Plasmon-Phonon Coupling in Electrostatically Gated β-Ga₂O₃ Films with Mobility Exceeding 200 cm² V^-1 s^-1.

Abstract

Monoclinic β-Ga₂O₃, an ultra-wide bandgap semiconductor, has seen enormous activity in recent years. However, the fundamental study of the plasmon-phonon coupling that dictates electron transport properties has not been possible due to the difficulty in achieving higher carrier density (without introducing chemical disorder). Here, we report a highly reversible, electrostatic doping of β-Ga₂O₃ films with tunable carrier densities using ion-gel-gated electric double-layer transistor configuration. Combining temperature-dependent Hall effect measurements, transport modeling, and comprehensive mobility calculations using <i>ab initio</i> based electron-phonon scattering rates, we demonstrate an increase in the room-temperature mobility to 201 cm² V^-1 s^-1 followed by a surprising decrease with an increasing carrier density due to the plasmon-phonon coupling. The modeling and experimental data further reveal an important "antiscreening" (of electron-phonon interaction) effect arising from dynamic screening from the hybrid plasmon-phonon modes. Our calculations show that a significantly higher room-temperature mobility of 300 cm² V^-1 s^-1 is possible if high electron densities (>10²⁰ cm^-3) with plasmon energies surpassing the highest energy LO mode can be realized. As Ga₂O₃ and other polar semiconductors play an important role in several device applications, the fundamental understanding of the plasmon-phonon coupling can lead to the enhancement of mobility by harnessing the dynamic screening of the electron-phonon interactions.