Optimizing n+ poly-Si layer doping and plasma treatments for enhanced electrical and thickness uniformity in n-type c-Si wafers

Abstract

This study investigates n⁺ polycrystalline silicon (poly-Si) layers, focusing on doping concentration and post-deposition treatments, to understand their electrical and structural properties on n-type crystalline silicon wafers. To address challenges in conductivity and film uniformity, the PH₃/SiH₄ gas ratio (1.2–4.0) was varied during PECVD deposition, followed by high-temperature annealing and selective NH₃ plasma exposure. Amorphous silicon (a-Si:H) was first deposited (on ultrathin SiO_x for optical-uniformity mapping) and subsequently crystallized into n⁺ poly-Si by annealing at 900 °C. Post-treatments (HF dip, NH₃ plasma) were then applied. Increasing dopant ratio raised carrier concentration (n) from 1.4 × 10²⁰ to 3.16 × 10²⁰ cm⁻³ while decreasing resistivity (ρ) and sheet resistance (R_s). Zone-dependent variations were evident across the 8-zone PECVD: Zone 4 generally exhibited lower R_s and higher μ, whereas Zone 8 lagged. Annealing improved crystallinity and electrical uniformity; NH₃ plasma, used here as a surface treatment on the poly-Si layer (not a full passivating-contact stack), produced a small increase in R_s that indicates the need for further plasma-parameter tuning for uniform electrical outcomes. Raman and ellipsometry confirmed crystallinity (X_c) and thickness distributions consistent with transport trends. This work focuses on layer-level optimization of n⁺ poly-Si; surface passivation quality and contact resistivity (ρ_c) of complete poly-Si/SiO_x stacks are not evaluated and will be addressed in future work. For electrical characterization, Hall parameters (n, μ) were extracted using glass-surrogate structures to avoid substrate conduction, and R_s was measured on wafers by four-point probe. The optimized layers are intended for rear-side electron-selective contacts in conventional n-type TOPCon, where optical penalties are modest; front-side use would require additional thickness minimization and optical re-optimization.