Photoreflectance investigation of exciton-acoustic phonon scattering in GaN grown by MOVPE

• Photoreflectance was used to investigate the optical properties of free exciton in GaN.
• A strong correlation between electron concentration and exciton linewidths is observed.
• The exciton-acoustic phonon coupling is strongly related to the exciton kinetic energy and to the strain level.

In this paper, we report a systematic investigation of the near band edge (NBE) excitonic states in GaN using low temperature photoluminescence (PL) and photoreflectance (PR) measurements. For this purpose, GaN films of different thicknesses have been grown on silicon nitride (SiN) treated c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Low temperature PR spectra exhibit well-defined spectral features related to the A, B and C free excitons denoted by FXA FXB and FXC, respectively. In contrast, PL spectra are essentially dominated by the A free and donor bound excitons. By combining PR spectra and Hall measurements a strong correlation between residual electron concentration and exciton linewidths is observed. From the temperature dependence of the excitonic linewidths, the exciton-acoustic phonon coupling constant is determined for FXA, FXB and FXC. We show that this coupling constant is strongly related to the exciton kinetic energy and to the strain level.

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