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5.3. Interface engineering I: Hydrogen passivation of the EuO/Si interface 107 Figure 5.18.: Surface and interface structure development of EuO on Si (001) and on H-passivated Si (001). RHEED photographs elucidate the EuO surface structure when grown on clean Si (001) (a), the cross-section is depicted by HR-TEM in (b). EuO growth on hydrogen-passivated Si (001) is documented by RHEED and LEED in (c), and the cross-section in real space and selected images of the reciprocal space are depicted in (d).
108 5. Results II: EuO integration directly on silicon 10–20 nm in the Si substrate occur frequently, and one is shown as an example in the TEM micrograph (Fig. 5.18b). We interpret these bulges as Eu diffusion regions, containing probably Eu silicides. The EuO/Si diffusion regions not only alter the Si and Eu chemistry readily towards metallic silicides, but also dramatically change the crystal structure affecting the EuO/Si interface (bulge), the lower EuO layer (polycrystalline layer), and also the upper EuO layer (instability with the EuO/Al interface). If, however, a complete hydrogen-passivation is applied to the Si (001) surface, the successive EuO growth steps are depicted in Fig. 5.18c. From the initial bulk-like Si (001) (1×1) pattern, first diffraction streaks of EuO are already observable at 1 nm EuO synthesis. Unlike the EuO growth on bare Si, here with H-passivated Si the EuO surface structure develops towards good crystallinity, as indicated by streaks in the RHEED photographs representing the EuO reciprocal lattice. After 10 nm EuO synthesis on H-Si (001), we observe the crystal structure of EuO (by RHEED and LEED) to be of a crystal quality comparable with EuO on cubic substrates under biaxial tensile strain (as discussed in Ch. 4.2). A cross-sectional HR-TEM study of the EuO/H-Si interface in Fig. 5.18d proves the successful prevention of diffusion effects, which were, in contrast, observed for EuO on bare Si (001). A 3 nm polycrystalline EuO layer forms in contact with Si, as indicated by the circular intensity of the Fourier transform picture. On top of this polycrystalline EuO, a single-crystalline EuO phase constitutes the residual EuO slab. However, the crystal structure of this EuO phase coincides best with Eu 2 O 3 which is deduced to be formed during sample transfer through ambient air. Without doubt, during MBE synthesis we observed divalent epitaxial EuO by its fcc rocksalt lattice, and this allows one to interpret the Eu 2 O 3 in the HR-TEM micrograph to represent the initial divalent EuO morphology. Thus, a complete hydrogen-passivation of Si (001) allows for persistent heteroepitaxial growth of EuO/H-Si structures with no indication of large diffusion areas in Si or the EuO slab. Only a polycrystalline EuO layer of about 3 nm remains in direct contact with Si. EuO layers above this poly-EuO layer are deduced to grow largely single-crystalline. Chemical optimization of the EuO/Si interface In order to achieve a minimization of the interfacial EuSi 2 in EuO/H-Si (001) heterostructures, we investigate the impact of two parameters: (i) the in-situ hydrogen passivation of the clean Si (001) surface “H-Si”, and (ii) the temperature of synthesis “T S ” as the general thermodynamic driving force. We vary the temperature between T S = 350 ◦ C and T S = 450 ◦ C, and we use the silicon (001) substrate either in situ hydrogen-passivated or with a clean (flashed) surface. These two parameters permit four variants of combinations, which we investigate in the following. For a correlation of passivation steps with the surface crystallinity, we first analyze the RHEED pattern: Figure 5.19b depicts the RHEED pattern after growth of the ultrathin EuO films (d = 2 nm) at an electron energy of 10 keV and the beam along the [100] direction. We observe RHEED patterns of the EuO fcc lattice at T S = 350 ◦ C only in the absence of hydrogen termination. The higher substrate temperature (T S = 450 ◦ C) allows one to observe sharper RHEED patterns than for the lower synthesis temperature. Here, the impact of H-passivation is a roughening of the surface as indicated by dot-like intensities inside the RHEED streaks. Thus, we conclude that excess Eu from an incomplete Eu distillation at lower T S hinders
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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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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
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Page 104 and 105: 5.1. Chemical stabilization of bulk
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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
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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
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Schlüsseltechnologien / Key Techno
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