Spectroscopy of charged donors and many-electron effects in semiconductor quantum wells

Far infrared magnetospectroscopic studies of negative donor ions (D-), and donors in the presence of many excess electrons in high:magnetic fields in GaAs/AlGaAs quantum wells are reviewed. Both singlet and triplet transition s of well-center D--ions were observed and are in good agreement with recen t theoretical calculations. For off-well-center D--ions evidence for a pred icted magnetic-field-induced "unbinding" of the second electron was found. In the presence of many excess electrons the D--singlet and -triplet transi tions are blue-shifted substantially and evolve into bound magnetoplasmon e xcitations. Cusps are observed at integral and fractional Landau-level fill ing factors (v) in a plot of normalized blue-shift of the D--singlet-like b ound magnetoplasmon transition vs, v. For v < 1, the singlet-like bound mag netoplasmon transition continuously approaches the isolated D--singlet tran sition with increasing magnetic field, while the triplet-like transition lo ses strength, irrespective of the electron density. Exact diagonalization s tudies of a donor ion with a few electrons in a parabolic lateral confining potential show the importance of electron-electron interactions and locali zation due to the long-range fluctuating potential in explaining this behav ior. High pressure studies in a specially designed diamond anvil cell exhib it a continuous evolution from bound magnetoplasmon transitions to isolated D--transitions to neutral donor transitions in a single sample as the pres sure is increased and the electron density in the wells is decreased.