In this letter, we have investigated the physical and electrical characteristics of atomic layer deposition of HfO2 on GaAs substrates. X-ray photoelectron spectroscopy (XPS) analysis revealed no significant reduction of arsenic oxides upon deposition of HfO2 on GaAs using tetrakis(dimethyl-amino)hafnium [Hf(NMe2)4] as the metallic precursor. However, XPS confirmed the absence of arsenic oxides at the interface of HfO2 and sulfide-treated GaAs. High-resolution transmission electron microcopy analysis verified a smooth interface between HfO2 and sulfur-passivated GaAs. In addition, frequency dispersion behavior of capacitors on p-type GaAs substrates was remarkably improved by employing an appropriate surface chemical treatment.