High-Purity Diamond Integration on $\beta$-Ga$_2$O$_3$ via Microwave Plasma CVD for Enhanced Thermal Management

The integration of diamond with $\beta$-Ga$_2$O$_3$ presents a promising pathway to enhance thermal management in high-power electronic devices, where the inherently low thermal conductivity of $\beta$-Ga$_2$O$_3$ can lead to localized self-heating and elevated junction temperatures. In this work, we demonstrate a scalable, low-damage method to integrate high-quality polycrystalline diamond films on (010) $\beta$-Ga$_2$O$_3$ substrates using microwave plasma chemical vapor deposition (MPCVD), enabled by dielectric interlayers and polymer-assisted electrostatic nanodiamond seeding. A systematic investigation of processing conditions was conducted to assess their effects on film morphology, grain evolution, sp3-phase purity, and optical characteristics. Diamond films grown at 800 degC exhibit grain sizes up to 126.6 nm, RMS roughness of 16.3 nm, and a sharp diamond Raman peak at around 1332 cm-1 with a full width at half maximum (FWHM) of 34.98 cm-1. The corresponding sp$^3$-phase purity exceeds 97.5 percent, with an optical bandgap up to 5.13 eV. Deposition time variation from 10 to 60 minutes at 800 degC results in thicknesses from 53 nm to 315 nm and corresponding grain coarsening. Interlayer comparison reveals that SiO2 yields slightly larger grains and higher purity than SiNx under identical conditions. Notably, films with more than 96 percent sp$^3$ content were achieved at temperatures as low as 480 degC, confirming the compatibility of this approach with reduced thermal budgets. These results provide a robust framework for scalable $\beta$-Ga$_2$O$_3$/diamond integration in thermally resilient high power and RF device technologies.