Physica status solidi: Volume 4, Number 2 1964 [Reprint 2021 ed.] 9783112500064, 9783112500057

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plrysica status solidi

VOLUME 4 • N U M B E R 2 • 1964

Contents Review Article K . N . R . TAYLOR

Page Field Quenching and Enhancement in ZnS and CdS Phosphors .

.

.

207

Original Papers K . W . BÖER a n d W . E . WILHELM

Artificial Initiation of Layer-Like Field Inhomogeneities in CdS Single Crystals 237

E . SEMXNEK a n d Z . SROTJBEK

L.

GOLD

On the Theory of the d-Electron Covalency in Ionic Crystals . . . . 251 An Empiricism for Establishing the Transition Temperatures of Superconducting Alloys and Compounds 261

G . BURGER, H . MEISSNER, a n d W . SCHILLING

The Influence of the Initial Defect Concentration on the Annealing of Low-Temperature Irradiated Metals 267

G . BURGER, H . MEISSNER, a n d W . SCHILLING

Analysis of Radiation Annealing Observed during Low Temperature Irradiation with Neutrons and Heavy Charged Particles 281

K . H . G . A S H B E E a n d R . E . SMALLMAN

The Fracture of Titanium Dioxide Single Crystals with Particular Reference to Non-Stoichiometry 289

A . A . I J B E T A E B , P . K . HYATKO H K). H .

TOJIOBAHOB

KimeTHKa peKOMÖHHaiiHH napHBix ae$eKTOB (JipeHKejiH npii OTHtnre «e$opMHpoBaHHHx MaTajijiOB 299

F . FORLANI a n d N . MTNNAJA

Thickness Influence in Breakdown Phenomena of Thin Dielectric Films 311

M . BOCEK u n d V . K A S K A

Die Orientierungs- und Temperaturabhängigkeit der Verfestigungskurven von Zinkkristallen 325

M . BOÖEK, P . LTJKIÖ u n d M . S V I B O V I

Zur Deutung der Orientierungsabhängigkeit des Verfestigungsanstiegs in Zinkkristallen ' 343

B . BURAS a n d J . L E C I E J E W I C Z

A New Method for Neutron Diffraction Crystal Structure Investigations 349

A . MÖSCHWITZER u n d S . WAGNER H . PUFF

Kalte Emission von Dünnfilmkatoden 357 Zur Theorie der Sekundärelektronenemission — Der Transportprozeß (II) 365

R . GEYERS, P . DELAVIGNETTE, H . B L A N K , a n d S . AMELINCKX

Electron Microscope Transmission Images of Coherent Domain Boundaries (I) 383

F . R . STEVENSON a n d H . R . PETFFER

Recovery of Defects Following Deformation of High Purity Cadmium at 78 °K 411

G . SCHOTTKY, A . SEEGER u n d G . SCHMID

Wanderungsenergien und Aktivierungsvolumina von Leerstellen in Edelmetallen 419

G . SCHOTTKY, A . SEEGER u n d G . SCHMID

J . MERTSCKENG

Bildungsentropie und andere Eigenschaften von Fehlstellen in Metallen 439 Vergleich der Ultraschalleffekte in piezoelektrischen und einfachen Deformationspotential-Halbleitern 453

Short Notes (listed on the last page of the issue) Pre-printed Titles and Abstracts of Papers to be published in this or in the Soviet journal „H3liKa Tßepnoro T e j i a " (Fizika Tverdogo Tela).

physica status solidi B o a r d of E d i t o r s P. A I G R A I N , Paris, S. A M E L I N C K X , Mol-Donk, W. D E K E Y S E R , Gent, W. F R A N Z , Münster, P. G Ö R L I C H , Jena, E. G R I L L O T , Paris, R. K A I S C H E W , Sofia, P. T. L A N D S B E R G , Cardiff, L. N E E L , Grenoble, A. P I E K A R A , Poznan, A. S E E G E R , Stuttgart, 0 . S T A S I W , Berlin, M. S T E E N B E C K , Jena, F. S T Ö C K M A N N , Karlsruhe, G. S Z I G E T I , Budapest, J . T A U C , Praha Editor-in-Chief P. G Ö R L I C H Advisory Board M. B A L K A N S K I , Paris, P. C. B A N B U R Y , Reading, M. B E R N A R D , Paris, W. B R A U E R , Berlin, W. C O C H R A N , Cambridge, R. C O E L H O , Fontenay-aux-Roses, H.-D. D I E T Z E , Aachen, J . D. E S H E L B Y , Birmingham, G. J A C O B S , Gent, J . J A U M A N N , Köln, E. K L I E R , Praha, E. K R O E N E R , Clausthal-Zellerfeld, M. M A T Y Ä S , Praha, H. D. M E G A W , Cambridge, T. S. M O S S , Camberley, E. N A G Y , Budapest, E. A. N I E K I S C H , Jülich, L. P A L , Budapest, M. R O D O T , Bellevue/Seine, B. Y. R O L L I N , Oxford, H. M. R O S E N B E R G , Oxford, R. V A U T I E R , Bellevue/Seine

Volume 4 • Number 2 • Pages 205 to 458 and K 53 to K 92 1964

A K A D E M I E - V E R L A G -

B E R L I N

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S c h r i f t l e i t e r u n d v e r a n t w o r t l i c h f ü r d e n I n h a l t : P r o f e s s o r D r . D r . h . c. P . G ö r l i c h , B e r l i n C 2, N e u e S c h ö n h a u s e r S t r . 20 b z w . J e n a , H u m b o l d t s t r . 26. Redaktionskollegium: D r . S. O b e r l ä n d e r , D r . E . G u t s c h e , D r . W . B o r c h a r d t . A n s c h r i f t d e r S c h r i f t l e i t u n g : B e r l i n C 2, N e u e S c h ö n h a u s e r S t r . 20, F e r n r u f : 4 2 6 7 8 8 . Verlag: Akademie-Verlag G m b H , B e r l i n W 8, L e i p z i g e r S t r . 3 - 4 , F e r n r u f : 2 2 0 4 4 1 , T e l e x - N r . 0 1 1 7 7 3 , P o s t s c h e c k k o n t o : B e r l i n 3 5 0 2 1 . — D i e Z e i t s c h r i f t „ p h y s i c a s t a t u s s o l i d i " e r s c h e i n t j e w e i l s a m 1. d e s M o n a t s . B e z u g s p r e i s e i n e s B a n d e s D M 60,—. B e s t e l l n u m m e r dieses B a n d e s 1 0 6 8 / I V . F ü r 1964 s i n d 4 B ä n d e v o r g e s e h e n . G e s a m t h e r s t e l l u n g : V E B D r u c k e r e i „ T h o m a s M ü n t z e r " B a d L a n g e n s a l z a . — V e r ö f f e n t l i c h t u n t e r d e r L i z e n z n u m m e r 1310 d e s P r e s s e a m t e s b e i m V o r s i t z e n d e n d e s Ministerrates der Deutschen Demokratischen Republik.

