Association

5.1. Chemical stabilization of bulk-like EuO directly on silicon 91
HAXPES: Analysis of the Eu 3d deep core-levels
Now we proceed with spectra of the more deeply bound Eu 3d core-levels, which helps to
provide a consistent quantification of the initial state valency in conjunction with the Eu 4f
results. We note, that analyzing the 3d core-level spectra has a significant advantage for the
determination of the initial state Eu valency compared e. g. to the 4d core-levels, because the
3d states show a much weaker multiplet splitting and larger photoexcitation cross section.
Moreover, the kinetic energy of Eu 3d photoelectrons is reduced by about 1150 eV, thus the
probe is more surface sensitive within the EuO layer, as sketched in Fig 5.2.
Eu 3d photoemission spectra for EuO compounds (i) and (ii), recorded in normal emission
geometry, are depicted in Fig. 5.3f and h. The spectra consist of a main peaks, the spin–orbitlike
final state groups 3d j= 5/2 and 3d j= 3/2, which are clearly separated by a large spin–orbit
coupling of 29.2 eV in agreement with previous studies. 159 The broad structure in the center
is assigned to extrinsic plasmon excitations caused by high energy 3d photoelectrons, which
dissipate the plasmon energy in the electron gas of the metallic Al top layer. 173 The plasmons
are located at the expected energy loss (16 eV) from the 3d5/2 peak and the 3d3/2 peak.
Stoichiometric EuO (i) shows one main peak in each of the Eu 3d5/2 and 3d3/2 groups in Fig. 5.3f.
Within the two spin–orbit split groups, the distribution of final states m J is asymmetrically
shaped, in perfect agreement with theoretical calculations of the divalent Eu 3d multiplet,
as depicted for comparison in Fig. 5.3j. 110 A satellite peak in the high binding energy region
of the Eu 3d5/2 (and 3d3/2) multiplet separated by 7.8 eV (6.3 eV) from the main peak belongs
to the divalent Eu 2+ multiplet of the 3d 9 4f 7 final state. Energy splitting as well as intensity
ratios of the satellites agree with previous reports on calculated and measured multiplet
spectra of divalent Eu compounds. 110,111
Evaluating the oxygen-rich EuO compound (ii) inFig.5.3h, we observe prominent double
peak structures in the 3d5/2 (3d3/2) regions, which are separated by 10.45 eV (10.90 eV). We
assign these features to divalent Eu 2+ (3d 9 4f 7 final state) and trivalent Eu 3+ (3d 9 4f 6 final
state) spectral contributions. Similar with the divalent Eu structure, the Eu 3+ 3d multiplet
consists of a doublet whith a satellite peak appearing at 6.9 eV higher binding energy below
the 3d5/2 main peak. The Eu 3+ 3d3/2 satellite structure is broadly distributed (not shown). The
energy positions and shape of the Eu 2+ and Eu 3+ 3d multiplet final states agree well with
previous experiments 176,180,181 and theoretical calculations, 110,111 as illustrated in Fig. 5.3j.
For a depth-sensitive analysis of the 3d core-levels, we calculate the energy and angle-depen-
Table 5.1.: 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 of the Eu 4f and 3d spectra.
Eu 4f
EB
Eu2+
EB
Eu3+
r Eu3+
4f
(i) EuO 1.8 eV 7.0–11.1 eV 7.4 ± 1.2%
(ii) EuO 1+x 1.64 eV 7.0–11.1 eV 66 ± 2%
Eu 3d
EB Eu2+ (eV) EB Eu3+ (eV) r3d
Eu3+
j = 5/2 3/2 E SO 5/2 3/2 E SO 5/2 3/2
(i) EuO 1124.9 1154.1 29.2 1134.7 1164.4 29.7 4.1% 3.2%
(ii) EuO 1+x 1125.0 1154.3 29.3 1134.8 1164.5 29.7 59% 49%