Substitutional or Vacancy Mechanism

The jumping of a substitutional atom to a neighboring substitutional site requires a vacancy to be created in the adjacent site, Fig. 9.11b. The probability of such an event is exp(-Ef/kT), where E( is the formation energy of a vacancy in the lattice. The movement causes bonds to break and new bonds to be formed after the jump. The energy barrier for this jumping process is Em (activation energy for migration). The value of Em + E is between 3 and 4 eV (Fair 1981). 

---