1.School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou 215009, China 2.School of Electronics and Information Engineering, Suzhou University of Science and Technology, Suzhou 215009, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11274054), the Jiangsu Key Disciplines of the Thirteenth Five-Year Plan, China(Grant No. 20168765), and the Suzhou Key Laboratory for Low Dimensional Optoelectronic Materials and Devices, China (Grant No. SZS201611).
Received Date:24 November 2018
Accepted Date:04 February 2019
Available Online:23 March 2019
Published Online:20 April 2019
Abstract:BaTiO3 (BTO) is a typical studying object both in ferroelectrics and in material science. By the GW method, optical property of BTO is investigated, and its volume effect under the case of iso-strain is also studied. It is found that the results of excited states are closer to the experimental results with the consideration of electron-hole interaction in the framework of GW method. Considering the volume effect, we obtain that the red shift of the peaks of optical absorption occurs under the expansion of volume, and the blue shift appears when the BTO is compressed. At the same time, the polarization and the hybridization between d orbital of Ti atom and p orbital of O atom are enhanced for the case of volume expansion, however, things will be opposite under the compression of volume. Furthermore, the volume effect in the iso-strain case is less dramatic than in the iso-stress case. Keywords:BaTiO3/ optical property/ volume effect/ GW method
表1不同体积下计算得到的BTO块材的能隙(单位:eV) Table1.Energy gaps (in eV) for selected BTO volumes and different theoretical method
23.3.光学性质 -->
3.3.光学性质
应变调控可以有效提高光电材料的光学吸收效率[33], 对BTO材料的应变实验研究表明[34], 当BTO外延生长在SrTiO$ _3 $ 基底时, BTO在$ xy $平面内获取的应变大致在0.3%—1.3%. 实验发现, 由于应变的引入, 原体材料在5 eV处的吸收峰发生了0.2—0.4 eV 的蓝移, 而在8.5 eV附近的吸收峰几乎没有受到应变的影响. 在理论处理方法上, 针对立方相和四方相BTO光学性质的第一性计算研究报道见文献[6, 8?10, 16], 其中前四篇是基于DFT的计算, 最后一篇是利用GW + BSE方法分别给出了BTO的介电常数和光学吸收谱. 本文考虑在等应变情形下改变四方相BTO的体积, 研究体积对材料光学性质的影响. 图4和图5分别给出了四方相BTO材料在V/V0 = 0.9, 1.0, 1.1情况下体系的介电常数虚部和光学吸收系数, 其中的蓝色实线是DFT-PBE的计算结果, 黑色实线代表的是考虑了准粒子相互作用的GW计算得到的数据结果, 红色实线表示的是在GW理论框架下进一步考虑电子-空穴相互作用(激子效应), 即求解BSE方程得到的四方相BTO的光学性质. 图 4 不同体积BTO的介电常数虚部谱($\epsilon_2^{\bot}$为垂直于极化方向即xy平面内的介电常数虚部, $\epsilon_2^{^{_{/\!/}}}$为z方向的介电常数虚部) Figure4. Spectrum of imagine part of dielectric constant for selected BTO volumes, where $\epsilon_2^{\bot}$ corresponds the direction perpendicular to the polarization, and $\epsilon_2^{^{_{/\!/}}}$ stands for the direction parallel to the polarization
图 5 不同体积下BTO块材的光学吸收谱($\alpha^{\bot}$为垂直于极化方向即xy平面内的光学吸收系数, $\alpha^{^{_{/\!/}}}$为z方向的吸收系数) Figure5. Spectrum of optical absorption of BTO for selected volumes, where $\alpha^{\bot}$ corresponds the direction perpendicular to the polarization, and $\alpha^{^{_{/\!/}}}$ stands for the direction parallel to the polarization
通过对图4和图5的分析, 得到的主要结果和结论如下. 1)无论是介电常数虚部还是光学吸收系数, 从起始峰角度来看, 当V = V$ _0 $ 时(无应变), 考虑了准粒子相互作用的GW 方法计算结果相对于DFT理论结果发生了2 eV的蓝移, 而激子效应引起起始峰相对于GW结果发生了1 eV左右的红移, 计算结果表明, 考虑了激子效应的理论计算结果与实验结果[11]更为接近. 2)由于四方相BTO原子结构和电子结构在平行于$ z $轴和垂直于$ z $轴呈现各向异性, 因此介电常数的虚部和光学吸收系数在两个方向上也不等价. 通过对照研究, 沿着$ z $方向的自发极化会使材料在$ z $方向产生内建电场, 从而导致介电常数虚部和光学吸收系数的起始峰在平行方向相对于垂直方向发生大约0.5 eV的蓝移. 3)体积压缩会使介电常数虚部和光学吸收系数发生蓝移(起始峰的蓝移现象最为显著), 而体积膨胀则导致介电常数虚部和光学吸收系数的红移, 我们的计算结果定性上与加静水压得到的结果一致[7]. 24.总 结 -->