Silicon carbide material quality

The electric furnaces, in which this reaction is carried out, are ca. 15 3 3 m- in size and are lined with refractory material. Electrodes at opposite ends are connected to a graphite core. The furnace is filled round this core with the reaction mixture and electrically heated to 2200 to 2400°C. The heating up time is ca. 18 h and the reaction time a further ca. 18 h. After cooling, the sides of the furnaces are removed and the unreacted material on the edges removed. The silicon carbide, which has formed round the graphite core, is broken up and separated into different qualities. 

It was shown that the efficiency for radial mixing depends on the gas phase as well (O. Eichstadt, Continuous Mixing of Fine Particles within Fluid Dynamic Vertical Tube Mixers, Dissertation, in German, ETH-Zurich, 1997). At best they operate with low volume concentration and for particles between 20 and 200 pm. Static mixers have been used for very abrasive free-flow materials such as silicon carbide. Since any rotating equipment is avoided inside static mixers, abrasion is limited. As will be shown below, mixture quality is dependent on feed consistency and residence time within the static mixer. Since the latter is very short in static mixers (seconds or fractions of a second), short-time feeding precision has to be very high to achieve high-quality mix. 

A new approach to the directed crystallisation of silicon carbide has been proposed by Vodakov and Mokhov [6]. Their idea was to exclude the conditions which could permit any uncontrolled nucleation. They employ a nearly flat source positioned close to the substrate and perform the growth under near-equilibrium conditions. This has ensured a high quality of the grown material. The method was named the sublimation sandwich method and it appeared to be very effective. 

Chemical and phase purity are also critical issues that drive SiC precursor design. However, keep in mind that sometimes the same end product may result from off-stoichiometry precursors or pyrolysis products by further processing steps after precursor pyrolysis. Phase purity is quite important to target such SiC materials which focus on reproducible semiconductor qualities. However, the fact that H- and N-doped silicon carbide films behave as high temperature semiconductors underlines that chemical and phase purity are not always desirable. 

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