Ion Implantation and Synthesis of Materials - Studium

7 Displacements and Radiation Damage7.1 IntroductionAs we have shown in the preceding chapters, collisions between ions and targetatoms result in the slowing down of the ion, and the energy loss resulting fromnuclear stopping contributes significantly to determining the ion’s range. In ourdiscussion of range theory we have to consider how the nuclear energy lossescontribute to the energy dissipation of the ion. From our discussion of elasticcollisions we know that the energy loss by the ion is transferred to lattice atoms. Inthese collisions, sufficient energy may be transferred from the ion to displace anatom from its lattice site. Lattice atoms that are displaced by incident ions arecalled primary knock-on atoms or PKAs. The PKAs can in turn displace otheratoms, i.e., secondary knock-on atoms, tertiary knock-ons, etc., thus creating acascade of atomic collisions. This leads to a distribution of vacancies, interstitialatoms and other types of lattice disorder in the region around the ion track. As thenumber of ions incident on the crystal increases, the individual disordered regionsbegin to overlap. At some point, a heavily damaged layer is formed. The totalamount of disorder and the distribution in depth depend on ion species,temperature, energy, total dose and channeling effects.7.2 Radiation Damage and Displacement EnergyRadiation damage theories are based on the assumption that a lattice atom struckby an energetic ion or recoiling target atom must receive a minimum amount ofenergy in the collision to be displaced from its lattice site. The energy required todisplace the lattice atom represents the displacement threshold and is called thedisplacement energy, E d . If in the collision process the energy transfer to thelattice atom, T, is less than E d , the struck atom undergoes large amplitudevibrations without leaving its lattice position. The vibrational energy of the struckatom is quickly shared with nearest neighbors and appears as a localized source ofheat. If, however, T is greater than E d , the struck atom is able to move out of the77