I. Introduction to Monocrystalline Silicon
Monocrystalline silicon, as the fundamental material for the solar photovoltaic industry, is primarily produced using the Czochralski (CZ) method. This article introduces the basic principles and processes involved in growing monocrystalline silicon using the CZ method.
II. History of Monocrystalline Silicon Production via the CZ Method
As early as 1918, Jan Czochralski proposed the CZ crystal growth method. However, it wasn't until more than thirty years later, around 1952, that Teal Buehler and others successfully produced monocrystalline silicon using the CZ method. In the field of crystal growth, CZ monocrystalline furnaces have evolved from initially handling hundreds of grams of raw materials, having quartz crucible diameters of 40-60 millimeters, and crystal growth lengths of a few centimeters to over 3 meters, with crucible diameters exceeding 600 millimeters, monocrystal diameters exceeding 200 millimeters, and multiple crystal growth processes. Control over the monocrystal growth process has progressed from manual to fully automated, reducing manual intervention and improving product consistency.
With continuous improvements in monocrystal growth equipment, larger thermal field sizes, optimized thermal field structures, and upgraded automation control systems, the unit cost and energy consumption of CZ monocrystal growth have significantly decreased, and product quality has greatly improved. In silicon wafer production, cutting methods have transitioned from circular cutting to multi-wire cutting, with sand-sawed steel wire cutting being the first technique used for both monocrystalline and multicrystalline silicon. As the industry has developed, there has been a growing demand for higher efficiency, lower costs, and better surface quality in silicon wafer cutting. In recent years, the diamond wire multi-wire cutting technique has been developed to take advantage of the unique properties of monocrystalline silicon. This technique has significantly increased cutting efficiency and quality and has greatly expanded the cost-reduction potential of monocrystalline wafers.
III. Principles of Monocrystalline Silicon Growth via the CZ Method
The process of growing monocrystalline silicon via the CZ method involves the solid-liquid phase transition of a polycrystalline silicon melt into a monocrystalline silicon solid. Initially, polycrystalline silicon raw material is loaded into a quartz crucible, with a rotatable and vertically adjustable seed crystal rod positioned above it. The lower end of the rod has a clamp to hold a seed crystal. After the raw material is melted by a heater, the seed crystal is inserted into the surface of the high-temperature silicon melt, allowing the seed crystal to fuse with the silicon melt. In the appropriate thermal field environment, the seed crystal is slowly pulled upwards through processes such as necking, shoulder formation, shoulder rotation, and equal diameter growth, resulting in the growth of monocrystalline silicon.
IV. Preparation Process of Monocrystalline Silicon via the CZ Method
The preparation process of monocrystalline silicon via the CZ method generally includes loading polycrystalline silicon, melting, seeding, necking, shoulder formation, equal-diameter growth, and tailoring.
(1) Loading: Polycrystalline silicon raw material is placed in a quartz crucible, and care must be taken to ensure the cleanliness of both the raw material and the quartz crucible. It's also essential to prevent crucible breakage. Additionally, if specific doped monocrystalline silicon is needed, doping should be performed according to a predetermined plan during this step. For example, adding trace amounts of Group III elements such as boron or gallium can produce positively (P)-doped silicon monocrystals, while adding trace amounts of Group V elements like phosphorus or arsenic can create negatively (N)-doped silicon monocrystals.
The production process for P-type silicon and N-type silicon in the CZ method is almost the same, but due to the ease of ensuring uniformity with boron in silicon, P-type silicon preparation is relatively simple, and the technology is more mature. Currently, P-type cells produced on P-type silicon substrates dominate the market. However, N-type cells, known for their good low-light response, low-temperature coefficient, and low light-induced degradation, offer greater potential for efficiency improvement, making them a major focus of solar cell technology development.
(2) Evacuation and Inert Gas Filling: To prevent impurity contamination, the heating chamber must be evacuated and filled with protective gases such as argon or nitrogen during the CZ monocrystalline silicon production process.
(3) Heating and Melting: The quartz crucible is heated in a protective gas environment to melt the polycrystalline silicon within. During this process, it's important to prevent edge sag, bridge formation, and silicon jumping.
(4) Seeding: A rotating seed crystal, preheated to prevent excessive thermal stress when it contacts the molten raw material, makes contact with the molten raw material, initiating the growth of a larger silicon monocrystal through thermodynamic processes.
(5) Necking: In the CZ method for monocrystalline silicon production, the temperature difference between the seed crystal and the molten raw material drives crystal growth but can also cause thermal shock, resulting in dislocations within the crystal. The presence of a necking region effectively concentrates and eliminates dislocations, producing low-dislocation silicon monocrystals.
(6) Shoulder Formation and Shoulder Rotation: Once the crystal growth reaches the desired monocrystal diameter, shoulder formation, and rotation ensure that the monocrystal enters a stable phase of equal diameter growth.
(7) Equal Diameter Growth: Once the crystal is in a state of equal diameter growth, it can grow automatically without dislocations. The presence or absence of dislocations can be determined by observing growth striations and flat planes (facet and facet edges) on the outer surface of the ingot. If the crystal grows along the <111> crystal orientation, it exhibits three main facets and three secondary facets, and during equal diameter growth, it shows growth striations and three flat planes. If the crystal grows along the <100> crystal orientation, it has four facet edges, and during equal diameter growth, it displays four facet edges continuously.
(8) Tailoring: The monocrystal diameter is gradually reduced, ideally ending with a point to prevent dislocation reverse propagation. If dislocation reverse propagation occurs, the yield of monocrystals will be correspondingly reduced.
(9) Cooling and Monocrystal Retrieval: After cooling the crystal for 1 to 4 hours in a protective gas atmosphere, the monocrystal can be removed.
Note: The CZ method is a well-established process for producing high-quality monocrystalline silicon, which serves as a critical material in the solar photovoltaic industry. The process involves various steps, including loading, melting, seeding, necking, shoulder formation, equal diameter growth, and tailoring, and is characterized by its ability to control dislocations and impurities in the resulting monocrystals, leading to improved product quality.
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