Chemistry of Formation, Structure, and Band Bending at Epitaxial Ni, Al, and NiAl/GaSe_xAs_1-x/GaAs(001)-(2xl) Interfaces

Abstract

We have investigated the relationship between interface chemistry, overlayer structure, and band bending when thin epitaxial overlayers of Ni, Al, and NiAI are grown by molecular beam epitaxy on anion-stabilized GaSe_xAs_1-x/GaAs(001)-(2xl) surfaces. The substrates were prepared by passivating GaAs(001) with H₂Se in a metal-organic chemical vapor deposition reactor. This treatment is now known to result in Se-As anion exchange in the top several anion layers, as well as a significant reduction in surface-state density. Chemistry of interface formation and band bending were monitored by x-ray photoemission. Overlayer structures were characterized by low-energy electron diffraction and x-ray photoelectron diffraction. Al, Ni, and NiAI overlayers assume fcc-R45°, metastable-bcc, and CsCl structures respectively. The free surfaces as prepared were nearly flat-band, exhibiting only ~0.180 meV of band bending. The growth of Al, Ni, and NiAI epitaxial films at a substrate temperature of 100°C increased the band bending, resulting in Schottky barrier heights of 0.48, 0.89, and 0.90 eV, respectively. The smaller increase in band bending when Al is depositied is correlated with a lack of disruption of the anion sublattice. Ga and As 3d core-level spectra for the growth of 25 monolayers of epitaxial NiAI on GaAs(001)-c(2X8) at 100C are virtually identical to those measured for NiAI growth under the same conditions on GaSe_x As_l-x/GaAs(001)-(2xl). Moreover, the band bending is the same, leading to a Schottky barrier height of 0.90 eV, and establishing that the rather high Schottky barrier height at the NiAI/GaAs(001) interface is a result of Fermi-level pinning deep in the forbidden gap.