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Chapter 9 Summary In the present work, the electronic properties of microcrystalline silicon films have been studied by using various experimental techniques and a number of concepts and models used to explain them. Paramagnetic states in µc-Si:H have been investigated for a large variety of structure compositions ranging from highly crystalline, with no discernable amorphous content, to predominantly amorphous material, with no crystalline phase contributions. For this material range, the density of states within the band gap was studied by electron spin resonance (ESR) and electrical conductivity. Moreover, the hole transport properties in highly crystalline material were studied by transient photocurrent measurements and have been successfully interpreted by the model of multiple trapping in an exponential band-tail. The spin density N S in µc-Si:H films is strongly dependent on the structure composition of the material. The highest N S is always found for material with the highest crystalline volume fraction. With increasing SC during the process, the spin density decreases, which was attributed to an increasing hydrogen content, terminating unsaturated dangling bonds. Moreover, the additional amorphous phase content incorporated between the crystalline columns acts as a passivation layer, leading to a better termination of unsatisfied bonds at the surface. The strong dependence of N S on the deposition temperature T S , found in HWCVD material, was attributed to an increasing desorption during the deposition. Generally, the ESR signal of intrinsic microcrystalline material shows contributions of two resonances at g-values of 2.0043 (db 1 ) and 2.0052 (db 2 ), independent of the particular deposition process and structure composition. The relative contributions of the individual lines changes as a function of the crystalline volume content. While the ESR spectra of low defect material in the highly crystalline regime are dominated by the resonance at g=2.0043, with increasing amorphous content the intensity ratio is clearly shifted towards the db 2 resonance. An 105
Chapter 9: Summary increasing intensity of the db 2 resonance is also observed as a result of increasing T S . Reversible and irreversible changes in the ESR signal and the conductivity found in µc-Si:H due to atmospheric effects are closely connected to the structure composition, in particular the active surface area. The porous structure of highly crystalline material leads to the in-diffusion of atmospheric gases, strongly affecting the density of surface states. Two processes have been identified, namely adsorption and oxidation. Both processes lead to an increase of N S . In the case of adsorption the increase could be identified as arising from changes of the db 2 resonance, while the intensity of the db 1 resonance remains constant. With increasing amorphous content the magnitude of both adsorption and oxidation induced changes decreases as a result of a higher compactness of the films. The adsorption of O 2 may be reversed by moderate temperature annealing, while a chemical treatment in HF is required to reverse the effects caused by oxidation. <strong>Measurements</strong> on n-doped µc-Si:H films were used as a probe for the density of gap states. The results confirm that the doping induced Fermi level shift in µc- Si:H, for a wide range of structural compositions, is governed by compensation of defect states for doping concentrations up to the dangling bond spin density. For higher doping concentrations a doping efficiency close to unity is found. It could be shown that in µc-Si:H the measured spin densities represent the majority of gap states (N S = N DB ). The close relationship between the CE resonance intensity and the conductivity is confirmed, which means the electrons contributing to the CE signal represent the majority of the charge carriers contributing to electrical transport. Transient photocurrent measurements were carried out on µc-Si:H material prepared in the highly crystalline regime. It was found that in these materials conventional time-of-flight interpretation is consistent and may be applied to obtain hole drift mobilities. It has been shown that in this material hole transport is dominated by effects associated with multiple trapping in valence band-tail states. Analyzing the transient photocurrents it was found that the density of valence band-tail states falls exponentially towards the gap with a typical band-tail width of ∆E V ≈ 31 meV. Combining the information derived in this work a schematic picture of the density of states in both a spatial and energetic sense has been obtained. 106
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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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2.2 Electronic Density of States Fi
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2.3 Charge Carrier Transport influe
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2.3 Charge Carrier Transport functi
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Chapter 3 Sample Preparation and Ch
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3.1 Characterization Methods Figure
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3.1 Characterization Methods Homoge
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3.1 Characterization Methods µ Fig
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3.1 Characterization Methods compar
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3.1 Characterization Methods bution
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3.1 Characterization Methods posite
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3.2 Deposition Technique admixture
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3.3 Sample Preparation 3.3.1 Sample
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3.3 Sample Preparation reverse bias
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Chapter 4 Intrinsic Microcrystallin
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4.2 Electrical Conductivity Figure
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4.3 ESR Signals and Paramagnetic St
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4.3 ESR Signals and Paramagnetic St
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4.4 Discussion - Relation between E
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4.4 Discussion - Relation between E
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Chapter 5 N-Type Doped µc-Si:H In
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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 76 and 77: Chapter 6 Reversible and Irreversib
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 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.
Page 126 and 127: Table B.3: Parameters of n-type µc
Page 128 and 129: Appendix C Abbreviations, Physical
Page 130 and 131: µ d Drift mobility µ d,h Hole dri
Page 132 and 133: Bibliography [1] E. Becquerel. Mém
Page 134 and 135: BIBLIOGRAPHY [21] D. Will, C. Lerne
Page 136 and 137: BIBLIOGRAPHY [45] H. Overhof and M.
Page 138 and 139: BIBLIOGRAPHY [66] P.W. Anderson. Ab
Page 140 and 141: BIBLIOGRAPHY [93] J.W. Orton and M.
Page 142 and 143: BIBLIOGRAPHY [121] J.R. Haynes and
Page 144 and 145: BIBLIOGRAPHY [148] W. Luft and Y.S.
Page 146 and 147: BIBLIOGRAPHY [170] G.N. Advani, R.
Page 148 and 149: List of Publications 1. T. Dylla, R
Page 150 and 151: Acknowledgments At this point, I wa
Page 152 and 153: Schriften des Forschungszentrums J
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