Optical phonons in InAs/AlAs structures with InAs and AlAs nanocrystals

Layered InAs/AlAs structures with InAs and AlAs nanocrystals (NCs) formed by Stranski-Krastanov growth on (001)-oriented GaAs substrates were investigated by Raman spectroscopy. Selection rules for the optical phonons in the structures were studied in a backscattering geometry from planar (001) and cleaved (110) surfaces using a micro-Raman setup. Transverse and longitudinal optical (TO and LO) phonons as well as interface (IF) phonons were observed for InAs (AlAs) NCs in AlAs (InAs) matrices. Optical phonon frequencies of InAs (AlAs) NCs are upshifted (downshifted) compared to the ones of the corresponding bulk materials. These shifts result from compressive (tensile) built-in strain in the NCs. The frequencies of optical phonons were used to determine deformationtensor components. It was found that optical phonons of InAs NCs obey Raman scattering selection rules for a superlattice consisting of materials with zinc-blende structure. However the selection rules are weakened since all the optical phonon peaks were detected in forbidden geometries (z(x,x) anti z and y'(z,z) anti y', where x, z and y' stand for [100], [001] and [110], respectively). Meanwhile, the selection rules for optical phonons of AlAs NCs are lifted. This effect can be caused by built-in strain and defects.