Schottky barrier height and thermal stability of the NiAl/n-Ge/GaAs(001) interface

Abstract

We have successfully grown epitaxial NiAl on single-crystal Ge interlayers, which were in turn grown on GaAs(001) substrates. The goal has been to use the single crystallinity of the NiAl overlayer with its characteristically low density of grain boundaries as a diffusion barrier to prevent outdiffusion of n-type dopant atoms from the Ge interlayer. Heavily As-doped Ge has proven effective in lowering the Schottky barrier height of GaAs substrates to 0.2–0.5 eV. The epitaxial overlayers and interfaces have been characterized in situ by means of x-ray photoemission, x-ray photoelectron diffraction, and low-energy electron diffraction. We have found that even at growth temperatures as low as 210 °C, outdiffusion of As from the Ge layer into the growing NiAl epifilm occurs, causing the barrier height to increase from ∼0.4 eV to ∼0.6 eV. Furthermore, limited Al atom indiffusion and Al–Ga cation exchange occur during the initial phase of NiAl growth, as does limited disruption of the Ge epilayer surface. Band alignment at the Ge/GaAs heterojunction is also perturbed as a result of the cross-diffusion that occurs, as evidenced by an ∼0.15 eV increase in the valence band offset. Annealing at 350 °C causes additional As loss from the Ge interlayer, a further barrier height increase, additional penetration of Al atoms to the Ge/GaAs interface, and enhanced Al–Ga cation exchange. Thus, it appears that such a system is of little value as a high-thermal-stability, low-barrier-height metal contact to GaAs. Interface degradation is dominated by interdiffusion and chemistry.