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 ( Q_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 | Q_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 | Q_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 ( S_eff ) is limited by minority carriers. This provides a possible solution for the empirical S_eff∝1/Q⁴_f 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 ( σ_n / σ_p ) is higher than 10 for p-type surfaces. For commonly used surface passivation films with a | Q_f | larger than ≈1 × 10¹²  cm⁻² , there is little space charge compression for b-Si. Consequently, S_eff simply scales with the surface area, i.e., there is no enhanced reduction of surface recombination by field-effect passivation on b-Si.