Molecular Dynamics Simulation Study Of Microstructure Formation Of Hydrogenated Amorphous Silicon Thin Films

Amorphous silicon/crystalline silicon heterojunction solar cells have extensive application prospect due to their low production cost and high photoelectric conversion efficiency.Hydrogenated amorphous silicon?a-Si:H?thin films,the passivation layer in the solar cell,whose structures and properties play a crucial role in the efficiency of the heterojunction solar cell.The current experimental studies focused more on the correlation between deposition process and the performance of a-Si:H thin films.However,the evolution of the thin film microstructure is lack of in-depth research and analysis,which results in the slow development of amorphous silicon/crystalline silicon heterojunction solar cells.In this paper,the influences of deposition parameters and post-treatment on the microstructure of hydrogenated amorphous silicon films were studied by molecular dynamics simulation method.The relationship between the deposition process and microstructure would be complemented for the existing experimental studies.The results are as follows:?1?The study about the effect of silicon substrate orientations indicated that:The a-Si:H thin films obtained at different substrate orientations are amorphous.However,the compactness and the degree of order on the atomic arrangement in the a-Si:H films have difference,the sequence was:?111?plane>?110?plane>?001?plane.The mass density analysis of a-Si:H thin films with different thicknesses reflected that the effect of silicon substrate orientations on the microstructure of a-Si:H thin films is limited to several atomic layers above the interface,after then the effect is obviously weakened.?2?The study about the effect of the incident angle indicated that:The a-Si:H thin film grew in a two-dimension mode at low or normal incidence?0°,15°and 30°?,however,it changed to follow a columnar or island-like mode at high incidence.As incident angles decreased,the surface roughness decreased,and the a-Si:H thin film had less void structure and dangling bond.The analysis of atomic diffusion indicated that the incident angle played a crucial role in affecting the diffusion of atoms:the diffusion ability of high incidence was greater than that of low incidence.As incident angles increased,the influence of shadowing effect on the growth mode,microstructure evolution of a-Si:H thin film,and atomic diffusion ability were enhanced.The analysis of Voronoi polyhedron indicated that the arrangement of the first neighboring atoms tended to be a tetrahedral structure and the arrangement of the second neighboring atoms tended to be closer as the incident angle decreased.?3?The study about the effect of the incident radical indicated that:During the growth of a-Si:H thin films,the adsorption rate was closely related to the difficulty degree of forming gas phase with the incident radical and hydrogen atom.The formation order from difficulty to easy was:Si₂H₄ radicals>SiH radicals>SiH₂radicals>SiH₃ radicals.The easier the formation of gas phase,the lower the adsorption rate.The a-Si:H/c-Si thin films obtained by depositing the SiH₃ radicals had the highest contents of H and SiH,the lowest contents of dangling bonds and floating bonds,and the highest contents of interface region H.?4?The study about the effect of annealing treatment on a-Si:H/c-Si thin films indicated that:Annealing treatment made the structure of a-Si:H thin films more compact and orderly,the arrangement of the first neighboring atoms tended to be a tetrahedral structure and the arrangement of the second neighboring atoms tends to be closer.As the annealing temperature increased,the film surface became smoother and the contents of dangling bond,SiH,and H in the film decreased.The hydrogen content of interface region increases firstly and then decreases with increasing annealing temperature,and the maximum H content is happened at 500 K.Internal short-range-diffusion of H atoms leaded to the increasing of the interface H atoms at low annealing temperature.Nevertheless,the long-distance-diffusion and the escape of H atoms leaded to the decreasing of the interface H atoms at high annealing temperature.?5?The study about the effect of hydrogen treatment on a-Si:H/c-Si thin films indicated that:Hydrogen treatment reduced the compact and orderly of a-Si:H thin films.Moreover,hydrogen treatment can improve the surface morphology of a-Si:H thin films,and the greater the kinetic energy of hydrogen,the smoother the film surface.The content of Post-H,H and SiH increases firstly and then decreases with increasing kinetic energy of hydrogen.The critical value of kinetic energy is 2.09 eV.In addition,when the films were treated by hydrogen with low kinetic energy?below2.09 eV?,the dangling bond content did not have obvious changing;conversely,the content of the dangling bond increased significantly,and the quality of the film decreased.