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5.4. Interface engineering II: Eu passivation of the EuO/Si interface 113 8 6 4 SiO x Si SiO x Si 40 35 30 104 102 100 98 1146 1142 1138 clean Si (fl ashed) 1 ML Eu EuO/Si 220 15 2 ML Eu EuO/Si 3 ML Eu EuO/Si 110 0 4.6 0.13 nm Si oxides 30% Si 3+ 0.56 nm Si oxides 38% Si 3+ 7.5 0 5.6 2.3 0.32 nm Si oxide 33% Si 3+ 0.31 nm Si oxide 54% Si 3+ 2.8 Si (001) (1x1), strained EuO (001) partially relaxed EuO (001) 0 0 1136 1148 1144 1140 1148 1144 1140 1136 Figure 5.23.: The EuO/Eu-Si interface analyzed RHEED and by peak fitting of the Si 2p and 1s corelevels measured by HAXPES. bulk diamond structure as well as an interstitial pattern of the Si (001) (2 × 1) surface reconstruction. The EuO/Si heterostructure with 1 ML protective Eu shows a streaky pattern of the EuO fcc lattice, whereas the rods have a larger distance in the upper region (θ =2 ◦ “grazing”) of the pattern than in the lower region (θ =6 ◦ ). From the lower end of the diffraction rods to the upper end the information depth of diffraction is altered from a few lattice parameters to the topmost layer. The diffraction features from the surface agree nearly with the EuO reciprocal lattice parameter, while the diffraction from deeper layers agrees with the lattice of Si (001). This is an example of strained growth (6% difference) of EuO on Si (001) maintaining epitaxy, where with the fourth ML of EuO (i. e. 20 Å EuO) the EuO lateral lattice parameter relaxes. The RHEED pattern for samples with 2 ML protective Eu at the EuO/Si interface also displays streaky rods of EuO. Here, EuO mainly adapts the Si (001) lattice parameter and shows only small relaxation. The EuO lattice parameter from the surface layer is determined as 5.3 Å which indicates the EuO lattice is only partially relaxed to the literature value of 5.144 Å. For the EuO/Si heterostructure with 3 ML protective Eu, the RHEED pattern of EuO consists of smooth rods and EuO is homogeneously strained in the lateral dimension adapting the Si (001) cubic lattice parameter. From these electron diffraction results we conclude that epitaxial growth of smooth EuO on clean Si (001) is feasible, where EuO mainly adapts the Si cubic lattice parameter. A detailed picture of the interfacial electronic structure is obtained from HAXPES core-level spectra of the EuO/Eu-Si (001) interface. In order to tune the information depth to selectively probe the EuO/Si interface layer, the low excitation energy of hν = 2.7 keV and off-normal
114 5. Results II: EuO integration directly on silicon Figure 5.24.: Quantitative results of the SiO x analysis in EuO/Eu-Si. emission are used (Fig. 5.23a). By increasing the off-normal emission angle further than to α =30 ◦ , the information depth underruns the minimum for an acquisition of Si 1s spectra; this indicates thus the threshold of probing exclusively the Si interface layer. Two core-level photoemission spectra of Si, Si 2p and 1s, are inspected as depicted in the two panels of Fig. 5.23c. A quantitative analysis, however, that demands for a smooth and well-resolved curve of the SiO x components is only feasible with the Si 1s deep core-level, as compiled in Fig. 5.23d. The chemical shifts of the silicon oxide are reduced due to final state effects, in agreement with a comprehensive SiO x /Si PES study 139 for ultrathin SiO x films in the 2 nm range. Evaluating the chemical shifts of the SiO x in detail reveals the shift to be not compatible with solely Si 4+ (SiO 2 ), but rather a mixture of oxidation states of Si. If one or two ML Eu were applied, the fraction of Si 3+ ranges from 33% to 38%, whereas this fraction is increased up to 54% on the cost of Si 4+ if three ML protective Eu were applied. The redistribution of the silicon oxide to the valency Si 3+ –different to the native oxide which is always Si 4+ – points out a characteristic low oxidation of the silicon wafer, which takes place during the oxygen-limited (1.5 × 10 −9 Torr) synthesis of EuO. In order to quantify the interfacial SiO x formation, we apply a least-squares peak fitting analysis after Levenberg-Marquardt for the three EuO/Si heterostructures with one up to three ML of protective Eu at the EuO/Si interface. The minimization of interfacial SiO x in the EuO/Si heterointerface with Eu passivation monolayers (ML) results in 0.67 nm SiO x for one ML Eu, 0.43 nm SiO x for two ML Eu, and a minimum of 0.42 nm SiO x if3MLEuwere provided onto Si (001) just before EuO synthesis (summarized in Fig. 5.24). The residual silicon oxide from a flashed clean silicon wafer was determined to be only 0.11 nm and is included in all heterostructures. In conclusion, two ML of protective Eu the silicon (001) surface has been passivated with are sufficient to limit the total thickness of interfacial Si oxidation to 0.43 nm. Moreover, the dominant oxidation state of the Si interface changes from Si 4+ to the Si 3+ valence state, and a EuO/Si (001) heteroepitaxy with adaption of the Si lattice parameter can be maintained.
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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 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
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Page 134 and 135: 5.5. Interface engineering III: SiO
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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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