Abstract
We have observed, for the first time, the confined LO m phonon modes in the ZnSe layer of the ZnSe-ZnTe strained-layer superlattice (SLS) and the folded longitudinal acoustic (LA) phonon modes in the ZnS-ZnSe SLS by off-resonance Raman scattering at room temperature. The relation between LO mode shifts and the superlattice structure parameters has been determined for the ZnSe-ZnTe SLS. We conclude that the critical thickness for the ZnSe-ZnTe SLS is about 40 Å. We have calculated the red shifts of LO phonon frequencies due to confinement and the shifts induced by the elastic strains, which are much larger than the red shifts due to confinement. The blue shifts induced by tensile strain in the ZnSe layer and the red shifts induced by compressive strain in the ZnTe layer made the confined LO modes in two individual layers overlap. Therefore, the folded LO mode had been observed in the ZnSe-ZnTe SLS. We have also studied the transverse optical (TO) phonon modes in the two SLS systems by means of far-infrared reflectivity spectra. ZnSe layers are under different stress in the two SLS systems.
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