Hydrogenated diamond-like carbon passivation thin films for interface trap suppression in GaN devices

The present work investigates the hydrogenated diamond-like carbon (a-C:H) films as an effective surface passivation strategy for AlGaN/GaN high electron mobility transistors (HEMTs). Diamond-like carbon (DLC) layers were deposited by RF plasma-enhanced chemical vapor deposition under varying CH4/H2 gas ratios to modulate the sp2/sp3 bonding environment. Increasing the CH4 flow rate produces an enhanced photoluminescence background and Raman evolution consistent with increased hydrogenation and reduced aromatic sp2 clustering. Across the investigated deposition series, the extracted interface trap density decreases 10.8-fold, reaching its minimum for the 20/80 sccm CH4/H2 condition. Integration of the optimized a-C:H layer into AlGaN/GaN HEMTs reduced the measured leakage current density to 8.9 × 10⁻⁸ A/cm² . These results indicate that the optimized a-C:H layer suppresses defect-mediated transport, consistent with a reduction in electrically active surface states and concurrent evolution of the sp2-bonded carbon network. Room temperature deposition allows the passivation to be applied to fully fabricated devices, offering a practical route to improved interface quality in GaN power and RF electronics.