In this study we demonstrated improved electrical characteristics of Gd2O3 dielectric thin films on n-GaAs substrate by manipulating wet-chemical clean and (NH4)(2)S passivation. With X-ray photoelectron spectroscopy analysis, the HCl-cleaned GaAs surface was characterized to possess oxide species mainly in the form of AS(2)O(x) near the outmost surface and Ga2Ox with elemental arsenic close to the interface. These undesirable components could be suppressed through rinsing in NH4OH alkaline solution and then performing sulfidization at 80 degrees C, resulting in alleviating the Fermi level pinning effect on Gd2O3/GaAs capacitor performance. Higher oxide capacitance and alleviated frequency dispersion at the accumulation/depletion regimes were achieved, accompanied by negligible charge trapping (< 100 mV). Accordingly, gate leakage J(g) was lowered to ca. 10(-5) A/cm(2) at gate voltage V-g = (V-FB + 1) V, which was comparable to the recently reported performance of HfO2/GaAs structure with an ultrathin Si/Ge interfacial layer. We attributed the electrical improvements to the enhanced stabilization of high-k/sulfur-terminated GaAs interface due to abatement of native oxides and excess arsenic segregation. (C) 2007 The Electrochemical Society.