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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.
112 5. Results II: EuO integration directly on silicon Figure 5.22.: Eu monolayers on top of the Si (001) surface under surface diffusion. The simulation is carried out for vanishing Eu–Si surface diffusion (top row), and for Eu–Si surface diffusion assumed to be equal with Eu–Eu self diffusion (bottom row). Programmed in HT-Basic by Ibach (2013). 207 crease island sizes but their large scale distribution does not change. As Eu is highly mobile near its sublimation point during the Eu distillation condition (T S = 450 ◦ C), a distinction between Eu–Eu diffusivity (ν large) and Eu–Si (ν small) is realistic. Therefore, in the upper row of Fig. 5.22 we assume the very first Eu layer in contact with the Si surface to show no surface diffusion. For comparison, in the second row the Eu–Si surface diffusivity is the same as for Eu–Eu. For vanishing Eu–Si surface diffusion (top row), a characteristic difference for 0.5 an one monolayer Eu is that Eu atoms on Si (001) form many single nucleation sites which are randomly distributed and will form islands upon ongoing coverage. The difference between the two initial values for surface diffusivity vanishes already after 1.5 monolayers Eu. Remarkably, no islands of height two are identified. In the pictures of the second Eu layer, no larger islands form, but the Eu atoms are rather diffusing and settling in sites with larger coordination number (holes, sites with only one ML Eu). From two monolayers Eu on, a closed coverage of the Si (001) surface is confirmed and the difference in island height is mostly not exceeding one monolayer. We consider Eu this coverage as a promising basis for an oxygen-protective passivation of the Si (001) surface. In Fig. 5.23b, we analyze the the surface crystal structure of 2 nm EuO/Eu-Si (001) heterostructures (Fig. 5.23a) by RHEED (E = 10 keV) pattern recorded immediately after EuO growth. As a reference, the clean (flashed) Si (001) surface exhibits a clear pattern of (1 × 1) No diffusion barrier at island edges (Ehrlich-Schwoebel barrier) is included in the calculation, this is a reasonable assumption for the highly mobile Eu ions.
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Member of the Helmholtz Association
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Forschungszentrum Jülich GmbH Pete
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Zusammenfassung In dieser Doktorarb
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Contents 1. Introduction 1 2. Theor
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List of Figures 2.1. Phase diagram
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List of Figures xi 5.12. Resulting
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1. Introduction Smart and powerful
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3 In the thesis at ha
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2. Theoretical background . . . The
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2.1. The challenge of stabilizing s
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2.2. Electronic structure of EuO 9
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2.3. Magnetic properties of EuO 11
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2.4. Hard X-ray photoemission spect
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2.5. Magnetic circular dichroism in
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3. Experimental details Die Pläne
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3.1. Molecular beam epitaxy for oxi
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3.2. In situ characterization techn
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3.2. In situ characterization techn
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3.3. Experimental developments at t
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3.4. Ex situ characterization techn
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4. Results I: Single-crystalline ep
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59 Table 4.1.: Parameters for stoic
Page 78 and 79: 4.1. Coherent growth: EuO on YSZ (1
Page 92 and 93: 4.2. Lateral tensile strain: EuO on
Page 94 and 95: 4.2. Lateral tensile strain: EuO on
Page 96 and 97: 4.3. Lateral compressive strain: Eu
Page 102 and 103: 5. Results II: The integration of t
Page 104 and 105: 5.1. Chemical stabilization of bulk
Page 114 and 115: 5.2. Thermodynamic analysis of the
Page 122 and 123: 5.3. Interface engineering I: Hydro
Page 130 and 131: 5.4. Interface engineering II: Eu p
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Page 134 and 135: 5.5. Interface engineering III: SiO
Page 136 and 137: 5.5. Interface engineering III: SiO
Page 138 and 139: 5.6. Summary 121
Page 140 and 141: 6. Conclusion and Outlook In the th
Page 142 and 143: 125 Outlook and perspectives In the
Page 144 and 145: A.1. EuO-related phases and cubic o
Page 146 and 147: A.1. EuO-related phases and cubic o
Page 148 and 149: A.2. In situ analysis of the Si (00
Page 150 and 151: Bibliography 133 [13] S. S. P. Park
Page 152 and 153: Bibliography 135 [36] R. E. Dickers
Page 154 and 155: Bibliography 137 [65] R. Sutarto, S
Page 156 and 157: Bibliography 139 [91] P. Braun, M.
Page 158 and 159: Bibliography 141 [116] S. Hüfner.
Page 160 and 161: Bibliography 143 [144] R. Feder and
Page 162 and 163: Bibliography 145 [171] H. Miyazaki,
Page 164 and 165: Bibliography 147 [197] J. Qi, T. Ma
Page 166 and 167: Software and Internet Resources [21
Page 168 and 169: List of own publications 151 [10] R
Page 170 and 171: List of conference contributions 15
Page 172 and 173: Schriften des Forschungszentrums J
Page 175: Schlüsseltechnologien / Key Techno
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