Review Article phys. stat, sol. 4, 207 (1964) Physics Department, Science Laboratories,

Durham University, Durham City

Field Quenching and Enhancement in ZnS and CdS Phosphors By K. N. R.

TAYLOR

Contents 1.

Introduction

2. Electric field distribution 3. Field quenching

in

phosphors

and enhancement

effects

3.1 Quenching 3.2 Enhancement 3.2.1 associated electroluminescence 3.2.2 Enhancement, Enhancement, not associated withwith electroluminescence 3.3 Summary 4. Field induced processes 5. Theoretical

approach

in phosphor to electric field

materials effects

5.1 Quenching 5.2 Enhancement 5.2.1 Enhancement, not associated with electroluminescence 5.2.2 Enhancement, associated with electroluminescence 6.

Conclusions

1. Introduction The effects produced by the simultaneous application of more than one type of stimulus to phosphor materials have been the subject of many investigations, and almost as many theoretical models. These include the application of electric or magnetic fields, illumination with infra-red radiation, or both together, while the specimen is being simultaneously excited with ultra-violet light, X-rays, or high energy particles. In addition, since the classic observations of electrophotoluminescence by G U D D E N and P O H L [I], experiments have been carried out using sequential application of various stimuli resulting in memory and storage effects. However, it is not the purpose of this review to consider this latter type of phenomenon although the nature of the results in themselves is probably no less important than those observed during simultaneous application. In the following, the observed phenomena induced by the presence of an electric field will be discussed. 14»

208

K . N. B . TAYLOR

Much of the work with ultra-violet excitation has been performed with 3650 Á radiation for experimental convenience. Unfortunately this wavelength is outside the absorption edge in ZnS and causes excitation from the localized levels in the forbidden gap. Consequently the observed effects depend on the impurity concentration and type, and wide discrepancies exist between the results of different workers. Experiments using higher energy radiation, for which the extent of the affected region can be accurately known are potentially more exact, although with the attendant high absorption coefficients inside the absorption edge the role of the surface becomes more important. For high energy particles and X-rays, the range and specific energy loss are known for many materials and it should be possible with these to obtain consistent results. In much of the work on field quenching and enhancement, both experimental and theoretical, intuitive guesses have been made about the distribution of the field within the specimen, assuming either uniform field distribution, a cathode exhaustion barrier or junction fields at many places throughout the body of the crystal. Many of the generalized models developed to explain the combined electrical and optical properties of phosphors assume a uniform field distribution. I t now seems certain that this is not the case in these materials. Electroluminescence, other than injection electroluminescence, can only be explained if electric field strengths comparable with the breakdown field strength exist in the material, and at the voltages at which this phenomenon is observed such field strengths can only be produced by non-uniform potential distributions. Consequently any examination of field effects should be preceded by an investigation of the field distribution in the specimen. In crystals in which the field distribution is not uniform it is possible for space charge layers to be formed at the electrodes, grain boundaries, or at stacking faults. When this happens the local field strength may be high enough to produce hot electrons. Multiplication processes may then occur due to collision ionization of the lattice atoms. Electroluminescence has been satisfactorily explained on this basis, the ionized atoms in this case being luminescence centres. Other high field effects which may occur are those of tunnelling induced by the field [la], or thermal excitation of an electron over a field-reduced energy gap. Consequently, before considering the field effects themselves, we shall examine the available results on field distribution in these materials. This has unfortunately only been possible in crystalline specimens, but the necessity for such information in all types of material used is apparent. 2. Electric Field Distribution in Phosphors Direct observations of potential distribution using a fine wire probe and of photosensitivity distribution using a narrow beam of light, have been made by several authors.

WATSON, DROPKIN, a n d HALPIN [2] reported t h e presence of

barriers in most of the zinc sulphide crystals studied, while LEMPICKI [3] in the same laboratory found that asymmetric voltage distributions are the exception in this material. FRANKL [4, 5] has found cathode barriers in some crystals and almost uniform field distribution in others. Both methods have been used by us [6] in the crystals later used for quenching measurements [7], and in seven out of nine crystals examined the voltage applied to the crystal appeared across a narrow region at the negative electrode. A typical variation of both potential and photosensitivity distribution observed in this work is shown in Fig. 1. The

Field Quenching and Enhancement in ZnS and CdS Phosphors

209

form of the distribution did not depend on the applied voltage from 50 to 700 V, nor did it change on illumination with ultra-violet light. In the remaining crystals, large fractions of the applied voltage were found to appear across narrow regions in the bulk of the specimen which were coincident with visible cracks or flaws in the crystal. On increasing the voltage applied to the specimen or illuminating with ultra-violet light the large changes in potential at these positions were removed and the field distribution became nearly uniform (Fig. 2). Further support for the existence of a potential barrier in the first crystals was obtained by the observation of a-particle scintillations [8]. That cathode barriers should exist in zinc sulphide crystals is not unreasonable and the application of the Schottky Exhaustion Barrier theory [9] to this material has been used many

210

K . N . R . TAYLOR

times in the past. Because of the nature of these results, and the possible changes in the field distribution caused by varying both illumination and applied potential, direct measurement of the field distribution under the experimental conditions used in examining field effects would seem to be necessary. The danger of assuming that a field distribution observed for one set of conditions remains constant on varying these conditions is adequately shown in Fig. 2. In the case of powdered specimens dispersed in a dielectric or cemented to a substrate no such direct evidence can be obtained and mechanisms used to account for experimental results must remain open to doubt. 3. Field Quenching and Enhancement Effects A decrease in the luminescence intensity caused by the application of an electric field to an excited phosphor was first conclusively observed by D E C H E N E [ 1 0 ] in 1935 using powdered ZnS specimens irradiated with ultra-violet light and with a static field applied to them. Since then other workers have observed quenching for excitation with X-rays [11], electrons [12], and a-particles [7]. An electric field induced enhancement effect was first reported by D E S T R I A U et al. [ 1 3 ] for X - r a y excitation of ZnS, for u.v. excitation by C U S A K O [ 1 4 ] in 1 9 5 5 , for A-particle excitation by M A T T L E R [ 1 5 ] , and for high energy cathode rays by . T A F F E [ 1 6 ] . In general since these early observations it has been found that if an electric field is applied to a ZnS type phosphor then quenching occurs regardless of the type of radiation used to stimulate the luminescence. For similar materials it has usually been reported that this quenching does not depend on the specimen being electroluminescent or non-electroluminescent, although in the work with a-particles [7] an effect could only be observed in those crystals which showed electroluminescence at sufficiently high voltages. In certain phosphors however, particularly those with manganese doping, or for certain experimental conditions, enhancement effects have been found. In some of these materials both quenching and enhancement have been found in a single material during one investigation, a reduction in intensity occurring at one part of the emission spectrum while an increase occurs at another [17, 18], Fig. 3 shows the typical behaviour of the output brightness with time for a specimen exhibiting quenching (Fig. 3 a) and enhancement (Fig. 3 b) for contiQUENCHING EFFECT

