In 0.2 Ga 0.8 As was effectively passivated using in situ molecular beam epitaxy deposited Al2O3/HfO2 and HfO2–Al2O3(HfAlO)/HfO2. HfO2 3 ML (monolayer) thick was epitaxially grown on InGaAs, as monitored by reflection high-energy electron diffraction. Al2O3 3 nm thick and HfAlO 4 nm thick were used to cap 3 ML epitaxial HfO2 due to their superior thermal stability up to 800 °C. Well-behaved capacitance-voltage characteristics with small capacitance dispersion between 10 and 500 kHz were obtained in both Al2O3/HfO2/InGaAs/GaAs and HfAlO/HfO2/InGaAs/GaAs, with the capacitance effective thickness values of the dielectrics being 1.46 and 1.18 nm, respectively. Particularly, HfAlO/HfO2/InGaAs/GaAs exhibited low leakage current density (2.9×10−4 A/cm2) at |VG-VFB|=1, good thermal stability up to 800 °C, and an equivalent oxide thickness of 1 nm.