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

TL;DR

This work addresses how oxygen stoichiometry in epitaxial UO$_{2+x}$ thin films alters the uranium local environment. The authors synthesize phase-pure UO$_2$ and tune x to $0.07 \le x \le 0.20$ via post-growth annealing, quantifying composition by XPS and probing local structure with EXAFS at the U L$_{3}$ edge, benchmarked against bulk UO$_2$. They show that near-stoichiometric films replicate the bulk local structure at low temperature, while increasing oxidation reveals non-trivial, temperature-dependent rearrangements of the O sublattice, with U–O and U–U distances subtly perturbed. This confirms the utility of epitaxial UO$_{2+x}$ thin films as a controlled platform for stoichiometry-tuned actinide studies and potential UO$_{2+x}$ device applications, capturing bulk-like behavior and dynamic interstitial oxygen effects in a solid-state system.

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.

Figures (13)

• Figure 1: XRD of an epitaxial UO$_{2}$ thin film deposited onto a CaF$_{2}$ substrate. (a) A $\theta-2\theta$ scan, with peak positions from both the thin film and substrate labelled with symbols. (b) An off-specular $\phi$ scan, showing reflections from the thin film (solid lines) and substrate (dashed lines).
• Figure 2: Fourier transformed U $L_{3}$ edge EXAFS $\chi\left(R\right)$ of a vacuum annealed epitaxial thin film UO$_{2}$ (blue) and bulk powder UO$_{2}$ (orange). (a) The modulus and real component of $\chi\left(R\right)$, with a dashed horizontal line representing the respective zero level. In the modulus the peaks correspond to scattering paths to nearest neighbours, while the real component of the Fourier transform highlights spectral components as a function of $R$. (b) The modulus of $\chi\left(R\right)$ shown relative to the multiple-scattering fit model (dashed lines).
• Figure 3: XRD of as-grown (black) UO$_{2}$ thin films with in-situ $\theta-2\theta$ characterisation as a function of vacuum annealing (blue) and sequentially longer oxygen annealing treatments (green, red). A non-indexed peak (star) emerged with O$_{2}$ annealing.
• Figure 4: X-ray photoemission spectra characterisation of annealed UO$_{2}$ epitaxial thin films. (a-c) O-$1s$ peak characterisation for SN$1-3$ respectively. (d-f) U-$4f$ peak characterisation for SN$1-3$ respectively. Each fit makes use of a Shirley background profile. Component parameters are detailed in Table \ref{['tab:Table3']}.
• Figure 5: X-ray photoemission spectra characterisation of annealed UO$_{2}$ epitaxial thin films. Note the SN$1$ data (blue) in (a-b) have been scaled by a factor of $50\times$ to aide comparison. (a) U-$4f$ peak characterisation. (b) O-$1s$ peak characterisation. (c) Estimated surface stoichiometry from the fitted U-$4f$ and O-$1s$ peak areas.