Association

4.3. Lateral compressive strain: EuO on MgO (100) 81
Figure 4.22.: EuO/MgO (100):
different epitaxial arrangements.
In (a), a
cube-on-cube relation is
illustrated inducing large
compressive strain to EuO.
In the EuO/MgO 4:5 configuration,
a minimum lateral
strain of +2.4% acts on EuO
(b). If however, EuO keeps
its native lattice parameter
(c), no simple cube-on-cube
relation is realized and
electrostatic distortions will
readily occur.
fraction, a certain roughness remains and hampers observation of a surface crystal structure
in low energy electron diffraction.
We elucidate the observed initial polycrystalline fraction and the quasi-relaxed EuO growth
at the EuO/MgO (100) interface in terms of geometrical and electrostatical aspects: In Fig. 4.22,
three configurations of the EuO/MgO (100) heteroepitaxy are illustrated assuming different
lateral lattice parameters (e. g. in [010] direction) of the initial EuO layers. From our
structural investigation, we can exclude the direct cube-on-cube heteroepitaxy (in Fig. 4.22a)
comprising large compressive strain, which can be explained as follows: the deposition of
one atomically flat EuO layer on an MgO (100) surface is equivalent to a substitution of the
Mg atoms of the top layer. Here, the ion-size difference 167 forms different sizes of the Mg–O
and the Eu–O bond: in the MgO surface, replacing the Mg 2+ ion with a Eu 2+ ion changes
the cation ionic diameter to 0.234 nm and increases the O–O nearest neighbor bond by 22%
from 0.298 nm to 0.363 nm. 168 In such a case, the ion-size difference would force the Eu or O
atoms to find equilibrium positions in a roughened structure. 27 Considering a second configuration,
a EuO/MgO lateral arrangement of 4:5 as sketched in Fig. 4.22b has a lateral strain
with a minimum value of +2.4%. From an electrostatic view point, strong Coulomb repulsion
between O 2− –O 2− and Eu 2+ –Mg 2+ ions at alternating position would certainly lead to
structural roughening at the interface. From our electron diffraction results, we observe in-