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5.1. Chemical stabilization of bulk-like EuO directly on silicon 91 HAXPES: Analysis of the Eu 3d deep core-levels Now we proceed with spectra of the more deeply bound Eu 3d core-levels, which helps to provide a consistent quantification of the initial state valency in conjunction with the Eu 4f results. We note, that analyzing the 3d core-level spectra has a significant advantage for the determination of the initial state Eu valency compared e. g. to the 4d core-levels, because the 3d states show a much weaker multiplet splitting and larger photoexcitation cross section. Moreover, the kinetic energy of Eu 3d photoelectrons is reduced by about 1150 eV, thus the probe is more surface sensitive within the EuO layer, as sketched in Fig 5.2. Eu 3d photoemission spectra for EuO compounds (i) and (ii), recorded in normal emission geometry, are depicted in Fig. 5.3f and h. The spectra consist of a main peaks, the spin–orbitlike final state groups 3d j= 5/2 and 3d j= 3/2, which are clearly separated by a large spin–orbit coupling of 29.2 eV in agreement with previous studies. 159 The broad structure in the center is assigned to extrinsic plasmon excitations caused by high energy 3d photoelectrons, which dissipate the plasmon energy in the electron gas of the metallic Al top layer. 173 The plasmons are located at the expected energy loss (16 eV) from the 3d5/2 peak and the 3d3/2 peak. Stoichiometric EuO (i) shows one main peak in each of the Eu 3d5/2 and 3d3/2 groups in Fig. 5.3f. Within the two spin–orbit split groups, the distribution of final states m J is asymmetrically shaped, in perfect agreement with theoretical calculations of the divalent Eu 3d multiplet, as depicted for comparison in Fig. 5.3j. 110 A satellite peak in the high binding energy region of the Eu 3d5/2 (and 3d3/2) multiplet separated by 7.8 eV (6.3 eV) from the main peak belongs to the divalent Eu 2+ multiplet of the 3d 9 4f 7 final state. Energy splitting as well as intensity ratios of the satellites agree with previous reports on calculated and measured multiplet spectra of divalent Eu compounds. 110,111 Evaluating the oxygen-rich EuO compound (ii) inFig.5.3h, we observe prominent double peak structures in the 3d5/2 (3d3/2) regions, which are separated by 10.45 eV (10.90 eV). We assign these features to divalent Eu 2+ (3d 9 4f 7 final state) and trivalent Eu 3+ (3d 9 4f 6 final state) spectral contributions. Similar with the divalent Eu structure, the Eu 3+ 3d multiplet consists of a doublet whith a satellite peak appearing at 6.9 eV higher binding energy below the 3d5/2 main peak. The Eu 3+ 3d3/2 satellite structure is broadly distributed (not shown). The energy positions and shape of the Eu 2+ and Eu 3+ 3d multiplet final states agree well with previous experiments 176,180,181 and theoretical calculations, 110,111 as illustrated in Fig. 5.3j. For a depth-sensitive analysis of the 3d core-levels, we calculate the energy and angle-depen- Table 5.1.: Binding energies E B and Eu 3+ valency ratios r Eu3+ for (i) stoichiometric EuO and (ii) oxygenrich Eu 1 O 1+x , as obtained by least-squares fitting of the Eu 4f and 3d spectra. Eu 4f EB Eu2+ EB Eu3+ r Eu3+ 4f (i) EuO 1.8 eV 7.0–11.1 eV 7.4 ± 1.2% (ii) EuO 1+x 1.64 eV 7.0–11.1 eV 66 ± 2% Eu 3d EB Eu2+ (eV) EB Eu3+ (eV) r3d Eu3+ j = 5/2 3/2 E SO 5/2 3/2 E SO 5/2 3/2 (i) EuO 1124.9 1154.1 29.2 1134.7 1164.4 29.7 4.1% 3.2% (ii) EuO 1+x 1125.0 1154.3 29.3 1134.8 1164.5 29.7 59% 49%
