Monte Carlo simulation of THz-pulse generation from semiconductor surface

An ensemble Monte Carlo simulation method has been used to study electron transport and dynamics of the depletion field screening at n-GaAs surface excited by a femtosecond laser pulse. THz-pulse generation caused by the ultrafast screening of the depletion field at n-GaAs surface under femtosecond photoexcitation has been also analysed. Electron transport under the time-varying inhomogeneous surface depletion field, which is found self-consistently from the Poisson equation, is simulated using the three-valley conduction band model and taking into account acoustic, optical (polar and nonpolar) and intervalley phonon scattering mechanisms. Numerical simulations show that the depletion field can decrease in time either monotonically or oscillatory depending on whether drift–diffusion or collisionless transport regime is realized during the screening; THz-pulse has respectively bipolar or oscillatory waveform. The oscillatory behaviour of the depletion field screening is explained in terms of plasma oscillations of both equilibrium and photoexcited electrons. Compared to the cold equilibrium electrons the hot photoelectrons are less implicated in the coherent plasma oscillations due to their higher scattering rate and effective mass.