Professor Robert Taylor

Abstract:

We present resonant femtosecond pump-probe reflectance measurements of excitons in high quality wurtzite GaN epilayers for a range of lattice temperatures and various pump intensities. From the density dependence of the excitonic bleaching we find that the Mott density is n(Mott) less than or equal to 2.2 x 10(19) cm(-3). At 4 K we find that the exciton dynamics is dominated by trapping at defects via acoustic phonon emission on a timescale of similar to 16 ps. At temperatures above 60 K we observe a much longer relaxation component of similar to 350 to 400 ps, which we ascribe to the radiative recombination of free excitons.

Abstract:

Hot carrier relaxation has been investigated in GaN using non-resonant femtosecond excitation where electrons are excited above and below the LO phonon energy. We observe remarkably rapid electron relaxation; the electron distribution is non-thermal, with a "cut-off" occurring near E-LO. In a preliminary Monte Carlo study we have simulations which reproduce the major experimental observations, confirming the development of a non-thermal electron distribution near E-LO due to the strong electron-LO phonon interaction. Hot phonon effects are pronounced at high carrier densities.

Abstract:

Measurements of edge emission on a GaN epilayer under N-2 laser excitation demonstrate stimulated emission and gain. The mechanisms involved are investigated by using laser pulses of 250 fs duration to measure simultaneously the photoluminescence and the time-resolved reflectance near the band edge, over a range of excitation densities. The excitons are bleached at high densities due to free carriers. A broad luminescence band is seen, extending similar to 100 meV below the low-density emission peak. We show that the onset of the stimulated emission coincides with the bleaching of the excitons (the Mott transition).

Abstract:

Hot carrier relaxation has been investigated in GaN using non-resonant femtosecond excitation where electrons are excited above and below the LO phonon energy. We observe remarkably rapid electron relaxation; the electron distribution is non-thermal, with a "cut-off" occurring near ELO. In a preliminary Monte Carlo study we have simulations which reproduce the major experimental observations, confirming the development of a non-thermal electron distribution near ELO due to the strong electron-LO phonon interaction. Hot phonon effects are pronounced at high carrier densities.