SiC Single Crystal Growth Technology
Under normal pressure, there is no liquid phase SiC with a stoichiometric ratio of Si
equal to 1:1. Therefore, the method using melt as the raw material, commonly used for silicon crystal growth, cannot be applied to bulk SiC crystal growth. Instead, the sublimation method (PVT, Physical Vapor Transport) is employed. In this process, SiC powder is used as the raw material, placed in a graphite crucible along with a SiC substrate as the seed crystal, and a temperature gradient is established with the SiC powder side being slightly hotter. The overall temperature is then maintained between 2000°C and 2500°C. The sublimation method using SiC seed crystals is now referred to as the modified Lely method, which is widely used for the production of SiC substrates.
Figure 1 shows a schematic diagram of SiC crystal growth using the modified Lely method. In a graphite crucible heated above 2000°C, the SiC powder sublimates into molecular states such as Si2C, SiC2, and Si, which are then transported to the surface of the seed crystal. The atoms supplied move across the seed crystal surface and are incorporated into positions where the crystal is forming, thereby growing bulk SiC single crystals. An inert atmosphere, typically low-pressure argon, is used, and nitrogen is introduced during n-type doping.
The sublimation method is currently widely used for the preparation of SiC single crystals. However, compared to the method using molten liquid as the raw material for the growth of Si single crystals, the growth rate is relatively slow. Although the quality is gradually improving, the crystals still contain many dislocations and other issues.
In addition to the sublimation method, attempts have also been made to prepare bulk SiC single crystals using methods such as liquid-phase growth through a solution or high-temperature chemical vapor deposition (CVD). Figure 2 shows a schematic diagram of the liquid-phase growth method for SiC single crystals.
First, regarding the liquid-phase growth method, the solubility of carbon in a silicon solvent is very low. Therefore, elements such as Ti and Cr are added to the solvent to increase the solubility of carbon. Carbon is supplied by a graphite crucible, and the SiC single crystal grows on the surface of the seed crystal at a slightly lower temperature. The growth temperature is typically set between 1500°C and 2000°C, which is lower than that of the sublimation method. It has been reported that the growth rate can reach several hundred micrometers per hour.
The advantage of the liquid-phase growth method for SiC is that, when growing crystals along the [0001] direction, dislocations extending in the [0001] direction can be bent to the vertical direction, sweeping them out of the crystal through the side walls. The screw dislocations extending along the [0001] direction are densely present in existing SiC crystals and are a source of leakage current in devices. The density of screw dislocations is significantly reduced in SiC crystals prepared using the liquid-phase growth method.
Challenges in solution growth include increasing the growth rate, extending the length of the grown crystals, and improving the surface morphology of the crystals.
High-temperature chemical vapor deposition (CVD) growth of SiC single crystals involves using SiH4 as the silicon source and C3H8 as the carbon source in a low-pressure hydrogen atmosphere, with growth occurring on the surface of an SiC substrate maintained at a high temperature (typically above 2000°C). The raw gases introduced into the growth furnace decompose into molecules such as SiC2 and Si2C in the hot-wall surrounded decomposition zone, and these are transported to the seed crystal surface, where single-crystal SiC is grown.
The advantages of the high-temperature CVD method include the ability to use high-purity raw gases, and by controlling the gas flow rate, the C/Si ratio in the gas phase can be precisely controlled, which is an important growth parameter that affects defect density. In bulk SiC growth, a relatively fast growth rate can be achieved, exceeding 1mm/h. On the other hand, the disadvantages of the high-temperature CVD method include the significant accumulation of reaction by-products inside the growth furnace and exhaust pipes, which adds a considerable maintenance burden on the equipment. Additionally, gas-phase reactions generate particles in the gas stream, which can become impurities in the crystal.
The high-temperature CVD method holds great potential as a method for producing high-quality bulk SiC crystals. Therefore, continuous development is underway to achieve lower costs, higher productivity, and lower dislocation density compared to the sublimation method.
Furthermore, the RAF (Repeated A-Face) method is reported as a sublimation-based technique that produces bulk SiC crystals with fewer defects. In the RAF method, a seed crystal cut perpendicular to the [0001] direction is taken from a crystal grown along the [0001] direction, and SiC single crystals are grown on it. Then, another seed crystal is cut perpendicular to this new growth direction, and further SiC single crystals are grown. By repeating this cycle, dislocations are swept out of the crystal, resulting in bulk SiC crystals with fewer defects. The dislocation density of SiC crystals prepared using the RAF method is reported to be 1 to 2 orders of magnitude lower than that of standard SiC crystals.
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A SiC wafer is a semiconductor material that has excellent electrical and thermal properties. It is a high-performance semiconductor that is ideal for a wide variety of applications. In addition to its high thermal resistance, it also features a very high level of hardness.
