Difference frequency Raman scattering and confined optical phonons in ultrashort-period (GaAs) n/(AlAs) n superlattices

Abstract

We present results of resonant Raman scattering on a series of ultrashort-period (GaAs) n /(AlAs) n superlattices with 1 ⩽ n ⩽ 5. The frequencies of confined longitudinal optical phonons of both GaAs and AlAs are compared with recent ab initio cakculations and mapped onto the bulk dispersion curve. The concept of unfolding is also found valid for ultrashort-period superlattices. The frequency dependence of the first confined phonon of GaAs and AlAs on layer thickness is well descibed by these ab initio calculations when interface disorder is taken into account. In all samples we observe second-order difference scattering from combinations of GaAs and AlAs optical phonons. The measurement of resonance profiles at various temperatures demonstrates that this scattering originates from second-order processes. The witdh and position of this difference scattering are explained by considering all possible combinations of the GaAs and AlAs longitudinal optical (LO), transverse optical (TO) and interface (IF) phonons.