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Sapphire Growth Method- Vertical Gradient Freeze Method (VGF)

Vertical gradient freeze method is similar to the level of zone melting, mainly in the way of moving the crucible, so that a temperature gradient within the molten metal, and then began to grow crystals. This method used by the heater is divided into two parts, the top of the furnace body heater temperature is higher, lower temperature below the temperature difference generated by the heater caused the temperature gradients, thus growing crystals. Because of the growth process, the temperature of the heater is constant, and the heater from the solid-liquid interface of the crystal growth when it is fixed, in which case you must make Bridgman, make molten metal through the solid-liquid interface, the use of VGF way, it solidified molten metal normal form a single crystal. Its growth principle is as the picture showing.

Vertical Gradient Freeze Method

The features of VGF method are as following:
1) The shape of the crystals may vary depending on the shape of a crucible for growing profiled crystals;
2) Plusing the seed crystal growth orientation can also be natural nucleation, based on the principles of geometry out of the crystal growth;
3) This method can be closed or semi-closed crucible to grow. Prevent evaporation melt dopant, resulting in compositional deviation and doping concentration, and can be on the surrounding environment pollution and hazardous substances;
4) It is suitable for large size, growing large number of crystals. A furnace can grow a few or dozens of different root, different sizes of crystals.
5) Operation process is relatively simple, easy to implement procedures and automation. VGF main drawbacks as follows:

a) The whole process of crystal growth is carried out inside the crucible, which is not easy to observe the growth because of the invisible material of tungsten. If the crucible is not made of tungsten, mo, or other metals, but glass, then it would be different.
b) Different crystals require specify physical and chemical properties of the crucible. Especially in matching the crucible and the thermal expansion coefficient of crystalline material to the right, while unsuitable for crystal growth of crystalline material volume expansion.
c) Tungsten crucible will stress and create parasitic crystal nucleation crystal during the sapphire growth. Therefore, the requirement of inner surface finish for crucible is very high.
d) When using Vertical Gradient Freeze method to grow sapphire crystal, the going down of crucible is not rotating, so that the quality of crystal is not better than the one grows out by grow out by Czochralski method.

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