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

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 improvem ents in the structural and electronic qualities of intrinsic nc-Si:H grown from hot-wire chemical vapour deposition. For examining a wide range of dep osition 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 > direct ion over a wide range of deposition conditions. General considerations on t he orientation and electronic activity of grain boundaries in polycrystalli ne silicon explain why the electronic quality of this < 110 > -oriented fil m is remarkably higher than the one previously grown, mixed-phase nanocryst alline silicon. Mobility-lifetime products of films from our novel filament s are two orders of magnitude higher than those of samples from Ta wires. I n photoluminescence spectra, no band tail contributions occur, and the amor phous and defect peaks are greatly reduced. Moreover, the transverse optica l Raman signal is red-shifted, and thereby indicates a reduction in mechani cal strain in our novel nanocrystalline silicon films. (C) 2001 Elsevier Sc ience B.V. All rights reserved.