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5.2 Electrical Conductivity Figure 5.2: Room temperature conductivity σ D of µc-Si:H films with doping concentrations of 0 ppm (), 1 ppm (◦), 5 ppm () and 10 ppm (▽) as a function of (a) Raman intensity ratio (IC RS ) and (b) gas phase doping concentration. The data of the undoped material were taken from reference [18]. be observed for all structure compositions from highly crystalline to amorphous. However, unlike the undoped material for the n-type doped samples the highest σ D values are not found for the samples with the highest crystallinity. In fact, for all doping concentrations PC = 1 − 10 ppm an increase of σ D by more than one order of magnitude can be observed if the crystallinity decreases from IC RS = 0.82 to 0.77. This indicates that the doping induced free charge carrier concentration is considerably less for the highly crystalline material. Between IC RS = 0.77 and 0.4 σ D stays almost constant before it drops down by about 5 orders of magnitude as result of the structural transition to predominantly amorphous growth, independent of the particular doping concentration. In Fig. 5.2 (b) the conductivity is plotted versus the gas phase doping concentration PC. In these plots the correlation between doping and conductivity becomes more obvious. Within the microcrystalline growth regime between IC RS ∼ 0.82−0.40 (upper three curves in the figure) the doping induced changes of σ D are highest for samples with IC RS ∼ 0.77 and 0.4 and considerably less for the sample with the highest crystallinity. Applying a doping concentration of PC = 1 ppm, the dark conductivity of the IC RS = 0.77 and IRS C = 0.4 material changes by more than three orders of magnitude, compared to the undoped material. On the other hand, for the highest crystallinity, where the highest N S is observed in the undoped material, σ D increases by only a factor of 6. Plotting σ D on a linear scale, one observes that for doping concentrations higher than PC = 1 ppm all microcrystalline samples (IC RS = 0.82 − 0.4) show an almost linear increase of the conductivity with a structure independent slope. For amorphous or almost 53
Chapter 5: N-Type Doped µc-Si:H amorphous samples (IC RS = 0.04 to 0) the dark conductivity shows much lower values over the whole doping range investigated. However, for the IC RS = 0.04 material σ D increases exponentially by about 4 orders of magnitude, while for the fully amorphous material σ D changes by almost 7 orders of magnitude from σ D = 2 × 10 −11 S/cm to 1 × 10 −4 S/cm. The dark conductivity follows nicely the expected behavior. Considering the characteristic of the spin density of the undoped material (see Fig. 4.5 (a)), the results are in agreement with the proposed doping mechanism of compensation of gap states as described in section 2.2.2. Doping induced changes are highest for material for which low spin densities are found in the undoped state. For highly crystalline material, which also exhibits the highest spin densities, the doping induced changes are considerably less, as more defect states have to be compensated before the Fermi level moves up into the conduction band-tail. 5.3 ESR Spectra Samples prepared for ESR measurements have been deposited on aluminum substrates in the same run as that used for conductivity measurements. The material was then powdered using the procedure described in section 3.3.1. ESR measurements were performed at a temperature of T = 40K. All spectra were normalized to the same peak height and plotted in Fig. 5.3. Fig. 5.3 shows stack plots of samples with different crystallinity ranging from IC RS = 0.82 − 0 and doping concentrations of 1, 5, 10 ppm. The fact that the Raman intensity ratio for the predominantly amorphous material IC RS = 0.08 deviates slightly from that found in Fig. 5.2 (IC RS = 0.04) is a direct result of the substrate dependence. The vertical dotted lines in Fig. 5.3 indicate the resonances at g=2.0043, g=2.0052, and g=1.996-1.998, typically found in µc-Si:H (see section 2.2.2). All spectra show contributions of the three well-known signals at g-values of 2.0043 (db 1 ), 2.0052 (db 2 ), and g=1.996-1.998 (CE) (for details see section 2.2.2), except for the spectra taken from the purely amorphous material (IC RS = 0). Quite surprisingly, the CE resonance can be observed in all spectra of the doped samples, in the highly crystalline material as well as in material where very little Raman intensity due to the crystalline phase is found (IC RS < 10%). Only samples with no detectable contribution from the crystalline volume to the Raman signal show only the dangling bond signal at g=2.0052. For a given doping level, the intensity of the CE-line is highest for highly crystalline material with an IC RS of 0.77 or 0.47. The CE line is much less pronounced for the highest crystallinity (IC RS = 0.82), which is in agreement with the high spin densities found in this material (see Fig. 4.5 (a)). As expected, the intensity of the CE resonance increases 54
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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
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
Page 22 and 23: 2.2 Electronic Density of States St
Page 24 and 25: 2.2 Electronic Density of States ta
Page 26 and 27: 2.2 Electronic Density of States Fi
Page 28 and 29: 2.3 Charge Carrier Transport influe
Page 30 and 31: 2.3 Charge Carrier Transport functi
Page 32 and 33: Chapter 3 Sample Preparation and Ch
Page 34 and 35: 3.1 Characterization Methods Figure
Page 36 and 37: 3.1 Characterization Methods Homoge
Page 38 and 39: 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 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
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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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