Insights on the bound states in the strained CdSe-ZnSe single-quantum wells

The bound states in the (CdSe)_NwZnSe(001) single quantum well are investigated versus the well width (N_w monolayers) and the valence-band offset (VBO). The calculation, based on the sp³s* tight-binding method which includes the spin-orbit interactions, is employed to calculate the band-gap energy (E_g), quantum-confinement energy (E_Q), and band structures. It is found that the studied systems possess a vanishing valence-band offset (VBO ≃ 0) in consistency with the common-anion rule, and a large conduction band offset of about (CBO ≃ 1 eV); both of which made the electronic confinement become predominant. The bi-axial strain, on the other hand, remains to control the hole states. Namely, the two highest (spin-degenerate) hole states are found to localize at the two interfaces due to the formation of two similar strain-induced potential dips at these interfaces, each of depth equal to the strain energy ∼35 meV. More importantly, the ultrathin CdSe wells (with N_w ≤ 4 monolayers) are found to contain only a single (spin-degenerate) bound state; but by increasing the well width further, a new (spin-degenerate) bound state falls into the well every time N_w hits a multiple of 4 monolayers (more specifically, for 4n+1 ≤ N_w ≤ 4 (n+1), the number of bound states is (n+1), where n is an integer). The rule governing the variation of the quantum-confinement energy E_Q versus the well width N_w has been derived. Our theoretical results are in excellent agreement with the available experimental photoluminescence data.