Phonon Scattering On The Mobility Of In Wurtzite AlGaN Multilayered Heterostructures

Wurtzite AlGaN/GaN heterostructures are the major structures in GaN-based high electron mobility transistors (HEMTs). The effect of strong spontaneous and piezoelectric polarizations induces a two dimensional electron gas (2DEG) with a high sheet density near the interfaces of HEMTs. The electronic mobility and sheet density of a2DEG in wurtzite AlxGa1-xN/AlN/GaN and AlxGa1-xN/GaN/AlyGa1-yN multilayer heterostructures with a wurtzite ternary mixed crystal (TMC) AlGaN can be modulated to improve the properties of HEMTs,Adopting a numerical method of solving self-consistently the Schrodinger equation and Poisson equation by taken into account of the realistic heterostructure potential including the influence of energy band bending and the finite thickness of barriers and built-in electric field induced by spontaneous and piezoelectric polarization, the eigenstates and eigenenergies of electrons in multilayered wurtzite AlGaN/GaN structures are obtained. Based on the continuous dielectric model, Loudon’s uniaxial crystal model and elastic continuum model, the Lei-Ting balance equation and Fermi golden rule are extended to investigate theoretically the size effect and TMC effect on the electronic mobility within the temperature range governed by the scattering form optical-phonon and acoustic-phonons. Our main contents and results are generalized as follows:(1) TMC and size effects on the distribution of2DEG in in a wurtzite AlxGa1-xN/AlN/GaN and AlxGa1-xN/GaN/AlyGa1-yN multilayer heterostructures are discussed. The results show that the increase of the thickness of inserting AlN layer and Al component of AlxGa1-xN in the barrier enhances the built-in electric field in the GaN layer to lead2DEG being much closer to the interface of a AlxGa1-xN/AlN/GaN heterostructure. With increasing the thickness of GaN layer and Al component of Ga1-xN, the2DEG slightly shift to the middle of the GaN layer from Ga1-xN/GaN interface in AlxGa1-xN/GaN/AlyGa1-yN multilayer heterostructures. The effects mentioned above may influence the transport properties of2DEG(2) TMC effect and size effect on electron mobility influenced by optical-phonons in a wurtzite AlxGa1-xN/AlN/GaN are investigated. It is found that the scattering from the interface phonons are stronger than other optical-phonons, the interface phonons play a dominant role in the total mobility. With increasing the thickness of inserted AlN layer and Al component of AlxGa1-xN in the barrier, the mobility slightly decreases. A higher electron mobility can be obtained by adjusting appropriately the thickness of inserted AlN layer and Al component.(3) TMC effect and size effect on electron mobility influenced by acoustic-phonons in wurtzite AlxGa1-xN/GaN/AlyGa1-yN multilayer heterostructures are investigated to show that the dominant scattering from the dilatational mode with the lowest frequency is stronger than that from the one with a higher frequency and flexural acoustic modes. The increase of the Al component in the AlxGa1-xN barrier changes the symmetry and vibration modes of acoustic phonons, leading an abrupt increase of the total mobility in a certain Al component region for an asymmetrical heterostructure. Besides, the total mobility reaches a maximum around a certain thickness of channel layer GaN.These effects give a guidance to improve the properties of HEMT devices.