La-doped ZrO2 thin films grown by O3-based atomic layer deposition directly on Ge(100) exhibit a dielectric constant of 29. Upon annealing in N2 at 400 °C, a high κ value >40 is extracted for film thickness below 15 nm. Compositional depth profiling allows to correlate this observation with a remarkable Ge interdiffusion from the substrate which is consistent with the stabilization of the tetragonal ZrO2 phase. Ge interaction with the oxide stack and the formation of a germanate-like interfacial region, which acts as an electrical passivation for the Ge surface, are also investigated.