**5.3. Evolution of defects in later stage**

14 Physics and Technology of Silicon Carbide Devices

FPI

from pore (a)

crystal growth front

dislocations to MPs.

(a) Open pore, (b) overgrown pore.

described below.

flux of vacancies from surface

Pore

(a) (b)

flux of vacancies

full-core dislocations

**Figure 12.** Mechanism of MP nucleation through coagulation of vacancies at the cores of threading dislocations. (a) Pore at the boundary of FPI starts to dissolve by emitting vacancies which migrate to neighboring full-core threading dislocations. (b) Vacancies from pore and growth surface migrate along the dislocation cores, meet and coagulate, thus transforming full-core

**Figure 13.** The pore of a convex shape has equal number of up-steps and down-steps and does not contain screw dislocation.

The surface regions around pores always contain surface steps. If the numbers of the steps up and down (let us call them up-steps and down-steps, respectively) are equal, there would be no screw dislocation inside the pore. Otherwise there should be a screw dislocation with a Burgers vector of the magnitude equal to the difference between the sum heights of up-steps and down-steps. Let us consider the first case as shown schematically in Fig. 13 and Fig. 14 for simple convex and complex pore shapes, respectively. If the pore shape is convex (say, circular or elliptical), its overgrowth can hardly lead to the formation of screw dislocations (Fig. 13). However, if the pore shape is complex with some concave fragments [Fig. 14(a)], one can expect that the pore starts to overgrow through a bridge between two opposite concave fragments in a narrow part of the pore [Figs. 14(b) or (c)]. This bridge can separate the initial pore into smaller ones. If the two new pores have different numbers of up-steps and down-steps on the growth surface, the difference can be compensated by large (or small) steps on the bridge [it is called S-bridge here, Fig. 14(b)] with no screw dislocations. Otherwise the bridge can have a smooth surface without steps and be distorted [it is called D-bridge here, Fig. 14(c)], forming a semi-loop of superdislocation. The edge segment of the semi-loop lies under the D-bridge, while the two screw segments are within their new pores.

Third, if pores have complex shapes (like those represented in Figs. 2–4, they can produce dislocated MPs during their lateral overgrowth even without full-core dislocations as

flux of vacancies from surface

flux of vacancies

crystal growth front

MP FPI

from pore (b)

The situation at the next stage of the crystal growth depends on whether new FPIs are generated or not. If generated, then the stages described above are repeated. However, though formed as explained in the intermediate stage, MPs can be attracted to FPIs and absorbed by their boundaries, producing extended pores there by agglomeration [10, 11]. If new FPIs are not nucleated, the processes of self-organization occur in the MPs ensemble: MPs elastically interact and react with each other as well as with full-core dislocations, as described in the section 4. As a result, some MPs annihilate or diminish their Burgers vectors and are finally healed. Otherwise they form separate dense groups of MPs which proceed to grow with the crystal.
