge carriers (generally electrons). The different excita- tion

(i)

(ii)

mechanisms proposed are: Field ionization of valence electrons and impurities. As the break down field competes with both of these processes, activator systems having a low ionization energy in dielectric phosphors (large band gaps), should be more conducive to EL by this process (Piper ef al., 1955). Injection EL. Injection of minority carriers may occur either at an electrode contact or a P-N junction. In a surface contact, the inter- face may possess ohmic characteristics or is capable of rectification and, thus, its be- haviour is governed by factors like the differ- ence in the work functions, the position of the Fermi level in the semiconductor, nature of the intermediate chemical or physical barrier layer, surface states and the concentration of the donors and the acceptors. For example, in an idealized case of the difference in work function being the dominant factor, an ohmic contact is formed between a n-type semi- conductor and a metal, if the work function of the metal is less than that of semiconductor. A

*These materials can be powders, single crystals or thin tihns.

215

216 SHASHI BHUSHAN

rectifying contact, however is formed for aninverse situation. The barrier formed in thelatter type of contact is known as Mott- N-tyl~z p-type N-type P-type

Sch°ttky barrier" In cases' where the w°rk ~ E c l ~~_ . . . / ~ Ec2function of metal is no more than a few "kT" Ec1less than the sum of the electron affinity andthe band gap of semiconductor, the Fermi - ~ 2 ~ ~ flevel approaches the valance band in the E v , . , . ~ k , , ~ x \ \\ v1 Ev2 . . . . . ~_qV_region near the surface ( P - T y p e ) a n d this 2~k\\XX\\XX~X~~ \.,..~7.~,,-,,~~\\\\,,~\\.x~Evlresults in an inversion layer. In this type of a {a) (hibarrier, minority carders can be injected intothe interior of the semiconductor. InjectionEL occurs in following structures:

(a) P-N junction: As shown in the Fig. 1, the FIG. 2. Conditions at a heterojunction (a) at equilibriumpotential energy of electrons in the N- and (b) under forward bias. Ect, Evt and Egl representregion is raised under a forward bias, conduction band edge, valence band edge and band gapwhich allows the flow of electrons in to of material number 1 respectively. A subscript 2 has beenthe P-region and similarly the flow of used for material number 2. Ef and V have their meaning

as in Fig. 1.electrons into the N-region. As soon as anoverlap of electrons and holes occurs,the i r recombination becomes possible.The Light Emitting Diodes (LED's) oftenhave one side of the junction more heavily ~ F - ~ " ~_ ~idoped than the other. Due to this, the . . . . . . . . .Ercarriers in the larger doped side have thebenefit of the reduction in the barrierheight.

(b) Heterojtmctions: As shown in the Fig. 2,the large band gap materials act as a Fio. 3. A Schottky barrier for an N-type semiconductor.source of injected carriers and that with Symbols mean as in fig. 1. Arrow indicates minoritysmall band gap as luminescent region, carrier injection during forward bias.

(c) Schottky barriers: Fig. 3 shows the inver-sion layer in an N-type material. U n d e r a forward bias, the band are flattened

allowing the injection of minority cardersinto the bulk matrix, where they re-combine radiatively.

N-type P-t.yp~ N-Lype P-type (d) Metal-Insulator-Semiconductor (MIS)y / " ~ - - - E c _f Ec structure: In this structure the band

bending can be controlled by an applied. . . . . . Ev- voltage and an inversion layer can be

introduced by one polarity. The carrier soaccumulated at the surface can be injectedby reversing the polarity of metal

la~ tbl electrode (Fig. 4). If the insulator is madethin (< 10 nm), the injection of electronscan take place by tunnelling.

