Micro-spectroscopy on silicon wafers and solar cells
© Gundel et al; licensee Springer. 2011
Received: 3 September 2010
Accepted: 4 March 2011
Published: 4 March 2011
Micro-Raman (μRS) and micro-photoluminescence spectroscopy (μPLS) are demonstrated as valuable characterization techniques for fundamental research on silicon as well as for technological issues in the photovoltaic production. We measure the quantitative carrier recombination lifetime and the doping density with submicron resolution by μPLS and μRS. μPLS utilizes the carrier diffusion from a point excitation source and μRS the hole density-dependent Fano resonances of the first order Raman peak. This is demonstrated on micro defects in multicrystalline silicon. In comparison with the stress measurement by μRS, these measurements reveal the influence of stress on the recombination activity of metal precipitates. This can be attributed to the strong stress dependence of the carrier mobility (piezoresistance) of silicon. With the aim of evaluating technological process steps, Fano resonances in μRS measurements are analyzed for the determination of the doping density and the carrier lifetime in selective emitters, laser fired doping structures, and back surface fields, while μPLS can show the micron-sized damage induced by the respective processes.
Silicon solar cells contribute by far the largest share to the world's photovoltaic facilities . An important property to classify these silicon solar cells is the base material, where two fundamentally different approaches can be observed in the photovoltaic industry: multicrystalline and monocrystalline cells. While the fabrication of monocrystalline silicon is more expensive, the efficiency potential of these cells is higher. The world record efficiency for monocrystalline silicon solar cells is 25.0%  and 20.4% for multicrystalline silicon . As different as these base materials are as different as the arising challenges in the industrial production: To realize the efficiency potential and to lower the price per Watt-peak of monocrystalline cells, sophisticated cell structures with doping microstructures including selective emitters, laser fired back surface fields , and backside contacts have been introduced and are partly already adopted in the industrial production. For multicrystalline silicon, the photovoltaic industry tries either to use less pure and cheaper silicon ("upgraded metallurgical grade silicon") and to improve this material during the solar cell process by high temperature and gettering steps, to reduce the costs, or to use multicrystalline material with low defect densities to increase the efficiency potential. From these strategies, two important fields of microscopic research emerge: the detailed characterization and improvement of doping microstructures and the research on microdefects, which limit the performance of multicrystalline cells. Both fields require the development and application of electrical characterization techniques which provide a high spatial resolution of at least 1 μm.
In this paper, we demonstrate the latest advances on these research fields, which are based on micro-Raman spectroscopy (μRS) and micro-photoluminescence spectroscopy (μPLS). First, we will introduce the measurement techniques and how the important parameters doping density, carrier lifetime and mechanical stress can be extracted from both techniques with a spatial resolution of down to 500 nm. In the second part, we will apply these techniques (1) for the characterization of technological doping microstructures and (2) for the fundamental research on the recombination activity of precipitates.
Experimental setup and samples
μRS and μPLS are based on the same scanning confocal microscope, which features a 532 nm laser as point excitation source, a ×50 lens with a numerical aperture of 0.65 for μPLS and a ×100 lens with a numerical aperture of 0.9 for highly resolved μRS measurements. The spotsize of the laser is less than 500 nm in diameter and the power on the sample can be varied between 0 and 27 mW. Details on the setup can be found in [5, 6].
The sample surfaces for the multicrystalline samples and the cross-sections of the back surface field (BSF) and the laser-processed BSF were polished mechanically. No surface passivation has been applied to all samples. The multicrystalline wafer is 1.5 × 1016 cm-3 boron doped and was intentionally contaminated with nickel.
Quantitative Raman and photoluminescence spectroscopy
In this section, the techniques to quantitatively determine the doping density, the Shockley-Read-Hall lifetime, and the residual stress with micron resolution are presented in the two following subsections. The Shockley-Read-Hall lifetime is highly correlated to the efficiency of multicrystalline silicon solar cells.
