Influence of the NH3:SiH4 ratio and surface morphology on the surface passivation of phosphorus-diffused C-Si by PECVD SiNx

We investigate the surface passivation of phosphorus (n⁺) diffused crystalline silicon that is either planar or textured and passivated with amorphous silicon nitride (SiN_x). A low and relatively constant saturation current density J₀ is attained on the n⁺-diffused planar surfaces over a wide range of refractive index (n = 1.9-2.9 at 632 nm), and a wide range of sheet resistance (39-320 Ω/□). The results demonstrate that the trade-off between the optical transmission and surface recombination at n⁺-diffusion can be circumvented. That is, with careful optimization of SiN_x properties, the optical advantages of SiN_x can be enjoyed without penalty in recombination. In specific, on a light diffusion with sheet resistance of 700 Ω/□, a record-low J₀ of 4 fA/cm² is obtained using a nearly-stoichiometric SiN_x with negligible absorption at short wavelengths. Moreover, it is shown that for lightly diffused surfaces, any additional recombination that occurs at textured surfaces relates strongly to the NH₃:SiH₄ ratio during the SiN_x deposition, and consequently correlates inversely with n. In other words, an increase in the NH₃:SiH₄ ratio leads to an increase in recombination on planar wafers, and an even larger increase in recombination on textured wafers. By contrast, at the heavily-diffused surface, recombination at textured and planar wafers is approximately the same, irrespective of the NH₃:SiH₄ ratio. In summary, the results in this work indicate that the additional recombination invoked by the textured surfaces is greater as (i) the NH₃:SiH₄ ratio increases, which also increases n, and (ii) the phosphorus diffusion is lighter.