Association

88 5. Results II: EuO integration directly on silicon
shifted towards higher binding energies by reduced Coulomb repulsion and couples antiferromagnetically;
this 4f 6 configuration will exhibit more complex final-state multiplet effects
that have been studied e. g. by Lang et al. (1981). 172
HAXPES experiments were carried out at the undulator beamline P09 at PETRA III (DESY
Hamburg) using a photon excitation energy of 4.2 keV. The spectra were recorded at room
temperature in normal emission geometry with an energy resolution of 0.5 eV. The binding
energy scale was calibrated to the metallic Fermi edge of an Au foil in contact with the
sample. A Shirley-type background was subtracted from the raw data to correct inelastic
photoelectron scattering.
Eu 4f valence band spectra in normal emission geometry are depicted in Fig. 5.3a and c
for both EuO compounds (i) and (ii). A center of spectral weight is located 1.8 eV below
E F in both spectra, which can be clearly identified as electron emission from the divalent
Eu 2+ initial state in a 4f 7 → 4f 6 final state configuration. The Eu 2+ peak agrees well with the
calculated divalent Euf 4f multiplet depicted in Fig. 5.3e. 173 Moreover, a doublet peak centered
at ∼4.8 eV below E F is assigned to the emission from O 2p orbital, which originates from
both EuO and Al 2 O 3 . The absence of a second multiplet in the higher binding energy region
in Fig. 5.3a indicates, that the Eu cations in compound (i) are mainly of divalent valency, as
expected for stoichiometric EuO.
In contrast, the 4f spectrum of the EuO compound (ii) in Fig. 5.3c shows a large additional
multiplet structure in the binding energy region from 5–13 eV below E F , which corresponds
to the 4f 6 → 4f 5 final state multiplet of trivalent Eu 3+ . The broad Eu 3+ final state structure
is in accordance with previous experimental studies on oxidized Eu compounds 159,160,174,176
and with calculated multiplet lines 172,175 (Fig. 5.3e). Due to over-oxidation in compound (ii),
the contribution of the overlapping O 2p peak has significantly increased. From the presence
of both 4f 6 and 4f 5 final state multiplets, we anticipate, that the initial state valency of Eu
in compound (ii) is of mixed divalent and trivalent nature, which is indicative for the Eu 3 O 4
oxidation phase.
A necessary criterion to identify the stoichiometry of the magnetic oxide EuO is its initial
state valency. Therefore, we quantify the spectral contributions of divalent (n = 7) and trivalent
(n = 6) Eu cations from the respective 4f n−1 peak intensities. However, the EuO surface
electronic structure may significantly differ from that of bulk EuO, due to changes in
local atomic geometry or coordination in direct proximity to the Al/Al 2 O 3 capping layer.
Thus, surface-shifted and bulk-like final states may contribute to the Eu photoemission peak,
shifted in binding energy E B by δ S = EB
surface − EB
bulk . To discriminate between surface and
bulk-related photoemission, we vary the depth sensitivity of the HAXPES experiment by
recording spectra in normal emission (ne) and 43 ◦ off-normal emission (oe) geometry. From
our calculated energy and angle-dependent information depths ID 4f of photoelectrons, † we
expect an enhanced surface sensitivity of 30% for 4f photoemission in (oe) geometry compared
to the bulk-sensitive (ne). Depth-dependent information can be extracted from Fig.
5.3b and d, which display the difference intensity curves of the normalized and backgroundcorrected
4f spectra recorded in (oe) and (ne) geometries, ΔI4f
∗ = Ioe ∗ − Ine. ∗ In these plots,
ΔI ∗ < 0(> 0) correspond to contributions from bulk (IB ∗ ) (surface (I S ∗ )) photoemission. For
We refer to Ch. 3.4.3 on pp. 48 for details on the beamline and the electron analyzer.
† see Ch. 3.4.4 on p. 51, or references Powell et al. (1999), Tanuma et al. (2011). 84,179