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3.1 Characterization Methods Homogeneous Broadening Because of the finite lifetime of an electron in the excited state, the uncertainty principle leads to a finite so called ”natural linewidth”. While an excited electron is re-enters its ground state its energy will be transferred to the surrounding lattice. Within a time T 1 the spin-lattice relaxation restores the system into its the equilibrium state. In addition, there is an interaction of the spins with each other. The time constant of this spin-spin relaxation process is typically denoted T 2 . From the spin-lattice and spin-spin relaxation the lineshape is of the form of a Lorentzian [118]. Inhomogeneous Broadening Differences from the Lorentzian line shape discussed above can arise from g-value anisotropy or an unresolved hyperfine interaction. Additionally broadening can also result from structural disorder. In this case the overall resonance line consists of a number of narrower individual lines, that are a result of the so-called spin packets. Each spin packet can be seen as an individual system of spins, having the same Lamor frequency ω i around ”their” magnetic field vectors B i . In general B i is given by the sum of the externally applied field B 0 and the local field B loc i . Due to inhomogeneities, like crystal irregularities, magnetic field inhomogeneities, or dipolar interaction between unlike spins, the local field B loc i differs for spins belonging to different spin packets. The observed line shape is therefore a contribution of several Lorentzian signals arising from different spin packets. For disordered semiconductors like amorphous and microcrystalline silicon both, the energetic position as well as the local environment of the spins may not be identical, which leads to numerous slightly different g-values. Both effects are expected to be statistically distributed and thus the resulting line shape of the convolution of the homogeneous lines is of the form of a Gaussian. 3.1.2.3 Experimental Setup ESR has been measured using a commercial X-band spectrometer (BRUKER ESP 380E). A reflex klystron working at a frequency of around 9.3 GHz with a maximum power output of about 200 mW was used as the microwave source. During the measurement the microwave frequency was kept constant while the resonance conditions were reached by scanning the magnetic field B 0 . The ESR signal was recorded using phase sensitive detection so that the measured signal intensity is proportional to the first derivative of the absorption signal. The area under the absorption curve, which is proportional to the spin density N S , was obtained by double integrating the measured signal, numerically. For a quantitative analysis 23
Chapter 3: Sample Preparation and Characterization of the ESR spectra (calculation of the g-value and the spin density N S ) the ESR signal was compared to that of a sample of un-hydrogenated sputtered amorphous silicon, which was calibrated to a standard of Picein and DPPH [120]. For temperature dependent measurements a He gas flow cryostat (Oxford ESR 900) was used. To avoid condensation of water at the walls the cavity was purged with dry nitrogen. ESR measurements were performed in a temperature range between 4.5 K and 300 K using a modulation frequency of 100 kHz and a modulation amplitude of 2 G. The microwave power could be attenuated in the range between 200 mW - 0.2 µW and was usually set such that saturation effects did not occur. Details of the sample handling and the preparation can be found in section 3.3.1. 3.1.3 Electrical Conductivity Conductivity measurements were performed on specimens deposited on roughened borosilicate glass prepared in the same run as the samples prepared for ESR measurements. As contacts, coplanar silver pads were evaporated under high vacuum conditions having a thickness of 700 nm, an electrode spacing b=0.5 mm, and a width l=4 mm. In order to avoid errors due to surface coverage all measurements were performed under high vacuum (p < 0.01 Pa) conditions after an annealing step of 30 min at 450 K (compare section 6 and 6.3). To ensure that the determined conductivity is voltage independent (ohmic contacts), I-V curves have been measured between V = ±100 V. Having determined the current I for an applied voltage V (usually set to V=100 V) the specific dark conductivity σ D is given by σ D = b · I l · d · V (3.4) where d is the film thickness and l, b are determined by the contact geometry (see above). Temperature dependent measurements were performed between 100 K and 450 K using a nitrogen cooled continuous flow cold finger cryostat. 3.1.4 Transient Photocurrent <strong>Measurements</strong> (TOF) The time-of-flight (TOF) technique was first described by J.R. Haynes and W. Shockley in 1951 [121] and was further improved by R. Lawrence and A.F. Ribson in 1952 [122]. The first application to an amorphous material (a-Se) was first done by W. Spear in 1957 [123, 124, 125] and since then it has widely been used to obtain valuable information on transport processes in a wide range of low mobility amorphous and crystalline solids. 24
Page 1 and 2: Forschungszentrum Jülich in der He
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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
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6.1 Metastable Effects in µc-Si:H
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6.2 Irreversible Oxidation Effects
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6.3 On the Origin of Instability Ef
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6.3 On the Origin of Instability Ef
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Chapter 7 Transient Photocurrent Me
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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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