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The electronic structure and the optical properties of Ca
_{2}Ge have been calculated by the first-principles pseudo potential method. The results of the electric structure show that Ca
_{2}Ge bulk is a direct semiconductor with the band gap of 0.306 eV, the conduction band is mainly composed of Ca 3d, the valence bands is mainly composed of Ge 3p. With Sr-doping, Ca
_{2}Ge bulk is a direct semiconductor with the band gap of 0.350 eV, the conduction bands are mainly composed of Ca 3d and Sr 3d, the valence bands are mainly composed of Ge 3p and Sr 3d. The results of the optical properties show that the dielectric constant of Ca
_{2}Ge bulk is reduced from 21.52 to 13.94, the reflectivity is decreased, and the absorption is increased with Sr-doping. The optical properties are improved with Sr-doping, the results offer theoretical guide for the optical properties control of Ca
_{2}Ge.

The alkaline-earth metal of Ca_{2}Ge is a new environmental friendly semiconductor material, the existence of multiple germander phases in the Ca-Ge system leads to the simultaneous formation of Ca_{2}Ge, Ca_{5}Ge_{3}, CaGe, Ca_{3}Ge_{4}, CaGe_{2}, Ca_{7}Ge_{6}_{ }and so on during growth process. However, Ca_{2}Ge is a direct semiconductor, has a simple orthorhombic structure and a cubic structure (The orthorhombic structure, which is a stable phase with an energy band gap of is 0.26 eV) [_{2}Ge has attracted much attention for its potential to create new classes of environmentally conscious electronics [_{2}Ge has a definite progress while is started relativity late, so the related literature reports and data available for reference. In 2003, the abinito method was used to study the geometric construction of Ca_{2}Ge by D. B. Migas, the results showed that Ca_{2}Ge was demonstrated had an orthorhombic and a cubic crystal system [_{2}Ge, the results showed that Ca_{2}Ge was a direct band gap semiconductor material with the band gap of 0.265 eV [_{2}Ge, the results showed that the photon frequency of Ca_{2}Ge was lower than Ca_{2}Si in the low energy region; the results provide reference for the photoelectric properties of Ca_{2}Ge in the next phase study [_{2}Ge, using the first principles pseudo-poten- tial method to study the electric structure of Ca_{2}Ge with Sr-doping, discuss the influence of dielectric function of Ca_{2}Ge with Sr-doping, and study the regulated mechanics of the optical properties of Ca_{2}Ge.

The simple orthogonal Ca_{2}Ge bulk belongs to the space group of Pnma (No.62), the lattice constant are a = 7.804 Å, b = 4.896 Å, c = 9.204 Å. Each primitive cell contains 8 Ca atoms and 4 Ge atoms [^{−6} eV/atom. Ultra-soft pseudo-potentials (USPP) is expanded within a plane wave basis set with 330eV, the iteration convergence accuracy is 1.0 × 10^{−6} eV. The energy of Ca_{2}Ge have been calculated based on the optimization of structural system, the minimum energy made to be chosen stable structure, the electronic structure and polarization of the dielectric function were calculation. The ionic and electronic interaction was calculated, the Ca 3p 4s, Ge 3p 4s electron made to be chosen valence electron, the k-point sampling are 4 × 5 × 3 according to the Monkhorst-Pack method in the Brillouin Zone (BZ) [

The lattice constants and volume of Ca_{2}Ge bulk for derails see

Sample | a/nm | b/nm | c/nm | V/nm^{3} |
---|---|---|---|---|

OrthCa_{2}Ge | 7.804 | 4.896 | 9.204 | 351.670 |

OrthCa_{2}Ge with Sr-doping | 7.912 | 4.931 | 9.228 | 360.022 |

shows that the lattice constants a, b, c and the original cell volume V are slightly changed with Sr-doping. Comparing with the results of Ca_{2}Ge bulk it shows that the lattice constant and volume of Ca_{2}Ge were increased, slightly. The reason is that the atom radius of Sr is larger than Ge and the bond length of Sr-Ge is longer than the Ca-Ge (the atomic radius of Sr is 2.45 Å, the atomic radius of Ge is 1.52 Å).

