The bound states in the (CdSe) Nw– ZnSe (001) single quantum well are investigated versus the well width (Nw 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 (Eg), quantum-confinement energy (EQ), 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 Nw ≤ 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 Nw hits a multiple of 4 monolayers (more specifically, for 4n+1 ≤ Nw ≤ 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 EQ versus the well width Nw has been derived. Our theoretical results are in excellent agreement with the available experimental photoluminescence data.
Determination of the valence band offset of Si/Si0.7Ge0.3/Si quantum wells using deep level transient spectroscopy