In-situ molecular beam epitaxy (MBE) Y2O3 films 1-2 nm thick were epitaxially grown on GaAs(001)- 4 x 6 reconstructed surfaces. Despite a large lattice mismatch, the hetero-structure exhibits outstanding thermal stability to 900 degrees C with excellent capacitance-voltage (C-V) characteristics. Low interfacial trap densities (D-it's) of (3-5) x 10(11) eV (1) cm (2) were obtained using the conductance method (G-V) without discernible peaks at the mid-gap. The frequency dispersion of the measured C-Vs of the Y2O3/GaAs(001) is similar to 4.6% for p-GaAs and similar to 12.4% for n-GaAs. In contrast, the atomic layer deposited Al2O3 on GaAs(001) shows large D-it with a peak at the mid-gap, large C-V frequency dispersion, and low thermal stability at temperatures higher than 600 degrees C. Synchrotron radiation photoemission results show intactness of the interfacial structure in the MBE-Y2O3/GaAs, while removal of the surface As atoms is found in the ALD-Al2O3/GaAs system. (C) 2017 Elsevier B.V. All rights reserved.