A nanosize effect upon Ge diffusion (850 to 1000C) in polycrystalline Si layers was investigated. The Ge diffusion coefficients in microcrystalline and nanocrystalline Si layers made of 30µm and 40nm wide grains, respectively, were measured. In the microcrystalline Si layer, the Ge diffusion coefficient in micrograin boundaries was measured using a conventional analytical solution of Fick's equations; corresponding to the Fisher model. In the nanocrystalline Si layer, the Ge diffusion coefficients in nanograins and in nanograin boundaries were measured via a method based upon 2-dimensional simulations using the Fisher model geometry. The diffusivities in nanograins and nanograin boundaries were an order of magnitude higher than in micrograins and micrograin boundaries. However, the nanosize effect appeared to be different in grains and grain boundaries; despite that the activation energy for diffusion in 40nm wide grains was at least 1eV lower than in Si bulk. The activation energy in nanograin boundaries was about the same as in micrograin boundaries.

Nanometric Size Effect on Ge Diffusion in Polycrystalline Si. A.Portavoce, G.Chai, L.Chow, J.Bernardini: Journal of Applied Physics, 2008, 104[10], 104910