The electronic structures of vacancies in amorphous silicon are studied by a tight-binding total-energy-minimization scheme. The most salient feature in the electronic structure is that deep gap states originating from the vacancies disappear in amorphous silicon while sustaining the vacant volume in most cases, unlike the vacancies in crystalline silicon. The origin of the disappearance of the deep gap states is explained by the relaxation of the neighboring atoms near the vacancy to enhance the strong ${\mathit{p}}_{\mathrm{\pi }}$ bonding character which gives large bonding-antibonding splitting, and ${\mathit{sp}}^2$+p bonding character where p states mostly contribute to the conduction bands.

• Received 9 November 1994

DOI:https://doi.org/10.1103/PhysRevB.51.5429