Off-center $D^{-}$ centers in a quantum well in the presence of a perpendicular magnetic field: Angular-momentum transitions and magnetic evaporation

We investigate the effect of the position of the donor in the quantum well on the energy spectrum and the oscillator strength of the $D^{-}$ system in the presence of a perpendicular magnetic field. As a function of the magnetic field, we find that when $D^{-}$ centers are placed sufficiently off-center, they undergo singlet-triplet transitions which are similar to those found in many-electron parabolic quantum dots. The main difference is that the number of such transitions depend on the position of the donor, and only a finite number of such singlet-triplet transitions are found as a function of the strength of the magnetic field. For sufficiently large magnetic fields the two-electron system becomes unbound. For the near center $D^{-}$ system, no singlet-triplet transition and no unbinding of $D^{-}$ is found with increasing magnetic field. A magnetic field vs donor position phase diagram is presented that depends on the width of the quantum well.