Optical Properties Of Micro-structured Silicon By Femtosecond Laser And Equipment

Arrays of conical spikes can be formed on a silicon surface with cumulative femtosecond laser pulse irradiation in a variety of ambient gases. This novel phenomenon has been paid more attention not only because of the interaction between femtosecond laser pulses and a semiconductor surface, but also because of the high absorption of micro-structured silicon in a very wide wave band than un-pattemed silicon. In this thesis, sets of experimental equipment have been established and the systematic studies for optical properties of the femtosecond laser pulses patterning silicon were carried out.The result shows that, micro-structured silicon formed in ambient of SF6 has strong absorption in the wavelength range of 0.3μm-200μm. In the ultraviolet-visible-near infrared bands (0.3μm-2.5μm), the average absorption ratio is greater than 90%; in the mid-infrared band (2.5μm-20μm), with an average absorption ratio greater than 80%; but, in the far infrared band (20μm-200μm), absorption curve is not flat as before, with the wavelength increases, it decreased rapidly. Even so, the minimum absorption ratio is still as high as 20%, several times higher than flat one. The study also demonstrates that as the absorption ratio of micro-structured silicon increases as the height of spike increases.The spatial distribution of reflected light of microstructure silicon shows that there is a retro-reflection characteristic of micro-structured silicon. The experiment also shows that even if the surface wave exists in micro-structured silicon, the proportion of its energy does not exceed 1% of the total reflected signal.In the research on normal emissivity characteristics of micro-structured silicon, experimental results show that the micro-structured silicon has a high normal spectral emissivity in the mid-infrared wavelength range, close to the ideal black body. As the height of spikes increase, the normal spectral emissivity increase. The normal spectral emissivity also increases with increasing temperature.Simulation results indicate that the micro-structured surface and high refractive index of silicon are the two key factors that effectively reduce the reflection, especially transmittance of the material by using software "TracePro". The transmittance matches well between the simulated and actual result; however, there is about 10% deviation for reflection and absoiption between the simulated and actual result although the trends are consistent. This shows that the structure does play an important role of the absorption, but far from all.Simulation also illustrates that and material relationship between the absorption and the microstructure is more complicated. Suppose the absoiption of bulk material is much higher, the absorption rises faster with the peak height increases. Generally, with the height of spike increases, the system absoiption ratio increase, but, in some part, there will be great shock significantly.These remarkable optical properties of surface-microstructured silicon open up new Possibilities for potential applications in optoelectronic and detectors fields, such as silicon solar cells, silicon-based wide band detectors, THZ, remote sensing, infrared irradiance and so on.