We show that using a density-functional supercell method we are able to follow the location of defect gap levels in SiGe alloys for different alloying compositions. The method is tested for several properties of the alloys, with special emphasis in the local bond-length relaxations. Our results indicate a predominant Pauling character for the alloys, with a topological rigidity parameter a** lying between 0.7 and 0.8. A comparative study between the electrical properties of the interstitial carbon ͑C i ͒ and carbon-oxygen ͑C i O i ͒ centers in the alloys, shows that these complexes interact weakly with Ge atoms. The C i O i defect is predicted to produce a hole trap that varies as ⌬E͑0/ +͒ = 0.41− 0.76x eV, implying its disappearance for Ge fractions x greater than ϳ0.5. PACS number(s): 61.66.Dk, 61.72.Ji, 61.80. Az FIG. 1. Structures of the C i (left) and C i O i (right) complexes in Si and SiGe alloys. Si, C, and O atoms are shown in white, gray, and black, respectively. Si atoms labeled with letters were replaced by Ge atoms, forming C i -Ge ␣ ͑a ഛ ␣ ഛ e͒ and C i O i -Ge ␣ ͑a ഛ ␣ ഛ j͒ complexes, respectively.PHYSICAL REVIEW B 70, 085201 (2004)
The local vibrational modes arising from single interstitial hydrogen centres in Si, Si-rich SiGe, Ge-rich SiGe, and Ge crystals are modelled by an ab initio supercell method. The stress response of the 1998 and 1794 cm −1 bands that appear in proton-implanted Si and Ge samples is well reproduced, further confirming their assignment to bond-centred H + defects. It is shown that H − in Ge is anti-bonded to a Ge atom, and is likely to be considerably less mobile than in Si. Although H + is not trapped by the minority species in both Si-rich and Ge-rich alloys, we find that H − can be stabilized by forming anti-bonded H-Si structures.
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