High-purity, isotopically enriched bulk silicon

The synthesis and characterization of dislocation-free, undoped, single crystals of Si enriched in all three stable isotopes is reported: ²⁸Si (99.92%), ²⁹Si (91.37%), and ³⁰Si (89.8%). A silane-based process compatible with the relatively small amounts of isotopically enriched precursors that are practically available was used. The silane is decomposed to silicon on a graphite starter rod heated to 700-750°C in a recirculating flow reactor. A typical run produces 35 g of polycrystalline Si at a growth rate of 5 μm/min and the silane to Si conversion efficiency exceeds 95%. Single crystals are grown by the float-zone method and characterized by electrical and optical measurements. Concentrations of shallow dopants (P and B) are as low as mid-10¹³ cm^-3. Concentrations of C and O can be lower than 10¹⁶ and 10¹⁵ cm^-3, respectively.