Cdse Theoretical Study On Electronic Structure And Optical Properties Of Doped System

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.