Ion Implantation and Synthesis of Materials - Studium

96 8 ChannelingMeV ION BEAMSUBSTITUTIONAL SCATTERINGSURFACE SCATTERINGINTERSTITIAL SCATTERINGFig. 8.3. Schematic of channeling trajectories and the interaction of the channeled particleswith surface atoms and impurities. Scattering from the first monolayers of the solid, surfacescattering, reveals details of the surface structure. The channeled ions, typically 98% of theincident beam, do not make close impact collisions with the host atoms or substitutionalimpurities. The dashed line, for substitutional scattering, indicates the small yield fromsubstitutional impurities. The channeled particles can interact strongly with displaced hostatoms or interstitial impurities8.2 General PrinciplesIf an energetic ion enters a crystal within a certain critical angle of a major axis,then each time it approaches one of the aligned rows of lattice atoms, thegradually increasing repulsion between the screened Coulomb fields of theprojectile and the lattice row is sufficient to steer it away again, thereby preventingviolent nuclear collisions from occurring. A reasonable estimate of this criticalangle within which channeling occurs can be obtained from the theoretical workof Lindhard (1965); for heavy ions at kilo-electron-volt energies, the channelingcriterion for a particle of energy E and angle of incidence ψ relative to a row isgiven by1/2c [( TF / )1]ψ ≤ ψ ≡ a d ψ(8.1)where1 1 22 1/2ψ = (2 ZZe / Ed )(8.2)