Resonant Impurity-Electron-Optical-Phonon Interactions in GaAs and GaAs/AlGaAs Quantum Wells and Superlattices

A review of recent systematic experimental studies of the well-width dependence of the resonant interaction of shallow donor electronic transitions with polar optical phonons in GaAs/AlGaAs multiple-quantum-well structures is presented. Particular attention is given to the effect of confinement on the strength of the observed interaction, and the question of whether or not interface phonons play a role in the narrow well samples. Primary emphasis has been placed on the hydrogenic 1s-2p+ transition of well-center donors, followed with far infrared photoconductivity spectroscopy, and tuned through resonances with GaAs optical phonons by magnetic fields up to 23.5T. Extremely large and asymmetric interaction gaps have been observed in both the “two-level” and “three-level” resonance regions for small well-width samples. These gaps decrease systematically as the well-width increases, with the largest well-width sample (450A) exhibiting structure and splittings close to those of a reference bulk sample. Results are consistent with enhancement of the interaction as confinement progresses from 3D to 2D and demonstrate that the extent of the electronic wavefunction is the most important factor determining the interaction strength. There is also some evidence supporting contributions of other phonon modes, presumably interface phonons, to the large interaction in the narrow well-width samples.