Photoconductivity in crystals of silver bromide

Pergamon

J. Phys. Chem. Solids

Press 1959. Vol. 8. pp. 297-300.

Printed in Great Britain

PHOTOCONDUCTIVITY IN CRYSTALS OF SILVER BROMIDE

K.5

E. A. BRAUN*

and J. W. MITCHELL

H. H. Wills Physical Laboratory, 1. INTRODUCTION

QUANTITATIVE measurements of the electronic photoconductivity of single crystals of silver chloride and silver bromide were first made by HECHT~) and by LEHFELDT.@)They determined the range of the photoelectrons at a temperature of -170°C but were unable to detect any drift of positive holes in the electric field. The photocurrents steadily decreased to values which could not be measured with their apparatus when the illumination was prolonged. This was attributed to polarization as a result of the establishment of space charges. The crystals which were used for these measurements had been exposed to light during preparation and this led to the separation of some photolytic silver. Before the measurements were made, they were heated in air for a period to “restore them to their original state.” Crystals of silver halides grown from the melt in air by the Kyropoulos method probably contain silver oxide either in the form of a very dilute solution or as segregated particles. If the coloration during exposure were due to the formation of colloidal silver, this would be converted to silver oxide when the crystals were annealed in air so that the original state might, in this case, be re-established. More recently, crystals which had been annealed in air in similar circumstances have been used as crystal counters by VAN HEERDEN,(~) ALLEMANDand ROSSEL@)and BROWN.(~)Although the sensitivity is high, so that the shapes of the electron pulses may be resolved with a high speed oscillograph, pulses which could be attributed to the displacement of positive holes have never been detected. HAYNES and SHOCKLEY@)(see also HAYNES(~)) * On leave of absence from the Department of Physics, the Hebrew

University,

Jerusalem,

Israel. 297

University

of Bristol

have also used air-annealed crystals for experiments in which they followed the displacement of the photoelectrons at room temperature by observations on the separation of photolytic silver. The object of the present investigation was therefore to study the photoconductive properties of large single crystals of silver bromide free from silver oxide and other possible sensitizing impurities, which have an extremely small photolytic sensitivity. The photochemical observations that holes and electrons, when suggest liberated in such crystals, recombine without the production of photochemical changes (CLARK and MITCHELL@)). 2. EXPERIMENTAL PROCEDURE The silver bromide used for this work was prepared by methods described in detail by CLARK and MITCHELL.(~) The material was kept molten for several hours in a high vacuum so as to pump away volatile impurities, including cuprous compounds, and to break down silver oxide into free silver and oxygen, which was pumped away. The silver was converted into silver bromide by admitting pure bromine to the molten silver bromide; the excess bromine was subsequently removed by condensation in a liquid air trap. This procedure appears to eliminate traces of silver oxide. The crystals were grown in vacuum by the Bridgman method and cut into slabs 1Ommx 7mmx 3mm, which were ground and polished. The slabs were annealed first in bromine and then in vacuum. Every care was taken to produce crystals of high purity and perfection. Particular attention was paid to the elimination of silver oxide and cuprous compounds. The crystals were provided with graphite

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electrodes such that the crystal was illuminated with the light beam either (1) at right angles to the electric field or (2) parallel to the field. Light of 436mm wavelength at a rate of quantum incidence of 1010 quanta set-1 was used. The specimens were mounted in a vacuum cryostat and cooled to various temperatures between -183°C and -78°C. Instead of using the recent pulse techniques for measuring the photoconductive response (ALLEMAND and RossE~(~) and BROWN(@),we used a current measuring circuit in which two electrometer triodes formed two arms of a Wheatstone bridge. The reason for this was twofold. First, the pulse technique is less sensitive than the current measurement and therefore useful only for crystals with fairly large electron ranges; we both expected and obtained rather small electron ranges in our crystals. Secondly, the d.c. measurement technique enabled us to study the behavior of the photocurrents over large periods, and this in turn gave information on the internal polarization which builds up in the crystals. One of the striking results of these measurements was the fact that the photocurrent did not decrease to zero even during periods of exposure to many hours.

