Czochralski is the founder of Tirana, and his paper was published in 19 18. This is one of the most commonly used methods for melt growth. Many important practical crystals have been prepared by this method. In recent years, several major improvements have been made to this method, and some volatile compounds, such as GaP and lead-containing crystals, as well as crystals with special shapes, such as octagonal silicon tubes and funnel-shaped sapphire crystals, can be successfully grown.
Second, the principle and device of pulling method
The schematic device of the Czochralski method is shown in Figure 9-3- 1. Growth equipment includes crucible, melt, seed crystal, Czochralski system, heating furnace, temperature and atmosphere control system, etc. Putting the premixed raw materials into a crucible, heating to above the melting point of the raw materials, and melting the raw materials into a melt; A pull rod capable of rotating and lifting is arranged above the crucible, and a chuck for loading seed crystals is arranged at the lower end of the pull rod; Lower the pull rod to insert the seed crystal into the melt, so long as the temperature is appropriate, the seed crystal will not melt and grow, and then slowly pull up the crystal bar and rotate. At the same time, when the heating power is slowly reduced, the seed crystal will gradually thicken and grow ("necking" process), and the required crystal can be obtained by constantly and carefully adjusting the heating power. The whole growth device is placed in a sealed shell to ensure the atmosphere and pressure required for the growth environment. Crystal growth can be observed through the window of the cover.
Fig. 9-3- 1 Schematic diagram of gem growth by melt pulling method
The technical requirements of each part of the growth device are as follows: the crucible is required to have the characteristics of high temperature resistance, melt corrosion resistance, easy processing and no crystal pollution. Molybdenum crucibles are generally used to grow corundum and other precious stones, while iridium crucibles are used to grow YAG, GGG and chrysoberyl. For heaters, stable and reasonable thermal temperature field is required, and medium frequency furnaces and graphite or tungsten resistance furnaces are often used. The atmosphere control consists of two parts: one is that the furnace body has a water-cooled stainless steel shell for heat preservation; The other part is filled with inert gases such as chlorine, helium and hydrogen. Lifting and rotating machinery is a set of precise mechanical devices.
Third, advantages and disadvantages
The main advantages of this method are:
(1) It is convenient to observe the crystal growth during the growth process.
(2) The crystal has good growth integrity, short growth time, large size and low stress.
(3) The dislocation density of the crystal is obviously reduced by directional seed crystal and necking process, and the optical uniformity of the crystal is improved.
In a word, the quality of crystal grown by Czochralski method is much better than that by flame melting method, but for those materials with strong chemical activity or extremely high melting point, it is difficult to find a crucible that does not pollute the melt, which limits the application scope of Czochralski method.
In the process of using the pulling method, the application of three technologies makes this method even more icing on the cake. That is, ADC-crystal diameter braking control technology, LEC-liquid capping and EFG-single crystal furnace technology and mold guiding technology. The following will introduce the guided mode technology (EFG technology) in detail.
Fourth, an example of artificial gem-sapphire.
Raw material: colorless corundum chips doped with TiO _ 2+Fe2O3 prepared by flame melting method. The amount of TiO _ 2 and Fe2O3 depends on the color, and colored corundum sintered blocks can also be used.
Crucible: molybdenum
Heater: graphite
Equipment: vacuum argon-filled crystal pulling furnace, and the thermal temperature field meets the interface design requirements.
Conditions: above 2050℃; 10 ~ 15 rpm; The casting speed can be adjusted from 1 to10mm/hour.
Put the raw material into a crucible, heat it to 2060℃ to melt it, and lower the pull rod with oriented seed crystal on the upper part to make the seed crystal contact with the melt, and control the temperature slightly higher than the melting point. Then, slowly pull and rotate, and carefully reduce the power to make the crystal thicker. By adjusting the power, the whole growth process of inoculation, necking, shoulder equal-diameter growth and ending is realized. The grown crystals often need post-heat treatment to eliminate thermal stress, dislocation and other defects formed during crystal growth, and sometimes unnecessary color centers can be eliminated, thus improving the quality of crystals.
Five, guide mode method
The full name of the guided mode method is the edge-limited film feed Czochralski growth (EFG) technology, which is essentially a Czochralski method to control the crystal shape. It is obviously of practical significance to prepare crystals according to the required shape and size. To this end, people have conducted research for more than 60 years. Great progress was made in the 1960s. Since 1980s, the technology has been greatly improved in order to meet the demand of solar cells for large-size silicon wafers and to grow sapphire crystals with complex shapes for special optical applications. It can produce 1mm×84mm× 170mm monocrystalline silicon wafer and 2mm×35mm×200mm sapphire flake crystal.
The film guiding process is to put the inert film with high melting point into the melt of the pulling method equipment, as shown in Figure 9-3-2. The melt is attracted to the upper surface of the mold due to capillary action, and then solidified and crystallized with the pulling of the seed crystal, while the edge of the upper part of the mold controls the shape of the crystal. In this way, flaky, banded, tubular and fibrous crystals grow.
Figure 9-3-2 Schematic Diagram of Crystal Growth by Guided Mode Method