Quantum-confinement versus strain effects in the Zn (Cd) S (Se) family of superlattices

We have investigated the quantum-confinement (QC) and strain effects on the band structures of the Zn (Cd) S (Se) family of binary-compound superlattices (SLs); particularly those with common anions and with common cations. All the studied cases of SLs are found to form type-I hetero-junctions, where one of the two constituents consists a well for both the holes and electrons. The calculation is based on the sp³ s^* tight-binding (TB) method with inclusion of spin-orbit coupling. We have simulated the quantum-confinement energy (E_Q) versus the well width (L_w) and fit some available photoluminescence (PL) data. Our results show that the strain-induced energy shifts to be small (E_str ∼ 25 - 35 meV) compared with confinement energies. Nevertheless, such small E_str can make a difference and play important roles to yield type-I SLs in the case of vanishing band-offsets, such as all cases of common-anion or special cases of common-cation SLs. The results also show the QC to manifest itself in three different ways: In case of common-anion SLs, such as ZnS / CdS and ZnSe / CdSe, the QC controls mainly the electrons. Whereas in common-cation SLs, whose conduction-band offset is small (CBO ≃ 0) such as ZnS / ZnSe, the QC controls only the holes. In the rest of common-cation SLs, whose valence-band offset (VBO) and CBO are comparable such as CdS/CdSe, the QC would involve both holes and electrons. Our theoretical results are shown to be in excellent agreements with the available PL data.