IMPROVED CONTROL OF SAPPHIRE CRYSTAL-GROWTH

A technique was developed to detect the crystallization of the upper-c entral surface of sapphire crystals grown in crucibles. This technique takes advantage of the large difference in optical absorption coeffic ients of molten and solid sapphire at the melting point, and is based on the recording of pyrometer readings throughout the course of growth . At the point of solidification of the observed sapphire surface, a k nee occurs on the recorded graph. The accumulated data enable the opti mization of growth rates at the end of the growth as well as the durat ion of the growth by proper timing of the beginning of crystal cool-do wn. It is also possible to detect and terminate unsuccessful growth ru ns at an early stage.