652 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 



Fig. 8 (a)- 

 from melt). 



Star Pattern on (111) plane (etched cross-section of crystal pulled 



The peripheral tension assumed in the above paragraph may be seen 

 to be quahtatively reasonable upon consideration of the heat flow pat- 

 tern of the crystal during growth. Heat must enter the crystal by 

 conduction through its hottest surface, the gro^\^ng interface, which 

 is a 940°C isotherm. It must leave through all the other surfaces by 

 radiation and conduction. Therefore, these surfaces must be cooler 

 than their adjacent interiors, and cross-sections of the crystal must 

 have cooler peripheries than cores because of the heat escaping from 

 the peripheral surfaces. Due to thermal contraction the cooler periphery 

 must be in tension and the core in compression. 



In zone leveled crystals the distribution of etch pits on a (111) section 

 was not dense or symmetric enough to display a star pattern. However, 

 it was reasoned that since thermal contraction stresses appeared to play 

 a major role in the production of dislocations in pulled crystals through 

 plastic deformation in the available slip systems, the same mechanism 

 might be playing a significant role in zone leveled crystals. 

 .' "-.The only stresses in a zone leveled ingot other than those due to the 

 weight of the crystal itself must be those due to non-uniformities in 



