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5.3. Interface engineering I: Hydrogen passivation of the EuO/Si interface 105 Second, a possible H supply from a H-passivated Si surface motivates an analysis of the hydrate Eu(OH) 3 . An extreme undersaturation of H far from equilibrium is given at the H-Si surface. Given Δμ ≪ 0forHandΔμ ≈ 0 for limited O 2 supply, the nucleation theory predicts a large formation barrier for Eu(OH) 3 clusters (either 2D or 3D), corresponding to red or orange lines in Fig. 5.14. Thus, from the classical nucleation theory, Eu(OH) 3 is not predicted to form clusters. Finally, the remaining compounds are the desired magnetic oxide EuO, and the interfacial contaminants EuSi 2 and SiO 2 . However, as mentioned before, a nucleation of these compounds cannot be evaluated due to the lack of nano-structural and surface energy data. Besides the classical nucleation theory, many kinetic surface effects between Si and the deposit will likely proceed at elevated temperatures of synthesis. These are sketched in Fig. 5.15. In particular, special kinetics at substrate imperfections and edges (the Ehrlich-Schwoebel barrier 205 ), as well as interdiffusion of the deposit with Si layers change the stoichiometry, structure and even magnetism, and render themselves as major antagonists to a functional transport interface. This motivates a comprehensive experimental study, in which we determine conditions for which the functional EuO/Si interface remains structurally sharp and chemically clean: nowadays often referred to as interface engineering. 7 5.3. Interface engineering I: Hydrogen passivation of the EuO/Si interface Figure 5.16.: Si (001) surface reconstructions. Clean Si (001) reconstructs as (2 × 1) dimers (a) and would have the least dense H-termination. The bulk (1 × 1) surface has the most dense H- termination (b). However, mixed variants like (3×1) are energetically favorable (c), and in reality, a canted (1 × 1) H-Si surface is usually observed (d). After Northrup (1991). 206 Dangling bonds of the Si (001) bulk (1 × 1) surface will either form a reconstructed (2 × 1) surface under clean UHV conditions, or react with air or organic adsorbates to form a bulklike yet contaminated surface layer. Here, we discuss the in situ hydrogen passivation of the Si (001) dangling bonds. Predicted reconstructions of the Si (001) surface with H-termination are compiled in Fig. 5.16. Experimentally, this is achieved in situ by a special atomic hydrogen supply as introduced in the experimental chapter (Ch. 3.3). Routinely, the Si wafer pieces of prime quality are treated with an in situ flashing procedure at a background pressure better than 5 × 10 −10 mbar, this yields a superior clean and atomically smooth Si (001) surface, as controlled by Auger electron spectroscopy and LEED.
106 5. Results II: EuO integration directly on silicon Figure 5.17.: Hydrogen termination of the clean Si (001) surface as observed by RHEED (blue), LEED (green), and simulated by LeedPat 224 and SSD 226 . The flashed Si (001) surface (a) shows diffraction pattern of the bulk and the (2x1) surface reconstruction dimers as indicated orange. These cleaned Si substrates show a (2 × 1) reconstruction dimers aligned along the [100] as well as [010] axes of the Si (001) single-crystal, as observed in Fig. 5.17a. After applying the in situ hydrogen passivation procedure, the surface reconstruction completely vanishes and only the (1 × 1) bulk structure of Si is observed by electron diffraction (Fig. 5.17b). Remarkably, the RHEED and LEED spots of the passivated H-Si (001) surface are equally sharp as for the freshly flashed Si surface, and no polycrystalline fraction is emergent. The latter would be indicated by diffuse circular intensities in LEED. A chemical characterization by Auger electron spectroscopy of the H-Si surface in comparison with other surface treatments can be found in the appendix (Ch. A.2). Structural optimization of EuO and the EuO/Si interface With a chemically clean and structurally crystalline H-passivated Si (001) surface at hand, we proceed to the synthesis of EuO on top of H-Si and as a reference on clean Si (001). We recorded electron diffraction pattern during EuO deposition (RHEED) and after EuO growth has finished (LEED), as compiled in Fig. 5.18. For bare Si (001), the clean and singlecrystalline surface allows for an initial heteroepitaxial growth of EuO, as indicated by streaks in the RHEED pattern (Fig. 5.18a). The EuO diffraction pattern, however, develops vertically aligned spots and a diffuse background from non-crystalline scattering already after 2 nm growth. Finally, after 7 nm EuO synthesis, the crystalline pattern are largely replaced by circular intensities as expected for a polycrystalline film. No LEED is observed after EuO growth. High-resolution transmission electron microscopy (HR-TEM) provides a crosssectional view of the EuO/Si heterostructure in the nanometer regime. In Fig. 5.18b, the Si substrate shows a perfect diamond structure, but in the 10 nm-thick EuO slab, a polycrystalline EuO layer of 3 nm thickness is identified in contact with the Si surface. Above this intermediate layer, crystalline EuO is observed. However, bulges of lateral dimension However, we remark that such a clean hydrogen passivation needs weeks of bake out of the H 2 supply lines and a cryo trap system for residual vapors such as oxygen or organics. The H 2 cracker itself must be properly cleaned and degased, too.
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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
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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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Page 168 and 169: List of own publications 151 [10] R
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Schriften des Forschungszentrums J
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Schlüsseltechnologien / Key Techno
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