Electronic Structure of Amorphous Semiconductors

Abstract

Until very recently, amorphous solids were a good deal more common in nature than in solid state theory textbooks. This can be only partially excused on the grounds that they are embarrassingly difficult substances to analyse theoretically in any fundamental way. The lack of periodicity (and hence Bloch’s Theorem) presents severe obstacles to the formulation of a theory that is at once fundamental, rigorous, realistic and tractable. One must always hope for the last of these qualities but the other three can hardly be attained simultaneously, so the problem must necessarily be investigated from different (complementary rather than competing) points of view, each of which has its own particular merits. We shall review these briefly in the next section, before embarking on an exposition of our own recent efforts. However it is first necessary to say something about structure. This obvious first step in the description of a solid is, in the case of amorphous solids, still a rather hesitant and uncertain one. While many of the remarks which we shall make are of wide validity, it is appropriate at this point to define our field of immediate interest as being confined to the very simplest amorphous semiconductors, namely Si, Ge and related compounds.