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 ∼107/cm2 in the GaAs layer. In addition, it was observed that threading dislocations were mainly confined within the first ∼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)2S, 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.