RESONANT ELECTRON OPTICAL-PHONON INTERACTIONS FOR IMPURITIES IN GAAS AND GAAS ALXGA1-XAS QUANTUM-WELLS AND SUPERLATTICES/

A systematic experimental study of confinement effects on the strength of electron-optical-phonon interactions is presented. The hydrogenic 1s-2p+1 transition of shallow donors in bulk GaAs and GaAs/AlxGa1-xAs multiple quantum wells (MQW) and superlattices has been tuned through resonances with the GaAs optical phonons by magnetic fields up to 23.5 T and followed with far-infrared photoconductivity spectroscopy. Extr emely large and asymmetric interaction gaps have been observed in both two-level and three-level resonance regions for small-well-width (125 angstrom) MQW samples. These gaps decrease systematically as the well width increases, approaching the bulk limit for the largest-well-widt h (450 angstrom) sample. For the superlattice sample (80-angstrom well and 9-angstrom barrier), the interaction is stronger than that for bu lk, but much smaller than that for the MQW samples with comparable wel l width. Results are consistent with enhancement of the interaction as confinement progresses from three dimensions to two dimensions and de monstrate that the extent of the electronic wave function is the most important factor determining the interaction strength. Other phonon mo des (interface phonons) may also contribute to the enhanced interactio n for small-well-width samples. The 1s-3p+1 transition was also used t o study the resonant polaron effects for strong-confinement MQW sample s; results are consistent with those from the ls-2p+1 transition.