Photoluminescence and reflectance studies of exciton transitions in wurtzite GaN under pressure

We have measured photoluminescence (PL) and reflectance spectra of high-quality wurtzite GaN on sapphire at pressures up to 8 GPa (Formula presented) All three intrinsic exciton transitions arising from the (Formula presented) and (Formula presented) interband transitions were observed in reflectance measurements. The PL spectra are dominated by the (Formula presented) and (Formula presented) free exciton transitions and the recombination of an exciton bound to a neutral donor. Transitions due to the excited (Formula presented) state of the (Formula presented) exciton and the first- and second-order LO phonon replicas of the (Formula presented) free exciton were also resolved. The pressure dependence of the (Formula presented) band gap and exciton binding energy is obtained from the intrinsic exciton energies. The experimental results clearly reveal a sample-dependent change of the biaxial strain in the GaN layers with increasing hydrostatic pressure.