Study On Preparation And Properties Of A-SiN_x Films Based On Soft Plasma Deposition System

Solar photovoltaics, serving as an important solution to the energy crises and environmental problems, have attracted wide attention. Nowadays, approximately 90% of solar cells are made of crystalline silicon materials. The conversion efficiency of crystalline Si solar cells is strongly affected by both surface recombination rate and surface reflection of crystalline Si substrates. Therefore, it is of great significance to passivate crystalline Si surface using a dielectric layer with appropriate refractive index. Amorphous hydrogenated silicon nitride is considered as the ideal passivation and antireflection dielectric material for crystalline Si solar cells due to its moderate passivation performances on crystalline Si surface and appropriate refractive index of 2.0. As a consequence, many efforts have been made to prepare high-quality amorphous silicon nitride in the past two decades. The mainstream fabrication methods for silicon nitride include plasma-enhanced(PE) chemical vapor deposition(CVD), inductively coupled plasma(ICP) CVD and microwave plasma CVD. However, these plasma assisted CVD methods may cause damages onto the growing surface and thus degrade the film quality because of the ion bombardment effect. In order to reduce the ion bombardment effect, we set up a soft plasma deposition system that can prevent the growing surface from ion bombardment damages. Based on this soft plasma deposition system, silicon nitride thin films are prepared and characterized. The detailed research content can be summarized as follows.(1) Firstly, we set up a self-designed soft plasma deposition system, which is composed of vacuum chamber, RF power generator, matching and tuning network, as well as inductive coil. The plasma is generated by the inductive coil but works at the capacitive mode(E-mode), so the statistic electric field governed by E-mode discharge dominates the plasma, and thus leads to lower dissociation rate and smaller plasma density. Furthermore, the direction of radial statistic electric field is parallel to substrate surface, making the ion movement parallel to substrate surface as well. As a result, this soft plasma deposition system can suppress the ion bombardment effect and thus reduce surface damage significantly via both lowering plasma density and restricting ion movement.(2) Silicon nitride films were prepared under different experimental parameters including cleaning methods, gas flow rates, RF power density using the soft plasma deposition system. The passivation performances of as-deposited silicon nitride films on crystalline Si substrates were systematically investigated to determine the optimal processing parameters by using the minority carrier lifetime measurement system. We conclude that RCA cleaning and HF dip before loading into vacuum chamber are more beneficial for improving the passivation performances of as-deposited silicon nitride films. The passivation performance of silicon nitride films is highly related to the nitrogen gas flow rate, which can be ascribed to the variation of N-H bonds in as-deposited films. In addition, RF power can also influence the passivation quality of silicon nitride films since the hydrogen content in as-deposited films is directly controlled by the plasma density.(3) We prepared silicon nitride films using the soft plasma deposition system under different flow rate ratios of nitrogen to silane(R=N2/SiH4). The surface morphology and chemical composition are systematically studied by scanning electron microscopy(SEM), x-ray photoelectron spectroscopy(XPS) and Raman spectroscopy, while the bond configuration, bond density, hydrogen content and the passivation performance before and after annealing are investigated by Fourier transform infrared spectroscopy(FTIR) and minority carrier lifetime tester(Sinton WCT-120). SEM results reveal that the deposited silicon nitride films feature homogeneous and smooth surfaces; XPS results indicate that the deposited silicon nitride films are composed of silicon, nitrogen; Raman results show that the deposited films are rich in amorphous silicon component due to the presence of strong 480 cm-1 characteristic peak. According to the calculated bond density and hydrogen content from FTIR results, we conclude that the films deposited at R=1.25 exhibit the highest hydrogen content of about 29%. After annealing, the crystalline Si substrates passivated by such silicon nitride film exhibit a minority carrier lifetime of 251 μs, a surface recombination velocity of 85 cm/s and an open-circuit voltage of 640 mV, showing the best passivation effect.