Contamination of silicon by iron at temperatures below 800 °C

Abstract

Iron‐related defects are deleterious in silicon‐based integrated circuits and photovoltaics, ruining devices and acting as strong recombination centres. Unless great care is taken, iron contamination will result from high temperature processing and so it is essential to understand the degree to which this can occur. Iron solubility data above ∼800 °C have been summarised by Istratov et al. (Appl. Phys. A 69, 13 (1999)), but many processes are performed at lower temperatures for which solubility data are scarce. We have studied iron contamination below ∼800 °C. Iron concentrations in intention‐ ally contaminated air‐annealed Czochralski silicon samples were determined from the change in minority carrier lifetime due to photodissociation of FeB pairs measured by quasi‐steady‐state photoconductance. In the ∼600 to 800 °C temperature range the iron concentration was found to vary according to $ 1.3 \times 10^{21} \; {\rm exp} \;\left (‐ { {1.8\;{\rm eV} } \over {kT} } \right)\;{\rm cm}^{‐3}. It is therefore the case that significantly more iron can dissolve in silicon at these temperatures than extrapolation of higher temperature data suggests, with the enhancement being by a factor of >20 at 600 °C. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrogénation effect on low temperature internal gettering in multicrystalline silicon

“…However, in general our results do not show the systematic decay in bulk iron concentration with annealing time and temperature observed by Krain et al [12]. This is surprising because at the temperatures used the interstitial iron is massively supersaturated [18,19], which provides a driving force for precipitation. The reason for the discrepancy with Krain et al's work is not clearly understood, and this study aims to start to address this.…”

“…At all temperatures studied, the measured interstitial iron concentrations are substantially in excess of the solubility values [18,19]. It is therefore surprising that the measured values increase upon annealing.…”

Competitive gettering of iron in silicon photovoltaics: Oxide precipitates versus phosphorus diffusion

“…The bulk iron concentration at the end of Stage 2 has a similar temperature dependence to the solubility value in oxide precipitate-free material. 19,20 At the very highest temperatures (> 800 C), the bulk iron concentration exceeds the solubility slightly. At all lower temperatures, the bulk iron concentration is lower than the solubility in precipitate-free material, and we have previously attributed this to iron segregation to precipitates during cooling.…”

“…5 This combined with the reversibility observation leads us to reinforce our previous conclusion that atomic (as opposed to precipitate-based) decoration of the oxide precipitates and C and 900 C. The solubility of iron in oxide precipitate-free silicon is also plotted. 19,20 It is noted that this solubility line is only directly relevant to the Stage 2 data as the plot considers contamination (not PDG) temperature. 2014) associated defects occurs.…”

Iron precipitation upon gettering in phosphorus-implanted Czochralski silicon and its impact on solar cell performance

“…The interstitial iron concentrations for wafers contaminated with 2·10 14 cm -3 were lower for each gettering condition than similar wafers that were prepared with a lower initial total iron concentration of 10 13 cm -3 , suggesting that precipitation played a role in the gettering process. However, interstitial concentrations remain above the solid solubility in p-type silicon at the gettering temperature [3], [4]. Secondary-ion mass spectrometry indicates an anomalously high iron concentration just below the surface [2].…”