Ion Implantation and Synthesis of Materials - Studium

88 7 Displacements and Radiation DamageNormal AtomInterstitial AtomPath of Primary ParticlePath of Primary Knock-OnFig. 7.5. Schematic of a highly damaged volume of material, formed when the mean freepath between collisions, λ d, approaches the atomic spacing of the target atoms. The densecascade is referred to as a displacement spike (after Brinkman, 1956)mean deposited energy density in the spike by3θ D = kT B2(7.14)where k B is Boltzmann’s constant.The quench time, t q , for a thermal spike of radius r is extremely small and canbe estimated bytq2= r4DT(7.15)(Davies 1983), where D T is the thermal diffusivity of the target. Using Au as anexample, with D T = 0.7 cm 2 s −1 . and assuming r = 8 nm, results in a quench time≈ 2 × 10 −13 s.