Occupancy of shallow donor impurities in quasi-two-dimensional systems: ${\mathit{D}}^0$ and ${\mathit{D}}^{\mathrm{-}}$ states

Far-infrared magnetotransmission and photoconductivity measurements combined with visible-light, photon-dose experiments on wide-barrier GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well samples planar doped with Si donors in both well and barrier centers have provided unambiguous evidence for the existence of ${\mathit{D}}^{\mathrm{-}}$ ion states and the evolution of the occupancy of the donor-ion electronic states in the quantum wells with excess electrons in this quasi-2D system.