Optical Phonon Modes And Ternary Mixed Crystal Effect In Multilayer Spherical Core-Shell Quantum Dots

In this thesis,we consider the one-and two-mode behavior of ternary mixed crystals?TMCs?.Within the framework of the dielectric continuum approach?DCA?and modified random-element-isodiplacement model?MREI?,the optical vibration modes and corresponding electron-phonon interaction in a multilayer spherical core-shell quantum dot?CSQD?consisting of TMCs are investigated.We select four core-shell structures for numerical calculation.They are InN/In_xGa_1-xN/InN,ZnS/Zn_xCd_1-xS/ZnS,GaAs/Al_xGa_1-xAs/GaAs and ZnSe/ZnS_xSe_1-x/ZnSe,respectively,in which In_xGa_1-xN and Zn_xCd_1-xS are "one-mode" TMC,Al_xGa_1-xAs and ZnS_xSe_1-x-x are "two-mode" TMC.The effects of component x,shell thickness,quantum number l and dielectric constant ?_d on optical vibration mode frequencies and electron-phonon interaction are discussed in detail.The numerical results obtained are as following:The interface/surface optical?IO/SO?phonon mode frequencies in the four structures have been studied.Firstly,the results show that there are 5 branches of IO/SO phonon modes in InN/In_xGa_1-xN/InN and ZnS/Zn_xCd_1-xS/ZnS structures,and 7 branches of IO/SO phonon modes in GaAs/Al_xGa_1-xAs/GaAs and ZnSe/ZnS_xSe_1-x/ZnSe structures,respectively.The dispersion curve of each phonon mode varies nonlinearly with the increase of component x.When x>0.8?x>0.9?,the frequencies of modes 6 and 7 are nearly coincident in the core-shell structure containing two-mode TMCs.Secondly,each phonon mode frequency varies significantly with increase of inner shell thickness,but the dependence on outer shell thickness is small.Finally,we found that each phonon mode frequency is strongly dependent on the smaller quantum number l.However,with increase of quantum number l,each phonon mode frequency gradually reaches its asymptotic value.We have investigated the electron-phonon interaction in the four structures.The results show that the contribution of each phonon mode to the electron-phonon interaction is different.The potential intensities are strongly dependent on component x.When the component x takes a certain value,the peak value of each phonon mode becomes more and more sharp with the increase of the quantum number l,while the phonon potential intensity gradually decreases and the contribution to the electron-phonon interaction weakens.It is also found that the phonon potential intensity is not only affected by component x and quantum number l,but also greatly affected by dielectric constant ?_d.With increase of ?_d,the phonon potential of each branch mode changes obviously.Especially in the structures with "two-mode" TMC,when ?_d=25,the localization of two branches of phonon modes changes.