It has been known for many years that bombardment of a crystal with energetic (kilo-electron-volts to mega-electron-volts) heavy ions produces regions of lattice disorder. The disorder can be directly observed by techniques sensitive to lattice structure, such as electron-transmission microscopy, MeV-particle channeling, and electron diffraction. The use of these and other techniques, along with the theoretical treatment of ion interactions in solids, has provided a basis for evaluation of implantation processes.
As an ion slows down and comes to rest in a crystal, it makes a number of collisions with the lattice atoms. In these collisions, sufficient energy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by incident ions are called primary knock-on atoms or PKAs. The PKAs can in turn displace other atoms, secondary knock-on atoms, tertiary knock-ons, etc. – thus creating a cascade of atomic collisions. This leads to a distribution of vacancies, interstitial atoms, and other types of lattice disorder in the region around the ion track. As the number of ions incident on the crystal increases, the individual disordered regions begin to overlap. At some point, a heavily damaged layer is formed. The total amount of disorder and the distribution in depth depend on ion species, temperature, energy, total dose, and channeling effects.
Radiation damage and displacement energy
Radiation damage theories are based on the assumption that a lattice atom struck by an energetic ion or recoiling target atom must receive a minimum amount of energy in the collision to be displaced from its lattice site.