Diffusion and Solubility of Impurities in SiC

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. 

The solubility limits of impurities in SiC and its various polytypes have not been studied in great detail. The most systematic study has been performed by Vodakov et al [1] in conjunction with the growth of 6H-SiC by using the sublimation sandwich [2,3] technique. No stronger scientific research has been done on the variation of the solubility with polytype. The data presented in this section can probably be used for other polytypes, but no clear study has shown this to be the case. 

Ga and P solubilities in 6H have shown a crystal face dependence [1], For Al, Ga and B the solubility limit is smaller on the [0001 ]C face. For P and N the solubility limit is smaller on the [0001]Si face. Vodakov et al [1] suggest that this dependence is caused by the very strong orientation doping anisotropy of 6H-SiC. The data for Al, B and Ga showed very good agreement when measured by the neutron-activation method and electrophysical measurements employing the Hall effect. The dependence on crystal direction of the solubility for N, B, Al, Ga and P in 6H-SiC is listed in TABLE 1. The maximum solubility limits for many of the impurities in 6H-SiC are listed in TABLE 2. 

The results of diffusion studies are summarized in TABLE 3. Some of the impurities can best be characterized by a fast and slow branch type diffusion and the information about each branch is provided where appropriate. Additional more recent studies [6] have been completed but provide no major new significant information about actual diffusion coefficients in SiC. 

Solubility with temperature data for nitrogen, aluminium, boron, gallium and phosphorus, on both Si and C faces, and maximum solubilities for a wider range of impurities at temperatures 2500°C in 6H-SiC have been detailed. No data for other polytypes are available. Diffusion rates of impurities in SiC are very slow (for temperatures between 1800 and 2300 °C) whether for those species, such as boron and nitrogen, which migrate via Si/C vacancies or for those, such as beryllium, lithium and hydrogen, which diffuse interstitially. Some impurities show 2-component diffusion profiles. 

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Diffusion and solubility of impurities in SiC TABLE 2 Maximum solubility limits for impurities in 6H-SiC (at temperatures above 2500 °C). 

A high solubility limit is appropriate to beryllium, the group II acceptor. It reaches 1020 cm 3 [40]. However, beryllium is usually not employed as a dopant in the sublimation growth. Firstly, it has an exceptionally high diffusivity in SiC (see Chapter 7). For the typical growth time of about 10-60 min the diffusion front will penetrate as far as 10- 100 pm. In addition, the activation energy of Be acceptors is considerable and this dopant has a tendency to self-compensation [51]. The results of a comprehensive study of impurity solubility in SiC performed by Vodakov, Mokhov and co-authors [40] are presented in TABLE 3. 

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