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 (SiNₓ). 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 (Seff,UL⁻¹$at an excess carrier density of 10$¹⁵$cm$⁻³)$. We also find that$Seff,UL$is higher for textured surfaces than for planar surfaces when the NH$₃$:SiH$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$= 1.83), the vertices and/or edges of the pyramidal texture drives a 3.5 times increase in$Seff,UL$. As the NH$₃$:SiH$₄$ratio decreases (and$n$increases),$Seff,UL$of the textured surfaces decreases rapidly and approaches the same$Seff,UL$as the planar surfaces when NH$₃$:SiH$₄$≤ 0.7 ($n$≥ 2.3). By contrast, we find that irrespective of NH$₃$:SiH$₄$ratio, and, therefore,$n, Seff,ULis equivalent on 100 and 111 planar surfaces. The results indicate that the increase inSeff,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 NH₃:SiH₄ratio and 2) the vertices and edges of the pyramids is primarily due to an increase in defect density rather than a decrease in SiNₓcharge density.