We report the first evidence that the $\Gamma -X$ transfer mechanism plays a significant role in carrier transport in type-I semiconductor superlattices under an electric field. An anomalously delayed photocurrent was observed in a GaAs/AlAs type-I superlattice under ultrashort optical pulse excitation. This phenomenon can be explained by a switch of the electron transport path from $\Gamma -\Gamma$ to $\Gamma -X-\Gamma -X$, caused by an electric-field induced change of the subband alignment of the second $\Gamma$ state ($\Gamma 2$) in the well and the $X1\mathrm{}$ state in the adjacent barrier.

• Received 7 July 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.4500