Diffusion along Small-Angle Grain Boundaries in Silicon

Diffusion fronts in samples containing grain boundaries are spike shaped. Velocity of spike advance and angle between spike and boundary are measured. The "spike-velocity method" of analysis permits evaluation of two effective widths, $W_D$ and $W_0$, which describe the diffusion properties of the boundary. This method has been used to analyze data on phosphorus diffusion into boron-doped silicon at 1200°C and 1050°C. It is concluded that an enhanced diffusion current flows along each dislocation of the grain boundary over a cross section less than one Burgers-vector square. The diffusion current density is about 300 000 times that of the bulk. This corresponds to an energy of 1.5 ev by which grain boundary diffusion is favored over the bulk diffusion. This enhancement is believed to be caused by enrichment of phosphorus and also partly by the extra concentration of vacancies near the dislocation cores. Some possible extensions of the studies to include saturation effects at the dislocation cores are discussed.