Photochemical wet etching of single-crystal c-plane AlN under UV illumination

The photochemical wet etching of 2.3 mu m thick c-plane AlN single crystal layers grown on sapphire substrate was examined at elevated temperatures of 80 similar to 95 degrees C using a 0.89 M potassium hydroxide (KOH) etchant solution under ultraviolet (UV) illumination. The etch rate was determined to be 1.86 & Aring;/min. at 80 degrees C, increasing to 11.8 & Aring;/min. at 95 degrees C, with an activation energy of 1.29 eV. This is indicative of reaction-limited kinetics, and represents a giant enhancement over conventional wet etching of AlN, where no significant etching of the c-plane occurs under these conditions. The basal c-plane etches slowly in KOH under photochemical conditions, but also with extensive formation of hexagonal etch pits related to dislocations in the AlN. These etch pits had a hexagonal column and inverse pyramid shape surrounded by {101<overline>0} and {101<overline>1<overline>} planes, and in some cases the underlying sapphire was exposed during etch times in which only a few hundred angstroms of AlN were removed in the c-plane direction.