Properties of iron-doped multicrystalline silicon grown by the float-zone technique

Multicrystalline Fe-doped Si ingots were float-zoned from high-purity feed rods. Fe was introduced by pill-doping, which gives uniform impurity content for small segregation coefficients (k~10¹⁵ for Fe in Si). Fe concentrations were calculated from the initial weight of the Fe pill, the molten zone geometry and the growth parameters. Values in the range of 10¹²-10¹⁶ atoms/cm³ were targeted. No additional electrically active dopants were introduced. Minority charge carrier lifetime (via YAG-laser-excited, 430 MHz ultra-high-frequency-coupled, photoconductive decay) was measured on the ingots, and wafers were cut to examine grain structure and electron-beam-induced current response of grain boundaries. Observed lifetimes decreased monotonically with increasing Fe content for similar grain sizes (from ~10 μs to 2 μs for <10 ^-3 cm² grains, from ~30 us to 2 us for ~5×10 ^-3 cm² grains, and from ~300 μs to 2 us for >10^-2 cm² grains) as the Fe content increased to 1×10¹⁶