F16. 1. Conditions at a P-N junction (a) at equilibriumand (b) under forward bias. Ec, Ev and Ef represent (iii) Radiative tunnelling: As is shown in Fig. 5,conduction band edge, valence band edge and Fermi levelrespectively. V is junction voltage under forward bias. electrons from the conduction band tunnels

The arrow represents a band-to-band recombination, into the gap where it makes a radiative

ELECTROLUMINESCENT MATERIALS 217

excitation of the luminescence centres. Its~ - - ~ E F ~ \ ~ various stages are shown in Fig. 6. A Mote-

EF Ec Schottky exhaustion barrier is suggested to, ~ , ~ ~EV account for the stability against breakdown at

Ec the field strength at which EL excitation takesEv place (Piper et al., 1958). The field in the

M I s M I s barrier and the thickness of the barrier(a) tb) increase as the square root of the voltage. The

source of charge carders may be deep donors:~ \ \ ~ Ec acceptors or traps which are field ionized.

Ev Excitation may also occur from hot carders(Gaffaux, 1956; Nagy, 1956). Three impact

EF processes relevant to high field EL i.e.ionization across the band gap, impurity

M 1 S excitation and ionization have been consideredtc) by Allen (1981). Carder acceleration may also

occur in semiconductors under reverse bias.Fxo. 4. Light emission at a metal--insulator-semiconductor(MIS) structure (a) without bias, (b) metal(-)ively biasedto form an inversion layer in which the surface (P-type)and bulk (N-type) have opposite conductivifies. Holes aregenerated in the surface region and (c) metal is(+)ively ~ _ __ _ 2 _biased. The holes accumulated at the surface are injected

and recombination takes place (shown by arrow).

I ~ -trapsN-type P-typ~ ~ c l - ivat.or

Ef ~ . . . . . . .x \ \ \ \ \ \ \ \ \ \ Y _ - ' t-- T- . . . .

~ ~ ~ ~ EfL('x'xxxxxxx',xx FIO. 6. Acceleration--colfision mechanism: (1)liberationof electrons from traps by the field and/or temperature,(2) acceleration of electrons, (3) collision with electrons inactivator centre, (4) ionization or excitation of activator

centre (emission occurs in reversing side of AC).Fie. 5. Radiative tunnelling across P-N junction underforward bias. haJmax and h'Omin show the maximum and

minimum energy of emission respectively. (v) Breakdown luminescence: By gaining suffi-

cient energy from the electric field, electronstransition to the valance band or to an empty and holes are multiplied and thereby form anstate in the band gap. For this process to avalanche breakdown. The pairs created inoccur, the semiconductor should be degener- this way, recombine radiatively and such kindatiy doped on both sides, as has been observed of emission has been observed in Si (Chyno-in the case of Ga(As) , (Pankove, 1964). weth et al., 1956), Ge (Chynoweth et al., 1960)

and GaAsl-xPx (Pilkhun et aL, 1965).(iv) Acceleration - collision EL: A process, found (vi) Luminescence from travelling high field

in most suitable materials like ZnS and is domains: According to Pankove (1971), ELbased on the acceleration of the electrons in may arise from travelling high field domains.the conduction band followed by a collision However no experimental results have been

218 S H A S H I B H U S H A N

reported supporting this. Several phenomena semiconductors it requires emission or absorp-convert low applied electric field into domains tion of a phonon to complete the lowestof intense electric fields, which then travel energy transition across the energy gap,through the specimen and generatc high (ii) Transition between band and impurity: Adensity of electron-hole pairs, either by impact radiative transition can occur between anexcitation or by Zener tunneling. In P-N impurity state and an intrinsic band and itsjunctions, the domains can cause a breakdown probability is high, if the impurity state isor injection, deep ,

(iii) Transitions at a localized centre: In large band2.2. Energy transfer gap materials impurities can be excited with-

