Chalcogenide Letters Vol. 1, No. 7, July 2004, p. 83 - 90 STEADY STATE PHOTOCONDUCTIVITY IN a-Se 90 Ge 10-x In x THIN FILMS S. Singh, R. S. Sharma, R. K. Shukla, A. Kumar * Department of Physics, Harcourt Butler Technological Institute, Kanpur-208 002, India Temperature and intensity dependence of photoconductivity is studied in a-Se 90 Ge 10-x In x thin films prepared by vacuum evaporation. Temperature dependence of photoconductivity shows a maximum at a particular temperature. The study carried out on intensity dependence of photoconductivity shows that the values of γ is found to be between 0.5 and 1.0 for all the samples. This indicates that there exists a continuous distribution of localized state in the mobility gap of a-Se 90 Ge 10-x In x thin films. Composition dependence of steady state photoconductivity shows that photoconductivity as well as photosensitivity is found to be minimum at 4 at. % of In. A discontinuity in various electrical parameters at 4 at. % of In is related to formation of mechanically stabilized structure at a particular average coordination number 2.2. This is consistent with the theory of Phillips and Thorpe for the topological model in case of chalcogenide glasses. 1. Introduction Chalcogenide glasses are promising materials for many applications in solid state devices. Recently, special interests have dedicated to in the amorphous films of chalcogenide glasses in connection with the modification of their properties on doping with metal impurities [1]. In thermally evaporated amorphous films of chalcogenide glasses these impurity atoms are electrically active and allow to obtain new materials with the improved properties. The changes in physical properties of amorphous films is caused by the modification of their structural and chemical disordering due to presence of high concentration of defects [2]. Trapping and recombination processes, which depend on the distribution and concentration of these defects, strongly influence the photoelectrical characteristics of the amorphous materials [3]. The electrical properties of chalcogenide glasses are not, in general, affected appreciably by the incorporation of impurities because the random network of atoms can accommodate an impurity without creating an extra electron or hole. This concept is based on the fact that an impurity atom can satisfy its valence requirements by adjusting its nearest neighbour environment, thus causing the negligible effect on the electrical properties [4]. Another argument is that the high density of localized states present in the forbidden gap effectively pins the Fermi level. However, experimental results reported by various researchers have shown that there are selected cases in which the addition of impurity atoms does change the electrical properties of chalcogenide glasses significantly [5-8]. It has also been found that the effect of impurities depends strongly on the composition of the glass, the chemical nature of the impurity and the method of doping. Impurity concentration obviously is a critical factor in such cases because all impurities cannot behave in an electrically active manner. Several of the physical properties are found to improve by the addition of certain impurities. Therefore, investigations on the influence of impurities on the properties of chalcogenide glasses are of relevance both from the basic science and application point of view. In the present paper the results of d. c. conductivity and steady state photoconductivity of ternary chalcogenide glassy system, namely Se-Ge-In, are presented. The system can be considered as consisting of the basic Se-Ge network with appropriate amounts of indium added as impurity in the glass forming region of the Se-Ge-In system. * Corresponding author: dr_ashok_kumar@yahoo.com