Ultrafast transport of electrons in GaAs: Direct observation of quasiballistic motion and side valley transfer

Temporally and spatially resolved femtosecond electron dynamics in GaAs is investigated, tracing ultrafast modifications of the Franz-Keldysh absorption spectrum. A complex heterostructure design allows for a study of unipolar transport as well as a nanometer scale definition of layers for both carrier injection and probe of the propagating electron distribution. Transit times through the structure are directly measured for electric fields between $7\mathrm{kV}∕\mathrm{cm}$ and $180\mathrm{kV}∕\mathrm{cm}$ comparing room temperature operation to results for $T_L=4\mathrm{K}$. For low lattice temperatures, quasiballistic electron motion with an average velocity of $5.4\times {10}^7\mathrm{cm}∕\mathrm{s}$ is observed over distances as large as $300\mathrm{nm}$. Realistic Monte Carlo simulations are in excellent agreement with our observations.