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3.1 Characterization Methods Figure 3.1: Typical Raman spectrum of a µc-Si:H film. shows a clear correlation between IC RS , X-ray diffraction and TEM [115, 116]. However, there is broad agreement that the crystallinity is underestimated by the Raman intensity ratio. 3.1.2 Electron Spin Resonance (ESR) Since the first spectrum recorded in 1945 [117], electron spin resonance (ESR) 2 has developed into a powerful spectroscopic technique used in many areas of modern physics, chemistry and biology. The subject matter of ESR is the interaction of electrons with magnetic fields and with each other. In this section the physical principles of ESR will be briefly discussed, followed by a brief description of the actual measurement. 3.1.2.1 Spin Hamiltonian For a system containing only one unpaired electron spins (S=1/2) the Hamilton operator can be written as H = g 0 µ B B 0 S + µ B B 0 [∆g]S. (3.2) where g 0 is the electronic g-value for a free electron (≈ 2.0023), µ B the Bohr magneton, B 0 is the flux density, [∆g] is the interaction tensor of the spin orbit 2 In the literature also referred to as Electron Paramagnetic Resonance (EPR) 21
Chapter 3: Sample Preparation and Characterization coupling, and S the spin operator [118]. For the following discussion, terms due to hyperfine interaction, the interaction with the nuclei spin, and the spin-spin interaction are neglected because they have no resolvable contribution to the spectra measured in this work. In Eq. 3.2 the second term describes the coupling of the electron spin with the magnetic moment of the orbital angular moment L, whereas the tensor quantity [∆g] describes the deviation of the g-value from that of the free electron g 0 . In covalent semiconductors, where the electronic eigenstates are usually described in terms of s- and p-state wavefunctions, L, whose eigenstates are degenerate, has a zero expectation value if the crystal field interaction greatly exceeds the Zeeman term [24]. This effect is known as quenching of the orbital angular momentum by the crystal field. In this special case the second term in Eq. 3.2 becomes zero. However, due to the interaction with a magnetic field the degeneracy of the eigenstates of L is lifted and the quenching is partly removed. For an unpaired electron in the ground state, the elements of the g-tensor g ij in the second term of Eq. 3.2 are given by [118] ∑ g ij = −2λ n(0) < Ψ 0 |L i |Ψ n >< Ψ n |L j |Ψ 0 > E n − E 0 (3.3) where λ is the spin orbit coupling parameter. The index n counts all other orbitals Ψ n , E 0 denotes the energy of the ground state and E n the energy of the state Ψ n . The g-value is therefore an important quantity in ESR measurements and serves to distinguish and identify electronic states. However, in disordered or powdered material the angular dependence is masked as all orientations can be observed at the same time. The obtained spectra are called ”powder spectrum”. 3.1.2.2 Lineshape and Linewidth Besides the g-value, the shape and the width of the resonance line contains a number of information about the spin system [119, 118]. It is important to note that in ESR one talks about the peak-to-peak width, which is defined as the width between the maximum and minimum of the derivative of the absorption line. Depending on the specific lineshape of the curve this value differs by a numerical factor from the linewidth at half maximum (FWHM) typically used in other spectroscopy methods. In general one distinguishes between two different mechanisms broadening the resonance line, first the ”homogeneous” broadening which is caused by the relaxation of the excited spin state and second the inhomogeneous broadening as a result of an unresolved overlap of different ESR lines. 22
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Page 6 and 7: Kurzfassung In der vorliegenden Arb
Page 8 and 9: Abstract The electronic properties
Page 10 and 11: Contents 1 Introduction 1 2 Fundame
Page 12 and 13: CONTENTS C Abbreviations, Physical
Page 14 and 15: Chapter 1 Introduction Solar cells
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Page 20 and 21: Chapter 2 Fundamentals In this firs
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Page 46 and 47: 3.2 Deposition Technique admixture
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Page 52 and 53: Chapter 4 Intrinsic Microcrystallin
Page 54 and 55: 4.2 Electrical Conductivity Figure
Page 56 and 57: 4.3 ESR Signals and Paramagnetic St
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Page 64 and 65: Chapter 5 N-Type Doped µc-Si:H In
Page 66 and 67: 5.2 Electrical Conductivity Figure
Page 68 and 69: 5.4 Dangling Bond Density Figure 5.
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6.1 Metastable Effects in µc-Si:H
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6.1 Metastable Effects in µc-Si:H
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6.2 Irreversible Oxidation Effects
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6.3 On the Origin of Instability Ef
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6.3 On the Origin of Instability Ef
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Chapter 7 Transient Photocurrent Me
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7.2 Transient Photocurrent Measurem
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7.3 Temperature Dependent Drift Mob
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7.4 Multiple Trapping in Exponentia
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7.5 Discussion Table 7.2: Multiple
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7.5 Discussion 7.5.3 The Meaning of
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Chapter 8 Schematic Density of Stat
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silicon as shown in Fig. 8.1 b, whi
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Chapter 9 Summary In the present wo
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Appendix A Algebraic Description of
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Eq. A.7 then becomes L(t) F = sin(
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Appendix B List of Samples Table B.
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Table B.3: Parameters of n-type µc
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Appendix C Abbreviations, Physical
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µ d Drift mobility µ d,h Hole dri
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Bibliography [1] E. Becquerel. Mém
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BIBLIOGRAPHY [21] D. Will, C. Lerne
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BIBLIOGRAPHY [45] H. Overhof and M.
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BIBLIOGRAPHY [66] P.W. Anderson. Ab
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BIBLIOGRAPHY [93] J.W. Orton and M.
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BIBLIOGRAPHY [121] J.R. Haynes and
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BIBLIOGRAPHY [148] W. Luft and Y.S.
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BIBLIOGRAPHY [170] G.N. Advani, R.
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List of Publications 1. T. Dylla, R
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Acknowledgments At this point, I wa
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Schriften des Forschungszentrums J
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Forschungszentrum Jülich in der He
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