Structure, chemistry, and band bending at the epitaxial NiAl/p-GaAs(001) interface

Abstract

We have investigated interface formation between epitaxial, intermetallic NiAl and As-stabilized, molecular-beam epitaxy (MBE)-grown, p-GaAs(001)-c(2×8)/(2×4). The epitaxial films were prepared by MBE growth of Ni and Al onto substrates at a temperature of ∼100 °C. The growth was interrupted every few monolayers in order to obtain measurements aimed at deducing interface structure, chemistry, and band bending as a function of coverage. High-energy resolution x-ray photoemission was used to measure chemistry of interface formation and band bending, and low-energy electron diffraction and x-ray photoelectron diffraction were used to follow surface and interface structure. Epitaxial growth at 100 °C was found to be essentially laminar, with no measurable indiffusion of Ni. The first NiAl bilayer bonds with the Ni layer down (i.e., in direct contact with the substrate) and the Al layer up. Ga disruption was observed at ∼1.4 monolayers, the lowest coverage grown, while As disruption was not observed until a coverage of ∼4.2 monolayers. Reacted Ga appears to remain trapped at the interface, possibly in the form of NiAlxGa1−x, whereas liberated As segregates at the surface. The Schottky barrier height goes from a free-surface value of 0.33±0.04 to 0.47±0.04 eV after growth of ∼1.4 monolayers, and then converges to a final value of 0.33±0.04 eV by a coverage of ∼6 monolayers, which persists at higher coverages. When added to the value determined previously for MBE-grown n-GaAs(001) with the same doping density in the buffer layer, this value sums to the band gap of GaAs, within experimental error. This fact establishes that the Fermi level is indeed pinned by NiAl deep in the forbidden gap of GaAs, rather than being unpinned.