What is monocrystalline silicon made of?
Silicon is currently the most widely used solar cell material, including single crystal silicon (sc-Si) and polycrystal silicon (polycrystal silicon), with a total solar market share of more than 95%. In the early days, solar cells mainly used Czochralski pulling technique (CZochralski pulling technique, CZ) to grow silicon crystals, but due to market price factors, more and more companies invested in the growth of large-scale polysiliconingot (polysiliconingot).
1.1 Growth of single crystal silicon
The growth methods of monocrystalline silicon mainly include the Czochralski method and the floating zone technique (FZ). In 1950, Teal and Little first applied the Czochralski method to the growth of silicon (Si) and germanium (Ge) single crystals. At present, about 80% of silicon single crystals are grown by the Czochralski method. The single crystal size can reach 12in. In the Czochralski method, because the molten silicon is in direct contact with the crucible and produces a chemical reaction, the silicon single crystal has serious oxygen and carbon pollution problems. Keck and Golay proposed the floating zone method in 1953 to grow silicon single crystals without oxygen and low metal pollution, which are mainly used in high-power transistor devices. However, due to the high cost of the floating zone method, the Czochralski method is still the main method for growing solar-grade silicon single crystals.
1.1.1 Chai-style lifting method
The Czochralski pulling technique was accidentally invented by Professor Jan Czochralski in 1916, originally to study the crystallization rate of metals such as tin, zinc and lead in solid-liquid contact. After the Czochralski method was invented, it was gradually forgotten. Until the end of the Second World War, the semiconductor industry was booming, which made the importance of semiconductor materials such as silicon and germanium increase. In 1950, Teal and Little of Bell Laboratories first applied the Czochralski method to grow silicon and germanium crystals, and obtained high-quality single crystals. In 1958, Dash proposed the use of necking technique to grow silicon single crystals with low dislocation density. At present, the size of silicon ingots has increased from 1in in the 1950s to 12in today. Dr. Lin Mingxian from Taiwan Sino-German Electronics Co., Ltd. once edited the book “Silicon Wafer Semiconductor Material Technology”, which is about the technology of silicon single crystal growth. Including the Czochralski method and the floating zone method, the growth defects of silicon crystals and the processing technology are all introduced in detail. It is a good reference book for those who are engaged in semiconductor devices and solar cell materials. In addition, the book “Crystal Growth Science and Technology” edited by Zhang Kecong and Zhang Leping has a very detailed explanation of the theory of melt growth and the thermodynamics involved.
Figure 1.1 Czochralski pulling technique
Figure 1.1 is a schematic diagram of the Czochralski method. The growth process is briefly described as follows: First, the polysilicon raw material is placed in a quartz crucible, and the crucible is placed in a graphite thermal insulation field; vacuum is drawn from the crystal growth furnace and a certain pressure range is maintained Use graphite resistance heater to melt silicon raw material into liquid (melting point is 1420℃), adjust the temperature so that the center of molten silicon becomes the cooling point in the whole thermal field. When the temperature of the molten silicon stabilizes, the positioned (100) or (111) direction seed crystal (seed) is gradually lowered until it contacts the surface of the molten silicon. Due to the seed crystal itself and the thermal stress when the seed crystal contacts the molten silicon Dislocation (dislocation), at this time, the temperature must be slightly increased to melt part of the seed crystal. At the same time, the seed crystal is rotated on one side and pulled up quickly at the same time, using the crystal neck technology
(Dashing technique or necking technique), to pull out the seed crystal with a smaller diameter (3 ~ 6mm) than the original seed crystal and low defect density. As long as the crystal neck is long enough, dislocations can be smoothly discharged from the crystal surface. After the crystal neck process is over, the pulling speed and temperature need to be reduced to gradually increase the crystal diameter to the required diameter. This step is called shoulder growth or crown growth. After the shoulders are placed, the cylindrical body of equal diameter is grown by adjusting the pulling speed and temperature. The most important work in this part is the diameter control. Finally, when the crystal grows to an appropriate length, the temperature must be increased or the pulling speed must be increased to gradually reduce the diameter of the crystal rod until it is completely separated from the liquid surface. Then the ingot is cooled for a period of time and then taken out to complete a life cycle.
The preparation equipment of the Czochralski method can be roughly divided into four parts.
(1) Furnace body. The water-cooled stainless steel furnace body can generally be divided into an upper chamber and a lower chamber.The upper furnace chamber is where the silicon crystal rods are cooled, and the lower furnace chamber is the place where crystals grow.
