  First principles electronic structure calculations are carried out to
investigate the band alignments of tensile strained (001) Ge interfaced with
(001) In$_{x}$Al$_{1-x}$As. The sensitivities of band offsets to interfacial
structure, interfacial stoichiometry, and substrate stoichiometry, are
investigated. Large qualitative variations of the valence and conduction band
offsets are observed, including changes of the band offset type, indicating the
importance of local structural variations of the interface for band offsets in
real samples. Our results explain recent measurements of band offsets derived
from XPS core level spectra in terms of As atoms penetrating through the first
few monolayers of the Ge film. Analogous studies are carried out for the
diffusion of other species across the interface, and in general the band
offsets vary approximately linearly with diffusion depth relative to the values
for pristine "sharp" interfaces, where the sign of the linear variation depends
on the diffusing species. This large sensitivity of the band alignments to
interface details indicates potential routes to chemically control the band
offset of this group IV/III-V interface by tuning the stoichiometry of the
substrate surface that the thin film is grown on.
