Zone Melting

A polycrystalline specimen is prepared, typically in the shape of a cylinder and placed into a crucible, with a seed crystal near the top of the crucible.

The sample cylinder is placed in a furnace with a very narrow hot zone (sometimes this is done using halogen lamps as heat sources).

The portion of the cylinder containing the seed crystal is heated to the melting point, and the rest of the cylinder is slowly pulled through the hot zone.

Zone melting setups are modifications of either the Bridgman or Stockbarger methods of crystal growth.

Bridgman ® Hot zone moves, crucible stationary
Stockbarger ® Crucible moves, hot zone stationary

An advantage of the zone melting technique is that impurities tend to be concentrated in the melted portion of the sample. Consequently, this process sweeps them out of the sample and concentrates them at the end of the crystal boule, which is then cut off and discarded. Thus this method is sometimes used to purify semiconductor crystals.