In this article, we have studied fabrication and characterization of GaAs metal-oxide-semiconductor (MOS) capacitors with Al2O3 gate dielectric. 300 nm thick GaAs layers were grown epitaxially on Ge/Si1-xGex/Si substrates. Cross-sectional transmission electron microscopy (TEM) confirmed a threading dislocation density of similar to 10(7)/cm(2) in the GaAs layer. In addition, it was observed that threading dislocations were mainly confined within the first similar to 50 nm of the GaAs layer, adjacent to the Ge film. Interfacial self-cleaning attribute of GaAs upon atomic layer deposition of Al2O3 was confirmed by x-ray photoelectron spectroscopy (XPS) analysis. However, the Al2O3/GaAs interface properties were remarkably improved by GaAs native removal in dilute HF (1%) followed by sulfur treatment in (NH4)(2)S, substantiated by probing electrical characteristics of the MOS capacitors and cross-sectional TEM analysis. Thermodynamic properties of Al2O3/sulfide-treated GaAs interface was also studied by monitoring the C-V characteristics of GaAs MOS capacitors implying excellent thermal stability of the Al2O3/GaAs interface. (C) 2008 American Vacuum Society.