Confined transverse-optical phonons in ultrathin CdTe/ZnTe superlattices

We report far-infrared reflectivity measurements on short-period (CdTe${)}_{\mathit{n}}$/(ZnTe${)}_{\mathit{n}}$ (n=2,3,4,5) superlattices. The superlattices were grown by atomic-layer epitaxy on a thick ZnTe buffer deposited on [001]-oriented GaAs substrates. The frequencies of transverse-optical phonon modes confined in the CdTe layers decrease systematically with increasing number of monolayers, n, whereas the corresponding confined modes in the ZnTe layers do not depend on n. It is shown that these data allow one to map out the dispersion of the transverse-optical modes along the [001] direction in both CdTe and ZnTe. Furthermore, the strain distribution in the superlattices is analyzed.