Dioxide, Uranium Experimental And Theoretical Studies Of Optical Properties

Uranium dioxide films were deposited on Si(111) substrates by dc magnetron sputtering method, and the films were grown at same conditions with different sputtering time. The morphology, roughness, structure, and chemical state of the films were studied by field emission scanning electron microscopy, atomic force microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The optical properties of the films were obtained by reflectometry and ellipsometry. The influences of film thickness and roughness on the morphology, micro structure, chemical state and optical properties were investigated. Different thickness measurement methods were used to obtain the thickness of the films. The results show that thickness, roughness and crystalline grain size increase with sputtering time. The optical properties measurements show that the refractive indices decrease with the film thickness, while the extinction coefficients have the opposite trend. The thickest film turns out to have strong absorption tendency. The results of different thickness measurement methods agree well with each other at low film thickness, while the discrepancy appears when the film thickness becomes larger.Theoretical calculations were based on density functional theory (DFT).Generalized gradient approximation (GGA) was used to optimize the structure of the unit cell, and then the local density approximation (LDA) was used to calculate the electronic structure, band gap and optical properties.Hubbard U is employed to evaluate exchange-correlation energy of 5f electrons.The calculated cell parameter was 5.39 A, bulk modulus was 240 GPa, band gap was 1.87 eV, which all agree well with the experimental results. Calculated density of states show that the valence band of UO2 consists mainly of U 6p, U 6d and O 2p orbitals, while the conduction band is mainly derived from U 6d and 5f orbitals. Furthermore, the dielectric function and the optical properties, such as reflectivity, refractive index, extinction coefficient, energy-loss spectrum and absorption coefficient were derived and analyzed.The calculated results were compared with experimental data from both published literature and our own results. Our calculated optical properties agree well with experimental data, especially in peak positions and trend.