Characterisation and optimisation of PECVD SiN_x as an antireflection coating and passivation layer for silicon solar cells

In this work, we investigate how the film properties of silicon nitride (SiN_x) depend on its deposition conditions when formed by plasma enhanced chemical vapour deposition (PECVD). The examination is conducted with a Roth Rau AK400 PECVD reactor, where the varied parameters are deposition temperature, pressure, gas flow ratio, total gas flow, microwave plasma power and radio-frequency bias voltage. The films are evaluated by Fourier transform infrared spectroscopy to determine structural properties, by spectrophotometry to determine optical properties, and by capacitance-voltage and photoconductance measurements to determine electronic properties. After reporting on the dependence of SiN_x properties on deposition parameters, we determine the optimized deposition conditions that attain low absorption and low recombination. On the basis of SiN_x growth models proposed in the literature and of our experimental results, we discuss how each process parameter affects the deposition rate and chemical bond density. We then focus on the effective surface recombination velocity S_eff, which is of primary importance to solar cells. We find that for the SiN_x prepared in this work, 1) S_eff does not correlate universally with the bulk structural and optical properties such as chemical bond densities and refractive index, and 2) S_eff depends primarily on the defect density at the SiN_x-Si interface rather than the insulator charge. Finally, employing the optimized deposition condition, we achieve a relatively constant and low S_eff,UL on low-resistivity (1.1 öcm) p- and n-type c-Si substrates over a broad range of n 1.85-4.07. The results of this study demonstrate that the trade-off between optical transmission and surface passivation can be circumvented. Although we focus on photovoltaic applications, this study may be useful for any device for which it is desirable to maximize light transmission and surface passivation.