Raman scattering by optical phonons in isotopic 70(Ge)n74(Ge)n superlattices.

Abstract

We present Raman spectra of a series of isotopic $^{70}$(Ge${)}_{\mathit{n}}^{74}$(Ge${)}_{\mathit{n}}$ superlattices with 2\ensuremath{\le}n\ensuremath{\le}32. In these novel structures only the phonons are subject to a periodic isotopic mass modulation, whereas the electrons retain their bulklike character. This differs substantially from conventional superlattices where both electronic and vibronic properties are affected by the reduction in dimensionality. Our results show clear evidence for optic phonon confinement in each of the two superlattice constituents. Observed frequencies, relative intensities, and mode mixing compare favorably with calculations based on a planar bond-charge model and the bond polarizability approach.