Shining a light on the uranium oxides:a structural, chemical, and photovoltaic study of epitaxially engineered UO_2+x thin films with tailored oxygen stoichiometries

Understanding the condensed matter physics of the uranium oxides remains an active area of research despite its long history. Within this thesis, the fundamental properties of these complex oxide systems are systematically explored through the use of stoichiometrically varied thin films of UO2+x. Through epitaxially matching the UO2 crystal structure to a suitable substrate material, these samples are synthesised by reactive DC magnetron sputtering in a partial oxygen atmosphere at high temperatures. This produces macroscopic single crystal surfaces, with the thickness of the thin films varied from nanometre to micrometre length scales.Post-growth, these thin films are treated through cycles of high temperature annealing, either under vacuum conditions or within an oxygen rich atmosphere, to alter the oxygen content of the UO2+x film. This process is then utilised to study the volume averaged alteration to the crystal structure in response to these treatment conditions as a function of sample thickness, crystal orientation, and substrate material through a combination of x-ray diffraction, reflectivity, absorption spectroscopy, and photoelectron spectroscopy measurements. This enables the alteration to the UO2 crystal structure to be probed on multiple length scales, ranging from angstrom-scale inter-planar distances to sub-angstrom inter-atomic separations, whilst also providing information on the surface state chemical speciation and depth-dependent oxygen content profiles of these oxidised thin films. Additionally these measurements probe the phenomenon of interstitial oxygen clustering reported in the bulk material within these idealised thin film systems.Finally, this sample growth and treatment route is employed for a series of measurements of the conductive properties of UO2+x. By employing an external photon source, the semiconducting behaviour of these samples are further studied as a function of incident wavelength, allowing the characterisation of these UO2+x materials as novel photovoltaic materials. Through the combination of the structural, chemical and photovoltaic experiments presented within this work, the possibility of further studies of thin film UO2+x systems are evaluated, as well as the potential of these systems for applications as functional devices.