| ‘Remote shallow donors,’ consisting of spatially separated semiconductor impurity ions and electrons, that are nevertheless bound as a result of their mutual coulomb attraction, have been investigated by means of magneto-photoconductivity, a tool that has so far remain unexplored in remote donor identification despite its obvious success in the characterization of bulk and quantum-well shallow donors. Remote donor configurations have been realized in molecular beam epitaxy grown barrier-doped quantum well heterostructures as well as coupled-well structures in GaAs/AlGaAs material system. High resolution and signal-to-noise ratio magneto-photoconductivity data has been obtained which clearly shows the advantage of the photoconductivity technique. Besides hydrogenic 1s-2p+1 transitions arising due to barrier and couple-well shallow donors, several additional transitions have been observed some of which are suggestive of binding of well electrons to an ionic pair in the barrier. The unidentified transitions in the barrier-doped samples are most likely due to binding between the well electrons and barrier donors since the possibility of shallow donor formation in the quantum-well has been eliminated due to doping of only the barrier region. Variational calculations based hydrogenic transition energies are found to be in good agreement with experimental results, especially when the diffusion of donors is taken into consideration. The feasibility of observing shallow donor transitions in a single period quantum-well structure is also reported. |
