Observation of above-barrier transitions in superlattices with small magnetically induced band offsets

Using magneto-optical absorption in ZnSe/${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$Se superlattices, we have observed a series of transitions involving above-barrier states of the conduction and valence bands, at the zone center and at the zone edge of the superlattice Brillouin zone. The Mn concentration in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$Se layers (x∼0.04) has been specifically chosen such that the bowing of the band gap with x and the strain in the superlattice together give rise to conduction- and valence-band offsets, which are nearly zero in the absence of a magnetic field. Owing to the large Zeeman splittings of band edges that occur in the ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$Se layers, the band alignment in these superlattices can be externally tuned by an applied magnetic field, providing a unique laboratory for investigating above-barrier subbands in a superlattice defined by very small periodic-potential variations (intermediate-dimensionality regime). The magnetic-field tuning, together with calculations based on an eight-band k⋅p model, allow us to unambiguously identify the transitions, and to compare the observed transition energies with theoretical predictions.