Contactless measurement of bulk free-carrier lifetime in cast polycrystalline silicon ingots

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Abstract
The spatial dependence of the bulk free-carrier lifetime in cast polycrystalline silicon ingots was determined from contactless measurements of optically modulated free-carrier infrared absorption. Using a cw Nd: yttrium aluminum garnet (Nd:YAG) laser (λ=1.319 μm) for a probe and a novel pulsed tunable infrared dye laser (λ=1.10–1.13 μm), with photon energies near the Si band gap, for a pump, the free-carrier lifetime was determined from transient absorption measurements to a maximum depth of 3.0 cm from the surface of polycrystalline silicon ingots. Unmodulated infrared probe transmission measurements were used to monitor relative changes in the bulk precipitate density in ingots due to infrared scattering by the precipitates. Free-carrier lifetime profiles measured along the growth axis of polycrystalline silicon ingots had the same qualitative features as the minority carrier lifetime profile measured from n+p junction solar cells fabricated on the wafered ingots. Differences between the ingot-measured free-carrier lifetimes and the minority carrier lifetime, apparently due to high injection and possibly carrier trapping effects, are discussed.