1. Introduction to Silicon Carbide
Semiconductor materials have become a focal point of global high-tech competition and major-power rivalry. Among them, wide-bandgap semiconductor materials, represented by silicon carbide (SiC), have been applied in large-scale applications in key fields such as next-generation mobile communications, smart grids, high-speed rail transportation, new energy vehicles, and consumer electronics. Silicon carbide features a wide bandgap, high breakdown electric field, high thermal conductivity, high electron saturation velocity, and strong radiation resistance. It can break through the performance limits of silicon-based semiconductor devices and is regarded as the "CPU" of power electronics and microwave radio-frequency devices, as well as the "core chip" of the green economy [1].
Crystal Structure of Silicon Carbide
Silicon carbide is a IV-IV compound with unique physical and chemical properties. Si and C atoms form covalent bonds by sharing electron pairs in sp³ hybrid orbitals, resulting in a tetrahedral bonding structure to form SiC crystals. Silicon carbide has more than 200 polytypes, which are constructed by stacking Si-C bilayers in different sequences. Known polytypes include 2H-SiC, 3C-SiC, 4H-SiC, 6H-SiC, 15R-SiC, etc. The polytype designation is based on the number of Si-C bilayers per unit cell combined with the crystal system (C for cubic, H for hexagonal, R for rhombohedral). Different SiC polytypes have different electronic, optical, and mechanical properties, thus offering distinct advantages in various applications [2-4].
3C-SiC: 3C-SiC is the simplest polytype, with a cubic close-packed structure. Si and C atoms are arranged in a specific three-dimensional pattern, forming a highly symmetrical crystal. Due to its symmetry, 3C-SiC has relatively poor electronic and optical properties, so it has limited practical application.
4H-SiC and 6H-SiC: Unlike 3C-SiC, 4H-SiC and 6H-SiC have hexagonal close-packed structures. Both polytypes are superior to 3C-SiC in electron mobility, thermal conductivity, and bandgap. 4H-SiC has higher electron mobility, making it more ideal for high-frequency and high-power electronic devices and widely used in new energy vehicles, 5G communications, etc. 6H-SiC has a larger bandgap, making it perform better in high-temperature and high-radiation environments [4].
Overview of Domestic and International Development of Silicon Carbide
In the 1990s, overseas companies such as Cree and Westinghouse had already taken the lead in developing and producing 2–4 inch SiC substrates. Entering the 21st century, with continuous technological iteration, 6-inch SiC wafers gradually became the mainstream in the market. Driven by the explosive growth of the electric vehicle and new energy industries, global demand for SiC materials has continued to rise. Domestic and foreign enterprises and research institutions have increased R&D investment in large-size, high-quality SiC wafers. Currently, international manufacturers such as Wolfspeed, II-VI, and Rohm have achieved mass production of 8-inch SiC substrates. Under the impetus of China's "14th Five-Year Plan," domestic 8-inch SiC substrate production has also entered the industrialization stage, with companies such as TankeBlue, SICC, and Jingsheng Mechanical & Electrical successively announcing batch supply of 8-inch SiC substrates [5].
At present, 6-inch SiC remains the global commercial mainstream, while 8-inch products are accelerating toward industrialization. The concentrated breakthroughs in the 12-inch SiC field mark a critical turning point for the third-generation semiconductor industry. Domestic companies including SICC, Jingsheng Mechanical & Electrical, Jingyue Semiconductor, Tiancheng Semiconductor, Keyou Semiconductor, Hoshine Silicon, Nansha Jingyuan, GlobalWafers, Shuoke Crystal, as well as U.S.-based Wolfspeed, have all made major progress in the development of 12-inch SiC single crystals and substrates.
2. Methods for Preparing Silicon Carbide Single Crystals
Currently, the three main methods capable of producing high-quality SiC single crystals are the Physical Vapor Transport (PVT) method, the High-Temperature Chemical Vapor Deposition (HTCVD) method, and the Top-Seeded Solution Growth (TSSG) method.
