Interfaces Between Crystalline and Amorphous Tetrahedrally Coordinated Semiconductors

Abstract

By carrying out a series of geometrical operations, we transform a superlattice composed of alternating slabs of ordered and random close-packed spheres into a superlattice composed of alternating crystalline and amorphous tetrahedrally coordinated networks. In this way we generate an atomic-scale model of the interface between the [001] face of crystalline Si and amorphous Si. The construction is fully automatic, being determined by computer algorithms which take account of physical and chemical constraints such as tetrahedral coordination and most probable bond lengths and angles. The first stage of the construction leads to a preliminary structural model having interfacial transition zones extending over a few atomic layers. The density of dangling bonds increases rapidly as one moves across the interfacial regions from the crystalline toward the amorphous sides. Calculations of the local electronic density of states indicate the presence of states in the thermal gap at the interface and in the amorphous region. These gap states arise primarily from dangling bonds, and to a lesser extent from structural disorder, i.e., statistical variations in bond geometries. Because of the conventions used to enumerate bonds, the preliminary model appears to have an excessive number of dangling bonds. Most of these dangling bonds are eliminated in the second stage, as neighboring pairs of unsaturated atoms are linked together, additional atoms are introduced into accommodating voids, and the lattice is concurrently relaxed.