The gettering capacity of cavities towards metals is exploited in order to purify mc-Si wafers. The purification sequence includes: i) cavity creation on the wafer surface by plasma immersion ion implantation (PIII) of hydrogen; ii) hightemperature annealing to promote cavity growth and trapping of metal impurities by chemisorption at the cavity surface and in the cavities; iii) etching of the metal-rich cavity layer (gettering zone). The formation of cavities and the trapping of metal impurities are studied by secondary ion mass spectroscopy and transmission electron microscopy. The electrical qualities of the resulting wafer substrates and the minority carrier lifetime are checked by microwave photoconductive decay after the surface de-capping (stage iii). The procedure is finally validated by manufacturing heterojunction solar cells. The proposed procedure proved to be effective for a substantial reduction of Cu and Ni impurities, for which the chemisorption is favored thanks to their high binding energy with the cavities. The critical role of the annealing temperature and time on impurity detrapping, as well as the importance of a low cooling rate emerged as key process parameters. The industrial feasibility is ensured by the application of PIII, a plasma-based implantation technique that allows to process large surfaces (30× 30 cm2) very rapidly (2 minutes of processing).
1 Jan 2013
Proceedings of the 28th EU PVSEC