Point Defects

Point defects take several forms as shown in figure. Any non-silicon atom incorporated into the lattice at either a substitutional (i.e., replacing a host silicon atom) or interstitial (i.e., between silicon atoms) site is considered a point defect.

Fig. 5 the location and types of point defects in a simple lattice.[2]

Missing atoms create a vacancy in the lattice called a "Schottky defect," which is also considered a point defect. A silicon atom in an interstitial lattice site with an associated vacancy is called a "Frenkel defect."

Vacancies and interstitials have equilibrium concentrations that depend on temperature. From thermodynamic principles the following relation gives the concentration of the point defect as a function of temperature. [2]

N_d = A exp(-E_a/kT)

Where

N_d is the concentration of the point defect

A is a constant

E_a is the activation energy

T is the absolute temperature

k is Boltzmann's constant

Point defects are important in the kinetics of diffusion and oxidation. The diffusion of many impurities depends on the vacancy concentration, as does the oxidation rate of silicon.