The Bakerian Lecture, 1988 - Amorphous semiconductors: a new generation of electronic materials

Abstract

The review deals with the electronic properties and recent applications of amorphous silicon (a-Si), which can be regarded as the first member of a new generation of electronically viable thin-film materials. After a brief introduction to the structure and the distribution of electronic states in a-Si the preparation of the material by the decomposition of silane in a radio-frequency glow discharge is discussed. The presence of hydrogen in the deposition process is of crucial importance; saturation of defect states, particularly of dangling bonds in the growing structure, leads to a material with a remarkably low density of gap states. Effective substitutional doping from the gas phase now becomes possible with wide-ranging control of the electronic properties. A brief discussion of the doping mechanism in amorphous solids is followed by a summary of carrier transport mechanisms in a-Si, investigated by fast transient techniques. The possibility of doping in a-Si has removed a major limitation in the a-semiconductor field and has, during the past 10 years, led to an upsurge in applied interest in this electronically controllable thin film material. A summary of the present state of applied developments, many already in industrial production, is given. Two groups are discussed in some detail. The first, the photovoltaic development, is based on the a-Si p–i–n junction, and forms part of a wide range of consumer products, but larger area photovoltaic panels are now in production. In the second major development a-Si field effect transistors are used as the addressable elements in large area liquid crystal displays. Remarkable progress has been made with thin film colour displays for small portable television sets. The use of a-Si elements in addressable linear image sensing arrays for telefax applications, coupled with a-Si high-voltage transistor arrays in the associated printers, represents an important step towards an integrated a-Si technology in large-area applications.