Overview of SiC Single Crystal Growth Technology

Overview of SiC Single Crystal Growth Technology

1. Introduction

Silicon carbide (SiC) single crystals have attracted widespread attention in recent years due to their superior performance in high-temperature, wear-resistant, and high-power electronic device applications. Among various preparation methods, the sublimation method (Physical Vapor Transport, PVT) is currently the primary method for growing SiC single crystals, though other potential growth techniques, such as liquid-phase growth and high-temperature chemical vapor deposition (CVD), also exist. This article will provide an overview of SiC single crystal growth methods, their advantages and challenges, and discuss the RAF method as an advanced technique for defect reduction.

2. Principles and Applications of the Sublimation Method

Since no stoichiometric liquid-phase SiC with a 1:1 Si-to-C ratio exists under normal pressure, the melt growth method commonly used for silicon single crystal growth cannot be directly applied to bulk SiC crystal production. Thus, the sublimation method has become the mainstream choice. This method uses SiC powder as the raw material, placed in a graphite crucible, and a SiC substrate as the seed crystal. A temperature gradient, slightly higher on the powder side, drives the material transport. The overall temperature is typically maintained between 2000°C and 2500°C.

Figure 1 shows a schematic of SiC single crystal growth using the modified Lely method. The SiC powder sublimates into molecular states, such as Si2C, SiC2, and Si, at temperatures above 2000°C inside a graphite crucible. These molecules are then transported to the surface of the seed crystal in an inert atmosphere (typically low-pressure argon). The atoms diffuse across the seed crystal surface and incorporate into growth sites, gradually growing the SiC single crystal. Nitrogen can be introduced during n-type doping.

3. Advantages and Challenges of the Sublimation Method

The sublimation method is currently widely used for preparing SiC single crystals. However, compared to the melt growth method for silicon single crystals, the growth rate of SiC crystals is relatively slow. While the quality is gradually improving, the crystals still contain a large number of dislocations and other defects. Through continuous optimization of the temperature gradient and material transport, some defects have been effectively controlled.

4. Liquid-Phase Growth Method

The liquid-phase growth method involves growing SiC through a solution. However, since the solubility of carbon in a silicon solvent is extremely low, elements such as titanium and chromium are typically added to the solvent to increase carbon solubility. Carbon is supplied by a graphite crucible, and the temperature on the surface of the seed crystal is relatively lower. The growth temperature is generally set between 1500°C and 2000°C, lower than that of the sublimation method. The growth rate of liquid-phase growth can reach several hundred micrometers per hour.

One major advantage of the liquid-phase growth method is its ability to significantly reduce the density of screw dislocations extending along the [0001] direction. These dislocations are densely present in existing SiC crystals and are a key source of leakage current in devices. By using the liquid-phase growth method, these screw dislocations are bent to the vertical direction and swept out of the crystal through the sidewalls, significantly lowering the dislocation density in SiC crystals.

The challenges of liquid-phase growth include increasing the growth rate, extending the length of the crystals, and improving the surface morphology of the crystals.

5. High-Temperature CVD Method

The high-temperature CVD method is another technique used for SiC single crystal production. This method is conducted in a low-pressure hydrogen atmosphere, with SiH4 and C3H8 serving as the silicon and carbon source gases, respectively. By maintaining the SiC substrate at temperatures above 2000°C, the source gases decompose into molecules such as SiC2 and Si2C in the hot-wall decomposition zone and are transported to the seed crystal surface, where they form a single-crystal layer.

The main advantages of the high-temperature CVD method include the use of high-purity raw gases and the precise control of the C/Si ratio in the gas phase by regulating the gas flow rate. This control is crucial for managing the defect density in the crystal. Additionally, the growth rate in bulk SiC can exceed 1mm per hour. However, one downside is the significant accumulation of reaction by-products in the growth furnace and exhaust pipes, which increases maintenance difficulty. Moreover, gas-phase reactions generate particles that can be incorporated into the crystal as impurities.

The high-temperature CVD method holds significant potential for producing high-quality bulk SiC crystals. Continuous development is ongoing to achieve lower costs, higher productivity, and reduced dislocation density compared to the sublimation method.

6. RAF Method: An Advanced Technique for Reducing Defects

The RAF (Repeated A-Face) method reduces defects in SiC crystals by repeatedly cutting seed crystals. In this technique, 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 gradually swept out of the crystal, resulting in bulk SiC crystals with significantly fewer defects. It has been reported that the dislocation density of SiC single crystals produced by the RAF method is 1 to 2 orders of magnitude lower than that of standard SiC crystals.

7. Conclusion

SiC single crystal preparation technology is evolving toward faster growth rates, reduced dislocation density, and higher productivity. The sublimation method, liquid-phase growth method, and high-temperature CVD method each have their advantages and challenges. With the application of new technologies such as the RAF method, the quality of SiC crystals continues to improve. In the future, with further optimization of processes and improvements in equipment, the technical bottlenecks in SiC crystal growth are expected to be overcome.

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