The Influence of Phonons and Phonon Decay on the Optical Properties of GaN

The temperature dependences of vibrational and optical properties of high-quality GaN are studied using Raman and photoluminescence (PL) spectroscopies in the range 20 to 325 K. The Raman-active A$_{1}$(LO) phonon has temperature dependence described well by combined two- and three-phonon decay. The temperature dependences of E$_{2}^{2 }$phonon are almost entirely dominated by the thermal expansion, and the contribution of three-phonon decay process is very small throughout interested temperature range. The shallow neutral donor-bound exciton (D$^{0}$,X) and two free excitons (X$_{A}$ and X$_{B})$ are observed at low temperatrue PL spectra. Also seen are two A$_{1}$(LO) phonon sidebands (PSBs), originating from the X$_{B}$ free exciton, with the characteristic asymmetry attributed to interactions between discrete and continuum states. Analysis of the band-edge excitons reveals that energy gap shrinkage and exciton linewidths are completely described based on electron-phonon interactions with phonon properties consistent with the Raman analysis. First and second PSBs have temperature dependence associated with the A$_{1}$(LO) phonon. The shift, broadening, and asymmetry of the PSBs are explained by Segall-Mahan theory adding the decay mechanism of A$_{1}$(LO) phonon and the exciton broadening from electron-phonon interactions. Work at Texas Tech University supported by National Science Foundation grant ECS-0323640.