ELECTRON TRAPPING AND IMPURITY SEGREGATION WITHOUT DEFECTS - AB-INITIO STUDY OF PERFECTLY REBONDED GRAIN-BOUNDARIES

We present the results of an extensive ab initio study of the SIGMA=5 tilt [310] grain boundary in germanium. We find that the boundary reli ably reconstructs to the tetrahedrally bonded network observed in high -resolution electron microscopy experiments without the proliferation of false local minima observed in similar twist boundaries. The reduce d density of bonds crossing the grain-boundary plane leads us to conje cture that the boundary may be a preferred fracture interface. Though there are no dangling bonds or miscoordinated sites in the reconstruct ion, the boundary presents electron-trap states just below the conduct ion band. Further, we show that lattice relaxation effects are irrelev ant to the segregation of impurities to tetrahedrally reconstructed de fects and that the interfacial electron-trap states give rise to an el ectronic frustration mechanism that selectively drives the segregation of only n-type dopants to the boundary.