What's the material for crucibles in Czokralski crystal growing process? I believe it's boron nitride, leaving about 1012at/cm3 of B in solid Si - thanks to the purifying effect of crystallization.
If this is still too much, you might use a "crucible" made of silicon - that is, melt only a small part of the crystal. At the time (upper neolithic) I worked with silicon, we had an electron gun to melt locally and vaporize clean aluminium to coat silicon.