Improved control of sapphire crystal growth

A technique was developed to detect the crystallization of the upper-central surface of sapphire crystals grown in crucibles. This technique takes advantage of the large difference in optical absorption coefficients 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 knee occurs on the recorded graph. The accumulated data enable the optimization of growth rates at the end of the growth as well as the duration of the growth by proper timing of the beginning of crystal cool-down. It is also possible to detect and terminate unsuccessful growth runs at an early stage.