Ion Implantation and Synthesis of Materials - Studium

9.4 Diffusion Mechanisms 121Fig. 9.11. Diffusion mechanismsa neighboring vacant site, Fig. 9.11b. It is also possible for a substitutionalimpurity atom to push a neighboring host atom into an adjacent interstitial site andmove into the vacant substitutional site thus created; this process is called theinterstitialcy mechanism.9.4.1 Interstitial MechanismThe interstitial sites of a diamond or zinc blend lattice are usually vacant. Theprobability of an interstitial atom finding an available site to jump into is veryhigh. As it jumps from one site to another, it faces a constriction due to the hostatoms (see Fig. 9.11a); the jumping atom needs a little push to squeeze by. Thissituation is described as having an activation or energy barrier. In this case thebarrier is also periodic in the lattice.9.4.2 Substitutional or Vacancy MechanismThe jumping of a substitutional atom to a neighboring substitutional site requires avacancy to be created in the adjacent site, Fig. 9.11b. The probability of such anevent is exp(− E f /kT ), where E f 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 E m (activation energy formigration). The value of E m + E f is between 3 and 4 eV (Fair 1981).9.4.3 Interstitial–Substitutional MechanismMany impurities have both an interstitial solubility, N i , and a substitutionalsolubility, N s . These interstitial impurity atoms and substitutional impurity atomscan diffuse independently or interdependently. Usually, N s is larger than N i , butsubstitutional diffusion is much slower than interstitial diffusion. In Si, these twocomponents of diffusion appear to move interdependently. They are related by a