Experimental Investigation Of The Ni-Si-P Ternary System And Evaluation Of Diffusion Mobilities In The Si-M(M=Al, Cu) Alloy

As photovoltaic industry has increased rapidly worldwide, crystalline siliconwafers are widely used for solar cells. However, the rapid growth of the globalphotovoltaics(PV) industry increasingly limited by the price of electronic gradesilicon. As the cheapest silicon raw material, metallurgical grade silicon contains alarge amount of transition metals, such as nickel, iron, cobalt, copper, aluminum, etc.Phosphorus gettering of Si wafers, which can remove mobile impurities effectively, isa feasible way to obtain new metallurgical silicon to substitute solar grade silicon.Gettering techniques remove the impurities from the active areas of the devices andtransport them in regions where they are not harmful, where they can be trapped oreliminated by a chemical etching. In order to know better about the thermodynamicsand kinetics basis of the phosphorus gettering of Si wafers, the thesis is comprised tothree parts:(1) Using diffusion couple method coupled with SEM-EDS methods, thephosphorus influence on the diffusion of metal impurity M (M=Ni, Fe, Co) in themulticrystalline silicon were studied. The diffusion coefficient of the impurity metalsdiffuse in the phosphorus-containing polysilicon and no phosphorus-containingpolysilicon at800oC,900oC and1000oC were compared. The studied real phosphoruscontent is in the range of0~0.67at.%. Phosphorus can accelerate the diffusion of themetal impurity significantly at800oC. However, phosphorus has little effect on thediffusion coefficient of the impurity metal in the multicrystalline silicon at1000oC.When the diffusion temperature is900oC, the effect is between the diffusiontemperature of800oC and1000oC.(2) By means of the mechanical alloying method, Ni-20at.%P and Ni-40at.%Palloys were prepared, and they are used as master alloys to prepare Si-Ni-P samples todetermine phase relations in this system. Meanwhile, the phase equilibria in theSi-Ni-P ternary system were investigated through equilibrated alloys combined withSEM-EDS, XRD analysis. Three new ternary compounds were found, named τ1, τ2and τ3. The composition range of Ni, Si, P in the τ1phase is20.0~21.5at.%,6.9~8.5at.%and70.7~72.5at.%, respectively, while which is5.8~7.0at.%,21.5~22.3at.%and71.5~71.9at.%in τ2phase.(3) As part of the studying on the metal impurity diffusion in multicrystalline silicon, the atomic mobilities of Si-M(M=Al, Cu) system were critically assessed as afunction of temperature and composition using DICTRA software package based onthe available experimental data. And the reasonable assumptions on the fcc siliconself-diffusion coefficient was used. The obtained mobility parameters can be used topredict the dynamic behavior (such as concentration profile, Kirkendall plane shift), itis turn out to be consistent with the experimental data, what is further confirmed thecorrectness of the assessed results.