二氧化铀光学性质的实验和理论研究

【作者】 陈秋云；

【导师】 赖新春；

【作者基本信息】 中国工程物理研究院 ， 核燃料循环与材料， 2010， 硕士

【摘要】 本文通过直流磁控溅射技术在单晶Si(111)基底上沉积UO2薄膜,在相同工艺条件不同沉积时间下分别制备了三组薄膜。通过扫描电子显微镜,原子力显微镜,X射线衍射仪以及X射线光电子能谱仪对所制备薄膜的表面形貌,粗糙度,相结构以及元素化学状态进行了分析。通过反射光谱仪和椭偏光谱仪对薄膜的光学性质进行了测量和分析。研究了不同厚度下薄膜的表面形貌,粗糙度,相结构以及元素状态和光学性质。利用几种不同的厚度测量方法分别对所制备的薄膜厚度进行了测量,并进行了分析比较。研究表明随着沉积时间的延长,薄膜的厚度增加,粗糙度增加,晶粒尺寸增大。光学性质测量结果表明薄膜的折射率随厚度的增加而减小,消光系数随厚度的增加而增加,最厚的薄膜的折射率出现了强吸收现象。几种厚度测量方法所测得的厚度结果在薄膜厚度较小的情况下符合得比较好,在厚度较大的情况下出现了一定的差别。在理论计算方面主要基于密度泛函理论,首先利用广义梯度近似(GGA)对UO2进行结构优化,再利用局域密度近似(LDA)方法对UO2的电子结构,能带以及光学性质进行了计算,引入了Hubbard U校正项来处理5f电子的强关联效应。计算所得的UO2晶体的晶胞参数为5.39(?),体模量为240 GPa,带隙为1.87 eV与实验结果吻合较好,态密度计算结果表明UO2价带主要由U 6p,U 6d和O 2p轨道组成,导带主要由U6d和U 5f轨道组成。在此基础上计算了UO2的介电函数,反射率,折射率,消光系数,能量损失谱和吸收系数,并与现有文献报道的实验值和自己的实验值进行了分析比较,本文的光学性质计算结果与文献报道值以及实验结果吻合较好,尤其在峰值位置和变化趋势上。更多还原

【Abstract】 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.更多还原