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3.1. Molecular beam epitaxy for oxides 37 Table 3.1.: Surface energies γ of materials used for magnetic oxide heterostructures in this thesis, i. e. silicon, oxides for substrates, Eu as constituent for the EuO synthesis, and metals used as capping or top electrodes. Remarkably, Eu has the lowest and YSZ the largest surface energy, this heterosystem thus likely forms a smooth Van der Merve interface. The same is true for Eu/Cu and Eu/Si, but here the wetting of Eu implicates a large reactivity. material application γ ( Jm -2) method reference Eu(100) EuO constituent 0.46 density functional theory D. Sander and H. Ibach (2002) 122 Eu(111) EuO constituent 0.524 density functional theory D. Sander and H. Ibach (2002) 122 Gd magnetic 0.9 cleavage experiments Himpsel et al. (1998) 123 YSZ (9%) substrate 1.63 multiphase equilibrium Tsoga and Nikolopoulos (1996) 124 MgO(100) substrate 1.15 cleavage experiments † D. Sander and H. Ibach (2002) 122 CaF 2 (100) substrate 0.45 cleavage experiments † D. Sander and H. Ibach (2002) 122 CaO substrate 0.75 equilibrium with H 2 O Paisley et al. (2011) 125 Si(100) substrate 1.2 cleavage experiments Himpsel et al. (1998) 123 Al 2 O 3 spacer oxide 1.4 cleavage experiments Himpsel et al. (1998) 123 Al capping 1.1 cleavage experiments Himpsel et al. (1998) 123 Cu electrode 1.52 cleavage experiments ‡ D. Sander and H. Ibach (2002) 122 Au electrode 1.41 cleavage experiments ‡ D. Sander and H. Ibach (2002) 122 at T = 450 ◦ C † at T = 77 K under HV ‡ at elevated T under HV In situ ultrathin thermal oxidation of silicon Surface treatments of Si are often necessary to engineer electric properties and tune the chemical reactivity and diffusion behavior, as investigated in Ch. 5. A thermal oxidation of singlecrystalline silicon is the most common process in semiconductor technology in order to create a thin layer of silicon dioxide. During thermal oxidation of Si, molecular oxygen diffuses into the Si surface and reacts to SiO 2 , which has been found to be ∼46% below and ∼54% above the surface of the original Si wafer. 222 Our approach aims at growing ultrathin silicon dioxide (d SiO2 2 nm) as a chemical passivation layer directly on clean Si (001), which is clearly thinner than native SiO 2 that forms on Si wafers upon exposure to air. Synthesis of ultrathin silicon oxide by Oxide-MBE is not well-described by the steady state model 126 which is established for thermal oxidation of silicon in the semiconductor industry. For ultrathin thermal SiO 2 growth on Si, Krzeminski et al. (2007) propose a time-dependent K reaction rate approach to model the oxidation kinetics for Si + O 2 −→ SiO2 . 127 Including the diffusivity D, the reaction rate constant K and the concentrations n, the oxidation reaction is then described by rate equations (i. e. changes by time), ∂n O2 = ∇ ( ) D ×∇n ∂t O2 − K × nO2 × n Si (3.3a) ∂n Si = −K × n ∂t O2 × n Si (3.3b) ∂n SiO2 = K × n ∂t O2 × n Si (3.3c) which correspond to the diffusion of molecular oxygen (3.3a), the reaction with silicon (3.3b), and finally the creation of SiO 2 (3.3c). As a result, this model reveals that the O 2 coverage on the silicon surface behaves like n O2 ≈ 0.2 p T , which clarifies that at a given oxygen
38 3. Experimental details pressure p the oxygen coverage is significantly larger for lower temperatures T . At higher temperatures T Si 800 ◦ C, however, the diffusion into the silicon wafer is significantly increased. Thus, the temperature is an essential parameter for successful in situ oxidation of Si surfaces. Experimentally, the model of the reaction rate approach is verified for ultrathin SiO 2 on single-crystalline Si (001) synthesized at T Si = 725 ◦ C (Fig. 3.2), which shows a time-dependent behavior n SiO2 ∝ t 0.8 for the oxide growth. 127 Aiming towards a controllable SiO 2 growth on Si with ultrathin oxide thickness in this work, we adopt this parameter set from the low end of the temperature range for thermal oxidation (T Si ∼ 700 ◦ C) and in the linear time behavior of oxide formation (∂n SiO2 ∝ t). However, the presented approach (eqs. (3.3)) is confirmed only for Si oxide thicknesses greater than 1 nm. Thus, the desired ultrathin Si oxide layers for Si surface passivation (0
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Member of the Helmholtz Association
Page 4 and 5: Forschungszentrum Jülich GmbH Pete
Page 6: Zusammenfassung In dieser Doktorarb
Page 10 and 11: Contents 1. Introduction 1 2. Theor
Page 12 and 13: List of Figures 2.1. Phase diagram
Page 14: List of Figures xi 5.12. Resulting
Page 18 and 19: 1. Introduction Smart and powerful
Page 20 and 21: 3 In the thesis at ha
Page 22 and 23: 2. Theoretical background . . . The
Page 24 and 25: 2.1. The challenge of stabilizing s
Page 26 and 27: 2.2. Electronic structure of EuO 9
Page 28 and 29: 2.3. Magnetic properties of EuO 11
Page 36 and 37: 2.4. Hard X-ray photoemission spect
Page 46 and 47: 2.5. Magnetic circular dichroism in
Page 52 and 53: 3. Experimental details Die Pläne
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Page 60 and 61: 3.3. Experimental developments at t
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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 94 and 95: 4.2. Lateral tensile strain: EuO on
Page 96 and 97: 4.3. Lateral compressive strain: Eu
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5.1. Chemical stabilization of bulk
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5.2. Thermodynamic analysis of the
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5.3. Interface engineering I: Hydro
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5.4. Interface engineering II: Eu p
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5.5. Interface engineering III: SiO
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5.5. Interface engineering III: SiO
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5.6. Summary 121
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6. Conclusion and Outlook In the th
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125 Outlook and perspectives In the
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A.1. EuO-related phases and cubic o
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A.1. EuO-related phases and cubic o
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A.2. In situ analysis of the Si (00
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Bibliography 133 [13] S. S. P. Park
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Bibliography 135 [36] R. E. Dickers
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Bibliography 137 [65] R. Sutarto, S
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Bibliography 139 [91] P. Braun, M.
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Bibliography 141 [116] S. Hüfner.
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Bibliography 143 [144] R. Feder and
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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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