Association

94 5. Results II: EuO integration directly on silicon
Proceeding with the most complex photoemission final state structure among the Eu corelevels,
we present the Eu 4d core-levels in Fig. 5.5c and d. Their complex multiplet structure
is distributed in a wide energy range from 125–170 eV and we focus on the most prominent
double peak structure at lower binding energy. Due to the strong 4d–4f exchange
interaction and much weaker 4d spin–orbit splitting, these two 4d peak shapes cannot be
assigned 4d 3/2 and 4d 5/2 spin–orbit components. By assuming a J = L − S multiplet splitting
caused by the 4d–4f interaction, the two peaks are denoted as 7 D J and 9 D J multiplets, respectively.
112,114,115 The fine structure of the 7 D final state is not resolved, whereas the J = 2–6
components in the 9 D state are easily identified. In agreement with the other Eu core-levels,
we clearly observe a mainly divalent Eu 2+ valency in EuO (i), but significant spectral contributions
from Eu 3+ cations in oxygen-rich EuO (ii).
Finally, we performed a quantitative peak analysis by least-squares fitting of the spectral
contributions with convoluted Gaussian-Lorentzian curves. The best fit is shown by the solid
lines in Fig. 5.5a–d and matches the experimental spectra quite well. From the integrated
spectral intensities of the divalent Eu 2+ and trivalent Eu 3+ components, we derive a relative
fraction of Eu 3+ cations of only 2.8% for stoichiometric EuO (i) and of 59% for oxygen-rich
Eu 1 O 1+x (ii), both in excellent agreement with the Eu 4s and 3d analysis, as summarized in
Tab. 5.2. Again, the larger ratio of Eu 3+ from the Eu 4f spectra is explained by the summation
of Al and Si valence band spectral weight to the trivalent Eu peaks (as illustrated in Fig. 5.4
on p. 90).
Table 5.2.: Binding energies E B and Eu 3+ valency ratios r Eu3+ for (i) stoichiometric EuO and (ii) oxygenrich
Eu 1 O 1+x , as obtained by least-squares fitting the Eu 4s and 4d spectra.
(
Eu 4s E B Eu 2+ ) (
E B Eu 3+ ) r4f
Eu3+
4s spins ‖ 4f 4f Estates
spin ‖ 4f 4f Estates
spin
(i) EuO 355.4 eV 362.7 eV 7.3 eV 363.5 eV 370.7 eV 7.2 eV 2.2%
(ii) EuO 1+x 355.4 eV 362.6 eV 7.2 eV 363.5 eV 370.7 eV 7.2 eV 53.0%
⇆
(
Eu 4d E B Eu 2+ ) (
E B Eu 3+ ) r3d
Eu3+
S+1 D J 7 D J 9 D 6 Estates
spin 7 D J 9 D 6 Estates
spin 7 D J + 9 D 6
(i) EuO 134.2 eV 128.0 eV 6.2 eV 141.9 eV 135.4 eV 6.5 eV 2.8%
(ii) EuO 1+x 134.3 eV 128.0 eV 6.3 eV 141.9 eV 135.4 eV 6.5 eV 59.0%
⇆
HAXPES: Impact of the EuO phases on the EuO/Si interface chemical state, observed
by Si 2p photoemission
Having confirmed the stoichiometric quality of the EuO film on top of Si, we now proceed to
study the chemical state of the interface formed between the EuO layer and the Si substrate,
which is crucial for any electrical and spin transport. Here, we probe the local chemistry
and bonding at the EuO/silicon transport interface by HAXPES. Photoemission from the
Si 2p core-level was recorded in normal (0 ◦ ) and off-normal (45 ◦ ) emission geometry with
hard X-ray (hν = 4.2 keV) excitation. In this way, the information depth of the escaping
photoelectrons is substantially varied between ∼18.4 nm and ∼13.2 nm, respectively, which
allows to distinguish bulk and interface-like electronic states of the buried Si substrate.
In oxygen-rich EuO/HF-Si (ii), next to the well-resolved Si 0 2p peak, a broader feature can