MAGNETIC-FIELD-INDUCED REDUCTION OF SINGLET BINDING-ENERGIES OF OFF-WELL-CENTER NEGATIVE DONOR IONS IN GAAS AL0.3GA0.7AS MULTIPLE-QUANTUM WELLS/

Far-infrared Fourier transform magnetotransmission spectroscopy has be en carried out on two GaAs/Al0.3Ga0.7As multiple-quantum-well (MQW) sa mples with delta-doped Si donors located away from the well centers. T emperature and magnetic field dependence studies have been used to ide ntify the features in the spectra. In contrast to well-center D- ions, the off-well-center D- singlet binding energy decreases with increasi ng magnetic field between 5.5 T and 15 T as reflected by the decreasin g strength of the D--singlet transition. The D-0 1s-2p(+) transition i ncreases in strength at the expense of the D- singlet in this field ra nge. These results reflect directly the importance of the repulsive el ectron-electron interaction in this unique system, where the balance b etween the electron-electron repulsion and the attractive interactions of the electrons with the positively charged impurity center can be ' tuned' by magnetic field. The experimental results are in good agreeme nt with theoretical predictions. (C) 1998 Academic Press Limited.