Determination of the Insulation Gap of Uranium Oxides by Spectroscopic Ellipsometry and Density Functional Theory

• Published in: Journal of physical chemistry. C 2013-08, Vol.117 (32), p.16540-16551
• Main Authors: He, Heming, Andersson, David A, Allred, David D, Rector, Kirk D
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electron states; Exact sciences and technology; Methods of electronic structure calculations; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of bulk materials and thin films; Physics
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp401149m

Detecting and understanding the complex signatures of species for attribution of highly enriched uranium, HEU, is challenging even though these compounds have been intensively studied for 65 years. Attempts to obtain, for example, chemical speciation signatures on uranium oxides are frustrated by the presence of extremely diverse phases, complex structures, and their tendency to form solid solutions with the coexistence of many nonstoichiometric oxides. More importantly, the spectroscopic signatures of many of these oxides, using common techniques such as X-ray diffraction or Raman scattering, are remarkably similar with each other. On the other hand, the effort to understand the U–O system also exhibits some of the most intriguing and challenging properties in theoretical and computational chemistry. This is due to the spatial extent between localization and delocalization of the 5f orbitals of the uranium atom. In this article, spectroscopic ellipsometry (SE) measurements and a comparison of six fitting methods as well as theoretical calculations are combined to examine the intrinsic electronic structure and the corresponding band gap of uranium oxides to determine the chemical speciation in a ∼102 nm thick reactively sputtered uranium oxide film. The SE results reveal that the UO x film exhibits two absorption edges, a primary absorption edge slightly above 2.6 eV and a secondary absorption at 1.7–1.8 eV. The optical band gaps compared with the theoretical calculations performed on UO2, U4O9, U3O7, α-U3O8, α-UO3, δ-UO3, and γ-UO3 suggest that the UO x film is composed of at least two components; the primary absorption is caused by the α-UO3 sublayer, which is superimposed on top of an adjacent α-U3O8 sublayer that is hypothesized to be heteroepitaxial growth of α-U3O8 along the UO x /substrate interface. Comparison to the ellipsometry measurements shows that the DFT+U and hybrid (HSE) calculations predict the correct trend for band gaps as a function of oxidation state and crystallography but they fail to capture the exact gaps. However, they provide important information for interpretation of the experimental results and highlight some of the structural complexity that prevails in the UO x compounds. The combination of theoretical and experimental methods to examine the intrinsic electronic structure and the band gap of the corresponding uranium oxides could benefit from the development of new methods for better distinguishing chemical speciation in ur