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6.2 Irreversible Oxidation Effects the overall signal increases. The absolute values, plotted in Fig. 6.11 (c), confirm that the magnitude of the changes strongly depends on the structure of the material. While for the porous material the line width increases from values of ∆H pp = 8.5Gto11.5G,∆H pp of the transition material increases only slightly. In contrast to the air-exposure/annealing cycles described in section 6.1, annealing of oxidized samples in argon at temperatures below T = 200 ◦ C does not have any effect on the ESR signal. For higher temperatures than T = 200 ◦ C bonded hydrogen, used to terminate vacant Si bonds, will desorb, resulting in a further increase of the dangling bond density. However, assuming that the increasing spin density, observed in Fig. 6.11, is a result of the oxidation, a dip into hydrofluoric acid (HF), well known from silicon wafer cleaning technology, provides a simple way to remove the oxide from the surface and by the way passivating dangling bonds by establishing Si-H bonds. 6.2.1 Reversibility by Chemical Reduction In order to remove the oxygen surface layer a 5% HF solution was used. The sample was etched in the acid for 30 seconds, carefully rinsed in distilled water, and dried by flushing with nitrogen. To avoid as much re-oxidation and post-contamination as possible the sample was immediately sealed in argon atmosphere and measured within 5 min after the cleaning process. For the ease of sample handling material has been prepared on glass substrates. The price to pay is a considerably lower signal to noise ratio due to the reduced sample volume. Additionally, the ESR spectra of the µc-Si:H is superimposed by signal traces from the borosilicate glass at g=2.001. However, still one can easily observe the effects as shown in Fig. 6.12 (a). The figure shows three spectra of highly crystalline porous material (i) taken right immediately after the deposition, (ii) after an annealing step in O 2 atmosphere for two hours at T = 160 ◦ C, and (iii) after the treatment in HF acid. The spectra shown are not normalized, so differences in the signal intensity are a result of a change in the spin density. After annealing in O 2 atmosphere for two hours the spectrum shows a higher intensity compared to the ”as deposited” signal, indicating an increase of N S . In fact, the spin density increases from values of N S = 5 × 10 16 cm −3 to N S = 2 × 10 17 cm −3 , resulting in the same line shape as observed for the O 2 -annealed material in Fig. 6.10. After the HF-dip the intensity decreases again and the line shape returns back to its initial form. Looking at the absolute values of N S , the g-value, and ∆H pp , plotted in Fig. 6.12 (b, c, d) the success of the HF-dip becomes more obvious. While all three quantities increase after annealing in oxygen, which in fact is the same effect as observed for the powder samples, all three quantities restore back to their initial values, after the treatment in HF acid. 77
Chapter 6: Reversible and Irreversible Effects in µc-Si:H Figure 6.12: (a) ESR spectra of a porous µc-Si:H sample (IC RS = 0.82) deposited on glass substrate measured right after deposition, annealing in O 2 , and after etching in HF. In panel (b) the measured values of N S , (c) the g-value, and (d) ∆H pp before and after the HF-dip are shown. 6.2.2 Charge Transfer caused by Oxidation of N-Type µc-Si:H As shown above, annealing µc-Si:H in oxygen atmosphere leads to an increase of the spin density. The fact that the corresponding resonance appears at higher g-values and exhibits a different line shape than observed for the resonances at g=2.0043 and g=2.0052 suggests that the additional spin states are somehow different. In Chapter 5 the use of phosphorous doping to probe the density of gap states was described and it was found that the shift of the Fermi level is governed by the compensation of gap states. In fact, states within the band gap have to be filled before states in the conduction band-tail can be occupied. The aim of this section is to use the n-type doping experiment just in the opposite direction. The question is, if and how the spin states created by the oxidation of the surface affect the position of the Fermi level and lead to a depopulation of conduction band-tail states. Annealing experiments have therefore 78
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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
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 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 92 and 93: 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.
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
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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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