Excitonic absorption of GaN epilayers on sapphire: Dynamics, intensity, and temperature dependence

Using test-pump measurements, we studied the excitonic absorption of GaN ep ilayers on sapphire substrate under nanosecond and picosecond pulsed excita tion conditions. We measured a short exciton lifetime of 25 ps, which is co nsistent with results obtained from degenerate four-wave mixing measurement s. Since the radiative lifetime should be much longer, this short character istic time shows the dominance of nonradiative processes. Under nonresonant excitation condition, we observed an intensity dependent shift and damping of the exciton resonance, attributed to an inhomogeneous temperature distr ibution. This thermal effect was not observed under resonant excitation con ditions. This behavior is tentatively explained by the presence of shallow centers that have a rather long lifetime and recombine nonradiatively. [S01 63-1829(99)10203-0].