SRPES spectra: (a) Ga 3d photoelectrons, (b) O 1s photoelectrons and (c) valence bands with surface treatments. The position of valence band maximum was determined by the linear extrapolation as shown in (c).
Change of Ga/N atomic ratio with the detection angle. The surface normal of sample was defined as 90°.
Schematic of energy band diagrams below the interfaces of Ti/n-type GaN: (a) the HCl-treated contact and (b) the plasma-treated contact.