The Influence Of Polarization On Exciton States In Coupled Quantum Dots

Recently, there is significant interest in quantum communication and information processing using quantum dots. In particular, it makes possible to quantum computing since the realization of quantum logic gate. Currently quantum logic can be realized via the entanglement of one exciton in QDs. Since this implementation for single QDs is limited to two qubits, the use of coupled QDs i.e. quantum dot molecule (QDMs) becomes more significant in order to scale this qubit system. In this thesis, we focus on the properties of exciton states of quantum dot molecule. And the exciton binding energy of quantum dot molecule is calculated via the use of the effective mass model. Then we provide a theoretical analysis of the influence of strain field on coulomb interaction and quantum coupling mechanism of the exciton state, and explore the mechanism of the polarization effect of symmetric/asymmetric QDMs on quantum ground state of the coupling system.In this paper, the exciton binding energy of InAs/GaAs and InAs/InP QDMs under the strain field is firstly calculated on coupling system. Considering the strong strain effect of the system, we focus on InAs/GaAs QDMs. The result shows that strain can significantly tune the binding energies of excitons by changing the height of QD in symmetric QDMs. However the variation of the binding energies in asymmetric structures with large size is2times higher than that in symmetric ones. And strain-induced carrier localization is responsible for the obvious tuning of the binding energies in the asymmetric QDMs. Moreover, while the InAs/InP QDMs has a weak strain field compared to InAs/GaAs system, both the strain induced confinement and quantum size confinement compete with each other, then work on the exciton binding energy.Then we further analyse the varying laws of the exciton binding energy of InAs/GaAs QDMs under strain effect as well as external electric fields. It is found that the exciton binding energy of asymmetric QDMs can be controlled effectively by external electric fields since considering the strain effect while the ability of symmetric system is very limited.Finally, both spontaneous and piezoelectric polarization is studied in wurtzite nitrides QDs. Before studying the QDs, we firstly calculate the polarization charge induced electrostatic potential of GaN/AlN quantum well. It is shown that the influence of spontaneous and piezoelectric polarization on the electrostatic potential is almost equal in GaN/AlN quantum well; then on this basis, the density of polarization charge in InGaN/GaN and GaN/AlGaN single QD under different alloy concentration is explored. The result shows that the density of two polarization charge is affected by the alloy concentration in two single QD. For InGaN QD, it is seen that the higher the indium content, the larger piezoelectric component, while the spontaneous polarization can be ignored. However a rapid increase of the spontaneous component is observed with increasing Al content in GaN/AlGaN single QD, which results in the difference of the strength of two polarization.