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about 100˚C lower than the source’s temperature so that sublimed SiC species condense and crystallize on the seed. Growth is usually performed at low pressure to enhance mass transport from the source to the seed. A high-purity inert gas such as argon (or helium) is employed for growth.
The growth rate in sublimation growth is mainly determined by the flux of the source supply (sublimation rate) and the transport efficiency from the source to the seed. The subli- mation rate is a function of the source temperature, while the transport efficiency depends heavily on the growth pressure, the thermal gradient, and the distance between the source and the seed.
Because the mass transport is diffusion-limited in sublima- tion growth, the growth rate is almost inversely proportional to the growth pressure. In other words, as the pressure is reduced, the vapor diffusion rate increases, and constitu- ents move faster along the concentration gradient from the source toward the seed. Here, the concentration gradient is basically determined by the source and seed temperatures (temperature gradient). Experimentally, a growth rate of 150 μmh–1 is possible with a pressure of 1 Torr (or 1/760 atm) and 2,000˚C of source temperature; at the same pressure but with a source temperature of 2,125˚C, the rate can be increased to 1,000 μmh–1.
Some precautions must be taken, however. The seed tem-
FIGURE 5: MODIFIED LELY METHOD CRUCIBLE
perature must be controlled to minimize the loss of Si from the surface by maintaining the overpressure of the Si vapor near the seed. The source temperature and pressure must be controlled to develop the appropriate temperature gra- dient and to ensure that the proper amounts of Si and SiC compounds are transported from the source to the seed. Any fluctuations in the temperature profile and pressure can result in the buildup of Si droplets, surface graphitization, and C inclusions. All these phenomena lead to the formation of macro- and micro-defects in SiC boules.
The main species moved from the SiC source to the seed are Si, Si2C, and SiC2 at a growth temperature of 2,300˚C to 2,400˚C. Thus, the gas phase in sublimation growth is usually rich in Si because of preferential evaporation of Si from the SiC source. This leaves the source with an excess of C, and graphitization of the source is the result during the growth. To avoid carbon inclusions in the growing crystal, silicon is added to the source to maintain a stoichiometric or Si-rich source surface. This is important because sufficient over- pressure of Si is also required to avoid graphitization of the growing surface on the seed. Carbon evaporation and trans- portation in the growth can be reduced by using a tantalum- carbon coating.5,6
Filippo Di Giovanni
is a strategic marketing, innovation, and key programs manager at STMicroelectronics.
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REFERENCES
1B. J. Baliga. “Silicon Carbide Power Devices,” 2005 World Scientific Publishing Co. Pte. Ltd.
2B. J. Baliga. “Power Semiconductor Device Figure of Merit for High-Frequency Applications.” IEEE Electron Device Letters, Vol. 10, No. 10, October 1989.
3Tsunenobu Kimoto and James A. Cooper. Fundamentals of Silicon Carbide: Technology Growth, Characterization, Devices, and Applications. 2014, John Wiley & Sons Singapore Pte. Ltd.
4Chris J. H. Wort and Richard S. Balmer. “Diamond as an electronic material.” Element Six Ltd., Ascot, U.K.
5Tsunenobu Kimoto. “Material Science and Device Physics for High-voltage Power Devices,” Japanese Journal of Applied Physics 54, 040103. 2015.
6V. D. Heydemann, N. Schulze, D. L. Barrett, and G. Pensl. “Growth of 6H and 4H silicon carbide single crystals by the modified Lely process utilizing a dual-seed crystal method.” Institut für Angewandte Physik, Universität Erlangen-Nürnberg, Erlangen, Germany. 1996.
Technology Analysis The Physics, Structure, and Properties of Silicon Carbide