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5.4. Interface engineering II: Eu passivation of the EuO/Si interface 115 Combination of Eu- and H-passivation of Si (001) to maintain a minimum of interfacial contamination 100 75 T S = 400 °C, 2 ML Eu 50 25 0.18 018nm silicide id 0.61 nm Si oxides 0 Figure 5.25.: The EuO/Si interface with a combined optimization against silicides and SiO x . Both H- passivation of the Si (001) are applied and oxygen protective Eu monolayers. As a free parameter, the temperature of synthesis T S is varied. After EuO growth, RHEED pattern are recorded (inset). 120 60 0 240 160 80 0 106 T S = 450 °C, 2 ML Eu 0.18 nm silicide 0.50 nm Si oxides T S = 500 °C, 2 ML Eu 0.17 nm silicide 0.84 nm Si oxides 104 102 100 98 96 In this section, we optimize the EuO/Si interface by means of two complementary methods: (i) the hydrogen-passivated Si (001) surface is capable to avoid silicides but has shown to allow for a certain Si oxidation, and (ii) the Eu monolayer-passivated Si interface shows a minimized SiO x contamination but establishes an environment for silicide formation. Thus, combining these two passivation techniques from the previous sections promises a simultaneous minimization of silicides as well as SiO x at the EuO/Si (001) interface. Taking the optimum set of interface passivation parameters for the silicide and the silicon oxide minimization from sections 5.3 and 5.4 into account, we point out the optimum parameters to be two monolayers of oxygen-protective Eu combined with a complete in-situ hydrogen passivation of Si (001). As a free parameter, we choose the temperature of synthesis T S , which constitutes the thermodynamic driving force for the exchange of heat and entropy, while the volume and system pressure are constant. Thus, a temperature increase pushes the possible interface reactions towards their thermodynamically expected results (as discussed in Ch. 5.2), but also promotes kinetics such as interdiffusion of Eu or Si atoms. The temperature of EuO synthesis T S is kept in a range in which the Eu distillation condition is still valid. We chose T S to vary around T S = 450 ◦ C, at which an optimum structural quality
116 5. Results II: EuO integration directly on silicon of EuO was observed in EuO on cubic oxide studies (Ch. 4). In this way, we prepare three EuO/H-Si heterostructures with two monolayers oxygen-protective Eu at temperatures T S = 400 ◦ C, 450 ◦ C, or 500 ◦ C. After synthesis of the EuO/Si (001) heterostructures, we analyze the surface structure of the EuO magnetic oxide layer by RHEED. The pattern of an fcc rocksalt lattice of EuO (001) is clearly observed after 2 nm EuO growth, with a lattice constant matching the silicon cubic lattice parameter. In the EuO reciprocal rods, the vertical intensities are distributed dot-like, which indicates an epitaxial growth with three-dimensional structures on the passivated Si (001). A quantitative analysis of the chemistry at the EuO/Si heterointerface with combined passivations is obtained from Si 2p spectra by HAXPES, in a geometry adjusted to probe precisely down to the Si interface. This is achieved by using the minimum excitation energy of hν = 2.3 keV of the HAXPES beamline (KMC1, see Ch. 3.4.3) and an off-normal emission angle of 30 ◦ . The Si 2p doublet provides the advantage that both the silicides and oxides can be well distinguished from bulk Si and quantitatively analyzed in one core-level (Fig. 5.25). For the silicides, the temperature variation does not affect the interfacial thickness of EuSi 2 , which we determined as d EuSi2 = 0.2 nm for all three heterostructures, in good agreement with the optimum value of the silicide optimization series (Ch. 5.3). The oxidation of the Si interface exhibits a minimum value of d SiOx = 0.69 nm for the heterostructure synthesized at T S = 450 ◦ C, while a maximum oxidation of the Si interface is observed for the EuO/Si interface exposed to the highest temperature of EuO synthesis, T S = 500 ◦ C. Interfacial silicide and SiO x thicknesses are summarized in Fig. 5.26. We identify a general shift to slightly higher values of interfacial SiO x in this study of combined Si passivation when compared to the explicit optimization regarding SiO x by protective Eu monolayers in Ch. 5.4. This can be explained by the additional hydrogen termination procedure of Si (001), which systematically introduces a fraction of at least 10 −6 oxygen in the hydrogen gas during the in-situ H-annealing process. The largest oxidation of the EuO/Si interface at T S = 500 ◦ C can be understood by an increased diffusion of Si surface atoms in the high temperature regime, changing the interface morphology and thus providing more reactive faces in contact with the EuO synthesis. In conclusion, we carried out three comprehensive HAXPES studies in order to investigate and optimize the chemical properties of the passivated EuO/Si interface by a quantitative analysis of silicon core-level spectra. The HAXPES results for the EuO/Si interface are compiled in Fig. 5.26. In a first step, the interfacial silicide was minimized by an increase of synthesis temperature from 350 ◦ Cto450 ◦ C (black spheres in Fig. 5.26), while the application of H-passivation to the Si (001) surface yields only a smaller reduction of the silicide content (red spheres in Fig. 5.26), resulting in a minimum of 0.14 nm silicide at the EuO/Si interface. In a second step, the SiO x contamination at the EuO/Si interface is diminished by oxygen-protective Eu monolayers, resulting in a minimum of 0.42 nm interfacial oxide (golden spheres). Finally, a combination of the advantageous passivation parameters from the silicide and the SiO x optimization studies reveal an optimum set (blue spheres in Fig. 5.26)
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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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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
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
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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