Phonon mode behavior in strained wurtziteAlN∕GaNsuperlattices

Abstract

We have studied phonons in $\mathrm{Al}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ superlattices with different periods but a constant well-to-barrier ratio using a combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The strain evolution in the superlattice structures is assessed by high-resolution x-ray diffraction and reciprocal space mapping. We have identified ${E}_{1}(\mathrm{TO})$, ${A}_{1}(\mathrm{LO})$ and ${E}_{2}$ localized, and ${E}_{1}(\mathrm{LO})$ and ${A}_{1}(\mathrm{TO})$ delocalized superlattice modes. The dependencies of their frequencies on in-plane strain are analyzed and discussed, and the strain-free frequencies of the superlattice modes are estimated. A good agreement between theory and experiment is found in the case of GaN localized modes, while large deviations between theoretically estimated and experimentally determined frequency shifts are observed for the AlN localized modes. The delocalization effect on the ${A}_{1}(\mathrm{TO})$ and ${E}_{1}(\mathrm{LO})$ phonons, as well as the free-carrier effect on the ${E}_{1}(\mathrm{LO})$ phonon are also discussed.