Polar heterojunction interfaces

A study of heterojunction interface geometry based on our measured differences in 3d core-state binding energies for germanium and gallium at Ge-GaAs heterojunctions of different crystallographic orientations is reported. For the interfaces which have been studied, i. e. , (110), (100) Ga, (100) As, (111) Ga, and (111) As, orientation-dependent variations in dipole contributions to valence-band discontinuities of about 0. 2 eV have been observed. From electrostatic considerations we deduce the simplest interface geometries consistent with the facts that the differences are small and no large charge accumulations can occur at the junction. An abrupt planar junction is allowed for the (110) interface, but the polar interfaces require at least two transition planes of atoms with compositions which are deduced from the two conditions above. The electrostatic calculations were based upon the differences in nuclear charge and are unaffected by the resulting polarization of the bonds if that polarization is described in an "electronegativity" approximation. In this approximation there would in fact be no dipole shift for the ideal geometries proposed. An improved treatment of the bond polarization based upon the bond-orbital model gives residual dipole shifts somewhat smaller than those observed, and in poor agreement with our measurements. Inclusion of lattice-distortion effects at the interface also fails to account for the observed dipole shifts. We conclude that the experimentally prepared junctions must contain deviations from the ideal atom arrangements. The number of these deviations required to account for the observed shifts is on the order of one for every fifteen interface atoms.