DEFECTS, TIGHT-BINDING, AND 1ST-PRINCIPLES MOLECULAR-DYNAMICS SIMULATIONS ON A-SI

We use ab initio pseudopotential local-density-approximation methods to create and study 63- and 216-atom models of a-Si. We examine the structure of defects encountered in these cells and pay particular attention to localization induced by the defects. In particular, we find that geometric defects and localized states in the gap are not identical and in some cases are grossly different. In addition, we compare ab initio results to several tight-binding models and find that tight-binding descriptions are often highly arbitrary and inadequate.