TIME a

ENHANCING EFFECT

TIME b

Fig. 3. Behaviour of emission brightness for a) quenching, and b) enhancement

211

F i e l d Quenching and E n h a n c e m e n t in Z n S a n d CdS P h o s p h o r s

nuous excitation of the phosphor. If a static field is applied between times and t2 after commencement of the excitation the behaviour is shown in curves 1, whereas if the field is applied throughout the excitation the brightness variation is shown in curves 2. Using these curves it is possible to define a brightness ratio R = BTjBL which for quenching is less than unity and for enhancement is greater than unity. Here BT is the emission brightness for both field and excitation together, and BL for the stimulating radiation only. More strictly this should be R = BTj(BL + Be), where BE is electroluminescent emission, but as much of the work has been performed below the electroluminescence threshold the earlier definition will in general hold. 3.1

Quenching

Since the early observations of D E C H E N E and DESTRIAU, many investigators have reported work on this subject and review articles have been prepared by I V E Y [19 to 21], Much of this work has been concerned with the output brightness waveform obtained when an alternating field is applied to the specimen. Many of these early experiments were restricted to alternating fields as only powdered specimens were available. These were either embedded in a dielectric or cemented to a substrate, and polarization effects occurred when a static field was applied to the specimens. More recent work with single crystals has extended the work to persistent quenching in static fields and the basic form of the results has remained the same. Too many results are available to attempt to summarise the work of various authors individually, but it is possible to outline the basic form of their results initially and then discuss exceptions to these generalisations. This is carried out below for those specimens which have been found to show quenching only. 1. The light output from a phosphor is found to continuously decrease as the applied electric field strength increases, regardless of the type of stimulus used to excite the phosphor. The detailed form of this variation is by no means constant, DESTRIAU and MATTLER [22] reporting a variation of the form R = [1 + eaV— — where a and jj are constants, while other workers find a linear variation with voltage. Typical results are shown in Fig. 4. With a-particle excitation the value of R dropped rapidly with increasing voltage but approached a saturation level above 800 V (d.c.) [7] (see Fig. 4). This effect was thought to be due to complete extinction within a cathode barrier, the remnant light level arising from emission in that part of the particle track outside the high field region. 2. With an alternating field applied to the specimen MATOSSI and NUDELMAN [23] found that the amount of quenching increases with the frequency of the 1.0

0.8 0.6 -

04 0.2 F i g . 4. Typical results of t h e voltage dependence of quenching

0

5

° Destriau s Mattier [22] « Alfreys Taylortf] • GobrechtiGumtichUS] * Dechene [10]

10

50

\ \

\

100 500 1000 APPLIED VOLTAOEiV!

5000 •

212

I

K . N . R . TAYLOR

6.0 LO 2.0 10 0.6

0Ä

0

30

60

90

120

150

180

tisi F i g . 5. L i g h t o u t p u t of Z n S C d S : Cu phosphor with t i m e , a f t e r the ultra-violet was turned off. 1 — n o r m a l d e c a y c u r v e ; 2 a n d 3 — a p p l i e d field frequency and AC - field o n ; BD —

800

5000 Hz;

field off (after OLSON [261)

F i g . 6. V a r i a t i o n of

— 1 a s a function of a p p l i e d

field a t v a r i o u s t e m p e r a t u r e s ( a f t e r DESTKIAU a n d MATTLER [22])

o

5

20

40

60

VOLTAGE iVl

80

35

applied field. Their results also show [24, 25] that for continuous ultra-violet excitation the quenched light output fluctuates at twice the field frequency and that the magnitude of this modulation decreases with increasing frequency at high frequencies, a maximum modulation occuring at 200 Hz. OLSON and DANIELSON [26] also reported an increase in quenching with frequency for measurements carried out in the phosphorescence decay as shown in Fig. 5. ALFREY and TAYLOR [7] found that the saturation value of R increased and in addition the voltage required for saturation, decreased with increse in applied frequency for «-particle excitation. For cathodoluminescence GOBRECHT et al. [27] found an increase in the amount of quenching in ZnS: Cu, Al with an increase in field frequency and a decrease in the electron beam current. Similar results were found by HALSTEAD [28] and JAFFE [12] in ZnS: Mn films, the dependence on beam current being ascribed to the neutralization of polarization fields by the bombarding electrons. At high frequencies MILLER [29] found a maximum amount of quenching at about 106 Hz. In addition above 107 Hz he observed maxima in the quenching curves which were thought to arise from a resonance of trapped electrons. 3. The variation of R with temperature has not been well examined and of the few reports which do exist, the majority cover only a very limited temperature range. These changes are complicated by the fact that the normal fluorescence intensity usually decreases with a temperature increase, however the intensity with a field applied is found to fall even more rapidly so that R is generally reported to decrease with increasing temperature. DESTRIAU and MATTLER [22, 30] first studied the dependence of quenching on temperature over the range from room temperature to about 70 °C. Their results plotted as 4- — 1 and shown in it Fig. 6, indicate the continuous decrease in the value of R with temperature increase. In the same work they showed that the curves of-^-— 1 against E at it various temperatures can be superimposed on one another by normalizing at any

