Coordination defect

Figure 4.12 Coordination defects in the fluorite structure (a) fluorite structure represented as two interpenetrating sets of XM4 tetrahedra pointing along the cube < 111 > directions (b) fragment of one subset of tetrahedra, all pointing in the same direction and (c) coordination defect. The central part of the defect, heavy outline, is the unoccupied tetrahedron core of the cluster. The cubic unit cell in (a) and (b) is outlined.

From thermodynamic insights won in the late 1970 s new attempts to craft general structural principles by expanding the basic ideas of the c.d. model were attempted by incorporating additional coordination defects of greater complexity (Manes et al., 1980 Sorensen, 1981). 

The presence of the hydrogen imposes a distinction between the atomic and the network coordination. All the silicon atoms are fourfold coordinated, apart from the small concentration of coordination defects. However, the network coordination is reduced by the presence of hydrogen which forms a single bond to the silicon and so does not help to link the network together. The average network coordination is. 

Fig. 4.25. Illustration of some of the possible coordination defects and their expected gap state structure.

Following the ideas of coordination defects introduced in Section... 

The divacancy is a stable defect in crystalline silicon. The six dangling bonds left by the removal of two adjacent atoms reconstruct to leave two strong bonds and a pair of weakly interacting dangling bonds which comprise the gap states. The defect has four electronic levels in the gap and its neutral state has paired electrons and is diamagnetic. The divacancy model was first proposed by Spear (1974) to explain the early field effect results. A two-fold coordinated silicon atom also has Az = — 2 and so is of the same type. Adler (1978) concluded from molecular orbital arguments that this defect has the lowest formation energy of any silicon coordination defect. 

Figure 2.8. Distribution of coordination defects in a 001 ]p layer of Cc O, with ihc unit of composition CejO j ouilmed. White rectangles can accommodate oxygen belonging respectively to the axial octant of eacn coordination defect situated above or below this layer.

Figure 2.9. Representation of unit containing coordination defect used to model Cc 02. 201 oxides, with their orientations along [tl21p plane. Module F has no vacancies module D, has one vacancy in lower octant (there are four possible bottom vacancies whose modules have b=l,2,3 and 4) module U has one top vacancy (there are four possible IT modules with t=l,2,3 and 4) W j is one of four possible modules with two vacancies, a lop vacancy and a vacancy tn the tower octant (W with t,b=3,l 4,2 U 2,4) <.

Si-Si bonds on the crystalline substrate surface also break to aid in the accommodation of strain in the clusters of deposited material, resulting in partial loss of the surface structural order. Tlie coordination defects of the Si atoms in the partially amorphized surface are passivated by transport of available H atoms that are bonded to overcoordinated Si atoms in the deposited clusters. These processes determine the structure and composition of the interface that forms between the deposited material and the crystalline substrate. The above relaxation mechanisms stabilize the amorphous pockets on the deposition surface, which grow and coalesce as more radicals arrive from the gas phase to form an amorphous film on the crystalline substrate. 

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