Conditions for Obtaining High-Quality Crystals by the Czochralski Method
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RYSTAL GROWTH
Conditions for Obtaining High-Quality Crystals by the Czochralski Method V. N. Matrosov Belarusian National Technical University, Minsk, 220107 Belarus e-mail: [email protected] Received May 11, 2017; revised October 23, 2017; accepted November 17, 2017
Abstract—The effect of two control contours on the shape of crystal‒melt interface has been examined. It is shown that the shape of crystal‒melt interface can be flexibly and efficiently governed using these contours with large and small time constants. The dependences of the shape of crystal‒melt interface on the fluctuations of cooling water temperature, growth room temperature, and input voltage have been established. The investigations made it possible to improve significantly the growth process parameters and obtain high-quality crystals. DOI: 10.1134/S1063774519010188
INTRODUCTION A crystal is formed at the interface between three phases (liquid, solid, and gaseous), which is called the crystal‒melt interface (CMI). This interface is an indicator of the temperature conditions settled in the growth crucible and above it. The most important property of CMI is its shape, which is specified by the kinetic and heat conditions and depends on the seed orientation and crystal structure. During crystal pulling, the CMI repeatedly changes its shape, being transformed from convex to plane and then concave, which leads to a change in the instantaneous crystallization rate and oscillations in the impurity distribution factor and refractive index of the medium. This may result in the formation of striations, which strongly distort the wave front in a laser element. In this case, the output laser spot is noncircular. and hot points occurring in the elements may cause optical breakdown. Therefore, it is very important to preserve the CMI shape during the entire growth process. This condition was a key one when designing a system for controlling the CMI shape. EXPERIMENTAL Researches have been tried to solve this problem for many years using different CMI control methods [1– 7]. Among them, noteworthy is the method of crystal diameter stabilization using a specified program of heater power variation in time. The power variation curve is plotted using the data obtained in a series of experiments. A drawback of this method is that it does not allow one to take into account the effect of different fluctuations that inevitably occur during crystal
growth. In the well-known system, the crystal diameter is controlled by tuning the crucible temperature with a special potentiometer according to a specified program. The efficiency of this method depends on the crystallizer quality and repeatability of all growth conditions from one experiment to another, which is very difficult to implement. Various optical techniques are also applied to control the crystal diameter, for example, using television systems. A significant drawback of all of them is the degradation of the interface visibility during the experiment because of the evaporation from the melt, which red
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