Charge confinements in CdSe-ZnSe symmetric double quantum wells

The bound states in the (CdSe)Nw(ZnSe)Nb(CdSe)Nw-ZnSe(001) symmetric double quantum wells are investigated versus the well width (Nw) and the barrier thickness (Nb). A calculation based on the sp3s* tight-binding method which includes the spin-orbit interactions is employed to calculate the bandgap energy, quantum-confinement energy, and band structures. The studied systems possess a vanishing valence-band offset (VBO = 0) in consistency with the well known common-anion rule, and a large conduction-band offset (CBO ≅ 1 eV), which plays an essential role in the confinement of electrons within the CdSe wells. The biaxial strain, on the other hand, plays another role in confining the holes at the interfaces (within the well regions) and thus enhancing the radiative efficiency. The induced-strain energy is estimated to be ∼35 meV. More importantly, the results show that, for a fixed barrier thickness, the double wells are able to confine a pair of bound states when they are very thin. By increasing the wells' width (Nw), further, a new pair of states from the conduction-band continuum falls into the wells every time Nw hits a multiple of four monolayers (more specifically, for 4n<Nw≤4(n+1), the number of bound states is 2(n+1), where n is an integer). On the other hand, the barrier thickness (Nb) is shown to have no effect on the number of bound states, but it solely controls their well-to-well interactions. A critical barrier thickness to switch off these latter interactions is estimated to occur at about Ncritb ≅ 9 (Lcritb ≅ 25∼AA. Rules governing the variation of the quantum-confinement energy versus both barrier thickness (Nb) and well width (Nw) have been derived. Our theoretical results are also shown to have excellent agreement with the available experimental photoluminescence data