Introduction to the first author: Sun Guangnian, the first and second members of the Artificial Products Professional Committee of China Insurance Association, the third vice chairman, and the director of Zhejiang Juhua Group Crystal Material Factory.

I. Introduction

Material science is one of the three pillars of modern civilization (energy, information and materials) and the material foundation of human civilization. Crystal growth belongs to the category of material science and is the frontier of its development. Facts have proved that the development of some high-tech science and technology is closely related to crystal materials; The development of military industry, such as the window materials of missiles, unmanned aerial vehicles, submarines, artificial satellites and spacecraft, requires high-quality crystal growth materials. The quality of these materials determines the level of technology. Only by making a breakthrough in materials can we hope to make a breakthrough in related technologies. High-quality colorless sapphire is widely used because of its special excellent properties. For example, sapphire single crystal has unique and excellent physical and chemical properties, especially in the 0.2 ~ 5.0 micron band, which can be widely used in infrared military equipment, satellites and space technology. Sapphire crystal has become one of the most widely used substrate materials because of its dielectric insulation and constant dielectric constant. To this end, all countries in the world are trying their best to carry out research and production. After many years' efforts, Crystal Material Factory of Zhejiang Juhua Group Co., Ltd. has produced high-quality colorless sapphire by using the fusion technology of melt kyropoulos method and melt pulling method. The product has a diameter of more than 220mm and a weight of more than 28kg. It can be used not only as a window material in the military field, but also as a substrate material and LED energy-saving and environmental protection industry, with unlimited potential and development prospects.

2. Brief introduction of high quality sapphire crystal growth technology

Sapphire is a corundum gem. Except ruby, other corundum gems are called sapphires. Colorless sapphire is a kind of sapphire, and its chemical composition is aluminum oxide (Al2O3), which has a tripartite crystal system. The polished surface has bright glass luster to diamond luster, and the uniaxial crystal has negative luster. Refractive index 1.762 ~ 1.770, birefringence 0.008 ~ 0.0 10, Mohs hardness 9.

The synthesis methods of sapphire mainly include flame melting method, flux method and melting method, and the melting method includes several methods. However, neither the flame melting method nor the flux method can grow high-quality sapphire crystals. The reasons are as follows: sapphire crystal grown by flame melting method has many mosaic structures and poor quality; The gem crystals grown by flux method are also relatively small, containing flux cations, and the quality is not very good. Only when the gem crystal grown by melt method has the characteristics of high purity, good integrity and large single crystal can it become a high-quality gem crystal urgently needed by modern high-tech and national defense industry. The principle of growing gems by melt method is that the raw materials that make up gems are heated and melted in a high-temperature crucible, and then the melt is supercooled by cooling under controlled conditions, thus growing crystals. Due to the different control conditions of cooling, the methods of growing high-quality gem crystals from melt are slightly different. At present, there are four main techniques for growing high-quality sapphire crystals by melt method in the world: ① Czochralski melt crystal pulling method; (2) EFG (Edge-Confined Film Feed Growth) melt-oriented die method; ③HEM (heat exchanger method) melting heat exchange method; ④ kyropoulos method kyropoulos method melt. The following briefly introduces the technical characteristics, advantages and disadvantages of the above four methods for growing gem crystals by melt.

