Femtosecond spectroscopy of unipolar nanometer-scale high-field transport in GaAs

Femtosecond high-field transport in GaAs is investigated tracing ultrafast modifications of the Franz-Keldysh absorption spectrum of AlGaAs heterostructure diodes. A sophisticated sample design allows to isolate unipolar transport contributions in combination with a nanometer scale definition of layers for both photoexcitation and detection of the propagating carriers. This novel all-optical technique is applied to various aspects of ultrafast charge dynamics in semiconductors: (i) Isolating the contribution of holes, we directly measure transient carrier velocities for electric fields between 15 kV/cm and 200 kV/cm. Especially, we compare room temperature operation to results for T-L = 4 K. Transient hole velocities are found not to exceed a value of 1.2 x 10(7) cm/s which is a result of ultrafast optical phonon emission with a scattering time below 25 fs. (ii) For the case of unipolar electron transport, we find an ultrafast velocity overshoot and a quasi-ballistic electron motion with an average velocity of 4 x 10(7) cm/s over distances as large as 200 nm. (iii) For electric fields F > 350 kV/cm the dynamical build up of a nonequilibriurn carrier avalanche due to impact ionization is directly analyzed in the time domain. Most interestingly, the timescale of the carrier multiplication is found to be in the order of 10 ps.