Hydrogen Doping Process Study Of Amorphous Silicon Thin Film By Magnetron Sputtering

Amorphous silicon(a-Si:H)has gained its popularity in thin film solar cell fabrication for its high absorption coefficient,high applicability on flexible substrates and practical feasibility for low-cost roll-to-roll mass fabrication.Working as the active layer,the optical-electrical characteristics of amorphous silicon film is crucial to the cell performance.In this work,the amorphous silicon film has been fabricated on PET substrate by magnetron sputtering method.The main optical-electrical characteristics,deposition rate and surface morphology have been systematically investigated under different fabrication conditions(including sputtering power,working pressure,working temperature,H2/Ar ratio).Meanwhile,the theory of doping of amorphous silicon thin films is studied and explained.Some theoretical calculation methods are introduced,explained and discussed along with the calculation parameters of this study are determined.The effect of hydrogen on the structure of the films is also studied by infrared spectroscopy.Therefore,we can calculate the content and atomic density of hydrogen in the films.The deposition rate and optical band gap of the films under different mixing ratios are calculated.The effects of different mixing ratios on the surface morphology of the films are investigated.The results indicate that the deposition rate increases remarkably from 1.88 to 8.74nm/min with the sputtering power increasing from 60W to 120W,while the light transmission rate decreases from 86%to 46%in the visible spectrum range(390nm to 780nm).Theoretical calculations have been carried out,showing a decreasing deposition rate under an increasing working pressure.A rising temperature provides a higher deposition rate and lower transmittance in the certain range.Moreover,the increase of the amount of hydrogen is not conducive to the aggregation between the silicon particle.With the hydrogen flow rate increases,the hydrogen content increases.When the hydrogen concentration reaches a certain value,the hydrogen concentration no longer increased.The hydrogen atom density increases with the increase of hydrogen flow rate,and when the mixing ratio is 2,the hydrogen atom density reaches the maximum value of 1.003%.The structure of the amorphous silicon thin film changes with the fluctuation of the mixing ratio.This structure is characterized by the configuration of Si-H bond condition.R*increases from 0.11 to 0.13 and then decreases to 0.06,this indicator finally climbs to 0.1 at mixing ratio of 3.The changes of hydrogen atom density has little effect on the Si-Hn bonding of the films.With the change of the proportion of the mixed gas,the optical bandgap value fluctuates.When the mixing ratio is 0.25,the bandgap of the film reaches 1.96 eV.The increase of the mix ratio make the bandgap value increase to 2.04 eV(H2/Ar),but the bandgap is significantly changed when the mixing ratio is 1 as it drops down to 1.53eV.Continuing to increase the flow ratio did not continue to reduce the bandgap value.With the change of hydrogen argon mixing ratio,the change trend of hydrogen density is very similar to that of optical bandgap.The hydrogen density has a direct effect on the optical bandgap value.The increase of hydrogen density can increase the optical Band gap,while the decrease in hydrogen density reduces the optical bandgap and produces a lower optical bandgap value.A lower hydrogen density is obtained under the condition that the mixing ratio is 1.0.In order to determine the optimal process parameters of amorphous silicon thin films,the results of the previous studies are discussed and compared.The optimum process parameters are obtained by considering the optimum film thickness and the optimal optical bandgap value.The process parameters were the parameters of sputtering power 80W,working pressure 1.0Pa,substrate temperature 120? and hydrogen argon mixing ratio of 1.0.Moreover,to refine the amorphous silicon thin film,Co-Sputtering method has been applied.By using Ti target,dangling bonds are passivated more efficiently.Meanwhile,deposition rate dropped down comparatively lower than that of samples deposited under direct sputtering method.Band gaps as well as R*are refined through this process.This study provides a basis for the roll-to-roll mass manufacture of amorphous silicon thin film solar cells.