  We report on the electronic properties of GaN$(1\bar{1}00)$ and $(0001)$
surfaces after three different and subsequent device processing compatible
cleaning steps: HCl etching, annealing at $400$ $^\circ$C in N$_2$ atmosphere,
and O$_2$ plasma exposure. The surface electronic properties are quantified, in
the dark and under ultraviolet illumination, using X-ray photoelectron
spectroscopy and a Kelvin probe. We find that the cleaning steps largely affect
the work function and the band bending of both GaN orientations. These
modifications are attributed to the presence of different surface states as
well as to the formation of adsorbates building up distinct surface dipoles.
Besides these results, we detect that under ultraviolet illumination the work
function of the surfaces exposed to HCl decreases by at least $0.2$ eV without
screening of the band bending. We thus attribute the observed surface
photovoltage to a photo-induced modification of the surface dipole. Overall,
these results emphasize the strong dependence of the electronic properties of
air-exposed GaN surfaces on adsorbates. As a result, we advocate the use of the
common cleaning steps analyzed here to re-initialize at will GaN$(1\bar{1}00)$
and $(0001)$ surfaces into pre-defined states.
