Study Of Femtosecond Laser-induced Semiconductor Internal Ultrafast Dynamic Process

In the present, the mechanism of momentum and energy relaxation,carrier-carrier scattering, valleys-valley scattering, optical phonon scattering and themicroscopic process during carrier diffusion have been investigated. As thesemiconductor is irradiated by ultrashort laser pulses, if the photon energy is largerthan the band gap of semiconductor, the electrons within the absorption depth willtransit from the valence band to the conduction band through the multi-photonabsorption process, generating holes in the valence band at the same time. Thesenon-equilibrium carriers (electron-hole pairs) will couple with the lattice systemtemporally and spatially, and finally most of their energy is transferred to the lattice,as a result, the system regains thermal equilibrium by assigning the heat between thecarriers and lattices. This is an ultrafast process which occurs in several picoseconds.The smaller the size of semiconductor device, the shorter its response time is,therefore, it is crucial to have a clear understanding for the dynamical energy transferprocess occurred inside the semiconductor devices over an ultrashort time. Generally,much attention has been paid to the understanding of the dynamics of thenon-equilibrium carriers, and it is commonly believed that non-equilibrium phononeffects can affect the release and delivery of the hot carriers. In practice, the numberof non-equilibrium phonons has been measured through the Raman scattering method,demonstrating the existence of nonequilibrium phonons generated by hot carriers. Huet al. used the van Driel system to study the change of carrier concentration, latticetemperature with the pulse duration, and found that carrier density and latticetemperature at the surface of semiconductor rise with the increase of pulse duration, which is in good agreement with the experimental results. However, the dynamicalprocess inside the semiconductor is less investigated, and it will be investigated inpresent work.In this paper, the ultrafast dynamical process of semiconductor Ge irradiated bythe femtosecond laser pulses is simulated by solving density-dependent twotemperature model (DDTTM) proposed by van Driel. The influence of pulse duration,intensity and fluence on the time-evolution of carrier density and lattice temperatureat the different depths in Ge is investigated. To further investigate the effect of pulseduration on the ultrafast dynamical process of Ge, the femtosecond double-pulsetechnique is adopted in the end.