Field Quenching and Enhancement in ZnS and CdS Phosphors

213

field value. The normalizing factor necessary for t h i s are u n f o r t u n a t e l y n o t a simple f u n c t i o n of t h e t e m p e r a t u r e . T h e f a c t t h a t these curves m a y be superposition is t h o u g h t t o indicate t h a t t h e quenching effect arises f r o m an internal competition between radiative transitions a n d field induced non-radiative transitions as will be discussed later in section 5. 1. 4 . DESTRIAU a n d MATTLEE [ 3 1 ] showed t h a t d u r i n g t h e quenching process relative spectral intensities change a n d t h e value of R, a n d of t h e m a g n i t u d e of t h e Gudden-Pohl peaks a t t h e beginning a n d end of t h e field action, d e p e n d on t h e wavelength of examination. T h e variation of R t h r o u g h o u t t h e emission s p e c t r u m m a y be sufficiently g r e a t in some materials for quenching t o occur in one region of t h e spectrum a n d e n h a n c e m e n t in a n o t h e r [17]. This t y p e of behaviour is discussed later u n d e r e n h a n c e m e n t . 5. R e p o r t s exist concerning t h e influence of particle size on t h e degree of quenching a n d a t t e m p t s h a v e been m a d e to correlate t h e two [32]. T h e results of this showed t h a t R decreases with a decrease in particle size. T h e i n t e r p r e t a t i o n of this t y p e of result is extremely difficult however as t h e electric field conditions within t h e phosphor will be v e r y complicated, a n d t h e possible existence of appreciable oxide contamination leads t o a consideration of t h e effects of chemical composition, for which t h e r e is y e t no satisfactory solution. 6. T h e results of a s t u d y of brightness waves in electroluminescent phosphors by MATOSSI a n d NUDELMAN [ 2 3 ] were considered t o show t h a t t h e t w o effects are of different origin. The results a n d i n t e r p r e t a t i o n of t h e a-particle work of A L F R E Y a n d TAYLOR [ 3 3 ] however, has indicated t h a t some connection between quenching a n d electroluminescence m a y exist. 3.2

Enhancement

Unlike t h e results of quenching experiments, no single description of t h e basic f o r m of t h e results can be given as more t h a n one t y p e of e n h a n c e m e n t effect has been observed in t h e p a s t . I t does a p p e a r possible however, t o divide these various effects i n t o two g r o u p s : a) those effects which are n o t connected w i t h t h e acceleration-collision mechanism of electroluminescence [ 1 3 ] a n d b) those of CUSANO [ 1 4 ] , THORNTON [ 3 4 ] , a n d WOODS [ 3 5 ] which appear t o be. DESTRIAU was t h e first t o point out t h e distinction between his results a n d those of CUSANO, a n d t h e difference has been stressed in t h e reviews of IVEY. I n f a c t it m a y be t h a t t h e t h r e e effects associated with electroluminescence listed u n d e r b) above, are one a n d t h e same, although it should also be considered t h a t a n y field induced process occurring in a non-electroluminescent phosphor should also occur in a n electroluminescent phosphor, so t h a t t h e division is not necessarily a complete one. (The work in refs. [14, 34, 35] mentioned are n o t alike in detail a n d I list these as t h r e e effects.) 3.2.1 Enhancement,

not associated with

electroluminescence

T h e first of these e n h a n c e m e n t effects was discovered b y DESTRIAU using powdered specimens of m a n g a n e s e activated Z n S a n d ZnSCdS, illuminated w i t h X - r a y s a n d subjected t o a n a l t e r n a t i n g field. Other a c t i v a t o r s showed n o effect b y themselves b u t t h e addition of gold to the m a n g a n e s e doped specimens caused an increase in t h e e n h a n c e m e n t response [17]. T h e value of R increased as t h e CdS content was increased in t h e mixed sulphide phosphors, reaching a m a x i m u m of R = 10 a t a sample composition h a v i n g equal p a r t s b y weight of b o t h sulphides. I n spite of these high values of R, t h e total o u t p u t was f o u n d t o be only slightly

214

K . N . R . TAYLOR

Temperature T(°K) a

b

Tig. 7. Change of emission brightness and R with field frequency and temperature, for yellow light at two current densities (a, b) and blue light at two current densities (c, d). 150 volts applied to the cell (after GOBRECHT et al. [38])

better than the normal emission from other phosphors, as the unaffected luminescence output from these mixed phosphors was found to be low. The details of the enhancement phenomena are summatized below in the same sections as those earlier for quenching. 1. As the field strength is increased, enhancement effects are first observed at quite low field strengths (50 V/cm) as applied to the specimen (this distinction is thought necessary throughout, because of the possible existence of much higher fields in potential barrier regions in the specimen). The value of R then increases to a saturation level which sets in at about 104 V/cm. This level then remains

Field Quenching and Enhancement in ZnS and CdS Phosphors

Temperature TCK) c

Fig. 7

215

»d

constant to field strengths sufficient great for electroluminescence to occur. Occasionally however a small decrease at high fields has been reported [36]. 2. The variation of the brightness ratio with frequency was somewhat confused in the early observations, as D E S T R I A U et al. [36] found no frequency dependence from 10 to 5000 Hz, whereas G O B R E C H T and G U M L I C H [18] reported a slight increase in B with frequency and later M A T T L E R [37] a slight decrease. More recently however, G O B R E C H T et al. [38] have examined the enhancement effect in CdS: Mn in considerable detail and find that for excitation with cathode-rays the variation of R, with frequency from 50 to 10000 Hz, is dependent both on the

216

K . N . R . TAYLOR

temperature and on the region of the spectrum examined (see Fig. 7). At low temperatures the blue emission is strongly enhanced (R = 4), the enhancement increasing with frequency but showing a tendency to saturate above 300 Hz. At the same temperature the yellow band is quenched and the value of R decreases with frequency increase. Above room temperatures the behaviour of the two emission bands changes, quenching occurring in the blue emission and enhancement in the yellow. The frequency dependence is also reversed and at these temperatures the enhancement (yellow emission) decreases with frequency increase and the quenching decreases with frequency increase (i.e. R increases). The behaviour is strongly dependent on the intensity of irradiation and detailed changes in the variation of R with frequency were found on increasing the cathoderay beam current from 3 X 10" 9 A/cm 2 to 200 x 10~9 A/cm 2 . Over part of the temperature range R passes through a maximum between 50 and 2000 Hz, but insufficient data is given to examine this more closely. Recently these workers [39] have reported a maximum in the enhancement of X-ray luminescence at 20 Hz in ZnCdS: Mn, Co. A similar effect has been found by HENCK and COCHE [40] for a-particle bombardment of ZnSCdS: Mn. Fig. 8 shows the enhancement ratio passing through a maximum at about 4 Hz in a frequency range covered from 0.005 to 500 Hz. The position of this maximum did not appear to depend on the degree of manganese doping, although the value of Rmax increased as the manganese content increased. Again these workers observed quenching in the blue light and enhancement in the yellow at room temperature. WINKLER et al. [41] have also observed this maximum for ZnSCdS: Mn phosphors excited with X-rays although no quenching was observed at any wavelength in contrast to the results of DESTRIAU [42] on similar phosphors. WEN DEL [43] reported similar observations for ultra-violet excitation, the position of the maximum shifting to lower frequencies for lower applied voltages. At sufficiently low illumination intensities the enhancement decreased and became a quenching effect as the frequency was raised. The maximum enhancement in this work was found for 4250 A wavelengths which is near to the expected absorption edge at 4280 A. 3. As in the case of quenching, little work has been carried out on the temperature dependence of this effect. DESTRIAU originally found no variation in R, for a given field strength, between 18 and 54 °C. Later MATTLER [44] examined the range from —150 to 100 °C reported an increase in R with temperature.

