Transport properties of amorphous hydrogenated silicon–carbon alloys

Abstract

We measure the electron and hole drift mobility and the recombination lifetime in hydrogenated amorphous silicon–carbon alloys by using the moving photocarrier grating (MPG) technique. Samples have been prepared by plasma enhanced chemical vapor deposition of methane/silane mixtures under 90% hydrogen 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 carbon (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 gap of the samples is larger than ∼1.95 eV. Both electron and hole drift mobilities are correlated with the Urbach energy. We find an increase 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 increase in the gap decreases the recombination lifetime, due to an increase in the density of defects acting as recombination centers.