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

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Abstract
Using test-pump measurements, we studied the excitonic absorption of GaN epilayers on sapphire substrate under nanosecond and picosecond pulsed excitation conditions. We measured a short exciton lifetime of 25 ps, which is consistent with results obtained from degenerate four-wave mixing measurements. Since the radiative lifetime should be much longer, this short characteristic 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 distribution. This thermal effect was not observed under resonant excitation conditions. This behavior is tentatively explained by the presence of shallow centers that have a rather long lifetime and recombine nonradiatively.