The mode of energy transfer from the point of out ionizing them i.e. without exciting theirexcitation to the site of emission occurs via electrons to the conduction band,numerous agents. The most obvious candidates are (iv) Transitions at isoelectronic centres: Suchcharge carriers. The flow of energy due to the centres are formed by replacing an atom of thetransport of charge carriers depends on the con- host crystal by another, but with the samecentration gradient, the electric field intensity and valence. Radiative transitions can occur in thethe concentration of the trapping states (Piper and exciton bound to an isoelectronic centre, and,Williams, 1958). Hall (1952) and Shockley and Read (v) Donor-acceptor transition: Efficient transi-(1952), obtained explicit expressions for the tions can also occur between donor andcapture and recombination process occurring in a acceptor levels and such transitions are foundmaterial, in which only one type of recombination in LEDs (Thomas et al., 1964) and incentre (having one level in the forbidden region) is phosphors (Bhushan et al., 1979).present. Haynes and Hornbeck (1954) extended Transitions from upper to lower energy statesthis analysis to the cases of a centre with more than occurring without emission of photons are termedone such level. Broser and Broser-Warminsky as non-radiative transitions and can occur through a(1955), analysed specific types of luminophors with number of processes such as:more than one type of centre. The theory of (i) Multiphonon emission at the intersection ofpositive hole migration between activators has also the ground and excited state configurationbeen developed. The transport of energy from the diagram. The electron can escape the excitedelectrically excited system to the emitting system state and return to its equilibrium position bycan also occur by mechanisms not involving generation of several phonons,electronic charge carriers, i.e. by (ii) Auger effect, wherein a recombining electron

(i) Cascade transfer mechanism, can transfer the energy (which it would have(ii) Resonance transfer mechanism, and normally radiated), to another electron in the

(iii) Exi ton migration mechanism (Klick and excited state. This electron goes to a stillSchulman, 1957). higher energy state and returns to lower

These depend on the relative proximity of the energy excited state by multiphonon emission;centres capable of luminescing, the overlap of and,absorption and emission bands and dipole--dipole, (iii) Non-radiative defects such as surface recombi-multipole interactions (Windischmann, 1970). nation crystal defects providing regions, where

a localized continuum of states can bridge the2.3. Recombination energy gap and cluster of vacancies (or a

In the process of recombination, various transi- precipitate) of impurities can form non-tions take place from upper energy state to empty radiative centres.lower energy states. These transition can beradiative or non-radiative. Radiative transitions

occur by the following processes: 3. ELECTROLUMINESCENT MATERIALS(i) Band to band transition: In direct semi-

conductors it occurs between conduction and EL is a meeting ground of semiconductor physicsvalence bands and in the case of indirect and insulator physics, and it has been observed in a

ELECTROLUMINESCENT MATERIALS 219

variety of materials (Henisch, 1962; Ivey, 1963; on its details. Zalm (1956), believed that theWindischmann, 1970), e.g. I I-VI group and III-V phosphor grains got uniformly covered with a verygroup compounds, alloy combination of II-VI thin layer of a semiconductor (e.g. CUES) and thegroup compounds, ionic salts, ferroelectric excitation was confined to the surface. Presence ofmaterials, ice and organic compounds, etc. the chemical barriers on the surface was alsoAlthough EL has been observed in different forms, suggested (Curie, 1953), but direct evidence to themost of the work has been concentrated on the contrary was afforded by microscopic observationspowder phosphors, which is also the commercially (Halpin et al., 1961). The emission was found topreferred form. The different kinds of EL materials occur mainly in the interior of the particles. CUES isare discussed below: a P-type semiconductor and it can also produce

P-N junctions. According to Georgobiani et al.3.1. P o w d e r p h o s p h o r s (1961). CUES segregations can play a direct role as

The most important phosphor has been ZnS:Cu. the contacts for the formation of the barriers or asAlthough a large number of impurity elements a source of electrons which tunnel into ZnS. Thisyield photoluminescence (PL), not all have been may also produce a local concentration of field byfound to give EL. The preparational method range geometrical effects as a result of high conductivityfrom the conventional precipitation of activated (or of CUES compared to that of ZnS (Maeda, 1959).unactivated) ZnS using hydrogen sulphide or According to Fischer (1966), the microcrystallitesammonium sulphide to a homogeneous precipita- of the type ZnS:Cu, C1, (which are nearly insulat-tion using organic precipitants like thio-urea, ing compensated semiconductors), contain imper-(Vechts et al., 1970). Other methods include solid fection lines which are decorated with CUES. If anstate organic reactions and the decomposition of electric field is applied, it relaxes inside theorganometallic compounds such as diethyl- conductors and leads to the intensification of thedithiocarbamate. The impurities as well as the field at the tips of the conducting needle. Further,variation in the stoichometry can be introduced at the opposite tip of these conducting needles, theduring one of the following stages: electrons and holes derived from the conduction

