To reduce the heat input to the bottom of the crucible and to control heat
extraction independently of heat input, a shield can be raised between a
heating element and a crucible at a controlled speed as the crystal
grows. Other steps could include moving the crucible, but this process
can avoid having to move the crucible. A temperature gradient is produced
by shielding only a portion of the heating element; for example, the
bottom portion of a cylindrical element can be shielded to cause heat
transfer to be less in the bottom of the crucible than at the top,
thereby causing a stabilizing temperature gradient in the crucible.