Preliminary Study Of The Crystalline Silicon Thin Films Prepared By Solid-phase Crystallization In Porous Silicon

As the ever increasing prices of oil and natural gas,the renewable energy resources, especially solar energy,have been attracting more and more attention internationally. With the further research into the third generation solar cells-thin film solar cells, fabricating high quality polycrystalline silicon (poly-Si) thin films is a most straight approach to improve the photovoltaic conversion efficiency.In this thesis, the fabrication of high quality poly-Si thin films is investigated. The transfer of monocrystalline Si films provides a new way for the fabrication of low -cost thin film solar cells, using a interlayer with low intensity between substrate and soar cell , detaching the device from substrate by mechamical stress when the solar have been finished. This is a new solar making method.This thesis aims to make better poly-Si thin film solars, first, make porous Si by electrochemical etching, then, recrystallize the a-Si films which were deposited on porous Si. Firstly, this experiment prepares porous Si by elechtrochemical etching. We study the effect of corrosion current, corrosion time, HF acid concentration, temperature on porosity and influence factor of porous Si surface topography, and make further study of the porous Si layer detachment.The results show: (1) the porousity will increase when the corrosion current increase, the corrosion time lengthen, the temperature arise, the HF acid concentration decrease. But it must be in a given dimension which asure that elecotropolishing doesn't happen. (2) the resistance has a great effect on pororsity,small resistance Si can easily form even porous Si layer than large ones. (3) the fresh porous surface will crack in the air because the capillary stress which generate when the liquid in porous layer volatilize. (4) increase the corrosion current to the critical current,then happen elecotropolishing ,the porous layer is stripped from Si wafer. we find that reflection index of porous layer is low in the whole wave band from near ultraviolet to near infrared band.Secondary, plasma enhanced chemical vapor deposition serves to deposit a-Si films on the porous Si wafer, then the thin films are further annealed to prepare poly-silicon films,1.he a-Si film are recrystallized by solid phase crystallization (SPC). Study the effect of sustrate,annealing method,temperature,doping,annealing time on crystallization of films. silicon thin films are characterized by Raman spectrum and X-ray diffraction(XRD) before and after thermal annealing. The result show:(l) the silicon films on porous substrate have better crystalline than ones on quartz substrate,(2) conventional thermal annealing resulted in slightly larger average grain size than rapid thermal processing (RTP),(3) each annealing temperature have its best annealing time,(4) the crystalline time will decrease when the temperature increase,(5) doping films have higher crystallinity than those without doping one, which is advantage for epitaxy.(6) The H in films escapes when annealing that form curl skin, which reduces the quality of thin films.