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wide-bandgap semiconductors. Furthermore, the ability of SiC to form silicon dioxide (SiO2) as a native oxide can also be exploited for device fabrication.
Growth of bulk SiC
Bulk crystal growth is the principal process for producing monocrystalline wafers, the base material for device fabrica- tion. The latest progress in SiC device development is based on the availability of relatively large SiC wafers (4- and 6-inch) with adequate quality. As of today, the standard tech- nique for SiC bulk growth is seeded sublimation, referred to as the modified Lely method. Sublimation is the direct phase transition from solid to gas.
Figure 4 shows the phase diagram of the SiC binary system. There is no SiC liquid phase, so it is impossible to employ congruent melt growth for SiC bulk growth at practical sys- tem pressures. SiC sublimes at very high temperatures, above 1,800˚C to 2,000˚C, and this represents the key step.
FIGURE 4: PHASE DIAGRAM OF SiC3
The phase diagram indicates that up to 15% of carbon can be dissolved in a Si melt at about 2,700˚C. Sublimation growth of SiC consists of three steps: sublimation of the SiC source, mass transport of sublimed species, and surface reaction and crystallization. In the gas phase, the dominant species are not stoichiometric SiC molecules but Si2C and SiC2 mol- ecules and atomic Si.
The method for the growth of single-crystalline SiC by sub- limation was first introduced by Lely in the 1950s. A brief description of the apparatus: In essence, the SiC source is placed along the inner walls of a cylindrical graphite crucible. The source material is normally SiC powder produced via the Acheson process. By heating the crucible to the temperature of about 2,500˚C, the SiC source sublimes and is transported to the inner part of the crucible. Under this nearly isothermal condition, many SiC platelets nucleate randomly along the vapor transport paths in the cavity. Nucleation in general is a self-organizing process that leads to a new thermodynamic phase or a self-assembled structure.
As of today, the standard technique for SiC bulk growth is ‘seeded sublimation,’ referred to as the modified Lely method.
The grown SiC platelets are of very good quality, and the typical dislocation density of good platelets is only 100 cm–2. However, the platelets are very small and irregular in shape, with typical areas of 1–2 cm2 and thicknesses of 0.3–0.5 mm. The platelets’ polytype is mainly 6H-SiC, but occasionally 4H- or 15R-SiC polytypes are also mixed in. Although these SiC platelets are of good quality, they cannot be used for device development. They can instead be reutilized as seed crys- tals for the early stage of the seeded sublimation growth, as explained below.
The modified Lely method illustrated in Figure 5 consists of the placement of a seed crystal at a slightly cooler place inside the crucible. The SiC source (in the form of powder or sintered polycrystalline SiC) is placed at the bottom of a cylindrical dense graphite crucible, and a SiC seed crys- tal is placed near the lid of the crucible. The seed crystal is a monocrystalline SiC slice that initiates the growth of a larger monocrystal, hence the term “seed.” Like the seed of a plant, this crystal passes on important “genetic informa- tion” to the growing crystal. The distance between the top of the SiC source and the seed crystal is about 30 mm. The crucible is heated, mainly by a radio-frequency induction coil, to 2,300˚C to 2,400˚C. The crucible is thermally insulated by graphite felt (an ideal material for vacuum furnaces or pro- cess temperatures above 2,000˚C with ash value <20 ppm) or with porous graphite.
By choosing an appropriate frequency, direct heating of this insulation can be avoided. The seed temperature is set at
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ASPENCORE GUIDE TO SILICON CARBIDE