Photoluminescence of planar and 3D InGaN/GaN LED structures excited with femtosecond laser pulses close to the damage threshold
We study the photoluminescence emission from planar and 3D InGaN/GaN LED structures, excited using a femtosecond laser with fluences close to sample’s damage threshold. For a typical laser system consisting of a titanium-sapphire regenerative amplifier, which is pumping an optical parametric amplifier delivering output pulses of a few tens of MW pulse power with ∼100 fs pulse duration, 1 kHz repetition rate and a wavelength of 325 nm, we determine the damage threshold of the InGaN/GaN LEDs to be about 0.05 J/cm2. We find that the relative intensity of the GaN photoluminescence (PL) and InGaN PL changes significantly close to the damage threshold. The changes are irreversible once the damage threshold is exceeded. As the damage threshold is approached, the InGaN luminescence band blue-shifts by several tens of meV, which is attributed to band filling effects. The PL decay time reduces substantially, by about 30%, when the excitation energy density is increased by approximately two orders of magnitude. The results are comparable for 2D and 3D LED structures, where in the latter case m-plane QWs exhibit different recombination dynamics because of the absence of the quantum confined Stark effect.