Work

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

Year: 2025

Type: article

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 PH 3 /SiH 4 gas ratio (1.2–4.0) was varied during PECVD deposition, followed by high-temperature annealing and selective NH 3 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, NH 3 plasma) were then applied. Increasing dopant ratio raised carrier concentration ( n ) from 1.4 × 10 20 to 3.16 × 10 20 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; NH 3 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.

Source: Results in Chemistry

DOI: 10.1016/j.rechem.2025.102726