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Imaging as-grown interstitial iron concentration on boron-doped silicon bricks via spectral photoluminescence

: Mitchell, B.; Macdonald, D.; Schön, J.; Weber, J.; Wagner, H.; Trupke, T.


IEEE Journal of Photovoltaics 4 (2014), No.5, pp.1185-1196
ISSN: 2156-3381
Journal Article
Fraunhofer ISE ()
Solarzellen - Entwicklung und Charakterisierung; Silicium-Photovoltaik; feedstock; Kristallisation und Wafering; Charakterisierung von Prozess- und Silicium-Materialien

The interstitial iron concentration measured directly on the side face of a silicon brick after crystallization and brick squaring can give important early and fast feedback regarding its material quality. Interstitial iron is an important defect in crystalline silicon, particularly in directionally solidified ingots. Spectral photoluminescence intensity ratio imaging has recently been demonstrated to independently provide high-resolution bulk lifetime images and is therefore ideally suited to assess spatially variable multicrystalline silicon bricks. Here, we demonstrate this technique to enable imaging of the interstitial iron concentration on boron-doped silicon bricks and thick silicon slabs. We present iron concentration studies for two directionally solidified silicon bricks of which one is a standard multicrystalline and the other is a seeded-growth ingot. This lifetime-based measurement technique is highly sensitive to interstitial iron with detection limits down to concentrations of about 1 × 1010 cm-3. Its accuracy is enhanced, as the injection level remains below 2 × 1012 cm-3 during the measurement and, hence, avoids the influence of injection level dependences on the conversion factor, although it remains dependent on the knowledge of the electron capture cross section of interstitial iron in silicon. Access to both bulk lifetime and dissolved iron concentration provides a valuable parameter set of as-grown crystal quality and the relative recombination fraction of interstitial iron via Shockley-Read-Hall (SRH) analysis. Simulated interstitial iron concentration profiles support the presented experimental data.