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Chapter 6 Reversible and Irreversible Effects in µc-Si:H Even though highly crystalline µc-Si:H does not suffer from light induced degradation, known as Staebler-Wronski-Effect [7, 86], the particular structure properties, described in section 2.1, suggest that instabilities and metastable phenomena may also occur in this material. In particular, µc-Si:H grown at high hydrogen dilution, which yields the largest grain sizes and the highest crystalline volume fraction, shows a pronounced porosity. The existence of crack-like voids in this material facilitates the diffusion of atmospheric gases into the structure. Adsorption and/or chemical reactions, e.g. oxidation, at the column boundaries, might lead to the creation or termination of surface states and might significantly affect the electronic properties of the material. In the following Chapter, we want to investigate and identify instability effects caused by adsorption and oxidation in µc-Si:H and want to relate them to changing structure compositions, ranging from highly crystalline porous, to highly crystalline compact and mixed phase amorphous/crystalline material. 6.1 Metastable Effects in µc-Si:H In this section, reversible phenomena in undoped µc-Si:H with various structure compositions are identified and their investigation by the use of electron spin resonance and electrical conductivity measurements is reported. 6.1.1 Influences of Sample Preparation Material with different structure compositions deposited on both Al and Mo substrates was prepared using VHF-PECVD and studied extensively by ESR. The 63
Chapter 6: Reversible and Irreversible Effects in µc-Si:H Figure 6.1: (a) Spin density N S of µc-Si:H powder obtained from Al substrates (⋆) and Mo (). In panel (b) the corresponding g-values are plotted. use of these different substrates requires a different handling of the material as described in section 3.3.1. While material deposited on aluminum foil is in contact with acids, water, and air for a fairly long time, material prepared on molybdenum substrates can be peeled off and sealed into inert gas atmosphere immediately without any further treatment. In Fig. 6.1 the spin densities N S and average g-values of samples deposited on both aluminum and molybdenum substrates are shown. Material prepared on Al and Mo, shows the same characteristic upon decreasing IC RS . The highest N S values are observed for the highly crystalline samples and N S decreases monotonically with increasing amorphous phase content. However, there are some remarkable influences of the etching (HCl + H 2 O) and drying process on the spin density N S and on the average g-value. Compared to samples deposited on Mo, for material prepared on Al substrates one observes a considerable increase of N S (Fig. 6.1 (a)) accompanied by a shift of g to higher values (Fig. 6.1 (b)). Influences can be observed for all structure compositions from highly crystalline to amorphous. The changes in N S and the g-value are highest for material with the highest crystallinity (IC RS = 0.81), that also possesses the highest porosity (compare section 2.1). For highly crystalline porous material deposited on Al substrates, the spin density is a factor of three higher than compared to that deposited on Mo. The increasing N S , from values of N S = 2.2×10 16 cm −3 (Mo substrates) to 7.2 × 10 16 cm −3 (Al substrates), is accompanied by a shift of the average g-value from g = 2.0044 to 2.0046. For higher amorphous phase content the absolute changes in N S are considerably less with ∆N S ≈ 10 16 cm −3 for IC RS = 0.71 and they further decrease with increasing amorphous content. To determine whether the HCl, the water, or the drying in air atmosphere causes the changes in N S , powder obtained from samples deposited on Mo sub- 64
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Forschungszentrum Jülich in der He
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Forschungszentrum Jülich GmbH Inst
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Kurzfassung In der vorliegenden Arb
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Abstract The electronic properties
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Contents 1 Introduction 1 2 Fundame
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CONTENTS C Abbreviations, Physical
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Chapter 1 Introduction Solar cells
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defect states provided that they ar
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Chapter 8: In this chapter, the inf
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Chapter 2 Fundamentals In this firs
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2.2 Electronic Density of States St
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2.2 Electronic Density of States ta
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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
Page 72 and 73: 5.6 Discussion concentration evalua
Page 74 and 75: 5.7 Summary Figure 5.7: Schematic b
Page 78 and 79: 6.1 Metastable Effects in µc-Si:H
Page 88 and 89: 6.2 Irreversible Oxidation Effects
Page 94 and 95: 6.3 On the Origin of Instability Ef
Page 96 and 97: 6.3 On the Origin of Instability Ef
Page 98 and 99: Chapter 7 Transient Photocurrent Me
Page 100 and 101: 7.2 Transient Photocurrent Measurem
Page 106 and 107: 7.3 Temperature Dependent Drift Mob
Page 108 and 109: 7.4 Multiple Trapping in Exponentia
Page 110 and 111: 7.5 Discussion Table 7.2: Multiple
Page 112 and 113: 7.5 Discussion 7.5.3 The Meaning of
Page 114 and 115: Chapter 8 Schematic Density of Stat
Page 116 and 117: silicon as shown in Fig. 8.1 b, whi
Page 118 and 119: Chapter 9 Summary In the present wo
Page 120 and 121: Appendix A Algebraic Description of
Page 122 and 123: Eq. A.7 then becomes L(t) F = sin(
Page 124 and 125: 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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