Schottky barrier height control at epitaxial NiAl/GaAs(001) interfaces by means of variable band gap interlayers

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This article discusses recent developments in the use of semiconducting interlayers to tailor the Schottky barrier height (SBH) at a metal/GaAs interface. We have used AlAs, GaSexAs1−x, n++-Ge, and n++-Si interlayers between n- and p-GaAs(001) substrates and epitaxial NiAl metallic overlayers to see if a variable SBH is achievable. The interlayers were grown by molecular beam epitaxy and metal organic chemical vapor deposition, while the NiAl overlayers were grown by molecular beam epitaxy. X-ray photoelectron spectroscopy was used to follow the chemistry of interface formation, as well as band offsets and SBH values. Low-energy electron diffraction and x-ray photoelectron diffraction were used to determine structural properties of both the interlayer and overlayer. It was found that thermally stable SBH values ranging from ∼0.5 eV for n++-Si interlayers to ∼1.2 eV for AlAs interlayers are readily achievable on n-GaAs. Different mechanisms were found to be responsible for these two extrema in the SBH. For instance, charge transfer from the n++-Si interlayer to the depletion region of the substrate was found to compensate the space-charge region, thereby lowering the band bending and the resulting SBH. In contrast, Fermi-level pinning rather deep in the gap, combined with a substantial conduction band offset at the heterojunction, created a high SBH when AlAs interlayers were used. n++-Ge interlayers, while causing a similar SBH reduction as n++-Si were not found to be thermally stable. Disruption of the anion sublattice and Fermi-level pinning deep in the gap nullified the band flattening effect of the GaSexAs1−x interlayer. Using these fundamental interface science results, we propose an application to complementary digital GaAs circuit design that may significantly reduce gate leakage relative to what can be achieved with conventional metallization schemes.