supplement material

1. SEM image of GaN stored under ambient conditions for ~2 days 2. XPS spectra of GaN in-situ sputtered by Ar+ 3. The spectrum of C 1s for GaN stored under ambient conditions 4. Schematic of the relationship between photoelectron emission angle and escape depth for ARXPS 5. XPS spectra for GaN treated by wet chemical methods 6. APXPS spectra for GaN treated by wet chemical methods 7. AFM images for different GaN samples