High mobility semiconductors such as Ge with high-k gates may be required to enhance performance of future devices. One of the biggest challenges for the development of a Ge metal-oxide-semiconductor (MOS) technology is to find appropriate passivating materials and methodologies for the Ge/high-k interfaces. Germanium oxynitride is frequently used as a passivating interlayer in combination with HfO2 and is found to be necessary for the fabrication of functional devices. However, it is also considered to be insufficient since electrical characteristics in capacitors are non-ideal and field effect transistors underperform, probably due to a the high density of interface defects. We show that alternative passivating rate earth oxide layers prepared by molecular beam deposition produce improved electrical characteristics and a significant reduction of the density of interface states. In the case of CeO2, a thick interfacial layer is spontaneously formed containing oxidized Ge, which is considered to be the key for the observed improvements. (C) 2006 Elsevier B.V. All rights reserved.