Structural Characterization of Amorphous Silicon

The structure of amorphous silicon in its pure and hydrogenated form has attracted wide interest over the last five decades. This substantial interest is twofold. Firstly, amorphous silicon is a technologically highly significant material with many applications such as photovoltaic cells or thin-film transistors. Secondly, physically it is a fundamentally interesting material which has been regarded as a model system of a covalently bonded continuous random network. While crystalline and amorphous silicon share some structural properties such as the mean bond-length, most other properties such as vibrational, thermodynamic and even mechanical differ substantially. The main feature which distinguishes amorphous materials from their crystalline counterparts is the disorder, i.e. the lack of long-range order. Hence a different theoretical approach is needed to describe amorphous materials to that of their crystalline forms. For an amorphous structure the chemical bonding between the atoms is used with emphasis lying on short-range order rather than on the long-range order used to describe crystalline materials, making a scientific description more challenging. Originally amorphous silicon was described as an ideal continuous random network, where the periodicity of the crystal is replaced by a random network in which each atom has full four-fold coordination as the only specific structural feature. More recently however, improvement of existing techniques and developement of new techniques have allowed greater insight into the structural properties of amorphous silicon and have revealed significant deviations from this ideal continuous random network. Intriguing deviations, not only from the ideal continuous random network, but especially between different forms of amorphous silicon have also been observed whereby the structure depends critically on the method of formation. Three principle different types of formation exist, namely condensation from a vapour, quenching from a melt and solidstate amorphization. However, to date it remains unclear to what extent the formation method of amorphous silicon influences its structural order. Another critically important parameter in the nature of the amorphous network is the thermal history. For example, amorphous silicon formed by self-ion implantation, a method of solid-state amorphization, has been observed to undergo short-range ordering upon thermal annealing to a new state that is close to an ideal continuous random network. This irreversible process is called structural relaxation and leads to changes in