Raman study of self-assembled GaAs and AlAs islands embedded in InAs

Vibrational spectra of self-assembled GaAs and AlAs islands grown by molecular-beam epitaxy on InAs (001) substrates have been investigated by Raman spectroscopy. We observed large strain-induced shifts of optical-phonon frequencies in GaAs and AlAs islands with respect to the bulk materials. The values of shifts are 36 and $24 {\mathrm{cm}}^{\ensuremath{-}1}$ for GaAs longitudinal-optical (LO) and transverse-optical (TO) phonons and 55 and $28 {\mathrm{cm}}^{\ensuremath{-}1}$ for AlAs LO and TO phonons, respectively. Comparison of experimental data with calculated optical-phonon frequencies in strained GaAs and AlAs islands of different shape demonstrates that the structures studied are coherently strained, i.e., they do not contain dislocations that could lead to strain relaxation. The features of interface phonons were observed in the polarized Raman spectra between the peaks of InAs TO and LO phonons. The frequency positions of interface phonon lines are well described by the dielectric continuum model and also give evidence of three-dimensional island formation. Doublets of folded acoustic phonons appear in the low-frequency region of Raman spectra of multilayer structures with GaAs and AlAs islands. These doublets are very similar to those typically observed in planar superlattices and well described by the elastic continuum model.