WELL-ABOVE BANDGAP TRANSIENT PHOTOREFLECTANCE CHARACTERIZATION OF LOW-TEMPERATURE-GROWN GAAS

Degenerate pump-probe photoreflectance (PR) was measured on as-grown a nd 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, esp ecially for the annealed sample. This dependence is attributed to the combined effect of absorption bleaching and enhanced absorption relate d to excess arsenic. A three-component decomposition procedure was use d 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 anneale d sample, the relaxation time increased slowly with the laser waveleng th. After comparing it to the carrier trapping time obtained from phot oconductive response, we conclude that the relaxation is dominated by trapping for excitation energies up to 1.58 eV and by both trapping an d cooling for excitation energies larger than 1.62 eV.