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Chapter 4 Intrinsic Microcrystalline Silicon The relation between the structure properties, in particular the ratio between crystalline and amorphous volume content, and the electronic properties of the material is of great interest. Especially since it has been shown, that contrary to what one might expect, material prepared close to the transition to amorphous growth yields the highest solar cell conversion efficiencies, rather than material with the largest crystalline grain size and the highest crystalline volume fractions. In this Chapter, paramagnetic states found in intrinsic µc-Si:H are identified and correlated to the structure properties of the material. ESR measurements, on intrinsic µc-Si:H prepared by HWCVD under various deposition conditions leading to material with structure compositions varying from highly crystalline to fully amorphous are described. Additionally, data from a PECVD series prepared and studied by Baia Neto et al. [17, 18, 160] is reanalyzed and compared with results obtained for the HWCVD material. 4.1 Raman Spectroscopy In Fig. 4.1 Raman spectra of µc-Si:H prepared on different substrates are shown. The silane concentration SC=[SiH 4 ]/([H 2 ]+[SiH 4 ]) was chosen to result in (a) highly crystalline material and (b) material at the transition between crystalline and amorphous growth. With increasing SC the amorphous phase content in the film increases, resulting in an increasing contribution of the amorphous signal in the Raman spectra around 480 cm −1 . While the highly crystalline material, shown in panel (a), shows no difference in the spectra of films prepared on glass or aluminum, the structure of the material prepared at the transition between crystalline and amorphous growth depends strongly on the substrate used (see panel (b)). Material deposited on glass substrates shows a much stronger contribution of the amorphous peak than that deposited on aluminum. This substrate dependence has 39
Chapter 4: Intrinsic Microcrystalline Silicon Figure 4.1: Raman spectra of material prepared on different substrates under deposition conditions resulting (a) in highly crystalline material and (b) material at the transition between crystalline and amorphous growth. to be kept in mind, as in the following chapters conductivity measurements are compared with results obtained from ESR, for which glass and aluminum substrates have been used. In particular, a comparison of material properties near the transition between microcrystalline and amorphous growth has to be made carefully. The structural properties of the deposited films are not determined solely by the silane concentration and the substrate. In Fig. 4.2 the Raman intensity ration IC RS , determined as described in section 3.1.1, is shown as a function of SC for a number of samples prepared with (a) PECVD (taken from reference [160]) and (b) HWCVD prepared at different substrate temperatures ranging from T S = 185 ◦ CtoT S = 450 ◦ C. For all series, the silane concentration was varied covering the complete range from highly crystalline to predominately amorphous growth. Except for the HW material prepared at T S = 450 ◦ C, for which glass substrates were used, all spectra were taken from material deposited on aluminum substrates. Independent of the particular deposition parameter or deposition process, one observes a decrease of IC RS with increasing SC. The qualitative behavior upon changes of SC are the same and is discussed for the VHF material, plotted in Fig. 4.2 (a). Between SC = 2 − 4%, IC RS decreases only slightly, from 0.80 to 0.74, indicating a highly crystalline structure. Above SC = 4% an increasing amorphous phase content results in a decreasing IC RS and for silane concentrations higher than 7% only an amorphous phase contribution can be observed in the Raman signal. The region SC= 4%−7% is referred to as the transition zone. Within this regime of deposition parameters, the material structure changes from crystalline to predominately amorphous. The position and width of the zone strongly depends on the deposition parameters used, e.g. the substrate temperature during 40
Page 1 and 2: 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
Page 22 and 23: 2.2 Electronic Density of States St
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Page 28 and 29: 2.3 Charge Carrier Transport influe
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Page 32 and 33: Chapter 3 Sample Preparation and Ch
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Page 46 and 47: 3.2 Deposition Technique admixture
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Page 54 and 55: 4.2 Electrical Conductivity Figure
Page 56 and 57: 4.3 ESR Signals and Paramagnetic St
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Page 60 and 61: 4.4 Discussion - Relation between E
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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 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
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7.2 Transient Photocurrent Measurem
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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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