Effects of Growth Parameters on Dislocations in CaF2

The relationship between dislocation morphology and variations of growth parameters has been studied in CaF2 ingots grown by the Czochralski technique. The growth parameters investigated were growth rate, seed orientation, crystal size, temperature gradient, use of an afterheater, and doping. The effects of mechanical damage at room temperature and of annealing the crystals are also presented. Normal handling has no effect on the production of new dislocations but severe stress concentrations cause marked slip on {100}. Three main sources of dislocations in a growing ingot have been found: (1) propagation of dislocations present in the seed, (2) thermal stresses, and (3) precipitates of CaO. Methods are discussed for minimizing the production of dislocations in a growing ingot.