Abstract

The advance of high-speed semiconductor optoelectronic devices has stimulated a considerable interest in studying photocarrier transport and electric field screening dynamics in semiconductors. The availability of femtosecond optical techniques has enabled to investigate these processes on a subpicosecond time scale and to observe effects such as ballistic acceleration and velocity overshoot of electrons in GaAs, coherent oscillations of photogenerated plasma in p-i-n structures and others. The dynamics of intensively studied second-harmonic (SH) generation and THz-pulse radiation from semiconductor surface excited by an ultrashort laser pulse is also fundamentally governed by the photocarrier-induced depletion field screening caused by charge-separation effect. The spatio-temporal evolution of the surface depletion electric field and photocarrier dynamics at n-GaAs surface excited by subpicosecond laser pulse (photon energy 1.5 eV) have been investigated self-consistently on the basis of a numerical simulation of the Boltzmann transport equation for electrons and Poisson equation for one-dimensional depletion field. The holes drift and recombination were put out of account on the considered time scale. The transport equation has been solved by using Monte-Carlo (MC) method. The three-valley conduction band structure and electron scattering by acoustic, polar optical and intervalley phonons were taken into account. The calculations were produced by using MC simulation of 1000 electrons trajectories in the time-varying inhomogeneous depletion field for 10 ps. The initial surface depletion field is determined by the semiconductor doping level and it has been calculated preliminary. The electric field has been found numerically from the Poisson equation every 0.02 ps. The numerical simulation shows that with increasing the photoexcitation level the temporal decay of the surface electric field varies from monotonic to oscillatory that can be explained as changing screening regime from hydrodynamics to collisionless. As an example the developed model has been applied to analysis of THz-pulse and SH generation from semiconductor surface with depletion layer excited by a subpicosecond laser pulse. It has been found that the radiated electric field emitted by the transient photocurrent in the far field region is determined by the second time derivative of the surface potential and its waveform is bipolar or oscillatory [1] depending on whether drift-diffusion or ballistic transport regime is realized. The SH generation induced by the surface depletion field has been analysed. The results of calculation showed that the photocarrier-induced screening of the depletion field caused the SH efficiency to decrease with increasing the pump intensity. The dynamics of the probe-generated SH in the depletion layer previously photoexcited by the pump laser pulse was also analysed. It was found that with increasing the pump intensity the probe-generated SH-signal dependencies on pump-probe time delay varied from monotonic to oscillatory, that could be explained as a change of the screening regime from drift-diffusion to collisionless

© 2000 IEEE