Boron activation in silicon thin films grown by PECVD under epitaxial and microcrystalline conditions

Abstract

Boron doping level and boron activation in silicon thin films grown by PECVD under epitaxial (p+ epi‑Si) and microcrystalline (p+ µc-Si) conditions have been investigated as functions of the substrate material and annealing in the range of 200°C – 300°C. Hall effect measurements show that in the as-deposited state, the conductivity is mainly governed by the carrier concentration, while the hole mobility is controlled by the crystalline quality. SIMS measurements reveal that dark conductivity is not directly proportional to the boron doping level, nor to the presence of B-H complexes, suggesting that carbon contamination and the formation of B-O complexes could play an important role in the electrical properties of the material. Annealing in air resulted in an increase in the dark conductivity for all samples. However, the increase was much higher (up to two orders of magnitude) for the samples deposited on silicon-on-insulator (SOI) substrates which have better crystalline quality and displayed the highest increase in hole carrier concentration, related to boron activation. Thus, the substrate material influences both the crystalline quality and the boron incorporation in the p-type material. At high boron incorporation, the mobility limitations are likely to be due to carrier concentration rather than to the crystalline quality, indicating that p+ µc-Si conditions allow a conductivity advantage over p+ epi‑Si conditions due to a higher doping level, which makes them suitable as hole-selective contacts in solar cells.