Spectroscopy of neutral and charged donors in semiconductor quantum wells: Many body effects

Studies of shallow donor impurities confined in semiconductor quantum-well (QW) structures are reviewed, concentrating on recent experimental (far infrared spectroscopy) and theoretical investigations of negative donor ions (D^-) and the effects of excess electrons on the donor-related transitions. With increasing excess electron densities in the QWs the two-electron D^--singlet and -triplet transitions are blue-shifted substantially, and cusps in the normalized blueshift versus filling factor (v) are observed at integer and fractional values of v. For v < 1 the many-electron (bound magnetoplasmon) transitions approach the isolated two-electron transitions. Exact numerical diagonalization calculations of a donor and confined electrons with v varying from 2 to <1 show the importance of electron correlations and localization at high magnetic fields in understanding this behavior. Similarities between some of these results and recent studies of internal transitions of neutral and negatively-charged excitons are noted.