Fig. 8. Frequency dependence of It for three values of manganese content. Mn content increases from D to A (after

HEJTCK and COCHE [40])

0.005

0.05

0.5 5 50 Frequency (Hz)

-

500

Field Quenching and Enhancement in ZnS and CdS Phosphors

a

217

b

Fig. 9. Spectral dependence of li for various manganese contents a) X-ray stimulation, field frequency 50 Hz, b) ultraviolet stimulation, field frequency 50 kHz (after GOBRECHT and GUMLICH [45])

et al. [ 3 8 ] studied the variation of R with temperature for the blue and yellow emission bands between 90 and 350 °K. The form of these results is shown in Fig. 7. 4. As mentioned in the discussion of the quenching effect the value of R depends on the region of the spectrum examined, and it is possible for quenching to occur at some wavelengths and enhancement at others. D E S T R I A U [ 4 2 ] and G O B R E C H T and G U M L I C H [ 4 5 ] showed that as the wavelength of examination increases, the value of R also increases. The latter workers found a small minimum in the value of R at short wavelengths (blue quenched) and a maximum in R at long wavelengths as shown in Fig. 9. The sizes of these maxima and minima changed for different amounts of added manganese, and also for changes from X-ray to ultra-violet illumination. W I N K L E R et al. [ 4 1 ] also found a maximum value of R at 5 8 0 0 A (in good agreement with the results of Fig. 9 for similar manganese concentrations ( 5 x 1 0 " ) , although in this case no quenching was observed at any wavelength. They also reported that the position of the maximum did not depend on the applied voltage, but Rmax did increase as the voltage increased, reaching a value of 6 at 2 0 0 V. G O B R E C H T and G U M L I C H [ 4 6 ] have also carried out valuable work on the effect of changes in the wavelength of the exciting ultra-violet radiation on the quenching effect, in both the manganese band and the host lattice band. Very little enhancement was found in the phosphors used (ZnS :Mn) and then only in the manganese band for wavelengths less than 3500 A when excitation across the forbidden gap occurred. The changes in the form of the curves with varying manganese content is complicated. 5 D E S T R I A U ' S original work reported an increase in R with X-ray illumination intensity, but no effect with ultra-violet excitation. G O B R E C H T and G U M L I C H [ 4 5 ] ,

GOBRECHT

4

218

K . N . R . TAYLOK

Mn CONTENT 2*10~3 Yzhu+VzCo

Mn CONTENT 2'10~3

--

%Au*% Co — — «

Au

^

Au Co

Co

Ag

c X sCr Ni to

7

2

3

4 - 5 0 1 AMOUNT OF ADDITIVE!* 10*)

2

3

4 - 5

Fig. 10. Influence of secondary activating elements on the luminescence sensitivity of (40 Zn, 60 Cd) S : Mn, CI excited with X-rays and under the influence of an alternating field (after DESTRIAU [47])

showed however, that at sufficiently high frequencies and illumination intensities a slight enhancement could be obtained in ZnS :Mn with ultra-violet illumination. Similar increases in the value of R have been found by other workers for ultraviolet excitation. The reverse is found for X-ray excitation, and M A T T L E R [37] reported that R decreased with increasing frequency. An increase in electron beam intensity by a factor of about 70 under the same conditions of voltage and frequency has been found to cause R to decrease by a sufficiently large amount for an enhancement effect to change to one of quenching. The details of this are complicated and are best seen in Fig. 7. The enhancement of a-particle scintillations observed by M A T T L E R [ 4 4 ] showed no tendency to saturate with increased voltage. However, in connection with these observations, M A T T L E R and M E S S I E R [52] later showed that the enhancement is not of the pulse height but of the background noise and the secondary scintillations. The possibility of enhancement due to an extension of the pulse tail has been suggested by T A Y L O R [33]. 6. Since the early work of D E S T R I A U on the addition of silver and gold to manganese doped phosphors, several papers have been published on the influence of various other additives on the light output from these materials with a field applied [47, 48]. The influence of these secondary doping agents is seen in Fig. 10 for X-ray excitation under an alternating applied field. 3.2.2 Enhancement, associated with electroluminescence The second type of enhancement effect is tiiat discovered by C U S A N O [ 1 4 ] using thin films of ZnS :Mn, CI. This appears to be related in some way with the electroluminescence phenomena and was considered by both C U S A N O and W I L L I A M S [ 4 9 ] as radiation controlled electroluminescence. The initial experiments used static fields applied to electrodes directly in contact with the films, and enhancement was observed for ultra-violet, cathode-ray and X-ray illumination. One of the electrodes was an evaporated T i 0 2 layer and the other a metal coating. Enhancement was only found if at least one of the electrodes was a rectifying contact, and then only when the rectifying contact was negative. Later experiments found

Field Quenching and Enhancement in ZnS and CdS Phosphors

219

a less pronounced enhancement effect with phosphorus [50], arsenic [50] and antimony [51] doping, the brightness increase occurring in the blue emission for these activators. 1. As the strength of the field applied to the specimen was increased the brightness ratio R increased rapidly with the field above 10 V/cm. Some of the brightness-voltage characteristics obtained by GUSANO for the rectifying contact negative are given in Pig. 11, for various activators. The effect of thickness on these curves was also investigated and the field required to produce a given amount of enhancement did not vary in any simple way with thickness, as shown in Fig. 12, where the numbers at the curves refer to the thickness in units of 10 (xm. T H O R N T O N [ 3 4 ] observed a more complex dependence of R on field strength for both thin electroluminescent films of ZnS (with a static field) and for electroluminescent suspensions of ZnS in a dielectric to which was applied an alternating electric field. As the field strength was increased from zero, R initially decreased from unity and then rapidly increased to an enhancement value. With further increase in field strength R then passed through a maximum and decreased towards unity. With thin films R approached a value of one asymptotically, before breakdown took place, whereas with alternating fields applied to the dielectric suspension R decreased sufficiently for the quenching effect to predominate again. These results also showed the polarity effect which C U S A N O had reported, however the work was limited to those specimens which would show electroluminescence. Recently S A T C H E L L [53] has examined the effect of electric fields on cathodoluminescence in single crystals of ZnS :Mn, CI and ZnS :Cu, CI and reported a strong polarity effect. In his results however, the field dependence of enhancement is comparable not to the results of C U S A N O and T H O R N T O N , but to those of D E S T B I A T J ,

APPLIED VOLTAGE(V) Tig. 11. Brightness-voltage characteristics for various activators (alter CUSANO [51])

APPLIED VOLTAGE(V) Fig. 12. Brightness-voltage dependence on cell thickness for ZnS: Mn, P, CI illuminated with 3650 A radiation ( a f t e r CUSANO [ 5 1 ] )

22 0

K . N . R . TAYLOR Fig. 13. Variation of brightness (yellow band) with voltage across the crystal for various beam currents. Excitation by 10 keV electrons A — crystal with bulk doping; B — crystal surface doped. 1 — Beam current 0.5 (IA; 2 — beam current 1.0 NA; 3 — beam current 200 ¡¿A (after SATCHELL [53])

0

10 20 30 W 50 60 70 80 90 100 Voltage across crystal (V)

10.1 mm gap!

