  We report the generation of photo-induced electromotive force (EMF) on the
surface of c-axis oriented InN epitaxial films grown on sapphire substrates. It
has been found that under the illumination of above band gap light, EMFs of
different magnitudes and polarities are developed on different parts of the
surface of these layers. The effect is not the same as the surface photovoltaic
or Dember potential effects, both of which result in the development of EMF
across the layer thickness, not between different contacts on the surface.
These layers are also found to show negative photoconductivity effect.
Interplay between surface photo-EMF and negative photoconductivity result in a
unique scenario, where the magnitude as well as the sign of the photo-induced
change in conductivity become bias dependent. A theoretical model is developed,
where both the effects are attributed to the 2D electron gas (2DEG) channel
formed just below the film surface as a result of the transfer of electrons
from certain donor-like-surfacestates, which are likely to be resulting due to
the adsorption of certain groups/adatoms on the film surface. In the model, the
photo-EMF effect is explained in terms of a spatially inhomogeneous
distribution of these groups/adatoms over the surface resulting in a lateral
non-uniformity in the depth distribution of the potential profile confining the
2DEG. Existence of such an inhomogeneity in the distribution of surface
potential has indeed been experimentally found for these layers.