Physical Vapor Transport (PVT) Method
In 1955, Lely first used the sublimation method to grow SiC single crystals. However, this method had many inherent drawbacks: extremely high growth temperatures, difficulty in precisely controlling nucleation, low growth efficiency, wide dispersion of electrical parameters in the resulting crystals, and generally small crystal sizes. As a result, the quality of SiC crystals produced by the Lely method was poor, while costs remained high. This technical challenge was not overcome until 1978, when Tairov and colleagues introduced a seed crystal into the growth process based on the traditional Lely method, enabling effective control over nucleation and reducing the growth temperature. This method became known as the modified Lely method, i.e., the Physical Vapor Transport method [6-9].
The PVT method features mature technology and strong controllability, and it is currently the primary method for producing high-quality, large-size SiC. In this method, SiC powder is placed at the bottom of a graphite crucible, and a SiC seed crystal is placed at the top. The graphite crucible is heated to the sublimation temperature of SiC, causing the powder to decompose into gaseous species such as Si vapor, Si₂C, and SiC₂. Driven by an axial temperature gradient, these species sublime to the top of the crucible and condense on the SiC seed surface, crystallizing into SiC single crystals [6]. This method can mass-produce large-size, high-purity SiC and is suitable for large-scale industrial production, but its disadvantages include slow growth rates and the need for substantial equipment, leading to high production costs.
For the PVT growth process, many factors affect SiC crystal growth, including temperature, temperature gradient, crucible pressure, distance between the seed and the polycrystalline powder, and powder purity.
1.1 SiC Powder Synthesis
SiC powder, as the raw material for synthesizing and growing single crystals, directly affects the quality and electrochemical properties of the grown crystals. Reducing compositional segregation during growth, lowering impurity content, and improving transport performance are important goals in powder preparation and pretreatment [8].
The SiC powder used for growing single crystals must meet very high purity requirements, with impurity content ideally below 0.001%. There are many methods for preparing SiC powder, which can be classified by the initial state of the raw materials into solid-phase, liquid-phase, and gas-phase methods. Solid-phase methods mainly include carbothermal reduction, mechanical crushing, and self-propagating high-temperature synthesis; liquid-phase methods include sol-gel and polymer decomposition; gas-phase methods include chemical vapor deposition and plasma methods. Among these, gas-phase methods can obtain high-purity SiC powder by controlling impurity levels in the gas sources; among liquid-phase methods, only the sol-gel method can produce powder meeting the purity requirements for single-crystal growth; among solid-phase methods, the improved self-propagating high-temperature synthesis method is currently the most widely used and mature process for SiC powder preparation [2,8].
1.2 Crucible
The internal structure of the crucible directly affects the size and quality of the grown SiC single crystals; the limited growth area inside the crucible is an important factor restricting diameter enlargement [7]. In addition, crucibles may introduce metal impurities due to material purity and machining factors. When such impurities are present, they cause uneven heating of the crucible, affecting the temperature field distribution inside the furnace and thus crystal quality. To avoid this, the crucible must undergo purification treatment [2].
1.3 Seed Crystal
For SiC crystals, different growth directions exhibit different growth rates. The choice and arrangement of the growth surface affect not only crystal quality but also crystal size. The morphology and structure of the seed surface directly affect the number of micropipes and defects in the crystal. To obtain large-size single crystals, a large seed crystal is a prerequisite. Seed attachment is generally achieved using adhesives. Typically, modified phenolic resin is dissolved in ethyl acetoacetate to form an adhesive, which is evenly applied to the back of the SiC seed and the lower surface of the crucible lid. Pressure is applied to the seed surface to expel bubbles and excess adhesive. The lid is then placed in an oven for heating and held for 2 hours, followed by carbonization of the adhesive in an argon atmosphere. Care must be taken to ensure uniform pressure during bonding to avoid cracking of the seed due to uneven stress [2].
