Resonant electron–optical-phonon interactions for impurities in GaAs and GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells and superlattices

A systematic experimental study of confinement effects on the strength of electron–optical-phonon interactions is presented. The hydrogenic 1s-2${\mathit{p}}_{+1\mathrm{}}$ transition of shallow donors in bulk GaAs and GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells (MQW) and superlattices has been tuned through resonances with the GaAs optical phonons by magnetic fields up to 23.5 T and followed with far-infrared photoconductivity spectroscopy. Extremely large and asymmetric interaction gaps have been observed in both two-level and three-level resonance regions for small-well-width (125 Å) MQW samples. These gaps decrease systematically as the well width increases, approaching the bulk limit for the largest-well-width (450 Å) sample. For the superlattice sample (80-Å well and 9-Å barrier), the interaction is stronger than that for bulk, but much smaller than that for the MQW samples with comparable well width. Results are consistent with enhancement of the interaction as confinement progresses from three dimensions to two dimensions and demonstrate that the extent of the electronic wave function is the most important factor determining the interaction strength. Other phonon modes (interface phonons) may also contribute to the enhanced interaction for small-well-width samples. The 1s-3${\mathit{p}}_{+1\mathrm{}}$ transition was also used to study the resonant polaron effects for strong-confinement MQW samples; results are consistent with those from the 1s-2${\mathit{p}}_{+1\mathrm{}}$ transition.