Oxygen in silicon: Switch in the diffusion-mediated mechanism

Abstract

The impact of a heavy doping on oxygen diffusion at 350 ${}^{\ensuremath{\circ}}\mathrm{C}\ensuremath{-}700{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ is widely discussed in literature, however, the retardation/enhancement mechanisms remains unclear at that temperature range. In this paper, we study the impact of heavy doping on the oxygen diffusion coefficient in silicon by using density functional theory calculations. While it is known that the lowering of temperature induces a switch in the diffusion mechanism from monomer mediated diffusion to dimer one, we have discovered that the reported enhanced oxygen diffusion in $p$-doped silicon is driven by a switch back from the dimer to monomer. We base our claim on extensive calculations of both pre-exponential factors and activation energies in various doping and stress conditions. We show that the stress has a negligible effect and we attribute the switch back to monomer diffusion at low temperatures in $p$-doped materials, to a charge assisted mechanism that reduces the migration energy of the monomer of 0.4 eV, while the diffusion rate is kept high thanks to the pre-exponential factor. We also provide comparisons to $n$-doped and isovalent cases.