Photoelectrochemically combined mechanical polishing of n-type gallium nitride wafer by using metal nanoparticles as photocathodes
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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.
KeywordsGallium nitride Photoelectrochemically combined mechanical polishing Metal nanoparticles Material removal rate Chemical mechanical polishing
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This work received financial support from the National Natural Science Foundation of China (No. 51575085, 91523102, 21273183, 21621091), the National key Research and Development Program of China (Grant No.2016YFB1102205), and the National Science and Technology Major Project of China (Grant No. 2014ZX02504001004).
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