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We report on the electronic properties of GaN$(1\bar{1}00)$ and $(0001)$surfaces after three different and subsequent device processing compatiblecleaning steps: HCl etching, annealing at $400$ $^\circ$C in N$_2$ atmosphere,and O$_2$ plasma exposure. The surface electronic properties are quantified, inthe dark and under ultraviolet illumination, using X-ray photoelectronspectroscopy and a Kelvin probe. We find that the cleaning steps largely affectthe work function and the band bending of both GaN orientations. Thesemodifications are attributed to the presence of different surface states aswell as to the formation of adsorbates building up distinct surface dipoles.Besides these results, we detect that under ultraviolet illumination the workfunction of the surfaces exposed to HCl decreases by at least $0.2$ eV withoutscreening of the band bending. We thus attribute the observed surfacephotovoltage to a photo-induced modification of the surface dipole. Overall,these results emphasize the strong dependence of the electronic properties ofair-exposed GaN surfaces on adsorbates. As a result, we advocate the use of thecommon cleaning steps analyzed here to re-initialize at will GaN$(1\bar{1}00)$and $(0001)$ surfaces into pre-defined states.

Electronic properties of air-exposed GaN$(1\bar{1}00)$ and $(0001)$ surfaces after several device processing compatible cleaning steps