This paper proposes an effective pretreatment process that combines wet ammonium sulfide ((NH4)2S) dipping and hydrazine (N2H4) vapor treatment before high dielectric constant (κ, kappa) deposition to reduce native oxide and elemental arsenic (As) before atomic layer deposition of ammonium oxide (Al2O3) gate dielectrics on n-doped indium gallium arsenide (In0.7Ga0.3As) layers to form metal oxide semiconductor capacitors (MOSCAPs). X-ray photoelectron spectroscopy (XPS) analysis confirms that sulfuration of In0.7Ga0.3As surface by (NH4)2S solution dipping can effectively reduce indium-oxygen (In-O), gallium-oxygen (Ga-O), arsenic-oxygen (As-O) bonds while elemental As still exist at the high-κ/In0.7Ga0.3As interface. Furthermore, N2H4 treatment on sulfurated In0.7Ga0.3As can effectively suppress the native oxides and elemental As. Accordingly, the obtained data indicate that the combination of chemical sulfur pretreatment and N2H4 treatment is advantageous to passivate the trap states on In0.7Ga0.3As metal-oxide-semiconductor capacitors (MOSCAPs). Moreover, forming gas annealing (FGA) process could further improve the capacitance-voltage (C-V) characteristics and the interface trap density (Dit) of In0.7Ga0.3As MOSCAP could be reduced significantly, achieving a value of 1.2 × 1012 cm-2 eV-1.