Measurements

More documents

Recommendations

Info

3.1 Characterization Methods posited on molybdenum and glass substrates the abrupt step necessary for the measurement was created by scratching the film and then applying and tearing off an adhesive tape. Thereby, a part of the film teared off from the substrate leaving a sharp edge. In the case of the pin diodes the step was created by mechanically scratching away the Si-layers using a scalpel. The bottom ZnO was then etched away with an HCl solution. Afterwards the thickness of the thin films or diodes could be measured using the mechanical step profiling system. To determine the i-layer thickness of the diodes, the thickness of the doped layers as well as the thickness of the ZnO has to be subtracted. The advantage of this method is that it measures the thickness directly with a precision of not less than 10 nm for thin films and about 50 nm in the case of the diodes. The shortcoming of this method is that an abrupt step right next to the position of interest has to be created, which especially in the case of the pin structures might destroy the device by creating shunt resistances. Measuring the capacitance of the pin structure provides a simple, nondestructive way to determine the i-layer thickness of the PIN-diodes using: C = εε 0A d w . (3.13) Here ε, A, and d w are the dielectric constant, the area of the contacts, and thickness of the depletion layer, respectively. The dielectric constant of silicon is ε=11. Capacitance measurements were performed using a pulse generator (Avtech, Av- 1023-C) and a digital oscilloscope (LeCroy, Model 9400). Two different methods were used to estimate the capacitance. The first is to determine the RC-time constant of the sample/load resistor system, by measuring the current decay following the application of the external field on the sample. The second approach was to determine the charge induced by a voltage step. Therefore a known capacitance (C ext. ) was connected in series with the sample. As the charge between the two capacitors can only be displaced, both capacitors C ext. and C Sample store the same amount of charge. By measuring the voltage V ext. across C ext , the capacitance of the sample can be calculated using: C Sample = C ext.V ext. V appl. − V ext. . (3.14) The induced charge has been measured 500 ns after the application of a voltage step. This method is commonly applied to amorphous silicon diodes and gives values which are in agreement with values obtained from physical measurements. However, it was found that for µc-Si:H structures sometimes the capacitance is up to one order of magnitude higher than calculated from the geometry of the specimen [143, 144]. 31
3.2 Deposition Technique Chapter 3: Sample Preparation and Characterization Two well established and widely used techniques for the deposition of thin films of silicon are plasma enhanced chemical vapor deposition (PECVD) and hot wire chemical vapor deposition (HWCVD). Both methods are based on the decomposition of silicon containing gases. The major difference between both processes is the way how the precursors are made. While in PECVD the gases are decomposed by a plasma, in HWCVD the reaction takes place at a hot wire usually made of tungsten or tantalum. Both deposition techniques will be briefly described in the following section. 3.2.1 Plasma-Enhanced Chemical Vapor Deposition (PECVD) A very common method for the preparation of microcrystalline silicon is plasma enhanced chemical vapor deposition (PECVD), also known as glow discharge deposition. Detailed information about this technique and the underlying physics can be found in the literature, e.g in the books by Chapman [145], Haefer [146], Frey and Kienel [147], or Luft and Tsuo [148]. In this work, a 6-chamber deposition system with designated chambers for p−,n−, and intrinsic layers was used. A detailed description of the technical realization can be found in the work by Vetterl [12]. In the PECVD process the source gases are decomposed by an electrical discharge. The main mechanism for the decomposition is the impact of electrons, that take up sufficient energy from an alternating electrical field with typical frequencies in the range between 13.56 and 150 MHz. The precursors arising diffuse and drift to the substrate, usually placed on one electrode, and contribute to the film growth after several secondary gas phase reactions. The detailed plasma chemistry and growth mechanism are of course much more complex. A major advantage of PECVD is that the activation energy for the dissociation of the source gases comes from an externally applied alternating electric field and does not need to be supplied thermally. Therefore the substrate temperature T S can be adjusted independently, allowing the use of low T S . Deposition Parameters The main source gas for the deposition of amorphous and microcrystalline silicon is silane (SiH 4 ). An overview about the possible reactions in a silane plasma was given by e.g. Perrin et al. [149]. The structure and the electro-optical properties of the silicon films depend on various deposition parameters. It has been shown that with the admixture of hydrogen (H 2 ) to the silane plasma or by the use of high discharge powers microcrystalline growth can be achieved [150]. In particular the 32
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
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 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 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 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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?