92 5. Results II: EuO integration directly on silicon dent information depths as ID ne 3d ≈ 10.7 nm and IDoe 3d ≈ 7.3 nm, indicating an increase in surface sensitivity of 43% for both emission geometries compared to 4f photoemission. The Eu 3d difference intensity curves ΔI3d ∗ = I oe ∗ − Ine ∗ for both stoichiometric and oxygen-rich EuO compounds are given in Fig. 5.3g and i. While in the 4f spectra the Eu 2+ ions show a symmetric shift from bulk to surface states, we observe a larger imbalance in intensity transfer of Eu 2+ photoemission in the 3d spectra such that the surface contribution is clearly dominant, |−ΔIB ∗ | | + ΔI S ∗ |. This spectral intensity from more surface-like states is explained by the significantly enhanced surface sensitivity of the 3d photoelectrons. Only for oxygen-rich EuO (ii) we find this accumulation of divalent ions in the surface region of the EuO layer, this effect coincides with “divalent surface states” of trivalent EuO compounds in literature. 111 In order to quantify the Eu 3d multiplet, we use convoluted Gaussian-Lorentzian lines with consistent intensity ratios, peak widths and energy differences. The Eu 2+ multiplet is in accordance with Cho et al. (1995). 110 For the Eu 3+ 3d line shapes, we employ single Gaussian- Lorentzian peaks, respectively, since the explicit fine structure of the trivalent Eu 3d multiplet is not published to date. Two satellite features are included besides the Eu 2+ and Eu 3+ main 3dmultiplets. They may be interpreted as follows: due to a photoionization of the 4f valence level, the rearrangement of 4f 6 electrons permits the acception of a conduction electron and is observed as an “apparent Eu (III) configuration” of the initial state, 4f 6 5d 1 . This yields a so-called shake-up satellite at about 6.5–7.0 eV higher binding energy with respect to the main divalent Eu photoemission peak, 3d 9 :4f 7 5d 0 . 180 Moreover, we include contributions from a well-known Eu (III) 3d final state effect 180 referred to as shake-down (SD and Δ ∗ SD in Fig. 5.3g and i). Here, unoccupied 4f -subshells are lowered in energy by the potential of the 3d photohole, which allows a conduction electron to occupy these 4f -subshells. This increases the occupation of 4f -subshells, corresponding to an “apparent change of the initial valence” as observable in the photoemission energy region of Eu (II) 3d. Thus, the shakedown satellite transfers intensity on the low binding energy side of the main photoemission peak, 176 and can be described within the Anderson impurity model. 182 Furthermore, for both Eu 2+ 3d and Eu 3+ 3d main peaks, a small surface spectral contribution S is included on the higher binding energy side. The best fit of the least-squares fitting is shown as solid line in Fig. 5.3f and h in very good agreement with the experimental spectra. We determine the relative fraction of Eu 3+ cations as r3d Eu3+ ≈ 3.7 ± 0.4% for EuO compound (i) and r3d Eu3+ ≈ 55±4% for EuO of type (ii). This result is in good agreement with the quantitative analysis of the 4f valence states, as summarized in Tab. 5.1. Our quantification reveals the excellent chemical quality of the MBE-deposited EuO thin films, with a homogeneous depth distribution of Eu cations. For (ii) oxygen-rich EuO, we extract an about 17% reduced fraction of Eu 3+ cations from the more surface-sensitive 3d compared to the 4f emission. This underlines again a small accumulation of Eu 2+ cations at the surface-near interface in contact with Al. The feature is consistently observed for both emission geometries in the Eu 4f and Eu 3d peak analysis and coincides with the grazing−normal emission difference intensity curves. Relating the formation energies 35,183 Gf ◦, Eu 3 O 4 (−2140 kJ/mol) ≪ α-Al 2 O 3 (−1582 kJ/mol) < Eu 2 O 3 (−1559 kJ/mol) we may identify the thermodynamically most probable origin of divalent Eu ions in compound (ii): the formation probability of Eu 3 O 4 exceeds these for alumina (Al 2 O 3 ) and europia (Eu 2 O 3 ). Indeed, Eu 3 O 4 is mixed-valent with 66% Eu 3+ ions, and thus fits perfectly
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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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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
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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
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 134 and 135: 5.5. Interface engineering III: SiO
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.
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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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