Ultrafast real-time tracing of surface electric field generated via hot electron transport in polar semiconductors

We trace an ultrafast real-space motion of photo-excited free carriers just near the surface of polar semiconductors GaAs and InAs by using the optical second-harmonic generation (SHG) technique. With a flat distribution of photo-carriers as an initial condition, ultrafast diffusion and drift motion followed by a rapid surface recombination develop a surface electric field which is efficiently probed by the electric-field-induced SHG. From the simulation based on the drift-diffusion equation combined with the ensemble Monte-Carlo approach, we could explain the experimental observation of the temporal evolution of the surface electric field, and extract time constants related to the hot electron cooling. Whereas the hot carrier cooling time is determined to be about 8 ps at room temperature for GaAs, it can be further extended to about 35 ps at 100 K due to the increase of the longitudinal optic phonon life time enhancing the hot phonon effect. Our work can be an important guide in designing optoelectronic nano-devices by providing a clear understanding of the ultrafast electron-hole separation near the surface of polar semiconductors in connection with the hot carrier cooling process.