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5.2. Thermodynamic analysis of the EuO/Si interface 99 EuSi 2 0 500 1000 1500 (a) Silicide formation during initialization of EuO growth: 1000 2(H-Si) + Eu (g) EuSi 2 + H 2 EuSi -2000 2 + 3Eu + O 2 2EuO + 2Si + 2Eu (evap) 0 500 1000 1500 2000 2Si + 2(H-Si) + 2Eu (g) 2EuSi 2 + H 2 500 2Si + Eu 2EuSi 2 0 (b) Silicide dissolution during oxygen-rich EuO growth: EuSi 2 + Eu + 3/2O 2 EuO + Si + SiO 2 -500 EuSi 2 + Eu + 3/2O 2 Eu 2 O 3 + 2Si -1000 1/2(EuSi 2 )+Eu+3/2O 2 1/2(Eu 3 O 4 +SiO 2 +Si) -1500 (c) Silicide dissolution during EuO distillation growth: temperature (K) ΔG reaction (T) (kJ/mol) temperature (°C) EuO formation (for orientation) Figure 5.10.: Resulting Gibbs free energies of EuO/Si interface reactions involving EuSi 2 . needed. Such a calculation has been conducted by Rushchanskii (2012) and Ležaić (2011) using the harmonic approximation in density-functional perturbation theory (DFPT). 191,192 The chemical environment when beginning EuO synthesis is an Eu-rich seed layer for epitaxial growth on top of the hydrogen-passivated silicon surface, H-Si (001). These conditions reveal possible silicide formation reactions, as assembled in Fig. 5.10a. Their resulting Gibbs free energy is ΔG300 ◦ K > 0 (purple and red circles), which renders the silicide formation on H-Si unlikely. However, we analyze three different hydrogen coverages of the Si surface, complete passivation (purple circle), half surface passivation (red circle), and in case no hydrogen covers the Si surface surface, the asymptotic case of Eu + Si alloying (orange circle). It is evident, that only the complete hydrogen passivation of Si shows a large positive G300 ◦ K 500 kJ/mol, while the results for fractional Si passivation follow G◦ 300 K 0, which is the thermodynamic threshold to a silicide formation probability. If no H passivation is left, the simple alloying reaction is thermodynamically favored to proceed (G300 ◦ K = −84 kJ/mol). This underlines the importance of a complete hydrogen passivation of the Si (001) surface in order to prevent EuSi 2 formation. Proceeding with the next stage of EuO synthesis on top of silicon, we analyze the two complementary EuO growth regimes (I) and (III), oxygen-rich growth in Fig. 5.10b and Eu distillation growth in Fig. 5.10c. During these EuO growth modes, EuO binds most of the supplied Eu, and any excess Eu is re-evaporated by the high synthesis temperature around the Eu sublimation point. Thus, during EuO synthesis silicides are thermodynamically unlikely formed (remember Fig. 5.9), and consequently we consider the disappearance of residual EuSi 2 from the initial Eu-rich stage of EuO growth. During oxygen-rich EuO growth (regime I. Red and yellow solid lines, red diamond in Fig. 5.10), the disappearance of EuSi 2 is very probable. Among the resulting phases, the formation of Eu 3 O 4 and Eu 2 O 3 are thermodynamically favored, yet not desired. During Eu distillation growth (regime III. Blue solid line in Fig. 5.10), Phonons of bulk EuSi 2 were calculated using DFPT 193 within local-density approximation 194 as implemented in Quantum-Espresso code. 225 A6×6×6 k-point mesh was used for Brillouin-zone integration with a plane-wave kinetic energy cut-off of 30 Ha. For pseudopotential construction, the following valence-electron configurations were considered: 5s 2 5p 6 4f 7 6s 2 for Eu and 3s 2 3p 2 for Si. In the structural relaxation, the Hellman- Feynman forces were minimized to 2 × 10 −4 Ry/Bohr. The thermodynamic potentials H and S were derived from the phonon density of state which was calculated at a 30×30×30 mesh in the Brillouin-zone.
100 5. Results II: EuO integration directly on silicon the disappearance of EuSi 2 is still favorable and yields EuO, Si, and metallic Eu, altogether constituting a suited basis for further synthesis of high quality EuO by the Eu distillation condition. All EuSi 2 disappearance reactions show GEuSi dissol. 2 (T ) 2 × GEuO f (T ), thus being significantly more probable than the formation of EuO, given for orientation as dashed blue line in Fig. 5.10. In conclusion, the only remaining condition when metallic europium silicide may be formed is the initial stage of EuO synthesis with excess of Eu and an incomplete passivation of the Si surface. Our thermodynamic prediction clearly prohibits a formation of EuSi 2 during EuO synthesis and in case of a complete hydrogen passivation of the Si surface. In short: the H-Si (001) surface is suited to provide a silicide-free EuO/H-Si hybrid structure. Silicon dioxide reactions at the EuO/Si interface Silicon oxide dissolution during initialization of EuO (Eu-rich seed layer): 0 0 temperature (°C) 500 1000 SiO 2 1500 6SiO 2 + 8Eu 4Eu 2 O 3 + 6Si 6SiO 2 + 9Eu 3Eu 3 O 4 + 6Si 6SiO 2 +12Eu 12EuO + 6Si ΔG reaction (T) (kJ/mol) -200 -400 -600 -800 EuO formation (for orientation) -1000 0 500 1000 temperature (K) Figure 5.11.: Resulting Gibbs free energies of EuO/Si interface reactions regarding SiO 2 in the initial growth stage of EuO (Eu seed layer). 1500 2000 While employed as chemically stable dielectric in many semiconductor applications, our approach is the avoidance of polycrystalline SiO 2 at the Si surface in order to maintain a structurally sharp and chemically well-defined functional interface of EuO directly on Si. From the Ellingham diagram (Fig. 5.7) we have already derived that all Eu oxides phases are thermodynamically more stable than SiO 2 , and this is true for any temperature. However, residual SiO 2 from the Si wafer may be located on the Si (001) surface, and due to diffusion and surface defects SiO x may form even under EuO growth conditions. This renders an indepth thermodynamic analysis of SiO 2 disappearance and formation reaction at the EuO/Si interface very reasonable. First, we consider the Eu-rich start of EuO synthesis directly on top of the Si wafer. No oxygen is supplied for the Eu seed layer, and only disappearance of residual SiO 2 may occur, as compiled in Fig 5.11. We analyze disappearance reactions leading to all possible europium oxide valency phases: they are formed with almost comparable probability, but divalent EuO is thermodynamically most favorable (grey solid line) with the most negative GSiO dissol. 2 (300 K) ≈ −280 kJ/mol. Remarkably, once oxygen supply initiates EuO synthesis, the EuO formation (blue dashed line) shows more than double the gain in Gibbs free energy and thus displaces any initial SiO 2 disappearance reaction. Therefrom, only in the initial stage of Eu deposition as a seed layer the SiO 2 disappearance is considerable, leading with the largest probability to
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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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3.4. Ex situ characterization techn
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Page 74 and 75: 4. Results I: Single-crystalline ep
Page 76 and 77: 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
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Page 96 and 97: 4.3. Lateral compressive strain: Eu
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Page 122 and 123: 5.3. Interface engineering I: Hydro
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Page 140 and 141: 6. Conclusion and Outlook In the th
Page 142 and 143: 125 Outlook and perspectives In the
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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
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Software and Internet Resources [21
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List of own publications 151 [10] R
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List of conference contributions 15
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Schriften des Forschungszentrums J
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