Crystal Systems use the Kyropoulos method I think (I never visited their place, but colleagues of mine did). I don’t think they do the zone heat-treatment on it though. They just start with ultra-pure feedstock and keep the whole furnace super-clean. The boule is grown as a single crystal, generally with the c-axis down the boule axis to avoid birefringence.
Zone-refining is critical for final purification of materials like silicon though. When we were working with BP Solar they were saying that one of the steps that was going to have to be taken in the photovoltaic industry was the setting up of plants to produce silicon for PV panels. The purity requirements on that stuff aren’t high. So up to that point (very late 1990’s IIRC) they’d been able to use wafers from the last 6" or so of zone-refined semiconductor industry boules. This was where the impurities from the rest of the boule ended up. It was useless for semicon work but perfectly good enough for PV. And it was close to being FOC. Now there’s nowhere near enough to satisfy PV demand though and the industry’s grown its own material for decades.
