强激光照射对2H-SiC晶体电子特性的影响

Effect of intense laser irradiation on the electronic properties of 2H-SiC

使用基于密度泛函微扰理论的第一原理赝势法,计算了纤锌矿结构2H-SiC晶体在强激光照射下的电子特性,分析了其能带结构和电子态分布.计算结果表明:2H-SiC平衡晶格参数a和c随电子温度Te的升高逐渐增大;电子温度在0—2.25 eV范围内时,2H-SiC仍然是间接带隙的半导体晶体,当Te超过2.25 eV达到2.5 eV以上时,2H-SiC变为直接带隙的半导体晶体;随着电子温度升高,导带底和价带顶向高能量或低能量方向发生了移动,当电子温度Te大于3.5 eV以后,价带顶穿越费米能级;电子温度Te在0—2.0 eV变化时,带隙随电子温度升高而增大;Te在2.0—3.5 eV范围变化时,带隙随电子温度升高而快速地减少,表明2H-SiC晶体的金属性随电子温度Te的继续升高而增强.在Te=0,5.0 eV处,计算了2H-SiC晶体总的电子态密度和分波态密度.电子结构表明Te=0 eV时,2H-SiC是一个带隙为2.3 eV的半导体;在Te=5.0 eV时,带隙已经消失而呈现出金属特性,表明当电子温度升高时晶体的共价键变弱、金属键增强,晶体经历了一个熔化过程,过渡到金属状态.

By using first-principles with pseudopotentials method based on the density functional perturbation theory, in this paper we calculate the electronic properties of wurtzite 2H-SiC crystal under the strong laser irradiation and analyze the band structure and the density of state. Calculations are performed by using the ABINIT code in the generalized gradient approximation for the exchange-correlation energy. And the input variable tphysel, which is a variable in the ABINIT code and relates to the laser intensity, is used to define a physical temperature of electrons Te. The size of Te is set to simulate the corresponding electron temperature of the crystal when intensive laser irradiates it in an ultrafast time. The high symmetry points selected in the Brillouin zone are along F-A-H-K-F-M-L-H in the energy band calculations. After testing, we can always obtain a good convergence of the total energy when choosing a 20 Hartree cut-off energy and a 4 × 4 × 2 k-points grid. Then, optimizing the structure, and the structural parameters and the corresponding electronic properties of 2H-SiC in the different electron-temperature conditions are studied using the optimized equilibrium lattice constant. The calculation results indicate that the equilibrium lattice parameters a and c of 2H-SiC gradually increase as the electronic temperature Te goes up. With the electronic temperature going up, the top of valence band is still at F, while the bottom of conduction band shifts from the K point with increasing electronic temperature, resulting in the fact that 2H-SiC is still an indirect band-gap semiconductor in a range of 0-2.25 eV and when the electronic temperature reaches 2.25 eV and even more than 2.5 eV, 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 exceeds 3.5 eV, the top of valence band crosses the Fermi level. When Te varies in a range of 0-2.0 eV, the forbidden bandwidth increases with temperature rising, and when Te varies in a range of 2-3.5 eV, the forbidden bandwidth quickly decreases. This variation shows that the metallic character of 2H-SiC crystals increases with electronic temperature Te rising. The total density of states （DOS） and partial density of states are calculated at Te = 0 eV and 5 eV. The DOS figures indicate that 2H-SiC is a semiconductors and its energy gap equals 2.3 eV. At Te= 5 eV, the gap disappears, exhibiting metallic properties. This result shows that the crystal covalent bonds weaken and metallic bonds strengthen with temperature rising and the crystal experiences the process of melting, shifting to metallic state.