Seeded sublimation growth

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. 

The principle is simple. A graphite crucible is partially filled with SiC powder and a seed is attached on the lid of the crucible. The whole system is closed and heated up to temperatures above which SiC starts to sublime appreciably. A thermal gradient is applied such that the seed is slightly colder than the powder source. Material will thus transport from the source to the seed where it will condense. The principal constituents during sublimation are Si, SqC, and SiC and the ratio between them is determined by the temperature. 

The temperature and temperature gradient are naturally very important factors for the growth. The growth rate is exponentially increasing with increasing temperature. Typical trends are comprehensively illustrated by Vodakov et al. [29], Barrett et al. [28], and Maltsev et al. [27]. 

Another even more interesting development was occurring at the same time. This was the development of a new growth technique, called the High Temperature Chemical Vapor Deposition (HTCVD) technique [34], that produced crystals that were intrinsically semi-insulating. In a paper by Ellison et al. [34], the authors reported on a defect with an activation energy of 1.15 eV yielding an extrapolated room temperature resistivity in excess of 10 il-cm. 

2-inch SI wafers without vanadium doping are sold commercially from several sources. Vanadium doped crystals will rapidly disappear and soon only be used as displays in museums or as book rests. 

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

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. 

Since 1990 high quality SiC wafers of 35 mm diameter are commercially available from single crystal 6H-SiC boules, produced via a seeded-sublimation growth technique [134]. In this process, nucleation occurs on a SiC seed crystal located at the top or bottom of a cylindrical growth cavity. As in the Lely process [135], SiC sublimes from a polycrystalline source at temperatures >2200°C under vacuum to form Si, Si2C, and SiC2 vapors... 

The modified Lely process. Despite the high crystalline quality that may be obtained with the Lely method, it has never been considered an important technique for future commercial exploitation on account of the low yield and irregular sizes. In the modified Lely process, which is a seeded sublimation growth process, these problems are overcome, though at the price of a considerably lower crystalline quality. In the modified Lely technique, SiC powder or lumps of SiC are placed inside a cylindrical graphite crucible. The crucible is closed with... 

On the other hand, bulk crystals of 4H-SiC and 6H-SiC are made by a physical vapor transport (seeded sublimation growth) technique known as the modified-Lely method [32]. High-quality bulk materials (threading dislocation... 

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]. 

One of the methods of growing bulk GaN is a sublimation method [1-3], In this method, the source GaN powder and a seed crystal or a substrate facing the source are loaded in a crucible. Upon heating the crucible, the source powder sublimates and recrystallises onto the seed crystal or on the substrate. When a system needs a reaction gas such as NH3 for growth, the system is not a sublimation but a VPE system. Now, a 4-inch SiC wafer grown by a sublimation method is commercially available [4],... 

In 1978, Tairov and Tsvetkov [24] reported a sublimation technique to produce SiC boules for device application. They produced an 8 mm diameter by 8 mm long boule of SiC on a seed crystal placed within a graphite crucible. In a further study, growth of SiC boules up to 14 mm diameter and 18 mm in length were obtained [15]. 

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