Ion Implantation and Synthesis of Materials - Studium

56 5 Ion Stopping5.5 Electronic StoppingAs we discussed in Sect. 5.2, the energy-loss rate of ions in solids is divided intotwo different mechanisms of energy-loss: the energy-transferred by the ion to thetarget nuclei (called nuclear stopping) and the energy-transferred by the ion to the targetelectrons (called electronic stopping). The relative importance of the variousinteraction processes between the ion and the target medium depends mostly onthe ion velocity and on the charges of the ion and target-atoms.A comparison of the nuclear and electronic stopping cross-sections expressedin reduced notation is shown in Fig. 5.3. Recall that ε is proportional to ion energyand that (ε) 1/2 is proportional to ion velocity.At higher velocities, the charge state of the ion increases and ultimately becomesfully stripped of all its electrons at v≥v0Z1. At this point, the ion can be2/3viewed as a positive point charge Z 1 , moving with a velocity greater than the meanorbital velocity of the atomic electrons in the shells or subshells of the targetatoms.When the projectile velocity v is much greater than that of an orbital electron(fast-collision case), the influence of the incident particle on an atom may beregarded as a sudden, small external perturbation. This picture leads to Bohr’stheory of stopping power. The collision produces a sudden transfer of energy fromthe projectile to the target electron. The energy-loss from a fast particle to a stationarynucleus or electron can be calculated from scattering in a central forcefield. The stopping cross-section decreases with increasing velocity because theparticle spends less time in the vicinity of the atom. In this high-energy, fastcollisionregime, the values of electronic stopping are proportional to (Z 1 /v) 2 .Stopping Power(a) k=0.20(b) k=0.15(c) k=0.10S e(b)S nS e(a)(c)v~v 0 Z 12/3S n (~10 –3 S e )0 1 2 3 10 20 30ε 1/2Fig. 5.3. The reduced nuclear and electronic stopping cross-sections as a function of ε 1/2 .The electronic stopping power variable, k, is dependent on the mass and atomic number ofthe ion and target