Normally silicon boules are round, but this one is square! I have several theories and maybe one of you readers will set me straight. One theory is that all silicon boules start out square like this, but turn round when they hit the edges of the spinning pot from which they are being drawn. This theory is supported by the fact that this one was obviously stopped before it finished growing any bigger than it currently is (click the turntable icon to the right to see the bottom side using the QTVR rotatable image). Another theory is that they accidentally mounted the seed crystal in the wrong orientation, causing the crystal to grow in the wrong shape, causing them to stop it early. All in all, I'm going with the first theory since there is no crystal axis that explains the round shape of other boules: That must come from the container, not the crystal growth itself.
And fortunately, reader Ian Haygood provided this explanation:
Hello,In other words, all my theories were wrong.
I was enjoying looking at your periodic table today when i read you comments about the square silicon boule you have. This crystal was not pulled using the Czochralski method like many of the other boules you own. I believe that it was made using the Stephanov process. In this process, a graphite shaper is inserted into the pool of liquid Si, and almost any shape can be produced. It is also possible that another technique called the edge-defined film-fed growth(EFG) process could have been used. The process is similar to the Stephanov method, however capillary feeding through the shaper is used to feed material to the growing crystal. Also, you mentioned that you believed that all boules start square. In the Czochralski process, a single crystal seed is introduced into a liquid silicon pool, and is the slowly lifted and rotated while the pool is counter-rotated. This rotation is what causes the resulting crystal to be round. In the other two methods the shaper defines the dimensions of the boule and no rotation occurs. I hope that this information is useful to you.
Source: eBay seller campgems
Contributor: Theodore Gray
Acquired: 10 September, 2004
Text Updated: 5 December, 2006