Association

90 5. Results II: EuO integration directly on silicon
Eu 3+ 4f
multiplet
Eu 2+ 4f
multiplet
Eu 2+ 4f
multiplet
Eu 3+ 4f
multiplet
Eu 2+ 4f
multiplet
Eu 3+ 4f
multiplet
Figure 5.4.: Valence bands of Al 2 O 3 , Al and Si in comparison with Eu 4f PES multiplets.
Miyazaki (2001), Drube (2012) and Ley et al. (1972). 99,177,178
From
both EuO compounds (i) and (ii) we observe a redistribution in peak intensity within the
divalent Eu 2+ 4f 6 final state from the lower binding energy side (−ΔIB ∗ ) towards the higher
binding energy side (+ΔIS ∗). This result suggests, that the 4f 6 final state multiplet of both EuO
compounds is composed of a bulk component B, located at a binding energy of E B 1.8 eV,
and a surface component S shifted towards higher binding energy due to the contact with
Al/Al 2 O 3 . The Eu 2+ 4f surface shifts for the EuO compounds (i) and (ii) are determined as
δ (i)
S
= 1.2 eV and δ(ii)
S
= 1.3 eV, respectively. A quantitative analysis of the Eu 3+ 4f interface
shift is impeded by the overlap with the O 2p states.
In order to quantify the chemical states of the Eu 4f photoemission peak in Fig. 5.3a and c,
we fit the 4f n−1 multiplets using convoluted Gaussian-Lorentzian curves. Three tunable parameters
were employed for least-squares fitting, namely the energy separation between the
divalent and trivalent Eu contributions, their intensity ratio and the spectral width (FWHM)
for every species. For Eu 2+ , a multiplet fine structure serves as reference for binding energy
in accordance with theoretical calculations by Cho et al. (1995). Least-squares best-fit curves
are shown by the solid curves in Fig. 5.3a and c, which match the experimental data points
very well, and the resulting intensity ratios and energy positions of Eu 2+ and Eu 3+ species
are summarized in Tab. 5.1. From the integrated spectral intensities A of the Eu 2+ and Eu 3+
components, we derive the relative fraction of trivalent Eu as r Eu3+ = A Eu3+ / (A Eu2+ + A Eu3+ ).
For the EuO compound (i), we determine mainly an integral divalent chemical state of the Eu
cations (trivalent contribution r4f
Eu3+ =7.4%). For the EuO compound (ii), in contrast, a mixed
initial valency with a trivalent contribution of r4f
Eu3+ ≈ 66% is derived. Thus, we confirm EuO
compound (i) asstoichiometric EuO and (ii) asoxygen-rich Eu 1 O 1+x .
A comparison of contributions to the valence level from the entire Al/EuO/Si heterostructure
is given in Fig 5.4. In Fig. 5.4a, the Al 2 O 3 valence band 177 coincides mainly with Eu 3+ PES
final states. The metallic Al valence band 99 (b) and the Si valence band 178 (c) coincide with
both Eu 2+ and Eu 3+ PES multiplets. 110,111,172,174,175 Thus, the oxidation state and thickness
of the Al overlayer influences the spectral weight observed at the energy position of Eu 4f
final states. Our Eu 3+ spectral fit does not include any Al 3s and Al 3+ 3s contributions 177
from the cap layer or the Si valence band 178 from the substrate in this energy region, which
may lead to a slight over-estimation of r4f Eu3+ .