Point Defects

POINT DEFECTS

1. What are Point Defects?

✓ The defects which take place due to imperfect packing of atoms during crystallisation are known as point defects.

✓ As the name implies, they are imperfect point-like regions in the crystal. Therefore, they are also called as zero-dimensional imperfections.

✓ Point defects are always present in crystals.

✓ The presence of a point defect in a crystal lattice causes an increase in its internal energy as compared to a perfect crystal.

✓ These defects also produce distortion inside the crystal structures.

2. Types of Point Defects

✓ The important types of point defects are given below.

Following types of point defects are important from the subject point of view:

1. Vacancies

✓ What is a vacancy?

● The simplest point defect is a vacancy. This refers to a missing atom or vacant atomic site. In other words, whenever one or more atoms are missing from a normally occupied position, as shown in Fig.0.26 (a), the defect caused is known as vacancy.

✓ Causes of vacancies: Vacancies may occur as a result of imperfect packing during the original crystallisation or they may arise from thermal vibrations of atoms at elevated temperatures. Because due to increase thermal energy causing individual atoms to jump out of their position of lowest energy. 

✓ Types of vacancies: The vacancies may be a single vacancy (if one atom is missing), di-vacancies (if two atoms are missing), tri-vacancies (if three atoms are missing), and shown.

(i) Schottky Defect

✓ When a pair of positive and negative ions is missing from an ionic crystal, as shown in Fig.0.26 (b), the defect caused is known as Schottky defect.

✓ Since a pair is missing, electrical neutrality is maintained.

2. Interstitial Defect

✓ Whenever an extra atom occupies interstitial position (i.e., voids) in the crystal system without dislodging the parent atom, as shown in Fig.0.27, the defect caused is known as interstitial defect.

✓ An atom can enter the interstitial space or void only if it is smaller than the parent atom. Otherwise it will cause atomic distortion or strain, because interstitial atom tends to push the surrounding atoms further apart.

✓ The interstitial atom may be either a normal atom from the same crystal called self interstitial [Fig.0.27 (a)] or an impure atom (i.e., foreign atom) not present in the lattice site [Fig.0.27 (b)].

(i) Frenkel Defect

✓ Whenever a missing atom (responsible for vacancy) occupies interstitial position (responsible for interstitial defect), as shown in Fig.0.27 (b), the defect caused is known as Frenkel defect. 

✓ A Frenkel defect is a combination of vacancy and interstitial defects.

3. Impurities (or Compositional Defects)

✓ Impurity defects are foreign atoms introduced into a crystal lattice either as an interstitial or a substitutional atom.

✓ A substitutional impurity is created when a foreign atom replaces a parent atom in the lattice, as shown in Fig.0.28 (a).

✓ An interstitial impurity is a small-sized atom occupying an interstice or space between the regularly positioned atoms, as shown in Fig.0.28 (b).

✓ These defects are responsible for the functioning of most semiconductor devices.

Note

Whenever a group of atoms is displaced from its ideal location, the defect caused is known as phonon. It is caused by thermal vibrations.

4. Electronic Defects

✓ Electronic defects are the result of errors in charge distribution in solids.

✓ These defects are free to move in the crystal under the influence of an electrical field.

✓ This accounts for some electronic conductivity of certain solids and their increased reactivity.

✓ The effects of the electronic defects are:

(i) Increase in the hardness and tensile strength (due to the distortion caused in the lattice).

(ii) Increase in electrical conductivity.

(iii). Increase in kinetics of diffusion and phase transfor- mation (due to the presence of vacancies).

✓ This effect is responsible for the operation of p-n junctions and transistors.