Electron-Phonon Interaction And Effects Of Ternary Mixed Crystals In Spherical Core-Shell Quantum Dots

Using the dielectric continuum approach (DCA), we have systematically investigated the optical vibration modes and electron-phonon interaction in a polar spherical multilayer core-shell quantum dot (CSQD) hosted in a non-polar material. The bound polarons in a bilayer CSQD have also been studied with a variational approach. On top of these, we introduce the polar ternary mixed crystals (TMC) into the spherical multilayer heterostructures, and investigate the TMC effects by applying the modified random-element-isodisplacement model (MREI) and effective-phonon-mode approximation (EPMA). The main results obtained are as following:1. We have studied the optical vibration modes in a spherical bilayer CSQD in the framework of DCA. The dispersion relation and the corresponding electron-phonon interaction Hamiltonians for interface optical (IO) or surface optical (SO) phonons are derived. Numerical calculations are performed for a spherical CdSe/ZnS CSQD, and the results show that there are three branches frequencies of IO/SO phonon in the system, and phonon frequencies depend on the host medium. As shell thickness increases, the potentials of the IO/SO phonon modes more localized at a certain interface or surface. It is also found that the ZnS-like IO/SO modes with higher frequencies, compared with CdSe-like IO mode, have a strong electron-phonon coupling when εd is smaller. However for the bigger εd, ZnS-like SO mode is transformed into ZnS-like10mode, that means the surface polaron effect is greatly reduced. As the same time, the IO/SO phonon contribution to the electron-phonon interaction is decrease with increasing the angular quantum number.2. The IO/SO phonon modes in a spherical three-layer CSQD are investigated by employing the DCA. The dispersion relation and the corresponding electron-phonon interaction Hamiltonians for IO/SO phonons are derived. Numerical calculations are performed for a spherical CdSe/ZnS/CdSe CSQD, and the results reveal that there are five branches frequencies of IO/SO phonon. Among these, the two modes with higher frequencies are ZnS-like IO modes, and the others with lower frequencies are CdSe-like IO/SO modes. The phonon frequencies depend on the geometrical and host material. It is observed that the electron-phonon coupling related to ZnS-like IO modes and CdSe-like SO mode is more significant when εd is smaller. Similarly, the surface polaron effect is much weaken for the larger εd, and CdSe-like SO mode switch to the outer interface.3. We have studied the effects of TMC in spherical multilayer CSQD consisting of TMC by using MREI and EPMA. The numerical computations are performed for the spherical s and GaAs/AlxGa1-xAs/GaAs system. The results show that the frequencies of IO/SO modes strongly depend on the components of TMC, and the frequencies of GaAs-like IO modes also vary with the components in a certain range. Our investigations also indicate that the effects of TMC result in different distributions of IO/SO phonon potentials. Thus, the effects of TMC should be taken into account in the electron-phonon coupling investigation.4. The confined longitudinal-optical (LO) phonon modes in a spherical three-layer CSQD have been studied in detail, and the corresponding electron-phonon interaction Hamiltonian is obtained by using the DCA. Numerical calculations are performed for a spherical CdSe/ZnS/CdSe CSQD, and the results reveal that the confined LO1phonon modes in the core material and the confined LO2phonon modes in the shell material, compared with the confined LO3phonon modes, are dominated in the electron-phonon coupling. At the same time, the potentials strength of LO2phonon is strongly dependent on the shell thickness, and the potentials strength decreases with the increasing of the shell thickness.5. With the electron-phonon interaction Hamiltonians derived above, we have studied the polaron effects of an impurity in a spherical bilayer CSQD within the effective mass approximation. A adiabatic variational method is used to calculate the binding energy of the bound polaron, and both the electron-and ion-phonon effects are considered. It is indicated that phonon effects reduced obviously the impurity binding energy and depend on the core radius and the shell well width. When the shell well width is smaller than a critical value, both the impurity binding energy and the phonon effects increase significantly.