强激光照射对6H-SiC晶体电子特性的影响

Effect of intense laser irradiation on the electronic properties of 6H-SiC

使用基于密度泛函微扰理论的第一性原理赝势法,模拟研究了纤锌矿6H-SiC晶体在强激光照射下电子特性的变化.研究结果表明,电子温度T_e在升高到3.89 eV及以上后,6H-SiC由间接带隙的晶体变为直接带隙的晶体;带隙值随电子温度T_e升高先是增大后又快速减小,当电子温度T_e大于4.25 eV以后,带隙已经消失而呈现出金属特性.

By using first-principle with pseudopotential method based on the density functional perturbation theory,in this paper we calculate the electronic properties of wurtzite 6H-SiC crystal under the strong laser irradiation and analyze the band structure and the density of states.Calculations are performed in the ABINIT code with using the generalized gradient approximation for the exchange-correlation energy.And the input variable tphysel is used to set up a physical temperature of electrons Te.The value of Te is set to simulate the corresponding electron temperature of the crystal when irradiated by intensive laser within an ultrafast time.The highly symmetric points selected in the Brillouin zone are along Γ-A-H-K-Γ-M-L-H in the energy band calculations.After testing,we can always obtain a good convergence of the total energy when choosing 18 Hartree cut-off energy and 3×3×3 k-point grid.By optimizing the structure and then using the optimized equilibrium lattice constant,the structural parameters and the corresponding electronic properties of 6H-SiC in the different electron-temperature conditions are studied.First of all,when the electron temperature stays in a range between 0 eV and 5.0 eV,we choose 23 groups of different electron temperatures to respectively test the values of equilibrium lattice parameters a and c of 6H-SiC.Within a temperature range between 0 eV and 4.25 eV,we continue to test 20 groups of the electrical properties of 6H-SiC under different electron temperatures,calculating the forbidden bandwidths at different electron temperatures and analyzing the changes of the bottom of conduction band and the top of valence band as the electron temperature goes up.Meanwhile,taking for sample two groups of the band structures in ranges of 0–2 e V and 3–4 eV,we comparatively analyze the changes of the energy and position of the bottom of conduction band and the top of valence band with electron temperature.The calculation results indicate that the equilibrium lattice parameters a and c of 6H-SiC gradually increase as electron temperature Te goes up.With the electron temperature going up,the top of valence band still stays there,while the bottom of conduction band shifts to the location between M and L point as electron temperature increases,leading to the fact that 6H-SiC is still an indirect band-gap semiconductor in a range of 0–3.87 eV,and as electron temperature reaches 3.89 eV and even more,the crystal turns into a direct band-gap semiconductor.With Te rising constantly,the bottom of the conduction band and the top of valence band both move in the direction of high energy or low energy.When Te is in excess of 4.25 eV,the top of valence band crosses the Fermi level.When Te varies in a range of 0–2.75 eV,the forbidden bandwidth increases with temperature rising,and when Te varies in a range of 2.75–3 eV,the forbidden bandwidth decreases slowly,and when Te varies in a range of 3–4.25 eV,the forbidden bandwidth quickly reduces.This variation shows that the metallic character of 6H-SiC crystal increases with electron temperature Te rising.The total densities of states（DOS）are calculated at Te = 0 eV and 5 eV.The DOS figures indicate that 6H-SiC is a semiconductor and its energy gap equals 2.1 eV.At Te = 5 eV,the gap disappears,presenting metallic properties.This result shows that the crystal covalent bonds are weakened and metallic bonds are enhanced with temperature increasing and the crystal experiences the process of melting,entering into metallic state.