Interlayer Transport of Photoexcited Electrons in Type II Gallium-Arsenide/Aluminum-Arsenide Multi-Quantum Well Structures

Optical pump-probe experiments on bulk GaAs and conventional type I GaAs/GaAlAs multi­quantum well structures (MQWS) have determined the time scales on which photoexcited carriers (1) attain thermal equilibrium among themselves, (2) scatter out of the zone-center Γ-valley to accessible X- or L-valleys, (3) relax their excess energy to the lattice, and (4) recombine.(1-3) In most cases, carrier thermalization (via carrier-carrier collisions) and intervalley scattering occur in less than 100 fs, lattice heating in picoseconds, and recombination in nanoseconds to microseconds and longer. In these direct gap systems, photoexcited electrons and holes remain in the same layer or region of the crystal. In type II structures, the highest valence band occurs in one layer and the lowest conduction band in the other; excited carriers spatially segregate, one carrier remaining in the narrower bandgap material, the other transferring to the lower energy states occurring in the adjacent layer. We have determined that in a type II GaAs/AIAs MQWS having 8 monolayers of GaAs alternating with 25 monolayers of AlAs photoexcited electrons transfer from the Γ-valley of the GaAs layers to the X-valley of adjacent AlAs layers within 100 fs.