Phonons in a strained hexagonal GaN–AlN superlattice

A GaN (6.3 nm)-AlN (5.1 nm) superlattice, grown by molecular beam epitaxy on a sapphire substrate and an AlN buffer layer, has been studied by means of micro-Raman spectroscopy. Most of the observed features have been identified and assigned to optical phonons of the superlattice layers. The average biaxial strain in GaN layers has been deduced from the detailed analysis of the frequency shift observed on the phonon lines. Additional measurements on the bevelled sample clearly suggest the significant increase of this strain for decreasing distances from the interface with the buffer layer.