亚皮秒脉冲激光辐照硅薄膜热效应的模拟研究

Simulation Study of Thermal Results of Si Film Irradiated by Sub-picosecond Pulses Laser

基于Boltzmann方程,采用了Chen J K等人建立的自相关模型,考虑了Si薄膜的热容、热导率、弛豫时间等热力学参量随温度非线性变化的影响.采用有限差分法,数值求解了脉宽为500fs的激光脉冲辐照2μm厚硅膜的自相关模型.分析了膜表面载流子浓度、载流子温度、晶格温度等随入射激光功率和脉宽等的变化规律.结果表明:在脉冲辐照初期(t<0.68ps),载流子和晶格之间存在着明显的非热平衡性,之后通过相互之间的弛豫碰撞,逐渐达到热平衡,载流子热容是引起载流子温度在早期迅速上升的原因;载流子温度速率方程中单光子吸收、载流子-晶格能量交换和载流子能流变化率对载流子温升影响较大,而多光子吸收、双极能流和带隙能量变化率对载流子温升的影响较小,可以忽略;较高脉冲激光能量(Ф>0.02J.cm-2)辐照Si膜,会引起载流子密度方程中的俄歇复合项增大,从而使载流子密度下降率增大,导致载流子温度出现双峰.

Based on the Boltzmann equation, taking the silicon film as example, the temperature effects on the heat capacity of carrier and lattice, thermal conductivity, relaxation time have been studied with the finite difference method. Solving the thermal response of 2 μm Si film irradiated by 500 fs laser pulse. Carrier-lattice interactions are simulated to obtain the temporal evolution of the carrier density and temperature as well as the lattice temperature. The numerical results indicate that before having been heated for t 〈 0. 68 ps, carrier-lattice nonequilibrium interactions are existed, then the temperature equilibration time of the whole film is going to be achieved. The temperature change of carriers is caused by the heat capacity at the beginning;Two peaks structure of the carrier temperature is caused by the drop of carrier density which effected by rise of rate of change of the carrier energy density when Φ〉0.02 J · cm^-2 ;the linear absorption, the rate of energy exchange and the changes of the carrier density are the critical factors that influence the temperature of carriers.