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Magnetic Oxide Heterostructures: EuO on Cubic Oxides ... - JuSER

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122 5. Results II: EuO integration directly on silicon<br />

Going one step further, for spin-functional interfaces allowing for an advanced tunnel approach<br />

(“coherent tunneling”) from EuO directly into silicon, we aim towards the the epitaxial<br />

integration of ultrathin EuO directly with Si (001). Thus, we focus on interface engineering<br />

of the chemically challenging EuO/Si (001) heterointerface. In response to the extremely<br />

high reactivity and surface kinetics of Eu, EuO, and Si during EuO synthesis at elevated<br />

temperatures, we conducted a thermodynamic analysis of the EuO/Si interface. Thereby,<br />

we selected three in situ passivation procedures for the Si (001) surface in order to prevent<br />

metallic and oxide contaminations at the EuO/Si interface, both being the main antagonists<br />

for spin-functional tunneling.<br />

As a first step, we engineer the functional EuO/Si interface with an in situ hydrogen passivation<br />

of clean Si (001), which is thermodynamically suited to prevent metallic silicides. We<br />

optimized interface-passivated EuO/H-Si (001) heterostructures under a variation of the synthesis<br />

temperature and hydrogen passivation procedure. By interface-sensitive HAXPES, we<br />

controlled the minimization of interfacial silicides: at higher temperature of EuO synthesis<br />

(T S = 450 ◦ C) and for a complete H-passivation of the Si (001) surface, the minimum thickness<br />

of interfacial silicides is determined from Si 2p core-levels to equal d opt (EuSi 2 ) = 0.16 nm –<br />

clearly below now monolayer coverage of this metallic contamination. The second interface<br />

contamination, SiO 2 , is treated by the consequent application of the Eu-rich growth regime.<br />

For this, we chose a Eu seed layer in the thickness regime 1–3 monolayers, in order to passivate<br />

the Si surface against oxidation. Those EuO/Si (001) heterostructures were fabricated by<br />

Oxide-MBE and quantitatively analyzed by HAXPES. Using Si 1s core-level spectra, we determined<br />

a minimization of interfacial Si oxide down to d opt (SiO x ) = 0.42 nm. The silicide EuSi 2<br />

and silicon oxides are contaminations originating from complementary growth regimes, either<br />

Eu-rich or oxygen-rich. Hence, these contaminants cannot simultaneously be diminished<br />

by a shift towards one of these chemical regimes. In order to optimize both contaminants,<br />

we combined the beneficial parameters of H-passivation, higher synthesis temperature, and<br />

Eu passivation in the monolayer regime. Using HAXPES, we evaluated Si 2p spectra and<br />

found an optimum of d opt (SiO x ) = 0.69 nm simultaneously with d opt (EuSi 2 ) = 0.20 nm, both<br />

of which clearly in the subnanometer regime.<br />

An alternative route to a chemical passivation of the EuO/Si interface is the application<br />

of ultrathin SiO x in the Ångström regime. This approach benefits from the high chemical<br />

and structural stability of SiO 2 and its electrical insulation, thus principally concordant<br />

with the intended tunnel functionality. We applied an in situ SiO x passivation to the<br />

clean Si (001) surface, and investigated the passivation and resulting contaminants at the<br />

EuO/Si heterointerface by HAXPES. The thickness of the SiO x passivation to ranges from<br />

10–13 Å. In this passivation regime, we observed a clear reduction of interfacial silicides<br />

down to d opt (EuSi 2 ) = 0.18 nm. With increasing SiO x passivation, the magnetic properties of<br />

EuO/SiO x -Si (001) heterostructures developed stepwise towards the EuO bulk magnetization<br />

with a maximum specific magnetic moment of 5μ B /EuO for 13 Å SiO x passivation.<br />

For all EuO/Si (001) heterostructures with optimized interface passivation in our studies, the<br />

EuO and Si reciprocal patterns could be observed by RHEED and indicate heteroepitaxial,<br />

yet partly three-dimensional growth with an adaption the Si (001) lateral lattice parameter.<br />

This is the first time, that a direct integration of EuO on silicon was experimentally<br />

realized – without additionally deposited oxide buffer layers. Such chemically and structurally<br />

optimized EuO/Si (001) heterostructures may be effectively utilized as spin-functional<br />

tunnel contacts for silicon spintronics devices such as a spin-FET in the near future.

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