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3.1 Characterization Methods bution. This can be written as the following equation, derived from calculating the electrostatic energy dissipated by a charge Q 0 in a uniform electric field F which is Q 0 Fx(t). Equating this with the energy furnished by the externally applied bias voltage Q(t)V one gets x(t) = d Q(t) Q 0 . (3.11) From Eq. 3.9 one concludes that the transit time is solely determined by the ratio L/F. Doubling both, electric field F and distance L, the transit time remains unchanged. Generalizing the distance L to x(t) Eq. 3.9 can be written as: L F = x(t) d2 = Q(t) F Q 0 V for (t ≪ t τ ) (3.12) Equating the transit photocharge, by integrating the photocurrent, L/F can be determined by using Eq. 3.12 and can be plotted as a function of time. These graphs are referred to as ”displacibiliy plots”; a typical example is shown in figure 3.3 (c). The transit time for a chosen value of L/F can be determined directly from the graph, as indicated in the figure. As this method is simply an enhancement of the half-charge method it is clear that transit times obtained are consistent with the other methods. However, there are two advantages of using this method. First, only a handful of transients are enough to obtain a continuous curve of transit times and secondly one can obtain the displacibility for transit times shorter than would be accessible by increasing the applied voltage. This method introduced by Schiff et al. in 1993 has been used by a number of other authors [131, 139, 140, 141, 142]. 3.1.4.4 Experimental Arrangement The experimental arrangement used for time-of-flight measurements is illustrated in Fig. 3.4. The charge carriers were excited using a nitrogen laser pumped dye laser (Laser Science Inc.) with a pulse width of 3 ns. As laser dye Coumarin 500 with an emission maximum at λ = 500 nm was used, so the carrier generation took place within the first 160 nm of the illuminated side of the intrinsic layer. Stronger absorbed light was intentionally avoided to prevent back diffusion problems. The intensity of the laser was attenuated with neutral density filters until the photogenerated charge was below the CV−limit (see section 3.1.4.2). The repetition rate of the laser was chosen to a value low enough to avoid a build up of charge in the material. A value of 2 Hz was allowable for temperatures between T = 300 K and 100 K. The specimens were mounted on the cold finger of a commercial vacuum cryostat (Oxford Instruments Model DN1754). This allowed measurements in the temperature range between T = 77 K and 350 K. Additionally, the cryostat acted 29
Chapter 3: Sample Preparation and Characterization Figure 3.4: Schematic view of the experimental setup for time of flight measurements used in this work. as a shield against electromagnetic interferences and also avoided influences of optical bias effects from accidental room light illumination. Great care was taken that the series resistance of the contacts was sufficiently small not to limit the time resolution of the system. To induce photocarrier drift an electric field was applied by a bias voltage across the sample. A step voltage was used to assure that the applied field was uniformly distributed (see section 3.1.4.2 and [128]). The photocurrent transients were measured by recording the voltage across a 50 Ω resistor in the time range between t = 0 − 10 µs. For longer times a larger resistor of typical 0.3−33 kΩ was used. In order to reduce the signal-to-noise ratio the average of 100 pulses was taken. For data acquisition and averaging of the transients a digital oscilloscope (LeCroy Model 9400, 500 MHz bandwidth) was used. The oscilloscope was connected to a computer for storage and analysis of the measured currents. 3.1.5 Thickness <strong>Measurements</strong> In this work, thin films of µc-Si:H as well as pin diodes containing a µc-Si:H i- layer were prepared and investigated. Details of the preparation and particular structures can be found in section 3.2 and 3.3. For most of the methods presented, a knowledge of the film or the i-layer thickness is of great importance. Two different methods for thickness measurements were applied, namely mechanical step profiling and capacitance measurements. The advantages and limitations of both methods will be discussed in the following. Mechanical measurements of the film thickness were performed using a mechanical step profiling system (Sloan DEKTAK 3030 Auto II). This method has been used for the measurements of both thin films and pin diodes. For films de- 30
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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Page 20 and 21: Chapter 2 Fundamentals In this firs
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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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