Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

Abstract

A phosphorus diffusion gettering model is used to examine the efficacy of a standard gettering process on interstitial and precipitated iron in multicrystalline silicon. The model predicts a large concentration of precipitated iron remaining after standard gettering for most as-grown iron distributions. Although changes in the precipitated iron distribution are predicted to be small, the simulated post-processing interstitial iron concentration is predicted to depend strongly on the as-grown distribution of precipitates, indicating that precipitates must be considered as internal sources of contamination during processing. To inform and validate the model, the iron distributions before and after a standard phosphorus diffusion step are studied in samples from the bottom, middle, and top of an intentionally Fe-contaminated laboratory ingot. A census of iron-silicide precipitates taken by synchrotron-based X-ray fluorescence microscopy confirms the presence of a high density of iron-silicide precipitates both before and after phosphorus diffusion. A comparable precipitated iron distribution was measured in a sister wafer after hydrogenation during a firing step. The similar distributions of precipitated iron seen after each step in the solar cell process confirm that the effect of standard gettering on precipitated iron is strongly limited as predicted by simulation. Good agreement between the experimental and simulated data supports the hypothesis that gettering kinetics is governed by not only the total iron concentration but also by the distribution of precipitated iron. Finally, future directions based on the modeling are suggested for the improvement of effective minority carrier lifetime in multicrystalline silicon solar cells.

Synchrotron-based investigation of transition-metal getterability in n-type multicrystalline silicon

“…l-XRF measurements of as-grown p-type mc-Si wafers with a very similar total Fe concentration of 4.4 Â 10 14 cm À3 revealed a similar precipitate size distribution of up to 30 nm radius particles. 18 After PDG at 870 C, precipitates of fastdiffusing species, specifically Cu, were readily removed from p-type mc-Si, but Fe precipitates persisted at a reduced level. 10 For p-type mc-Si with a total Fe concentration of $10 15 cm À3 , the median reduction in precipitate size of Fe-rich precipitates identifiable both before and after PDG increased as the PDG temperature increased.…”

Influence of n+ and p+ doping on the lattice sites of implanted Fe in Si

“…Ref. 13, many fundamental questions on the basic underlying microscopic mechanisms are still poorly understood, among them the detailed knowledge of Fe lattice sites in different types of silicon, or when forming complexes with other impurities, such as dopants, or implantation defects, such as vacancies. For instance, the P-diffusion gettering process, which is nowadays widely used in the fabrication of n-p-junction Si solar cells [13][14][15][16][17][18][19][20] is known to create on top of a low p-doped multi-crystalline Si wafer highly-doped n + -regions, in which Fe is trapped.…”

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

“…f1-XRF has proven useful in probing the behavior of iron in multicrystalline silicon especially [5], given its occurrence at readily observable grain boundaries [6] and dislocations [7]. The evolution of the iron distribution as a function of silicon solar cell processing has been investigated for p-type multicrystalline silicon with standard tube-furnace gettering and firing processes [8], for in-line gettering of red zone material [9], and in a systematic investigation of higher gettering temperatures by several authors [10], [11]. Iron precipitates in contaminated regions of n-type multicrystalline silicon ingots are also under investigation to better understand their performance impact [12].…”