  The growth of NiO on GaN(00.1) substrates by plasma-assisted molecular beam
epitaxy under oxygen rich conditions was investigated at growth temperatures
between 100 $^{\circ}$C and 850 $^{\circ}$C. Epitaxial growth of NiO(111) with
two rotational domains, with epitaxial relation
$\normalsize{}\mathrm{\mathrm{\mathrm{NiO}(1\bar{\mathrm{{1}}}0)}\:||\:\mathrm{\mathrm{GaN}(11.0)}}$
and
$\mathrm{\mathrm{\mathrm{NiO}\mathrm{(10\bar{\mathrm{{1}}})}\:||\:\mathrm{GaN(11.0)}}}$,
was observed by X-ray diffraction (XRD) and confirmed by in-situ reflection
high-energy electron diffraction as well as transmission electron microscopy
(TEM) and electron backscatter diffraction. With respect to the high lattice
mismatch of 8.1 % and a measured low residual tensile layer strain, growth by
lattice matching epitaxy or domain matching epitaxy is discussed. The
morphology measured by atomic force microscopy showed a grainy surface,
probably arising from the growth by the columnar rotational domains visible in
TEM micrographs. The domain sizes measured by AFM and TEM increase with the
growth temperature, indicating an increasing surface diffusion length. Growth
at 850 $^{\circ}$C, however, involved local decomposition of the GaN substrate
that lead to an interfacial
$\mathrm{\beta}$-Ga$\mathrm{_{2}}$O$\mathrm{_{3}}$($\bar{\mathrm{{2}}}$01)
layer and a high NiO surface roughness. Raman mesurements of the
quasi-forbidden one-phonon peak indicate increasing layer quality (decreasing
defect density) with increasing growth temperature. The results above suggest
optimum growth temperatures around 700 $^{\circ}$C for high layer and interface
quality.
