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Improvement of charge minority-carrier lifetime in p(Boron)-type Czochralski silicon by rapid thermal annealing

: Lee, J.Y.; Peters, S.; Rein, S.; Glunz, S.W.


Progress in Photovoltaics 9 (2001), No.6, pp.417-424
ISSN: 1062-7995
Journal Article
Fraunhofer ISE ()

In p-type Czochralski-grown (Cz) silicon a light-induced degradation of the minority-carrier lifetime is well known in the literature. Reducing the extent of this degradation would significantly improve the stable effective lifetime and thus the related-performance of solar cells. In this work, the reduction of the density of the metastable defect underlying the degradation is performed by rapid thermal annealing (RTA). For a proper analysis it is extremely important to avoid contamination by the RTA furnace. Both, SiN/sub x/, and SiO/sub 2/ were examined as a barrier layer. A 60 nm SiN/sub x/, layer was proven to act as the most effective barrier layer, allowing maintenance of a very high lifetime of 700 mu s on 1.25 Omega cm p-type FZ material. A design-of-experiments (DOE) study was used to analyze the effect of five process parameters on the stable effective lifetime. Especially, the plateau temperature shows a strong correlation with tau /sub d/,, the stable effective lifetime after light-induced degradation. The effect of plateau temperature on tau /sub d/ of Cz- and FZ-Si wafers is examined in the temperature range of 700-1050 degrees C for plateau time 120 s. It was found that the stable effective lifetime of all RTA-treated Cz-wafers is increased compared with the initial stable effective lifetime before processing. The highest increase of stable effective lifetime (by a factor of around 2) is obtained at 900 degrees C with a process time of 120 s. This increase in lifetime is reflected in a reduced concentration of the metastable defect