Association

18 2. Theoretical background
Figure 2.10.: Schematic band structure of EuO for the Δ direction in k space (a, left). For coherent
tunneling, evanescent waves in the imaginary part κ need to match the real band structure q of
the EuO conduction band. Majority and minority spins band are at different energy heights, as
highlighted in (a). 75 Evanescent waves in the imaginary part exhibit different decays rates in the
electrode for majority and minority spins, as emphasized in (b). 76 These differences are the basis
for spin-selective coherent tunneling.
In practice, the spin injection interface must be crystalline and epitaxial in order to allow
for a matching of symmetry bands. EuO thin films can be grown epitaxially with singlecrystalline
structural and magnetic properties by reactive MBE on cubic oxides, 25,27,32 thus
forming a basis for a possible coherent tunnel functionality. This may also be achieved on
cubic silicon crystals. Therefore, we investigate EuO directly on silicon with the demand of
high interface quality in Ch. 5.
Moreover, to date, no band structure for coherent tunneling is reported for tunnel contacts
of epitaxial EuO on Si (001). This motivates spin-dependent density functional theory calculations
of EuO symmetry bands matching Bloch states of Si (001), in order to compare the
spin filter efficiency of experimental epitaxial EuO tunnel contacts with models in the frame
of band structures.
2.4. Hard X-ray photoemission spectroscopy
A characterization of buried EuO layers and interfaces is necessary in order to understand
their chemical and electronic properties. In response to the experimental need for a large
and tunable probing depth, we select a specialized spectroscopic technique in this thesis.
Hard X-ray photoemission spectroscopy (HAXPES) is perfectly suited to characterize our EuO
heterostructures, which we introduce in the following.
The story of photoemission already began in 1887, when Heinrich Hertz observed the emission
of electrons by light from solids, 77 which was not compatible with Maxwell’s equations
describing a continuous wave theory. Later in 1905, this photo-induced electron emission