Association

5.1. Chemical stabilization of bulk-like EuO directly on silicon 95
Figure 5.6.: Si 2p core-level
photoemission spectra
for stoichiometric EuO (i)
and oxygen-rich EuO (ii),
recorded at hν = 4.2 keV
in normal (α ≈ 0 ◦ ) and offnormal
(α =45 ◦ ) electron
emission geometry.
be observed on the high binding energy side as depicted in Fig. 5.6. It is attributed to the
emission from Si n+ 2p states, which indicates the presence of the silicon oxide, SiO x . The
chemical shift of ΔE ∼ 3.0 eV corresponds to Si 3+ oxidation states, and ΔE ∼ 4.0 eV is the
dioxide with Si 4+ ions 139 . The Si 4+ 2p spectral intensity is enhanced for off-normal emission,
which confirms that the silicon dioxide signal mainly originates from the EuO/Si interface
rather than deeper Si regions.
For stoichiometric EuO/HF-Si (i), we observe the well-resolved Si 2p doublet structure for
both emission geometries, as depicted in Fig. 5.6. This doublet is indicative for an exclusively
integral Si 0 valency both in the bulk and interface regions of the substrate. Any other valencies
are absent in these Si spectra, which confirms the chemical stability of the EuO/HF-Si
interface, which is in agreement with the predicted thermodynamic stability of EuO in contact
with the Si surface. 14
We finally conclude, that the chemical state of the EuO/HF-Si interface directly correlates
with the specific EuO growth conditions at elevated substrate temperatures. In particular,
any oxygen excess during EuO synthesis not only leads to the formation of antiferromagnetic
EuO phases (favorably Eu 3 O 4 ), but also promotes an oxidation of the EuO/Si interface. Only
if the Eu distillation process and a precise oxygen supply is maintained, stoichiometric EuO
thin films can be grown directly on Si without interface oxidation. The results of this section
are published in Caspers et al. (2011, 2012). 3–5
The highly sensitive interface chemistry observed for oxygen-rich EuO in contact with silicon
motivates further investigations of the chemical interface properties of EuO on Si – both of
which featuring high reactivity and and atomic mobility at elevated temperatures. Therefore,
we conduct a thermodynamic analysis in order to elucidate the interface chemistry of the
functional EuO/Si (001) heterointerface in the following section.