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

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We have measured photoluminescence (PL) and reflectance spectra of high-quality wurtzite GaN on sapphire at pressures up to 8 GPa $(T=10 \mathrm{K}).$ All three intrinsic exciton transitions arising from the $A,$ $B,$ and $C$ interband transitions were observed in reflectance measurements. The PL spectra are dominated by the $A$ and $B$ free exciton transitions and the recombination of an exciton bound to a neutral donor. Transitions due to the excited $n=2$ state of the $A$ exciton and the first- and second-order LO phonon replicas of the $A$ free exciton were also resolved. The pressure dependence of the $A$ 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.