1. melting crystal pulling method

Melt pulling method (referred to as melt pulling method) is a method of pulling and growing crystals from melt by using seed crystals. This method can grow high-quality single crystals without dislocation in a short time. It was first invented by J.Czochalski in 19 17, so it is also called Czochralski, which is one of the most commonly used methods to grow crystals in melt. Its main technical feature is that all crystals grown by Czochralski method have the same basis, which is briefly described as follows: putting raw materials of gem components into a crucible, heating to above the melting point of the raw materials, and then melting the raw materials in the crucible into a melt; There is a rotatable lifting pull rod above the crucible, and a chuck is arranged at the lower end of the pull rod, and the seed crystal is installed on the chuck. Lower the pull rod to insert the seed crystal into the melt, adjust the temperature to make the seed crystal neither melt nor grow, and then slowly pull the seed crystal rod and rotate it. At the same time, the heating power is slowly reduced, and the crystal with the required diameter is obtained through the whole growth process of necking, shoulder expanding, equal-diameter growth and ending. The whole growth device is placed in a closed jacket, so that the growth environment has the required atmosphere and pressure; Growth can be observed through the window of the outer cover. Its growth principle is shown in figure 1. The main advantages of this method are: ① it is convenient to observe the crystal growth during the growth process; (2) The crystal grows on the surface of the melt and does not contact with the crucible, which can significantly reduce the crystal stress and prevent the crucible wall from nucleation; ③ Directional seed crystal and necking process can be used conveniently. The dislocation of the seed crystal after necking can be greatly reduced, so that the dislocation density of the crystal grown after shoulder expanding can be reduced and the crystal with high integrity can be obtained. The main disadvantage of this method is that the crystal is relatively small, and the maximum diameter can reach 2 ~ 3 inches 1 inch = 25.4 mm.

, can not meet the requirements of modern high-tech and national defense industry for large-size crystals. The other three methods are better than Czochralski method in growing large-size crystals.

In 1970s, due to the need of laser material research, China began to develop the crystal growth technology of artificial yttrium aluminum garnet (YAG) and artificial gadolinium gallium garnet (GGG) by melt pulling method. Due to the development of military industry and the need of cutting-edge scientific research and application, the growth technology of gem crystals by melt Czochralski method has been further developed and perfected, and now many practical gem crystals (Figure 2 and Figure 3) can be successfully grown, such as synthetic colorless blue.

Figure 1 Schematic diagram of crystal growth by melt pulling method

Fig. 2 Sapphire crystal grown by melt pulling method

Fig. 3 Laser crystal grown by melt pulling method

2. Melt guiding method

Melt-guided mold method is an improved melt-pulling method, which can control the crystal shape. Its main technical feature is that an inert mold with high melting point is placed in the melt, the upper surface of the mold has a "pattern" with the required shape, and the lower part of the mold has a thin tube that goes straight through the top of the mold. The melt is attracted to the upper surface of the mold due to capillary action. After contacting with the seed crystal, when the melt is higher than the surface of the mold, it can automatically expand to the edge of the "pattern" and grow a crystal with the shape of the top of the mold in the subsequent pulling. Its main advantage is that it can grow crystals of various shapes according to our requirements. Saint-Gobain can use this technology to grow sapphire optical wafers with a diameter of 450mm to 500mm, while Kyocera Corporation of Japan can use the improved technology to grow C-plane wafers for LED substrates, and has a patent for this technology. Its principle is shown in Figure 4. The equipment and technology for growing crystals by this method are difficult and difficult to popularize.

Fig. 4 schematic diagram of crystal growth by melt guided mode method

Fig. 5 350 mm sapphire crystal grown by melt heat exchange method.

3. Melting heat exchange method

The essence of melt heat exchange method is to control the temperature and let the melt solidify and crystallize directly in the crucible. Its main technical characteristics are: temperature gradient furnace, a heat exchanger made of tungsten and molybdenum is installed at the bottom of vacuum graphite resistance furnace, and cooling helium flows through it. The crucible filled with raw materials is placed at the top of the heat exchanger, the centers of the heat exchangers coincide with each other, and the seed crystal is placed at the center of the bottom of the crucible. When the raw materials in the crucible are heated and melted, helium flows through the heat exchanger for cooling, so that the seed crystal does not melt. Subsequently, the flow rate of helium gas is increased to take away more heat of the melt, so that the seed crystal grows gradually, and finally the melt in the whole crucible solidifies. The main advantages of this method are: when the crystal grows, the crucible, the crystal and the heating zone do not move, thus eliminating the crystal defects caused by mechanical movement; At the same time, the cooling rate can be controlled to reduce the thermal stress of the crystal and the resulting defects such as crystal cracking and dislocation, which is a good method to grow high-quality large crystals. However, this method has not been widely used because of its high equipment conditions, complex whole process and high operating cost. This process is a patented technology of crystal system, which mainly provides fairing for the US military. Sapphire crystals with a diameter of 350mm have been grown (Figure 5).

