Association

124 6. Conclusion and Outlook
we focus on the characterization and careful optimization of the spin-functional EuO/Si heterointerface.
First, we explore how to stabilize metastable EuO on Si using Oxide-MBE. We adapt a common
strategy from silicon technology, the surface etching with hydrofluoric acid (HF), in
order to remove native SiO 2 and chemically passivate the Si surface (H-Si). On such passivated
Si substrates, we synthesized polycrystalline EuO using different growth parameters.
By means of HAXPES, we clearly distinguished two EuO valency phases: stoichiometric EuO
and over-oxidized Eu 1 O 1+x . For the stoichiometric EuO heterostructures, we confirmed a
bulk-like ferromagnetism – this is an important prerequisite for magnetic EuO tunnel contacts.
This result paves the way for a following interface study of ultrathin EuO tunnel barriers
directly on Si (001).
In the next step of our integration of ultrathin EuO directly with silicon, we focus on experimentally
realizing a high structural and chemical quality of the EuO/Si heterointerface. Such
atomically sharp interfaces of high crystalline quality may lead to coherent tunneling. Thus,
we carefully studied how to chemically control the highly reactive EuO/Si heterointerface
in order to permit an epitaxial integration of EuO on Si (001). Our thermodynamic analysis
of the EuO/Si interface – taking into account the high chemical reactivity of Eu, EuO, and
Si during EuO synthesis at elevated temperatures – gives us a guideline of probable reaction
paths. In this way, we decided to compare three complementary in situ passivation procedures
of the Si (001) surface: (i) hydrogen passivation, (ii) Eu passivation in the monolayer
regime, and (iii) SiO x formation in the Ångström regime. We apply these passivations selectively
against interfacial silicon oxides and silicide formation – both being major antagonists
to efficient spin filter tunneling.
In order to quantify the impact of different chemical passivations of the Si (001) surface
on the EuO/Si interface properties, we conducted optimization studies comprising the control
of surface crystalline structure and chemical interface properties by electron diffraction
techniques and HAXPES, respectively. We conclude, that the respective Si surface passivations
have minimized silicon oxides to below 1 nm and interfacial silicides to 2 Ångströms,
which is clearly in the sub-nanometer regime. In particular, for an optimum passivation
using the SiO x method, the ultrathin EuO layer showed a magnetic saturation moment of
5μ B /EuO, comparable to that of bulk EuO. Thus, our work demonstrates how to successfully
apply chemical passivations to the Si (001) surface and – at the same time – maintain a
mainly heteroepitaxial integration of EuO with the Si (001) wafer. This is an important result
for possible spin-dependent coherent tunneling in EuO/Si contacts. Such optimized EuO/Si
heterointerfaces may be used as spin-functional EuO tunnel contacts for silicon spintronic
devices in the near future.