Magnetic-field-induced reduction of singlet binding energies of off-well-center negative donor ions in GaAs/Al_0.3Ga_0.7As multiple quantum wells

Far-infrared Fourier transform magnetotransmission spectroscopy has been carried out on two GaAs/Al_0.3Ga_0.7As multiple-quantum-well (MQW) samples with δ-doped Si donors located away from the well centers. Temperature and magnetic field dependence studies have been used to identify the features in the spectra. In contrast to well-center D^- ions, the off-well-center D^- singlet binding energy decreases with increasing magnetic field between 5.5 T and 15 T as reflected by the decreasing strength of the D^- - singlet transition. The D⁰ 1s-2p⁺ transition increases in strength at the expense of the D^- singlet in this field range. These results reflect directly the importance of the repulsive electron-electron interaction in this unique system, where the balance between 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 agreement with theoretical predictions.