Dynamics of Below-Band-Gap Carrier in Highly Excited GaN

Femtosecond time-resolved reflectivity was used to investigate below-band-gap (3.1 eV) carrier dynamics in a nominally undoped GaN epilayer under high excitation. A 2.5-ps rising process can be observed in the transient trace. This shot rising time results from the hot phonon effects which can cause a delayed energy relaxation of the initial photocarriers toward the band edge. From the density dependence of the carrier dynamics, the Mott density was estimated to be 1.51-1.56 x 10¹⁹cm^-3. Below the Mott density, the initial probed carrier dynamics was explained to the effect of acoustic phonon-assisted tunneling for localized states, where a significant excitation density dependence of the tunneling probability was observed due to the optically-induced bandtail extension to lower energies. Above the Mott density, the measured carrier dynamics reflected the relaxation of an electron-hole plasma, in which a distinct fast decay component of 2.3 ps was observed due to the onset of nonlinear relaxation processes such Auger recombination.