Association

5.4. Interface engineering II: Eu passivation of the EuO/Si interface 111
tures: (i) we clearly observe a Brillouin-shaped magnetization curve for the bulk EuO film
with the native silicon oxide as diffusion barrier (green curve). The blue and red lines represent
the M(T ) curves of ultrathin EuO on H-passivated Si (001) with the only difference
being the temperature of synthesis: a low temperature of 350 ◦ C vs. a high temperature of
450 ◦ C are chosen in order to provide consistency with the EuO/Si heterostructures investigated
by HAXPES. Both curves of ultrathin EuO exhibit a reduced Curie temperature of
T C = 58 K for the high T S heterostructure (blue curve) and T C ≈ 10 K for the low T S structure
(red curve). The T C of the high temperature EuO (T S = 450 ◦ C) agrees well with the magnetic
results obtained for 2 nm-thin EuO films on Si from a previous work; 48 therefore, we
ascribe the reduction of the magnetic ordering temperature to the reduced nearest-neighbor
(NN) interaction in ultrathin EuO films. The high magnetic moment at T → 0 K may be
partly induced by residual EuSi 2 which is predicted to be paramagnetic. 191 Moreover, the
specific magnetic moment per EuO formula unit (f.u.) shows a moderate reduction to about
4.4μ B /f.u. for the high T S sample, whereas the low temperature EuO (T S = 350 ◦ C) shows a
clearly reduced magnetic moment, saturating at only 0.15μ B /f.u. This is in agreement with
the HAXPES quantification of interfacial silicides (Fig. 5.19) and conveys the crucial influence
of the interfacial silicide on the magnetic properties of ultrathin EuO on H-Si (001).
In conclusion, an optimized magnetic EuO/Si (001) heterostructure should be treated with a
complete in situ hydrogen passivation and a high temperature of synthesis (450 ◦ C) at which
the Eu distillation condition is perfectly maintained.
5.4. Interface engineering II: Eu monolayer passivation of the EuO/Si interface
In this section, we aim to minimize the interfacial SiO x which – from all reaction products
considered in this thesis – is chemically most stable on the Si surface. From its thermodynamic
stability, a hydrogen-passivated Si (001) surface is not stable enough to prevent SiO 2
formation. Thus, the only remaining approach is to shift the chemical regime at the beginning
of EuO synthesis away from any oxidation regime, in order to render an SiO x formation
least probable. In the Gibbs triangle (Fig. 5.8), consequently this means a shift towards Eurich
EuO growth regimes II or III in the initial stage. In practice, we apply one up to three
monolayers (ML) of metallic Europium to the Si surface immediately before EuO synthesis.
In order to analyze, whether an Eu monolayer coverage of the Si (001) surface is energetically
favorable at all, we carry out a simulation of Eu monolayers on the Si (001) surface after
the solid-on-solid model including surface diffusion. The two-dimensional hopping ratio per
time is referred to as surface diffusivity and defined as
ν = ν 0 ·e − E barrier
k B T
. (5.4)
In Fig. 5.22, the images for 0.5 up to three monolayers Eu coverage of the Si (001) surface are
depicted. We present calculated meshes of 50 × 50 cells, in which one cell corresponds to the
area of one atomic site of the Si (001) surface. The ratio of Eu surface diffusivity over arrival
of new Eu atoms is ν/F = 1250. Larger values of ν which may be apparent in Eu–Eu will in-
as depicted in Fig. 5.7 on p. 96.