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5.5. Interface engineering III: SiO x passivation of the EuO/Si interface 117 1.0 0.8 0.6 0.4 0.2 I. minimize silicides EuO / pure Si EuO / H-Si II. minimize Si oxide EuO / Eu Si III. combined optimization EuO / Eu H-Si 300 350 400 450 500 550 0 0.2 0.4 0.6 0.8 1.0 Figure 5.26.: Summary of EuO/Si interface optimization including silicides, SiO x , and the combination of them. The aim of the optimization is to reduce interface contaminants into the greenshaded slot of silicide and SiO x thicknesses. (I) Silicide minimization is accomplished by the tuning of the synthesis temperature and the surface hydrogen termination of Si (001). (II) Silicon oxides are diminished by protective monolayers of Eu on Si (001). (III) Finally, optimum parameters including H-Si and a protective Eu monolayer are checked against synthesis temperature to yield a combined optimization accounting for both the silicides and SiO x . for the synthesis of EuO on passivated Si (001) heterostructures: H-passivated Si (001), 2 ML protective Eu, a medium T S = 450 ◦ C. During all three studies, EuO on Si (001) could be observed to grow heteroepitaxially on Si (001) by RHEED. The sharpest EuO diffraction pattern are observed in the combined passivation series – this is due to the net contamination (EuSi 2 + SiO x ) being smallest only in this combined interface optimization series. 5.5. Interface engineering III: SiO x passivation of the EuO/Si interface The challenge to prevent metallic contaminations at the EuO/Si interface involves two major aspects: the avoidance of (i) silicides due to Eu–Si reactions, and (ii) excess Eu from a seed layer for EuO synthesis in order to initialize the Eu distillation growth. We address these issues by an alternative passivation method, the in situ application of ultrathin SiO x in
118 5. Results II: EuO integration directly on silicon Figure 5.27.: Magnetization behavior of 4 nm EuO on passivated Si (001) with interfacial silicide. Si (001) is passivated with hydrogen, SiO 2 , or left clean after flashing. In (i), the change of magnetization from hydrogen passivation to ultrathin SiO 2 is highlighted, and in (ii) from ultrathin SiO 2 to the thick native SiO x . the sub-nanometer regime to the clean Su surface. This approach shifts the thermodynamic environment at the EuO/Si interface towards the oxygen-rich regime. We characterize the SiO x passivation by presenting magnetic properties of four different EuO/Si heterostructures, as compiled in Fig. 5.27. By excess Eu and an elevated EuO synthesis temperature, we pursue a significant formation of interfacial silicide which is paramagnetic 191 at T → 0 K. As a result, we find a gradual optimization of the Si passivation methods with respect to their chemical stability as against silicide formation: while EuO/H- Si preserves a T C of the magnetization curve only 2 K higher than for EuO/flashed Si, we observe a significant increase of T C for the EuO on 1 nm SiO x -Si hybrid structure – from ∼12 K to ∼40 K. A Brillouin-shaped magnetization curve without any paramagnetic contamination is observed only for EuO on natively oxidized Si (d SiOx 2 nm). This preservation of ferromagnetism motivates the study of this section, which combines chemical (by HAXPES), structural (by RHEED) and magnetic (by SQUID) optimization of EuO/Si hybrid structures using ultrathin in situ SiO x passivation of the Si (001) surface. HAXPES analysis of the SiO x -passivated EuO/Si interface First, we clean Si (001) by thermal flashing (T Si 1100 ◦ C), followed by the in situ surface passivation with either molecular oxygen (p ∼ 10 −7 mbar, T Si = 700 ◦ C) or atomic hydrogen (p ∼ 10 −2 mbar, T Si = RT). We confirmed the chemical cleanliness after the Si passivation step using Auger electron spectroscopy. The crystalline structure of the passivated Si surface is checked by LEED which reveals exclusively a (1×1) Si (001) bulk diamond structure, in contrast, a flashed Si (001) surface would show the (2×1) surface reconstruction. Second, 4 nm EuO thin films are grown directly on the Si (001) substrates. In order to stabilize stoichiometric EuO, a low oxygen partial pressure (p O2 ∼ 1.5 × 10 −9 Torr) and the Europium distillation condition 32,45 are applied, in agreement with the optimum EuO synthesis yielding stoichiometric EuO in Ch. 4. During EuO synthesis, the surface crystal structure is mon- referred to as thermodynamic region “I” in the Gibbs triangle (see Fig. 5.8 on p. 97).
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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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4. Results I: Single-crystalline ep
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59 Table 4.1.: Parameters for stoic
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4.1. Coherent growth: EuO on YSZ (1
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Page 84 and 85: 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 114 and 115: 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 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
Page 144 and 145: A.1. EuO-related phases and cubic o
Page 146 and 147: A.1. EuO-related phases and cubic o
Page 148 and 149: A.2. In situ analysis of the Si (00
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
Page 166 and 167: Software and Internet Resources [21
Page 168 and 169: List of own publications 151 [10] R
Page 170 and 171: List of conference contributions 15
Page 172 and 173: Schriften des Forschungszentrums J
Page 175: Schlüsseltechnologien / Key Techno
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