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5.1. Chemical stabilization of bulk-like EuO directly on silicon 89 6 P 6 F7/2 6 F5/2 6 H7/2 6 H5/2 Eu 3+ 4f multiplet 7 F6 ... 7 F 0 Eu 2+ 4f multiplets Eu 2+ 3d Eu 3+ 3d 1172.0 1162.5 1153.0 1143.5 1134.0 1124.5 1115.0 Figure 5.3.: Ultrathin EuO films (d = 45 Å) directly on HF-Si investigated by HAXPES. For the Al/EuO/HF-Si heterostructures, HAXPES spectra of the Eu 4f valence level (a–d) and of the Eu 3d deep core-level (f–i) are analyzed by quantitative peak fitting. Plotting the difference of spectral weight in grazing and normal emission geometries (b, d and g, i) exploits any surface and bulk contributions of the EuO thin film. For curve fitting, calculated Eu 2+ and Eu 3+ multiplets 110,111,172,174,175 (e and j) serve as reference of binding energies for the photoemission final states.
90 5. Results II: EuO integration directly on silicon Eu 3+ 4f multiplet Eu 2+ 4f multiplet Eu 2+ 4f multiplet Eu 3+ 4f multiplet Eu 2+ 4f multiplet Eu 3+ 4f multiplet Figure 5.4.: Valence bands of Al 2 O 3 , Al and Si in comparison with Eu 4f PES multiplets. Miyazaki (2001), Drube (2012) and Ley et al. (1972). 99,177,178 From both EuO compounds (i) and (ii) we observe a redistribution in peak intensity within the divalent Eu 2+ 4f 6 final state from the lower binding energy side (−ΔIB ∗ ) towards the higher binding energy side (+ΔIS ∗). This result suggests, that the 4f 6 final state multiplet of both EuO compounds is composed of a bulk component B, located at a binding energy of E B 1.8 eV, and a surface component S shifted towards higher binding energy due to the contact with Al/Al 2 O 3 . The Eu 2+ 4f surface shifts for the EuO compounds (i) and (ii) are determined as δ (i) S = 1.2 eV and δ(ii) S = 1.3 eV, respectively. A quantitative analysis of the Eu 3+ 4f interface shift is impeded by the overlap with the O 2p states. In order to quantify the chemical states of the Eu 4f photoemission peak in Fig. 5.3a and c, we fit the 4f n−1 multiplets using convoluted Gaussian-Lorentzian curves. Three tunable parameters were employed for least-squares fitting, namely the energy separation between the divalent and trivalent Eu contributions, their intensity ratio and the spectral width (FWHM) for every species. For Eu 2+ , a multiplet fine structure serves as reference for binding energy in accordance with theoretical calculations by Cho et al. (1995). Least-squares best-fit curves are shown by the solid curves in Fig. 5.3a and c, which match the experimental data points very well, and the resulting intensity ratios and energy positions of Eu 2+ and Eu 3+ species are summarized in Tab. 5.1. From the integrated spectral intensities A of the Eu 2+ and Eu 3+ components, we derive the relative fraction of trivalent Eu as r Eu3+ = A Eu3+ / (A Eu2+ + A Eu3+ ). For the EuO compound (i), we determine mainly an integral divalent chemical state of the Eu cations (trivalent contribution r4f Eu3+ =7.4%). For the EuO compound (ii), in contrast, a mixed initial valency with a trivalent contribution of r4f Eu3+ ≈ 66% is derived. Thus, we confirm EuO compound (i) asstoichiometric EuO and (ii) asoxygen-rich Eu 1 O 1+x . A comparison of contributions to the valence level from the entire Al/EuO/Si heterostructure is given in Fig 5.4. In Fig. 5.4a, the Al 2 O 3 valence band 177 coincides mainly with Eu 3+ PES final states. The metallic Al valence band 99 (b) and the Si valence band 178 (c) coincide with both Eu 2+ and Eu 3+ PES multiplets. 110,111,172,174,175 Thus, the oxidation state and thickness of the Al overlayer influences the spectral weight observed at the energy position of Eu 4f final states. Our Eu 3+ spectral fit does not include any Al 3s and Al 3+ 3s contributions 177 from the cap layer or the Si valence band 178 from the substrate in this energy region, which may lead to a slight over-estimation of r4f Eu3+ .
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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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Page 60 and 61: 3.3. Experimental developments at t
Page 62 and 63: 3.4. Ex situ characterization techn
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 114 and 115: 5.2. Thermodynamic analysis of the
Page 122 and 123: 5.3. Interface engineering I: Hydro
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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
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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
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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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