Abstract
Time-resolved photoreflectance (PR) and photo-transmittance (PT) of a GaAs layer grown on r-cut sapphire substrate at 220oC were measured. Similar to our previous observation from LT-GaAs grown on semi-insulating GaAs substrates, the PR traces show strong spectral dependence and can be well fitted by the sum of a fast positive, a fast negative and a slow positive component, each being calculated by convolving a single-sided exponential function with the measured laser autocorrelation. The fitting parameters indicate that the spectral dependence results mainly from the change in amplitude of the negative component. Since the LT-GaAs grown on sapphire is amorphous, as determined by X-ray diffraction, we conclude that the negative component does not come from the long-range crystallinity of the sample. The time constant of the fast positive component is consistent with the interpretation that the carrier trapping dominates the relaxation in the first few picoseconds. The implications will be discussed.