Step 4 Melt kyropoulos method

The molten kyropoulos method was invented by kyropoulos method in 1926. After decades of continuous improvement and perfection by researchers, it is one of the methods to solve the problem that the melt crystal pulling method can not expand the diameter crystal. The principle of crystal growth is shown in Figure 6, and its main technical features are as follows: put the crystal raw material to be grown into a high-temperature crucible for heating and melting, and then adjust the temperature field in the furnace to make the upper part of the melt slightly higher than the melting point; The seed crystal is placed on the seed crystal rod so that the seed crystal contacts the surface of the molten liquid. When the surface of the seed crystal melts slightly, the surface temperature is lowered to the melting point, and the seed crystal rod is pulled and rotated, so that the top of the melt is supercooled and crystallized on the seed crystal. In the process of continuous pulling, columnar crystals grow. The crystal does not contact the crucible during the growth process or at the end of the growth, which greatly reduces the stress of the crystal and can obtain high-quality large-diameter crystals. Different from the melt crystal pulling method, the diameter of the crystal is relatively large when expanding the shoulder, which is almost the same as that of the crucible (compare figure 1 and figure 6), and the crystal does not contact with the crucible. These are the technical characteristics and difficulties of melt kyropoulos method. Crystal Material Factory of Zhejiang Juhua Group Co., Ltd. has developed this high-quality sapphire growth technology by combining melt kyropoulos method with melt Czochralski method. The main product is to grow colorless high-quality sapphire crystals. Some people also call this method "molten bubble raw pulling method" At present, high-quality colorless sapphire crystals with a diameter of more than 220mm and a weight of more than 28kg have been grown, and the products are shown in Figure 6.

Kyropoulos method crystal growth in melt and schematic diagram of its products.

Thirdly, the process of growing high-quality colorless sapphire crystal by melt kyropoulos method.

1) Put the pure α-Al2O3 raw material into the crucible. A pull rod capable of rotating and lifting is arranged above the crucible. There is a seed crystal fixture at the lower end of the rod, which is equipped with directional colorless sapphire seed crystal (note: colorless sapphire should be used as seed crystal when growing colorless sapphire, without colorant, and colorless sapphire crystal is more useful than colored sapphire crystal).

2) heating the crucible to above 2050℃, lowering the pull rod, and inserting the seed crystal into the melt.

3) Control the temperature of the melt so that the liquid surface temperature is slightly higher than the melting point, and melt a small amount of seed crystals to ensure that they can start to grow on the clean surface of seed crystals.

4) After the seed crystal and the melt are fully wetted, the liquid surface temperature reaches the melting point, and the seed crystal rod is slowly pulled upward and rotated. When the casting speed and rotation speed are well controlled, the seed crystal will grow gradually.

5) Carefully adjust the heating power to make the liquid surface temperature equal to the melting point, so as to realize the whole growth process of gem crystals from necking-shoulder expanding-equal-diameter growth.

The whole growth device is placed in the outer cover, which can be filled with inert gas after vacuum pumping, so as to keep the required gas and pressure in the growth environment. The crystal growth can be observed through the window on the cover, so as to adjust the temperature at any time and make the crystal growth process normal. In this way, colorless sapphire crystals with large diameter and high quality can be grown.

Technical points of growing high quality sapphire by melt kyropoulos method.

Sapphire belongs to cubic crystal system, and there are two main slip systems in crystal structure (bottom slip system and column slip system). Therefore, in the process of growing sapphire single crystal by Czochralski method, the temperature gradient of temperature field and the reasonable choice of crystal growth direction will have a key impact on the quality of sapphire single crystal.

1. Establishing a reasonable temperature gradient is the first condition for growing high-quality crystals.

Thermal regime is the decisive factor of temperature gradient and the basic condition for growing high-quality crystals. When the crystal grows at constant temperature, according to the interface stability condition:

Man-made products, China

but

Man-made products, China

So there is

Man-made products, China

Therefore, the maximum growth rate of interface stability is

Man-made products, China

Where: and are the temperature gradients of melt and crystal near the interface, respectively; K 1 and Ks are the thermal conductivities of melt and crystal, respectively; L is the latent heat of crystallization; ρ is the crystal density.