(i) a slow addition of solution of impurities during and the valence band of CUES respectively areprecipitation or by an addition of appropriate injected, which are then responsible for an ELorganometallic complexes to Zinc compound emission. Brovetto et al. (1971) and Busca et al.thereby synthesizing the doped sulphide, (1971), believe that an explanation of EL pattern

(ii) by slurrying the raw material in solution of the can be found in terms of the properties of stackingrequired impurities followed by a firing stage, fault type linear defects in luminor crystallites andIn non-acqueous systems organo-metallic ad hoc entities of CUES can be avoided. Evidencecomplexes of impurity can be added, and, for the existence of copper sulphide has been

(iii) by firing the doped (undoped) raw material in presented by May (1981), who monitored thean appropriate atmosphere, device conductance and/or luminance as a function

Some important requirements of a good EL of temperature. Bhushan and Chandra (1984)phosphor are: suggested that the formation of a conducting phase

(i) its electron traps should be shallow, like CUES is important and a transport process of(ii) its activation energy for the escape of holes Schottky emission type is largely effective. The EL

from the ionized activator levels should be as efficiency, p, on the basis of this transport processlarge as possible, and, may be given by:

(iii) it should have a cubic structure, which is morefavourable for EL emission, log p = log Bo - log A - log V - 0.43

Another important requirement is an intimate [(b/X/-V-)+ (aX/V)] (1)contact between the ~phosphor and the electrical The constants A and a are determined from I - Vconducting phase, like CUES. This fact provides an plot and Bo and b from B - V plot. The theoreticalexplanation to the question of the formation of and experimental curves for CaS:Cu, Er are shownbarrier in the case of phosphors embedded in a in Fig. 7.dielectric. However there has been little agreement One of the systems of recent interest has been

220 SHASHI BHUSHAN

'z~ ~ ~ - - o - - ~ - o -~ . . 1.o

15 o

X /

/ e----e E x •P

>-I0

0.5

5 / w

0 2"8 2.6 24 2,2 2"05 10 15 20 25

VolLage ( x l O O V ) PhoLon e n e r g y (~V)

l~o. 8. Photoluminescence (1) and electroluminescence (2)FIO. 7. Dependence of EL efficiency p on the applied spectra of undoped ZnO phosphors prepared by twovoltage V for CaS:Cu, Er phosphors..--- Experimental methods, by burning metal zinc in the atmosphere of

.... Theoretical (equation 1). pure oxygen (method 1) and --- by firing the raw materialZnO (method 2).

ZnO phosphor. It presents an interesting system 3.2. Thin f i lmsbecause of its emission in different spectral regions. Light emitting thin film displays are extremelyThere are however controversies on their origin, attractive and these also have the advantage of aBhushan et al. (1979) proposed that the green- variable brightnes, fast speed, thin package size,yellow, green and blue bands of self activated ZnO high temperature and humidity specifications, highare due to centres formed by cation vacancies, contrast ratio, low cost and extremely long lifestoichiometric Zinc and edge emission in the (Sigmatone, Inc. 1973). The cold cathode displayspresence of some impurity levels respectively. This suffer from poor contrast besides severe parallaxmodel has been favoured by Takata et al. (1981) on problems. LED displays have the disadvantage ofthe basis of their work on the sputtered films of this segmental size limitation, negative temperaturesystem. In their later work (Bhushan and Asare, coefficient and high cost. Further, the liquid crystal1981a), it has been emphasized that the break-up of displays have considerable temperature limits, slowcrystallites due to grinding generally takes place on display speed and short operational life and arethe boundaries of coherent zones. The centres dependent on an additional light source.responsible for green band are mostly located near In the earlier studies, three principal methodsthe boundaries of coherent zones while those for were used for the preparation of thin films:yellow bands lie inside the coherent zones. Further, (i) direct evaporation of activated ZnS (Golov-rare earth doped ZnO gives broad bands (Bhushan kina et al., 1965),et al., 1979), which are explained in terms of a (ii) chemical reaction of Zinc and activator halidesdonor-acceptor model, with rare earths forming with sulphur containing compounds (Studer etthe donor levels. Intensity shifts has been observed, al., 1955) and,which further support this model. However, Vu (iii) embedding and firing of a thin film in aXuan Quang et al. (1983), have found that line powder phosphor containing the requiredemission in rare earth doped ZnO is observed in activators (Vechts, 1966).the presence of V-group ions. It is further reported Some recent methods used for the preparation of(Bhushan and Asare, 1981b), that ZnO prepared thin films are:by burning metal zinc in pure oxygen produces (i) Sputtering (Takata et al., 1981; Warren et al.,efficient phosphors (Fig. 8). 1983),