-

rising to a saturation level at about 104 V/cm as applied to the specimen (Fig. 13). Again, unlike C U S A N O (see later) only quenching was ever found with the copper activated specimens. 2. Although the enhancement effect has been observed under alternating field conditions, little or no work has been carried out to investigate its dependence on the frequency of the applied field. T H O R N T O N showed however that the dependence of R on the alternating field strength was essentially the same as that obtained for static fields. 3. T H O R N T O N ' S work represents the first examination of the effect of temperature on R for this type of enhancement. His results showed an increase in R with decrease in temperature giving a value of R = 40 at 90 ° K for a static applied field. 4. Changes in the emission spectrum are also found to occur in these materials during enhancement. A phosphor capable of emitting in both a blue (electroluminescence) band and a green (photoluminescence) band has enhancement largely in the blue region. Further study showed that in fact the added light was entirely electroluminescent in origin, and that any quenching which occurred was in the yellow. These observations on ZnS:Cu, CI provide conclusive evidence of the association, if not identification of this effect with electroluminescence. For ZnS :Mn, CI the form of the spectral shift is the same, the peak in the emission spectrum for ultra-violet or cathode-ray excitation shifting to shorter wavelengths. The results of S A T C H E L L are again in contradiction to these trends, as he finds enhancement at long wavelengths (yellow manganese band) and quenching at shorter wavelengths (green band). Unfortunately insufficient date are given for further comparison of the results. 5. Both C U S A N O and T H O R N T O N found that over a wide range of illumination intensities the enhancement ratio varies inversely as the square root of the excitation intensity, for static field conditions. In other words the enhanced output was proportional to the square root of the excitation intensity. T H O R N T O N also

Field Quenching and Enhancement in ZnS and CdS Phosphors

221

reported that the dependenc of R on field strength changed with excitation intensity in such a way that the quenching and enhancement regions moved to higher field strengths as the intensity increased. At very high intensities the initial quenching vanished and enhancement was observed at low field strengths followed by quenching at higher field strengths. For cathode-ray bombardment SATCHELL found a slight decrease in R with increasing beam current, the decrease being greater for surface doped crystals than for bulk doping. 6. CUSANO'S early work showed that enhancement could be obtained with manganese, arsenic, phosphorus or antimony as activator and chlorine as coactivator. Neither copper nor silver doping were found to show any enhancement although THORNTON found maximum enhancement in ZnS :Cu, CI. While ZnS :Mn is electroluminescent, H A L S T E A D [28] could find only quenching in this material, whereas in ZnS:Mn, CI enhancement is always found. Similar effects occur for other activating elements in place of manganese, indicating the need for a coactivator for this type of enhancement. For completeness however it should be mentioned that WOODS [35] found enhancement, with R = 4, for ZnS which had not been intentionally activated. However, no composition details are given so that the possibility of activation can not be ruled out. For copper activation no enhancement was found under any conditions. With mixed activators, e.g. ZnS:Mn, P, CI, quantum efficiencies as high as 50 have been obtained, i.e. many visible photons were emitted for every 3650 A photon incident on the specimen. THORNTON observed enhancement in a large number of materials under alternating field conditions. For specimens with both activator and coactivator present, electroluminescent ZnS:Cu, CI, ZnS :Cu,Mn,Cl, and Zn(S, Se) :Cu,Cl showed enchancement but ZnS :Cu, ZnS :Ag,Cl, or non electroluminescent ZnS :Cu, CI showed only quenching. 3.3

Summary

The results which have been described above show a wide variety of detailed form, much of which is undoubtedly due to variations in specimen composition and individual experimental conditions from author to author. The necessity for closely specifying illumination and emission intensity values, known composition amounts, electrode geometry, particle size and examining the spectral dependence of both effects is very obvious when attempting to compare the results of two workers. In spite of this however, it is possible to summarize the form of these results. In general when a steady, alternating, or pulsed electric field is applied to a phosphor in the form of a powder, a film, or a single crystal, excited with ultraviolet light, X-rays, cathode-rays or «-particles, a quenching of the emitted light occurs. However, for some conditions, such as manganese activation and chlorine coactivation, an increase in the luminescence intensity can be induced by the field. The mechanism by which these effects occur is by no means simple, as it is possible for both enhancement and quenching to be observed in a single specimen and for the spectral dependence of the effects to change with field frequency, illumination intensity and temperature. The observation of polarity effects in both quenching and enhancement is perhaps not unexpected, but that separate types of enhancement effect should occur for those showing a polarity effect and those not, poses a difficult problem for theories of these phenomena. This problem is not simplified by the abundance of varied information and the absence of detailed studies of the 15

physica

222

K . N . R . TAYLOR

dependence of R on temperature, frequency, illumination intensity and wavelength, and observed wavelength for one and the same set of crystals. Some work of this kind has been carried out in the first type of enhancement effect, however even this is limited in extent but its value is obvious. 4. Field Induced Processes in Phosphor Materials Before considering the possible mechanisms which could account for the quenching and enhancement effects in phosphor materials it will be useful to consider, as I V E Y [20] has done, the possible effects which an electric filed applied to an excited phosphor can have on the electron distribution within the material. Many of these processes involve quantum mechanical tunnelling between two energy levels, an effect which is likely to be assisted by the thermal energy available in the crystal lattice. The remainder are associated with the motion of electrons or holes in the applied field. To simplify the discussion of these processes, Pig. 14 shows the types of allowed energy levels involved in the field effects and the various transitions induced by the field as discussed below. a) The tunnelling of an electron from valence band to conduction band requires very large field strengths for these materials with large energy gaps (2.4 to 3.7 eV) and is unlikely to occur before breakdown in the crystal. b) The tunnelling of an electron from an unionized luminescence centre to the conduction band ( F R A N Z [54]) is also unlikely to occur because of the energies involved. c) Tunnelling of an electron from the valence band to the ground state of an empty luminescence centre can occur at relatively low field strengths and is the basis of several proposed mechanisms [33, 38, 46], Transitions across these relatively small energy gaps will be greatly influenced by temperature and will normally result in quenching. d) The emptying of an electron trap by the applied field is governed by the same considerations as c) above. Such a transition may cause enhancement due to the increased number of electrons available for recombination, or quenching by field removal of the electrons away from the ionized centres (see h) below). e) Acceleration of electrons in the conduction band has been proposed in the electroluminescence process, where it is assumed that the local field values may be sufficiently great for the electron to ionize a luminescence centre (that is to

a.

