Observation of nonstandard Fickian diffusion at the interface of isotopically pure amorphousB11onB10by neutron reflectometry

As part of a larger study to investigate atomic-diffusion behavior in both elemental boron and refractory transition-metal borides, neutron reflectometry was used to examine the temperature-induced self-diffusion of isotopically enriched thin films of amorphous ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ on ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ deposited by electron-beam evaporation. The reflectometry studies were performed and model boron density profiles for samples annealed at various times and temperatures of 360 and 400 °C were fit to the reflectivity data. Although the ${}_{}{}^{10}{}_{}{}^{}\mathrm{/}_{}^{11}{}_{}{}^{}$B interface did not move relative to the air/boron interface upon annealing, the expected standard Fickian diffusion for the annealed samples was not observed. A pinned Fickian diffusion model, which imposes the boundary conditions of a fixed composition of ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ to ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ at the interface, fit the reflectivity data accurately and consistently. A typical equilibrium diffusion constant was determined to be ∼${10}^{\mathrm{-}17}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{\mathrm{-}1}$, measured at an annealing temperature of 360 °C. The measured diffusion constants are inconsistent with the high melting temperature of elemental boron, but are consistent with measured boron diffusion constants in other amorphous thin films. The presence of clusters in the boron film is proposed to explain the observed results.