Spectral dependence of time-resolved photoreflectance of low-temperature-grown GaAs

Time-resolved photoreflectance (PR) traces taken from a low-temperature-grown (LT-)GaAs were observed to exhibit strong dependence on the laser wavelength as well as the growth condition and the annealing process to which it was subjected. The change in reflectance changes sign from negative to positive as the laser wavelength decreases across a certain crossover wavelength. Postgrowth annealing of LT-GaAs was found to result in blue shift in this crossover wavelength. The long tails of the PR traces can be explained using a simple model calculation which takes into account band filling, band-gap renormalization, and free-carrier absorption effects. The PR in the first few picoseconds was found to be dominated by hot-carrier effects. By decomposing the PR traces, 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 photoinduced carriers by photoreflectance technique under similar experimental conditions.