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5.2. Thermodynamic analysis of the EuO/Si interface 97 Figure 5.8.: Three growth regimes of EuO, illustrated in a Eu–O–Si Gibbs triangle (inset). (I) describes the oxygen-rich regime, (III) the Eu-rich EuO regime, and (II) can be referred to, when EuO is stabilized on (H-passivated) Si, as the stoichiometric EuO regime. significant shift to lower Gibbs free energy from the Eu oxides to SiO 2 by 260 kJ/mol, which clearly exceeds energies of the Si surface or surface diffusion kinetics. This energy shift is nearly temperature independent. Thus we can conclude, that EuO is not only statically stable on Si, but is also under gaseous oxygen supply and under elevated temperature significantly more stable than the formation of SiO 2 . We remark, that this prediction is of thermodynamic nature, thus the result (stable Eu oxides) may be reached after a long time or even mixed up by interface kinetics. The Gibbs phase rule 36 connects the number of constituents C and resulting phases P with the degrees of freedom F of a system under constant pressure as F = C − P +1. (5.1) In our system, the constituents are O 2 –Eu–Si at constant and limited O 2 pressure, and the resulting phases are assumed to be SiO 2 + EuO + EuSi 2 during Eu distillation growth † .For this three-constituent system, a Gibbs triangle serves as a perfect visualization as depicted in Fig. 5.8a. The Eu–O–Si phase diagrams are constrained to contain only one, two, and at most three-phase regions. A one-phase region exists at the vertex of the triangle, e. g. Si. A two-phase region exists on any tie line where phases thermodynamically meet, e. g. Si and Eu. The interior of any triangle is a three-phase region, including probabilities for e. g. EuO + Eu + EuSi 2 in region III. Crossing tie lines are not allowed because at the point of their intersection the simultaneous coexistence of four phases would contradict the Gibbs phase rule, this for example negates the simultaneous existence of EuO + Eu + SiO 2 + EuSi 2 in thermodynamic equilibrium, as read from a fictitious crossing of lines SiO 2 → Eu with EuO → EuSi 2 from Fig. 5.8a. Hence, from the Gibbs triangle, we obtain a limit to three chemical regimes (I, II, and III) of resulting phases, which limits the number of co-existing phases to those being in the same subtriangle. The Gibbs phase rule for constant pressure is sometimes referred to as the “solid state phase rule”. It is particularly suited for solid systems under low pressures in which one constituent is gaseous. † as introduced in Ch. 4
98 5. Results II: EuO integration directly on silicon What is the free parameter which drives the system into one of these regimes? The phase rule for constant pressures (5.1) evaluates F = 1, a degree of freedom in only one dimension. This degree of freedom is the temperature of synthesis T S . Via this temperature, the Eu distillation condition during EuO synthesis is strongly affected by means of partial or complete re-evaporation of excess Eu, this corresponds to a shift along the O–Eu tie line in the Gibbs triangle. Therefore, the chemical phases can range from oxidized phases over stoichiometric EuO to metallic phases. This variation is plotted in Fig. 5.8b with different background colors revealing the chemical regimes (I), (II), and (III), all depending on the temperature degree of freedom. We proceed with the quantitative analysis of EuO/Si interfacial reactions products. The reactions are evaluated by balancing the Gibbs free energies of formation, ΔG ◦ = ∑ products nG ◦ f − ∑ reactants mGf ◦ . (after Hess’ law) (5.2) These balances are not limited to the comparison of bare formation energies G f (T ) directly from the constituents as presented in the Ellingham diagram. We rather compare oxygen-rich (I) with Eu-distillation (II–III) chemical regimes during the EuO synthesis in the initial stage and during sustained growth. Herein, the distillation condition is persistently expressed by the term (3Eu + O 2 ) and the oxygen-rich synthesis as (Eu + 3/2O 2 ). First, we address the native metallic silicide EuSi 2 , and then the silicon dioxide SiO 2 . Among ternary compounds, we limit the discussion to Eu(OH) 3 which is the most probable europium hydroxide. In the case of a two-dimensional structure, we remark that the energy gain ΔG ◦ is reduced compared to a volume reaction by the surface energy of the substrate: ΔG ◦ (1×1) = 113 kJ/mol for (1 × 1)-Si (001), or ΔG ◦ (2×1) = 124 kJ/mol for (2 × 1)-Si (001).190 Europium silicide reactions at the EuO/Si interface temperature (°C) 0 500 1000 1500 200 Gibbs free energy of formation, G f ° (kJ/mol) 0 -200 -400 -600 -800 -1000 -1200 0 EuSi 2 H-Si (001) surface EuO 500 1000 temperature (K) decomposition 1500 2000 Figure 5.9.: Gibbs free energies of formation for EuO, the H-Si (001) surface, and EuSi 2 . The silicide shows crossing to positive Gibbs free energy, which indicates a favorable decomposition at high temperature. We begin with the thermodynamically least stable compound (Fig. 5.9), Europium silicide EuSi 2 , notwithstanding being the most serious antagonist to any tunnel functionality due to its metallic conductivity and paramagnetic behvavior 191 . Despite the likely silicidation of Si surfaces, thermodynamic data for the native europium silicide EuSi 2 are still missing. Therefore, a calculation of the formation energy including the temperature dependence is
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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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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
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 116 and 117: 5.2. Thermodynamic analysis of the
Page 122 and 123: 5.3. Interface engineering I: Hydro
Page 128 and 129: 5.4. Interface engineering II: Eu p
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
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Page 146 and 147: A.1. EuO-related phases and cubic o
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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,
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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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Schlüsseltechnologien / Key Techno
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