Electronic structure of defects in amorphous arsenic

The cluster-Bethe-lattice method, self-consistent pseudopotentials, and tight-binding Hamiltonians are used to study the electronic structure of various possible defects in amorphous As. The electronic densities of states of isolated twofold coordinated As atoms, fourfold coordinated As atoms, "intimate valence alternation pairs," and vacancies are calculated. The origin, character, and localization of the resulting gap states are discussed. It is found that isolated fourfold coordinated As atoms give rise to states in the gap that are nondegenerate, fairly localized, and predominantly $p$-like in character. Moreover, our investigation indicates that the character and localization of the gap states arising from the "valence alternation pair" are consistent with the properties of optically induced paramagnetic states observed in amorphous As, while the electronic gap states of the vacancy defect may be consistent with the recently observed thermally generated paramagnetic states in this material.