In the present work we report on the structural and electrical properties of metal oxide semiconductor (MOS) devices with HfO2/Dy2O3 gate stack dielectrics, deposited by molecular beam deposition on p-type germanium (Ge) substrates. Structural characterization by means of high-resolution Transmission Electron Microscopy (TEM) and X-ray diffraction measurements demonstrate the nanocrystalline nature of the films. Moreover, the interpretation of the X-ray reflectivity measurements reveals the spontaneous growth of an ultrathin germanium oxide interfacial layer which was also confirmed by TEM. Subsequent electrical characterization measurements on Pt/HfO2/Dy2O3/p-Ge MOS diodes show that a combination of a thin Dy2O3 buffer layer with a thicker HfO2 on top can give very good results, such as equivalent oxide thickness values as low as 1.9 nm, low density of interfacial defects (2-5 x 10(12) eV(-1) cm(-2)) and leakage currents with typical current density values around 15 nA/cm(2) at V-g=V-FB-1V. (C) 2009 Elsevier B.V. All rights reserved.