Anomalies in the resonant magnetopolaron region for confined impurities in multiple-quantum-well structures

Far infrared photothermal ionization spectroscopy of well-center doped shallow donors in GaAs/AlCaAs'multiple-quantum-well structures has been carried out to investigate the electron-optical-phonon interaction. The ls-2p(m=+1) and ls-3p(m=-i-1). transitions are tuned through the resonant region with the GaAs optical phonons by magnetic fields up to 23 .5T . Both two-level and three-level resonance measurements show similar anti-level-crossing behavior with extremely large interaction gaps. The lower branches are well below (-50 cm ) the resonant energies involving the bulk zone centr LO phonons. These anomalous features strongly suggest that electrons in the wells interact with phonon modes at several different frequencies, or a phonon band, in contrast to the single frequency LO phonons in the bulk. A partial explanation may be found in terms of electron-interface-phonon and electron-confined-LO-phonon interactions. Other possible mechanisms are also discussed.