The authors present experimental evidence on the impact of three different chemical surface treatments on the interface between the GaAs substrate and the aluminum oxide dielectric layer used in the fabrication of metal-oxide-semiconductor capacitors. The three different chemical surface treatments studied prior to atomic layer deposition (ALD) of the dielectric layer include (a) GaAs native oxide removal in a dilute HF solution only, (b) HF etch followed by a NH4OH treatment, and (c) HF etch followed by a (NH4)(2)S treatment. Moreover, interfacial self-cleaning of nontreated GaAs wafers upon ALD of aluminum oxide using trimethyl aluminum precursor was examined. Transmission electron microscopy, electron energy loss spectroscopy (EELS) and capacitance-voltage (C-V) data showed slight differences among the nontreated, HF-only, and NH4OH treated samples. However the (NH4)(2)S treated sample showed improved capacitance-voltage characteristics as well as an improved aluminum oxide/GaAs interface compared to the other three samples. Additionally, the characteristic oxygen K EELS peak suggests the presence of a thin additional layer close to the center of the high-kappa layer containing oxygen, tantalum, and aluminum, as a consequence of probable plasma damage to the high-kappa layer during the TaN metal gate deposition.