Abstract
Degenerate pump-probe photoreflectance (PR) was measured on as-grown and in situ annealed low-temperature-grown GaAs at photon energies well-above bandgap (1.54-1.71 eV) to investigate the effects of annealing and excitation energies on the initial carrier relaxation. The change in reflectivity was found to depend strongly on the photon energy, especially for the annealed sample. This dependence is attributed to the combined effect of absorption bleaching and enhanced absorption related to excess arsenic. A three-component decomposition procedure was used to analyze all the measured PR traces. With this procedure, we found that the initial carrier relaxation time was nearly a constant 160 fs for the as-grown sample, indicating that trapping would dominate the relaxation for excitation energies as high as 1.71 eV. For the annealed sample, the relaxation time increased slowly with the laser wavelength. After comparing it to the carrier trapping time obtained from photoconductive response, we conclude that the relaxation is dominated by trapping for excitation energies up to 1.58eV and by both trapping and cooling for excitation energies larger than 1.62 eV.