Perpendicular transport of photoexcited electrons and holes in GaAs/AlAs short-period superlattices: Barrier-thickness and temperature dependence

Perpendicular transport of photoexcited carriers, which sink into an intentionally enlarged quantum well, is investigated in a set of GaAs/AlAs short-period superlattices with systematically varied AlAs barrier thicknesses as a function of the lattice temperature between 4.2 and 200 K. Excitation-power dependence of the luminescence observed at low temperatures indicates that the ambipolar transport is operative at carrier densities in excess of 10¹⁵--10¹⁷ cm^-3. We find that tunneling-assisted hopping conduction prevails at low temperatures via localized states and that the tunneling probability is correlated with the calculated heavy-hole miniband width. We also find a crossover from the hopping conduction to the Bloch-type transport at higher temperatures, which critically depends on the barrier thickness, as a result of thermal activation of carriers to the extended miniband states.