The flux and temperature dependence of titanium silicide islands formed by
reactive deposition near 500 degreesC indicate a critical nucleus containin
g 2 Ti atoms and a single activation energy of E-d + 1/2 E-2 = 1.4 +/- 0.2
eV, where E-d and E-2 are the surface diffusion and cluster binding energie
s, respectively. These values are not consistent with STM observations of T
i dimer-vacancy hopping at lower temperatures and show that silicide island
nucleation involves a different, highly mobile Ti species.