Electronic levels of interstitial carbon and carbon–oxygen centers in SiGe alloys

Density-functional supercell calculations are employed to follow the location of the donor levels arising from interstitial carbon and interstitial carbon–oxygen complexes in SiGe alloys. We show that these complexes interact weakly with neighboring Ge atoms, and that energetics rules out the existence of defect-Ge complexes involving direct GeC or GeO bonds. The CiOi defect is predicted to produce a hole trap that varies as E(0/+) − Ev = 0.41 − 0.76x eV, implying its disappearance for Ge fractions x greater than 0.5.