Direct determination of free exciton binding energy from phonon-assisted luminescence spectra in GaN epilayers

Investigation of variable-temperature longitudinal optical (LO) phonon-assisted luminescence spectra of free excitons and free electrons in heteroepitaxial GaN thin films has been conducted. It is found that thermal broadening of the LO phonon-assisted photoluminescence peaks is much slower than those of the peaks of their parents so that the first-order LO peaks of the free exciton transition and the band-to-band transition can be well resolved even at room temperature, leading to a direct determination of the band A free exciton binding energy as 25.4±0.9 meV. At the same time, we demonstrate that the simple hydrogenlike model still is a good approximation to describe the energy level structure of free excitons in GaN.