Association

5. Results II: The integration of the magnetic oxide EuO directly
on silicon
The interface is still the device.
(H. Y. Hwang and J. M. Chakhalian,
Nat. Mat. editorial, 2012 146,169,170 )
Despite its tremendous potential as tunnel contacts with spin functionality, the successful
stabilization of stoichiometric EuO thin films directly on silicon without any buffer layer is
not reported to date. At present, a number of studies have used interfacial barriers of several
nanometers thickness, which prevent diffusion and serve as epitaxial seed for EuO synthesis
on silicon. 7,25,27 This, however, largely extends the tunneling path, as the total thickness
of the oxide barrier and the magnetic oxide EuO can easily exceed 5 nm. This may result in
hopping transport and scattering of spin-polarized electrons rather than tunnel transmission.
Therefore, our goal is the integration of ultrathin EuO on Si (001) without any buffer layer.
This study is based on the predicted thermal stability of the magnetic oxide EuO in contact
with silicon. 14 A moderate lateral strain of 5.6% provided by Si (001) to the EuO lattice is
in the usual range which allows for heteroepitaxy of oxides on semiconductors. Remarkably,
this lattice mismatch is clearly lower than for the dielectric oxide HfO 2 epitaxially integrated
with Si (strain = 6.9%), which is presently attracting great interest. Having established a
high quality EuO synthesis on cubic oxides (Ch. 4) with single crystalline quality and bulklike
magnetism, now we proceed stepwise towards the application side: epitaxial integration
of EuO directly on Si (001). This system combines lattice strain with a challenging interface
chemistry.
We start the study with a chemically well-defined system: polycrystalline EuO on HF-passivated
Si. Herein, EuO is grown with the well-established EuO distillation growth mode as
used for oxide substrates, yielding a bulk-like EuO thin film. A detailed picture of the valence
state of Eu ions in EuO and the electronic structure depending on the chemical composition
is still missing. A major reason is the difficulty to probe this highly reactive compound by
conventional surface-sensitive photoelectron spectroscopy, due to the necessity for a protective
capping layer in EuO/Si hybrid structures. Here, we control the chemical integrity of
the buried EuO bulk and surface layers by hard X-ray photoemission (HAXPES) experiments
carried out at the high brilliance synchrotron light source PETRA III.
In the next step, we prepare and investigate ultrathin EuO on atomically smooth Si (001).
We remove any oxides and contaminations from the Si substrate by in situ flashing procedures.
For the system EuO directly on Si, the interface chemistry is the crucial challenge,
as all the constituents Eu, O 2 , and Si easily react to either metallic silicides, various oxides
or over-oxidized EuO phases. Thus, we dedicate one section explicitly to a thermodynamic
analysis of the EuO/Si interface. For different regimes of EuO synthesis, we identify the most
probable interface reaction products and quantify balanced interface reactions by means of
their resulting Gibbs free energies. In this way, we derive three different passivation meth-
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