Resonant quasiconfined optical phonons in semiconductor superlattices

We point out that resonant phonon modes with quasiconfined behavior may arise in semiconductor superlattices in the continuum frequency range, i.e., where both constituents have allowed bulk frequencies with real wave vector. With reference to the cases of Si/Ge and InAs/GaSb (001) superlattices, we show that such modes appear close to the edge of overlapping optical frequencies, with displacement patterns and Raman strengths comparable to those of true confined modes. However their degree of confinement and their actual number and frequency location are found to be more sensitive to the adjacent layer and to the details of the interfaces. Their study can therefore yield additional structural information on the interface region with respect to the study of true confined modes.