Three SiC single crystal growth technologies

The main methods for growing SiC single crystals are: physical vapor transport (PVT), high temperature chemical vapor deposition (HTCVD) and high temperature solution growth (HTSG). As shown in Figure 1. Among them, the PVT method is the most mature and widely used method at this stage. At present, the 6-inch single crystal substrate has been industrialized, and the 8-inch single crystal has also been successfully grown by Cree in the United States in 2016. However, this method has limitations such as high defect density, low yield, difficult diameter expansion, and high cost.

The HTCVD method uses the principle that Si source and C source gas react chemically to generate SiC in a high temperature environment of about 2100 ℃ to achieve the growth of SiC single crystals. Like the PVT method, this method also requires a high growth temperature and has a high growth cost. The HTSG method is different from the above two methods. Its basic principle is to use the dissolution and reprecipitation of Si and C elements in a high temperature solution to achieve the growth of SiC single crystals. The currently widely used technical model is the TSSG method.

This method can achieve the growth of SiC in a near-thermodynamic equilibrium state at a lower temperature (below 2000 °C), and the grown crystals have the advantages of high quality, low cost, easy diameter expansion, and easy stable p-type doping. It is expected to become a method for preparing larger, higher-quality and lower-cost SiC single crystals after the PVT method.

Figure 1. Schematic diagram of the principles of three SiC single crystal growth technologies

01 Development History and Current Status of TSSG-grown SiC Single Crystals

The HTSG method for growing SiC has a history of more than 60 years.

In 1961, Halden et al. first obtained SiC single crystals from a high-temperature Si melt in which C was dissolved, and then explored the growth of SiC single crystals from a high-temperature solution composed of Si+X (where X is one or more of the elements Fe, Cr, Sc, Tb, Pr, etc.).

In 1999, Hofmann et al. from the University of Erlangen in Germany used pure Si as a self-flux and used the high-temperature and high-pressure TSSG method to grow SiC single crystals with a diameter of 1.4 inches and a thickness of about 1 mm for the first time.

In 2000, they further optimized the process and grew SiC crystals with a diameter of 20-30 mm and a thickness of up to 20 mm using pure Si as a self-flux in a high-pressure Ar atmosphere of 100-200 bar at 1900-2400 °C.

Since then, researchers in Japan, South Korea, France, China and other countries have successively carried out research on the growth of SiC single crystal substrates by the TSSG method, which has made the TSSG method develop rapidly in recent years. Among them, Japan is represented by Sumitomo Metal and Toyota. Table 1 and Figure 2 show the research progress of Sumitomo Metal in the growth of SiC single crystals, and Table 2 and Figure 3 show the main research process and representative results of Toyota.

This research team began to carry out research on the growth of SiC crystals by the TSSG method in 2016, and successfully obtained a 2-inch 4H-SiC crystal with a thickness of 10 mm. Recently, the team has successfully grown a 4-inch 4H-SiC crystal, as shown in Figure 4.

Figure 2. Optical photo of SiC crystal grown by Sumitomo Metal's team using the TSSG method

Figure 3. Representative achievements of Toyota's team in growing SiC single crystals using the TSSG method

Figure 4.  Representative achievements of the Institute of Physics, Chinese Academy of Sciences, in growing SiC single crystals using the TSSG method

02 Basic principles of growing SiC single crystals by TSSG method

SiC has no melting point at normal pressure. When the temperature reaches above 2000 ℃, it will directly gasify and decompose. Therefore, it is not feasible to grow SiC single crystals by slowly cooling and solidifying SiC melt of the same composition, that is, melt method.

According to the Si-C binary phase diagram, there is a two-phase region of "L+SiC" at the Si-rich end, which provides the possibility for the liquid phase growth of SiC. However, the solubility of pure Si for C is too low, so it is necessary to add flux to the Si melt to assist in increasing the C concentration in the high-temperature solution. At present, the mainstream technical mode for growing SiC single crystals by HTSG method is TSSG method. Figure 5 (a) is a schematic diagram of the principle of growing SiC single crystals by TSSG method.

Among them, the regulation of the thermodynamic properties of high-temperature solution and the dynamics of solute transport process and crystal growth interface to achieve a good dynamic balance of supply and demand of solute C in the entire growth system is the key to better realize the growth of SiC single crystals by TSSG method.

Figure 5. (a) Schematic diagram of SiC single crystal growth by TSSG method; (b) Schematic diagram of the longitudinal section of the L+SiC two-phase region

03 Thermodynamic properties of high-temperature solutions

Dissolving enough C into high-temperature solutions is the key to growing SiC single crystals by the TSSG method. Adding flux elements is an effective way to increase the solubility of C in high-temperature solutions.

At the same time, the addition of flux elements will also regulate the density, viscosity, surface tension, freezing point and other thermodynamic parameters of high-temperature solutions that are closely related to crystal growth, thereby directly affecting the thermodynamic and kinetic processes in crystal growth. Therefore, the selection of flux elements is the most critical step in achieving the TSSG method for growing SiC single crystals and is the research focus in this field.