It can be seen from Formula (3) that the maximum growth rate of the crystal depends on the size of the temperature gradient in the crystal, and it is necessary to increase the temperature gradient to improve the growth rate of the crystal. However, the temperature gradient in the crystal is too large, which will increase the thermal stress of the crystal, cause the dislocation density to increase, and even lead to crystal cracking.

Therefore, according to the thermal conductivity and other properties of colorless sapphire single crystal, establishing a reasonable temperature gradient is the premise of growing a complete single crystal.

2. The choice of crystal growth direction is very important.

Colorless sapphire belongs to cubic crystal system, and there are two main slip systems: (000 1) bottom slip system along the direction and cylindrical slip system along the direction. Slip is most likely to occur along the crystal direction with high atomic density, so when the angle between the crystal growth interface and the (000 1) plane is large, a large number of grain boundaries are easily generated due to bottom slip; When the slip is serious, it may produce slip zone and form twins; On the contrary, it is not easy to slip and grain boundaries are not easy to form.

When growing along the 0 orientation (000 1), although the symmetrical cross section of the crystal shape is easy to be hexagonal, defects will preferentially proliferate in the optical axis direction, which is easy to form mosaic structure and destroy the integrity of the crystal structure.

It can be seen that it is necessary to choose a suitable crystal growth direction. According to many experiments, we found the crystal growth direction for growing high-quality colorless sapphire. We believe that according to the given temperature gradient, choosing the appropriate crystal growth direction is the key to grow high-quality colorless sapphire single crystal.

Verb (abbreviation of verb) The application of melting kyropoulos method to grow high-quality colorless sapphire.

High-quality sapphire grown in melt kyropoulos method is usually used in the fields of national defense industry, military science and technology and cutting-edge science and technology research, and its scraps or unqualified raw materials can be used in jewelry industry. High-quality sapphire grown by melt kyropoulos method is widely used in the fields of national defense industry, military science and technology and cutting-edge science and technology research, which is determined by the excellent performance of colorless sapphire crystal itself. See table 1 for some performance parameters of colorless sapphire single crystal.

Table 1 Properties of Colorless Sapphire Single Crystal

1. application of high quality colorless sapphire crystal in substrate

High-quality colorless sapphire crystal has become the first choice for some substrate applications because of its excellent properties. Mainly in the following aspects:

1) blue LED LED substrate (BLED' S)- growing III-V and II-VI compounds on colorless sapphire substrate;

2) infrared detector-colorless sapphire crystal can be used as the substrate for growing HgCdTe crystal;

3) Substrate (GaAs) of gallium arsenide wafer;

4) Microwave integrated circuit materials.

On the one hand, in the application of microelectronic integrated circuits, R-plane colorless sapphire substrate is the first choice for heteroepitaxial silicon. Colorless sapphire single crystal is especially suitable for microwave and high-speed integrated circuits and pressure sensing because of its high and stable dielectric constant. On the other hand, mixed superconducting compounds such as thallium can be epitaxially grown on colorless sapphire single crystals to make high-resistance devices, and can also be used to grow GaAs or be used as carriers of other materials.

In addition, A-plane-crystal plane colorless sapphire single crystal substrate: Because colorless sapphire single crystal has stable dielectric constant and high insulation, it can be used as a carrier of high temperature superconducting materials.

Furthermore, single or double-sided polishing of colorless sapphire single crystal substrate with C-{000 1} crystal plane is widely used for epitaxial growth of III-V and II-VI compounds, such as GaN substrate for blue LED (white LED is made by using phosphor effect on the basis of blue LED). Secondly, it is also used as a carrier for making mercury cadmium antimony compound devices for infrared detection.

