Festk6rperprobleme XXI (1981) Structure and Electronic States in a-Si: H John D. Joannopoulos, Douglas C. Allan Department of Physics, Massachusetts Institute of Technology, Cambridge,Mass. 02139, USA Summary: An overview of recent theoretical accomplishments in the study of the electronic structure of hydrogenatedamorphous silicon is presented. Special emphasis is placed on attempts to elucidate the relationships among bonding, structure and electronic states. This includes the search for an understandingof the nature of the properties of a variety of silicon- hydrogen bonding conformations and bonding coordination defects. Particular attention is paid to the interpretation ofresultsin terms of the effects of buik-tike and surface-like environ- ments, the effects of quantitativeand topological disorder, and the formation of band-like and gap-like states. 1 Introduction Since many years ago, it was thought that it was a basic and fundamental property of an amorphous semiconductor that it could not be doped. Mott had explained [I] in a simple and elegant way that the lack of periodic steric constraints could allow the valency of any impurity t o be satisfied. It was therefore a great surprise when in 1975 W. Spear and P. LeComber [2, 3] managed to dope amorphous silicon both n-type and p-type. One very interesting property of this material was that it was prepared by glow discharge silane (SiI-I4) decomposition and consequently contained a large amount of hydrogen. It has been of great importance, therefore, t o understand the influence of the hydrogen on the doping process and consequently understand the structural aspects of the material. Is the hydrogen actually allowing the doping of an amorphous network? Do impurities normally occupy substitutional sites in an amorphous network and the hydrogen aids in the doping process by removing gap states? If this is the case, why doesn't the hydrogen also aids in satisfying the valency of the impuri- ties? Could it be that the presence ofhydrogen helps to createsmall microcrystal- line regions which can be doped normally? It is still not certain what the answers to these questions are; nevertheless, these questions and the exciting possibilities asso- ciated with doping amorphous semiconductors [ 2 - 1 3 ] and the construction of devices and solar cells [14-30], have inspired alarge number of experimental in- vestigations [31-143] of the structural [31-49], electronic [50-68], vibrational [69-821, optical [83-93], luminescent [94-112], magnetic [110-121 ], and trans- port and photoconductive [ 122-144] properties of this material. Recently amode- 167