Strain effects in the common-cation II-VI heterostructures: Case of ZnS/ZnSe superlattices

The electronic band-structures of the strained-layer ZnS/ZnSe (001) superlattices (SLs) have been investigated using the sp³s* tight-binding method, which includes the strain and spin-orbit effects. The SL band-structures are studied versus the biaxial strain, layer thickness, and band offsets. The results suggest that the common-cation II-VI heterojunction exhibit a vanishingly small conduction-band offset (CBO). It is shown that the SL valence-band top state is always a heavy-hole localized within ZnSe slabs: whereas the conduction-band edge state (electron) is sensitive to the biaxial strain (or VBO). To assess the strain effects, we considered three differently strained SLs corresponding to the three substrates: (i) ZnSe; (ii) ZnS_0.5Se_0.5; and (iii) ZnS. The results show that all the studied SLs are of type-I except those strained to ZnS (case iii), that may exhibit type-I to type-II transition. One striking result obtained here is the existence of a critical VBO (V_c ≃ 0.76 eV) that predicts such transition, and particularly the fact that this value is independent of the strain state (substrate) (i.e. all SLs whose VBO is smaller than V_c are of type-I. else are of type-II). The comparison of our theoretical results to the photoluminescence experiments yields valuable information about the strain morphology as well as the structural and optical qualities of the experimental samples.