The electrical quality of the GeO 2/Ge interface, prior to and after Gd 2 O 3 deposition, has been investigated as a function of the oxidizer (atomic O, O 2, O 3) used for the GeO 2 based passivation of the Ge surface. In particular, the density of interface traps depends on the details of the Ge oxidation process and on the reactivity of the GeO 2 passivation layer with the overlying Gd 2 O 3 film. Complementary compositional depth profiling analysis shows that the oxygen content in the interfacial layer varies as a function of the type of oxidizer and plays a key role in dictating the interface chemistry and the electrical features of the MOS structures.