Ion Implantation and Synthesis of Materials - Studium

94 8 Channeling1.00.8Non-aligned Implant(Non-channeled)N(x)0.60.4∆Rp-aligned Implant(Channeled)0.2Rp≅ exp(–x/λc)00 1.0 2.0 3.0 4.0x/RpFig. 8.1. Range distributions for channeled ion implanted along the 〈100〉 axis of Si. Thedashed line shows the Gaussian distribution for incident ions aligned away from anychanneling directionof the outermost atoms to shadow the underlying atoms and hence shield theseatoms from direct interactions with the beam.As the ions penetrate deeper in the crystal they make only small anglecollisions with the atomic rows. The initial motion is oscillatory, with the particlesbouncing from row to row with a wavelength between bounces of some hundredsof Ångstroms. These channeled particles (~98% of the beam) cannot get closeenough to the atoms of the solid to undergo close encounters such as large anglescattering. Hence the yield of backscattered particles decreases by almost twoorders of magnitude.The reduction in scattering yield associated with channeling can be applied todetermine the lattice site position of impurity atoms and defects in the crystal(Fig. 8.3). An impurity on a lattice site has a reduction in scattering yield equal tothat of the bulk crystal; interstitial impurities or atoms located more than 0.1 Åfrom a lattice site are exposed to the flux of channeled ions. Consequently, thebackscattering yield from such nonsubstitutional atoms does not exhibit the samedecrease as that of the host crystal.The effects of lattice disorder defects and crystal imperfection on channelingare used to analyze ion-implanted samples. Host atoms displaced from their latticesites can interact with the channeled beam, leading to an increase in the scatteringyield.