Void nucleation and motion
Voids nucleate at the interface between the SiC seed and the graphite crucible lid. Seeds are usually affixed to the lid using graphitized sucrose. However, any attachment agent contains some volatile components which are likely to form small cavities during high temperature growth. An optical image of a sucrose attachment layer after short growth run (with seed removed) is shown in Fig. 3.
Fig. 3 Sucrose attachment layer after 1 hour annealing at 2300 oC.
Since the heat is extracted through the lid, there is a temperature gradient across cavities in the attachment layer that drives evaporation of the seed and deposition of SiC on the crucible lid. One can then expect that the cavities will be filled with SiC and voids produced in the silicon carbide seed. The early stage of void formation is shown in Fig. 4. The graphite lid was mechanically lapped down to about 100 mm and the rest of graphite was burned off in oxidizing ambient at 1100 oC. The original seed surface was polished flat mechanically and did not exhibit any depressions.
Fig. 4 (a) Early stage in formation of hexagonal void at the SiC seed /graphite crucible interface.
Fig. 4(b) Magnified part of the void nucleation process showing initial stage of void closing.
Once inside the crystal, void continues moving up the temperature gradient (toward higher temperatures). The rate of motion is controlled by the evaporation process at the top facet of the void and the void size. Screw dislocations intersecting the top facet serve as negative step sources which then propagate toward the void edges. An AFM image of the top of the void is shown in Fig. 5.
Fig. 5 AFM image of a depression on the void top surface.
In the center of the image several elementary screw dislocations produce negative 1.5 nm high steps winding counterclockwise. Morphology is consistent with evaporation on the top surface. The rate of void motion is comparable to the growth rate of silicon carbide crystals. In some conditions voids can reach growth surface of the SiC boule. Although in such case they disappear from the crystal, the path of their passage remains in form of dislocation arrays and micropipes.
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