Optical investigation of a strain-induced mixed type-I–type-II superlattice system: CdTe/${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te

We present a systematic optical study of strained, CdTe/${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$Te (x≊0.1) superlattices grown by molecular-beam epitaxy. We have observed the intrinsic heavy- and light-hole exciton transitions of these superlattices, as well as the excited states (2s) of the heavy-hole exciton. By studying the energy variation of these transitions as a function of the period, we point out the mixed nature of the superlattice band structure (type I or II for heavy- or light-hole exciton transitions, respectively) due to the opposite strain experienced by the two kinds of layer (CdTe and ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$Te); this is revealed by the relative variation of the light-hole exciton binding energy as a function of the superlattice period compared with that of the heavy hole. All these data provide a determination of the partition of the band-gap discontinuities between the valence and the conduction bands, found to lie between 1/9 and -1/11. Because the valence-band configuration is essentially influenced by the strain, we change the respective energy positions of the direct and indirect exciton transitions just by changing the average strain in the superlattice (namely, by growing the structures on buffer layers of different zinc concentrations); therefore we observe a strain-mediated type-I–type-II transition.