The atomic oxygen-assisted molecular beam deposition of Gd2O3 films on Ge (001) substrates has been performed at various growth temperatures. The compositional aspects, the interface details and the surface structure have been investigated by in situ X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectroscopy and in situ atomic force microscopy, and ex situ. The interface layer of GeO2 has been subsequently fabricated by means of atomic oxygen exposure in order to passivate the high-k/Ge interface. The electrical characterization on the final Gd2O3/GeO2/Ge structure has been reported. The electrical characterization on the Al gate/Gd2O3/GeO2/Ge structure exhibits a MOS behavior, indicating the beneficial effect of GeO2 passivation.