Thermal Conductivity of β-Phase GaO and (AlGa)O Heteroepitaxial Thin Films.

Heteroepitaxy of β-phase gallium oxide (β-GaO) 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 β-GaO heteroepitaxial thin films were investigated. Unintentionally doped β-GaO thin films were grown on -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 β-GaO thin films were characterized using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The thermal conductivities of the β-GaO 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 β-GaO 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 β-GaO films on 6° offcut sapphire substrates was found to result in higher crystallinity and thermal conductivity than films grown on on-axis -plane sapphire. However, the β-GaO films grown on 6° offcut sapphire exhibit a lower thermal boundary conductance at the β-GaO/sapphire heterointerface. In addition, the thermal conductivity of MOVPE-grown (2̅01)-oriented β-(AlGa)O thin films with Al compositions ranging from 2% to 43% was characterized. Because of phonon-alloy disorder scattering, the β-(AlGa)O films exhibit lower thermal conductivities (2.8-4.7 W/m·K) than the β-GaO thin films. The dominance of the alloy disorder scattering in β-(AlGa)O 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 β-phase GaO and (AlGa)O thin films and lays the groundwork for the thermal modeling and design of β-GaO electronic and optoelectronic devices.