Interfacial hole-flux regulation enables uniform slurry-less photoelectrochemical mechanical polishing of doping-inhomogeneous GaN

Highly doped GaN substrates are essential for low-resistance vertical power devices, but their intrinsic carrier-concentration inhomogeneity causes spatially nonuniform oxidation and consequently nonuniform removal in electrochemical mechanical polishing, which limits surface quality and process controllability. This study investigated the oxidation behaviour of OVPE-grown n-type GaN under chemical, electric-field-assisted, and photo-assisted reaction fields and proposed a slurry-less photoelectrochemical mechanical polishing (PECMP) process for uniform finishing of doping-inhomogeneous substrates. The results show that anodic oxidation and UV-assisted oxidation exhibit opposite carrier-concentration dependences: anodic oxidation was accelerated in high-carrier regions because of enhanced current density and hole supply, whereas UV-assisted oxidation was suppressed in high-carrier regions because intensified recombination shortens the effective lifetime of photogenerated holes. By balancing these opposite responses, spatially uniform surface modification was achieved across the substrate surface. Based on this mechanism, a slurry-less PECMP process using a soft fixed CeO 2 abrasive was developed to selectively remove the modified layer without damaging the GaN substrate. Under an electric-field-dominant condition, the material removal rate reached 3.81 µm h⁻¹, but carrier-dependent topography remained. Under the balanced condition, the formation of characteristic floral-like topography was suppressed and an ultra-smooth surface with S a = 0.397 nm was obtained. Stress mapping using Raman spectroscopy further confirmed the elimination of scratch-related residual-stress features. The results demonstrated that uniform and damage-free polishing of highly doped GaN was achieved not by simply increasing the oxidation rate, as in conventional PECMP, but by controlling the relative contributions of reaction fields with opposite carrier-concentration responses. This process design provides an effective route for uniform, efficient, and slurry-less finishing of semiconductors and other hard materials with spatially inhomogeneous reactivity.