Defects in Covalent Amorphous Semiconductors

Abstract

Before any analysis of the physical properties of an amorphous solid can be undertaken, a knowledge of the structure is essential. Of course, this is true of a crystalline solid as well. However, because of the lack of long-range periodicity in amorphous materials, it is much more difficult to determine the structure. In addition, the preparation methods necessary to obtain the material in the amorphous phase require the consideration of several issues that are not usually contemplated when investigating crystals. First and foremost of these is the question of composition: we must determine what is in the material. If the solid has been prepared as a bulk glass, there are not usually too many compositional surprises. However, many of the most important amorphous solids cannot be fabricated as bulk glasses but are generally deposited as thin films directly from the vapor phase, while even compositions which are relatively easy glass formers are often also deposited in thin-film form for convenience or for commercial applications. In such cases, the composition must be carefully determined and preferential deposition and even unsuspected impurities are the rule rather than the exception. The composition can be a sensitive function not only of the preparation technique but also of the many deposition parameters and even of the geometry of the system.