  NiO layers were grown on MgO(100), MgO(110), and MgO(111) substrates by
plasma-assisted molecular beam epitaxy under Ni-flux limited growth conditions.
Single crystalline growth with a cube-on-cube epitaxial relationship was
confirmed by X-ray diffraction measurements for all used growth conditions and
substrates except MgO(111). A detailed growth series on MgO(100) was prepared
using substrate temperatures ranging from 20 {\deg}C to 900 {\deg}C to
investigate the influence on the layer characteristics. Energy-dispersive X-ray
spectroscopy indicated close-to-stoichiometric layers with an oxygen content of
~47 at. % and ~50 at. % grown under low and high O-flux, respectively. All NiO
layers had a root-mean-square surface roughness below 1 nm, measured by atomic
force microscopy, except for rougher layers grown at 900 {\deg}C or using
molecular oxygen. Growth at 900 {\deg}C led to a significant diffusion of Mg
from the substrate into the film. The relative intensity of the quasi-forbidden
one-phonon Raman peak is introduced as a gauge of the crystal quality,
indicating the highest layer quality for growth at low oxygen flux and high
growth temperature, likely due to the resulting high adatom diffusion length
during growth. The optical and electrical properties were investigated by
spectroscopic ellipsometry and resistance measurements, respectively. All NiO
layers were transparent with an optical bandgap around 3.6 eV and
semi-insulating at room temperature. However, changes upon exposure to reducing
or oxidizing gases of the resistance of a representative layer at elevated
temperature were able to confirm p-type conductivity, highlighting their
suitability as a model system for research on oxide-based gas sensing.
