Polymorphysm in doped ZrO2 plays a major role in the tailoring of the dielectric constant (k) of the oxide. In particular, the tetragonal phase is associated with the highest k value. In the present study, we analyze La-doped ZrO2 (La 5% at.) films grown by atomic layer deposition on Ge (001). X-ray diffraction analysis has been systematically conducted using synchrotron radiation on 14-31 nm thick films both as deposited and annealed at 400 C. Using a grazing incidence methodology the tetragonal an monoclinic phases can be unambiquously detected. Results are corroborated by chemical and electrical analysis. We show that the diffusion of Ge not only stabilizes the tetragonal phase, but also inhibits the formation of the undesired, low-k, monoclinic structure. The stabilizing role of low percentages of La is also discussed by the complementary study of ZrO2 grown on Ge (001) and of La-ZrO2 grown on Si (001)