Any irregularity in the crystal structure is known as crystal imperfection or crystal defects.

Defects can be classified as:

1. Points defects (Zero dimensional defects):

A point defect in a crystal is an entity that causes an interruption in the lattice periodicity. This can occur due to many events

If an atom is removed from its regular lattice site; the defect is a vacancy.

If an atom is in a site different from a regular lattice (substitutional) lattice site; the defect is an interstitial. An interstitial defect can be of the same species as the atoms of the lattice (it is an intrinsic defect, the self-interstitial) or of a different nature (it is then an extrinsic defect, an interstitial impurity).

An impurity can occupy a substitutional site.

Anything other than a silicon atom on crystal lattice constitutes a Point defect.

Crucial role in diffusion & ion implantation and very less in oxidation kinetics.

2. Line defects (One dimensional defects):

One-dimensional defects in crystals are known as dislocations. The crystal contains an extra plane of atoms, which terminates at a dislocation.

The dislocation itself then is a linear defect in the direction into the paper. Dislocations either terminate at the edge of the crystal (edge dis- location or they form a closed loop within the crystal (dislocation loops).

Dislocations are active defects in crystals, i.e. they can move when subjected to stresses or when excess point defects are present. The process of "climb" occurs when excess point defects are absorbed by the dislocation.

3. Surface or plane defects (Two dimensional defects):

The most common kind of 2D or area defect found in silicon is the stacking fault.

Stacking faults always forms along {111} planes and are simply the insertion or removal of an extra {111} plane.

In a perfect crystal, the stacking order is ABCABC, and so on. When a stacking fault is present, either an extra plane is inserted (ABCACBC, etc.) or a plane is missing (ABCABABC, etc.).

Such faults are referred to as "extrinsic" if there is an extra plane of atoms, or "intrinsic" if a plane is missing.

Stacking faults are bounded by dislocations and, when they intersect the wafer surface, are usually referred to as surface stacking faults.

4. Volume defects (Three dimensional defects):

Volume or Bulk defects occur on a much bigger scale than the rest of crystal defects.

Void is a common bulk defect. Voids are regions where there is large number of atoms missing from the lattice.

When void occur due to air bubbles becoming trapped when a material solidifies, it is commonly called porosity.

When a void occurs due to shrinkage of a material as it solidifies, it is called cavitation.