Amorphous semiconductors voids

The characterization of amorphous semiconductors is further compU-cated by the fact that the same chemical composition can exist in many structural states which depend on the thermal history and preparational variables. Moreover, compositional heterogeneity, voids, cracks, as well as the preservation in some materials of molecular structures with relatively extended short-range order makes real amorphous materials quite different from the ideal disordered systems presently studied by theorists. 

We do not know whether these problems are common to all tetrahedral amorphous films. The similarity of their properties and the anneaUng behavior suggest that this is indeed the case. Most information about the electrical behavior of tetrahedral films has been obtained with material containing internal voids. It appears nevertheless important to review the properties and to analyze to what extent they reveal properties inherent to this group of amorphous semiconductors. 

An ideal amorphous semiconductor is thought of as a uniform three-dimensional random network structure in which the valence requirements of each atom are satisfied. Any real material contains, however, imperfections and deviations from the ideal disordered network in the form of dangling bonds, voids, or density fluctuations. Furthermore, some vitreous semiconductors contain structural units which are bonded together by weak van der Waals type forces. This section deals in particular with structural and compositional inhomogeneities whose own size or whose effect (by forming a... 

The surface potential of the internal void structure must be screened within the characteristic length X and thus leads to internal potential fluctuation. It will be recalled that the tetrahedral amorphous semiconductors have properties quite dissimilar from those of chalcogenide glasses. They exhibit, for instance, a high density of free spins, on... 

This book is divided into two parts. Part A deals with hydrogen in amorphous semiconductors. After a review, by several speakers, of the theoretical understanding of electronic and transport properties in these systems, there follow lectures dealing with the distribution of hydrogen in external and internal surfaces and its effect on defect structure. Finally, attention is given to the formation and trapping of molecular hydrogen in voids in the host matrix. 

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