Germanium and hafnium-dioxide (HfO2) stack structures' physical and electrical properties were studied based on the comparison of germanium and silicon based metal-oxide-semiconductor (MOS) capacitors' electrical properties. In germanium MOS capacitor with oxide/oxynitride interface layer, larger negative flat-band-voltage (Vfb) shift compared with silicon based MOS capacitors was observed. Secondary ion mass spectrum (SIMS) characteristics of HfO2-germanium stack structure with germanium oxynitride (GeON) interfacial layer showed germanium out diffusion into HfO2. These results indicate that the germanium out diffusion into HfO2 would be the origin of the germanium originated negative Vfb shift. Using Ta3N5 layer as a germanium passivation layer, reduced Vfb shift and negligible hysteresis were observed. These results suggest that the selection of passivation layer strongly influences the electrical properties of germanium based MOS devices.