{H,B}, {H,C}, and {H,Si} pairs in silicon and germanium

The interactions between interstitial H and substitutional B, C, and Si in crystalline silicon and germanium are studied in molecular clusters at the ab initio Hartree-Fock level with large basis sets. The energetics, electronic structures, and relative stabilities of these pairs are determined. Our results show that the {H,B} pair is virtually identical in Si and Ge. Substitutional C is a strong trap for interstitial H in the two hosts. There are two trigonal, nearly energetically degenerate, and electrically active configurations of the electrically neutral {H,C} pair in each host. Substitutional Si in Ge is a weak trap for H. There are four configurations with trigonal symmetry of the {H,Si} pair in Ge within some 0.25 eV of each other. Two of these are electrically active with H covalently bound to the Si or to a Ge atom. The other two have H near tetrahedral interstitial sites. The {H,Si} pairs are much less stable than the {H,C} ones, and the reasons for this difference are discussed.