FAR-INFRARED MAGNETOOPTICAL STUDIES OF D- IONS AND MANY-ELECTRON EFFECTS ON DONOR IMPURITIES IN QUASI-2D SEMICONDUCTOR STRUCTURES

A systematic study of shallow donor states in the presence of excess electrons has been carried out on a series of Si-doped GaAs/AlGaAs multiple quantum well samples by far-infrared magneto-spectroscopy. The evolution of the occupancy of the one-electron (D0) and two-electron (D-) impurity bound states with increasing population of electrons in the well has been clearly observed through the use of a controllable photon-dose technique and a sequence of modulation and well-center doped samples. After all impurities become D ions, increasing the density of two-dimensional electrons in the wells shifts the 'D-' transition to higher energy, opposite to predictions for static screening of D0 states. The magnetic field dependence of the D- transitions and the dependence of the transition energies on the filling factor are also described.