Structural Characterization of Amorphous Silicon

Abstract

The structure of amorphous silicon (a-Si) has attracted wide interest over the recent decades. This substantial interest is twofold. Firstly, a-Si has many, highly significant, technological applications. Secondly, physically it is a fundamentally interesting material which has been regarded as a model system of a covalently bonded continuous random network (CRN). Such a CRN is a random network in which each atom has full four-fold coordination as the only specific structural feature. More recently, improvement of techniques has allowed greater insight into the structural properties of a-Si. Intriguing deviations, not only from the ideal CRN, but especially between different forms of a-Si have been observed. However, to date it remains unclear to what extent the formation method of a-Si influences its structural order. Another critically important parameter in the nature of a-Si is its thermal history. For example, a-Si formed by ion-implantation undergoes structural relaxation – or short-range ordering – upon thermal annealing to a new state that is close to an ideal CRN. It remains unclear however, if other forms of a-Si undergo structural relaxation to the same degree. Thus, despite its widespread use and decades of research, the exact nature of a-Si is still not fully understood and this thesis addresses this topic. Different forms of a-Si was prepared by deposition techniques, rapid quenching from the melt and solid-state amorphization. These different forms were investigated in their as-prepared state as well as in their thermally annealed. A combination of techniques was used, namely nanoindentation, electron-energy-loss spectroscopy, Raman microspectroscopy, electron diffraction and fluctuation electron microscopy. All forms of a-Si were first probed for their uniformity. Films prepared by plasma-enhanced chemical vapour deposition and by rapid quenching from the melt were found to contain voids and nanocrystals which prevented the study of their structural properties. More uniform films prepared by magnetron-sputtering, ion-implantation and the so-called pressure-induced (PI) a-Si however, were studied in depth for their structural properties. Each as-prepared form of a-Si was found to have a unique network with very different structural properties. The magnetron-sputtered a-Si was observed to have significant microstructure. The pure ion-implanted a-Si however, is free of such microstructure although some inhomogeneities are clearly present within the network. Interestingly, PI a-Si possesses very little order on the entire length-scale. Only uniform, pure forms of a-Si without any microstructure undergo structural relaxation upon annealing. In the case of the other forms of a-Si, the presence of voids and nanopores seems to prevent the formation of a more ideal CRN. Intriguingly, for the pure cases however, the structural relaxation results in essentially the same properties for both networks over the entire length-scale. These findings were used to build a framework for the understanding of the…