An experiment was performed where UMG-Si and modified Siemens Si were mixed (20/80) and compared in [C], [O], and [N] before and after directional solidification , using silicon-nitride-coated quartz crucibles. SIMS was used for the measurements. (Note that the modified Siemens Si was thought by the manufacturer to be 11N but SIMS detected S at high enough level to make the material 7N). A control of the modified Siemens Si was included before and after the directional solidification. A schematic of the experiment is shown in Figure 6.7 and the SIMS data are in Table 6.17.
The conclusions of the experiment were:
(A) The directional solidification process itself (with the nitride liner) dominates the C, O and N levels in the mc-Si solar wafers, in good agreement with the Olsen and 0verlid  results discussed before.
(B) Specifications for very low levels of C, O and N in PV Si feedstock are not warranted if a directional solidification process with nitride liner is used.
(C) The opportunity to reduce C, O and N in mc-Si wafers is to focus on the directional solidification process, not the feedstock.
Table 6.17 SIMS impurity concentrations (x1016 atoms cm 3) in PVSi feedstock.