Vibrational spectroscopy of InGaAs/AlGaAs quantum dot superlattices

We present a study of the phonon spectrum of self-assembled InGaAs/AlGaAs quantum dot (QD) superlattices grown by molecular beam epitaxy. Raman scattering and IR absorption by confined optical phonons in InGaAs and AlAs QDs were observed for InGaAs/AlGaAs QD structures. Optical phonons of QDs are strongly affected by both strain and confinement. IR spectra of the QD structures are described within the model of the effective dielectric function using Bruggeman approximation. Raman study reveals a two-mode behavior of optical phonons in the whole composition range for both InGaAs QDs and the AlGaAs matrix. An analysis of the Raman spectra of InAs QDs embedded in an AlxGa1-x As matrix allows determination of Ga incorporation in the QDs. Interface InAs- and GaAs-like phonons as well as AlAs- and GaAs-like phonons were also observed in InGaAs QDs and the AlGaAs matrix, respectively. Their frequency positions agree well with the calculated ones within the dielectric continuum model. Low frequency resonant Raman scattering by acoustic phonons was observed in InGaAs/AlGaAs QD superlattices. The periodic oscillations seen in the Raman spectra are well described by the elastic continuum model. (authors)