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) ZnS0.5Se0.5; 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 (Vc ≃ 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 Vc 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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Acoustics and Ultrasonics
- Surfaces, Coatings and Films