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 are compared to numerical simulations of hotcarrier 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.