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 Σ=5 tilt [310] grain boundary in germanium. We find that the boundary reliably 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 reduced density of bonds crossing the grain-boundary plane leads us to conjecture that the boundary may be a preferred fracture interface. Though there are no dangling bonds or miscoordinated sites in the reconstruction, the boundary presents electron-trap states just below the conduction band. Further, we show that lattice relaxation effects are irrelevant to the segregation of impurities to tetrahedrally reconstructed defects and that the interfacial electron-trap states give rise to an electronic frustration mechanism that selectively drives the segregation of only n-type dopants to the boundary.