SUBPICOSECOND ELECTRIC-FIELD DYNAMICS IN LOW-TEMPERATURE-GROWN GAAS OBSERVED BY REFLECTIVE ELECTROOPTIC SAMPLING

For the first time we investigate the subpicosecond electric field and coherent LO phonon dynamics in as-grown and post-growth th annealed low-temperature (LT) GaAs by reflective electro-optic sampling (REOS) with femtosecond time-resolution. This technique allows a direct observation of the screening of built-in surface fields after pulsed optical excitation of fret carriers and the built-up of an electric field due to the different mobilities of electrons and holes (photo Dember effect) in this high resistivity material. After trapping of mobile carriers the decay of the signal gives an accurate measure of the carrier lifetime in trapped states. The observed differences in annealed and as-grown LT GaAs are related to the microscopic form of excess arsenic and the density of point defects. The generation of coherent LO phonons allows the determination of the LO phonon frequency and lifetime in as-grown and annealed LT GaAS. Besides a red-shift of the LO phonon in as-grown LT GaAS we observe an additional local-mode below the LO frequency, which gives evidence for structural defects.