Amorphous Semiconductors and Their Defects

For the reader unfamiliar with amorphous semiconductors, we present a brief summary of some of the essential principles needed as a context for the treatment of metastable defects in hydrogenated amorphous Si (a-Si:H). For more detailed discussions of properties of amorphous solids the reader is referred to books by Elliott (1983) and Zallen (1983); for theory of tetrahedral semiconductors to that by Overhof and Thomas (1989); and for broad descriptions of a-Si:H to a four-part set edited by Pankove (1984), a two-volume set edited by Fritzsche (1989), and a book by Street (1991b). For additional historical background there is the landmark book of Mott and Davis (1979).

It is essential to recognize from the start that, although amorphous materials lack the long-range order of crystals, atoms in amorphous structures do not have random locations as occur in a gas. Rather, they retain the same shortrange order that characterizes the local atomic relationships of a crystal of the same material; in this respect amorphous and liquid semiconductors have much in common. When some liquids are cooled sufficiently to solidify, they may form a glassy state, which is defined by such high viscosity as to provide structural rigidity (i.e., shear strength) although crystallization does not occur.

Most semiconductors, when cooled from the melt, form crystallites rather than glasses; but other more rapid cooling techniques can force them to become amorphous solids, although these are then limited to thin films.