Professor Robert Taylor

Abstract:

Hot carrier relaxation has been studied in GaN using femtosecond time-resolved optical transmission and reflection. A non-thermal electron distribution with a pronounced `cut-off' at ELO is observed for up to approximately 3 ps after photoexcitation. Monte Carlo simulations show that this behaviour is due to a remarkably strong electron-LO phonon interaction. Excitonic effects are also pronounced, and strongly influence the dynamics.

Abstract:

Hot carrier relaxation has been studied in GaN using femtosecond time-resolved optical transmission and reflection. A non-thermal electron distribution with a pronounced "cut-off" at E-LO is observed for up to similar to 3 ps after photoexcitation. Monte Carlo simulations show that this behaviour is due to a remarkably strong electron-LO phonon interaction. Excitonic effects are also pronounced and strongly influence the dynamics. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

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.