Crystalline silicon surface passivation by intrinsic silicon thin films deposited by low-frequency inductively coupled plasma

Abstract

Amorphous and microcrystal hydrogenated intrinsic silicon (a-Si:H/μc-Si:H) thin films with good silicon surface passivation effect were deposited using a precursor gases of silane and hydrogen, which were discharged by low frequency inductively coupled high density plasma source. With regard to silicon surface passivation, the effect of discharge power on thin films properties, including the optical band gap, the crystal fraction, and bond configuration, as well as the deposition rate were thoroughly investigated. It was found that the best passivation effect was obtained at the region near the transition regime from a-Si:H to μc-Si:H with a minimized incubation layer between the passivation layer and substrate. Cz-silicon wafer passivated by as-deposited μc-Si:H thin films without any post-deposition thermal annealing possesses minority carrier lifetime of about 234 μs. This is attributed to the chemical annealing from the high-density hydrogen plasma during the deposition process. Subsequent thermal annealing in hydrogen flow increased the lifetime to 524 μs with a suppressed maximum surface recombination velocity of as low as 60 cm/s. Throughout the process flow covering the pre-deposition H plasma treatment, the film deposition from H2 diluted feedstock gases and the post-deposition annealing, hydrogen plays a vital role to enhance the minority carrier lifetime by improving the interface properties. The injection level dependent surface recombination velocity was also extracted from the lifetime measurement. The effectivity of the a-Si:H/μc-Si:H for silicon surface passivation in a practical heterojunction solar cell was further validated by the excellent photovoltaic performance.