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