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.