Sublimation growth

S. Yu Karpov, N. Yu Makarov, M. S. Ramm. Simulation of sublimation growth of SiC single crystals. Physica Status Solidi B 202 201, 1997. 

Figure 1.3 The seeded sublimation growth or modified Lely growth invented byTairov and Tsvetkov.

As previously mentioned in Section 1.2, Tairov and Tsvetkov [22] invented seeded sublimation growth in 1978. The technique is almost exclusively used today to manufacture SiC wafers. 

A lot of research is being conducted on the seeded sublimation growth technique. The material properties are improving steadily and there should be no reason for worries. Yet, one worry is the need for off-axis substrates for high-quality epitaxial... 

The source material will release excess silicon in the beginning of the growth cycle and be more carbon-rich in the end due to preferential depletion of silicon. This is a known problem and it is a matter of detailed control and an understanding of the dynamic transport mechanisms in combination with thermodynamics. Nevertheless, the result is invariably that SiC boules grown by seeded sublimation growth are Si-rich in the beginning and C-rich near the end, which creates yield issues. Simulation of the process is necessary to improve the situation. 

Seeded sublimation growth is a mature and needed tool for the SiC industry today. There are still major challenges. Specifically, boules will need to be grown on off-axis substrates, or the off-axis angle needs to be eliminated, which will only be possible if a combined effort of improving wafer quality, polishing procedures, and epitaxial procedures is pursued. 

The formed microparticles of Si C will move into the hot chamber or the sublimation zone with the aid of the inert helium carrier gas. Once in the sublimation zone, the microparticles will sublime to form Si, Si C, and SiQ, as in the case of seeded sublimation growth. A thermal gradient is applied, as illustrated in Figure 1.9, so that the sublimed species will condense on the seed, as in the case of seeded sublimation growth. 

There are similarities between seeded sublimation growth and HTCVD in that solid particles sublimate in the reactor and the vapor condenses on a seed crystal maintained at a lower temperature. However, the differences are quite dramatic and the outcome even more so. Take, for instance, the dynamics governing the growth... 

One of the prime advantages of the HTCVD approach is the resulting crystal properties. Due to the high purity of the gases, the material comes out intrinsically semi-insulating. Also, since the source material is produced on demand, the stoichiometry can always be kept the same, unlike the case with seeded sublimation growth. This will improve the yield of the grown material. 

It is also interesting to note that the technique is shown to reduce micropipes by 80% during each run [35]. The micropipes close at the interface by some type of hollow core-closing effect. Very low micropipe densities have been recorded using this technique, which is clearly much faster in improving material quality than seeded sublimation growth. 

The growth rates today can be approximated to about 1 mm/hr, which means that roughly 20 mm of material may be grown each day, including 4 hours for the turnaround heat-up and cool-down time. Although this is better than what the seeded sublimation growth can achieve, growth rates will need to be increased further to drive wafer costs down. 

In the commercial development of Si devices, diffusion is an important semiconductor fabrication process. This process does not play a major role (except in the case of the sublimation growth of SiC discussed in Chapter 8) in the development of SiC, because the diffusion coefficients for the most part are negligible at temperatures below approximately 1800 °C. As a result of this commercial insignificance, the diffusion process in SiC and its various polytypes has not received a great deal of scientific attention and diffusion data are incomplete. It does, however, appear that the solubility of impurities and their diffusive mobilities in different SiC polytypes are very similar. 

Transport mechanisms in sublimation growth are complicated. Growth rate increases with increasing source temperature, increasing source to seed distance, decreasing pressure, and decreasing crystal-source distance [19,20,27-33]. 

FIGURE 1 Sublimation growth crucibles (a) single-wall graphite type and (b) double-wall type. 

The studies of growth by the sandwich method have provided a better understanding of the sublimation growth peculiarities and they have formed the basis of the new approach to the bulk crystal growth of silicon carbide. The first successful results in this direction were reported by Tairov and Tsvetkov [7,8]. Currently, similar studies are being performed by a number of research groups and rather impressive progress has been achieved thus far see Datareview 8.1. 

Furnaces of two types are employed for the sublimation growth resistively heated furnaces and induction heated ones. Schematic diagrams of typical furnaces are shown in FIGURES 1(a) and 1(b). The peculiarities of their construction can be found in [12-17] as well as in the reviews [1,4]. We shall outline only the basic features of their design. 

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