Temperature dependent photon emission spectra of free excitons in phonon   field of GaN
  Temperature-dependent radiative recombination of free excitons involving one or two LO phonons in GaN is investigated in detail. It is found that both phonon sidebands possess asymmetric lineshape and their energy spacings from the zero phonon line strongly deviate from the characteristic energy of LO phonons as the temperature increases. Furthermore, the deviation rates of one and two phonon sidebands are significantly different. Segall-Mahan theory, taking the exciton-photon and exciton-phonon interactions into account, is employed to calculate the sidebands of one or two LO phonons for free excitons in a wide temperature range. Excellent agreement between theory and experiment is achieved by using only one adjustable parameter (effective mass of free excitons). The obtained effective mass indicates that the free excitons in GaN are likely much lighter than estimated by available theoretical studies. 
S. J. Xu, G. Q. Li, S.-J. Xiong, and C. M. Che