First Principles Simulations of ε-Ge/In_xAl_1-xAs Interfaces: Band Alignments and Interface Structure

Tensile strained Ge (ε-Ge) grown on III-V substrates is currently the focus of substantial research efforts due to the range of potential technological applications, from high performance, low power tunnel FETs, to on-chip optical interconnects based on III-V/ε-Ge/III-V quantum well light emitters. In this work, we investigate the sensitivity of quasiparticle band offsets to interface structure for the ε-Ge/(001)In_xAl_1-xAs interface. Utilizing first principles methodologies for atomistic simulations, namely density functional theory (DFT) combined with many body perturbation theory within the GW approximation, valence and conduction band alignments are calculated for a range of interface configurations. Considering the combined effects of cation stoichiometry of the substrate, interface stoichiometry, diffusion of mixed layers across the interface, and the strain induced movement of quasiparticle valleys in Ge, band alignments recently measured from core level XPS spectra for epitaxial thin films of ε-Ge on (001)In_0.25Al_0.75As substrates are explained in terms of the of diffusion of substrate species into the Ge film.