Insight into the initial oxidation of UN1.85 thin films

Abstract

High N-content uranium nitride of UN1.85 thin films were successfully fabricated by radio frequency magnetron deposition on Si substrate, and the samples were characterized by Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS), respectively. Furthermore, the in-situ oxidation of UN1.85 was carried out under UHV condition with controlled oxygen exposure, which yields insights into the initial oxidation behavior of UN1.85. A gradual oxidation of UN1.85 is identified, where the N is displaced by O resulting in the formation of a series of uranium oxy-nitrides under various oxygen exposures. Three characteristic U4f7/2 peaks locating at 378.6 eV, 379.9 eV and 380.4 eV are corresponding to different intermediate uranium oxy-nitrides, where the higher the U4f7/2 peak, the higher O content in the intermediate uranium oxy-nitrides. The intermediate uranium oxy-nitride with U4f7/2 peak at relative lower position is further oxidized to higher energy side by the gradual displacement of N by O. The U4f7/2 peaks at 380.4 eV could be originated from the overlap of uranium oxides and uranium oxy-nitrides with the least N content among the three intermediates mentioned above. The coexistence of U4O9 and other uranium oxides in higher states UO2+x on UN1.85 oxidized surface is further identified by Raman spectroscopy, where the former is characterized with vibration signals at ~460 cm−1 and ~630 cm−1. A three-layered model is applied for the description of the oxidized UN1.85 surface, and an excellent resistance of UN1.85 against ambient corrosion is proved, where the oxidized thickness on the UN1.85 sample exposed under laboratory ambience for 18 months is similar with that oxidized with 2000 L O2 exposure under UHV condition.