Acoustic Phonon Modes In Wurtzite AlGaN Multilayered Heterostructures And Scattering On Electrons

Wurtzite asymmetric AlxGa1-xN/GaN/AlyGa1-yN multilayered heterostructures not only have higher two dimensional electron gas (2DEG) density and electronic mobility than an AlxGa1-xN/GaN single heterostructure but also approach to the actual high electron mobility transistors (HEMT), thus they are widely applied in HEMT devices.In this thesis, the effect of ternary mixed crystal (TMC) of acoustic phonon mode dispersion relations and the phonon potential as well as distribution of 2DEG in wurtzite asymmetric AlxGa1-xN/GaN/AlyGa1-yN multilayered heterostructures have been studied as a basis to explore electron scattering rate by acoustic phonons. The main contents and conclusions are summarized as follows:(1) Based on the elastic continuum model, TMC effect on the acoustic phonon mode dispersion relations and the phonon potential in wurtzite asymmetric AlxGa1-xN/GaN/AlyGa1-yN heterostructures are discussed. The results show that, for symmetric structures, acoustic phonon mode dispersion relation curves approach to four extremums; while for asymmetric structures they tend to six ones, and have a more complex effect of acoustic phonon mode superposition. For the acoustic phonon potential, with the increase of the Al component in the left barrier, phonon modes will transform, and acoustic phonons are confined gradually, so as to move to the right barrier, there will be a maxmum value at the interface of a GaN/AlyGa1-yN heterostructure. It is also found that the acoustic phonon potentials of dilatational modes are stronger than that of other acoustic phonon modes, acoustic phonons of dilatational modes play a dominant role in scattering on electrons.(2) Adopting a numerical method of solving self-consistently the Schrodinger equation and Poisson equation, the conduction band profile, electronic distribution of 2DEG and the relationship varying with the change of Al component in wurtzite asymmetric AlxGa1-xN/GaN/AlyGa1-yN multilayered heterostructures are obtained. It is found that with the increase of mixed crystal component, the barrier height and built-in electric field intensity will be improved. As a result, the electron gas distribution slightly shift to the middle of the GaN layer from AlxGa1-xN/GaN interface in AlxGa1-xN/GaN/AlyGa1-yN multilayer heterostructures and reduce the effects of the left interface on the 2DEG, and enhance its density.(3) Combining with electron-phonon interaction Hamiltonian and the Fermi golden rule, electron scattering rates by acoustic phonons are obtained. The results show that with the increase of electron energy the scattering rate rapidly decrease in the beginning, and then the decrease becomes weak. Meanwhile with the Al component increase, the scattering rate will increase.The above effects give a theoretical guidance to improve the properties of mixed crystal HEMT devices working at low temperatures.