There are many binary high-temperature solution systems reported in the literature, including Li-Si, Ti-Si, Cr-Si, Fe-Si, Sc-Si, Ni-Si and Co-Si. Among them, the binary systems of Cr-Si, Ti-Si and Fe-Si and the multi-component systems such as Cr-Ce-Al-Si are well developed and have obtained good crystal growth results.

Figure 6 (a) shows the relationship between SiC growth rate and temperature in three different high-temperature solution systems of Cr-Si, Ti-Si and Fe-Si, summarized by Kawanishi et al. of Tohoku University in Japan in 2020.

As shown in Figure 6 (b), Hyun et al. designed a series of high-temperature solution systems with a composition ratio of Si0.56Cr0.4M0.04 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rh and Pd) to show the solubility of C.

Figure 6. (a) Relationship between SiC single crystal growth rate and temperature when using different high-temperature solution systems

04 Growth kinetics regulation

In order to better obtain high-quality SiC single crystals, it is also necessary to regulate the kinetics of crystal precipitation. Therefore, another research focus of the TSSG method for growing SiC single crystals is the regulation of the kinetics in high-temperature solutions and at the crystal growth interface.

The main means of regulation include: rotation and pulling process of seed crystal and crucible, regulation of temperature field in growth system, optimization of crucible structure and size, and regulation of high-temperature solution convection by external magnetic field. The fundamental purpose is to regulate the temperature field, flow field and solute concentration field at the interface between high-temperature solution and crystal growth, so as to better and faster precipitate SiC from high-temperature solution in an orderly manner and grow into high-quality large-size single crystals.

Researchers have tried many methods to achieve dynamic regulation, such as the "crucible accelerated rotation technology" used by Kusunoki et al. in their work reported in 2006, and the "concave solution growth technology" developed by Daikoku et al.

In 2014, Kusunoki et al. added a graphite ring structure as an immersion guide (IG) in the crucible to achieve the regulation of high-temperature solution convection. By optimizing the size and position of the graphite ring, a uniform upward solute transport mode can be established in the high-temperature solution below the seed crystal, thereby improving the crystal growth rate and quality, as shown in Figure 7.

Figure 7: (a) Simulation results of high-temperature solution flow and temperature distribution in crucible; 

(b) Schematic diagram of experimental device and summary of results

05 Advantages of TSSG method for growing SiC single crystals

The advantages of TSSG method in growing SiC single crystals are reflected in the following aspects:

(1) High-temperature solution method for growing SiC single crystals can effectively repair microtubes and other macro defects in the seed crystal, thereby improving the crystal quality. In 1999, Hofmann et al. observed and proved through optical microscope that microtubes can be effectively covered in the process of growing SiC single crystals by TSSG method, as shown in Figure 8.

Figure 8: Elimination of microtubes during the growth of SiC single crystal by TSSG method:

(a) Optical micrograph of SiC crystal grown by TSSG in transmission mode, where the microtubes below the growth layer can be clearly seen; 

(b) Optical micrograph of the same area in reflection mode, indicating that the microtubes have been completely covered.

(2) Compared with PVT method, TSSG method can more easily achieve crystal diameter expansion, thereby increasing the diameter of SiC single crystal substrate, effectively improving the production efficiency of SiC devices and reducing production costs.

The relevant research teams of Toyota and Sumitomo Corporation have successfully achieved artificially controllable crystal diameter expansion by using a "meniscus height control" technology, as shown in Figure 9 (a) and (b).

Figure 9: (a) Schematic diagram of meniscus control technology in TSSG method; 

(b) Change of growth angle θ with meniscus height and side view of SiC crystal obtained by this technology; 

(c) Growth for 20 h at a meniscus height of 2.5 mm; 

(d) Growth for 10 h at a meniscus height of 0.5 mm;

(e) Growth for 35 h, with the meniscus height gradually increasing from 1.5 mm to a larger value.

(3) Compared with PVT method, TSSG method is easier to achieve stable p-type doping of SiC crystals. For example, Shirai et al. of Toyota reported in 2014 that they had grown low-resistivity p-type 4H-SiC crystals by the TSSG method, as shown in Figure 10.

Figure 10: (a) Side view of p-type SiC single crystal grown by TSSG method; 

(b) Transmission optical photograph of a longitudinal section of the crystal; 

(c) Top surface morphology of a crystal grown from a high-temperature solution with an Al content of 3% (atomic fraction)

06 Conclusion and Outlook

The TSSG method for growing SiC single crystals has made great progress in the past 20 years, and a few teams have grown high-quality 4-inch SiC single crystals by the TSSG method.

However, the further development of this technology still requires breakthroughs in the following key aspects:

(1) In-depth study of the thermodynamic properties of the solution;

(2) The balance between growth rate and crystal quality;

(3) The establishment of stable crystal growth conditions;

(4) The development of refined dynamic control technology.

Although the TSSG method is still somewhat behind the PVT method, it is believed that with the continuous efforts of researchers in this field, as the core scientific problems of growing SiC single crystals by the TSSG method are continuously solved and key technologies in the growth process are continuously broken through, this technology will also be industrialized, thereby giving full play to the potential of the TSSG method for growing SiC single crystals and further promoting and driving the rapid development of the SiC industry.