E L E C T R O L U M I N E S C E N T M A T E R I A L S 221

(ii) Organometallic chemical vapour deposition 3.4. L i g h t emitting diodes(Cattella et al., 1983), The most developed LEDs are from I I I - V group

(iii) Atomic layer epitaxy (Tauninen et al., 1981), compounds. A list of important LEDs is given inand Table 1 (Bhargava, 1975). Such LEDs are usually

(iv) Molecular beam epitaxy (Yao et al., 1983). Of fabricated from epitaxial layer grown on a singleall the different thin film materials, the most crystal substrate cut from melt grown ingots.important to-date are ZnS:Cu, Mn and Epitaxial layers are shown by, ( i ) V a p o u r phaseZnSe :Mn systems, epitaxy ( V P E ) and (ii) Liquid phase epitaxy (LPE)

methods (Bergh and D e a n , 1976). Although VPE3.3. Single crystals m e t h o d has been well developed, the LPE method

Single crystals are important from the view point yields LEDs with higher efficiencies. P-N junctionof understanding the mechanism of EL. Single is formed ei ther by N- and P-layer grown by eithercrystals of ZnS, one of the most important of the two methods or by Zn diffusion into an N-materials, have been prepared from vapour or type layer grown by any of the two methods.melt. G r o w t h from vapour phase is preferred For an enhanced utility, L E D s , capable ofbecause of a high melt ing point. Piper et al. (1952) emission over the ent ire visible spectrum areprepared single crystals of ZnS from its vapour in a needed. For this property, a semiconductor mustsealed tube placed in a temperature gradient, have a large band gap (~> 1.8 eV). In this contextwhereas, G r e e n e et al. (1958) used the atmosphere the large band gap I I - V I group compounds (suchof argon, hydrogen-sulphide, hydrogen or nitrogen, as CdS, Z n T e , ZnSe and ZnS) are interestingKremhel ler (1955), t ransported ZnS vapour in a because they,gas flow of H2S and H2 (or He). Addamiano et al. (i) possess excellent luminescent properties,(1960) obtained the crystals from melted ZnS at (ii) have band gaps, which range throughout thepressures as high as 7 atmospheres u n d e r argon, visible spectrum,All these crystals lack reproducibility, owing to (iii) can be produced in the form of mixed crystalshigh density of defects and poor control on (alloy) combinations, andstoichiometry. Others have used static sublimation (iv) are direct band gap materials.(Anderson et al., 1965), vapour phase (Girton et However , not much progress has been made due toal., 1969) and hydrothermal synthesis (Minami et the difficulty in achieving a low resistivity ampho-al., 1979) methods, teric doping. This arises due to the compensation of

Table 1. Some important LEDs and their characteristics

Colour (peak P-N junction Luminous External quantumS. No. Materials wavelength, growth process output efficiency Brightness

nm) N-layer P-layer (lumens/W) Optimum Commercial L/A cm-2(%) (%)

