Point defects in pure amorphous silicon and their role in structural relaxation: A tight-binding molecular-dynamics study

Structural relaxation in pure amorphous silicon (a-Si) produced by ion implantation has been attributed to the annihilation of point defects (vacancies and interstitials) introduced during the amorphization process. We have studied this problem by using tight-binding molecular-dynamics simulations. We find that structural defects can, indeed, be identified in a-Si - they manifest themselves through a strong correlation between the charge and the volume of nearby atoms. The relaxation of these defects proceeds via the recombination of the dangling bonds. This results in an increase in the coordination number at constant density; the relaxation of a-Si, therefore, results from local, rather than global, structural changes, in full agreement with the high-precision x-ray diffraction experiments of Laaziri [Phys. Rev. Lett. 82, 3460 (1999)].