Synthesis and characterization of silicide nanowire materials using chemical vapor deposition of single source precursor molecules.

摘要

Transition metal silicides represent an extremely broad set of refractory materials that are currently employed for many technologically relevant applications including complementary metal oxide semiconductor devices, thin film coatings, bulk structural components, electrical heating elements, photovoltaics, and thermoelectrics. Many of these applications may be improved by making one-dimensional nanomaterials. General and rational chemical synthesis of silicide nanomaterials is challenging, due in part to the multiple stoichiometries and complex phase behavior exhibited by many silicide compounds. To overcome this challenge, single source precursor molecules that comprise all the elements necessary to form silicide nanowires were utilized. Using the single source precursor, trans-Fe(SiCl3)2(CO)4 (Co(SiCl 3)(CO)4) we synthesized FeSi (CoSi) nanowires and performed x-ray and electrical characterization of the wires. Surface selective deposition of the precursor molecule was demonstrated along with patterned growth of nanowires on substrates with a variety of shapes and feature sizes. Because of the interesting magnetic properties of the alloys between FeSi and CoSi (Fe1--xCoxSi is a solid solution over the entire range of x), we prepared Fe1--xCoxSi nanowires using a solution of individual precursors and studied their magnetoresistance. X-ray magnetic circular dichroism was performed to probe the elemental and electronic contribution to the spin polarization in the material. Nanowires were mounted to conductive silicon posts and atom probe tomography was used to determine the microstructure of the wires with A and ppm spatial and atomic resolution, respectively. The spin polarization of the nanowires was measured using point contact Andreev reflection spectroscopy by making nanowire devices with Nb metal leads. Through various silicide nanowire growth experiments, we now have a good, but incomplete picture of the NW growth mechanism in play. A review of these experiments, our current growth mechanism hypothesis and future experiments aimed at elucidating this process is included. Lastly, progress and future direction towards using silicide nanowires as silicon spin injection sources is given including future device concepts utilizing the inherent spin polarization in Fe1--xCo xSi and other magnetic silicide nanowires.