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

Vibrational spectra of self-assembled GaAs and AlAs islands grown by molecu lar-beam epitaxy on InAs (001) substrates have been investigated by Raman s pectroscopy. We observed large strain-induced-shifts of optical-phonon freq uencies in GaAs and AlAs islands with respect to the bulk materials. The va lues of shifts are 36 and 24 cm(-1) for CaAs longitudinal-optical (LO) and transverse-optical (TO) phonons and 55 and 28 cm(-1) for AlAs LO and TO pho nons, respectively. Comparison of experimental data with calculated optical -phonon frequencies in strained GaAs and AlAs islands of different shape de monstrates that the structures studied are coherently strained, i.e., they do not contain dislocations that could lead to strain relaxation. The featu res of interface phonons were observed in the polarized Raman spectra betwe en the peaks of InAs TO and LO phonons, The frequency positions of interfac e phonon lines are well described by the dielectric continuum model and als o give evidence of three-dimensional island formation. Doublers of folded a coustic phonons appear in the low-frequency region of Raman spect-ra of mul tilayer structures with GaAs and AlAs islands. These doublets are very simi lar to those typically observed in planar superlattices and well described by the elastic continuum model.