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5.6 Discussion concentration evaluated from the gas phase doping concentration PC. This is in agreement with the decreasing defect density observed in undoped material (compare Fig. 4.5 (a)), proving that the occupation of conduction band-tail states is governed by the compensation of gap states. 5.6 Discussion In section 4.4 the spin densities N S of intrinsic microcrystalline material of various structure compositions were discussed. However, as ESR only detects single occupied states D 0 it remains unclear to what extent the measured N S is related to the real defect density of the material. In this Chapter, ESR in combination with conductivity measurements were applied on n-type µc-Si:H with different phosphorous doping concentrations PC and different structure compositions IC RS to study the density of gap states and the influence of these states on the free charge carrier density. PC was chosen to be close to the defect density, where the doping induced Fermi level (E F ) shift is determined by the compensation of gap states. The results confirm that in µc-Si:H, like in a-Si:H, the doping induced Fermi level shift is governed by the compensation of gap states for doping concentrations up to the dangling bond density N DB . Doping induced changes can be observed in both, the ESR signal and the electrical conductivity. While the electrical conductivity increases, in ESR the increasing intensity of the CE resonance indicates a shift of E F as a function of PC. This confirms the close relation between the dark conductivity σ D of µc-Si:H at 300K and the spin density of the CE resonance N CE , that have led authors to assign the CE signal to localized states close to the conduction band [30, 29, 36, 72]. However, this is known for a long time. Far more interesting is the fact that both σ D and N CE are moderated in the same way by the defect density N DB . Doping induced changes are highest for material where low spin densities are found in the undoped state. For highly crystalline material, that also exhibits the highest spin densities, the doping induced changes are considerably less. In other words, the higher DB density observed in highly crystalline material results in lower values of conductivity and lower N CE as a function of the doping concentration. On the other hand, for samples with a low crystalline volume fraction of IC RS = 0.08 the much lower DB density allows much higher σ D and N CE , the latter normalized to the crystalline volume fraction (Fig. 5.6). It is surprising, that by doing this normalization the maximum N CE obtained at IC RS = 0.08 is in very good quantitative agreement with the values of the maximum dopant concentration calculated from the gas phase doping concentration, PC. 59
Chapter 5: N-Type Doped µc-Si:H This suggests a built-in coefficient of phosphorous into the Si-matrix and active donor concentration of unity, if one excludes charge transfer from the amorphous phase. Keeping in mind the characteristic of the spin density N DB of the undoped material, the results are in agreement with the proposed doping mechanism and confirm that the measured N DB represents the defect density in the material. In a simple picture, defects in the gap of µc-Si:H, or more precisely, states in energy below the phosphorus donor states, have to be filled up first before the Fermi level can be shifted into conduction band-tail states, where the majority of the electrons is located which contribute to the CE resonance at 40 K. Tacitly also this assumes, that the phosphorus donor states are located in a position close to the conduction band like in crystalline silicon and that the majority of defects contributing to N DB , no matter where they are located (inside the crystalline cluster columns; at the cluster boundaries; in the disordered regions) affect the E F shift. It seems important to note, that the observed built-in ratio of P into the Si-matrix of unity somewhat differs from values observed in earlier investigations, where within some scatter a value of about 0.5 has been found [30, 32]. As these authors studied only highly crystalline material, the built-in ratio might vary upon different growth conditions. From great importance is the energetic distribution of the paramagnetic states within the band gap of the µc-Si:H material. While the intensity of the CE resonance increases with increasing doping concentration, surprisingly the DB resonance is observable in all spectra. From the measurements presented in this chapter it is, however, not possible to distinguish between both DB centers (db 1 and db 2 ) and therefore only their sum will be discussed here. The rather strong overlap of more than 0.1 eV suggest that the CE and DB states are spatially separated from each other. This further supports the suggestions made in section 4.4, where the DB resonance, in particular the db 2 , was attributed to states located in hydrogen reach regions at the column boundaries. On the other hand, it is a widely accepted fact that the CE signal originates from the crystalline phase of µc-Si:H. Additional support for this thesis comes from Zhou et al., who used electron-spin-echo envelope modulation (ESEEM) to measure the interaction of the unpaired electron with its surrounding nuclear spins [165]. The finding was that the echo decay of the DB signal is modulated by the 1 H nuclei, while no modulation of the CE echo decay could be found. This suggests, that the DB centers are located in hydrogen rich regions, whereas the CE centers arise from hydrogen depleted parts of the µc-Si:H films. Although these results could not be confirmed yet, they still support the considerations made in this work. Why does the spatial separation lead to an energetic overlap of the db 2 and the CE states? Keeping in mind that the column boundaries also represent the interface between the crystalline phase and the disordered material a schematic 60
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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
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
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
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Page 60 and 61: 4.4 Discussion - Relation between E
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Page 66 and 67: 5.2 Electrical Conductivity Figure
Page 68 and 69: 5.4 Dangling Bond Density Figure 5.
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Page 74 and 75: 5.7 Summary Figure 5.7: Schematic b
Page 76 and 77: Chapter 6 Reversible and Irreversib
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Page 100 and 101: 7.2 Transient Photocurrent Measurem
Page 106 and 107: 7.3 Temperature Dependent Drift Mob
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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
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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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