Femtosecond high-field transport in compound semiconductors

A study of nonequilibrium transport of carriers in GaAs and InP at electric fields up to 130 kV/cm and with a temporal resolution of 20 fs is presented. All measurements are carried out at room temperature. The THz radiation originating from the ultrafast current change in a photoexcited semiconductor device is measured by ultrabroadband electro-optic detection. We probe the influences of two important lattice scattering processes on electron acceleration. Distinct differences are seen between GaAs and InP and interpreted in terms of the different band structures and coupling strengths of these important materials. The maximum velocities and carrier displacements achieved under nonequilibrium conditions are measured directly. Peak velocities of $6\times {10}^7$ and $8\times {10}^7\mathrm{c}\mathrm{m}/\mathrm{s}$ are obtained in GaAs and InP, respectively. The distances achieved during the overshoot regime are found to depend strongly on electric field and material. A displacement as large as 120 nm builds up in less than 200 fs at a field of 60 kV/cm in InP. These findings are important for the design of modern high-speed devices. Coherent excitation of the polar crystal lattice is observed and demonstrated to result from the coupling between free carrier displacement and material polarization via the linear dielectric function. Our experiment is sensitive to collective displacements of the lattice ions with an amplitude as small as ${10}^{-16}\mathrm{m}.$