TABLE OF CONTENTS Amorphous materials are not homogeneous on a microscopic scale. They may well be described as solid solutions of defects and atoms. Because there is a spectrum of defects, it is very difficult to assign a structure to correspond to measured structural parameters. Further, the processing variables can markedly affect the measured structural parameters and one can find in the literature what appear to be contradictory results for apparently the same experiments. Hence, the reader is warned to maintain a critical attitude to what will be described as "facts" in the remainder of this chapter. Because both of the complexity of ?-Si:H and its applicability to industrial products, its defects are probably the most studied to date.
The major factor limiting the properties of amorphous semiconductors is the density of defects in the energy gap. Usually, the lower this density the better are the properties. For example, the dark conductivity in amorphous silicon is reduced from 10 -7 S/cm to about l0 -11 S/cm by hydrogenating amorphous silicon while the dangling bond density correspondingly decreases from 10 19/cm 3 to 10 15/cm 3.
There is good evidence for the belief that the main defect adversely affecting properties is the dangling bond (a three-coordinated Group IV atom). The other main class of defects consists of distorted bonds involving both length and bond angle distortions. The latter are believed to affect the number density...
