Journal of Non-Crystalline Solids 109 (1989) 255-261 255 North-Holland, Amsterdam PHOTOACOUSTIC INVESTIGATION OF THE OPTICAL ABSORPTION AND THERMAL DIFFUSIVITY IN SixTelo o_ x GLASSES K.N. MADHUSOODANAN and Jacob PHILIP Department of Physics, Cochin University of Science and Technology, Cochin 682022, India S. ASOKAN, G. PARTHASARATHY and E.S.R. GOPAL Instrumentation and Sert~ices Unit, Indian Institute of Science, Bangalore-560012, India Received 17 July 1987 Revised manuscript received 8 January 1988 The photoacoustic technique is used to determine the optical energy gap E 0 of bulk SixTel0o_ x glasses in the glass-forming region 10 < x < 28. The thermal diffusivity a of these samples has also been measured. The variation of E 0 and a with x is reported. It is found that E o increases with x nearly linearly with a sharp decrease in the rate of increase beyond x = 20. The thermal diffusivity also increases with x up to x = 20 but decreases for compositions with higher values of x. The observed behaviour is explained on the basis of a chemical bond approach. It is accounted for in terms of the increase in the number of Te-Te bonds and formation of SiTe 4 tetrahedra with an increase in the chalcogen content. 1. Introduction Chalcogenide glasses are finding extensive ap- plications as electronic and optoelectronic device materials [1]. This class of materials, containing a large percentage of chalcogen elements, is widely used in switching and memory devices [2,3]. An understanding of the dependence of various prop- erties of such chalcogenide glasses on composition is important, because the continuously variable composition of these alloys may be utihsed to prepare materials for particular applications. In this paper we report the results of a study of the variation of optical energy gap and thermal diffu- sivity of bulk Si-Te glasses using the photoacous- tic technique. SixTel00_ x glasses with x in the glass-forming region 10 < x < 28 are the subject of the present investigations. The crystallization be- haviour and electrical conductivity of this system have been reported before [4]. As amorphous materials lack long-range order, the chemical bond approach [5] which considers the local bonding and short-range order is consid- ered to be the most appropriate to explain the 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) various properties exhibited by these materials. This approach has proved to be useful in the study of covalent semiconductors such as chalcogenide glasses [6]. In chalcogen-rich chalcogenide glasses, the top of the valence band is formed from the lone-pair non-bonding states and the bottom of the conduction band is formed from the antibond- ing states of the chalcogen atom [7]. The energy difference between the top of the valence band and the bottom of the conduction band closely corresponds to the optical energy gap E 0. The photoacoustic technique [8] which depends upon the absorption of modulated incident light by the sample and subsequent production of acoustic waves in the surrounding gas through a nonradiative relaxation process is used for the present studies. When the incident photons are absorbed by the sample, internal energy levels within the sample are excited. Upon subsequent de-excitation of these energy levels, all or part of the absorbed energy is transformed into thermal energy through nonradiative de-excitation proces- ses. In the gas medium surrounding the sample, this periodic heat flow in the sample appears as