A theoretical study of defects in amorphous arsenic

Abstract

Abstract Clusters of about 55 atoms have been built to model various defects which may be present in amorphous arsenic (a-As). These clusters have been relaxed and their electronic properties calculated by two complementary methods—by a self-consistent linear combination of atomic orbitals (LCAO) method, and by attaching Bethe lattices to the cluster surfaces and then calculating the electronic density of states (DOS) by the recursion method. Both methods are initially applied to a perfect 56-atom cluster to show that they are valid for calculating the DOS of a-As. They are then used to study the electronic states associated with isolated twofold and fourfold coordinated atoms, neighbouring twofold and fourfold coordinated atoms (‘intimate valence alternation pairs') and the vacancy. We have in addition examined the stability of the vacancy and find it is probably unstable. We therefore conclude that the optically induced E.S.R. signal observed in a-As is consistent with the presence of valence alternation pairs while the thermally generated E.S.R. signal is probably due to the presence of neutral twofold coordinated centres.