Observation of type-I excitons and related confinement effects in type-II superlattices

CdSe/${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te superlattices are characterized by type-II band alignment. We have used these structures to demonstrate that type-II superlattices can exhibit type-I excitons, i.e., excitons which are confined in the same semiconductor layer (either in CdSe or in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te in the present example). Such spatially direct excitons form in a type-II structure when one of the carriers (electron or hole) originates from a well, while the other (hole or electron) originates from a state localized in the barrier, which is typical for subbands at above-barrier energies.