Technique for controlling temperature uniformity in crystal growth apparatus

A method of producing a crystalline material comprising:

• providing a crystal growth apparatus comprising a chamber, and a hot zone within the chamber, the hot zone comprising a heating element having a circular cross section, a crucible having a square or rectangular cross section and containing a solid feedstock, and a heat exchanger positioned beneath the crucible;
• providing a muffle having a square or rectangular cross section inside of the heating element and surrounding the crucible, wherein the muffle is thicker in each of a plurality of corners thereof than in each of a plurality of sides thereof;
• covering a top of the muffle with a muffle cover disposed on an upper surface of the muffle and in contact with walls of the muffle, the muffle cover having an exit port formed therein;
• fitting the exit port with a tube extending from the muffle cover to an outer surface of the chamber;
• heating the solid feedstock in the crucible through the muffle with the heating element to form a liquid feedstock melt;
• controlling a temperature uniformity in the crucible by orientating the muffle such that at least one corner of the plurality of corners of the muffle is positioned between a center of one side of the crucible and the heating element;
• and
• cooling the liquid feedstock melt to produce the crystalline material.