Nanocrystalline silicon from hot-wire deposition — a photovoltaic material?

Abstract

Nanocrystalline silicon (nc-Si:H) attracts a great deal of attention due to the hope for more efficient and stable solar cells, as well as better thin-film transistors and optical sensors. In this study, we report on improvements in the structural and electronic qualities of intrinsic nc-Si:H grown from hot-wire chemical vapour deposition. For examining a wide range of deposition parameters, we use a design-of-experiments approach. In contrast to our previous films obtained from tungsten and tantalum filaments, a novel type of filament greatly enhances preferential growth in the 〈110〉 direction over a wide range of deposition conditions. General considerations on the orientation and electronic activity of grain boundaries in polycrystalline silicon explain why the electronic quality of this 〈110〉-oriented film is remarkably higher than the one previously grown, mixed-phase nanocrystalline silicon. Mobility–lifetime products of films from our novel filaments are two orders of magnitude higher than those of samples from Ta wires. In photoluminescence spectra, no band tail contributions occur, and the amorphous and defect peaks are greatly reduced. Moreover, the transverse optical Raman signal is red-shifted, and thereby indicates a reduction in mechanical strain in our novel nanocrystalline silicon films.