Effect Of Porous Silicon Gettering On The Conductive Properties Of Monocrystal Silicon

Crystalline silicon is the prime material for solar cells. However, impurities and defects can be introduced during the crystal growth of crystalline silicon, which has great effect on the conversion efficiency of solar cell. Gettering is an effective method to significantly improve the quality of silicon wafers. In mono-crystalline silicon, the electrical property of silicon is determined by impurities concentration and defects. The important parameter to measure electrical property is resistivity and there is a functional relationship between resistivity and impurities concentration. So far, the impact of porous silicon preparing method on morphology and porosity as well as the influence of porous silicon structure on gettering ability has not been systematically studied. In this work, different methods are employed to prepare porous silicon and investigated. Moreover, the removal efficiency of impurities and defects in silicon by gettering method is analyzed. SEM, XRD, FTIR, weighing method and four probe resistivity tester are carried out to evaluate the morphology, porosity and structure of porous silicon. Contents and main conclusions are presented as followed:(1) As for the fabrication of porous silicon by electrochemical etching, the pore size and density increase with the enhancement of current density, It can be judged from XRD pattern that the porous silicon still remains single crystal silicon structure. The gradual increase of FWHM shows that the grain size decreased. In addition, the diffraction peaks shifted to the direction of the small-angle, indicating a larger lattice parameter and lattice expansion. With the increase of corrosion time, the pore size and density increased accordingly. The porosity is basically proportional to the corrosion time. The thickness of porous silicon layer also increased correspondingly, but the phenomenon of collapse may occur after reaching a certain time. As for the fabrication of porous silicon by chemical etching, pore size of porous silicon increased gradually with the corrosion time. Nevertheless, there are not significant changes for porosity and pore size to a certain corrosion time. After reaching up to the maximum corrosion time, an extension of time may lead to the porous silicon collapse phenomenon.(2) For the gettering treatment of porous silicon by electrochemical etching, the sample resistivity was significantly increased with increasing current density and the resistivity increased the most by51.2%at the current density of100mA/cm2. With the increase of corrosion time, the resistivity increased correspondingly which is in consistence with the trend of porous silicon morphology and porosity. Heat treatment temperature also has some influences on the effect of porous silicon gettering. The resistivity increased the most at850℃while decreased at950℃, which is mainly due to that long time of porous silicon under high temperature annealing process can result in the coarsening, densification and collapse of the pore structure. For the gettering treatment of porous silicon by chemical etching, the resistivity values were all improved after heat treatment under different corrosion time conditions. And the resistivity reached up to the maximum after corrosion for11min while decreased at14min. The gettering law presented above has a direct relationship with the morphology and porosity of porous silicon.(3) Preliminary study on the effect of porous silicon gettering by electron beam bombardment was conducted. The morphology of porous silicon changed after electron beam bombardment. Through3min gettering treatment, the resistivity changed significantly, which fully improves that electron beam bombardment have the key thermal effect on the porous silicon. Although electron beam bombardment worked for the removal of impurities, the experimental processes and parameters need to be further studied in details.