Surface-induced charge at a Ge (100) dimer surface and its interaction with vacancies and self-interstitials
The behavior of intrinsic point defects near the Ge (100) surface was investigated by using ab initio calculations. A thin plate model with clean Ge surfaces including a dimer structure on both sides was examined. A decrease of the formation energies of both types of intrinsic point defects near the surface is obtained similar as was reported before for Si. An important difference, however, is that the impact of the electric charges at the Si surface vanishes around the fifth layer, while for Ge, the effect of negative charges near the surface remains and positive charges are observed even deeper than the fifteenth layer from the surface. In bulk Ge, negatively charged vacancies are reported to be stable. Opposite to this, the neighbouring atoms around a vacancy near the Ge surface have a strong positive charge, compared with the case of Si. Taking these facts into consideration, the difference of the charge state of a vacancy in the bulk and that near the surface can be explained by band bending due to the surface-induced charge. A self-interstitial in bulk Ge most likely has a positive charge state (+2 or +1). It is also shown that a self-interstitial except for the position in the second layer away from the surface is positively charged. This charging can lead to a reflection of self-interstitials by the Ge surface because of the repulsive force between the positive charges of surface atoms and those of self-interstitials. Such interstitial reflection was suggested to explain experimental diffusion observations when self-interstitials are generated inside a bulk crystal by knocking-on Ge atoms in an implantation or irradiation process during a thermal anneal.