On the surface passivation of textured C-Si by PECVD silicon nitride

We investigate the surface passivation of crystalline silicon (c-Si) wafers that are textured with random upright pyramids and passivated with amorphous silicon nitride (SiN_x ). Over a large range of refractive indices (n = 1.89-4.1 at 632 nm), we achieve a low upper limit to surface recombination velocity on textured samples (S_{\rm eff,UL} < 10 cm s ^-1 at an excess carrier density of 10 ¹⁵ cm^{-3}). We also find that S_{\rm eff,UL} is higher for textured surfaces than for planar surfaces when the NH₃:SiH ₄ ratio is high (and, hence, n is low). For example, when passivated by an N-rich SiN_x deposited with NH ₃:SiH ₄ = 4.7 (n = 1.83), the vertices and/or edges of the pyramidal texture drives a 3.5 times increase in S_{\rm eff,UL}. As the NH ₃:SiH₄ ratio decreases (and n increases), S _{\rm eff,UL} of the textured surfaces decreases rapidly and approaches the same S_{\rm eff,UL} as the planar surfaces when NH ₃:SiH₄ ≤ 0.7 (n ≥ 2.3). By contrast, we find that irrespective of NH₃:SiH₄ ratio, and, therefore, n, S _{\rm eff,UL} is equivalent on {100} and {111} planar surfaces. The results indicate that the increase in S_{\rm eff,UL} of the textured surfaces is related to the presence of vertices and/or edges of the pyramids rather than to the presence of {111}-orientated facets. By depositing varying degrees of corona charge on the samples, it is found that the increase in recombination introduced by 1) a higher NH ₃:SiH₄ ratio and 2) the vertices and edges of the pyramids is primarily due to an increase in defect density rather than a decrease in SiN _x charge density.