Local Structure of Epitaxial Single Crystal UO$_{2+x}$ Thin Films

Abstract

The influence of oxygen stoichiometry on the uranium local environment is explored in epitaxial single crystal uranium oxide thin films grown by DC magnetron sputtering. Through post-growth annealing, the stoichiometry of as-grown UO$_{2}$ films are tuned over an approximate stoichiometry range of $0.07 \leq x \leq 0.20$, estimated with X-ray photoelectron spectroscopy measurements of the U$-4f$ and O$-1s$ peaks. The local structure of the thin films are then probed using extended X-ray absorption fine structure measurements at the U $L_{3}$ absorption edge. We observe both the evolution of the U local environment of as a function of oxidation in UO$_{2+x}$, and that the near stoichiometric UO$_{2}$ film replicates the local structure of bulk UO$_{2}$ material standards well. The series of stoichiometrically varied samples highlights the non-trivial transitional behaviour of the UO$_{2+x}$ oxygen sublattice with increasing oxygen content in this stoichiometric regime, while also demonstrating the efficacy of this thin film synthesis route for actinide studies beyond their established use as idealised surfaces, which could be readily adapted for further stoichiometrically tailored material studies and UO$_{2+x}$ device fabrication.