Resonant donors in semiconductors: Sc impurity in CdSe and ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se

The electronic structure of the ground state associated with a resonant Sc donor in CdSe and ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se was investigated by far-infrared reflectivity and millikelvin magnetoresistance measurements. Similarities in the results for samples doped either with Sc or with In, particularly in the spectral region of the 1s-2p excitations and near the metal-insulator transition, demonstrate that the ground state of the Sc impurity is hydrogenic. Thus, our findings prove that the Coulombic part of the potential of a resonant donor impurity can create hydrogenic states in the forbidden energy gap. Furthermore, the data provide support for the two-fluid model of electronic states in the vicinity of the metal-insulator transition.