.si / Ge Nano-thin Film Optical And Electrical Properties Of The Simulation Analysis

In this thesis, we use the software CASTEP which based on density functional theory and the support of first principles method, optimize the structure of Si,Ge,Si/Ge, and systematically study their geometric structure, electronic structure and optical properties.Analyze the structure, energy band, density of states and absorption spectra of the calculated single crystal Si. The results indicate that the density of states of the P orbit is much greater than the S orbital in the Fermi energy, the absorption to visible light of Si crystal is very weak at room temperature,because solving the Kohn-Sham equation system don't often consider the excited states,making the fundamental band gap is smaller than the experimental value, the calculated Si lattice parameters is fit in with the experimental, it is indicates that the selected method have reliability for this paper. It is established the doping of Si and Ge model, the Ge atom is consisted in Si crystal with the form of substitutional impurity, the doping mole ratio of about 3.1%. Study results and get the following conclusions: Ge/Si embedded structures is an indirect band gap semiconductor, band gap is smaller than Si; the absorption coefficient of Ge/Si embedded structure change significantly, after doped a Ge atom, the valence electron configuration of Ge is 4s4p, it makes the electronic density of states of the embedded Ge/Si structure near the Fermi level has got to weaken, so that the absorption coefficient of Ge/Si embedded structure is decrease, but the absorption band edge has obvious blue shift, and the absorption bandwidth becomes narrower.Analyze the structure, energy band, density of states and absorption spectra of the calculated single crystal Ge. The results show that the absorption peak of Ge is at 5.0eV at the room temperature, located in the ultraviolet region and the absorption to visible light is very weak; Ge is an indirect band gap semiconductor, band gap is about 0.677eV. Then we study the energy band structure and optical properties of Si/Ge layered structure. The Si/Ge layered structure is similar to the Ge/Si embedded structure, it is an indirect band gap semiconductor, about 0.37648eV; through the analysis of the populaion,after joining the Ge layer, Si-Ge bond length is larger than Si-Si bond, this is because atomic radius is increases with the atomic number in the familyⅣatoms; the absorption coefficient of the Si/Ge layered structure after joining Ge layer change significantly, because of the valence electron configuration, it makes the electronic density of states of the Si/Ge layer structure near the Fermi level has got to weaken, so that the absorption coefficient of Si/Ge layer structure is increase, but the absorption band edge has obvious red shift, and the absorption bandwidth becomes narrower.