Ion Implantation and Synthesis of Materials - Studium

124 9 Doping, Diffusion and Defects in Ion-Implanted Sidirection, the depth profile of interstitials would naturally be slightly deeper thanthat of the vacancies. SRIM simulations of 50 keV or 500 keV Si implanted intoSi reveals the formation of a net vacancy-rich region with a depth of about half ofthe Si projected range. The average separation between vacancy locations andinterstitial locations increases when the implantation energy increases.The implantation of MeV Si ions into a Si substrate can also suppress boronenhanceddiffusion normally associated with a high boron concentration layer(Shao et al. 2003). Junction depths of 20 nm were achieved in samples implantedwith 0.5 keV B ion at a dose of 10 15 cm −2 following a 1,000°C thermal anneal.9.6 Irradiation-Enhanced DiffusionIon irradiation is quite efficient in forming vacancy-interstitial pairs, as shownin Chap. 7 and in the simulation in the previous sections. The atomic displacementsresulting from energetic recoiling atoms can be highly concentrated intosmall, localized regions containing large concentrations of defects well in excessof the equilibrium value. If the defects are produced at temperatures where theyare mobile and can, in part, anneal out, the balance between the rate formation andthe rate of annihilation leads to a steady state of excess concentration of defects.Since the atomic diffusivity is proportional to the defect concentration, an excessconcentration of defects leads to an enhancement in the diffusion process.Enhanced diffusion has been found in Si. It is featured in alloys. Atomic motionin most metals and solid-solution alloys usually occurs by the interchange betweenatoms and neighboring vacant sites. For such a diffusion process, the atomicdiffusion coefficient is given byD =Γ d 2 / 6(9.7)(Shewmon 1963), where Γ is the atomic jump frequency for uncorrelated jumps,and d is the atomic jump distance. For diffusion driven by the vacancymechanism, Γ is given byΓ = fCv vΓ v(9.8)where C v is the vacancy concentration, Γ v is the vacancy jump frequency, which isvvproportional to exp − Em/ kBT , where Em is the vacancy migration energy, andf v is a correlation factor (which is nearly unity).The vacancy diffusion coefficient can be described by an expression similar tothe atomic diffusion coefficient,