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5.2. Thermodynamic analysis of the EuO/Si interface 101 (a) Silicon oxide formation during O-rich EuO growth: 2(H-Si) + 2Eu + 3O 2 2SiO 2 + 2EuO + H 2 (g) (b) Silicon oxide formation during EuO distillation growth: 2(H-Si) + 2(3Eu+O 2 ) 2SiO 2 + 6Eu + H 2 (g) (c) Silicon oxide dissolution during EuO distillation growth: 2(3Eu+O 2 ) + SiO 2 2Eu 2 O 3 + Si + 2Eu (evap) 2(3Eu+O 2 ) + SiO 2 Eu 3 O 4 + 2EuO + Si + Eu (evap) 2(3Eu+O 2 ) + SiO 2 6EuO + Si (dummy) (d) Silicon oxide formation when stable EuO on H-Si or Si: 2(H-Si) + 2EuO (g) SiO 2 + 2Eu + Si + H 2 Si + 2EuO SiO 2 + 2Eu ΔG reaction (T) (kJ/mol) 1000 500 0 -500 -1000 -1500 -2000 -2500 0 0 500 temperature (°C) 500 1000 SiO 2 EuO formation (for orientation) 1000 1500 temperature (K) Figure 5.12.: Resulting Gibbs free energies of EuO/Si interface reactions involving SiO 2 during EuO synthesis, and for stable EuO on Si (001). 1500 2000 the divalent EuO phase. In the next step, we proceed with reactions involving SiO 2 during EuO synthesis. First, the analysis of oxygen-rich EuO (regime I in the Gibbs triangle) growth is depicted in Fig. 5.12a and reveals in conjunction with divalent EuO a highly favorable formation of SiO 2 (red circle). This denotes thus an exclusively oxidic result of the initial EuO layers without any metallic silicide or Eu. We proceed with the complementary EuO growth mode, the Eudistillation growth (regime III in the Gibbs triangle) of EuO on H-Si as depicted in Fig. 5.12b– c. Herein, the resulting Gibbs free energy of the SiO 2 formation during Eu distillation growth equals almost the bare EuO formation energy. Which one is the thermodynamically favored result? If we analyze the energy gain of the EuO formation, we find it at GEuO f (300 K) = −559 kJ/mol, while the SiO 2 formation reaction results in GSiO f 2 (300 K) = −546 kJ/mol, which is only 2% less gain of Gibbs free energy and thus a weak argument for a possible dominance of the EuO formation. In order to elucidate the SiO 2 behavior from another viewpoint, we consider its disappearance during Eu distillation growth in Fig. 5.12c. We find, that the SiO 2 disappearance is very probable yielding all different EuO valencies, with an energy gain of GSiO dissol. 2 (300 K) ≈ −2500 kJ/mol. However, upon SiO 2 disappearance the most favorable products are the mixedvalent Eu 3 O 4 and divalent EuO. This underlines the importance of minimization of any SiO x at the EuO/Si interface, because SiO 2 may act as constituent for oxidation yielding antiferromagnetic higher Eu oxides. We remark, that disappearance reactions of SiO 2 need an activation energy and are more likely at elevated temperatures. Finally, when layers of stoichiometric EuO have been grown on top of the Si surface, we investigate the thermodynamic stability of this system. In Fig. 5.12d, stable EuO on either bare Si (001) or hydrogen-passivated Si (001) are analyzed with respect to a possible SiO 2 formation. In case of H-Si as the substrate, the resulting Gibbs free energy is GSiO f 2 (300 K) = +900 kJ/mol ≫ 0 and thus extremely unfavored. Here, it is safe to state that EuO is thermodynamically stable directly integrated with H-Si (001), this coinciding with a previous thermodynamic prediction. 14 Furthermore, when we consider EuO on bare This is evident from the large gain of Gibbs free energy upon SiO 2 formation as already summarized in the introductory figure 5.7 on p. 96.
