Germanium is studied as a replacement candidate for silicon as channel material to enhance transistor performance. Interface states that cannot be passivated by standard forming gas anneals have been widely reported. In this work we demonstrate that thermal processing of a germanium/HfO2 stack leads to the formation of a hafnium germanate (HfGeOx) that appears to be linked to these interface states. Our results suggest that interactions between HfO2 and germanium can only be avoided by passivating the germanium with a capping layer. As an example, we discuss the use of a thin epitaxially grown Si layer that leads to greatly improved CV-characteristics. (c) 2006 Published by Elsevier B.V.