Raman Scattering Study of Ultrathin Si-Ge Superlattices

We present a Raman scattering study of CdTe-ZnTe superlattices. The Raman spectra show confined and interface optical phonons. The strain configuration of the samples can be deduced from the frequency shifts of the Raman phonons with respect to unstrained samples. For CdTe thicknesses d1 ~50A and ZnTe thicknesses d2 ~70A, our results are consistent with a free standing configuration, as previously proposed. However, for d1 ~20A, d2 ~28A, we find the surprising result that the CdTe layers can elastically accommodate a lattice mismatch as high as 5%. In resonance with the fundamental gap of the superlattices, we observe an enhancement of the Raman cross section not only for CdTe phonons, but also (in the samples with the thinnest layers) for phonons confined in ZnTe. This behavior suggests that the hole wavefunctions are not well localized in the CdTe layers and can couple to ZnTe phonons.