Optics of charged excitons in quantum wells: Free versus donor-bound complexes

We theoretically study localization of quasi-two-dimensional negatively charged excitons X^- on isolated charged donors in magnetic fields. We consider donors located in a barrier at various distances L from the heteroboundary as well as donors in the quantum well. We establish how many different singlet X_s^- and triplet X_t^- bound states a donor ion D^+ can support in magnetic fields B > 6 T. We find several new bound states, some of which have surprisingly large oscillator strengths.