Ion Implantation and Synthesis of Materials - Studium

8 Channeling8.1 IntroductionAll theories examined earlier concerning the ranges of ions and radiation damageof materials were based on the assumption that the stopping medium is disordered,i.e., amorphous. In practice, we are dealing with polycrystalline or monocrystallinesubstances. The main parameters determining the range of an ion are its energy, E,its atomic number, Z 1 , and the atomic number of the substrate, Z 2 . In the case ofsingle crystals, the orientation of the substrate and the vibrational amplitude (i.e.,temperature) of the lattice atoms are also important parameters.With single-crystal substrates of Si or GaAs, for example, the orientation ofthe ion beam with respect to the crystallographic axes of the substrate can have apronounced effect on the range distribution. Figure 8.1 shows the rangedistribution in Si for 100 keV As implanted with the beam aligned parallel to the〈100〉 crystal axis (solid line) and oriented away from any crystal axes or planes(dashed line). As is evident in the figure, implantation along crystal axes can leadto a fraction of the total number of ions that penetrate several times Rp.The crystal orientation influence on ion penetration is called channeling or thechanneling effect. When an ion trajectory is aligned along atomic rows, thepositive atomic potentials of the line of atoms steers where the positively chargedion goes within the open space, or channels, between the atomic rows. Thesechanneled ions do not make close-impact collisions with the lattice atoms andhave a much lower rate of energy loss, dE/dx, and hence a greater range than thoseof nonchanneled ions. The depth distribution of channeled ions is difficult tocharacterize under routine implantation conditions. The channeling distributiondepends on surface preparation, substrate temperature, beam alignment, and thedisorder introduced during the implantation process itself. The channeled ions thatpenetrate beyond R p often have a distribution that falls off exponentially withdistance as exp(−x/λ c ), where λ c R .pAnother consequence of channeling is that the energy spectrum of particlesscattered back from a crystal aligned with the beam is dramatically different fromthat of a noncrystalline solid (Fig. 8.2). In the aligned spectrum the scattering yieldfrom the bulk of the solid is reduced by almost two orders of magnitude, anda peak occurs at a position corresponding to scattering from the surface atoms.Both the surface peak and the reduction in yield are due to shadowing – the ability