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 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
$S_{\rm eff,UL}$
. As the NH
$_{3}$
:SiH
$_{4}$
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
$_{3}$
:SiH
$_{4}$
≤ 0.7 (
$n$
≥ 2.3). By contrast, we find that irrespective of NH
$_{3}$
:SiH
$_{4}$
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
$_{3}$
:SiH
$_{4}$
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.