Association

80 4. Results I: Single-crystalline epitaxial EuO thin films on cubic oxides
study 27 investigates ultrathin EuO directly on MgO (100) and finds that epitaxial EuO can
be observed only after 2 nm of EuO deposition. This study also reveals a formation of EuO
on MgO (100) in the monolayer regime without oxygen supply, a similar effect as observed
for YSZ, 32,45 which supplies ionic oxygen at elevated temperatures. Facing the problem of
“interrupted epitaxy”, Swartz et al. (2012) focus on an additional oxide interlayer in order to
integrate the EuO unstrained lattice with the MgO substrate. 27
In this section, we focus on high-quality epitaxial EuO grown directly MgO (100) in order to
investigate structural and magnetic effects of the large lateral compressive strain.
Structural characterization of epitaxial EuO on MgO (100)
Figure 4.21.: EuO/MgO (100): EuO deposition under large compressive strain. Beginning from one
monolayer, EuO shows its native in-plane lattice parameter. A seamless EuO/MgO heteroepitaxy
is observed by RHEED, accompanied by a small fraction of polycrystalline EuO as indicated
by circular intensities. From 4 nm EuO on, reciprocal pattern improve towards largely singlecrystalline
EuO; however, the accumulated roughness prevents the observation of the EuO surface
structure by LEED after growth.
In Fig. 4.21, the in situ deposition of EuO directly on MgO (100) is monitored by electron
diffraction. The annealed MgO substrate shows sharp RHEED and LEED pattern indicating
a single-crystalline and smooth surface. Accounting for possible oxygen diffusion from the
substrate, one monolayer of Eu metal is deposited onto the heated MgO just before EuO
growth as a seed preventing over-oxidized EuO phases. When EuO growth begins, for 1–2
monolayers EuO we observe a RHEED pattern of MgO and EuO simultaneously without any
polycrystalline or amorphous phase. This is the first time, that a good heteroepitaxy of EuO
directly on MgO (100) without any non-crystalline stages is achieved. The observed RHEED
streaks of the first monolayer EuO exhibit the native EuO lattice parameter, thus large areas
of unstrained and epitaxial EuO have formed directly on MgO. Proceeding with the EuO
synthesis, from two monolayers EuO on, a small polycrystalline fraction indicated by circular
intensities next to the unstrained EuO pattern is observed in the RHEED photograph. Thus,
a small fraction of disorder or cracks must be present to allow the unstrained EuO to arrange
heteroepitaxially directly on MgO (100). From two monolayers EuO until the end of EuO
deposition, the surface crystal structure improves, and only EuO fcc pattern are observable
when EuO growth is finished (d = 15 nm). However, inherited from the initial polycrystalline