3. EXPERIMENTAL

RESULTS

Results of measurements at temperatures between -183°C and -150°C will be described first. The general pattern of the photoconductive response of the crystals was as follows: When a field of about 1000 Vcm-1 was applied, the current rose abruptly on illumination from zero to a maximum value Im and then slowly fell according to a law of the form I = Is+l/b[{b(~m--*))-~+t]2 to an equilibrium value &; t is the time and b a constant. Equilibrium was reached within a few minutes. The value of the equilibrium current was about a tenth or more of the maximum current and was stable as long as the illumination and applied field were left unchanged. When the light was switched off, the current fell rapidly to zero. No dark current was ever observed at low temperatures. When the field

CRYSTALS

was removed and the crystal illuminated again, a transient photocurrent in a direction opposite to that of the original photocurrent was observed. This “back-current” was due to the internal polarization caused by holes and electrons trapped during the flow of the forward photocurrent. The peak value of the forward photocurrent corresponds to the primary photocurrent measured by HECHT, LEHFELDT and others. We have applied HECHT’S formula to calculate the mean range of carriers in unit field from the extrapolated peak value of the photocurrent. The initial mean range for electrons was of the order of 10-s cm2 V-1 and for holes it was 10-Y cm2 V-1. The fact that the photocurrent, although decreasing after the initial peak due to internal polarization, reaches an equilibrium value different from zero, provides further proof of some finite, if small, range for the holes. The internal polarization could be detected even after very short exposures and reached its equilibrium value at the same time as the photocurrent. This equilibrium value clearly could not eliminate completely the applied field in the illuminated region. The “back-current” was always proportional to the steady state photocurrent. At low temperatures, the internal polarization which decayed very slowly over periods of hours during dark storage scarcely decayed at all if the electric field was maintained. At higher temperatures, the internal polarization was less persistent, but no single trap depth could be ascertained from thermal decay of the polarization. The voltage dependence of the equilibrium photocurrent followed a law of the type I = AEb where I is the current, E the field and b a constant. The value of b was unity or slightly higher for the case of parallel illumination and about 1.5 for the perpendicular case. At temperatures above -15O”C, the photoconductive response remained very much the same as at low temperatures as long as the ionic current did not exceed the electronic current by orders of magnitude. Above -100°C the surface photoconductivity was markedly increased compared with the bulk photoconduction. This is consistent with previous observations by EVANS, HEDGES and MITCHELL@)on the displacement of the latent image in single crystals of silver bromide by electric fields. The photochemical sensitivity

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of these crystals was remarkably low and the printing out of photolytic silver during even prolonged periods of intense illumination was almost impossible to achieve.

4. CONCLUSIONS

It has now been shown that crystals of silver chloride and silver bromide, of the highest attainable purity and perfection, have two characteristic properties. Their sensitivity for the separation of photolytic silver is extremely small and the range of the conduction electrons which are liberated under illumination is 3 x 10-s cm2 V-r. It would be surprising if there were no correlation between these properties. The absence of any photochemical change shows that conduction electrons and positive holes, if they are actually liberated by the absorption of energy in the crystals must recombine. The bulk of the energy of the absorbed photons is therefore converted into lattice vibrational energy. Crystals of silver halides may be sensitized for both photochemical change and photoconductive response by heating them in oxygen in contact with a piece of silver foil. Silver oxide is then formed which diffuses into the crystals to form a very dilute solid solution and possibly also segregated particles. Such sensitized crystals darken, due to the separation of discrete particles of silver, during exposure and the range of the conduction electrons is found to be of the order of 10-a cm2 V-r. The drift of the positive holes which may be liberated with the conduction electrons cannot be detected and their range in these sensitized crystals must be extremely small. It therefore becomes clear that there must be a fundamental difference between the properties of the positively charged centres produced by the trapping of the positive holes in the pure and in the sensitized crystals which results in a reduction in the amount of recombination between trapped positive holes and electrons and in an increase in the lifetime of the conduction electrons. Before we discuss this in greater detail, we shall refer to an alternative interpretation of the sensitizing influence of silver oxide which has been suggested by SUPTITZ(~@and by BROWN. (11) Following the general lines of argument of the

