Necking

Here is an X-ray topograph of the first part of crystal growth

An X-ray topograph is similar to a transmission electron microscope image - it shows the interior of the sample and dislocations are visible as dark lines.

You see a dislocation-free seed crystal, followed by region full of dislocations. This is unavoidable because dipping a solid seed in a melt that has by definition a higher temperature, always causes a "thermal shock" with stress and strain and therefore plastic deformation.

The the diameter of the now growing crystal is made as small as possible (it still must be able to carry the weight of the finished crystal - up to 250 kg or so). This is the "necking" or Dash process.

The dislocations disappear after a few cm, the question is why? The picture almost shows it. For the usual <100> oriented crystal, the glide planes of the dislocations (the {111} planes) are all inclined to the growth direction, and the dislocations, still feeling some stress, will simply move out of the crystal.

This is where the art part comes in - or better came in. Keep enough stress to move the dislocations, but not that much that new ones will be generated.

Courtesy of Wacker / Siltronic; Burghausen, Germany

With frame

4.1.3 Silicon Crystal Growth and Wafer Production

Czochralski Crystal Growth Process

The Science and Art of Si Crystal Growth

Silicon Crystal