Amorphous materials do not have the regular atomic structure characteristic of a crystal. Instead, the specific bonding arrangement within a particular volume of material represents one of many alternative configurations. Hydrogenated amorphous silicon has the added variability of a hydrogen content which can reach 50 at %. Most features of the a-Si: H network structure are defined at the time of growth and therefore depend on the details of the deposition process. Thus it is anticipated that the electronic properties vary with the growth conditions and that a detailed understanding of the growth mechanisms is essential for the optimization of the electronic properties. Indeed, a-Si:H does exhibit a great range of specific properties. However, the optimization of the growth process produces films which are remarkably independent of the detailed growth process. The best films are all similar, while low quality films are defective in many different ways. It is now recognized that the electronic structure is influenced by defect reactions taking place within the material after growth, which are largely independent of the growth process. These are described in Chapter 6 and explain why the properties of low defect density material are not so sensitive to the deposition method.

Growth of a-Si:H

The usual method of depositing a-Si:H is by plasma decomposition of silane gas, SiH4, with other gases added for doping and alloying. Silane decomposes in the absence of the plasma above about 450 °C and high temperature pyrolitic decomposition is used to make polycrystalline or epitaxial silicon.