Nucleation and growth of nanoscale metal silicides in nanowires of silicon.

摘要

Transition metal silicides have been used in the SALICIDE process to form gate and source/drain contacts in MOSFET devices. How to control silicide formation in shallow junction devices and the kinetics of single silicide phase formation between the Si and metal thin films have received extensive attention and study. As the trend of miniaturization of Si devices moves from 45 rim to smaller sizes, the formation of nanoscale metal silicides has attracted renewed interest. Nanostructures in Si nanowires have been studied for basic components in electronic and optoelectronics devices, especially for biosensors. Well-defined nanoscale building blocks such as ohmic contacts and gates on Si nanowires must be developed in order to be assembled into functional circuit components in future nanotechnology. It requires a systematic study of solid state chemical reactions in the nanoscale to form these circuit components. In the dissertation, the nucleation and growth of epitaxial silicides in Si nanowires has been discussed and the comparison of silicide formation in thin films and in nanowires has been made. The difference of silicide formation between the thin film case and the nanowire case, especially the kinetics of nucleation and growth, will be emphasized.;In this dissertation, I focused on the nucleation and growth of Ni and Co silicides formation in Si nanowires and used in situ high resolution transmission electron microscopy (TEM) to investigate kinetics of the silicides formation. Heterostructures of silicide/Si/silicide, such as NiSi/Si/NiSi, NiSi2/Si/NiSi2, and CoSi2/Si/CoSi 2, have been produced with sharp interfaces. Periodic multi-heterostructures of NiSi/Si and NiSi2/Si have been synthesized by in-situ TEM. The NiSi, NiSi2, and CoSi2 were found to have epitaxial growth with Si and the growth can be decomposed into nucleation periods and growth periods instead of continues growth as thin film silicides formation on Si wafers. The silicides grow atomic layer by atomic layer. After growing one atomic layer, incubation time is required for supersaturation and grows the next atomic layer. This is supply limit or nucleation limit reaction because of the limited contact area in point contact reactions. The nucleation of silicides was found to be homogeneous nucleation, which is rare except in theory. The experimental and theoretical results are in good agreement. The supersaturation was calculated to be very large.;Both homogeneous and heterogeneous nucleation was found in the NiSi2 formation in [110] Si nanowires by in situ TEM. Two epitaxial interfaces were observed in heterogeneous nucleation where a giant step exists. Without the step, homogeneous nucleation occurs. Incubation time of heterogeneous nucleation of NiSi2 has been measured by high resolution video to be much shorter than that of homogeneous nucleation. The overall growth rate of NiSi 2 for the case of heterogeneous nucleation is faster than that for the case of homogeneous nucleation. Kinetic analysis of both types of nucleation is presented for a direct comparison in order to have a better understanding of the nucleation events.