Electrical and material properties of hydrothermally grown single crystal (111) UO2

Abstract

The semiconductor and optical properties of UO2 are investigated. The very long drift carrier lifetimes, obtained from current-voltage I(V) and capacitance-voltage C(V) measurements, along with the well-defined optical properties provide little evidence of an abundance of material defects away from the surface region. Schottky barrier formation may be possible, but very much dependent on the choice of contact and surface stoichiometry and we find that Ohmic contacts are in fact favored. Depth resolved photoemission provided evidence of a chemical shift at the surface. Density functional theory, with the Heyd-Scuseria-Ernzerhof (HSE) functional, indicates a band gap of a 2.19 eV and an anti-ferromagnetic ground state. Ellipsometry measurements indicates at UO2 is relatively isotropic with a band gap of approximately 2.0 eV band gap, consistent with theoretical expectations.

Band-to-band transitions and critical points in the near-infrared to vacuum ultraviolet dielectric functions of single crystal urania and thoria

“…In the case of UO 2 , this experimentally determined lowest critical-point transition energy shows excellent agreement with the previously published results of band structure calculations, which predicted a bandgap energy of 2.19 eV. 3 Several studies have been conducted of the band structure of ThO 2 . [22][23][24] The calculated values of the bandgap span a broad range of 3.3-6.9 eV.…”

“…Until recently, experimental studies of optical and electronic properties of UO 2 and ThO 2 have been limited due to the nonavailability of single-crystal samples of optical quality. Our previous research on these samples indicates high-quality ordered structures of near stoichiometric UO 2 as indicated by core level photoemission spectroscopy and other techniques [3][4][5][6][7] and compared to previous measurements of growth by other processes. 8 This study is a continuation of the previous work given that we have evidence that we have nearstoichiometric UO 2 3,5,9,10 and ThO 2 6,7,9 permitting detailed analysis of their optical properties.…”

“…11 He et al used spectroscopic ellipsometry and density functional theory to explore the bandgap of uranium oxide thin films with various compositions. 12 Siekhaus and Crowhurst 13 and Dugan et al 3 used spectroscopic ellipsometry to determine the critical-point structures in e. Critical-point structures are associated with singularities in the joint density of states and caused by electronic band-toband transitions. 14 Critical-point features have different shapes corresponding to the type of the band structure singularities and can be characterized by critical-point transition energy, broadening, and amplitude parameters.…”

“…For UO 2 , significant shifts were observed previously in the optical critical-point features obtained by reflectivity 8 and by ellipsometry. 3,13 No measurements of optical properties have been reported thus far for ThO 2 .…”

“…E is the photon energy, and e 1 is the static contribution to the dielectric function. 3,18 The hydrothermal synthesis technique was used to grow ThO 2 and UO 2 single crystals for this study. Growth procedures for ThO 2 and UO 2 were quite similar in temperature, pressure, and mineralizer solution.…”