Transport properties of amorphous hydrogenated silicon-carbon alloys

We measure the electron and hole drift mobility and the recombination lifet ime in hydrogenated amorphous silicon-carbon alloys by using the moving pho tocarrier grating (MPG) technique. Samples have been prepared by plasma enh anced chemical vapor deposition of methane/silane mixtures under 90% hydrog en dilution. The optical gap of the samples varies between 1.72 and 2.09 eV . We find a decrease in the electron drift mobility when small amounts of c arbon (leading to optical gaps <1.95 eV) are incorporated into the silicon matrix, and a saturation when the carbon content is such that the optical g ap of the samples is larger than similar to 1.95 eV. Both electron and hole drift mobilities are correlated with the Urbach energy. We find an increas e in the recombination lifetime when the optical gap increases from 1.72 to 1.85 eV, which we assign to a widening of the band tails. A further increa se in the gap decreases the recombination lifetime, due to an increase in t he density of defects acting as recombination centers. (C) 2000 Elsevier Sc ience B.V. All rights reserved.