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 presente d. All measurements are carried out at room temperature. The THz radiation originating from the ultrafast current change in a photoexcited semiconduct or device is measured by ultrabroadband electro-optic detection. We probe t he influences of two important lattice scattering processes on electron acc eleration. Distinct differences are seen between GaAs and InP and interpret ed in terms of the different band structures and coupling strengths of thes e important materials. The maximum velocities and carrier displacements ach ieved under nonequilibrium conditions are measured directly. Peak velocitie s of 6 x 10(7) and 8 x 10(7) cm/s are obtained in GaAs and InP, respectivel y. The distances achieved during the overshoot regime are found to depend s trongly on electric field and material. A displacement as large as 120 nm b uilds up in less than 200 fs at a field of 60 kV/cm in InP. These findings are important for the design of modem high-speed devices. Coherent excitati on of the polar crystal lattice is observed and demonstrated to result from the coupling between free carrier displacement and material polarization v ia the linear dielectric function. Our experiment is sensitive to collectiv e displacements of the lattice ions with an amplitude as small as 10(-16) m .