102 5. Results II: EuO integration directly on silicon Si, GSiO f 2 (300 K) ≈ +200 kJ/mol, which is in the range of the reconstruction energy of the Si (001) surface. Thus, under unpropitious conditions like surface contamination or roughness, some SiO 2 may form on thermodynamic average for bare Si (001). Again, we remark that the SiO 2 formation needs an activation energy, thus is likely only at elevated temperatures of several hundred ◦ C. From our thermodynamic balances, we conclude that in the oxygen-rich EuO growth regime the Si dioxide will form as well. Thus, the growth regime of choice is the Eu distillation growth mode. Here, EuO is thermodynamically more favored. Nonetheless, SiO 2 formation cannot be excluded because the resulting energy gain of EuO is larger only by 2% than for the SiO 2 formation. Fortunately, during ongoing Eu distillation growth, SiO 2 disappearance is evaluated to be extremely favorable. However, SiO 2 disappearance yields EuO and also higher Eu oxides. Finally, EuO on H-Si is confirmed to be clearly stable. Europium hydroxide and silicates at the EuO/Si interface (a) Eu hydroxide formation during O-rich EuO growth: 3(H-Si) + (Eu+3/2O 2 ) Eu(OH) 3 + 3Si 0 0 temperature (°C) 500 1000 Eu(OH) 3 1500 (b) Eu hydroxide formation during EuO distillation growth: 2(H-Si) + (3Eu+O 2 ) 2/3Eu(OH) 3 + EuSi 2 + 4/3Eu (evap) (c) Eu hydroxide dissolution during EuO distillation growth: 2Eu(OH) 3 + (3Eu+O 2 ) Eu 3 O 4 +Eu 2 O 3 +3H 2 (g) +1/2O2 (g) (d) Eu hydroxide dissolution when stable EuO: EuO + Eu(OH) 3 Eu 2 O 3 + H 2 O + 1/2H 2 (g) 2EuO + Eu(OH) 3 Eu 3 O 4 + H 2 O + 1/2H 2 (g) ΔG reaction (T) (kJ/mol) -200 -400 -600 -800 EuO formation (for orientation) 0 500 1000 temperature (K) Figure 5.13.: Resulting Gibbs free energies of EuO/Si interface reactions involving Europium hydroxide. 1500 2000 In this section, we consider ternary phases at the EuO/Si interface, which are commonly formed in case of equilibrium, i.e. if every constituent is provided and the reactants may form in their standard state (solid). First, we analyze the most likely Europium hydroxide (often referred to as “the hydride”) Eu(OH) 3 as depicted in Fig 5.13. Again, we start from preconditions of either or oxygen-rich EuO growth or Eu-distillation growth (regimes I or III, respectively), and we assume a hydrogen-passivated Si substrate. In case of oxygen-rich EuO synthesis (in Fig. 5.13a, red circle), Eu hydroxide may form with a gain of Gibbs free energy of G form. Eu(OH) 3 (300 K) ≈−290 kJ/mol, which is only half the gain for EuO formation. Still thermodynamically possible is the formation of Eu(OH) 3 during Eu distillation growth (in Fig. 5.13b, blue circle), however the energy gain is small (ΔG r ≈−90 kJ/mol). Moreover, during Eu-distillation growth, a disappearance of the hydroxide (green cross) is thermodynamically more favorable than the EuO formation itself. When we consider stable EuO together with Eu(OH) 3 , we even find disappearance reactions to be thermodynamically favorable in the range ΔG r ≈−60 kJ/mol. All disappearance reactions, however, yield preferably higher oxidized Eu oxide phases rather than divalent EuO. For most lanthanide hydroxides, the hydroxide disappearance occurs at elevated temperatures (T 300 ◦ C) and is basically an evaporation of water. 195,196 While H-passivation of Si or different growth regimes of EuO
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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 76 and 77: 59 Table 4.1.: Parameters for stoic
Page 78 and 79: 4.1. Coherent growth: EuO on YSZ (1
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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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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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