# SiC potpourri

| History
|  | Silicon carbide is the only stable compound in the Si-C equilibrium system at atmospheric pressure. SiC was first observed in 1824 by Jöns Berzelius. The properties and potential of the material were, of course, not understood at the time. The growth of polycrystalline SiC with an electric smelting furnace was introduced by Eugene Acheson around 1885. He was also the first to recognize it as a silicide of carbon and gave it the chemical formula SiC. The only occurrence of SiC in nature is found in meteorites. Therefore, SiC cannot be mined but must be manufactured with elaborate furnace techniques. In its polycrystalline forms, silicon carbide has long been a well proven material in high-temperature, high-strength and abrasion resistant applications. Silicon carbide as a semiconductor is a more recent discovery. In 1955, Jan Antony Lely proposed a new method for growing high quality crystals which still bears his name. From this point on, the interest in SiC as an electronic material slowly began to gather momentum; the first SiC conference was held in Boston in 1958. During the 60's and 70's SiC was mainly studied in the former Soviet Union. Year 1978 saw a major step in the development of SiC, the use of a seeded sublimation growth technique also known as the modified Lely technique. This breakthrough led to the possibility for true bulk crystal preparation. The first blue LED was fabricated already in 1979 and in 1987, Cree Research Inc., the first commercial supplier of SiC substrates, was founded. Today, there are a few companies and many active research groups in the field, but SiC industry still remains a small business. In the beginning, the SiC industry was concentrated around the blue LEDs that are impossible to manufacture using conventional silicon technology. The potential of the power electronics industry is much larger and its interest has grown only in recent years as the progress of silicon devices have began to stagger. This also largely explains the rapidly increasing research on SiC growth.
| The most common polytypes are 3C and 6H; 4H, 15R and 2H have also been identified, but are much more rare. All other polytypes are combinations of these basic sequences. The only cubic polytype (3C) is also referred as -SiC.
| The thermal conductivity of SiC is larger than that of copper.
| While the polytype 6H is the easiest to grow, 4H would be favored by the power electronics industry. In the field of SiC, the problems with micropipes and polytypes dominate to such a degree that the research of dislocations, vacancies and impurities still remains an academic activity. When the material develops
| Growth from Melt Most commercially utilized single crystal semiconductor boules are grown from a melt or solution, but this is not a feasible option for SiC growth. SiC does not have any liquid phase in normal engineering conditions. Calculations have indicated that stoichiometric melting is possible only under pressures exceeding bar at temperatures higher than 3200 C. Even if the solubility of carbon in silicon melt ranges from 0.01% to 19% in the temperature interval from 1412 to 2830 C, at high temperatures the evaporation of silicon makes the growth unstable. The solubility of carbon can be increased by adding certain metals to the melt (e.g., praseodymium, terbium, scandium). This would, in principle, enable the use of crystal pulling techniques, such as Czochralski growth. Unfortunately there is no crucible material available that would be stable with these melts. It is also speculated that the solubility of the added metals in the growing crystal is too high to be acceptable in semiconductor materials [10, 20]. In spite of all the problems, SiC was grown from melt at 2200 C and 150 bar in a recent study. The crucible was made of graphite and it also acted as the carbon source. A 1.4-inch crystal was demonstrated [21]. The technology is very expensive and might never be economically feasible. However, growing from a solution would avoid many of the problems related to the growth techniques from gas phase.
| Chemical Vapor Deposition A well established method for growing thin crystalline layers directly from gas phase is chemical vapor deposition (CVD) [14]. In the process a mixture of gases is injected to the growth chamber. The higher the temperature the larger is the probability that the initial bonds will crack and the radicals will attach to the surface thus leading to epitaxial growth. When temperature is increased the probability of sticking increases but also the etch rate from the surface increases. The growth rate is therefore determined by the desorption of the reaction products and by the etch rate of the surface, and by the diffusion dominated mass transport of the source molecules. Generally, the growth rates in CVD are too low to allow boule production, usually tens of micrometers an hour [22]. By increasing the temperature the growth rate increases, but at the same time problems related to the controlling of the growth become more severe, and problems such as homogeneous nucleation in the gas phase may occur. These problems might be overcome by a very careful control of the thermal and thermodynamic conditions. This technology is not yet available, even though research on high temperature CVD (HTCVD) is under way [23, 24].
| Lely Growth The Lely growth method is used even nowadays to grow the crystals of the highest quality. A schematic Lely geometry is presented in Figure 3.1. In CVD the growth is driven by the initial gas composition, whereas in the Lely method the growth is due to temperature gradients within the system. The system is close to chemical equilibrium and the partial pressures of the SiC forming species are higher where also the temperature is higher. This leads to a pressure gradient that results in mass transport from the hot parts of the crucible to the cooler parts of the crucible [25, 26]. Figure 3.1: The Lely method for growth of SiC crystals In the Lely growth the temperature distribution is such that in the cylindrical crucible the temperature minimum is at the origin. Therefore the gases travel towards the origin. The porous graphite holding the source provides nucleation centers for infinitely small seed crystals. They will eventually grow larger and usually obtain an energetically favorable hexagonal form. Unfortunately, the Lely grown crystals are limited and random in size, in average they are about the size of a nail. Because the Lely growth method does not require any seed crystals it is the method which all the other SiC crystals originate from. The resulting crystals have low defect and micropipe densities.

SiC - Morphologien