Local structure, bonding, and electronic properties of covalent amorphous semiconductors

The many types of chemical-bonding arrangements which can characterize primarily covalent amorphous semiconductors are enumerated and discussed. Emphasis is given to local structures which are difficult to achieve in significant concentrations in crystalline solids but are readily attainable in amorphous materials via controlled composition and preparation techniques. These include undercoordination, overcoordination, valence alternation, multicentre bonding, and complexing, and their consequences can be important whatever group of the Periodic Table the constituent atoms represent. In each case, the effects on the electronic properties of the materials are discussed. It is shown how these unusual chemical configurations are necessary to understand recent experimental results on both doped and chemically modified amorphous semiconductors. The overall approach represents a unified method for understanding the electronic structure of all of these solids.