Confinement behaviors of charge carriers in strained CdTe-ZnTe single-quantum wells

The electronic band structures of the (CdTe)_Nw single-quantum wells (SQWs) embedded in ZnTe are investigated versus the well width (N_w) and the valence-band offset (VBO). The calculation, based on the sp³ s^* tight-binding method including the spin-orbit interactions, is employed to calculate the band-gap energy, quantum-confinement energy, and band structures. Two values of VBO are used: (i) VBO = 0, which is consistent with the well-known common-anion rule, and (ii) VBO = 0.284 eV, which corresponds to the model-solid approach (MSA) and should be the largest possible estimate. In both these cases, the CdTe slab forms a well for both electrons and holes. However, the charge carriers are found to completely behave in different ways. We have found that by increasing the well width, a new (spin-degenerate) electronic state becomes localized within the CdTe well every time N_w hits a multiple of 4 monolayers when VBO = 0 and 5 monolayers when VBO = 0.284 eV. The holes were found to remain delocalized for VBO = 0, where they form part of the valence-band continuum; while for VBO = 0.284 eV, they alternate from doublet to singlet of (spin-degenerate) states to localize into the CdTe well as N_w hits a multiple of 4 monolayers. Interestingly, the band-gap energy was found to decrease exponentially with the increasing N_w. A characteristic length was identified which was shown not to be so sensitive to VBO and is estimated to be about ξ ∼ 20 Å, which is correlated to the localization length of the e-h ground states and agrees, in turn, with the critical-barrier thickness to decouple double quantum wells measured in photoluminescence (PL) experiments. Finally, while our theoretical results are comparable to the PL data obtained from SQWs, they also yield relevant information about the structural and optical qualities of the experimental samples.