Hydrogenic impurity states in quasi-two-dimensional electron-gas systems

We use the local-spin-density-functional method to study theoretically hydrogenic impurity states in a quantum-well structure containing a quasi-two-dimensional free-electron gas. We find through numerical studies that over a wide range of free-electron density there is a doubly occupied spin-degenerate electron-bound state for an impurity ion located at the center of the quantum well. This ground state with two bound electrons is shown to be sensitive both to the position of the impurity ion and to the free-electron density. We have also calculated the transition energies from the ground state to excited states with the application of the transition-state technique.