1.4 Growth Parameters
The settings of temperature, pressure, temperature gradient, gas supply, and other parameters during SiC single-crystal growth directly affect the uniformity, crystallinity, and defect properties of the deposited crystals. Research and design of crystal growth parameters have always been a key focus. Crystal growth processes mainly involve controlling temperature and pressure. SiC crystal growth includes heat transfer, mass transfer, and chemical reactions. The temperature field distribution inside the furnace largely determines the quality and growth rate of the SiC single crystals. Accurate understanding of the temperature field and vapor transport processes is crucial for obtaining high-quality, large-size crystals [7].
Top-Seeded Solution Growth (TSSG) Method
The TSSG method, also known as the liquid-phase method, grows SiC single crystals at a slower rate, but the crystals produced have high structural perfection. The TSSG apparatus consists of an induction coil, graphite insulation, a graphite crucible, a Si melt, a SiC seed crystal, and a pulling rod. The graphite crucible is used because it is heat-resistant and corrosion-resistant, serving as a container for the Si melt, and it also supplies the carbon source for crystal growth. Silicon raw material and dopants are placed in the graphite crucible. Since the temperature at the crucible wall is high while that at the seed rod is low, carbon dissolves from the graphite crucible and combines with melted silicon at the seed, forming SiC crystals. Compared to the PVT method, the liquid-phase method offers advantages such as lower dislocation density, easier diameter expansion, and the ability to obtain p-type crystals. However, issues remain regarding impurity content control and selection of transition elements.
Because the essence of the TSSG method is the dissolution and recrystallization of carbon in the silicon melt, enhancing carbon solubility in the Si solution is key to successful SiC single-crystal growth. However, the solubility of carbon in silicon is extremely low, only about 13% even at the peritectic temperature of 3037 K. Moreover, silicon vaporizes directly above 2273 K, so other transition or rare-earth elements must be added to the Si solution to increase carbon solubility. Therefore, selecting an appropriate solvent is the most critical step for successful SiC crystal growth by the TSSG method. Another key research focus is controlling the kinetic processes during growth. Additionally, solvent inclusion and polytype control are important issues in solution growth [5,10].
High-Temperature Chemical Vapor Deposition (HTCVD) Method
The HTCVD reactor consists of a graphite crucible with a seed crystal placed at the top, external insulation, and induction heating. The raw materials are gaseous silicon- and carbon-containing compounds such as SiH₄, SiCl₄, C₃H₈, and C₂H₂. By controlling the temperature and pressure inside the reactor, SiC grows on the seed at temperatures of 2200°C–2500°C. Compared with the PVT method, the HTCVD method offers advantages such as high purity, convenient control of the C/Si ratio, and continuous supply of source materials. Its disadvantages are that both silicon and carbon sources come from gases, and the high growth temperature leads to high power consumption and production costs [2-3].
3. Development Trends of Silicon Carbide Single Crystals
Looking ahead, high-quality SiC single-crystal preparation technology will continue to upgrade in four major directions: large-size, high-quality, low-cost, and intelligent [11]:
Large-size: The diameter of SiC single crystals has expanded from early millimeter-scale to the current 6-inch, 8-inch, and even 12-inch and beyond. Large-size crystal preparation can significantly improve production efficiency, reduce unit manufacturing costs, and better meet the needs of high-power power electronic devices.
High-quality: High-quality SiC substrates are the core foundation for reliable high-performance devices. Although SiC single-crystal quality has improved greatly, crystal defects such as micropipes, dislocations, and impurities remain widespread, directly affecting device performance and long-term reliability. Future efforts will focus on continuous breakthroughs in defect control.
Low-cost: Currently, the relatively high cost of SiC single-crystal preparation limits its large-scale application in more scenarios. Future cost reductions can be achieved by optimizing crystal growth processes, improving yield and production efficiency, and reducing raw material and consumable costs throughout the industry chain.
Intelligent: Empowered by artificial intelligence, big data, and other technologies, the SiC crystal growth process will gradually become more intelligent. Through the deployment of in-line sensors and automated control systems, real-time monitoring and precise regulation of the entire growth process can be achieved, improving process stability and consistency. Combined with big-data analysis, growth parameters can be iteratively optimized to further enhance crystal quality and production efficiency.