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6.3 On the Origin of Instability Effects in µc-Si:H Figure 6.14: Energy band bending and Fermi level shift due to (a) the adsorption and the (b) desorption of oxygen on surface of µc-Si:H. band bending, which could result in a bond breaking via weak-bond dangling bond (WB-DB) conversion, similar to the field effect and doping induced defect creation found in a-Si:H [167]. However, it is difficult to fit the reversibility of the observed effects with the WB-DB conversion. Taking the reversible changes of σ D into account, the adsorption of oxygen seems to be a most likely candidate [20] to explain the observed metastable effects in ESR and electrical conductivity. In fact, while the material is in contact with air, additional surface states can arise due to the adsorption of atmospheric gases. The following models have been mainly developed and have widely been used to explain the gas sensing properties of semiconducting oxides, like SnO 2 and ZnO [168, 169, 170, 171, 172]. As determined by Hall measurements, for intentionally undoped µc-Si:H one generally observes n-type behavior. The in-diffusion of oxygen along the column boundaries and the adsorption of O 2 on the surface of the crystalline columns will therefore capture an electron from the conduction band forming an O − 2 state. The capturing of the electron is equivalent to the occupation of a localized surface state, induced by the adsorption of O 2 . This is possible, because the energy level of the O − 2 state is below the Fermi level of the material without the presence of oxygen. This can be described by the following reactions O 2(gas) ⇒ O 2(ads) e − + O 2(ads) ⇒ O − 2(ads) . (6.1) The transfer of charge carriers will lower the Fermi level and the accumulation of trapped charge carriers at the column boundary will lead to a band bending in the material. This is shown schematically in Fig. 6.14. The changes of the dark conductivity σ D are therefore a result of two different, however interconnected effects. While the trapping of charge carrier at the surface leads to changes in 81
Chapter 6: Reversible and Irreversible Effects in µc-Si:H the free carrier concentrations, the increasing barrier height handicaps the charge carrier transport. As dark conductivity in µc-Si:H is usually measured perpendicular to the growth direction, a charge carrier has to cross a number of column boundaries before it reaches the other electrode (compare Fig. 2.1). To overcome the barriers formed at the column boundaries, a charge carrier has to be thermally excited. This model of barrier-limited transport is based on the ideas of Seto [41] and has been successfully applied to explain the transport behavior of polycrystalline silicon. In the case that the free charge carrier concentration in the columns is higher than the density of surface states induced by adsorption of O 2 the current is given by ( −EB ) I = const · exp (6.2) kT and is so extremely sensitive to the barrier hight E B . This assumption is plausible if one bears in mind that a typical surface adsorption will exchange charge carriers of the order of 10 10 − 10 12 e/cm 2 [173]. Taken a charge carrier concentration of N D = 10 17 cm −3 in the bulk of µc-Si:H [96] and a column diameter of 200 nm the free charge carrier concentration is about one order of magnitude higher than the number of surface states created by adsorption. Upon annealing in argon atmosphere, the desorption of oxygen will lead to a re-emission of the trapped charge back into the bulk. The band bending will decrease to its initial state and restoring the values of the electrical conductivity. The considerations made above can explain the reversible effects observed in highly crystalline porous material IC RS = 0.82 very well. For highly crystalline but compact material IC RS = 0.71, on the other hand, the conductivity shown in Fig. 6.9 strongly deviates from the characteristic found in IC RS = 0.82 material. While for the later one σ D decreases, for IC RS = 0.71 material σ D increases upon contact with air. The key-issue for the different behavior is the porosity. While in highly crystalline porous material atmospheric gases can easily diffuse along the column boundaries leading to the creation of surface states, the compact structure of IC RS = 0.71 prevents the diffusion of atmospheric gases into the material. One would therefore not expect any strong influence on the barrier height at the column boundaries upon contact with air atmosphere. However, oxygen can still be adsorbed at the film surface. Providing that the charge carrier density within the film is small, i.e. the Fermi level position is close to the mid gap, the creation of O − 2 states at the film surface might lead to the induction of holes. In fact, due to the capture of an electron by the surface state, holes are induced into the film, resulting in an increasing σ D . Adsorption processes could therefore also account for the meta-stability of σ D in highly crystalline compact material. This assumes that the O − 2 surface states lies energetically below mid gap, which however re- 82
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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
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 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 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.
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
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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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