Photoelectrochemically combined mechanical polishing of n-type gallium nitride wafer by using metal nanoparticles as photocathodes

Abstract

In this study, we developed a photoelectrochemically combined mechanical polishing (PECMP) method to process n-type gallium nitride (GaN) wafer. The methodological features include (1) the pre-adsorption of metal nanoparticles (MNPs) within the polishing pad; (2) the use of ultraviolet (UV)-light to irradiate the wafer under the polishing solution containing oxidizing reagents and abrasives. During polishing, a tight contacting between MNPs and newly exposed GaN surface may construct a photoelectrochemical (PEC) circuit, similar as a solar cell system. In terms of PEC oxidation/etching mechanism, MNPs with a suitable work function can act as the photocathodes to facilitate the extraction of the photogenerated conduction band (CB) electrons of GaN by the oxidizing agents; thus, the photogenerated valence band (VB) holes may oxidize the GaN surface. We designed a novel equipment to enable the alternating of GaN wafer between PEC oxidation and mechanical polishing. Results show that when PECMP employing gold (Au) MNPs and 0.18 wt% SiO2 abrasives, the fastest material removal rate (MRR) attains 198.6 nm/h and the surface roughness (Ra) reaches 1.55 nm (5 × 5 μm2). PECMP has the distance-sensitive material removal ability due to the higher polishing pressure generated at the tops of rough surface, at which the tighter MNP-GaN contact causes the faster PEC oxidation. The presented results reveal that PECMP is a promising approach to polish inert semiconductors in high efficiency and high quality.