SPECTRAL DEPENDENCE OF TIME-RESOLVED PHOTOREFLECTANCE OF LOW-TEMPERATURE-GROWN GAAS

Time-resolved photoreflectance (PR) traces taken from a low-temperatur e-grown (LT-)GaAs were observed to exhibit strong dependence on the la ser wavelength as well as the growth condition and the annealing proce ss to which it was subjected. The change in reflectance changes sign f rom negative to positive as the laser wavelength decreases across a ce rtain crossover wavelength. Postgrowth annealing of LT-GaAs was found to result in blue shift in this crossover wavelength. The long tails o f the PR traces can be explained using a simple model calculation whic h takes into account band filling, band-gap renormalization, and free- carrier absorption effects. The PR in the first few picoseconds was fo und to be dominated by hot-carrier effects. By decomposing the PR trac es, an extra enhanced optical absorption besides the scattering of the carriers off their initial states was observed. Our results indicate that caution must be taken when measuring the lifetime of the photoind uced carriers by photoreflectance technique under similar experimental conditions.