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6.2 Irreversible Oxidation Effects been performed on IC RS = 0.82 material with phosphorous doping concentrations of 10 17 cm −3 . The highly crystalline porous material was chosen because it showed the biggest changes upon oxidation (see Fig. 6.10). The measured spectra taken after different stages of the oxidation process are shown in Fig. 6.13 (a). In the initial state the ESR signal is dominated by the CE resonance, which arises from electrons trapped in conduction band-tail states. As expected from the experiments on intrinsic material the intensity of the ESR signal around g=2.005 increases as a function of the annealing period. With the creation of additional states, the CE centers become depopulated resulting in a decrease of the signal intensity around g=1.998. This will happen when the states created by the oxidation are located lower in energy than the CE states. Providing that these states can accommodate additional electrons they will become occupied, while states in the conduction band become depopulated. The assumption that the electrons are transferred from CE states to states created by oxidation of the surface is supported by absolute values of the spin density plotted in Fig. 6.13 (b). The spin density was determined by integrating the ESR spectra over the entire range measured. It is surprising, that Figure 6.13: ESR signal of a n-type highly crystalline porous µc-Si:H sample with 10 ppm PH 3 doping for different periods of storage in oxygen atmosphere. The measurements were taken at T = 40K. In (b) the number of spins (N S ), contributing to the spectra shown in panel (a) is shown; the spectrum taken in inert gas is arbitrarily set at 0.01 days. 79
Chapter 6: Reversible and Irreversible Effects in µc-Si:H upon annealing the total number of spins observed in ESR does not vary, while the g-value changes considerably. As the value of g is strongly connected to the microscopic environment surrounding the paramagnetic state, this indicates that only the microscopic location of the resonant electrons changes and defect states created by oxidation are not involved in compensation effects. The reason for this remains unclear and requires further investigation. 6.3 On the Origin of Instability Effects in µc-Si:H In- and out-diffusion of impurities and atmospheric gases leads to numerous instability and metastable phenomena on the ESR signal and the electrical conductivity in µc-Si:H at or close to room temperature. It appears, that the amplitude of these changes is connected to the surface area of the material. Reversible and irreversible changes in N S are highly pronounced for IC RS = 0.81 material which exhibit a very porous structure (compare section 2.1). In highly crystalline material with only a minute amount of amorphous phase content, which has shown to be compact, the effects are much less prominent and are almost absent for still lower IC RS . In the following section the observed meta-stable and irreversible effects in µc-Si:H of various structure compositions will be discussed. The main effects to consider are adsorption and oxidation on surfaces, caused by atmospheric gases. 6.3.1 Adsorption of Atmospheric Gases The results presented in section 6.1 show that the spin density N S , the average g- value, as well as the electrical conductivity σ D strongly depend on the particular treatment and the history of µc-Si:H material. While N S increases upon storing in air atmosphere, the effect can be reversed by a simple annealing step in argon atmosphere. A detailed analysis of the ESR spectra shows that the changes in N S can be traced back to a varying number of spins with a g-value of g=2.0052, while the db 1 resonance at g=2.0043 remains unaffected. On the other hand, for σ D not only the amplitude, but also the direction of the changes depends on the structure of the material. While for the IC RS = 0.82 material σ D decreases, an increase is observed for the IC RS = 0.71 material as a result of a storage in air atmosphere. The airbreak/annealing cycles can be observed still after many cycles with no fatigue or the appearance of any irreversible effect. The question arises, how far the observed effects of the various structure compositions can be related to each other and/or have the same origin. Taking into account the low energies required for the air-exposure/annealing cycles one can exclude the possibility of breaking up and annealing of Si-DB as a possible origin for the varying N S . One could also speculate that adsorption leads to a strong 80
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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
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 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
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Page 94 and 95: 6.3 On the Origin of Instability Ef
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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.
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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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