An attempt was made to clarify the controversial issue of the interaction of Cd impurities with vacancies and interstitials in Si and Ge. Two complementary ab initio methods (all electron Korringa-Kohn-Rostoker Green function method, pseudopotential plane-wave method) were applied within the framework of density-functional theory in order to search for the correct local geometry. It was found that, in both Si and Ge, the substitutional Cd-vacancy complex was unstable and relaxed into a split-vacancy complex; with Cd on the bond-center site. This complex had a very small EFG, permitting a unique attribution of the small measured electric-field gradients of 54MHz in Ge and 28MHz in Si. For the Cd-self-interstitial complex, a highly symmetrical split configuration with large electric-field gradients was obtained; in reasonable agreement with experiment.

Cd-Vacancy and Cd-Interstitial Complexes in Si and Ge. H.Höhler, N.Atodiresei, K.Schroeder, R.Zeller, P.H.Dederichs: Physical Review B, 2004, 70[15], 155313 (8pp)