High field drift velocities of two-dimensional carriers in GaAs/AlGaAs quantum well structures.

We report direct measurements of the high field drift velocities of photoexcited electrons in p-type multiple quantum well structures of GaAs/Al0.48Ga0.52As by picosecond time-of-flight techniques. Negative differential mobility is observed at electric fields > 8kV/cm, caused by real-space and valley transfer of hot carriers into Al(x)Ga(1-x)As, which is an indirect semiconductor at x=0.48. From the photoluminescence spectra the carrier temperatures, at which the transfer effects occur, are determined.