Structural and Optical Properties of Phase-Pure UO<sub>2</sub>, α-U<sub>3</sub>O<sub>8</sub>, and α-UO<sub>3</sub> Epitaxial Thin Films Grown by Pulsed Laser Deposition
Erik Enriquez, Gaoxue Wang, Yogesh Sharma, İbrahim Sarpkaya, Qiang Wang, Di Chen, Nicholas Winner, Xiaofeng Guo, J. Dunwoody, Joshua T. White, Andrew Nelson, Hongwu Xu, P. C. Dowden, Enrique R. Batista, Han Htoon, Ping Yang, Q. X. Jia, Aiping Chen
Abstract
Fundamental understanding of the electronic, chemical, and structural properties of uranium oxides requires the synthesis of high-crystalline-quality epitaxial films of different polymorphs of one material or different phases with various oxygen valence states. We report the growth of single-phase epitaxial UO2, α-U3O8, and α-UO3 thin films using pulsed laser deposition. Both oxygen partial pressure and substrate temperature play critical roles in determining the crystal structure of the uranium oxide films. X-ray diffraction and Raman spectroscopy demonstrate that the films are single phase with excellent crystallinity and epitaxially grown on a variety of substrates. Chemical valance states and optical properties of epitaxial uranium oxide films are studied by X-ray photoelectron spectroscopy and UV–vis spectroscopy, which further confirm the high-quality stoichiometric phase-pure uranium oxide thin films. Epitaxial UO2 films show a direct band gap of 2.61 eV, while epitaxial α-U3O8 and α-UO3 films exhibit indirect band gaps of 1.89 and 2.26 eV, respectively. The ability to grow high-quality epitaxy actinide oxide thin films and to access their different phases and polymorphous will have significant benefits to the future applications in nuclear science and technology.