Thermal Conductivity of β-Phase Ga2O3 and (AIxGa1-x)2O3 Heteroepitaxial Thin Films

Heteroepitaxy of beta-phase gallium oxide (beta-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 ((2) over bar 01)-oriented beta-Ga2O3 heteroepitaxial thin films were investigated. Unintentionally doped beta-Ga2O3 thin films were grown on c-plane sapphire substrates with off-axis angles of 0 degrees and 6 degrees toward (11 (2) over bar0) via metal-organic vapor phase epitaxy (MOVPE) and low-pressure chemical vapor deposition. The surface morphology and crystal quality of the beta-Ga2O3 thin films were characterized using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The thermal conductivities of the beta-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 beta-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 beta-Ga2O3 films on 6 degrees offcut sapphire substrates was found to result in higher crystallinity and thermal conductivity than films grown on on-axis c-plane sapphire. However, the beta-Ga2O3 films grown on 6 degrees offcut sapphire exhibit a lower thermal boundary conductance at the beta-Ga2O3/ sapphire heterointerface. In addition, the thermal conductivity of MOVPE-grown ((2) over bar 01)-oriented beta-(AlxGal)(2)O-3 thin films with Al compositions ranging from 2% to 43% was characterized. Because of phonon-alloy disorder scattering, the beta-(AlxGa1-x)(2)O-3 films exhibit lower thermal conductivities (2.8-4.7 W/m.K) than the beta-Ga2O3 thin films. The dominance of the alloy disorder scattering in beta-(AlxGa1-x)(2)O-3 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 beta-phase Ga2O3 and (AlxGa1-x)(2)O-3 thin films and lays the groundwork for the thermal modeling and design of beta-Ga2O3 electronic and optoelectronic devices.

Automated summary

This study investigates the thermal conductivity of heteroepitaxial beta-Ga2O3 films, and finds that the thermal conductivity is strongly influenced by film thickness, crystallinity, and substrate offcut angles. Additionally, the thermal conductivity of ((2) over bar 01)-oriented beta-(AlxGal)(2)O-3 thin films grown via MOVPE was characterized, with results showing lower conductivity due to phonon-alloy disorder scattering. These findings provide fundamental insights for the development of beta-Ga2O3 electronic and optoelectronic devices.