Carrier recombination in m-plane GaN thin films

We report the ultrafast time-resolved photoluminescence (TRPL) study of m-plane GaN thin films grown on $\gamma $-LiAlO$_{\mathrm{2}}$ substrates by molecular beam epitaxy. The TRPL was measured by a time-correlated single-photon counting instrument with temporal resolution of 150 ps using laser pulses of energy 4.5 eV from a Ti:sapphire laser. Two major PL peaks were found in all the three GaN samples with different N/Ga ratios. The PL contributed by the bandgap recombination was found first blue-shift below 100 K and then red-shift as temperature increases. We found that the internal quantum efficiency as well as the nonradiative recombination rate decreased with N/Ga ratio may be due to the large defect concentration in high N/Ga ratio. The radiative recombination rate was constant below 100 K in all samples and was dependent on temperature with T.$^{\mathrm{-3/2}}$ The temperature dependence of radiative recombination time is consistent with theoretical prediction. The carrier localization, for both holes and electrons, is responsible for the blue-shift in PL spectra and constant of radiative rates below 100 K.