  We report on the optoelectronic properties of GaN$(0001)$ and $(1\bar{1}00)$
surfaces after their functionalization with phosphonic acid derivatives. To
analyze the possible correlation between the acid's electronegativity and the
GaN surface band bending, two types of phosphonic acids, n-octylphosphonic acid
(OPA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA), are grafted on
oxidized GaN$(0001)$ and GaN$(1\bar{1}00)$ layers as well as on GaN nanowires.
The resulting hybrid inorganic/organic heterostructures are investigated by
X-ray photoemission and photoluminescence spectroscopy. The GaN work function
is changed significantly by the grafting of phosphonic acids, evidencing the
formation of dense self-assembled monolayers. Regardless of the GaN surface
orientation, both types of phosphonic acids significantly impact the GaN
surface band bending. A dependence on the acids' electronegativity is, however,
only observed for the oxidized GaN$(1\bar{1}00)$ surface, indicating a
relatively low density of surface states and a favorable band alignment between
the surface oxide and acids' electronic states. Regarding the optical
properties, the covalent bonding of PFOPA and OPA on oxidized GaN layers and
nanowires significantly affect their external quantum efficiency, especially in
the nanowire case due to the large surface-to-volume ratio. The variation in
the external quantum efficiency is related to the modication of both the
internal electric fields and surface states. These results demonstrate the
potential of phosphonate chemistry for the surface functionalization of GaN,
which could be exploited for selective sensing applications.
