Study Of Ultra-Fast Energy Transport In Silicon Nano-Films During Femtosecond Laser Heating

Silicon nanostructures are a central part of modern integrated circuits, detectors and sensors which are widely used in microprocessors, high speed communications, portable electronics, micro-photodetectors, satellites and weapon control systems, etc. To obtain an optimal and steady-going performance of these devices, a further understanding of energy transport in silicon nanostructures is needed. The researches in nanoscale energy transport and conversion have become a hotspot around the world.In this work, a study of non-Fourier energy transport in silicon nanoscale thin films during femtosecond laser heating is carried out based on the Boltzmann transport equation. First, a numerical model is established to simulate the ultra-fast electron energy transport during the laser heating. It is observed that the electronic ballistic transport dominates the energy transport. The coupling between electrons and lattice is non-local due to the ballistic transport of the unbalanced electrons, and the coupling coefficient is found to be much more strongly dependent on electron temperature than lattice temperature. Then, the modified characteristic method is introduced for simulating the nonequilibrium phonon radiative heat transfer during ultra-fast laser heating. The investigation of the EPRT results show that the assumption of local equilibrium for the Fourier's law is questionable because the phonon energy transport in the region close to the light absorption region is ballistic instead of diffusive, and the speed of thermal wave predicted by the EPRT is equal to the sound speed. By considering the nonequilibrium transport of nonequilibrium electrons and phonons as well as their energy conversion processes into the BTE, a coupled energy transport model is established. By applying this unified model to simulate ultra-fast energy transport and conversion in silicon thin films, a more detailed microscale energy transport picture is developed. During ultra-fast laser irradiation, electrons are excited to an extremely high equivalent temperature, and then relax their energy through electron-phonon scattering processes. Electrons and phonons are highly nonequilibrium. Because emitted phonons carry only little energy, multiple scattering processes is needed for electrons and phonons to reach thermal equilibrium. After the direct effect of laser irradiation, ballistic phonon transport dominates the thermal transport in thin films, and so, determines the temperature distribution.