We have studied the structural relaxiation of vacancies in amorphous silicon (a-Si) using a tight-binding molecular-dynamics method. The most significant difference between vacancies in a-Si and those in crystalline silicon (c-Si) is that the deep gap states do not show up in a-Si and those in crystalline silicon (c-Si) is that the deep gap states do not show up in a-Si.This difference is explained through the unusual behaivor of the structural relaxation near the vacancies in a-Si,which enhances the sp~2 + p bonding near the band edges.We have also observed that the vacancies do not migrate below 450 K although some of them can still be annihilated,particularly at high defect density due to large structural relaxation.
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