Sulfur point defects in crystalline and amorphous silicon

We present first-principles calculations for the behavior of sulfur point defects in crystalline and amorphous silicon structures. By introducing the sulfur point defects at various representative positions in the samples, including substitutional and interstitial sites in the crystal and fourfold coordinated or miscoordinated sites (dangling bond and floating bond sites) in the amorphous, we analyze the energetics in detail and determine the most stable structures. Two important conclusions we draw are: (a) in crystalline Si, the S defects form pairs in which the two S atoms are energetically bound but not covalently bonded; (b) in amorphous Si, they preferentially occupy threefold coordinated sites, even when the starting configuration has higher coordination (four- or fivefold). The implications of these results for the electronic structure of sulfur-doped Si samples are also analyzed in the context of the present calculations.