Dopant incorporation, Fermi-level movement, and band offset at the Ge/GaAs(001) interface

We have used high-energy Auger electron diffraction and x-ray photoelectron diffraction to obtain a direct structural determination of $n$-type dopant atoms coevaporated with ultrathin Ge epilayers on GaAs(001). Angular distributions of photoelectron intensity from the isovalent dopant atoms P and Sb establish that P atoms uniformly incorporate into the epilayer and occupy lattice sites, whereas Sb atoms surface segregate. These structural results are strongly correlated with Fermi-level movement at the interface. The Fermi-level energy within the band gap is critically dependent on overlayer structure. However, the valence-band offset remains constant at 0.60±0.05 eV, independent of dopant kind, quantity, and spatial distribution in the epilayer. Significant Schottky-barrier-height reduction (0.4-0.5 eV) occurs only when dopant atoms occupy Ge lattice sites.