Influence Of Optical Phonons On Electron Mobility And Transition In Asymmetric ?-N Semiconducotor Heterostructures

Wurtzite ?-N semiconductors possess the characteristics of wide bandgap,high breakdown voltage,strongly piezoelectric and spontaneous polarizations.High electron mobility transistors(HEMTs)and quantum cascade lasers(QCLs)consisting of these kinds of semiconductors are potentially applicable within high power and high frequency regions.This thesis focuses on the influence of optical phonons on the electron mobility and transition in asymmetric multilayered heterostructures as the basic units of the above devices.The main research contents and obtained results are generalized as follows:(1)For electron states in asymmetric quantum wells(QWs)and heterostructures,considering the build-in electric fields(BEFs)induced by the strong polarizations near interfaces,the electronic eigen-energies and wave functions are obtained by solving self-consistently the Schr?dinger equation and Possion equation.The results show that in asymmetric Al_xGa_1-xN/Ga N/Al_yGa_1-yN QWs,the BEFs increase as x increases,the electrons locate in theGaN well and gather near the left interface,resulting in the enhancement of quantum localization.In Al.₂₇Ga.₇₃N/In_yGa_1-yN/Ga N/Al N heterostructures,positive charges are induced at the AlGaN/Ga N interface by inserting an InGaN layer,leading to that the concentration of two dimensional electron gas near the left interface increases as y increases.Whereas the electrons move toward the right interface with the increase of well width,and the confinement on electrons increases at first and then subsequently decreases.In step Al N/Ga N/Al_xGa_1-xN/Al N QWs,modulating Al composition can move the electrons into theGaN well,so as to modify the energy of an electron transiting from the first excited state to the ground state within the THz range.(2)For optical phonons in the asymmetric multilayered heterostructures,adopting the continuum dielectric model and Loudon's uniaxial model,the dispersion relations and electrostatic potentials of phonons are obtained by using the transfer matrix method.The results show that due to the anisotropy of wurtzite structures,there can be four kinds of optical phonons in certain composition regions,including interface(IF),confine(CO),half space(HS)and prorogation(PR)modes.Differing from symmetric QWs,in the asymmetric heterostructures,the asymptotic frequencies of symmetric and asymmetric phonon modes seperately tend to the resonant frequencies of different interfaces.In the multi-heterointerface structures,the varieties of phonon modes increase,and the mode transform appears as the composition and wave vector increase.The potentials and localizations of phonon modes are related to the resonant frequencies of the different interfaces.(3)As for asymmetric quantum wells and multilayered heterostructures,the influence of optical phonons on the electron mobility is obtained by solving the Lei-Ding force balance equation.The results show that in asymmetric Al_xGa_1-xN/Ga N/Al_yGa_1-yN QWs,IF and CO phonons in consideration of two-mode behavior for transverse modes play a main role on the electron mobility.The mixed crystal effect induces transformation of IF and HS phonon modes to result in a step shape of the total mobility.The total electron mobility decreases at first,then decreases in the step shape as x increases for a fixed y.For y=0.58,the asymmetric QWs can modulate the total mobility up to 30%higher than that in symmetric QWs.In Al.₂₇Ga.₇₃N/In_yGa_1-yN/Ga N/Al N heterostructures,the scattering from CO phonons on the electron is stronger than that from IF phonons in high frequency regions,so that CO modes play a leading role on the total mobility.The total mobility elevates at first as In composition y increases,and then bends down slightly at middle and larger y intervals.Inserting a composite channel of InGaN/Ga N can elevate electron mobility more than a pure conventionalGaN does.As temperature increases,the scattering from phonons on the electron enhances to lower the total mobility.The electron mobility modulated by the size is also given.Our results not only agree with experiments better,but also indicate how to modulate this kind of structures to achieve higher electron mobility,so as to guide optimizing the performances of ?-N HEMTs.(4)For step Al N/Ga N/Al_xGa_1-xN/Al N QWs,the influences from different kinds of optical phonons on electron intersubband transition are given by using Fermi golden rule.The results show that the electron-phonon interactions can be modulated by mixed crystal composition to lower the probability of non-radiation transition assisted by phonons,thus the photon radiation transition could become the main mechanism.The transformations among different kinds of IF modes dependent on compositions and wave vectors result in a slight bending on the electron intersubband transition assisted by phonons.In the lager composition interval of x>0.73,each branch of phonon modes reduces the probability of electron non-radiation transition.In addition,the THz frequency also enhances as x increases.The above properties can be helpful for designing the single well as the active region in a THz QCL.