Experimental evidence for phonon folding in compositional amorphous superlattices

Abstract

We give experimental evidence for the folding of LA-phonons in a-Si:H/a-SiN x:H compositional superlattices with periods ranging from 60 to 400 Å. We observe two peaks between 5 and 70 cm −1 in the Raman spectra of these samples which we identify with transitions into the second and third branch of the folded longitudinal acoustic phonon dispersion curve. The dispersion of these branches was determined by varying the superlattice period and the laser frequency. The result is well described by a continuum theory that takes the differences in density and sound velocity of a-Si:H and a-SiN x:H into account.