  Orientation, structure and morphology in the early growth stages of CuO films
on strontium titanate were studied by synchrotron radiation X-ray diffraction.
Nanostructured CuO films were obtained by ex situ heat treatment at 970 K in
oxygen flow at ambient pressure after an initial deposition of Cu on
SrTiO$_3$(001) at $10^{-4}$ Pa oxygen pressure using the molecular beam epitaxy
technique. These films at low coverages grow forming islands a few tens of
nanometers wide, with the [010] direction perpendicular to the substrate
surface. Two types of epitaxies are observed, CuO[001]//SrTiO$_3$[100] and
CuO[100]//SrTiO$_3$[100], the former being the preferred one. The combination
of these epitaxies with the P4mm surface symmetry of the substrate generates 16
different orientations of nanostructures. The lattice constant values obtained
from X-ray diffraction are very close to those of bulk tenorite, with the
exception of the monoclinic angle $\theta$, which is reduced from 99.54$^\circ$
to 96.76(8)$^\circ$ by the epitaxy constraints. This angle is a key parameter
in determining the magnetic properties of CuO.
