ABSTRACT
We present a Raman study of the phonon spectra of periodical structures with (In,Ga)As self-assembled semiconductor quantum dots in (Al,Ga)As matrix as well as AlAs quantum dots embedded in InAs. The nanostructures studied were grown by molecular beam epitaxy in the Stranski-Krastanov growth mode on GaAs substrates.
Low frequency resonant Raman scattering by acoustic phonons was observed in (In,Ga)As/(Al, Ga)As quantum dot (QD) superlattices. The periodic oscillations seen in the Raman spectra can be described by the Raman scattering interference model. The features occur when acoustic phonons interact with the localized electronic states in the QDs.
Raman scattering by confined TO and longitudinal optical (LO) phonons in InAs and AlAs QDs was observed in both AlAs/InAs and (In,Ga)As/(Al,Ga)As QD structures. Optical phonons of QDs are strongly affected by both strain and confinement. The Raman study reveals a two-mode behavior of optical phonons in the whole composition range for both InGaAs QDs and the AlGaAs matrix. Interface (IF) InAs- and GaAs-like phonons as well as AlAs- and GaAs-like phonons were observed in InGaAs QDs and the AlGaAs matrix, respectively. Their frequency positions were analysed as a function of the alloy content within the dielectric continuum model. The positions of IF phonons in the QD structures observed in the experiment agree well with calculated ones assuming the QDs have a shape of oblate ellipsoids.
Calculations also show the dependence of the IF band positions on the QD shape, which allows using the Raman spectroscopy as an additional tool to determine the preferential shape of quantum dots.
