  In this paper, the Ni Schottky barrier on GaN epilayer grown on free standing
substrates has been characterized. First, transmission electrical microscopy
(TEM) images and nanoscale electrical analysis by conductive atomic force
microscopy (C-AFM) of the bare material allowed visualizing structural defects
in the crystal, as well as local inhomogeneities of the current conduction. The
forward current-voltage (I-V) characteristics of Ni/GaN vertical Schottky
diodes fabricated on the epilayer gave average values of the Schottky barrier
height of 0.79 eV and ideality factor of 1.14. A statistical analysis over a
set of diodes, combined with temperature dependence measurements, confirmed the
formation of an inhomogeneous Schottky barrier in this material. From a plot of
FB versus n, an ideal homogeneous barrier close to 0.9 eV was estimated,
similar to that extrapolated by capacitance-voltage (C-V) analysis. Local I-V
curves, acquired by means of C-AFM, displayed the inhomogeneous distribution of
the onset of current conduction, which in turn resembles the one observed in
the macroscopic Schottky diodes. Finally, the reverse characteristic of the
diodes fabricated in the defects-free region have been acquired at different
temperature and its behaviour has been described by the thermionic field
emission (TFE) model.
