Temporary surface passivation for characterisation of bulk defects in silicon : a review

Accurate measurements of the bulk minority carrier lifetime in high-quality silicon materials is challenging due to the influence of surface recombination. Conventional surface passivation processes such as thermal oxidation or dielectric deposition often modify the bulk lifetime significantly before measurement. Temporary surface passivation processes at room or very low temperatures enable a more accurate measurement of the true bulk lifetime, as they limit thermal reconfiguration of bulk defects and minimize bulk hydrogenation. In this article we review the state-of-the-art for temporary passivation schemes, including liquid immersion passivation based upon acids, halogen-alcohols and benzyl-alcohols, and thin film passivation usually based on organic substances. We highlight how exceptional surface passivation (surface recombination velocity below 1 cm s−1) can be achieved by some types of temporary passivation. From an extensive review of available data in the literature, we find p-type silicon can be best passivated by hydrofluoric acid containing solutions, with superacid-based thin films showing a slight superiority in the n-type case. We review the practical considerations associated with temporary passivation, including sample cleaning, passivation activation, and stability. We highlight examples of how temporary passivation can assist in the development of improved silicon materials for photovoltaic applications, and provide an outlook for the future of the field