Microscopic effects at GaAs/Ge(100) molecular-beam-epitaxy interfaces: Synchrotron-radiation photoemission study

We have measured the evolution of abrupt GaAs/Ge(100) interfaces to study the relationship between the valence-band discontinuity (Δ$E_v$) and growth properties on an atomic scale. We used soft x-ray photoemission, low-energy electron diffraction, and Auger electron spectroscopy. The interface was obtained by evaporating Ge (at <${10}^{\mathrm{-}9}$ Torr and intentionally coevaporating at an overpressure of ${\mathrm{As}}_4$) in situ onto a molecular-beam-epitaxy- (MBE-) grown GaAs sample under epitaxial growth conditions. The MBE-grown GaAs(100) provided different surface reconstructions with controllable starting anion-to-cation ratio. We present a new core-intensity-analysis method for determining thin-film growth independent of the adlayer thickness calibration. For all interfaces, a monolayer of segregated As was observed at the free Ge surface. This suggests that the As-stabilized Ge surface phase formation plays an important role in determining the interface growth. The value we obtain for Δ$E_v$ is (0.47±0.05) eV, independent of both the initial clean GaAs(100) surface properties and the evolution of the Fermi level. We propose that the driving force for MBE interface formation always yields a unique, low-energy equilibrium structure at the materials transition region, although its extent may vary by as much as a couple of atomic layers. However, the observed constancy of Δ$E_v$ suggests that the contribution of any perturbation caused by variations in the local microscopic properties of the interface (e.g., dipoles) is very small compared to the intrinsic (perhaps bulk) mechanism that determines Δ$E_v$.