Professor Robert Taylor

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.

Abstract:

We present optical gain and loss spectra measured over a range of carrier densities at low temperature in hexagonal GaN epilayers. We have determined the optical loss directly to be ∼80 cm-1. Photoluminescence spectra show that stimulated emission in our samples arises from electron-hole plasma recombination. © 1998 American Institute of Physics.

Abstract:

We present time-, temperature- and intensity-dependent photoluminescence measurements of undoped, n-type and p-type GaN epilayers. In the nominally undoped samples we observe at low temperatures the trapping of free excitons by neutral donors and subsequent radiative recombination. In the n-type Si-doped samples bound-exciton luminescence is dominant over a wide range of temperatures. The luminescence from p-type Mg-doped samples is dominated by shallow-donor-shallow-acceptor pair recombination and by a deep blue centres at 3.0 eV. These two emission bands show identical temperature and linear intensity dependence.