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This paper demonstrates how the treatment of III-V semiconductor surface affects the number of defects and ensures the conformal growth of the high-k dielectric thin film. We present the electrical properties of an HfO₂/InGaAs-based MOS capacitor, in which growth temperatures and surface treatments of the substrate are two key factors that contribute to the uniformity and composition of the HfO₂ thin films. A remarkable asymmetry observed in capacitance versus voltage measurements was linked to the interface defects and charge redistribution, as confirmed from X-ray photoelectron spectroscopy. The GaAs substrates that were etched with only NH₄OH showed a large frequency dispersion and a higher surface roughness; however, the HfO₂ thin films grown on GaAs pre-treated with both NH₄OH etching and (NH₄)₂S passivation steps produced a desirable surface and superior electronic properties.