1. GaP:(Zn,0) Red (699) LPE LPE 20 15 2.0-4.0 3502. GaP:N Green Yellow LPE LPE 610 0.7 ~0.5-0.1 470

(570)3. GaP:NN Yellow (590) VPE Zn-diff -450 0.1 -0.5 - -4. GaAso.6P0.4 Red (649) VPE Zn-diff 75 0.5 0.2 - -5. GaAso.asPo.65:N Orange (632) VPE Zn-diff 190 0.5 ~0.2 - -6. GaAso.15Po.as:N Yellow (589) VPE Zn-diff 450 ~0.2 0.05 - -7. Gao.TAlo.3As Red (675) LPE LPE ~ 35 1.3 - - 1408. Ino.3Gao.TP Orange VPE Zn-diff - - 0.1 - - - -9. SiC Yellow (590) VPE Al-diff ~500 0.003 - - 610

10. GaN Blue (440) VPE - - - 20 0.005 - - 400011. GaN Green (515) VPE - - 420 0.1 - - 4000

222 SHASHI B H U S H A N

native defects during growth, low solubility of bands at 2.808, 2.788, 2.781, 2.692, 2.680 and 2.660added impurities and high activation energies of eV at 77 K.donors or acceptors. Thus only CdTe has beenprepared in both low resistivity N- and P-type 3.7. Hetero]unctionsforms. CdS, ZnSe and ZnS have been prepared A heterojunction is made from materials ofonly in low resistivity N-type form and ZnTe in low different band gaps. The most successful devicesresistivity P-type form by conventional doping are made from different compositions of misciblemethods. However such difficulties can be over- alloys having similar lattice constants. Some recentcome through the use of techniques such as ion developments in this direction are:implantation doping, in which the amount of (i) Au/ZnSe:Mn/n--GaAs heterostructure withdopant depends only on the energy and the beam brightness 200 fL and power conversion effi-current of the ions. Some of the materials prepared ciency of 10-5 W/W at 20.4 DC and 7 mA,by this technique are already described by Pankove (Ohnishi et al., 1978),(1977). Other important materials are: (ii) In GaAs-InP heterostructure LED (Ueda et

(i) ion implanted ZnS:Er diode having the green al., 1980) and,emission characteristics of Er, (Yu and (iii) ZnSe-ZnTe heterojunction with emissions atBryant, 1978), and 2.32 eV and 1.95 eV at 40 K (Firszt et al.,

(ii) Nd implanted ZnSe diodes with external power 1983).efficiencies of about 5 x 10-5 and capable ofoperation at very low voltage and at very low 4. APPLICATIONS AND FUTURE SCOPEtemperatures, e.g. 77 K, (Zhong and Bryant,1982). The powder phosphors are the most convenient

form for the applications. But its scope is limiteddue to low light output. Although, Lehmann (1958)

3.5. Meta l - s emiconduc tor d iodes found the maximum efficiency for green phosphorSuch kind of diodes are easy to fabricate. A as 14 lumens/W, which is about 18-19 lumens/W if

point contact or an evaporated layer on the surface the correction for optical absorption in the cell isof a semiconductor forms a M-S diode. Important considered. In practical EL lamps, the efficiency isrecent progress made in this direction are: usually 3 lumens/w. On the energy basis, the figure

(i) Blue emitting S+ implanted A u - Z n S Schottky obtained by Lehmann corresponds to an efficiencybarrier diode, with room temperature external of 4 .5%, which is not significantly appreciable. Thisefficiency of 2 x 10-6 at 3.7 V and 50 mA, figure however can be compared with those from(Lawther et al., 1980), o ther light sources e.g. 100 W incandescent lamp

(ii) ZnSe Schottky diode (Ando et al., 1981) and, with 16 lumens/w; 40 W fluorescent lamp with 75(iii) efficient M-S diodes from ZnO (Takata et al., lumens/w, for white light in each case. Some

1981). present and anticipated applications of ELmaterials and devices (phosphor display panels,

