Association

116 5. Results II: EuO integration directly on silicon
of EuO was observed in EuO on cubic oxide studies (Ch. 4). In this way, we prepare three
EuO/H-Si heterostructures with two monolayers oxygen-protective Eu at temperatures
T S = 400 ◦ C, 450 ◦ C, or 500 ◦ C.
After synthesis of the EuO/Si (001) heterostructures, we analyze the surface structure of the
EuO magnetic oxide layer by RHEED. The pattern of an fcc rocksalt lattice of EuO (001) is
clearly observed after 2 nm EuO growth, with a lattice constant matching the silicon cubic
lattice parameter. In the EuO reciprocal rods, the vertical intensities are distributed dot-like,
which indicates an epitaxial growth with three-dimensional structures on the passivated Si
(001).
A quantitative analysis of the chemistry at the EuO/Si heterointerface with combined passivations
is obtained from Si 2p spectra by HAXPES, in a geometry adjusted to probe precisely
down to the Si interface. This is achieved by using the minimum excitation energy
of hν = 2.3 keV of the HAXPES beamline (KMC1, see Ch. 3.4.3) and an off-normal emission
angle of 30 ◦ . The Si 2p doublet provides the advantage that both the silicides and oxides can
be well distinguished from bulk Si and quantitatively analyzed in one core-level (Fig. 5.25).
For the silicides, the temperature variation does not affect the interfacial thickness of EuSi 2 ,
which we determined as d EuSi2 = 0.2 nm for all three heterostructures, in good agreement
with the optimum value of the silicide optimization series (Ch. 5.3). The oxidation of the Si
interface exhibits a minimum value of d SiOx = 0.69 nm for the heterostructure synthesized at
T S = 450 ◦ C, while a maximum oxidation of the Si interface is observed for the EuO/Si interface
exposed to the highest temperature of EuO synthesis, T S = 500 ◦ C. Interfacial silicide
and SiO x thicknesses are summarized in Fig. 5.26.
We identify a general shift to slightly higher values of interfacial SiO x in this study of combined
Si passivation when compared to the explicit optimization regarding SiO x by protective
Eu monolayers in Ch. 5.4. This can be explained by the additional hydrogen termination procedure
of Si (001), which systematically introduces a fraction of at least 10 −6 oxygen in the
hydrogen gas during the in-situ H-annealing process. The largest oxidation of the EuO/Si
interface at T S = 500 ◦ C can be understood by an increased diffusion of Si surface atoms in
the high temperature regime, changing the interface morphology and thus providing more
reactive faces in contact with the EuO synthesis.
In conclusion, we carried out three comprehensive HAXPES studies in order to investigate
and optimize the chemical properties of the passivated EuO/Si interface by a quantitative
analysis of silicon core-level spectra. The HAXPES results for the EuO/Si interface are compiled
in Fig. 5.26. In a first step, the interfacial silicide was minimized by an increase of
synthesis temperature from 350 ◦ Cto450 ◦ C (black spheres in Fig. 5.26), while the application
of H-passivation to the Si (001) surface yields only a smaller reduction of the silicide
content (red spheres in Fig. 5.26), resulting in a minimum of 0.14 nm silicide at the EuO/Si
interface. In a second step, the SiO x contamination at the EuO/Si interface is diminished
by oxygen-protective Eu monolayers, resulting in a minimum of 0.42 nm interfacial oxide
(golden spheres). Finally, a combination of the advantageous passivation parameters from the
silicide and the SiO x optimization studies reveal an optimum set (blue spheres in Fig. 5.26)