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2.2 Electronic Density of States Figure 2.3: Typical ESR-Spectra of (a) undoped µc-Si:H with contributions at g=2.0043 and g=2.0052 and (b) n-doped material with an additional contribution at g=1.998. Both spectra were taken from material investigated in this work. well understood. Besides the unknown microscopic location, there are also uncertainties regarding the energy positions of defects within the gap, which have been taken into account in the schematic DOS of µc-Si:H shown in the right panel of Fig. 2.2. For intrinsic µc-Si:H, the most important paramagnetic defect is the Si dangling bond (DB). An ESR spectrum of high quality intrinsic material shows an asymmetric line shape with contributions at g=2.0043 and g=2.0052. A typical spectra taken of undoped µc-Si:H material is shown in Fig. 2.3 (a). The origin of these two contributions is still controversially discussed. While it has been suggested by several authors that the anisotropy arises from two independent dangling bond states in different microscopic environments [21, 33, 35, 34, 36, 32], Kondo et al., on the other hand, assigned the two components contributing to the ESR signal to an axial symmetric g-tensor of P b -like 1 defects located on < 111 > oriented grain surfaces with components of g ‖ = 2.0022 and g ⊥ = 2.0078 [31]. A more recent publication from de Lima et al. [84] also suggested the signal arising from an axial-symmetric center, but extracted g-values of g ‖ = 2.0096 and g ⊥ = 2.0031, relating the signal with defects in the crystalline phase. As microcrystalline silicon can consist of a considerable amount of amorphous phase, also dangling bond defects located in the a-Si:H fraction may contribute to the ESR signal. The DB defect found in a-Si:H has a characteristic g-value of g=2.0055 and a typical peak to peak line width of ∆H pp = 10 G in X-band 2 [85]. Another aspect of the increasing amorphous phase is the Staebler-Wronski- Effect (SWE) [7]. The SWE describes the light induced breaking of weak Si-Si bonds in the silicon network which leads to the creation of additional dangling 1 P b centers are silicon dangling bonds at the Si/SiO 2 -interface of oxidized silicon wafers. 2 For details of the notation see section 3.1.2 13
Chapter 2: Fundamentals bond defects [86]. For highly crystalline µc-Si:H, it has been shown that it does not suffer from the SWE [19]. However, due to the presence of amorphous phase, this material might also be susceptible to light-induced metastable effects, which in fact was recently confirmed by Klein [87]. For n-type doped and also for illuminated intrinsic µc-Si:H samples, another resonance with a g-value of g=1.996-1.998 can be observed (Fig. 2.3 (b)). Since the intensity of this signal is correlated with the dark conductivity σ D at 300 K and the g-value is close to the one of free electrons in crystalline silicon, this signal was first attributed to electrons in the conduction band [27, 29]. The resonance has therefore been referred to as the conduction electron (CE) resonance. Later on, this signal has also been attributed to localized states in the conduction bandtail [88, 35, 38, 39, 72]. Substitutional Doping Controlled incorporations of impurities are typically used to provide additional free charge carriers. Typical donors and acceptors used to dope silicon are phosphorus and boron, respectively. In crystalline silicon (c-Si), the inclusion of dopants immediately lead to a shift of the Fermi level E F up to an energy position, located between the energy level of the dopant and the band edge, even for very low doping concentrations. In contrast to crystalline silicon, the high concentration of intrinsic defects has a major influence on the free carrier concentration achieved from doping. Additional incorporation of e.g. donors will be compensated by the defects as they act as acceptors by creating D − states, accumulating the charge carriers. This is the reason why deep defect states within the band gap first have to be compensated before the Fermi level can shift to the conduction band edge. For amorphous silicon another complication prevents that high doping densities can be achieved. The doping induces deep states in the gap preventing a shift of the Fermi level [89, 90]. Different, independent experiments have shown that the total defect density increases with the square root of the doping concentration [65]. 2.3 Charge Carrier Transport In crystalline silicon, the charge carrier transport takes place in the extended states of the band and can be described using effective mass theory [91]. In µc-Si:H, on the other hand, the presence of localized states within the bandgap has a major 14
Page 1 and 2: Forschungszentrum Jülich in der He
Page 4 and 5: Forschungszentrum Jülich GmbH Inst
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
Page 16 and 17: defect states provided that they ar
Page 18 and 19: Chapter 8: In this chapter, the inf
Page 20 and 21: Chapter 2 Fundamentals In this firs
Page 22 and 23: 2.2 Electronic Density of States St
Page 24 and 25: 2.2 Electronic Density of States ta
Page 28 and 29: 2.3 Charge Carrier Transport influe
Page 30 and 31: 2.3 Charge Carrier Transport functi
Page 32 and 33: Chapter 3 Sample Preparation and Ch
Page 34 and 35: 3.1 Characterization Methods Figure
Page 36 and 37: 3.1 Characterization Methods Homoge
Page 38 and 39: 3.1 Characterization Methods µ Fig
Page 40 and 41: 3.1 Characterization Methods compar
Page 42 and 43: 3.1 Characterization Methods bution
Page 44 and 45: 3.1 Characterization Methods posite
Page 46 and 47: 3.2 Deposition Technique admixture
Page 48 and 49: 3.3 Sample Preparation 3.3.1 Sample
Page 50 and 51: 3.3 Sample Preparation reverse bias
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
Page 58 and 59: 4.3 ESR Signals and Paramagnetic St
Page 60 and 61: 4.4 Discussion - Relation between E
Page 62 and 63: 4.4 Discussion - Relation between E
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.
Page 70 and 71: 5.5 Conduction Band-Tail States Fig
Page 72 and 73: 5.6 Discussion concentration evalua
Page 74 and 75: 5.7 Summary Figure 5.7: Schematic b
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Chapter 6 Reversible and Irreversib
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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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