The energy band structure of Ca_{2}Ge bulk for derails see _{2}Ge bulk is a direct semiconductor with the band gap of 0.306 eV at the Γ-point. The ^{6}4s^{2}, the configuration of extra nuclear electron of Sr is 4p^{6}5s^{2}, and the lose electron of Sr is easy than Ca. the results show that Ca_{2}Ge is a direct band gap semiconductor material with the band gap of 0.35 eV, with Sr-doping.

The density of states of Ca_{2}Ge bulk for derails see _{2}Ge bulk with Sr-doped is that the Ge 3p active state is increased in the valence bands and the Ca 3d active state is in-

creased in the conduction bands. The valance bands of Sr-doped Ca_{2}Ge are mainly composed of Ge 4p, Ca 3d and Sr 4d. The conduction bands are mainly composed of Ca 3d, Sr 4d.

_{2}Ge bulk. _{2}Ge bulk is 21.5 and the dielectric function department is formed two dielectric peaks with the photoelectron energy increasing. The maximum dielectric peak is appeared at 1.3 eV, and the maximum dielectric peak is 42.5. The dielectric function imaginary part is forthcoming when the photoelectron energy is higher than 0.8 eV is formed four dielectric peaks with the photoelectron energy increasing, and it reflected the transition of the electron. _{2}Ge bulk with Sr-doping is 13.9, and the dielectric function department is formed two

dielectric peaks with the photoelectron energy increasing. The maximum dielectric peak is appeared at 2.1 eV, and the maximum dielectric peak is 38.5. The dielectric function imaginary part is forthcoming when the photoelectron energy is higher than 1.8eV is formed two dielectric peaks with the photoelectron energy increasing. The effect of the dielectric function of Ca_{2}Ge bulk with Sr-doped is that the dielectric constant and the maximum dielectric peak are decreased, and it due to the configuration of extra-nuclear electron of Sr is active than Ca. The results show that the dielectric constant of Ca_{2}Ge bulk is 21.5. The results show that the dielectric constant Ca_{2}Ge bulk with Sr-doping is 13.9.

_{2}Ge bulk. _{2}Ge bulk is appeared at the photoelectron energy of 0.8 eV, and in the energy range of 1.8 - 7.3 eV the absorption spectrum more than 10,000. The absorption edge of Ca_{2}Ge bulk with Sr-doping is appeared at the photoelectron energy of 1.8 eV, and in the energy range of 2.3 - 10.2 eV the absorption spectrum more than 10,000. The absorption range of Ca_{2}Ge bulk is increased and the absorption edge is moved to high energy

direction with Sr-doping. The results show that the absorption coefficient decreased.

_{2}Ge bulk. _{2}Ge bulk is exceed 80% is appeared at the photoelectron energy range of 6.3 - 8.2 eV. The reflectivity of Ca_{2}Ge bulk with Sr-doping is exceed 80% is appeared at the photoelectron energy range of 7.6 - 8.7 eV. Comparing the reflectivity spectrum found that the reflectivity is moved to the high direction and is decreased the high reflection area with Sr-doping. The results show that the reflection spectrum decreased.

The electronic structure and optical properties of orthorhombic Ca_{2}Ge bulk are calculated by first-principles pseudo potential method based on density functional theory. The results show that the forbidden bandwidth is increased, and the optical properties are enhanced with Sr-doping.

Project supported by the science and technology foundation of Guizhou Province, China (The contract LH of Guizhou No. [

Wei, Y.J., Yang, Y.Y., Cen, W.F., Li, R.J. and Lv, L. (2016) The Effect on the Electric Structure and Op- tical Properties of Ca_{2}Ge Bulk with Sr-Do- ping. Journal of Materials Science and Che- mical Engineering, 4, 20-26. http://dx.doi.org/10.4236/msce.2016.411003