Ion Implantation and Synthesis of Materials - Studium

9.5 Transient Enhanced Diffusion of Boron 12310 22-3Boron Concentration (cm)10 2110 2010 190.5 keVannealed10 18 0 20 40 60 80 100 120 140Depth (nm)Fig. 9.12. Profiles of boron concentrations for a dose of 10 15 B cm −2 (a) as-implanted and(b) as-annealed at 1,050°C for 10 s (Ziegler 2000)limits. The diffusion length, (Dt) 1/2 , is orders of magnitude greater than thatpredicted from D ~ 10 −13 cm −2 s −1 at 1,050°C.Silicon self-interstitials emitted from the ion implantation damages interactwith the boron atoms to form B–Si pairs that diffuse very fast until they aretrapped or dissociated. Enhancement of boron diffusion can be up to 10 7 times thenormal diffusion value during postimplant annealing. Observation of transientenhanced diffusion (TED) of boron, the mechanisms of TED, and experimentalresults have been reviewed recently by Jain et al. (2002) and Shao et al. (2003).For light ions and low doses, ion implantation produces Frenkel pairs of vacanciesand interstitials. The diffusing interstitials may encounter and kick out substitutionalboron atoms during the first moment of annealing, leading to ultra-fast boronenhanceddiffusion. When ion implantation doses are high enough, rod-likeextended defects appear. These defects, called {311} defects, precipitate on the{311} planes along 〈100〉 directions (Eaglesham et al.). With low implantationdoses and hence low damage, the intrinsic TED is driven by the annealing of smallclusters, while {311} defects contribute to TED with higher dose implantation.Most of the implantation damage is removed during the early stage of annealingvia point defect recombination, leaving excess interstitials approximately equal innumber to the implanted dose. These interstitials then coalesce into extendeddefects. After a short annealing, these extended defects are primarily {311} defects.Transmission electron measurements of the total interstitial density in {311}defects as a function of annealing time show that the emission of interstitials from{311} defects is the main source of interstitials responsible for TED afterannealing of small clusters (Eaglesham et al.).Nearby vacancies and interstitials recombine either dynamically during irradiationor subsequently during postimplant annealing. Extra atoms corresponding tothe implant dose at end-of-range defect positions are responsible for TED. However,since the momentum transfer from an incident ion to a target atom is in the forward