By dividing the PL intensity around the center of the band-to-band PL peak I 1 (large pinhole) by the PL intensity I 2 (small pinhole), we obtain information about the depth profile. Using the ratio of two measurements has the advantage that unknown parameters such as the absolute quantum efficiency of the detector system and the emissivity of the sample surface cancel out. The measured ratio Q = I 1 /I 2 is compared to numerical two dimensional simulations of the injection density and the resulting Q. We call these techniques micro-photoluminescence lifetime mapping  and micro-doping density mapping .
Furthermore, μPLS can be utilized to measure the bandgap energy. Since the bandgap energy depends on the residual stress [9, 10] and the doping density, these parameters can be extracted from the μPLS measurement. For this the PL spectrum at 300 K is empirically fitted with three overlapping Gaussians with fixed relative spectral distances and the relative peak shift is extracted. From the relative peak shift, the stress level can be calculated if the doping density is homogeneous (variations below 1017 cm-3, where the influence on the bandgap energy becomes significant). In , we could show that the measured stress is in agreement with μRS stress measurements. If no stress is present, the doping density can be estimated.
In samples with unknown doping densities, the calibration curves are used to determine the doping density. Since Γ is more robust against fitting errors than q, we rely on this parameter for the measurements below.
At high injection, the Fano resonance is not solely governed by the doping density but also by the injected holes. With simulations in analogy to [7, 8] and the calibration tables in Figure 4, the Shockley-Read-Hall lifetime can be measured at injection densities above 1018 cm-3. An excellent agreement between μPLS and μRS Fano measurements was demonstrated in .
The advantage of μRS compared to μPLS is the higher spatial resolution of 500 nm or less. μPLS offers the advantages to measure not only p-type doping but also n-type doping and the measurements are typically less noisy. Furthermore μPLS has the ability to measure the defect luminescence within the same measurement.
Aluminum back surface field
Laser doping from a dopant containing passivation layer (PassDop)
Figure 8b shows the Shockley-Read-Hall lifetime on a 100 × 100 μm2 area at the triple point of three grain boundaries, which was measured by micro-photoluminescence lifetime mapping. The measurement shows the strongly different recombination activities of the three grain boundaries and reveals micron-sized denuded zones around the left grain boundary. The linescan across this grain boundary highlights the spatial resolution of micro-photoluminescence lifetime mapping. Micron-sized denuded zones could not be detected prior to the application of μPLS and μRS. The origin could be slowly diffusing impurities, which are internally gettered at the grain boundary during the block casting, which cleans the area around the grain boundary from these impurities. The lower right grain boundaries are highly recombination active, which is probably caused by a high metal decoration. Metal precipitates are also the most likely origin of the round structures along this grain boundary.
Stress and recombination activity
with the maximum measured hole density p max and the hole density p.
We presented an overview about the most recent developments of micro-Raman (μRS) and micro-photoluminescence spectroscopy (μPLS) and their successful application on technological microstructures and on fundamental problems of recombination at defects in silicon. We demonstrated the high resolution (< 1 μm) measurement of (1) the Shockley-Read-Hall lifetime by μRS and μPLS, (2) of the doping density by μRS and μPLS, and (3) of stress with both methods.
μRS has the advantage of a higher spatial resolution (about 0.5 μm compared to 0.8 μm) and is not influenced by defect luminescence, which can make the extraction of the bandgap energy and thus of the doping density and the stress from PL measurements difficult. μPLS has the advantages to be able to measure both n- and p-type doping and exhibits less noise in carrier lifetime measurements for comparable measurement times. Furthermore, the analysis of the defect luminescence can give a deeper insight in the carrier lifetime limiting defects.
We were able to detect high recombination activities within an aluminum-doped back surface field and the damage caused by a laser firing contact process, which shows ways to improve the processes.
On multicrystalline silicon, we investigated the recombination activity of grain boundaries and were able to measure micron-sized denuded zones around a grain boundary. We could explain the observed effect that recombination activity is significantly increased by tensile stress and reduced by compressive stress, by the high piezoresistivity of silicon.
We gratefully acknowledge sample preparation by Aleksander Filipovic, Gisela Räuber, Miroslawa Kwiatkowska and Markus Hecht. This work was supported by internal funding of the Fraunhofer Society.
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