Delocalization of phonon-plasmon modes in GaAs/AlAs superlattices with tunnel-thin AlAs barriers

The technique of Raman spectroscopy has been used to investigate doped (n-type) and undoped GaAs/AlAs superlattices with AlAs barrier thicknesses from 17 to 1 monolayers. The peak corresponding to the scattering by a two-dimensional plasmon was found in the Raman spectrum of a doped superlattice with relatively thick barriers. The position of the experimental peak corresponded to the value calculated in the model of plasma oscillations in periodic planes of a two-dimensional electron gas. The electron tunneling effects played an increasingly prominent role as the AlAs barrier thickness decreased. The peaks corresponding to the scattering by coupled phonons with three-dimensional plasmons were found in the Raman spectra for a superlattice with an AlAs thickness of 2 monolayers; i.e., the delocalization of coupled modes was observed. In this case, the folding of acoustic phonons was observed in the superlattice under consideration, indicative of its good periodicity, while the localization of optical phonons in GaAs layers was observed in undoped superlattices with an AlAs thickness of 2 monolayers.