Thermal stability of uranium nitride and oxynitride films in an ultra-high vacuum environment

Uranium nitride (UN1.66) and oxynitride (UN1·42O0.23) films have been prepared on the substrate of Si by radio frequency (RF) magnetron sputtering. The films have been studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Auger electron spectroscopy (AES). Then, vacuum heat treatment of uranium nitride and oxynitride films were in situ investigated in an ultra-high vacuum chamber of AES. The experimental results show that the UN1.66 and UN1·42O0.23 films fabricated by radio frequency magnetron sputtering method are dense and uniform. The surface of the UN1.66 films changes into UNxOy at the beginning of the heat treatment. Then the surface changes into UO2 with the increasing temperature. When the temperature exceeds 573 K, UO2 phase gradually changes back into UNxOy phase due to the out diffusion of decomposed N atom in UN1.66 subsurface with increased N content of the surface. UN1.42O0.23 film exhibits good stability until 573 K. UNxOy and UO2 mixed phases form on the surface of the UN1·42O0.23 after initial oxidation. Owing to the decomposed nitrogen from the UN1·42O0.23 subsurface, the UO2 phase turns back into UNxOy again when the temperature exceeds 573 K.