Hydration effects in the photoassisted wet chemical etching of gallium nitride

In this paper, we report a pH dependent study of the wet chemical etching of unintentionally doped n-type gallium nitride (GaN) layers grown on sapphire substrates. When illuminated from a 253.7-nm mercury line source, etching of GaN is found to take place in aqueous potassium hydroxide (KOH) solutions of pH values ranging from 11 to 15. The measured GaN etch rate reveals a peak value of 90 nm/min at pH=14.25 and drops rapidly on both sides of the peak position. The etch effect ceases in KOH/ethanol solutions of similar molarities. These observations are attributed to a hydration related phenomenon in which competing effects from free water molecules (H/sub 2/O)/sub f/ and hydroxyl ions (OH/sup -/) result in a peak etch rate. Our analysis indicates the slowly falling tail of the GaN etch rate in the dilute pH regime is characterized by a (H/sub 2/O)/sub f/ dominant process while the fast variation of etch rate at pH>14 is due to a competition between (H/sub 2/O)f and OH/sup -/. PACS numbers: 68.55.Jk, 73.50.Pz, 81.65.Cf, 81.65.Hq.