<title>Transient photocurrent in α-As<formula><inf><roman>2</roman></inf></formula>Se<formula><inf><roman>3</roman></inf></formula> thin films with optical bias</title>

Abstract

Relaxation of pulsed photocurrents under optical bias is studied in amorphous films of glassy alloys AsSe3:Sn (x=O to 3.5 at%) by the time-of-ifight technique and steepness band-to-band light illumination. In frame of multiple4rapping model it is shown that by adding tin to the glass former As2Se3, the hole drift mobility is strongly increased and hamper the recombination.. The kinetics of the long-term photocunent decay can be described by stretched exponential fimction. The dispersion parameter a, which can be deduced from the time dependence of the photocurrent I (t) exp[-(r/t) } and, is about 0.47 for undoped samples, and 0.35 for tin-containing samples. The obtained results indicate the variation in occupation of deep localized centers. For the investigated samples, the width of distribution of the deep traps is approximately kT/a5O-7O meV. L INTRODUCTIONArsenic serenades glasses are well known as high photosensitive materials with a wide range of application in optoelectromcs and information storage systems. Besides that, the physical and practical interest of the impurity effect in amorphous materials arises from its structure, which has a higher degree of disordered than that of bulk glasses. The influence of impurities on transient photocurrents in amorphous As2Se3 films was reported'. It was found that the impurities influence the dispersion parameter a, which describe the exponential density oflocalized states in amorphous material.One of the methods for determination the energy distribution of localized states in amorphous semiconductors is socalled transient photoconductivity tethnique2'. The experimental results on x-As2Se3 films usually were interpreted in frame of multiple-trapping model when the photoconductivity relaxation is determined by trap-controlled recombination. Moreover, it was shown that the Sn and other metal impurities influence the relaxation of photocurrent due to appearance ofnew localized centers as result ofdoping ofthe amorphous material3'5. The time-of flight experiments also can be used to investigate photoinduced changes in electronic properties of the amorphous materials9. It was shown that bias light induces an increases in the trap density in a-As2Se3 and modifications ofthe electric field in a-Se7. Our first experiments on time-of-ifight measurements of the hole drift in xAsS3 show that under additional illumination distributed uniformly over the sample volume, the drift mobility increase, especially at lower temperatures6. This increase of the drift mobility and decreasing of the activation energy was attributed to the filling ofdeep traps under high levels ofadditional illumination. Because Sn create new localized centers in the forbidden gap of the amorphous material, the goal of this paper is to investigate the distribution of localized states in a-Aa2Se3 doped with different concentrations of tin. For this purpose the time-of-flight technique and long-term relaxation ofphotocurrent were applied. IL EXPERIMENTALThe amorphous ifims of glassy alloys As2Se3:Sn (x=O to 3...