Electrical study of Schottky barriers on atomically clean GaAs(110) surfaces

Abstract

We report here a systematic study of the electrical properties of a large number of metal/n-type GaAs (Cr, Mn, Sn, Ni, Al, Pd, Cu, Ag, Au) diodes. Diodes were fabricated on cleaved GaAs(110) surfaces under ultrahigh-vacuum conditions with in situ metal deposition. Using current-voltage (I-V) and capacitance-voltage (C-V) measuring techniques, we were able to obtain very reliable and consistent determinations of the barrier height ${\ensuremath{\varphi}}_{b}$ and ideality factor n. All of the metal-semiconductor systems formed on lightly doped (5\ifmmode\times\else\texttimes\fi{}${10}^{16}$/${\mathrm{cm}}^{3}$) n-type GaAs substrates were characterized by near-unity (1.05) ideality factors. A decrease in the effective I-V barrier height, an increase in the ideality factor in forward bias and a strong voltage dependence on the thermionic emission currents in reverse bias were found for diodes formed on the more heavily doped samples.These changes are essentially metal independent, but depend strongly on the doping of the substrate. The characterization (and elimination in some cases) of peripheral leakage currents from the thermionic emission current is found to be essential in obtaining consistent results in our work and in reinterpreting some of the prior work in the literature. The dominant leakage current flows through a small area, low barrier at the periphery of the device and can be eliminated by mesa etching. The consistent and reproducible barrier-height determinations reported in this study, when combined with the results of recent surface-sensitive studies, are a particularly critical test of models of Schottky-barrier formation.The barrier heights measured from the electrical characteristics of thick-metal-film diodes were found to be essentially identical to those reported during the initial stages (submonolayer to several monolayers of metal) of Schottky-barrier formation by photoemission spectroscopy. This agreement indicates that the physical mechanism responsible for Fermi-level pinning in the thick-metal Schottky diodes is first established at submonolayer to several-monolayer coverages of adatoms and an atomic scale model is therefore necessary to account for the available experimental data. No strong correlation between the barrier heights and the work function of the metal or chemistry at the interface was found. Also, diodes formed on clean GaAs(110) surfaces were found to have essentially identical barrier heights to those formed on clean GaAs(100) surfaces and on contaminated (i.e., chemically prepared) GaAs(100) and GaAs(110) surfaces. Several currently popular models concerned with the physical mechanism responsible for the formation of the Schottky barrier are discussed, and the unified defect model is found to be most consistent with the experimental data.