Photoelectric memory in epitaxial n-GaAs films

Abstract

The residual (“frozen”) conductivity (RC) effect is the retention, over a long period of time, of the conductivity state that is induced by illumination (this phenomenon is called photomemory). In the case of epitaxial GaAs films this can be explained by a model of “collective” repulsive barriers of recombination centres”‘, but the “barriers” are mainly the effect of band bending near the sample surface3A. In order to establish the mechanism of RC observed in epitaxial n-GaAs films, in this work the characteristics of the RC were studied over wide ranges of film thickness and temperature (4.2-l 50 K), with simultaneous control of the surface properties of the samples, through the capacitance-voltage characteristics, field effect measurements and photo-e.m.f. measurements. The use of surface-sensitive methods makes it possible to measure directly the height of the surface potential barriers that are induced by charged surface states and surface recombination centres (BISC) both in the dark and under RC conditions. Epitaxial n-GaAs films grown on insulated substrates by the gas transport method were used for investigation. The initial film thickness was 10-l 5 m; the carrier concentration and mobility were n = 1015 -1016 cm-3andp=(4-5) x IO3 cm2 V-‘s-l, respectively; the substrate orientation was (100). At film temperatures of less than approximately 150 K and for illumination with white light, an RC appears with a relaxation time of 104-lo6 s. If we decrease the temperature to 4.2 K, the value Aua this bending provides the surface barrier mechanism of the RC. If we illuminate with light from the fundamental