Time-resolved reflectivity characterization of polycrystalline low-temperature-grown GaAs

Using femtosecond time resolved reflectivity, we have characterized the dyn amics of photoinduced generated carriers in a polycrystalline low-temperatu re-grown GaAs sample. Our measurements are fitted with an analytical expres sion reliable for low pump power experiments. The sample, which presents no As precipitates, shows an ultrafast subpicosecond response together with a longer picosecond tail that we attribute to the midgap defect states. More over, we have observed the influence of surface roughness on the differenti al reflected signal. (C) 1999 American Institute of Physics. [S0003-6951(99 )04817-2].