Effect of pressure on the Raman scattering of wurtzite AlN

High-pressure Raman spectra of wurtzite aluminum nitride were measured up to 25 GPa. The Raman-active modes in wurtzite-AlN were obtained up to the wurtzite-to-rocksalt transition at 20 GPa and their pressure coefficients compared with those obtained from ab initio lattice dynamics calculations. A slight decrease of the LO-TO splitting under pressure has been measured. We also report Raman scattering of the rocksalt phase of AlN during downstroke. The comparison of previously reported Raman scattering in rocksalt-GaN and ab initio calculations suggests that the Raman scattering we have measured in rocksalt-AlN likely corresponds to the one-phonon density of states of the rocksalt phase. A partial return to the wurtzite phase is found at ambient pressure after fully releasing pressure.