Study On The Mechanisms And The Properties Of Femtosecond-laser Processing Silicon

We have studied the transient processes and the mechanisms of modifying (ablation) silicon by femtosecond laser. The properties of the modified (microstructured) silicon were studied as well.By using steady-state and time-resolved spectroscopy, we investigated the plume induced by femtosecond laser in air, nitrogen, sulfur hexafluoride (SF6), and vacuum. We discovered the generation and evolution of the plume, leading to a better understanding of the mechanisms of the femtosecond laser interaction with matter. Moreover, we simulated the thermal dynamics in the silicon surface between and after femtosecond laser-silicon interaction by the two temperature model (TTM), which provided some useful references. Based on our researches, we can depict the whole micro-processes of femtosecond laser modifying silicon. After impacted by a femtosecond laser pulse, the electron density in silicon surface reaches the peak value (～1027/m3) within tens of femtoseconds, and the electron temperature reaches up to the maximum (105K) within100fs. This high temperature exceeds greatly the threshold (～3.7×104K) of electron escape. This causes a large amount of electrons escape from the surface, leading to the electrical field formation near the surface. The residual hot electrons transmit the energy to the lattices by electron-lattice interaction, so that the lattice temperature increases rapidly and exceeds the melting temperature (1683K) of silicon. As a result, the ultrafast melting takes place, causing the ejection of the materials that contain lots of atoms and monovalent ions of silicon. The ions have a higher speed due to the electrical field. The plasma plume forms on the silicon surface within0.2ns. And then, the plume is confined by the ambient gas during expansion. The confinement effect causes the redeposition of the ejected material as well. The redeposition affects the interaction between the successive laser pulses and the silicon surface. Due to the feedback mechanism (incubation effect), the different microstructures form in various ambient gases.To understand the optoelectronic properties and the carrier dynamics of the microstructured silicon, we also studied the photoluminescence (PL) of the microstructured silicon, and discussed the transport and recombination of the carriers in this material. We observe that the peak wavelength of the luminescence is at530nm. This emission band is attributed to the silicon nanocrystals. The oxygen-related defects, which locate at the interface of the core and the surface oxide layer of the nanocrystals, form the localized states. These localized states form a band tail with a width of～14meV. The carriers can be localized in these states and recombine radiatively. The decay of the PL, which can be fitted well with a stretched exponential function, has a time constant of1-2ns. This proves that the PL originates from the defects. We have established a model of transport and recombination of carriers to depict the carrier dynamics and the mechanisms of PL. The complex dynamics of the carriers contain:carrier transport, localized, reexcited, relocalized, trapped by nonradiative center, and so on. The nonradiative recombination is influenced greatly by the complex dynamics and should be carrier density-related. Hence, we described the nonradiative recombination coefficient as a function of carrier density, and obtained the mathematical expression of the PL decay. Our experimental results can be perfectly fitted with this model. This encourages us to draw a conclusion that the non-exponential decay of the PL is due to the carrier density-related nonradiative recombination. The temperature and the defect density of the sample affect the nonradiative recombination.Based on these researches, we preparated the sulfur-doped silicon nanoparticles by ablating the modified silicon with femtosecond laser. Comparing with the silicon nanoparticles, the sulfur-doped silicon nanoparticles have different properties of reflection and transmition. The near-infrared absorption of doped nanoparticles is a little higher. We studied the PL of these nanoparticles and observed that the spectrum was continuous from450nm to780nm. It is interesting that the PL decays so fast that the equipment cannot resolved it. Therefore, we believe that the optical emission originates from the supercontinuum caused by the pump laser pulse. It means a strong nonlinear effect of the nanoparticles.These results give us a deeper understanding of the processes and the mechanisms of femtosecond laser modifying silicon and lead us to a better understanding of the carrier dynamics in the modified silicon. These works can help us to obtain the better micro-and nano-scale silicon materials and are useful in the application of this material.