This paper proposes a chemical pre-treatment process to reduce sulfate prior to atomic layer deposition of Al2O3 gate dielectrics on n-doped In0.7Ga0.3As layers to form metal oxide semiconductor capacitors (MOSCAPs). Cleaning with methanol was done after hydrogen chloride etching and ammonium sulfide passivation. X-ray photoelectron spectroscopy performed on the sample with methanol cleaning indicated that its native oxide regrowth was effectively prevented on the sulfur-passivated In0.7Ga0.3As surface under air exposure. By analysis using atomic force microscopy of the sample cleaned using methanol, no further increase of surface roughness was observed after 5 months of air exposure. In addition, a mid-gap interface defect density (Dit) of 1.3 × 1012 cm−2 eV−1 was obtained from this sample. After forming gas annealing (FGA), the Dit was improved significantly, to a value below 1012 cm−2 eV−1. The obtained data indicate that the combination of chemical pre-treatment and FGA is advantageous to passivating the trap states on In0.7Ga0.3As MOSCAPs.