Femtosecond pump-probe transmission spectroscopy was performed at 10 K to study the nonequilibrium carrier dynamics in a GaN thin film for carrier densities ranging from $4\times {10}^{17}$ to ${10}^{19}{\mathrm{cm}}^{\mathrm{-}3}.$ Spectral hole burning was initially peaked roughly at the excitation energy for an estimated carrier density of $4\times {10}^{18}{\mathrm{cm}}^{\mathrm{-}3}$ and gradually redshifted during the excitation. Because of hot phonon effects, a very slow energy relaxation of the hot carriers at these densities was observed. The hot carriers were strongly confined in a nonthermal distribution and they relaxed collectively to the band edge for ∼1 ps. We observed remarkable persistence of the excitonic resonances in GaN at carrier densities well above the Mott density at early time delays, indicating that the excitons do not strongly couple to the nonthermal electron-hole plasma.

• Received 3 September 1999

DOI:https://doi.org/10.1103/PhysRevB.63.115315