3.6. Meta l - i n su la to r - semiconduc tor d i o d e LEDs and diode arrays) are low intensity lightProgress has also been made in achieving sources, solid state indicators (e.g. digital clocks,

emission from MIS structures. In these devises, meter read outs), display systems (e.g. instrumentmethod used to provide insulating layers vary panels and TV displays), image amplifiers and logicconsiderably e.g. proton bombardment for produc- control circuits etc. The application in each case ising a semi insulating ZnTe region or A1 vapour determined by the light output and the cost of thediffusion to produce insulating regions in ZnTe particular device. Kazan (1958) in his earlier work(Tonomura et al., 1 9 7 3 ) a n d use of Collodion developed a solid state panel, using photo-(cellulose dinitrate), or SiOx as insulator in CdS, conductive and EL materials. This panel was(Wheeler et al., 1973). Apar t from the materials comparable in its form and size to the conventionalreported by Pankove (1977), recently blue EL in fluoroscope screen and produced a high contrastZnSe MIS structures has been reported by image with the X-ray intensities used in medicalKorotkov et al. (1981), which consists of emission fluoroscopy. It took a few seconds for the image to

ELECTROLUMINESCENT MATERIALS 223

build up and the image persisted for 30 s or longer Anderson W. W., Razi S. and Walsh D. J. (1965)after the X-ray exposure was cut off. In terms of Luminescence of rare earth activated ZnS. J. Chem.doses about 5 to 50 millirontgens were required to Phys. 43, 1153-1160.produce the image. In his later work on image Ando K., Yamamoto A. and Yamaguchi M. (1981)

Surface field effect of PL intensity in ZnSe diode.storage panels, Kazan (1968) used an EL powder Jpn. J. Appl. Phys. 20, 679-680.layer for the generation of an output image and Bergh A. A. and Dean P. J. (1976). Light EmittingZnO powder layer for the control of storage D/odes, p. 560. Clarendon Press, Oxford.

Bhargava R. N. (1975) Recent advances in visible LEDs.properties. In this, first the ZnO surface was IEEE Trans. on Electron Devices ED-22, pp.uniformly corona charged to a negative potential, 691-701.which reduced the conductivity and thus erased the Bhushan S., Pandey A. N. and Kaza B. R. (1979) PL andprevious informations. After this the panel was EL of undoped and rare earth doped ZnO electro-

luminors. J. Luminescence 20, 29-38.exposed to an optical image which produced a Bhushan S. and Asare R. P. (1981a)Effect of grinding onstored charge pattern on ZnO surface and which in the PL and EL of ZnO electroluminophores. Czech.turn controlled the luminescent output of adjacent J. Phys. il31, 931-937.phosphor layer. Incident radiation in the wave Bhushan S. and Asare R. P. (1981b) PL and EL of ZnOlength region 350-400 nm of intensity of the order phosphors by burning metal zinc. Ind. J. Pure Appl.

Phys. 19, 694--697.of 1 0J cm -2 was used and stored images having Bhushan S. and Chandra F. S. (1984) EL and PL of CaSbrightness as high as 20 fL with a maximum phosphors. J. Phys. D 17, 589-595.contrast ratio 100:1 was seen. The size of the panels Broser I. and Broser-Warminsky R. (1955) Statisticalused by him was 30 cm x 30 cm and the limiting kinetic theory of lum. and electrical conductivity ofresolution was between 400 to 800 TV lines. LEDs defect semiconductors. Ann. Physik 16, 361.

Brovetto P., Busca C. and Cortese C. (1971) EL patternshave also been used as opto isolators which consists and crystal defects in ZnS. Lett. Nuovo Cimento 1,of a LED and a photoconductor coupled optically 211-216.and isolated electrically (Bergh and Dean, 1976). Busca C., Cortese C. and Maxia V. (1971) TheTwo potential areas exist for I I -VI group mechanism of AC EL in ZnS. Lett. Nuovo Cimento

1,217-220.compounds: Cattella A. F., Cockayne B., Dexter K., Kirtor J. and

(i) DC EL phosphors and, Wright P. J. (1983) EL from films of. ZnS:Mn(ii) Blue LEDs. prepared by organometallic CVD. IEEE Trans.

The operational capabilities of DC EL phosphors Electron Devices ED 30, pp. 471--475.have already been demonstrated (Vecht, 1973). For Chynoweth A. G. and McKay K. G. (1956) Photon

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