FIRST-PRINCIPLES STUDY OF ELECTRIC-FIELD GRADIENTS AT THE CD SITE FORNEUTRAL HYDROGEN-CADMIUM COMPLEXES IN CRYSTALLINE SILICON

In the present work, a first-principles investigation of electric-fiel d gradients (EFG's) at the Cd site for possible neutral complexes of C d-H in crystalline silicon has been performed. This was motivated by e xperimental results observed by perturbed angular correlation spectros copy, and with the aim of trying to establish the validity of the conf igurational models proposed in the literature. We use the full-potenti al linear-muffin-tin-orbital method, within the local-density function al theory, and the supercell approach. We have allowed local relaxatio ns of the Cd and Si nearest and next-nearest neighbors for different p ositions of H in the high and low electron-density regions. We have fo und that H in an intrabond site is unstable (saddle point), and that a bridgelike configuration is the energetically favorable one, where th e Cd atom and the nearest silicon suffer relaxations along the [111] d irection. The antibond positions of low electronic density regions wer e also studied, and have been found to be also local minima. The calcu lated EFG's in these configurations that locally minimize the total en ergy of the supercell (H at bridgelike and H in antibond sites to Cd a nd Si-1) give quadrupolar coupling constants which are in agreement wi th the experimental results.