Finite-size effects on Raman scattering from GaAs-AlAs superlattices

Abstract

We have investigated finite-size (total number of periods) effects on Raman scattering by longitudinal acoustic (LA) and longitudinal optic (LO) phonon modes in GaAs-AlAs superlattices. Every superlattice has the same period (10-monolayer GaAs and 10-monolayer AlAs), while the total number of periods ranges from 3 to 100 periods. Even in the superlattice with three periods, folded LA phonon modes are induced by the superlattice period. Raman linewidths of folded LA phonon modes increase as the total number of periods decreases. The Raman linewidth broadening becomes remarkable below 20 periods. The dependence of linewidth broadening on the total number of periods results from the relaxation of the Raman selection rule for phonon wave vectors induced by the finiteness of superlattices. On the other hand, for LO phonon modes the finite size does not affect Raman line shapes; this originates in the confinement of LO phonon modes in individual layers.