2. Application of high-quality colorless sapphire in the field of light-emitting diodes.

LED has a wide range of applications, including communication, consumer electronics, automobiles, lighting, signal lights and so on. We can be roughly divided into five areas: backlight, lighting, electronic equipment, display screen and automobile. Today, when the earth's resources are exhausted, environmental protection and energy conservation are the focus of the development of various industries, especially the lighting industry, which needs to consume a lot of electricity. In the research and development of light sources, it tends to be environmentally friendly and energy-saving. The appearance of light-emitting diode (LED) is a revolution in the field of human lighting, which has a great influence and change on the future of mankind. In addition to low energy consumption and long service life, LED has the following advantages:

1) the application is very flexible: it can be made into light, thin and short products with various shapes of points, lines and surfaces;

2) Good environmental protection benefit: because there is no ultraviolet ray and infrared ray, there is no heat and radiation in the spectrum, which belongs to a typical green lighting source, and the waste can be recycled without pollution;

3) The control is very convenient: as long as the current is adjusted, the light can be adjusted at will, and there are various combinations of different light colors. By using the time sequence control circuit, colorful dynamic change effects can be realized.

LED can be used not only for large advertising display screens, but also for building and traffic lighting. The appearance of white LED is a substantial step for LED from identification function to illumination function. White LED is closest to sunlight, which can better reflect the true color of the illuminated object. Therefore, from a technical point of view, white LED is undoubtedly the most advanced technology of LED.

White LED will be widely used in the market, and it is also the "killer" of incandescent tungsten bulbs and fluorescent lamps. At present, white LED has entered some practical applications, such as emergency lights, flashlights, flashlights and other products have come out one after another.

According to the forecast of the US Department of Energy, about 55% of incandescent lamps and fluorescent lamps in the United States will be replaced by white LEDs around 20 10, and the annual energy saving value can reach 35 billion US dollars, which can form a large industry of 50 billion US dollars. Japan proposes that white LED will replace traditional incandescent lamps on a large scale around 2008. In order to seize the commanding heights of the future market, three global lighting giants, such as General Electric, Philips, and Osram, have taken actions to set up LED lighting enterprises in cooperation with semiconductor companies, with the goal of increasing the luminous efficiency of LEDs by 8 times and reducing the price by 99% by 20 10. What an attractive prospect!

3. Application of high quality colorless sapphire in GaN epitaxial substrate materials.

High-quality colorless sapphire crystal is the only commercialized GaN epitaxial substrate material at present, and the new achievements of semiconductor luminescence technology have opened up new prospects for the application of high-quality colorless sapphire crystal. Blue light emitting diodes can be fabricated by epitaxial GaN on colorless sapphire crystal substrates.

At present, the important uses and prospects of LED are gradually recognized by people. With the rapid development of LED industrialization, sapphire crystal with large size and high quality will become the new favorite of the market.

Conclusion of intransitive verbs

We have grown high-quality colorless sapphire with large diameter by melt kyropoulos method and melt pulling method. In order to obtain high-quality colorless sapphire single crystal, a growth process with reasonable temperature gradient and crystal growth direction was established based on the characteristics of the crystal itself. High-quality colorless sapphire single crystal grown by melt kyropoulos method has a wide range of applications, which can be used in the fields of national defense industry, military science and technology and cutting-edge science and technology research, especially in the fields of substrates and light-emitting diodes (LED), showing excellent development prospects.

refer to

He and Shen. 2005. Gemstone synthesis technology. Beijing: Chemical Industry Press.

Ma Shengli et al. 1998. Observation of defects in sapphire single crystal grown by guided mode method. Journal of Inorganic Materials, 13( 1).

Wang. 1983. sapphire single crystal. Tianjin: Tianjin Science and Technology Press.

Yu Xudong et al. 2006. Study on the growth direction of sapphire crystal. Journal of Synthetic Crystals, Volume 35, No.2..

Cong et al. 1997. Crystal growth science and technology. Beijing: Science Press.

Zhang Yulong et al., 2005. Intraocular lens-growth technology, performance and application. Beijing: Chemical Industry Press.

Zhang beili 2006. Systematic Gemmology (2nd Edition). Beijing: Geological Publishing House.

Patent literature. Japan yamaki precision gem co., ltd P2003-3 13092A.

LED lighting has outstanding advantages and wide applications. Communication information news, February 2006-10.

Publication number: US 2005/0227117a1,USA, and patent application is published.

Schmid F. et al. J.Amer. 1970. Selma Football Club, 53,528 people

Xie Haijie, Fu Tiantai, 2003. Development of Crystal Growth Technology, Page 3~ 14.