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FREDERICK C. BROWN
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ELECTRONIC PROPERTIES ,4ND BA4NDSTRUCTURE OF THE SILVER HALIDES’ BY FREDERICK C. BROWS Department of Physics, Vniversity of Illinois, Urbana, Illinois Recezved June 80, 1962
The band structure of AgCl and AgBr is discussed with emphasis on recent experiments which bear on our understanding of the solid state. These include external photo-cmission, magneto-resistance at low temperature, and optical absorption. Certain additional phenomena are mentioned such as luminescence, electron-hole pair production by ionizing radiation, and the temperature dependence of electron mobility. The importance of high purity material such as is produced by zone refining is pointed out.
I. Introduction Several recent articles have reviewed the silver halides from t’he point of view of solid state p h y s i ~ s . ~ -It~ is not’ our intent,ion to go over t’he ground col.ered in these earlier papers a t this time. Rather we will discuss some new developments and make a few suggestions for future investigations. Our remarks will apply mainly to fundamental processes in large single crystals but it is hoped t,hat they will bear in part on what goes on in the photographic process. For information on conduction phenomena in emulsion type material, one can turn to other papers of this conference as well as t’o a recent review, “Elementary Photographic Processes in an Electric Field,” by A. L. Kartuzhanskii.6 The new developments referred to above are some optical absorption and magnetoconductivit,y experiments which bear upon the band st,ructure of AgBr and ,4gC1. In addition, progress has been made recently in t,he preparation of crystals of high purity. Pure materials are needed in the study of t’he mobility of electrons, especially a t low temperature. The factors which cont’rol the mobility of carriers a t room t,emperature and below are now at least qualitatively understood. 11. Band Properties A. Band Structure and Electron Affinity.It’has been pointed out. by Taft, Phillip, and Apker7 that the silver halides are a particularly intriguing case compared to other non-metallic photo-emitters. These workers and an earlier investigat,or* have published results on the photo-yield of electrons from AgBr into a vacuum. Photo-emission begins a,t a photon energy of about’ p‘ = 6.1 e.v. a t 300°1i. Apparently this is the energy required t’o excite an electron from the top of the filled or valence band and remove it to infinity. On the (1) (a) I’reaented a t a Symposium on Photographic Processes sponsored by the ACS Division of Physical Chemistry, Washington. I).C . , hlarch 22-24, 1962; (b) Supported in part by the U. 8. Air Force Office of Soientific Research. ( 2 ) F. Seitz, Rea. M o d . Phys., 83, 328 (1931). (3) (a) F. Seitz, “Photographic Sensitivity,” Vol. I. Maruzen. Tokyo, 1936, p. 5 : (b) F. C. Brou-n and F. Seitz, “Photographic Sensitivity,” ’Bol. 11, JIaruzen, Tokyo, 1958, p. 11. (4) F. C . Brown and F. Seitz, “Conference on Scientific Photography,” Liege, :I960 (to be published). (5) J. W.hlitchell, in A. F. Gibson, F. 9.Kroger, and R. E. Burgess. “Progress in Semiconductors,” Vol. 3 , Heyaood & Co.. 1968. p. 5 5 .
( 8 ) .I.1,. h:artiizhanskii. Societ P h y s . - l - s p e k h i , A.I.P. translation, 4 , 205 t l Y 6 l ) . (7) IC. .!I ‘raft, J€. R. Phillip, and I,. Apker, Phys. RPI,.,110, 876
other hand, optical absorption and photo-conductivity data (discussed below) show that the band gap of AgRr is E, = 2.6 e.v. This means that the electron affinity, the energy to remove an electron from the bottom of the conduction band a t E , to infinity, is x = 3.5 e.v. for AgBr. This is of the same order of magnitude as the energy gap. The situation is midway between two extremes, as shown in Fig. 1. Cesium iodideYbelongs to the alkali halides, a class of substances which have electron affinities less than their band gaps. Actually CsI is an extreme rase in that it has a very low affinity, x = 0.3 e.v., and a rather high yield or quantum efficiency. Other alkali halides have affinities of the order of one elertron volt.’O On the other hand, semiconductors such as Ge and Si have affinities much larger than band gaps. The sketch for Si in Fig. 1 is drawn from new data taken on cleaved surfaces under ultra high vacuum.I’ It is also true in the (lase of Si and Ge but not the alkali halides that the threshold for emission, p’, is larger than the energy above which a conduction electron will scatter appreciably with valence band electrons as deduced from breakdown phenomena. I t may be that this scattering threshold occurs about 8 e.v. above the valence band in the rase of AgBr.’ The energies of Fig. 1 are estimated assuming little or no bending of the bands a t the surface due to space charge or a dipole layer.’? This assumption may be checked in the case of AgBr by considering the emission from silver directly into AgBr. It is reasonab1el3 l 4 to assume that thc threshold of this process is just given by the difference in the vacuum work function of -ig metal, p = 4.7 e.v.,lj and x. Emission from deposited silver into AgBr has not been reported, hut the threshold of the Herschel effect in both AgBr and AgCI emulsions is near hv = p - x = I 2 e.v.,lfi which would give x = 3.5 c 1- Moreover, Gilleo has given a value of p - x = 1 1 e.v. for photoemisiion from silver into large crystals of AgCl (9) H (1956)
R Phillip and E
4 Taft J
Phqs
Chem Solzds, 1, 159
(10) E A Taft and H R Phillip, ihad S, 1 (1957) (11) F G Allen and G W Gobelli, Bull A m P h y s Sac 11, 6 , 421 (1961), paper E3 (12) J Bardeen P h y s R 117) U r Rlott and R tronic Processes in Ionlo Cr)stals ” Oxford, 1940 i l l ) W T Turner I’hus Re5 101, 1653 i l q i b ) (15) €I F rarnsilorth and R P T5inch i h d 58, 812 (1940) (le) 0. Bartlttt and H ICliig .Z Phusik 89, 779 (19941, B H. C ‘ < r ~ ~ ltnd l C hf I
