We have systematically investigated the impact that postdeposition annealing (PDA) has on the physical and electrical properties of Hf Ox Ny thin films sputtered on Ge and Si substrates. These two substrates display contrasting metal-oxide-semiconductor characteristics that we attribute to the different compositions of their interface layers (ILs). We observed an increased Ge O2 incorporation into the Hf Ox Ny dielectric and severe volatilization of the IL on Ge after higher PDA processing. These undesired phenomena in the Hf Ox Ny Ge gate stacks may be responsible for their different electrical properties with respect to those of the Hf Ox Ny Si gate stacks, i.e., a further scaling of the capacitance-equivalent thickness, a significant presence of fixed positive charges and electron-trapping sites, and a degradation of dielectric reliability. In addition, the anomalous low-frequency-like behavior of the high-frequency capacitance-voltage curves in inversion for the Ge capacitors was predicted from theoretical calculations.