The stability of the tetragonal phase of Ge doped HfO2 thin films on Si(100) was investigated. Hf(Ge)O-2 films with Ge atomic concentrations varying from 0% to 15% were deposited by remote plasma chemical vapor deposition. The atomic structure on the oxide after rapid thermal annealing was investigated by x-ray absorption spectroscopy of the O and Ge K edges and by Rutherford backscattering spectrometry. The authors found that Ge concentrations as low as 5 at. % effectively stabilize the tetragonal phase of 5 nm thick Hf(Ge)O-2 on Si and that higher concentrations are not stable to rapid thermal annealing at temperatures above 750 degrees C. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3430562]