Miniband effect on optical vibrations in short-period InxGa1−xAs∕InP superlattices

The formation of the miniband electron energy structure and its effect on optical vibrational modes were explored in doped InxGa1−xAs∕InP superlattices with different periods. The analysis of the high resolution x-ray diffraction, Raman and magnetotransport data allowed us to conclude that in spite of the defect structure of the layers constituting the superlattices, their superperiodicity was well defined. The blueshift of the coupled plasmon-LO phonon modes was observed with decreasing superlattice period consistent with the development of the minibands. The coherence lengths of the coupled modes were found to be considerably longer than those of the optical phonons. This provided a quantitative proof of the conditions for breakdown of the Raman selection rules. Due to the defect structure of the layers no Raman selection rules were observed for the longitudinal optical phonons in long-period superlattices. In contrast, the selection rules of the coupled plasmon-longitudinal optical phonon vibrations observed in short-period superlattices were demonstrated to occur due to the increase of the coherence lengths of the coupled modes with respect to the coherence lengths of the optical phonons.