SUBPICOSECOND CARRIER TRANSPORT IN GAAS SURFACE-SPACE-CHARGE FIELDS

Above-band-gap pulsed optical excitation of electron-hole pairs within the surface-space-charge region of semiconductors alters the surface-space-charge field via free-carrier transport. We report on the direct observation of this ultrafast transient screening and the associated charge-carrier transport by applying reflective electro-optic sampling (REOS) with subpicosecond time resolution to (100)-oriented GaAs surfaces. The REOS measurements performed under different initial surface field conditions and various optical excitation densities axe compared to numerical simulations of hot-carrier transport, including the calculation of the optical response. The simulations, which are based on a simple drift-diffusion model for optically excited electron-hole pairs, are in quantitative agreement with the experiment. The strength and sign of the static built-in field can be determined and the carrier drift velocities can be derived on a subpicosecond time scale.