Ab initio calculations of vacancies in SixGe1-x

Ab initio calculations are used to investigate the structural and electronic properties of vacancies in SixGe1-x. The (+ +), (+), (0), and (-) charge states are studied and the substitutional disorder of the alloy is considered explicitly. We found a linear relationship between the effective-U for the system formed by the ( + +), ( +), and (0) charge states and the number of Si atoms in the first neighborhood of a vacancy (N-Si). The effective-U is positive when N-Si is zero, and it is negative when N-Si is 2 and 4. In all cases, the absolute value of the effective-U in the alloy is significantly smaller than its value for pure Si and pure Ge. (C) 2002 American Institute of Physics.