QSS-μPCD measurement of lifetime in silicon wafers: advantages and new applications (original) (raw)
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Limitations in the accuracy of photoconductance-based lifetime measurements
Solar Energy Materials and Solar Cells, 2012
The accuracy of photoconductance-based minority carrier lifetime measurement techniques is studied in detail. Regarding their accuracy and comparability, the quasi steady state photoconductance (QSSPC) as well as the novel steady state microwave detected photoconductivity (MDP) method are compared with the traditional microwave detected photoconductance decay (mPCD). We show that differences in measurement conditions and analyzing procedures lead to deviating results. Calculations based on a generalized rate equation system are used to model the photoconductivity and the resulting effective lifetime for different measurement conditions and defect models. The simulation results are compared to measurements on several mono-and multicrystalline silicon samples with and without surface passivation. Our results clearly show, that for low as well as high injection conditions deviations occur for the measurement and analysis techniques investigated. To allow for a comparison of lifetime data, we recommend to report exact measurement conditions and analysis procedures as well as to perform measurements in a certain injection range. Only if no trapping effects are present and the penetration depth of the applied method is significantly larger than the sample width, accurate and comparable lifetime results can be achieved.
Applied Physics Letters, 1996
A simple method for implementing the steady-state photoconductance technique for determining the minority-carrier lifetime of semiconductor materials is presented. Using a contactless instrument, the photoconductance is measured in a quasi-steady-state mode during a long, slow varying light pulse. This permits the use of simple electronics and light sources. Despite its simplicity, the technique is capable of determining very low minority carrier lifetimes and is applicable to a wide range of semiconductor materials. In addition, by analyzing this quasi-steady-state photoconductance as a function of incident light intensity, implicit current-voltage characteristic curves can be obtained for noncontacted silicon wafers and solar cell precursors in an expedient manner.
Solar Energy Materials and Solar Cells, 2010
mPCD MDP a b s t r a c t Simulations of time dependent carrier profiles for thick as-grown silicon samples (e.g. ingots) were computed for the excitation conditions of two different transient photoconductance lifetime measurement methods. The simulations were performed using a partial differential equation system that allows computing also non-steady state conditions. The specific effective lifetimes for different measurement conditions can be extracted and compared. Simulation results and measurement results for mPCD (microwave detected photoconductivity decay), a non-steady state method and MDP (microwave detected photoconductivity), which operates typically with a steady state photo generation, were simulated and measured. It was found that the effective lifetimes measured at thick samples with each method may differ strongly. This discrepancy can be attributed to the different penetration depths of the laser light and microwave, but first and foremost to a varying light pulse length and its influence on the developing carrier profile. Altogether the MDP measurements or methods with a steady state photo generation in general are less prone to the surface impact and accordingly better suited for investigating the bulk properties of silicon samples.
Optical and Other Measurement Techniques of Carrier Lifetime in Semiconductors
International Journal of Optoelectronic Engineering, 2012
In this paper, various methods for characterization of semiconductor charge carrier lifetime are reviewed and an optical technique is described in detail. This technique is contactless, all-optical and based upon measurements of free carrier absorption transients by an infrared probe beam following electron-hole pair excitation by a pulsed laser beam. Main features are a direct probing of the excess carrier density coupled with a homogeneous carrier distribution within the sample, enabling precision studies of different recombination mechanisms. The method is capable of measuring the lifetime over a broad range of injections (10 13-10 18 cm-3) probing the minority carrier lifetime, the high injection lifetime and Auger recombination, as well as the transition between these ranges. Performance and limitations of the technique, such as lateral resolution, are addressed while application of the technique for lifetime mapping and effects of surface recombination are also outlined. Results from detailed studies of the injection dependence yield good agreement with the Shockley-Read-Hall theory, whereas the coefficient for Auger recombination shows an apparent shift to a higher value, with respect to the traditionally accepted value, at carrier densities below 2×10 17 cm-3. Data also indicate an increased value of the coefficient for bimolecular recombination from the generally accepted value. Measurement on an electron irradiated wafer and wafers of exceptionally high carrier lifetimes are also discussed within the framework of different recombination mechanisms.
