Field-Effect Passivation of Undiffused Black Silicon Surfaces

Black silicon (b-Si) surfaces typically have a high density of extreme nanofeatures and a significantly large surface area. This makes high-quality surface passivation even more critical for devices such as solar cells with b-Si surfaces. It has been hypothesized that conformal dielectrics with a high fixed charge density ( ${{\bm{Q}}_{\bm{f}}}$ ) are preferred as the nanoscale features of b-Si result in a significant enhancement of field-effect passivation. This article uses 1-D, 2-D, and 3-D numerical simulations to study surface passivation of b-Si, where we particularly focus on the charge carrier control by | ${{\bm{Q}}_{\bm{f}}}$ | up to 1 × 1013 cm−2 under accumulation conditions. We will show that there is a significant space charge region compression in b-Si nanofeatures, which affects the charge carrier population control for moderate | ${{\bm{Q}}_{\bm{f}}}$ | up to ≈1 × 1012 cm−2. The average surface minority charge carrier density can be reduced by 70% in some cases, resulting in an equivalent reduction in area-normalized surface recombination losses if the effective surface recombination velocity ( ${{\bm{S}}_{{\rm{eff}}}}$ ) is limited by minority carriers. This provides a possible solution for the empirical ${{\bm{S}}_{{\rm{eff}}}} \propto 1/{\bm{Q}}_{\bm{f}}^4$ reported previously. We will also show that the situation is more complicated for surface passivation films where the ratio between the electron and hole capture cross section ( ${{\bm{\sigma }}_{\bm{n}}}$ / ${{\bm{\sigma }}_{\bm{p}}}$ ) is higher than 10 for p-type surfaces. For commonly used surface passivation films with a | ${{\bm{Q}}_{\bm{f}}}$ | larger than ≈1 × 1012 cm−2, there is little space charge compression for b-Si. Consequently, ${{\bm{S}}_{{\rm{eff}}}}$ simply scales with the surface area, i.e., there is no enhanced reduction of surface recombination by field-effect passivation on b-Si.