Electronic mobility limited by optical phonons in symmetric MgxZn1-xO/ZnO quantum wells with mixed phases

Abstract

In symmetric MgxZn1-xO/ZnO quantum wells (QWs), which are the basic structures of high electronic mobility transistors (HEMTs), the electron states and optical phonon modes are clarified with the dielectric continuum model, uniaxial model, and force balance equation. Then, the electronic mobility affected by optical phonons is obtained around room temperature by a weight model combined with Lei-Ting's force-balance equation, in consideration of mixed phases in MgxZn1-xO (0.37<x < 0.62). Our results show that the potentials of QW's barriers, sharply influenced by the built-in electric field, present a novel fluctuation in mixed phases region with increasing Mg composition x, contrasting to the results only influenced by ternary mixed crystals effect. Meanwhile, compared rocksalt (RS) phase barriers with wurtzite (WZ) ones in QWs, the confinement on both optical interface and confined phonons is stronger in the well and weaker in the barriers. Furthermore, the total electronic mobility in the QWs is mainly influenced by WZ and RS phases, in small (x < 0.37) and large (x > 0.62) Mg composition regions, respectively. In WZ phase, the mobility first reaches a minimum due to the strong polarizations, then rises to a maximum in RS phase. It indicates that the restriction of electronic mobility from different phases should be a primary consideration for the designation of HEMTs. Strong temperature and size dependences of the mobility are also revealed as well. Relatively thicker well width of QWs is more beneficial to increase electronic mobility.