Solid-State Electronics, 1999
In this paper an all-optical measurement procedure for the characterization of minority carrier recombination lifetime and surface recombination velocity is presented as a reliable tool to monitor the fabrication process of a standard crystalline silicon solar cell. In the methodology presented here, there are no stringent requirements concerning the state of wafer surface. The IMEC (Interuniversity Microelectronics Centre, Leuven, Belgium) fabrication process is taken as an example of the capability of this method to monitor the whole process from the silicon wafer to the ®nished cell. It is shown that the cell process does not degrade the bulk recombination lifetime and that the eect of the external surfaces is eectively screened. #
Measurements of the open-circuit photovoltage decay in a silicon solar cell
Solar Cells, 1983
Experimental results are presented for the open-circuit photovoltage decay in a silicon p-n junction solar cell for monochromatic light of five different wavelengths and for composite light. Analysis of the results in terms of the existing theories gives a reasonable estimate of the minority carrier lifetime and other material parameters of the solar cell.
Fast Wafer-Level Characterization of Silicon Photodetectors by Photoluminescence Imaging
IEEE Transactions on Electron Devices
Photoluminescence imaging (PLI) technique is conventionally used in silicon (Si) photovoltaics (PV) for device characterization and inline quality control, providing substantial assistance for a wafer-level process monitoring from as-cut wafers to fully fabricated devices. Surprisingly, employing this method has not spread outside PV, and thus, its potential remains largely unknown in other fields. In this case study, a fully processed Si photodetector wafer, consisting of photodiodes with various sizes, has been chosen as an example to explore the potential of PLI beyond PV. First, we show that the standard PLI measurement is able to provide a high-resolution full-wafer luminescence image of the complete devices only within a couple of seconds. The image reveals various types of inhomogeneities present in the devices, such as furnace contamination and other processing-induced defects. The measured data are then converted to an effective lifetime image followed by benchmarking with a conventionally measured recombination lifetime map obtained by microwave-detected photoconductance decay (µ-PCD), demonstrating further superiority of PLI in terms of the spatial resolution and the measurement time. Finally, correlation with diode leakage current and photoresponse measurements show that PLI is able to provide useful information on the final device performance without a need for traditional electrical contact measurements. While this study has focused on Si photodetectors, the results imply that PLI also has potential in other semiconductor devices for fast wafer-level process monitoring purposes as well as for a single device characterization either before or after wafer dicing.
Materials Science and Engineering: B, 2013
The state-of-the art lifetime measurement technique MDP (microwave detected photoconductivity) is presented with its latest developments in sensitivity, measurement speed and data simulation. Several applications and examples in the field of inline material characterization, defect recognition and real time statistical process control in silicon bricks and wafers are presented, demonstrating the practical use of MDP measurements and of the data obtained by it. The measured lifetime itself combined with its spatial distribution and the measured steady state photoconductivity enable a good correlation to the cell efficiency.
Camera-based photoluminescence lifetime imaging of crystalline silicon wafers
2009
Photoconductance-calibrated photoluminescence lifetime imaging (PC-PLI) is a fast and easy-to-apply method for spatially resolved carrier lifetime measurements of crystalline silicon wafers. The photoluminescence signal in arbitrary units is converted into absolute values of the actual carrier lifetime by measurements of the photoconductance of the silicon wafer. We determined a calibration function which is valid for wafers of arbitrary dopant densities by utilizing a fundamental relationship between photoluminescence and dopant density. In principle, a single measurement of the relation between photoluminescence signal and excess carrier density is sufficient to obtain this calibration function due to its dependence on the dopant density. We demonstrate that PC-PLI allows high resolution lifetime measurements down to injection levels of 10 11 cm -3 and is in agreement with light-biased microwave-detected photoconductance decay (MW-PCD) lifetime mappings.