Electron-phonon Interaction In Rectangular Quantum Wires

In this Thesis, we investigate the impurity states and excitons in a rectangular quantum wire by a variational approach. The ion-and electron phonon effects on the binding energies of bound polarons in the infinite polar rectangular quantum wires are disussed. Then, bound polarons in RQWs under an electric field are studied. Finally, the exciton-phonon coupling effects on Wannier excitons in the infinite RQWs are investigated. Some main results obtained in the thesis are generalized as following:1. We have investigated the ion-and electron-LO phonon effects on the binding energies of bound polarons in polar rectangular quantum wires. The results indicate that the phonon effect reduces obviously the binding energy and cannot been neglected. The ion-phonon coupling is dominant in various phonon effects. The results for the binding energy without phonon influences agree with the previous theories. The phonon effect visibly reduces the binding energy and increases with decreasing the section size for the square quantum wire. The phonon contribution is sensitive to the shape of the rectangular quantum wires, and obviously weakens when the wire shape changes from being square.2. We have investigated the LO phonon effects on the binding energies of bound polarons in polar RQW under an electric field, by taking both the ion-and electron-phonon couplings into account. The numerical results for the square and rectangular GaAs QWWs show that the binding energy and the phonon effect depend not only on the applied electric field, but also on the area, shape of the wire section as well as the position of the impurity. The binding energy increases with decreasing the area of the wire section. The LO phonon effect lowers the binding energy of the on-center impurity but enhances that of the impurity near the wire surface, and gives a qualitatively similar contribution to the Stark energy-shift.3. We have investigated the exciton-LO-phonon coupling effect on Wannier excitons in RQWs and arrive at the following conclusions:The binding energy is reduced by the exciton-phonon interaction because of the phonon screening on the Coulomb potential. The phonon contribution to the binding energy of the light-hole exciton is smaller than that of the heavy-one. The exciton-phonon effect is sensitive to the dimension of the QWW, and increases with decreasing the section size. The exciton binding and the phonon effect in Ⅱ-Ⅵ QWWs are both stronger than those in III-V compound systems. The calculation involving the phonon contribution improves obviously the theoretical results for the exciton binding energy in QWWs. Our calculated results for the GaAs QWW are lower than those given by the previous theories and coincide with the experimental results. The theoretical results for ZnSe QWW are qualitatively in agreement with the experiments. It is worth mentioned that a Gaussian-type "orbital" function along the wire axis has been chosen for the exciton binding and the confined LO phonons have been considered for the exciton-phonon interaction in the calculations. These approximations may deduce somewhat higher exciton binding energy. The theory could be improved by using a wave function bound in the three directions of the QWW for the exciton ground state and taking surface (interface) optical phonons into account in the calculations.4. We have investigated the LO and SO phonon effects on the binding energies of bound polarons in polar rectangular quantum wires. The results indicate that the phonon effect reduces obviously the binding energy and cannot been neglected. The LO phonon effect is dominant in various phonon effects. Compared to our previous work, the binding energy reduces more by taking into the SO phonon effect. The amendment of the dielectric constant by the ion-phonon interaction is between ε∞and ε0.