Structural relaxation process in pure amorphous silicon

Abstract

Amorphous silicon (a-Si) is a material of major scientific and technological interest. It has been a very active topic of investigation for several decades, frequently crossing the border between physics and materials science. Research has been motivated partly by the fact that a-Si is an excellent model of a covalently bonded continuous random (CRN) network; thus a detailed understanding of the structure and properties of this material may aid the understanding of many other disordered materials. Notwithstanding many years of research and widely accessible experimental techniques, the structural properties of as-implanted a-Si and its relaxation mechanism remain poorly understood and are indeed subject of lively debate. This thesis addresses this issue by looking at the transition from unrelaxed to relaxed states which may provide insight into the nature of the structural relaxation process. In all three forms of a-Si studied: ion-implanted, pressure-induced, and re-irradiated relaxed a-Si, analysis of Raman spectra indicated a local ordering of the material approaching a continuous random network in the fully relaxed material due to thermal annealing. The bond-angle distortion is found to reduce at temperatures above 250 degree Celsius, whereas previous studies show reduction below this temperature. Electrical conductivity measurements of ion-implanted a-Si showed a decrease in the number of dangling bonds upon annealing. This phenomenon is observed during low temperature annealing up to 250 degree Celsius, before the bond-angle ordering commenced. Differential scanning calorimetry measurements show a heat release during low temperature annealing that is predominately due to defect annealing from the structure, but the heat release continues up to annealing temperature where the bond-angle distortion is reduced. Indentation tests showed that the transition in the deformation mechanism in ion-implanted a-Si mechanism from plastic to phase transformation related to the reduction of defects in the structure. This transition occurred at annealing temperatures before the bond-angle distortion reached its minimum value. The effect of in-diffused hydrogen upon annealing was also extensively investigated in this current study. It is observed that in-diffused hydrogen does not contribute significantly to the short-range ordering and hence structural relaxation. Finally, the results from all studies, namely Raman spectroscopy, electrical measurements, calorimetry, and indentation are brought together to develop a new model for structural relaxation in a-Si. Broadly, it is found that there are two main steps in structural relaxation: defect removal at low temperatures up to 250 degree Celsius, which is accompanied by significant heat release, and a reduction in bond-angle distortion at higher temperatures, where the amount of heat release is smaller. In terms of existing models, the current findings are consistent with elements of the previous defect/lattice strain models but differ in that a more distinct separation of defect removal and reduction in strain (bond-angle distortion) is found in the present study.