Crystallographic wet chemical etching of GaN

We demonstrate well-controlled crystallographic etching of wurtzite GaN grown on c-plane sapphire using ${\mathrm{H}}_{\mathrm{3}}{\mathrm{PO}}_{\mathrm{4}},$ molten KOH, KOH dissolved in ethylene glycol, and NaOH dissolved in ethylene glycol between 90 and 180 $°$ C, with etch rates as high as 3.2 $\mu$ m/min. The crystallographic GaN etch planes are {0001}, $\mathrm{\{101̄0\}}$ , $\mathrm{\{101̄ 1̄\}}$ , $\mathrm{\{101̄ 2̄\}}$ , and $\mathrm{\{101̄3\}}$ . The vertical $\mathrm{\{101̄0\}}$ planes appear perfectly smooth when viewed with a field-effect scanning electron microscope. The activation energy is 21 kcal/mol, indicative of reaction-rate limited etching.