1. Field of the Invention
The present invention relates to a sapphire single crystal growing apparatus using the Kyropoulos method, and more particularly, to a Kyropoulos sapphire single crystal growing apparatus using an elliptic crucible, which can increase the recovery rate by the elliptic crucible and anisotropic heating.
2. Description of the Related Art
A sapphire single crystal substrate is an alternative substrate for gallium nitride (GaN), and is used as basic elements, including data storage devices, such as a blue or green light emitting diode (LED), a blue laser diode (LD), a DVD, etc., and various optical devices, such as a white light emitting device, a photo-detectors (PD), etc. In addition, a sapphire single crystal is an a-alumina single crystal, and is also used as biomaterial for artificial joints, artificial tooth, etc because causing no harm to human bodies.
However, since sapphire is an a-alumina single crystal having a rhombohedral structure, the sapphire has the optical and thermal anisotropy. Therefore, it is impossible to make an ingot from a sapphire polycrystal and, although possible, it is impossible to obtain a superior property, so that it is urgently required to develop a method of economically growing a sapphire single crystal.
Until now, the Czochralski method, the Edge-defined Film-fed Growth (EFG) method, the heat exchanging method, the Kyropoulos method, etc. have been known as the growth methods which make it possible to obtain such a sapphire single crystal.
The Czochralski method is called a rotation pulling method, which is to bring a seed crystal into contact with the surface of alumina solution and then to grow a single crystal by rotation pulling. Such a growth method makes it possible to produce a single crystal having a relatively high etch pit density (EPD), but limits the shape of the single crystal to a cylinder shape. Also, in the case of using the growth method, a vibration caused by the puller or shaking in the crucible may cause a defect in a crystal, and a high temperature gradient may cause a crack on a crystal when the crystal has a diameter of 5 cm or more.
The EFG method, which is intended to solve the defect of the Czochralski method, is to grow a single crystal with a desired shape in such a manner as to deposit a molybdenum die with the desired shape in alumina solution, to bring a seed crystal into contact with melted alumina rising to the surface of the die due to surface tension, and then to pull up. This method also does not make it possible to lower the defect density.
The heat exchanging method is to grow a single crystal in such a manner as to install a heat exchanger on the lower part of a growth furnace having a uniform temperature, and to precisely control the amount of flow of helium gas passing through the heat exchanger and the temperature of a heating element (e.g. heater). The growth method does not require a crucible to move a crystal itself, and thus it is possible to obtain a single crystal of good quality. However, since helium gas used as a heat exchange medium is high-priced, there is a fatal defect in that production costs are high.
The Kyropoulos method is similar to the Czochralski method, but does not rotate a single crystal and performs only a little pulling to grow the single crystal. This growth method has advantages in that a grown crystal has a few defects, the growth of a large-sized crystal is possible, and the equipment costs thereof are lower than those of the Czochralski method. Recently, it has been known that 60% of produced sapphire single crystals have been grown in the Kyropoulos method.
However, the Kyropoulos method has a disadvantage in that the recovery rate is low. That is to say, a sapphire single crystal substrate for deposition of GaN thin film generally requires a c-plane, while a sapphire crystal by the Kyropoulos method has a characteristic that the growth rate of a-plane is two or more times faster than the growth rate of c-plane. Therefore, there is an inconvenience in that an ingot grown into an a-plane has to be processed into a c-plane, which process is called “core drilling”.
In addition, since the Kyropoulos uses a cylindrical crucible, a grown crystal also has a cylindrical shape. Therefore, the cylindrical crystals are disadvantageous in terns of the cylinder recovery rate in comparison with rectangular block-type crystals. Moreover, the Kyropoulos method has a problem in that, when the diameter is raised, a crystal extension region is formed and the region is not used for good cylinders.