Polariton effects in reflectance and emission spectra of homoepitaxial GaN

Reflectance and photoluminescence spectra of exciton polaritons in GaN homoepitaxial layers, grown by metalorganic chemical-vapor deposition on GaN single crystals, are reported. Polariton modes involving excitons $A,$ $B,$ and $C,$ which correspond to split valence bands of ${\Gamma }_9,$ ${\Gamma }_7,$ and ${\Gamma }_7$ symmetries, are resolved. Energies of the transverse excitons are found to be, respectively: $E_{\mathrm{TA}}=3.4767\mathrm{}\mathrm{eV},$ $E_{\mathrm{TB}}=3.4815\mathrm{}\mathrm{eV}$ and $E_{\mathrm{TC}}=3.4986\mathrm{}\mathrm{eV},$ at the temperature $T=1.8\mathrm{}\mathrm{K}.$ The shape of the measured emission spectra depends upon donor concentration. This is explained in terms of interbranch polariton scattering.