Efficient thermal management is critical for reliable ultrawide-bandgap β-Ga2O3 devices, as their intrinsically low thermal conductivity limits high-power operation. Integration with diamond, a material with exceptional thermal conductivity, provides a promising pathway for heat dissipation; however, achieving high-quality β-Ga2O3/diamond interfaces remains challenging due to thermal expansion and lattice mismatches, as well as β-Ga2O3 degradation during harsh-environment and high-temperature diamond growth. In this work, we investigate β-Ga2O3/diamond heterostructures consisting of epitaxial β-Ga2O3 films on c-plane sapphire, nanocrystalline diamond layers deposited by chemical vapor deposition, and intermediate SiO2 or SiC layers. Three diamond seeding approaches (nanodiamond, ultrasonic, and polymer-assisted) were evaluated to determine their impact on diamond quality, thermal boundary resistance (TBR), and mechanical adhesion. The structures were comprehensively characterized using X-ray diffraction, electron microscopy, scratch adhesion testing, and time-domain thermoreflectance measurements. Our results demonstrate that both the interlayer material and the seeding technique strongly influence interface quality, mechanical stability, and thermal transport. SiO2-based structures exhibited the lowest effective TBR (∼47 m2 K/GW), while SiC interlayers provided superior mechanical adhesion. The overall thermal resistance was found to be primarily governed by the buried interlayer and its associated interfaces. Based on these findings, we propose an optimized two-step β-Ga2O3/diamond integration approach, employing a thin protective nanocrystalline diamond layer followed by highly-thermally-conductive diamond overgrowth. When maintained at thicknesses of 20 – 50 nm, the initial nanocrystalline diamond layer contributes only ∼3 – 9 m2 K/GW to the total thermal resistance. This approach establishes an optimized interface-forming technological platform for future β-Ga2O3-based power device applications.
