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

Abstract

In this work, we investigate how the film properties of silicon nitride (SiNx) 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 SiNx properties on deposition parameters, we determine the optimized deposition conditions that attain low absorption and low recombination. On the basis of SiNx 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 Seff, which is of primary importance to solar cells. We find that for the SiNx prepared in this work, 1) Seff does not correlate universally with the bulk structural and optical properties such as chemical bond densities and refractive index, and 2) Seff depends primarily on the defect density at the SiNx-Si interface rather than the insulator charge. Finally, employing the optimized deposition condition, we achieve a relatively constant and low Seff,UL on low-resistivity (≤1.1 Ωcm) 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

Gettering of interstitial iron in silicon by plasma-enhanced chemical vapour deposited silicon nitride films

“…39 While Petroff et al conjectured the gettering mechanism to be due to the internal stress and non-stoichiometry of the Si 3 N 4 films, 39 Jourdan et al attributed the gettering effect to the Si 3 N 4 /Si interface which develops local stress and creates dislocations upon annealing. 43 In addition to the debated gettering mechanism of the CVD Si 3 N 4 films, the PECVD SiN x films deposited at low temperatures (below 300 C in this study) have different properties to the CVD Si 3 N 4 films deposited at 700-800 C. 26,27 PECVD films are generally less dense and contain more defects and voids, which could aid their ability to segregate impurities. Further investigation is required to understand the details of the gettering mechanism of the PECVD silicon nitride and the effects of the film properties on the gettering efficiency.…”

“…The resulting film thickness was 90 6 20 nm, with a refractive index of 1.93 at a wavelength of 632 nm, a hydrogen content of $15%, and a Si-N bond density of $9 Â 10 22 cm À3 . Note that the deposited SiN x films were optimised for the surface passivation and anti-reflection purposes, 26,27 not for the most effective removal of iron from the silicon bulk.…”

Totally Vacuum-Free Processed Crystalline Silicon Solar Cells over 17.5% Conversion Efficiency

“…SiN x films or SiN x based stack layers are commonly used in the commercial solar cells for ARC purposes with their effective antireflective behavior and good passivation effect [7,8]. However, its deposition technique of plasma-enhanced chemical vapor deposition (PECVD) [9,10] has some drawbacks, including the need for toxic and hazardous gases such as SiH 4 and NH 3 with vacuum processing for chemical vapor deposition operation, difficult handling, and high costs. Besides, the common use of vacuum processes for ARC formations for single junction crystalline silicon solar cells, especially heterojunction solar cells, basically relies on sequential delicate vacuum-processed applications [11].…”

Atomic Layer Deposition TiO2 Films and TiO2/SiNx Stacks Applied for Silicon Solar Cells

“…SiN x thin film deposited by PECVD not only is the mainstream option for anti-reflection coatings in c-Si solar cells, but it also serves as the passivation layer due to the atomic hydrogen in the film. During the process of solar cells, hydrogen atoms can diffuse and serve in the chemical passivation [40]. For the silicon surface passivation of TiO 2 /SiN x stacks, Figure 9 shows the effective carrier lifetimes of bare Si wafer, TiO 2 /SiN x stacks, 80-nm-thick SiN x films deposited by PECVD, and 66-nm-thick TiO 2 films deposited by ALD on Si at the injection level of 1.0 × 10 15 cm −3 .…”