Uniform Surface Smoothing of OVPE-GaN with Nonuniform Doping Concentration via Plasma-Assisted Polishing

Gallium nitride (GaN), owing to its wide bandgap and high electron saturation drift velocity, has shown great potential for applications in high-frequency power devices. Among various growth methods, oxide vapor phase epitaxy (OVPE) enables the fabrication of GaN substrates with high doping concentrations and low dislocation densities, making it particularly suitable for vertical GaN power devices with low on-resistance. However, as a substrate material for power devices, OVPE-GaN imposes extremely stringent requirements on surface quality. Thus, achieving ultrasmooth and damage-free surfaces is essential to fully exploiting its superior properties. As a widely used polishing technique, chemical mechanical polishing (CMP) has also been applied to the surface finishing of OVPE-GaN. Nevertheless, because of the intrinsically high hardness and chemical inertness of GaN, the material removal rate (MRR) of CMP is extremely low. Moreover, during crystal growth, OVPE-GaN inevitably develops an in-plane hexagonal floral-patterned distribution of doping concentration. This leads to a doping-dependent variation in the local MRR during CMP, resulting in the formation of corresponding floral-patterned protrusions on the surface after polishing. Consequently, obtaining ultrasmooth surfaces required for high-performance device applications using CMP is difficult. To simultaneously enhance polishing efficiency and eliminate the removal nonuniformity induced by doping concentration distributions, plasma-assisted polishing is proposed. An O 2 plasma that can generate low-electron-affinity O radicals is employed to form an easily removable Ga 2 O 3 modified layer on the OVPE-GaN surface. Benefiting from the self-limiting modification behavior during plasma treatment, a uniform modified layer can be formed even under nonuniform in-plane doping concentrations. Subsequently, the modified layer was selectively removed using soft SiO 2 abrasives under mechanical polishing conditions that do not damage the underlying OVPE-GaN substrate. The results demonstrate that this method enables the formation of ultrasmooth and damage-free surfaces on OVPE-GaN without floral-patterned protrusions and without being affected by the doping concentration distribution. The average surface roughness S a is significantly reduced from 0.75 to 0.21 nm. This approach enables a high-quality finishing process for the ultraprecision polishing of OVPE-GaN substrates.