(2) Hot field. Including quartz crucible, graphite crucible (supporting quartz crucible), graphite resistance heater and thermal insulation materials. The problem with the quartz crucible is that it will react with molten silicon at high temperatures, causing oxygen contamination of the silicon crystal rod; the graphite crucible is used to fix the quartz crucible to prevent its softening and deformation. The thermal field, also known as the thermal gradient, can generally be divided into external thermal gradient and internal thermal gradient, the after-heater in the furnace or the radiation shield ) Belongs to the external temperature gradient, while the temperature distribution in the crystal and molten soup belongs to the internal temperature gradient. Because the heating method is resistance heating, heat energy is provided to the quartz crucible and the surrounding insulation materials at the same time, the heating effect on the crucible is more uniform, and it is easy to produce a small temperature gradient, and the position of the crucible has little effect on the temperature gradient.
(3) Ar atmosphere and pressure control system. Because the quartz crucible reacts with molten silicon to produce silicon monoxide (SiO), the reaction of SiO and graphite devices will produce carbon monoxide (CO), and Ar is to take away both SiO and CO gases.
(4) Growth control system. The control parameters can include the diameter of the crystal rod, the pulling speed, the rotation speed and the temperature. Generally, the change in the shape of the crystal aperture (meniscus) is used to adjust the temperature or the pulling speed to control the diameter of the crystal rod. The crucible or silicon crystal rotates at the same time, and its purpose is to cause forced convection in the molten silicon, make the dopant concentration uniform, and make the temperature distribution in the furnace more symmetrical. Generally speaking, when making a crystal growth furnace, the furnace body itself will always have slight asymmetries. These asymmetries will cause excessive facet growth, striation, and difficult crystal growth. Controlling equal diameter growth and other shortcomings, rotating crystals and crucibles can effectively reduce these effects.
It is necessary to rely on the matching of the above four parts to grow silicon single crystals with low defect density.
In addition to defects such as dislocation, vacancy and stacking fault in silicon crystals grown by the Czochralski method, the most important defects are non-metallic impurities such as oxygen and carbon. Pollution. The quartz crucible reacts with molten silicon to form SiO2, which will affect the resistance value, conversion efficiency and carrier lifetime of silicon. But when the oxygen concentration reaches a certain level, it will enhance the mechanical strength of the silicon crystal. Because the Si-O bond in SiO2 vibrates and has a strong absorption at the infrared wavelength of 900nm, the oxygen content in the silicon crystal can be measured with an infrared spectrometer. The carbon in the silicon crystal is reacted by SiO and graphite heat insulation material to generate CO and merge into the molten silicon. The carbon content accelerates the deposition of oxygen, which in turn causes microscopic defects. In order to reduce carbon pollution, the gap between the quartz crucible and the graphite can be minimized to make the CO concentration at the contact point higher and prevent the quartz crucible and graphite from continuing to react. In addition, the flow rate of Ar gas can be adjusted to take away the generated CO gas.
The advantage of the Czochralski method is that it is easier to grow large-size and high-doped silicon single crystals, but for the application of solar cells, the most important factor is the price. The price of a single chip can be reduced to the minimum by increasing the crystal size, continuous feeding, and improving the cutting, grinding, and polishing process to minimize the loss of silicon raw materials, so that it is possible to reduce the price of silicon chips.
1.1.2 Floating zone method
The floating zone method was proposed by Keck and Golay in 1953, and Theuerer applied it to the growth of high-purity silicon single crystals. The biggest advantage of the FZ method is that no crucible is needed. In the CZ pulling method, molten silicon inevitably comes into contact with the crucible and reacts. Therefore, only a few substances can be used as crucible materials, such as quartz (SiO2), Si3N4, silicon carbide (SiC), graphite (graphite), and so on. Even so, the carbon, oxygen and other metal impurities in the crucible will still flow into the molten silicon, causing pollution to the silicon single crystal.
Figure 1.2 Floating zone method
The growth device of the floating zone method is shown in Figure 1.2. The polycrystalline silicon raw material rod is fixed above the high frequency coil (RFcoils), and the silicon single crystal seed crystal is placed under the polycrystalline silicon raw material rod. When the polysilicon raw material rod is heated by the coil, the bottom will begin to melt. At this time, the raw material rod is lowered, so that the molten area is attached to the seed crystal, forming a solid-liquid phase equilibrium, and the molten area is supported by surface tension. Then, the seed crystal and the raw material rod are rotated in opposite directions to make the distribution of impurities in the molten zone uniform. The melting zone moves from top to bottom, or from bottom to top, so that the polysilicon raw material rod can completely pass through the heating coil to complete the crystallization process. In the floating zone method, the stability of the melt zone is maintained by the balance of surface tension and gravity. The advantage of the floating zone method is that it can grow high-purity and defect-free silicon single crystals. The content of oxygen, carbon and other transition metals can be less than 1011cm-3, and its resistance can easily reach 300Ω·m, which is suitable for high-efficiency solar materials. Although the defect density of the silicon crystal grown by the floating zone method is low, its lifetime is only 0.5ms, which is still far below the theoretical value. The main reason is that the high-purity silicon crystal has many microscopic defects caused by the growth and cooling process. Another disadvantage of the floating zone method is that because the melting zone is only at the top of the crystal rod, only crystals with a smaller diameter can be grown.