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

Monoclinic ß-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 ß-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 ab initio 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.

Effect of extended defects on photoluminescence of gallium oxide and aluminum gallium oxide epitaxial films.

In this work, a systematic photoluminescence (PL) study on three series of gallium oxide/aluminum gallium oxide films and bulk single crystals is performed including comparing doping, epitaxial substrates, and aluminum concentration. It is observed that blue/green emission intensity strongly correlates with extended structural defects rather than the point defects frequently assumed. Bulk crystals or Si-doped films homoepitaxially grown on (010) ß-Ga2O3 yield an intense dominant UV emission, while samples with extended structural defects, such as gallium oxide films grown on either (-201) ß-Ga2O3 or sapphire, as well as thick aluminum gallium oxide films grown on either (010) ß-Ga2O3 or sapphire, all show a very broad PL spectrum with intense dominant blue/green emission. PL differences between samples and the possible causes of these differences are analyzed. This work expands previous reports that have so far attributed blue and green emissions to point defects and shows that in the case of thin films, extended defects might have a prominent role in emission properties.

Thermal Conductivity of ß-Phase Ga2O3 and (AlxGa1-x)2O3 Heteroepitaxial Thin Films.

Heteroepitaxy of ß-phase gallium oxide (ß-Ga2O3) thin films on foreign substrates shows promise for the development of next-generation deep ultraviolet solar blind photodetectors and power electronic devices. In this work, the influences of the film thickness and crystallinity on the thermal conductivity of (2̅01)-oriented ß-Ga2O3 heteroepitaxial thin films were investigated. Unintentionally doped ß-Ga2O3 thin films were grown on c-plane sapphire substrates with off-axis angles of 0° and 6° toward ⟨112̅0⟩ via metal-organic vapor phase epitaxy (MOVPE) and low-pressure chemical vapor deposition. The surface morphology and crystal quality of the ß-Ga2O3 thin films were characterized using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The thermal conductivities of the ß-Ga2O3 films were measured via time-domain thermoreflectance. The interface quality was studied using scanning transmission electron microscopy. The measured thermal conductivities of the submicron-thick ß-Ga2O3 thin films were relatively low as compared to the intrinsic bulk value. The measured thin film thermal conductivities were compared with the Debye-Callaway model incorporating phononic parameters derived from first-principles calculations. The comparison suggests that the reduction in the thin film thermal conductivity can be partially attributed to the enhanced phonon-boundary scattering when the film thickness decreases. They were found to be a strong function of not only the layer thickness but also the film quality, resulting from growth on substrates with different offcut angles. Growth of ß-Ga2O3 films on 6° offcut sapphire substrates was found to result in higher crystallinity and thermal conductivity than films grown on on-axis c-plane sapphire. However, the ß-Ga2O3 films grown on 6° offcut sapphire exhibit a lower thermal boundary conductance at the ß-Ga2O3/sapphire heterointerface. In addition, the thermal conductivity of MOVPE-grown (2̅01)-oriented ß-(AlxGa1-x)2O3 thin films with Al compositions ranging from 2% to 43% was characterized. Because of phonon-alloy disorder scattering, the ß-(AlxGa1-x)2O3 films exhibit lower thermal conductivities (2.8-4.7 W/m·K) than the ß-Ga2O3 thin films. The dominance of the alloy disorder scattering in ß-(AlxGa1-x)2O3 is further evidenced by the weak temperature dependence of the thermal conductivity. This work provides fundamental insight into the physical interactions that govern phonon transport within heteroepitaxially grown ß-phase Ga2O3 and (AlxGa1-x)2O3 thin films and lays the groundwork for the thermal modeling and design of ß-Ga2O3 electronic and optoelectronic devices.