Structure, chemistry, and Fermi-level movement at interfaces of epitaxial NiAl and GaAs(001)

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Abstract
We have used x-ray photoemission and low-energy electron diffraction (LEED) to investigate the chemical, structural, and electronic properties of ultrathin epitaxial NiAl overlayers on GaAs(001) grown at ∼250 °C. Approximately 30%–35% of the incident Ni in the first two monolayers (ML) of NiAl diffuses into the substrate. At the same time, most of the first ML of Al binds to the surface arsenic layer to form AlAs, leaving the LEED pattern essentially identical to that exhibited by the substrate. As coverage proceeds, NiAl nucleates on the interfacial AlAs layer and approaches a nearly 1:1 stoichiometry by a coverage of∼10 ML. Interestingly, there is no Al–Ga cation exchange at low coverages. Rather, Ga disruption at the substrate begins at 4 ML, and As disruption is first seen at 10 ML. The Schottky barrier height is within experimental error of the substrate value (0.75±0.05 eV) for coverages up to 6 ML, at which point it begins to increase, reaching a final value of 0.9–1.0 eV by the time the NiAl crystal structure is fully developed in the epilayer. The exact final value of the barrier height depends on the structural quality and doping level in the substrate. NiAl epilayers on GaAs are rather robust with respect to thermal treatments. Although annealing 30 ML films at temperatures up to 570 °C causes some Ni indiffusion, the basic NiAl crystal structure remains in tact and the barrier height stays constant at ∼1 eV. The observed barrier height is consistent with the expectations of Mönch’s modified metal-induced gap states model which takes into account charge transfer across the interface.

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