Neutron reflectometry was used to study the temperature-induced self-diffusion, of isotopically enriched thin films of amorphous 11B, on 10B which had been deposited by means of electron-beam evaporation. Predicted B density profiles for samples which had been annealed at 360 or 400C for various times were fitted to the reflectivity data. Although the 11B/10B interface did not move, relative to the air/B interface, during annealing, the expected normal Fickian diffusion behavior of the annealed samples was not observed. A pinned Fickian diffusion model, which imposed the boundary condition of a fixed composition of 10B at the interface, was found to fit the reflectivity data closely. The equilibrium diffusion constants were typically of the order of 10-17cm2/s at 360C. The measured diffusion coefficients were inconsistent with the high melting point of B, but were consistent with measured B diffusivities in other amorphous thin films. The suggested presence of clusters in the B film was used to explain the results.

S.M.Baker, G.S.Smith, N.J.S.Brown, M.Nastasi, K.Hubbard: Physical Review B, 1997, 55[11], 7255-63