Using the plane wave pseudopotential approach, the structure and mechanism of migration of an interstitial boron pair are proposed. The energy of the proposed configuration is lower by at least 0.2 eV in comparison to other interstitial boron pairs. The proposed model, which is equivalent to a B 2 I 2 pair ͑using the notation where B m I n refers to m boron atoms and n silicon atoms occupying m regular sites͒, migrates in the ͗110͘ channel with energy barriers between intermediate sites not greater than 0.1 eV and a total energy barrier of 0.6 eV between stable sites. Conventional continuum and kinetic Monte Carlo models of formation of boron interstitial clusters ͑BIC's͒ do not consider the mobility of the proposed configuration in the growth process. Its stability against dissociation could have considerable implications for the modeling of transient enhanced diffusion of boron in silicon.The relentless decrease of feature sizes of semiconductor devices introduces a serious problem in precise control of dopant distribution due to the phenomenon of transient enhanced diffusion ͑TED͒. Although this ultrafast diffusion occurs only for a short period after thermal annealing, swiftly diffusing species have sufficient amount of time to migrate for distances that are comparable to device feature size, creating a problem in monitoring the depth of dopant penetration into the material. It is believed that TED is induced by interactions between dopants and native defects ͑self-interstitials or vacancies͒, which are produced as a result of bombardment by energetic ions during the process of ion implantation. 1 In silicon, TED of boron, a widely used acceptor dopant, is caused predominantly by interstitials as generally agreed on the basis of experimental observations, 2 kinetic Monte Carlo simulations, 3 and ab initio modeling. 4 -7 On an atomic level it was proposed that boron diffuses via alternating ''kick-out'' and ''kick-in'' of boron atom by a self-interstitial. 4,5 In such a mechanism the boron atom at a substitutional site is displaced by a self-interstitial ͑kick-out͒ and forced to migrate until it falls into another substitutional site releasing a self-interstitial ͑kick-in͒. Using experiment, Cowern et al. estimated that kick-out is associated with an energy barrier less than 0.3 eV, 8 which is substantially lower than ''kick-in'' ͑1 eV͒, as obtained by ab initio calculations. 4 Furthermore, Monte Carlo simulations have shown that a kick-in reaction is usually followed by immediate kick-out and therefore boron atoms are not trapped at substitutional sites but migrate as boron-interstitial pairs. 9 In this study the migrational barrier of boron diffusion energy required to fit mean migrational path length was estimated to be 0.55 eV and kick-out and kick-in barriers were 0 and 0.8 eV, respectively. 9 Subsequent ab initio calculations 6,7 confirmed that boron diffuses forming a mobile boron-self-interstitial pair with energy barrier in the range 0.4 -0.7 eV and kick-in dissociation barrier of 0.8 eV.To realize ...