Silicon surface passivation of industrial n-type CZ Si (111) by Al₂O₃ layers deposited by thermal ALD process for application in carrier selective contact solar cells

c-Si based carrier selective contact solar cells are achieving high efficiency with a crucial step of silicon surface passivation, reducing the electronic recombination losses occurring at the interface of Si and the passivation layer. The ultrathin passivation layers of Al2O3 are deposited by atomic layer deposition (ALD), known for its conformance and homogeneity. ALD is an attractive technique for low-temperature deposition of layers required for the development of c-Si carrier selective contacts and passivation. The deposition temperature is fixed at 200 degrees C, which lies in the ALD window for Al2O3 films. In this study, deposition of Al2O3 films is done by thermal ALD process where the growth per cycle of the film is calculated to be 0.1 +/- 0.01 nm using spectroscopic ellipsometry. Al2O3 films were deposited on n-type CZ Si (111) (2 omega cm) wafers of thickness 170 mu m after processing with saw damage removal, standard RCA clean, and HF dip. Post-deposition annealing was done in forming gas environments at various temperatures to probe the passivation quality. A monotonic improvement was obtained with annealing in such an environment, reaching an excellent lifetime of about 1.24 ms (measured by WCT-120 Sinton lifetime tester) at a minority carrier concentration of 1 x 10(15) cm(-3) for samples annealed in forming gas environment at 310 degrees C. This is an excellent value for an industrial-type CZ wafer with a measured bulk lifetime of only similar to 3 ms. The corresponding effective surface recombination velocity obtained is 3.7 cm/s. An implied open-circuit voltage (iV(OC)) of 0.704 V is achieved for the same. These results demonstrate that the passivation obtained here is of device quality for CZ Si wafers and facilitates the development of high-efficiency Si heterojunction (SHJ) solar cells.