Study Of Black Silicon Prepared By Microsecond Pulse Laser

Crystalline silicon is one of the most important basic materials of the information industry, however, it is a kind of indirect band gap semiconductor whose reflectivity is above30%in the wavelength range of250nm-2500nm.It is difficult to absorb the light longer than1100nm because of its large band gap. Due to these defects of silicon itself, it is limited for Si in the application and development of photovoltaic devices. In1998Mazur obtained arrays of sharp conical spikes on the silicon surface unexpectedly by cumulative irradiation of femtosecond laser pulses in the presence of ambient gases. This kind of micro-structured silicon was called black silicon and its absorption is more than90%in the wavelength range of250nm-2500nm. With board-spectrum absorption black silicon will be well used to improve the application of Si in infrared band. In recent years, scientists fabricated black silicon by kinds of methods such as femtosecond laser scanning, electrochemical etching and plasma immersion ion implantation and femtosecond laser scanning method is current research focus. Fabricating black silicon by laser scanning method without mask can be embedded in the semiconductor manufacturing process. While femtosecond laser equipment is too expensive to product large area black silicon at low cost. In this paper, we prepare black silicon with inexpensive microsecond pulse laser system and realize effective control of micro-structure on silicon surface induced by microsecond pulse laser. The main research work is as follows:1. We analyze physical principle of the interaction between pulse laser and solid material and the growth mechanism of laser induced silicon surface microstructure. Arrays of conical spikes are formed on the silicon surface by the irradiation of microsecond pulse laser in the presence of ambient gases. The research result that is indexed by El journal provides a new approach to producing black silicon at low cost.2. We studied the influence of energy density, repetition rate, pulse width, laser beam scanning speed and the spacing, ambient gas and the pressure on the morphology and properties of black silicon. The result shows that:(l) When the laser energy density is greater than0.15J/cm2and each spot was irradiated with1000laser pulses, conical spikes are formed;(2) Within a certain range, with the increase of energy density or repetition rate the height of microstructure will increase. But when any one of the tow factors is too lager microstructures become cluttered. The aspect radio decrease with the increase of pulse width, laser beam scanning speed and the spacing;(3) Differences in morphology of black silicon that is prepared in different working atmosphere is big. SF6atmosphere is more effective for the formation of microstructure. Within a certain range the increase of pressure favors the formation of complete conical microstructure.3. We studied the irradiation of different crystal structures of Si with microsecond pulse laser. The result shows that we can obtain surface conical microstructure on several kinds of silicon with microsecond pulse laser irradiation.4. By controlling cumulative laser pulses of each spot the evolution process and growth mechanism of laser induced silicon surface microstructure is investigated. The results reveal that:(1) At the beginning laser induced periodic surface structure is formed and then thermal stress and boiling vaporization cause surface cracks and holes;(2) Science middle and late of the process silicon surface morphology change quickly because of laser-assisted chemical etching and re-deposition. At this stage working atmosphere plays an important role in final morphology.5. The preliminary study of reflectivity of black silicon microstructure shows that the average reflectivity of micro-structured silicon produced in SF6is lower than6%. In visible wavelength range reflectivity of samples prepared in air and vacuum is about7%, while it increases rapidly with wavelength in the near infrared wave band. Further experiments find that reflectivity of black silicon is similar to changing with the wavelength before and after annealing. Visible light reflectivity of the samples is always about3%before and after annealing. In the near infrared reflectivity increases after annealing, and the effect of annealing temperature on the reflectivity is small.