Far-infrared magnetoabsorption study of weakly bound electrons in GaAs/AlxGa

Far-infrared magnetoabsorption experiments have been performed at 4.2 K on $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ multiple-quantum-well (MQW) heterostructures that were selectively doped with Si donors in the centers of the quantum wells and barrier layers. Data were obtained for several values of dopant concentration in the barriers. The results provide strong evidence for the binding of electrons in the wells to positively charged ions in the barriers.