Measurements

More documents

Recommendations

Info

defect states provided that they are paramagnetic and it has been successfully applied to amorphous silicon and its alloys (see e.g. [24, 25, 26]). First applied to µc-Si:H in the 80s [20, 27, 28], systematic studies have only been performed in the last recent years, and various ESR signals have been identified [29, 30, 31, 32]. Intrinsic µc-Si:H shows an asymmetric signal with contributions at g=2.0043 (db 1 ) and g=2.0052 (db 2 ). The origins of these lines are still under discussion. While it has been suggested that the asymmetry results from an axial symmetric g-tensor of defects on grain surfaces [31], there are a number of indications that these lines originate from two independent states located in different microscopic environments [21, 33, 34, 35, 36, 32]. For n-type material and also for illuminated material a third resonance at g=1.996-1.998 can be observed. According to a similar resonance found in polycrystalline silicon this resonance has been called CE-Line and has been attributed to electrons in the conduction band [27, 29] and later also to shallow localized states in the conduction band-tail [35, 36, 37, 38]. A number of reports on ESR properties of highly crystalline n-type µc-Si:H have been published and show that highly crystalline n-type material shows a nearly linear dependence of the dark conductivity σ d on phosphorous doping concentrations for PC =[PH 3 ]/([PH 3 ]+[SiH 4 ]) higher than 10 ppm [30, 39, 40]. For lower doping concentrations the conductivity deviates from this linear dependence. It is likely that within this doping regime the Fermi level shift is governed by the compensation of gap states. However, this has not been proven yet and will be a key task of this work. Moreover, the presence of localized states within the bandgap has a major influence on the transport properties and has to be considered in order to explain transport features. In contrast to c-Si, the occurrence of band-tail states and deep defects open additional transport paths, they might act as traps for charge carriers, or form barriers. There is, of course, a wide range of possible structures in microcrystalline silicon materials. This explains the large spread in reported drift mobilities and transport properties. In the past, various models have been proposed to describe the transport in µc-Si:H. These models adopt and combine former approaches successfully applied for either polycrystalline or pure amorphous material, e.g. for n-type µc-Si:H the so called ”grain boundary trapping model” [41], successfully applied to poly-crystalline silicon, has been used to describe the transport behavior [42, 43] and also percolation models were applied to interpret conductivity and Hall effect data [44, 45]. On the other hand, similarities between a-Si:H and µc-Si:H suggest that structural disorder are from constitutional importance and transport might take place by direct tunneling between localized states (hopping) or by trap-limited band motion (multiple trapping) [46, 47, 48, 49]. This work provides a comprehensive study of paramagnetic centers in µc-Si:H. Material with different structure compositions and doping levels have been inves- 3
Chapter 1: Introduction tigated by ESR and electrical conductivity. It will be shown that structural changes influence the nature as well as the density of the defects. Accompanied by structural changes the material tends to be susceptible of instabilities due to adsorption and chemical reactions of atmospheric gases. The present work investigates and identifies instability effects caused by adsorption and oxidation in state of the art material, with a wide range of structure compositions. The application of additional n-doping will be used as a probe for the density of gap states. Additionally, the transport properties of highly crystalline µc-Si:H will be studied using transient time-of-flight experiments. This thesis is organized as follows: Chapter 2: A short summary of the structural properties as well as their impact on the electronic structure of microcrystalline silicon is given. In the second part, the influences of the electronic properties on electrical transport will be treated. Different transport models proposed for µc-Si:H material are shown and compared. Chapter 3: A short presentation of the experimental techniques, used in this work, is followed by a brief description of the deposition process and the particular preparation of the samples. Chapter 4: In Chapters 4-8, the results of the material characterization are presented and discussed. Chapter 4 addresses the properties of paramagnetic states in intrinsic µc-Si:H with varying structure compositions ranging from highly crystalline to fully amorphous. Chapter 5: In this Chapter films with different structure compositions and doping levels are studied by ESR and electrical conductivity. n-Doping densities in the range of the intrinsic defect density are used as a probe for the density of gap states. Chapter 6: Electron spin resonance and conductivity measurements are used to study adsorption and oxidation effects on µc-Si:H with different structure compositions. The magnitude of observed meta-stable and irreversible effects will be discussed with respect to changes of the active surface area. Chapter 7: The hole transport properties of highly crystalline material are studied in this Chapter. Transient photocurrent measurements are presented and consistently analyzed using the model of multiple trapping in an exponential band-tail. 4
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 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 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 80 and 81:
Page 82 and 83:
Page 84 and 85:
Page 86 and 87:
Page 88 and 89:
6.2 Irreversible Oxidation Effects
Page 90 and 91:
Page 92 and 93:
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 102 and 103:
Page 104 and 105:
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
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
Page 122 and 123:
Eq. A.7 then becomes L(t) F = sin(
Page 124 and 125:
Appendix B List of Samples Table B.
Page 126 and 127:
Table B.3: Parameters of n-type µc
Page 128 and 129:
Appendix C Abbreviations, Physical
Page 130 and 131:
µ d Drift mobility µ d,h Hole dri
Page 132 and 133:
Bibliography [1] E. Becquerel. Mém
Page 134 and 135:
BIBLIOGRAPHY [21] D. Will, C. Lerne
Page 136 and 137:
BIBLIOGRAPHY [45] H. Overhof and M.
Page 138 and 139:
BIBLIOGRAPHY [66] P.W. Anderson. Ab
Page 140 and 141:
BIBLIOGRAPHY [93] J.W. Orton and M.
Page 142 and 143:
BIBLIOGRAPHY [121] J.R. Haynes and
Page 144 and 145:
BIBLIOGRAPHY [148] W. Luft and Y.S.
Page 146 and 147:
BIBLIOGRAPHY [170] G.N. Advani, R.
Page 148 and 149:
List of Publications 1. T. Dylla, R
Page 150 and 151:
Acknowledgments At this point, I wa
Page 152 and 153:
Schriften des Forschungszentrums J
Page 154 and 155:
Page 156 and 157:
Page 159:
Forschungszentrum Jülich in der He
show all

Measurements

Create successful ePaper yourself

Delete template?

Save as template?