X-ray diffraction, high resolution transmission electron microscopy, and resistivity measurements were used to demonstrate a modification of the Ti/Si reaction path consisting of direct nucleation followed by diffusion limited growth of low resistivity C54 TiSi2 without nucleation of high resistivity C49 TiSi2, for the reaction of Ti with Mo doped polycrystalline or Mo doped amorphous Si by rapid thermal processing at 650 °C. We also report the mechanism of early C54 nucleation. We demonstrate that MoSi2 and an unidentified silicide phase lattice matched to C54 TiSi2, with spacings of 4.15 and 2.26 Å, nucleate along the Ti/Si interface at early stages of the reaction and act as templates on which C54 TiSi2 nucleates and grows epitaxially. In contrast, the conventional phase sequence, nucleation and growth of C49 TiSi2 preceding nucleation of C54 TiSi2, was observed for the Ti/Mo doped single crystal (100) Si reaction and for all samples without Mo. © 1998 American Institute of Physics.