Chemical polishing of bulk and epitaxial GaN

Bulk single crystals of GaN and heteroepitaxial GaN layers were subjected to free-etching and mechano-chemical polishing in aqueous solutions (10-1N) of KOH and NaOH. It has been established that free-etching of bulk crystals is kinetically controlled and strongly anisotropic, resulting in the formation of numerous stable pyramids. Etching is terminated when the {0 0 0 1}-oriented surface of GaN is completely covered by these pyramids. On the other hand, when a soft polishing pad and pressure above 2 kg/cm(2) are employed, all surface irregularities (etch pyramids, roughness after mechanical polishing, growth hillocks on epitaxial layers) are removed. The procedure is very effective: removal of a few tenths of a micron from the surface are usually sufficient to polish out irregularities 200 nm in height. When optimized mechano-chemical polishing is used, atomically flat surfaces of bulk GaN have been reproducibly obtained (RMS = 0.1 nm), as measured by Atomic Force Microscopy.