STUDY OF TUNGSTEN NANOWIRES GROWN BY FIELD EMISSON INDUCED METHOD

Abstract

This thesis concerns tungsten nanowires grown by the field-emission-induced growth technique. The method allows individual nanowires to be grown at intended locations, and has the potential for nanowiring of nanodevices. The objective of the current work is to study the nanowire properties such as structural morphology and electrical resistivity. From transmission electron microscope (TEM) imaging, the nanowires are polycrystalline and contain grains of variable sizes depending on the nanowire diameter. 4-terminal current-voltage measurements of nanowires of diameters ranging from 10 to 50nm show size effects in which the resistivity is much higher than that of bulk tungsten due to enhanced electron scattering at both nanowire surface and grain boundaries. Analyses of the experimental results reveal grain boundary scattering to be the dominant contributor to increased resistivity. Morphology studies conducted in situ in the TEM show the processes of grain growth and grain grooving in nanowires as a result of annealing. The polycrystalline tungsten nanowires transform to bamboo-like structure, with transversal grain boundaries normal to the nanowire axis, at particular temperatures which depend on the nanowire diameter. As a result of grain grooving, a bamboo-like nanowire eventually breaks up at developed grain grooves, giving rise to a form of morphological instability peculiar to polycrystalline nanowires at elevated temperatures. We also experimentally identified that breakage occurs at grains with a larger initial aspect ratio of grain length to grain diameter of about 3, which is in good agreement with theoretical predictions based on the kinetic process of atomic diffusion at the grain surface. In addition, we propose a methodology to estimate the surface diffusion coefficient of one-dimensional nanowires structure. By overcoating a carbonaceous layer onto the nanowire surface, the grain grooving process in the tungsten-carbon core-shell structure can be prevented due to inhibition of surface diffusion. As a result, the morphology stability of nanowire can be improved, and the nanowire can sustain considerably higher current density and temperature arising from Joule heating, until another form of instability takes place, finally leading to nanowire failure by electromigration. Oxidation of polycrystalline tungsten nanowire was carried out by heating in a low-pressure oxygen environment. The oxidation is shown to be kinetically limited and the metallic core is seen to shrink to an asymptotically small diameter, arising from accumulated stress in oxide. Theoretical analysis suggests that this self-limited oxidation mechanism is mainly due to a reduction of the oxidation rate by the compressive stress at tungsten/oxide interface. The results from this project provide useful directions for further explorative studies on the application of polycrystalline metallic nanowires.