CdSe掺杂系统电子结构和光学性质的理论研究

【作者】 王亚超；

【导师】 苏希玉；

【作者基本信息】 曲阜师范大学 ， 凝聚态物理， 2010， 硕士

【摘要】 近几十年来,随着新型半导体光电材料的发展和应用,Ⅱ-Ⅵ族化合物材料已成为人们研究的热点问题。硒化镉(CdSe)是一种性能优良的直接带隙半导体材料,它具有宽的红外透过波段、高的激光损伤阈值、宽的禁带、大的非线性系数和透明波段适宜的双折射等优点。同时,CdSe的化学稳定性好、不易潮解、机械强度适中、加工性能好,常被用于制作太阳能电池、激光探测器等。为进一步提高CdSe的应用价值、扩展其应用范围,有必要对CdSe的光学带隙进行调节、改变其导电类型、提高载流子浓度。我们的研究表明,Mg元素掺杂CdSe可以调节系统的光学带隙,使其光响应范围基本覆盖整个可见光区；Ag和In掺杂CdSe可以改变其导电类型,提高载流子浓度。我们的结果与实验符合。与现代计算机技术相结合的材料设计和材料模拟计算是现代材料科学研究的重要方法之一。本文应用基于密度泛函理论的第一性原理平面波赝势计算方法对CdSe及其掺杂问题进行了研究。论文的主要内容如下。(1)介绍了CdSe的结构、基本性质、研究现状及其应用发展情况。简要介绍了密度泛函理论和ABINIT程序包。(2)研究了不同结构CdSe系统的电子结构和稳定性。分别计算了闪锌矿结构,纤锌矿结构,岩盐结构和CsCl结构CdSe的能带、几何参数、电子态密度和内聚能。结果表明,闪锌矿结构和纤锌矿结构CdSe为直接带隙半导体材料；盐岩结构CdSe为间接带隙半导体材料；CsCl结构CdSe为半金属材料。通过对这四种结构CdSe的内聚能的比较,发现闪锌矿结构和纤锌矿结构CdSe的稳定性好；盐岩结构和CsCl结构CdSe的稳定性相对较差。(3)研究了Mg掺杂纤锌矿结构CdSe系统的电子结构和光学性质。研究表明,Cd1-xMgxSe系统的价带顶主要由Se4p态决定,其位置基本不随掺杂浓度改变；导带底由Se4s态和Cd5s态共同决定,并随掺杂浓度的增加向高能区移动,结果使得带隙展宽,系统介电函数虚部的峰值和折射率实部的峰值向高能区移动。(4)研究了Ag、In掺杂闪锌矿结构CdSe系统的电子结构和光学性质。研究表明,Ag掺杂为p型掺杂,Ag4d电子和Se4p电子轨道杂化,提供受主杂质能级,杂质能级距价带顶约0.211 eV,为深受主能级。In掺杂为n型掺杂,In5s态提供施主杂质能级,杂质能级位于禁带的中心附近,为深施主能级。与纯CdSe相比,两种掺杂系统的带隙均变小,吸收边都有明显的红移。特别地,Ag掺杂系统在2.1eV(590nm)出现了新的吸收峰,在黄光区的吸收明显增强,这与实验结果是一致的。更多还原

【Abstract】 In recent decades, with the development and application of new optoelectronic semiconductor materials,Ⅱ-Ⅵcompound materials have become a hot topic. CdSe is a direct band gap semiconductor with good properties. CdSe has broad infrared band, high laser-induced damage threshold, wide band gap, large nonlinear optical coefficient, and suitable birefringence of transparent waveband. Additionally, CdSe has good chemical stability, non-deliquescence, excellent mechanical strength, and good processability. CdSe is ofen used for making solar batteries, laser detector and so on. With a view to further improving the application value and application areas, it is necessary to adjust the band gap, to change the type of conduction, and to raise the concentration of charge carrier of CdSe. It is shown that the band gap of Mg doped CdSe can be modulated to cover almost the whole visible light region. For the Ag and In doped CdSe systems, the type of conduction can be changed, and the concentration of charge carrier can be raised. Our results are in good agreement with experiments.Material design and material simulation based on the advanced computer technology is very important in modern material science. In this thesis, CdSe and its doped systems are studied by the first-principles ultra-soft pseudo-potential plane wave approach based upon the density functional theory.The main contents of the thesis are as follows.(1) The structures, basic physical properties, research situation, and the applications and development of CdSe systems are introduced. In addition, the density functional theory and ABINIT package are briefly discussed.(2) The electron structures and stabilities of the pure CdSe systems with different structures are investigated. The energy band, geometry parameters, density of states, and cohesive energy of CdSe with zinc blende structure, wurtzite structure, rock-salt structure, and CsCl structure are calculated, respectively. The obtained results indicate that CdSe with zinc blende structure and wurtzite structure are direct band gap semiconductors; CdSe with rock-salt structure is indirect band gap semiconductor; CdSe with CsCl structure is semimetal. By the comparison among the cohesive energies of different systems, we find that CdSe with zinc blende structure and wurtzite structure have good structural stabilities, CdSe with rock-salt structure and CsCl structure have poor structural stabilities.(3) The electron structures and optical properties of the Mg doped wurtzite CdSe systems are investigated. The obtained results show that the top of the valence band of Cd1-xMgxSe systems is fundamentally determined by the Se4p electrons and is nearly unchanged; the bottom of conduction band is determined by Se4s, Cd5s electrons and is removed to higher energy direction with the Mg concentration increasing, thus the band gap is broadened. The peaks in the imaginary part of dielectric function and the peaks in the real part of refractive index are found to have blue-shifts as Mg concentration increasing.(4) The electron structures and optical properties of the Ag and In doped zinc blende CdSe systems are investigated. The obtained results indicate that Ag doping is p-type, Ag4d electrons and Se4p electrons are hybridized, the deep acceptor energy level is introduced, and the impurity energy level is about 0.211eV above the top of the valence band; In doping is n-type, deep donor energy level is introduced by In5s electrons, and the impurity energy level is near the center of the band gap. The band gaps of the two doped systems become smaller and the absorption edges shift obviously to the infared region compared with that of the pure CdSe system. Particularly, A new absorption peak at 2.1eV(590nm) is found in Ag doped CdSe system, this is in agreement with experiment.更多还原