Size effects in the thermal conductivity of gallium oxide (β-Ga₂O₃) films grown via open-atmosphere annealing of gallium nitride

Gallium nitride (GaN) is a widely used semiconductor for high frequency and high power devices due to of its unique electrical properties: a wide band gap, high breakdown field, and high electron mobility. However, thermal management has become a limiting factor regarding efficiency, lifetime, and advancement of GaN devices and GaN-based applications. In this work, we study the thermal conductivity of beta-phase gallium oxide (β-Ga₂O₃) thin films, a component of typical gate oxides used in such devices. We use time domain thermoreflectance to measure the thermal conductivity of a variety of polycrystalline β-Ga₂O₃ films of different thicknesses grown via open atmosphere annealing of the surfaces of GaN films on sapphire substrates. We show that the measured effective thermal conductivity of these β-Ga₂O₃ films can span 1.5 orders of magnitude, increasing with an increased film thickness, which is indicative of the relatively large intrinsic thermal conductivity of the β-Ga₂O₃ grown via this technique (8.8 ± 3.4 W m⁻¹ K⁻¹) and large mean free paths compared to typical gate dielectrics commonly used in GaN device contacts. By conducting time domain thermoreflectance (TDTR) measurements with different metal transducers (Al, Au, and Au with a Ti wetting layer), we attribute this variation in effective thermal conductivity to a combination of size effects in the β-Ga₂O₃ film resulting from phonon scattering at the β-Ga₂O₃/GaN interface and thermal transport across the β-Ga₂O₃/GaN interface. The measured thermal properties of open atmosphere-grown β-Ga₂O₃ and its interface with GaN set the stage for thermal engineering of gate contacts in high frequency GaN-based devices