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2.2 Electronic Density of States Studies have also shown that these voids may extend from the surface deep into the film and allow for in-diffusion of atmospheric gases along the column boundaries [20, 22]. An increasing amorphous phase content leads to a reduction of the column diameter, while an extended disordered phase is incorporated in the increasing incubation layer and at the column boundaries. Concerning the porosity there is some not yet understood discrepancy between results obtained from TEM and IR. While TEM shows cracks and voids, there is no indication of oxygen in-diffusion or low temperature hydrogen effusion in such material, suggesting a rather compact structure. At the site of transition between crystalline and amorphous growth, the material structure changes significantly. The columns no longer extend throughout the entire film thickness. In fact, the crystalline regions are frequently interrupted and embedded in an amorphous matrix. The size of the crystalline domains decreases as the size of the coherent regions forming them. Finally only amorphous growth is obtained and no crystalline contribution can be found in the material. However, the electronic properties of the amorphous phase found in this kind of material differs from standard a-Si:H and is therefore often referred to as ”protocrystalline”, ”polymorphous” or ”edge material” [61, 62, 63]. 2.2 Electronic Density of States The structural properties of µc-Si:H, in particular the disorder, lead to some phenomena in the electronic density of states (DOS) that cannot be found in the crystalline counterpart. The lack of translational symmetry leads to some major consequences for the electronic properties of the material. However, as the electronic structure is mostly determined by the short range order, the overall electronic properties are very similar compared to the equivalent crystal. But, due to the lack of long range order, the abrupt band edges found in the crystal are replaced by a broadened tail of states extending into the forbidden gap. On the other hand, the deviation from the ideal network structure also results in electronic states deep within the gap (dangling bonds). As microcrystalline silicon is a phase mixture of crystalline and disordered regions separated by grain boundaries and voids, the particular band structure dependents on the particular spatial position within the material, and an overall DOS-diagram can not be drawn easily. In the following section a brief description of the main features of the DOS is given. On the basis of the simplified picture for the DOS in a-Si:H, shown in Fig. 2.2, band-tail and defect states are discussed and adopted for a description of the DOS of µc-Si:H. Note, while the schematic DOS for a-Si:H shown in Fig. 2.2 is sufficient to describe a number of experimental results including electron spin 9
Chapter 2: Fundamentals Figure 2.2: Schematic density of states of amorphous (left) [65] and microcrystalline (right) silicon [39]. resonance (ESR) very well, there are other models for the distribution of defects, e.g. the so-called defect-pool model (see e.g. [64] for a review), which however will not be treated here. 2.2.1 Band-Tail States One consequence of a missing long range order is the existence of band-tail states. Local fluctuations in the interatomic distances and the bonding angles result in spatial fluctuations of the band edges. This leads to regions within the band, where charge carriers can be trapped. The existence of localized states in disordered material was first predicted by Anderson [66], and it has been shown by Mott that any random potential introduces localized states in the tails of the band [67, 68]. The resulting DOS is schematically shown in Fig. 2.2, where the usually sharp band edges are replaced by a broad tail extending deep into the bandgap. Within the band-tail localized and extended states are separated by mobility edges at energies E C or E V , respectively. The mobility edge derives its name from the fact that at zero temperature only charge carriers above E C (for E V below) are mobile and contribute to transport [68]. While these ideas have been developed and experimentally proven mainly for amorphous material, it has been shown by Werner et al. [69, 70, 71] that for poly-crystalline silicon, the spatial distribution of defects at grain boundaries also leads to potential fluctuations, resulting in band- 10
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 24 and 25: 2.2 Electronic Density of States ta
Page 26 and 27: 2.2 Electronic Density of States Fi
Page 28 and 29: 2.3 Charge Carrier Transport influe
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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
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5.6 Discussion concentration evalua
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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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