Optical vibrational modes of ZnSe-ZnSxSe

Abstract

Resonant-Raman-scattering experiments were carried out at low temperatures to study the optical lattice modes of ZnSe-${\mathrm{ZnS}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ strained-layer superlattices, which were grown by molecular-beam epitaxy on (100) surfaces of GaAs substrates. The observed Raman modes can be classified into two groups: One group corresponds to vibrations with amplitudes localized either in the ZnSe or in the ${\mathrm{ZnS}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ layers. These are the confined modes. In the second group, the phonons with amplitudes in both layers are included, namely, interface vibrations and folded optical modes. The measured dependence of the confined and delocalized phonons on sample parameters (individual layer thicknesses, superlattice period, total superlattice thickness, and interfacial strain) and on resonant excitation is presented and discussed in detail. The trends that were established are compared with the theoretical predictions of lattice-dynamic models for multilayer structures. Experimental data on the lattice-dynamic properties of ${\mathrm{ZnS}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ alloys are included for comparison purposes and to unequivocally distinguish superlattice effects. These data were gathered with ternary layers grown and measured under similar conditions as the superlattices. Concomitant with vibrational characteristics, this work has also yielded an insight into other intrinsic properties of strained-layer superlattices. The interplay between confinement and strain in the renormalization of phonon frequencies was elucidated from an experimental point of view and this information was applied to characterize the nature of the superlattice transition from pseudomorphic to free standing.