Neutron reflectometry study of thermally induced boron diffusion in amorphous elemental boron

We have begun a series of neutron reflectometry studies to investigate atomic diffusion behavior in both elemental boron and refractory transition-metal borides. In this paper, we report on the results for boron-atom self-diffusion in amorphous elemental boron. The samples used for the measurements were thin-film bilayer structures consisting of alternating layers of isotopically enriched 10B and 11B, deposited by electron-beam evaporation. The reflectometry studies were performed with a beam of pulsed neutrons having a broad thermal spectrum incident on the samples at an angle of approximately 1.00 degree(s). Specularly reflected neutrons were detected as a function of their time-of-flight with a position-sensitive detector. This data was reduced to obtain a plot of the reflectivity vs. the perpendicular wavevector. Because of the different nuclear potentials for the two boron isotopes, an interference pattern was observed, analogous to those obtained by X-ray measurements on samples with a modulated index of refraction. Reflectivity profiles were measured and analyzed as a function of annealing time for temperatures of 360 degree(s)C and 400 degree(s)C. Variations in the interference patterns were then used to determine the diffusion coefficient, D, at 360 degree(s)C, and hence the intrinsic diffusion parameters. A typical equilibrium result was determined to be D approximately 10-1 angstroms2s, measured at an annealing temperature of 360 degree(s)C. The measured diffusion constants are inconsistent with the high melting temperature of elemental boron, but are consistent with measured diffusion constants in other amorphous thin films.