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4.2 Electrical Conductivity Figure 4.2: Raman intensity ratio of (a) VHF-PECVD material and (b) HWCVD-material deposited at different substrate temperatures ranging from T S = 185 ◦ CtoT S = 450 ◦ C taken from reference [17, 18, 160] and [13], respectively. the process plays an important role for the structural development of the films. This is shown in Fig. 4.2 (b), where the IC RS of HW-material prepared at different T S is shown. An increasing T S leads to a shift of the transition to higher values of SC. The dependency on T S has been also observed for PECVD material [161]. The observed shifts of the transition under different deposition conditions make it difficult to use the silane concentration as a single parameter to predict the electronic properties of samples prepared under different conditions. Even material prepared in the same run, but on different substrates, may show different properties. Therefore, in the following sections the Raman intensity ratio IC RS , rather than SC, will be used to compare electronic properties of different samples. 4.2 Electrical Conductivity Within this section, a short review of the conductivity data of both the PECVD and the HWCVD material taken from references [17] and [13], respectively, will be given. These results are of particular importance for the following Chapters and will therefore briefly be reviewed in this context. The transition in growth is directly reflected in the conductivity as shown in Fig. 4.3. Here, results of dark conductivity σ D measurements are plotted versus the Raman intensity ratio IC RS for material prepared using (a) VHF-PECVD and (b) HWCVD. Plotting σ d versus IC RS has recently become a widely used method to compare material prepared under different deposition conditions and in different systems (see e.g. [16, 13]). All samples show the same general depen- 41
Chapter 4: Intrinsic Microcrystalline Silicon Figure 4.3: Dark conductivity σ D as a function of IC RS of material prepared with (a) VHF- PECVD and (b) HWCVD at various substrate temperatures, taken from reference [17, 18] and [13], respectively. dency of σ D upon IC RS . Material prepared in the crystalline growth regime shows a dark conductivity of 10 −4 to 10 −7 S/cm, which is of the same order of magnitude as observed for crystalline silicon. However, in c-Si the electron mobilities (µ ≈ 1500 cm 2 /Vs [91]) exceed by far the electron mobilities in µc-Si:H (a few cm 2 /Vs [49]). This suggests a considerable background doping in intentionally undoped µc-Si:H. As a consequence of the structural transition, for material with an IC RS lower than 0.4 σ D decreases by several orders of magnitude, resulting in values below 10 −10 S/cm, which is typical for a-Si:H [65]. When comparing material prepared in the transition regime, the drop in σ D occurs at slightly higher values of IC RS for the material prepared with PECVD compared to that prepared with HWCVD. For material prepared with HWCVD one observes subsequent lower values of the conductivity for substrate temperatures of T S = 450 ◦ C and 185 ◦ C. Anticipating results obtained in the next section, the low σ D of the T S = 450 ◦ C material can be explained by a high defect density. On the other hand, the T S = 185 ◦ C material shows very low spin density. The low values of σ D might therefore be a result of a reduced impurity level or reduced mobility caused by grain boundary effects. 4.3 ESR Signals and Paramagnetic States in Intrinsic µc-Si:H ESR measurements were performed on powdered material deposited on aluminum substrates. Details of the powder preparation process can be found in section 3.3.1. 42
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Forschungszentrum Jülich in der He
Page 4 and 5: 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
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Page 52 and 53: Chapter 4 Intrinsic Microcrystallin
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Page 68 and 69: 5.4 Dangling Bond Density Figure 5.
Page 70 and 71: 5.5 Conduction Band-Tail States Fig
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Page 74 and 75: 5.7 Summary Figure 5.7: Schematic b
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7.2 Transient Photocurrent Measurem
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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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Forschungszentrum Jülich in der He
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