Study On Carrier Selective Films Of Poly-silicon By Magnetron Sputtering

Polycrystalline silicon（poly-Si）film is currently regarded as the core technology material of new high-efficiency crystalline silicon solar cells,which can be compatible with high temperature process,and has been widely used in the photovoltaic industry because of its excellent application value.A doped poly-Si film is prepared on the monocrystalline silicon（c-Si）substrate.as the carrier selective contact structure of crystalline silicon high-efficiency solar cells,based on doped Si film is also a material to achieve the best performance of selective contact,commonly used silicon film preparation methods such as chemical vapor deposition method have problems such as large energy consumption,winding plating and easy to produce toxic gases.In this paper,the“magnetron sputtering+high temperature annealing”two-step process was used to prepare poly-Si films on fused quartz and monocrystalline silicon substrates respectively.This method is simple,safe and has low consumable material,which can reduce cost and increase efficiency.P-type thin films were prepared by in situ doping and spin coating doping respectively.The growth and electrical properties of the films were mainly studied to obtain the variation rules of the preparation process and properties.Firstly,the structure and composition of doped silicon films deposited by sputtering method were analyzed.After annealing,the Si films showed the preferential growth phenomenon of（111）crystal direction,and the amorphous phase began to crystallize toward the polycrystalline structure.Subsequently,XPS was used to analyze the elemental composition and chemical state of Si thin films prepared by different sputtering processes.The results of multi-peak fitting of Si2p orbitals showed that the silicon thin films were oxidized,but Si was the main element.It is concluded that lower substrate temperature and higher background vacuum can effectively reduce the risk of oxidation and improve the electrical properties of silicon films.In addition,in order to control the thickness of Si film,the deposition rate of Si film deposited by different sputtering process parameters was studied and the causes were explained.Continue to optimize the electrical properties of the thin films and improve the sputtering process.Through comprehensive discussion of the test results of the deposition of thin films under different processes,the optimal sputtering process for the preparation of thin films with the best electrical characteristics can be obtained as follows:The sputtering power is set to 100W,the local vacuum degree is 8×10^-4Pa,the substrate temperature is set to room temperature,and the working pressure is 0.5 Pa.The lowest sheet resistance（Rs）of the deposited silicon film with 80 nm thickness after annealing at 950℃is 542Ω·sq^-1in situ doped and 79.2Ω·sq^-1for the spin-coated doped sample.Carrier concentration（Ns）was as high as 4.24×10²⁰cm^-3,and the mobility（Mob）was 31.9 cm²V^-1s^-1.The poly-Si thin films with different thickness（40～150 nm）were studied.It was observed by SEM that the grain size and cluster phenomenon of the films increased with the increase of the film thickness,and the grain size of the films ranged from 16 nm to 23 nm.Raman spectroscopy was used to test the crystallization rate of the films above 47%.Time of flight secondary ion mass spectrometry（TOF-SIMS）was used to test the B content doping and depth distribution of poly-Si films.The doping atom concentration at poly-Si/c-Si interface with 80 nm and 40 nm thickness doped by spin coating is greater than 2.98954×10²⁰cm^-3and 7.08193×10¹⁹cm^-3,which is similar to the result of Hall test carrier concentration.By changing the annealing process parameters,the process route of Si film growing during heat treatment was optimized.The change of the electrical properties of poly-Si film and the influence mechanism of the annealing process（annealing temperature and holding time）were studied.With the increase of temperature and holding time,the proportion of silicon element shows an upward trend.The longer the holding time,the deeper the diffusion of B atoms in c-Si substrate.Finally,based on the matching of sputtering and annealing processes,the lowest square resistance of the optimized poly-Si film doped in situ is 213Ω·sq^-1,the lowest square resistance of the spin-coated doped film is 72.4Ω·sq^-1,and the doping atom concentration is 4.846×10²⁰cm^-3,showing excellent electrical properties.