Ab initio modeling of interstitial oxygen in crystalline SiGe alloys

We have investigated the structural and vibrational properties of interstitial oxygen in Si1−xGex alloys by using a pseudopotential density-functional code and 128 atom supercells. First, to test the method, well-established data for interstitial oxygen in Si and Ge were reproduced. Secondly, to model the alloys, Si atoms were randomly replaced by Ge atoms in an otherwise perfect 128 Si atom supercell. Atomic and volumetric degrees of freedom were allowed to relax. We found from energetics that interstitial oxygen prefers to bond with silicon atoms for a large range of Ge concentrations (x = 25%, 50%, 75%). The defect model accounts well for the observed red-shift of the asymmetric stretch mode band when increasing the Ge concentration. On the other hand, highly energetic Ge–Oi–Si and Ge–Oi–Ge structures produce frequencies not seen experimentally. Therefore we conclude that oxygen only forms Si–Oi–Si units in crystalline SiGe alloys for a large range of Ge compositions.