Applications of SIMS and GDMS for PV Si Feedstock Studies

6.1.1 Impurity Segregation in Directional Solidified (DS) Silicon Blocks

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Reported measurements of impurity segregation in directional solidified blocks have shown significant scatter that may be due to the solidification process or the measure­ment precision, or a combination of both. This difficulty has led some to believe that the selection of the measurement method must involve the maximum volume of sampled material. However, with a well-controlled directional solidification process both GDMS and SIMS are able to generate precise enough data that representative sampling can be possible. To support this statement Figures 6.3 through 6.6 show GDMS and SIMS data of P, B, O, and Ga through the growth axis of large, directional solidification UMG-Si blocks. The distance between measurement points is 10 mm. Note in Figure 6.6 that the effective segregation coefficient of Ga is 0.45, much greater than the equilibrium

Figure 6.3 Both GDMS and SIMS provide good precision of phosphorus segregation in a large-scale, directional-solidification block from UMG-Si.

GDMS: B, P and Ga distribution

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Figure 6.6 GDMS provides good precision of Ga along with B and P in a large-scale, directional-solidification block from UMG-Si.

segregation coefficient of 0.008. In addition, the effective segregation coefficient of P is 0.60, about twice its equilibrium segregation coefficient. These effective segrega­tion coefficients occurred from an innovative growth process in directional solidification (private communication).