b.

e

Fig. 14. Possible field-induced processes in phosphor materials

Field Quenching a n d E n h a n c e m e n t in ZnS a n d CdS P h o s p h o r s

223

provide the energy for transition b) b y impact). CURIE [ 5 5 ] has opposed this mechanism as he believes t h a t the acceleration occurs in the bulk of the crystal in lower field strengths. The results of electroluminescence work [56, 57] which show local regions emitting electroluminescence in the bulk of the crystal, do not oppose this mechanism. The absence of any effect caused b y a magnetic field, in the work of DESTRIAU [ 5 8 ] and INCE V [59] also contradicts CTZBIE'S hypothesis. f) I n addition to transition b), t h e accelerated electron could also induce transitions a), c) and d) by collision. Holes liberated in the valence band by a) or c) could then ionize luminescence centres b y transition f). g) The motion of electrons liberated into the conduction band m a y lead to trapping at other traps sufficiently deep for reexcitation t o be unlikely. h) The removal of the electrons in the conduction band away from the region of excitation will lead to quenching in a static field as discussed under d) above, or to a modulation of the emission intensity due to their periodic removal and return in an alternating field. i) If the accelerated electrons gain sufficient energy to become 'hot', then the transition probabilities for recombination with a hole in the valence band, an ionized centre, or an e m p t y t r a p will be affected. This is only likely to occur in regions of the crystal where very high local fields exist, such as in potential barriers. j) Finally the possibility of Stark splitting has been suggested (and observed in Ge and Si) b y WILLIAMS [60] t o account for a 100 A shift of the manganese emission band to shorter wavelengths in ZnS:Mn, CI in a static field caused by applying 100 V to the specimen. These processes are only for one type of luminescence centre and traps a t two depths. If more centres are involved or if surface states are considered, the total number of mechanisms which m a y be operating concurrently becomes extremely large. I n addition we have postulated the possible motion of holes as affecting the luminescence emission through transitions equivalent to those of g) and h) for electrons. The work on infra-red quenching [61] has led to the idea of a luminescence centre-electron t r a p complex consisting of a trapped electron, an excited level a n d the ionized centre. Clearly if this type of coupled system occurs in infra-red quenching it must also be considered in effects induced by electric fields. Consequently the number of possible processes which m a y have to be considered can rapidly become too large t o be of value. I t might also be added, t h a t with this number of variables, given the time and inclination, it should be possible to derive a mechanism to account for almost any observed effect. F o r t u n a t e l y it is possible to eliminate some of these processes as being improbable in the materials under consideration. I n this way field induced transitions a) a n d b) m a y be neglected for normal fields in ZnS a n d CdS. However c) appears probable for typical experimental conditions and has been used successfully as p a r t of the mechanism giving quenching of a-particle scintillations in ZnS. Similarly d) can occur giving the possibility of recombination (either radiatively a n d therefore enhancement, or nonradiatively, leading to quenching) or retrapping a t a deeper t r a p (process g)) which results in quenching. Processes b) and f) induced by impact excitation leave empty luminescence centres which may contribute to enhanced emission. 15*

224

K . N . R . TAYLOR

The possibility of hot electrons existing in the conduction band can not be dispensed with as local regions of high field strength undoubtedly exist in the specimens. I t m a y also be possible t o restrict the consideration of various mechanisms further, by taking account of the fact t h a t in the enhancement effect discovered by DESTBIATJ, manganese doping plays an essential role. T h a t is, in these specimens the luminescence is confined entirely t o the centre a n d they m a y be considered a p a r t from those in which transitions t o the conduction band are involved in luminescence, a n d which also show photoconductivity. I n addition, those phosphors showing electroluminescence effects are more likely to have high intensity and strongly localized fields, t h a n non-electroluminescent samples. This however does make certain inferences about the electroluminescence mechanism which m a y not be justifiable. F u r t h e r it must be remembered t h a t more t h a n one process m a y be occurring simultaneously and contributing to the overall observed phenomena, or as I V E Y [20] points out " a n excited phosphor in the steady state condition is nevertheless in a state of dynamic equilibrium". I n spite of t h e inherent difficulties involved in attempting to account for t h e various field effects, m a n y a t t e m p t s have been made to understand the observed phenomena using both the kinetic approach to luminescence [64] and more specific models designed to explain a given set of results. Some of these models we shall discuss below. First however let us consider briefly the problems entailed in the kinetics of luminescence. Because of the uncertain crystal composition in these phosphors, the kinetics of carrier recombination which are now accepted for semiconductors [63, 64] are not applicable in ZnS and CdS. A t t e m p t s t o arrive a t theoretical solutions have been made by several workers without any outstanding success. The complete solution of even a simple model is often complex and the necessary work can only be justified if the material conforms to the standard of purity required by the model. As this unlikely in practice, such solutions m a y only be misleading. Alternative approaches to the problem have been adopted by both D U B O C and R O S E who use a generalized system as a basis from which t o develop more definite models based on observed results. D U B O C [65] exhaustively examined a family of simple models derived from two kinds of centre in the forbidden gap. This involved calculations on sixteen basic models in all, some of which had already been considered b y other workers. K L A S E H S [66] has used this twocentre model to analyse photoconductivity results, and prefers the approach to t h a t of R O S E [67] in which a distribution of ground states is necessary. R O S E attempted to understand the photoconducting properties of non-perfect crystals by considering the effects of centres distributed throughout the forbidden gap. These centres are considered in two energy regions, shallow levels which represent trapping states and deep lying levels representing the ground states of luminescence centres. B U B E [68] has divided the impurity centres into two classes having different capture cross sections, a n d used this approach to explain various properties of cadmium sulphide and selenide. Provided t h a t the observed effects occur in a uniform electric field, it appears t h a t the combined optical and electrical properties of the sulphide phosphors m a y be explained in a semiquantitative way by using these general models. As it seems certain t h a t in m a n y specimens the field is not uniform, allowance for the probable field distributions should be made in interpreting observed results. This has been carried out successfully in work on bombardment conductivity [69].

