The (NH4)2S treatment can reduce native oxides and passivate GaAs. Atomic-layer-deposited (ALD-Al2O3) can further remove the residue native oxides by self-cleaning. For the first time, native oxides can be sufficiently removed by the combination of (NH4)2S treatment and ALD-Al2O3. Stacked with high dielectric constant TiO2 prepared by atomic-layer deposition on a Al2O3/(NH4)2S-treated GaAs MOS capacitor, the leakage current densities can reach 1.9 × 10−8 and 3.1 × 10−6 A/cm2 at ±2 MV/cm. The dielectric constant is 25 and the interface state density is about 2.4 × 1011 cm−2 eV−1.