Study On The Morphology And Mechanisms Of Sapphire Crystal Growth By SAPMAC

Sapphire crystals were grown through Micro-Pulling and Shoulder-Expanding at Cooled Center (SAPMAC) with the voltage decreasing rate of 10mv/h, 12mv/h, 14mv/h and 16mv/h. Based on further observation of the shape of the crystals, the effect of voltage decreasing rate, so the voltage decreasing rate, on the crystal shape was studied in detail. It was shown that the diameter of crystal was of an increase trend under a constant voltage decrease rate, which means the crystal grew faster and faster in the radial direction under that condition. This conclusion was further confirmed by observations that the shoulder-expanding angle and the equal-diameter angle were increased, and the angle of the bottom of crystal was decrease when the voltage decreasing rate increased. According to these results, a conclusion was drawn that the temperature must be controlled strictly to decrease; moreover the decreasing rate could be constant in the shoulder-expanding and must descend in the equal-diameter period. This is accordant with the analyzing based on the one dimensional crystallization model.To explain the shape evolution under different process, a geometrical model describing the crystal growth was also founded according to the observation above and the layer profile of crystal side wall. The model shows increasing the crystal growth rate in radial direction; increasing the bulgy extent or decreasing the pulling speed will increase the shoulder-expanding angle and equal-diameter angle. Combing this model with the one dimensional crystallization model, the effect of voltage decrease rate on the crystal shape was explained successfully.Observation of the microtopographies of the bottoms (last crystallization surfaces) of the crystals indicates that the last crystallization surfaces of SAPMAC growing A{11 20}sapphire crystals always compose smooth region and rough region. The smooth region is grown by 2D nucleation mechanism and with no obvious structure. The rough region could be flat and terrace like, and is mainly composed by step bunch which were formed by the collaboration of many spiral hillocks. Moreover, the step bunches aline in the direction [ 8 80 3]; their width is in the range of 20~200μm and their slop was in the range of 3°~30°. The steps on the bunch are in width of 1~5μm and in height of about 1μm. Among the bunch are grooves with main impurity detected as carbon element, which means impurity depress the growth of crystal. A measurement of the size of the bunch showed that the width and height of the bunch go up with the decrease of undercooling in the flat rough region and the terrace rough region. And the width the height of the bunch fluctuate around a vaule in the region between terrace rough and flat rough region. The results above indicate the face (11 20) of sapphire crystal grow by the spreading of steps, that means it is a smooth face. This is consistent with the evaluation of phase transform entropy factor which is 2.03.