Picosecond response of photoexcited GaAs in a uniform electric field by Monte Carlo dynamics

The transient electrical response of GaAs photoexcited by a subpicosecond pulse, in the presence of a uniform biasing electric field, has been studied with use of a Monte Carlo calculation. Noninteracting electron transport is considered, using the three-valley model for the conduction band. Scattering from acoustic, optical, and intervalley phonons is included. The valence-band dispersion relations and valence- to conduction-band momentum matrix elements needed to treat the optical absorption were obtained from a full-zone k⋅p calculation. The optical absorption has been given a realistic treatment by including an effective energy linewidth resulting from the combination of the Fourier transform of the driving pulse, electron-phonon scattering, and the effect of the applied electric field. The average electron velocity is found to overshoot its steady-state value only if the electric field is larger than a critical value which increases with the photon energy. For example, these calculations indicate that at 5.0 kV/cm overshoot occurs for a photon energy of 1.5 eV but not for 1.7 eV. Velocity overshoot is seen to occur when the steady-state average electron energy (for the given applied field) is larger than the average electron energy of the initial photoexcited distribution. The regime of applied field and photon energy necessary for overshoot is mapped out.