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2.4. Hard X-ray photoemission spectroscopy 27 Figure 2.16.: Comparison of the intensity profiles of elastic photoelectron forward diffraction. The intensity plumes are drawn as expected for C 1s photoemission at hν = 1 keV or hν = 10 keV from a vertical CO molecule on the surface. Adapted from Thompson and Fadley (1984). 102 Figure 2.17.: Calculated Debye-Waller factors for valence-level photoemission. The values are strongly dependent on the photoelectron kinetic energy. At high kinetic energy, cooling well below room temperature reveals significantly larger Debye-Waller factors. Adapted from Plucinski et al. (2008). 103
28 2. Theoretical background III. The inelastic mean free path A critical parameter for the probing depth in any photoemission experiment is the inelastic mean-free path (IMFP) λ of hot electrons in a solid. Calculated curves of λ versus E kin are depicted in Fig. 2.18. The similarity in the slope for many elemental solids justifies the use of the term “universal” curve. We recognize that with energies of 10 keV IMFPs of up to 200 Å can be reached. Figure 2.18.: Universal curve of photoelectron emission in solids. The inelastic mean free path (IMFP) λ is the key parameter to describe inelastic scattering of photoelectrons in solids. Extending the escape depth of the photoelectrons up to several tens of nanometers (with λ ∝ Ekin 0.78 ), 85 the HAXPES regime is perfectly suited to investigate the electronic properties of buried interfaces or bulk properties. This recent calculation of IMFPs is from Tanuma et al. (2011). 84 It is evident that, in reality, the IMFP curves can be very different depending on the physical properties of an individual material. Density, dielectric function and band gap of the solid as well as the elastic scattering can also alter the trajectory of a photoexcited electron. If these effects are included into λ, one obtains the effective attenuation length (EAL). One can then express a mean escape depth down to which unscattered photoelectrons can be expected as λ ∗ = λ EAL (E kin )· cosα, (2.26) where λ EAL denotes the experimentally determined effective attenuation length, 80 and α the off-normal exit angle of the photoelectron. As evident from eq. (2.26), the bulk-vs-surface sensitivity of the photoemission measurement can be controlled the kinetic energy or by varying the photoelectron’s off-normal exit angle α. The behavior is determined by cosine, which gives e. g. a halving of the mean escape depth at λ ∗ (60 ◦ )=1/2λ ∗ (0 ◦ ). This phenomenon is extremely useful, since simply by tilting the sample one can obtain depth-resolved information, 86 which we apply for chemical depth profiles of EuO/Si in Ch. 5. Most intuitive for depth-selective measurements, however, is the information depth ID, from which 95% of the detected photoelectrons originate. This definition coincides, in practice, with the detection limit for chemical species in buried layers (see Ch. 5.1), and equals approximately ID 95% ≈ 3λ ∗ . Concluding, hard X-ray excitation offers the advantage of a largely tunable probing depth up to tens of nanometers, thus allowing to investigate buried layers and functional interfaces. Minor drawbacks are reduced photoemission cross-sections and Debye-Waller factors. Those, however, can be compensated by nowadays highly brilliant hard X-ray radiation sources, as introduced in Ch. 3.4.3.
Page 1 and 2: Member of the Helmholtz Association
Page 4 and 5: Forschungszentrum Jülich GmbH Pete
Page 6: Zusammenfassung In dieser Doktorarb
Page 10 and 11: Contents 1. Introduction 1 2. Theor
Page 12 and 13: List of Figures 2.1. Phase diagram
Page 14: List of Figures xi 5.12. Resulting
Page 18 and 19: 1. Introduction Smart and powerful
Page 20 and 21: 3 In the thesis at ha
Page 22 and 23: 2. Theoretical background . . . The
Page 24 and 25: 2.1. The challenge of stabilizing s
Page 26 and 27: 2.2. Electronic structure of EuO 9
Page 28 and 29: 2.3. Magnetic properties of EuO 11
Page 36 and 37: 2.4. Hard X-ray photoemission spect
Page 46 and 47: 2.5. Magnetic circular dichroism in
Page 52 and 53: 3. Experimental details Die Pläne
Page 54 and 55: 3.1. Molecular beam epitaxy for oxi
Page 56 and 57: 3.2. In situ characterization techn
Page 58 and 59: 3.2. In situ characterization techn
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
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4.2. Lateral tensile strain: EuO on
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4.3. Lateral compressive strain: Eu
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5. Results II: The integration of t
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5.1. Chemical stabilization of bulk
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5.2. Thermodynamic analysis of the
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5.3. Interface engineering I: Hydro
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5.4. Interface engineering II: Eu p
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5.5. Interface engineering III: SiO
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5.5. Interface engineering III: SiO
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5.6. Summary 121
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6. Conclusion and Outlook In the th
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125 Outlook and perspectives In the
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A.1. EuO-related phases and cubic o
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A.1. EuO-related phases and cubic o
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A.2. In situ analysis of the Si (00
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Bibliography 133 [13] S. S. P. Park
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Bibliography 135 [36] R. E. Dickers
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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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