Association

114 5. Results II: EuO integration directly on silicon
Figure 5.24.: Quantitative results of the SiO x analysis in EuO/Eu-Si.
emission are used (Fig. 5.23a). By increasing the off-normal emission angle further than to
α =30 ◦ , the information depth underruns the minimum for an acquisition of Si 1s spectra;
this indicates thus the threshold of probing exclusively the Si interface layer. Two core-level
photoemission spectra of Si, Si 2p and 1s, are inspected as depicted in the two panels of
Fig. 5.23c. A quantitative analysis, however, that demands for a smooth and well-resolved
curve of the SiO x components is only feasible with the Si 1s deep core-level, as compiled in
Fig. 5.23d.
The chemical shifts of the silicon oxide are reduced due to final state effects, in agreement
with a comprehensive SiO x /Si PES study 139 for ultrathin SiO x films in the 2 nm range. Evaluating
the chemical shifts of the SiO x in detail reveals the shift to be not compatible with
solely Si 4+ (SiO 2 ), but rather a mixture of oxidation states of Si. If one or two ML Eu were
applied, the fraction of Si 3+ ranges from 33% to 38%, whereas this fraction is increased up to
54% on the cost of Si 4+ if three ML protective Eu were applied. The redistribution of the silicon
oxide to the valency Si 3+ –different to the native oxide which is always Si 4+ – points out a
characteristic low oxidation of the silicon wafer, which takes place during the oxygen-limited
(1.5 × 10 −9 Torr) synthesis of EuO.
In order to quantify the interfacial SiO x formation, we apply a least-squares peak fitting
analysis after Levenberg-Marquardt for the three EuO/Si heterostructures with one up to
three ML of protective Eu at the EuO/Si interface. The minimization of interfacial SiO x in
the EuO/Si heterointerface with Eu passivation monolayers (ML) results in 0.67 nm SiO x for
one ML Eu, 0.43 nm SiO x for two ML Eu, and a minimum of 0.42 nm SiO x if3MLEuwere
provided onto Si (001) just before EuO synthesis (summarized in Fig. 5.24). The residual
silicon oxide from a flashed clean silicon wafer was determined to be only 0.11 nm and is
included in all heterostructures.
In conclusion, two ML of protective Eu the silicon (001) surface has been passivated with
are sufficient to limit the total thickness of interfacial Si oxidation to 0.43 nm. Moreover, the
dominant oxidation state of the Si interface changes from Si 4+ to the Si 3+ valence state, and a
EuO/Si (001) heteroepitaxy with adaption of the Si lattice parameter can be maintained.