Ion Beam Studies of Noble Metal Diffusion in Amorphous Silicon Layers.

The diffusion of Au, Ag, and Cu were measured in amorphous silicon prepared by MeV Ar ion-implantation. Diffusion in the temperature range 150-600^circ C was observed to be Arrhenius in nature with a single activation energy of ~1.5 eV which is approximately the same as the corresponding values in crystalline Si. Moreover there was no evidence of relaxation of the amorphous phase. There is a weak concentration dependence evidenced by a slight increase in D_0 with increasing impurity concentration. Passivating the dangling bonds by hydrogenation had no effect on the activation energy and only slightly increased the D_0 values. During solid phase epitaxial growth of Au-containing amorphous layers, segregation of Au was observed at the moving amorphous-crystal interface. The velocity of the interface increased with Au concentration until it halted when the solid solubility of Au in amorphous Si was exceeded and the Au precipitated resulting in nucleation sites for polycrystal formation. The solubility of Au in amorphous Si at 515^circC is some 6 to 8 orders of magnitude greater than in crystalline Si.