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Gurney-Mott theory of photolysis, SUPTITZ and focus their attention upon the capture of the conduction electrons by traps of an unspecified nature as the process which determines the range. They do not consider recombination between conduction electrons and trapped positive holes. With this approach, it is necessary to postulate that the electron traps are inactivated or removed, possibly by segregation, when the crystals are annealed in oxygen. In our view, heating the crystals under conditions which allow silver oxide to diffuse into them produces a very dilute solid solution of silver oxide in the silver halide, together with segregated silver oxide, if the treatment is prolonged. The function of the dissolved silver oxide is to provide traps for positive holes which are not recombination centres. In the crystal, the excess negative charge of a doubly charged oxide ion is compensated by an interstitial silver ion and the drift of these ions in electric fields is responsible for the enhanced conductivity of crystals containing silver oxide compared with pure crystals. During illumination, the positive holes are trapped by the oxide ions to give singly negatively charged ions. The positive charge of the interstitial silver ion is then compensated by the negative charge of the conduction electron for which both the interstitial silver ion and the singly charged oxide ion have small capture cross sections. The lifetime of the conduction electron at room temperature is determined by the time required for its combination with an interstitial silver ion on the external surface or on an internal surface of the crystals. That other trapping processes are unimportant is established by the equality between the Hall and the drift mobilities for electrons in silver chloride found by HAYNESand SHOCKLEY.(~)This accounts directly for the correlation between the increased photosensitivity and the increased photoconductive response of crystals containing silver oxide at room temperature. BROWN

REFERENCES 1. HECHT K. 2. Phys. 77, 235 (1932). 2. LEWFELDT W. Nachr. Ges. Wiss. Gb’ttingm 1, 171 (1935). 3. HEERD~ P. J. VAN,Physica. 16. 505 (1950). 4. ALLEMAND (2%.a& Ro&. j.kelv.$hys.‘Acta 27, 212, 519 (1954).

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5. BROWN F. C. Phys. Rev. 97, 335 (1955). 6. HAYNES J. R. and SHOCKLEY W. Report on Confemme on Strength of Solids p. 151. Physical Society, London (1948); Phys. Rev. 82, 935 (1957). 7. HAYNF.SJ. R. Rev. Sci. Instr&~ 19, 51 (1948).

J. Phys. Chem. Sotids

K.G

CRYSTALS

8. CLARK P. V. McD. and MITCHELL J. W. g. Phor. &i. 4, 1 (1956). 9. EVANS T., PLEDGESJ. M. and MITCH J. W. J. Phot. Sci. 3, 75 (1955). 10. SIJPTITZP. Naturwissenschaften (1957). 11. BROWN F. C. J. Phys. Chem. 4, 206 (1958).

Pergamon Press 1959. Vol. 8. pp. 300-304.

ELECTRONIC

PROCESSES

Printed in Great Britain

IN THE SILVER HALIDES

AT

LOW TE~P~RAT~RE~ F. C. BROWN

and

K. KOBAYASHI

University of Illinois

THE mobility of electrons at room temperature in ionic crystals is considerably smaller than in atomic semiconductors because of the strong interaction between conduction electrons and optical mode vibrations. Below the Debye temperature 0 associated with the longitudinal optical mode, mobility increases rapidly with decreasing temperature according to p = A exp[(O/T)-l].(r) If impurity scattering is not too important acoustical mode scattering with a T-5 dependence will predominate at very low temperatures.@) Electron drift mobility has been measured over a wide range of temperatures by transit time techniques in the case of AgCl and AgBr.(s-s) The results are not in agreement with the above expectations. Drift mobility is found to be small at low temperatures and to have a maximum in the vicinity of 50 to 90°K depending upon sample preparation. Scattering by imperfections such as ionized impurities could cause such an effect, but an unreasonably high density of impurities (1017 to 101s cm-s) is required. It seems more reasonable to ascribe the drift mobility results to multiple trapping effects. Recently the situation has been clarified by

observations of the Hall effect for photo-carriers using a modification of the Redfield technique.(T) Details will be given elsewhere,(*) but some essential results are shown in Fig. 1 for a simple crystal of AgCl. Both Iiall mobilityl and drift mobility for electrons are plotted as a function of temperature. Note that the Hall mobility, which is independent of trapping effects, rises to very high values at low temperature. The sign of the observed Hall effect was negative since holes make a small or negligible contribution to photoconductivity in all samples studied so far. This result is in agreement with transient photoconductivity experiments.(*) The drift of holes in an applied field has not been observed and, if present at all, must be several orders of magnitude less than the drift of electrons. The Hall mobility data shown in Fig. 1 can be approximated by a T-” dependence below about 50°K indicating the importance of acoustical scattering. There is also evidence for scattering by other imperfections at the lowest temperatures. The crystals are relatively pure but may contain ionized impurities of the order of 101s cme3. Optical mode scattering becomes apparent at the

* Supported in part by the U.S. Air Force Office of Scientific Research.

t Obtained from the Hall angle in the range of linear dependence on magnetic field.