Field Quenching and Enhancement in ZnS and CdS Phosphors

225

5. Theoretical Approach to Electric Field Effects 5.1

Quenching

Since quenching has been observed in non-electroluminescent phosphors, and in view of the results of MATOSSI and NUDELMAN it seems reasonably certain that the two phenomena are not related. TAYLOR'S work [ 3 3 ] however showed a possible dependence of 'pre-electroluminescence' phenomena on the quenching process, although this was essentially the production of electron injection caused by local enhancement of the barrier field due to holes liberated by the a-particle. B y the nature of the quenching effect, the presence of the electric field must be to either introduce non-radiative transitions which would otherwise not be present, or to suppress the radiative transitions in favour of non-radiative transitions already possible in the specimen. The steps by which this may occur have already been considered in section 4 and are those given by field induced processes c), g), h), and i). MATOSSI has used the kinetic theory of luminescence in attempts to account for quenching [25] enhancement [70] and brightness wave [71] phenomena. In the work on alternating field quenching, he introduces into the relations of RANDALL and WILKINS [72] a term which allows for field emptying of filled traps (process d), responsible for the Gudden-Pohl effect and a term which takes into account the field transport of electrons to surface states where they can recombine non-radiatively. The importance of the action of the surface in quenching emission has been shown directly in germanium crystals by NEWMAN [ 7 3 ] and indirectly by the work of LEHMANN [ 3 2 ] on particle size effects. The effect of non-radiative transitions which occur in the normal luminescence emission were assumed to be in equilibrium and would not contribute to the field effects. The solutions of these equations were enough to explain the alternating modulation of the field affected output and predict that the modulation would reach maximum at some intermediate frequency. In addition the time variation of the basic quenching was also explained up to the time of "field-off" and the subsequent recovery.

In a later approach applicable to static fields only, and dealing with the time dependence of the quenching of luminescence and photoconductivity, MATOSSI [74] considers that the quenching agent (he deals both with field and i.r. effects), in addition to the transitions in the earlier work, may induce de-excitation of centres by lifting an electron from the valence band to the ground state of the centre, and encourage non-radiative transitions from trap to centre, conduction band to centre, or conduction band to hole in the valence band. This empirical treatment gave a satisfactory description of observations in terms of transition parameters which describe the average properties of aggregates of energy levels A simplified approach to the steady state quenching has been made by

IVEY

[ 2 0 ] , based on the kinetic model of SCHON to which has been added three field

induced transitions, namely, field induced non-radiative transitions (including those already present and those brought into action by the field), field induced filling of empty luminescence centres (process c)) and the release of trapped electrons by the field. Fig. 15 shows the energy level diagram for the non-electroluminescent, photoconducting phosphor considered by IVEY, where the tran-

226

K . N . R . TAYLOR

/ / / / / / / / / / / / / / , ./CONDUCTION BAND, VZ////////////,

LUMINESCENCE CENTER(N) t £1

Fig. 15. Energy level diagram for a non-electro luminescent photoconducting phosphor (after IVEY [20])

h . 7777777777,; VALENCE BAND

yyyyyyyyyyy

sitions are denoted by the symbols: a0 ax a2 (3 y1 y2 d e0 e1 e2 E1 E2 I B M N

non-radiative recombination production of empty centre by hole in valence band trapping of conduction band electron radiative recombination thermal filling of empty centre thermal release of trapped electron exciting transition due to energy absorption field induced non-radiative transition field induced filling of empty centre field release of trapped electrons energy of luminescence centre above the valence band energy of trap below the conduction band intensity of incident radiation light output number of traps number of centres

He shows that in the presence of a field the light output will be given by BT =

M (a0 + e„) «2 (yi_+_fiT|—1 P ' ai ' (y, + i2)|

I

compared with the result of

SCHON [ 7 7 ]

BL =

I 1 +_L

for no applied field

N ß a.,' Y2

(2)

Consequently quenching will result if 2 ^ fo. _J_ _fl_ e0 1 £ )'2 «o Vi o ei ' and enhancement if the reverse is true. Contrary to observation, these results lead to values of R which are independent of illumination intensity. However, the effect of the illumination in removing or increasing space charge effects has not been considered. As in the models of M A T O S S I , no attempt is made to consider

Field Quenching and Enhancement in ZnS and CdS Phosphors

227

the nature of the field induced transitions, however it should be possible t o introduce the probability of field excitation given by FRANZ [ 5 4 ] and used in theories of electroluminescence. I n this way the variation of B with field strength m a y be evaluated. D E S T R I A U and MATTLER [ 2 2 ] have discussed the superposition of R versus field strength curves a t various temperatures a n d conclude t h a t the quenching effect arises from competition between radiative and non-radiative transitions induced by the field. If Pr is the probability of radiative transition occuring, Pnr(T) the probability of a thermally induced non-radiative transition, and Pnr(E) the probability of a field induced non-radiative transition ; then the efficiency without a field can be written Pr Pr + Pnr(T)

'

and with a field applied pr %

Then

DESTRIAU'S

reduction

1 R

1=

Pr + Pnr(T) + Pnr(E)

=

'

W

PnriE)

(5) K>

factor

Bl~~

Bt

Bt

= HL -

1 =

Pr+Pnr(T)

has a functional similarity to the observations. The probability of field induced non-radiative transitions will depend on the temperature a n d is written Pnr(E) = f(T).

g(E),

where / and g are increasing functions. As increase in / will counteract the effect of an increase in Pnr(T), consequently they consider t h a t a thermal 'step b y step' process of the type envisaged by VIGEAN and CURIE [ 7 5 ] is necessary. I n addition D E S T R I A U [ 7 6 ] has shown t h a t Pnr(E) has an almost constant character. Both these results, and the early work of MATOSSI, allow for the quenching effect only, a n d cannot account for the observation of simultaneous quenching and enhancement a t different wavelengths in the same experiment, or t h e observation of quenching of ultra-violet luminescence and enhancement of X-ray luminescence in one phosphor. The work of IVEY, and later calculations of MATOSSI do take this into account b y adjusting the values of transition probabilities etc. for different purposes. I n addition to these kinetic theories, more specific models have been developed to account for particular sets of observations. D A N I E L et al. [ 7 7 ] observed large amounts of quenching a t low static voltages in both powders and crystalline specimens provided t h a t the exciting radiation was strongly absorbed, a n d t h a t hole mobility was possible. This quenching occurred when the electrode under illumination was negative and it was assumed t h a t the luminescence recombinations were controlled by extraction of the positive holes at one electrode and electrons at t h e other. W i t h alternating voltages 80% modulation was reached a t about 15 V applied and remained constant for increase in voltage above this value. B L E I L a n d S N Y D E R [ 7 8 ] interpreted a very complicated set of results in which the peaks in the emission spectrum are shifted to longer wavelengths, t h e green bands being slightly quenched and the red bands strongly quenched, as being due to field induced self trapping of electrons in the lattice accompanied

228

K . N. R . TAYLOR

by a non-radiative process due to the field presence. In the work of ALFREY and TAYLOR on the field quenching of A-particle scintillations [7] is was assumed that the mechanisms occurred in the high electric fields in a barrier layer. The high the