Crystal Defects in Silicon - Page 3 of 3 - Area Defects and Volume Defects

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Crystalline Defects in Silicon (Page 3 of 3)

Area defects in crystals consist of stacking faults, grain boundaries, and twin boundaries. A 'stacking fault' pertains to a disturbance in the regularity of the stacking of planes of atoms in a crystal lattice. This usually occurs when a plane is inserted into or removed from the lattice. The insertion of an extra plane in the stacking is known as an 'extrinsic' stacking fault, while the removal of a plane is referred to as an 'intrinsic' stacking fault.

Stacking faults can become electrically active when decorated by impurity atoms. Electrically active stacking faults can cause device degradation, examples of which are higher reverse bias currents in p-n junctions and storage time reduction in MOS circuits.

Figure 1. Photo of a Stacking Fault

Image Source: http://lmass.uah.edu

- J. A. Gavira-Gallardo, J. D. Ng and M.A. George

A 'twin' is an area defect wherein a mirror image of the regular lattice is formed during the growth of the silicon ingot, usually caused by a perturbation. The 'twin boundary' is the mirror plane of the twin formation.

A 'grain boundary' refers to the transition or interface between crystals whose atomic arrangements are different in orientation with respect to each other.

Volume defects in a crystal are also known as 'bulk' defects, which include voids and precipitates of extrinsic and intrinsic point defects.

Every impurity introduced into a crystal has a certain level of solubility, which defines the concentration of that impurity that the solid solution of the host crystal can accommodate. Impurity solubility usually decreases with decreasing temperature. If an impurity is introduced into a crystal at the maximum concentration allowed by its solubility at a high temperature, the crystal will become supersaturated with that impurity once it is cooled down. A crystal under such supersaturated conditions seeks and achieves equilibrium by precipitating the excess impurity atoms into another phase of different composition or structure.

Precipitates are considered undesirable because they have been known to act as sites for the generation of dislocations. Dislocations arise as a means of relieving stress generated by the strain exerted by precipitates on the lattice. Precipitates induced during silicon wafer processing come from oxygen, metallic impurities, and dopants like boron.

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