States in the gap and defects in amorphous semiconductors

Abstract

The development of our understanding of amorphous semiconductors over the past ten years has in many ways paralleled that of crystalline semiconductors about thirty years earlier. For both classes of materials, initial studies were plagued by poor agreement between results obtained in different laboratories on ostensibly the same material a feature that was attributed in many cases to inadequate characterization of samples in terms of structure, stoichiometry, and impurity content, and to the presence of defects of various kinds. For crystalline semiconductors it became clear, as time progressed, that many properties could be associated with such local departures from ideality in an otherwise perfect material. Subsequent careful control over conditions of growth and the development of elaborate techniques for reducing the impurity content, particularly in the case of Ge and Si, eventually allowed investigation of their intrinsic properties. Later, the controlled addition of impurities, coupled with an understanding of the role they played, paved the way towards the application of these materials in devices. For amorphous semiconductors a similar road has been followed and, although there is clearly still a long way to go, it seems fair to say that sufficient advances have been made to allow a separation of intrinsic from extrinsic properties at least as far as certain simple elemental and binary materials are concerned. Just as for crystals, defects play a major and often dominating role.