A Study of the Photoelectrochemical Etching of n-GaN in H₃PO₄ and KOH Electrolytes

We investigated the photoelectrochemical etching of n-GaN in H₃PO₄ and KOH as a function of electrolyte concentration, potential and light intensity. Etch rates measured by stylus profilometry were compared with coulometric and amperometric values. In both electrolytes, etch rates increased with concentration, reaching a maximum at 3.0 mol dm^-3 and decreasing at higher concentrations. The increase in etch rate with concentration of either H₃PO₄ or KOH reflects the amphoteric nature of gallium and the decrease above 3.0 mol dm^-3 is attributed to common-ion effects. Profilometric etch rates were lower than coulometric and amperometric etch rates reflecting formation of a surface film. SEM and profilometry demonstrated that thick surface films are formed at lower concentrations. Etch rates increased linearly with light intensity indicating a carrier-limited etching regime: a quantum efficiency of 57.6% was obtained. At light intensities greater than ∼35 mW cm^-2 the etch rates showed evidence of saturation. AFM and SEM images of the etched GaN surfaces showed a distinctive ridge-trench structure with a hexagonal appearance. Photoluminescence spectra of the etched GaN show a significant increase in the defect-related yellow luminescence peak suggesting correlation to the formation of the ridge structures, which may represent dislocations terminating at the surface.