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 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. I. INTRODUCTION The transport dynamics of optically generated free electron-hole pairs in semiconductors and the associated screening of electrical fields are of great importance for the performance of high speed optoelectronic devices. Clear data on hot-electron dynamics are extremely im portant for a better understanding of the nonequilibrium phenomena involved, such as high-field transport, veloc ity overshoot, and ballistic transport. Time-resolved optical techniques using femtosecond laser pulses are a unique tool for the determination of carrier dynamics on a subpicosecond time scale. The in creasing temporal resolution to only some ten femtosec onds allows the time-resolved observation of dissipation processes such as carrier-carrier, carrier-phonon, and in tervalley scattering. Recently, experiments with ultra short laser pulses have been performed for studies of sub picosecond carrier transport dynamics. In these exper iments the temporal evolution of average carrier trans port parameters such as the drift velocityl or the electron mobility 2,3 have been investigated. Reflective electro-optic sampling (REOS) is ideally suited for the study of transient field changes after opti cal excitation of free electron-hole pairs in surface-space charge fields. 4 At bare semiconductor surfaces a static