An equivalent oxide thickness about 1 nm for Ga2O3(Gd2O3) (GGO) on In0.2Ga0.8As has been achieved by employing a thin in situ deposited 3 nm thick Al2O3 protection layer. The dual gate oxide stacks of the Al2O3/GGO (33, 20, 10, 8.5, and 4.5 nm)/In0.2Ga0.8As/GaAs metal-oxide-semiconductor (MOS) capacitors remain amorphous after rapid thermal annealing up to 800-850 degrees C, accompanied with atomically sharp smooth oxide/semiconductor interfaces. Well behaved capacitance-voltage (C-V) curves of the MOS diodes have shown sharp transition from depletion to accumulation with small flatband voltage (1.1 V for Au metal gate and 0.1 V for Al), and weak frequency dispersion (1.5%-5.4%) between 10 and 500 kHz at accumulation capacitance. Low leakage current densities [3.1x10(-5) and 2.5x10(-9) A/cm(2) at V=V-fb+1 V for Al2O3(3 nm)/GGO(4.5 and 8.5 nm)], a high dielectric constant around 14-16 of GGO for all tested thicknesses, and a low interfacial density of states (D-it) in the low 10(11) cm(-2) eV(-1) have also been accomplished. (C) 2008 American Institute of Physics.