Study On Doping Behavior In Crystalline Silicon By Using Liquid And Solid Source Diffusion

The n-type and p-type doping are important in the process of the preparation of solar cells.There novel doping source with boric acid mixed with SiO₂ nanosphere,phosphorus acid with SiO₂ nanosphere and boron papers were introduced,and achieved excellent diffusion performance.In order to improve the quality of B doped layer formed by diffusion,a novel boron source mixed with SiO₂ nanosphere was introduced.Scanning electronic microscope,four probe method and minority carrier lifetime measurement were adopted to study the influence of SiO₂ nanosphere,boric acid concetration and diffusion parameters on the performance of p+ layer formed by B diffusion from boric acid solution.It was found that the thickness of boron rich layer（BRL）decreased from 130 nm to 15 nm with the help of SiO₂ nanosphere.At the same time,the uniformity of diffusion increased from 88.17% to 96.68%.In addition,junction depth decreased slightly and minority carrier lifetime in the samples increased apparently after using mixed boric acid solution with SiO₂ nanosphere.All the results above indicated that SiO₂ nanosphere could evidently enhance the property of the p+ layer formed by B diffusion from boric acid solution.In addition,concentration of boric acid could affect the performance of p+ layer.Sheet resistance of diffused silicon samples reduced with increasing concentration of boric acid till the concentration reaches 0.4 g/ml.This implies that even a 0.4 g/ml boric acid solution contains enough boron to saturate the silicon surface.Sheet resistance reduced and uniformity increased gradually with increasing temperature.In addition,junction depth increased with increasing temperature.The influence of diffusion time on the performance of B diffusion was similar with that of temperature.The performance of B diffusion using B40 SC boron paper produced in Filmtronics was studied next.Boron paper was placed in a self-made quartz box that could adjust the distance between boron paper and silicon.Sheet resistance was invariable with changing the distance,and diffusion uniformity was the best when the distance between boron paper and silicon was 0.3 cm.Scanning electronic microscope,four probe method and minority carrier lifetime measurement were adopted to study the influence of diffusion temperature and time on the performance of p+ layer formed by B diffusion from boron paper.It was found that Sheet resistance decreased with increasing diffusion temperature,which was as low as 6.9 Ω/□ at 1050 ℃,and diffusion uniformity was 99.173%.The junction depth increased from 195 nm to 517 nm with diffusion temperature raised from 900 ℃ to1050 ℃,but the thickness of BRL increased and minority carrier lifetime in the samples decreasedapparently with increasing temperature.The influence of diffusion time on the performance of B diffusion from boron paper was similar with that of temperature.Phosphorus diffusion in p-type silicon was mainly used in preparation of p-n junction which was the core of the solar cell structure.In order to improve the quality of P doped layer formed by diffusion,a novel phosphorus source mixed with SiO₂ nanosphere was introduced.Scanning electronic microscope,four probe method and minority carrier lifetime measurement were adopted to study the influence of SiO₂ nanosphere,concentration of phosphorus acid and diffusion condition on the performance of n+ layer formed by P diffusion from phosphorus acid solution.It was found that the smaller particle size of SiO₂ nanosphere we used,the better diffusion uniformity the silicon obtained.And Sheet resistance of diffusion reduced with increasing volume concentration of phosphorus acid till H3PO4: 60wt% SiO₂=3:1.It was found that diffusion uniformity increased from84.49% to 94.9% compared with that produced without addition of SiO₂ nanosphere.The existence of SiO₂ nanosphere could effectively control the production of heavy doping layer,but minority carrier lifetime decreased slightly compared with that produced without of SiO₂ nanosphere.In addition,Sheet resistance decreased obviously and uniformity evidently increased with increasing temperature.At the same time,doping concentration of phosphorus and junction depth both increased with increasing temperature,it reached 571 nm at 925 ℃.With the increase of diffusion time,sheet resistance decreased sharply at first and then reduced slowly,and diffusion uniformity increased slightly.The variation tendency of doping concentration was same with sheet resistance as increasing difffusion time.But diffusion junction depth increased from 250 nm for 15 min to 486 nm for 60 min,which indicated diffusion time has deep influence in junction depth.