Text anzeigen (PDF) - Universität Duisburg-Essen

108 5. Results II: EuO integration directly on silicon
10–20 nm in the Si substrate occur frequently, and one is shown as an example in the TEM
micrograph (Fig. 5.18b). We interpret these bulges as Eu diffusion regions, containing probably
Eu silicides. The EuO/Si diffusion regions not only alter the Si and Eu chemistry readily
towards metallic silicides, but also dramatically change the crystal structure affecting the
EuO/Si interface (bulge), the lower EuO layer (polycrystalline layer), and also the upper EuO
layer (instability with the EuO/Al interface).
If, however, a complete hydrogen-passivation is applied to the Si (001) surface, the successive
EuO growth steps are depicted in Fig. 5.18c. From the initial bulk-like Si (001) (1×1) pattern,
first diffraction streaks of EuO are already observable at 1 nm EuO synthesis. Unlike the EuO
growth on bare Si, here with H-passivated Si the EuO surface structure develops towards
good crystallinity, as indicated by streaks in the RHEED photographs representing the EuO
reciprocal lattice. After 10 nm EuO synthesis on H-Si (001), we observe the crystal structure
of EuO (by RHEED and LEED) to be of a crystal quality comparable with EuO on cubic
substrates under biaxial tensile strain (as discussed in Ch. 4.2). A cross-sectional HR-TEM
study of the EuO/H-Si interface in Fig. 5.18d proves the successful prevention of diffusion
effects, which were, in contrast, observed for EuO on bare Si (001). A 3 nm polycrystalline
EuO layer forms in contact with Si, as indicated by the circular intensity of the Fourier transform
picture. On top of this polycrystalline EuO, a single-crystalline EuO phase constitutes
the residual EuO slab. However, the crystal structure of this EuO phase coincides best with
Eu 2 O 3 which is deduced to be formed during sample transfer through ambient air. Without
doubt, during MBE synthesis we observed divalent epitaxial EuO by its fcc rocksalt lattice,
and this allows one to interpret the Eu 2 O 3 in the HR-TEM micrograph to represent the initial
divalent EuO morphology.
Thus, a complete hydrogen-passivation of Si (001) allows for persistent heteroepitaxial growth
of EuO/H-Si structures with no indication of large diffusion areas in Si or the EuO slab. Only
a polycrystalline EuO layer of about 3 nm remains in direct contact with Si. EuO layers above
this poly-EuO layer are deduced to grow largely single-crystalline.
Chemical optimization of the EuO/Si interface
In order to achieve a minimization of the interfacial EuSi 2 in EuO/H-Si (001) heterostructures,
we investigate the impact of two parameters: (i) the in-situ hydrogen passivation of the
clean Si (001) surface “H-Si”, and (ii) the temperature of synthesis “T S ” as the general thermodynamic
driving force. We vary the temperature between T S = 350 ◦ C and T S = 450 ◦ C, and
we use the silicon (001) substrate either in situ hydrogen-passivated or with a clean (flashed)
surface. These two parameters permit four variants of combinations, which we investigate in
the following.
For a correlation of passivation steps with the surface crystallinity, we first analyze the RHEED
pattern: Figure 5.19b depicts the RHEED pattern after growth of the ultrathin EuO films
(d = 2 nm) at an electron energy of 10 keV and the beam along the [100] direction. We observe
RHEED patterns of the EuO fcc lattice at T S = 350 ◦ C only in the absence of hydrogen
termination. The higher substrate temperature (T S = 450 ◦ C) allows one to observe sharper
RHEED patterns than for the lower synthesis temperature. Here, the impact of H-passivation
is a roughening of the surface as indicated by dot-like intensities inside the RHEED streaks.
Thus, we conclude that excess Eu from an incomplete Eu distillation at lower T S hinders