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3.4. Ex situ characterization techniques 53 Figure 3.18.: Photoelectron yield for low or high kinetic energy and off-normal emission. In case of a high kinetic energy of the photoelectrons, the escape depth is large due to less probable inelastic scattering. Lowkinetic energy electrons experience inelastic scattering more likely on their escape, such that the depth of unscattered photoelectron is smaller. Off-normal emission reduces the escape depth by cosα and renders this configuration surface-sensitive, too. For an off-rotation of α =60 ◦ , the photoelectrons travel two times the normal emission path to escape the solid. oxide on top of a silicon wafer calculates as ( Iox c = −λ ox cosα ·ln · λSin ) Si . (3.12) I Si λ ox n ox Here, λ denotes the effective attenuation length, 84 α is the off-normal emission angle, n is the density of the element from which the spectra arise (e. g. Si), and I are the measured spectral weights of either the bulk material or the surface layer of interest. However, if the thin layer of interest is buried under multiple top layers (e. g. EuO and a capping), we have to extend the derivation of eq. (3.12) in order to obtain a model for a threelayer system: this includes intensities of a substrate, a thin buried reaction layer, and accounts for the exponential damping of the photoelectrons by top layers. First, we consider the photoelectron intensities I of the substrate (sub) and a possible reaction layer (rl) as depicted in Figure 3.19.: HAXPES probe of a buried interface. The photoelectrons from either the substrate element are evaluated (a), or from an element of the tunnel barrier (b).
54 3. Experimental details Fig. 3.19a, I rl (c,d,λ)=e − I sub (c,d,λ)=e − ∫ d d+c λcap cosα e − d ∫ ∞ d+c λcap cosα e − d+c x λ rl cosα dz, and (3.13) x λ sub cosα dz. (3.14) In the next step, we express the fraction of spectral contribution of the reaction layer f rl with respect to the bulk intensity. Here, the measured intensities I have to be normalized by λ and the density n of contributing atoms in the particular layer, I rl (c,d,λ) calc meas = f I sub (c,d,λ) rl = I rl · λsubnsub . (3.15) I sub λ rl n rl This equation (3.15) contains the thickness of the buried reaction layer c. Because it is not analytically solvable, a Taylor expansion of c to the 2nd order is applied which lets c be expressed as λ λ rl λ cap ∗λ sub cosα e rl cosα λ sub cosα √ λ rl λ cap ∗ c = + · λ sub λ cap ∗ − 2λ sub λ rl − 2λ rl λ cap ∗ λ sub λ cap ∗ − 2λ sub λ rl − 2λ rl λ cap ∗ d d λ √2f rl λ cap ∗λ sub e rl cosα λ − 4f rl λ rl λ sub e rl cosα λ − 4f rl λ rl λ cap ∗e rl cosα λ − λ rl λ cap ∗e sub cosα ( ) d d 2 . λ −e sub cosα λ e rl cosα d d d (3.16) In order to determine the thickness of possible reaction layers of EuO on a silicon surface (SiO x or EuSi 2 ), this expression is applied to Si 1s or Si 2p HAXPES spectra of EuO/Si heterostructures investigated in this work. A consistent chemical characterization of the reaction layer at the functional EuO/Si interface can be achieved by the additional evaluation of HAXPES spectra from the magnetic oxide layer in direct contact with the Si substrate. Here, the photoelectrons from the bottom layer in the EuO oxide (ox) layer carry information about the electronic structure of a possible reaction layer (rl). Hence, the HAXPES intensities are modeled according to Fig. 3.19b as I rl (a,b,c,λ)=e − I ox (a,b,λ)=e − ∫ a+b a+b+c λ cap ∗ cosα e − x a+b ∫ a a+b λcap cosα e − x a λ rl cosα dz, and (3.17) λox cosα dz. (3.18) The measured photoelectron spectra have to be normalized (similar to eq. (3.15)), and this fraction is compared with the modeled fraction, so that I rl(a,b,c,λ) calc meas = f I ox (a,b,λ) rl = I rl I · λoxn ox ox λ rl n can be rl solved analytically to extract the thickness c of the reaction layer as ⎛ ⎞ c = −λ rl ln⎜⎝ f λ ox rl e − aλox+aλcap+bλcap λcapλox cosα + e − (a+b)(λ cap ∗ +λ rl) λ rl λ cap ∗ cosα λ − f ox λ rl e − a(λox+λcap) λcapλox cosα rl λ rl ⎟⎠ · cosα (3.19) + aλ rl + bλ rl + aλ cap ∗ + bλ cap ∗ . −λ cap ∗
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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
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
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
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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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