Homoepitaxial silicon growth in a non-ultra-high vacuum environment by ion-assisted deposition on Si wafer and seeded glass substrates

Abstract

Eliminating the requirement of ultra-high vacuum (UHV) conditions and achieving high-rate crystalline silicon (c-Si) growth are important targets for cheap mass production of semiconductor devices such as thin-film solar cells. In this paper we report on the achievement of high-quality, high-rate, epitaxial Si growth in a non-UHV environment by ion-assisted deposition (IAD) on (100) Si wafer substrates as well as on glass substrates featuring a polycrystalline Si seed layer. This was achieved by high-rate growth to suppress the contamination in the growing Si film and by a sacrificial protective layer which protects the initial growth surface prior to start of epitaxy. On (100) Si wafer substrates good structural quality of the epitaxially grown Si films was achieved, resulting in 1-Sun open-circuit voltages of up to 550 mV. On planar seeded glass substrates the heating procedure had to be modified to take the large thermal mass and the thermal stress caused by the underlying glass substrate into account. It is shown that the pre-heating phase must be extended by more than a factor of 4 and that high growth temperatures have to be avoided to minimise thermal stress. Taking these considerations into account and using two post-deposition treatments (rapid thermal annealing and hydrogenation), open-circuit voltages of 420 mV and short circuit-current densities of about 10 mA/cm 2 under 1-Sun illumination are achieved on glass substrates.