Nature of the disordered state of hydrogenated amorphous silicon as revealed by the study of anelastic relaxation behavi
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R. O. Dusane and V. G. Bhide School of Energy Studies, University of Poona, Pune-411 007, India
S.B. Ogale Department of Physics, University of Poona, Pune-411 007, India (Received 11 July 1988; accepted 14 December 1988) The nature of the disordered state of hydrogenated amorphous silicon is examined for the first time by measurement of anelastic relaxation behavior. It is demonstrated that local structural units and their modifications control the relaxations in these films under different conditions of deposition, aging, and light exposure. Specifically, the light-induced state in this material is shown to be characterized by four distinct relaxations.
I. INTRODUCTION The physics of disordered solids has been a subject of immense scientific interest in recent times in view of a rich variety of phenomena generated by such systems via intriguing structure-property relationships. Even though significant efforts have been undertaken in the past to study these relationships, there are several outstanding issues which remain to be examined. Amongst such issues an important one is the nature of defect states in amorphous materials.1 There is indirect evidence to believe1 that defect states which can be and have been accessed by optical, electrical, or magnetic measurements form only a small subset of the totality of defect configurations in such structures, and this situation calls for use of alternative defect characterization probes to explore these material systems. Amongst such alternative probes the internal friction technique2 stands out for the unique information it can provide about the anelastic relaxation behavior of defect structures.2"6 Berry et al.5 have previously applied this method to the study of metallic glasses and have demonstrated occurrence of point defect-like relaxations in these systems in spite of their being disordered materials. This raises interesting questions about the definition of structural defect states in disordered materials and their relation to local structural units. In the work reported here we have used for the first time the internal friction technique to study the anelastic relaxation behavior of hydrogenated amorphous silicon. This disordered material exhibits a number of interesting properties such as low defect density,7 high photoconductivity gain, and both n- and p-type dopability. Another interesting property of this material is the susceptibility to light (or radiation) induced property modifications called the Staebler-Wronski effect.8 These properties are known to be significantly dependent on the preparation conditions.9 We have shown that all these effects are char612
J. Mater. Res., Vol. 4, No. 3, May/Jun 1989
acterized by the emergence of distinct anelastic relaxation signatures in this material. To study the anelastic relaxation behavior of a-Si:H films, we have used the vibrating reed technique developed by Berry and Pritchet,2 wherein one essentially looks for the damping of various resonant flexural modes of oscillation of a reed (sample) as a function of
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