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4.3 ESR Signals and Paramagnetic States in Intrinsic µc-Si:H Figure 4.4: Typical ESR spectra of VHF-PECVD and HWCVD material with structure compositions varying from highly crystalline (IC RS ≈ 0.75) to fully amorphous (IRS C ≈ 0). The discrepancy for the crystalline HW-material is a result of post-oxidation of these highly porous structures. A detailed study of these effects can be found in Chapter 6. Fig. 4.4 shows typical ESR spectra taken on material with different crystallinity prepared with VHF-PECVD and HWCVD. The structure varies from highly crystalline (IC RS ≈ 0.75) to material at the transition between crystalline and amorphous growth (IC RS ≈ 0.4) and finally to material which shows no contribution of the crystalline phase in the Raman spectra (IC RS = 0). All spectra show the typical asymmetric line shape with contributions at g=2.0043 and g=2.0052, in the following denoted as db 1 and db 2 , respectively. Numerical fits to the measured data are included in the graphs, as gray lines. Interestingly, the line width of the two resonances changes only little upon different structure compositions and the width is also similar for material prepared with either PECVD or HWCVD. Optimum fits of the superimposed lines could be performed using Gaussian lines with line widths of ∆H pp = 5.6 ± 0.3 G for the db 1 and ∆H pp = 9.7 ± 0.5 G for the db 2 resonance. However, while these fits work perfectly for the PECVD material, some deviations can be observed for the HWCVD material. In particular, at the high g-value site of the spectra, the two resonances does not fit the spectra correctly. This is a result of post-oxidation of these highly porous structures and will be extensively studied in Chapter 6. Looking at the VHF-PECVD material (uppermost spectra), one observes that for the highly crystalline material the spectrum is dominated by the db 2 resonance, while for material prepared at the µc-Si:H/a-Si:H transition with IC RS ≈ 0.40 the resonance at g=2.0043 contributes much more to the overall ESR signal, resulting 43
Chapter 4: Intrinsic Microcrystalline Silicon Figure 4.5: Spin density as a function of IC RS for µc-Si:H prepared with (a) VHF-PECVD and (b) HWCVD deposited at various substrate temperatures. The values for the VHF- PECVD material were taken from reference [17]. in a shift of the average g-value 1 with respect to lower values. For material with no discernable crystalline signal in the Raman spectra (IC RS = 0, right panel), the g-value shifts to higher values, however, without reaching the value of g=2.0055 characteristically observed in a-Si:H. The signal is clearly dominated by the resonance at g=2.0052. The same trend can be also observed for the HW material prepared at T S = 285 ◦ C (see Fig. 4.4). For the highly crystalline and amorphous material the spectra are dominated by the db 2 resonance at g=2.0052, while for the material at the µc-Si:H/a-Si:H transition an increasing contribution of the db 1 line can be observed. Material prepared at substrate temperatures as high as T S = 330 ◦ C, regardless of the structure composition all spectra are dominated by the db 2 resonance. It is important to note, that for a fixed IC RS the increasing contribution of the db 2 resonance goes along with an increasing overall spin density N S . This issue will be discussed in more detail below. The spin density N S obtained from numerical integration of the ESR spectra is plotted in Fig. 4.5 versus the Raman crystallinity IC RS . Remarkably, independent of the deposition technique and deposition conditions used, the highest spin densities are always observed for the material with the highest crystallinity. For the VHF- PECVD material, plotted in Fig. 4.5 (a), N S decreases slightly between IC RS = 0.78 and 0.39. At the highest crystallinity, N S increases steeply from 2.5 × 10 16 cm −3 at IC RS = 0.78 to values of 7.2 × 1016 cm −3 at IC RS = 0.80. Beyond the transition to amorphous growth (IC RS < 0.39), the spin density drops further down by one order of magnitude to values as low as 2 × 10 15 cm −3 . 1 The average g-value is defined as the zero-crossing of the ESR signal. 44
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Forschungszentrum Jülich in der He
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Forschungszentrum Jülich GmbH Inst
Page 6 and 7: Kurzfassung In der vorliegenden Arb
Page 8 and 9: Abstract The electronic properties
Page 10 and 11: Contents 1 Introduction 1 2 Fundame
Page 12 and 13: CONTENTS C Abbreviations, Physical
Page 14 and 15: Chapter 1 Introduction Solar cells
Page 16 and 17: defect states provided that they ar
Page 18 and 19: Chapter 8: In this chapter, the inf
Page 20 and 21: Chapter 2 Fundamentals In this firs
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Page 46 and 47: 3.2 Deposition Technique admixture
Page 48 and 49: 3.3 Sample Preparation 3.3.1 Sample
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Page 52 and 53: Chapter 4 Intrinsic Microcrystallin
Page 54 and 55: 4.2 Electrical Conductivity Figure
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7.3 Temperature Dependent Drift Mob
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7.4 Multiple Trapping in Exponentia
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7.5 Discussion Table 7.2: Multiple
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7.5 Discussion 7.5.3 The Meaning of
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Chapter 8 Schematic Density of Stat
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silicon as shown in Fig. 8.1 b, whi
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Chapter 9 Summary In the present wo
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Appendix A Algebraic Description of
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Eq. A.7 then becomes L(t) F = sin(
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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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