Spontaneous Chemical Vapor Growth of NiSi Nanowires and Their Metallic Properties

Abstract

One-dimensional metallic nanostructures offer challenging opportunities to investigate their possible applications as interconnects in electronic circuitry and as field emitters in field emission displays. In particular, metallic silicide nanowires are promising candidates, because their growth can be easily integrated with silicon processing technology. Indeed the synthesis of single-crystalline, transition metal silicide nanowires, such as TaSi2, [6] e-FeSi, and CoSi, has been recently reported using diverse methods. The synthetic methods employed involve relatively high temperature processes, and may not be compatible with the low temperature processes required for their practical applications into electronic and display devices. In contrast, single-crystalline Ni-silicide nanowires, which have been experimentally verified to possess intrinsically low resistivity at low dimensions compared with the others, can be processed at relatively low temperatures, thus can provide practical advantages. For example, Wu et al. reported highly metallic properties of single-crystalline NiSi nanowires fabricated by a solid-state reaction at 550 °C between individual Si nanowires and Ni overshells. Here a simpler and controlled synthesis at low temperatures (∼400 °C) is reported, where single-crystalline NiSi nanowires spontaneously grow by SiH4 chemical vapor deposition (CVD) on Ni thin films predeposited on various substrates, such as SiO2/Si, quartz, and indium tin oxide (ITO) substrates. In fact, the Ni-catalyzed vapor phase thin film deposition has been previously investigated for applications into Si microelectronics and photovoltaics. Specifically, the dimensionality of the Ni-silicides have been reproducibly directed from thin films to single-crystalline nanowires, as well as their phases, by fine-tuning of growth parameters during CVD of a SiH4 gas precursor on predeposited Ni thin films. It is noted that Decker et al. and Kim et al. recently reported similar Ni-silicide nanowire growth by CVD and sputtering, however, the detailed growth mechanism, which is the main focus of this study, is unavailable to date. Here, by providing close observations of the morphological and phase evolution of Ni-silicide nanowires under optimized growth conditions, the spontaneous nanowire growth mechanism is discussed based on one-dimensional nucleation and growth of NiSi at a low supersaturation limit along with one-dimensional Ni diffusion during the chemical vapor reaction. Singlecrystalline NiSi nanowires in this study exhibit typical metallic behaviors, and show promising field-emission properties. It is suggested that our simple method to spontaneously grow NiSi nanowires at low temperatures can provide a practical strategy to fabricate metallic nanostructures based on bottom-up synthetic approaches. Our synthetic approach employs the Ni-catalyzed decomposition of SiH4, [14] which can occur well below the thermal decomposition temperature of SiH4 at above 600 °C. Thermally evaporated Ni thin films of 60–80 nm in thickness are used. During the CVD of SiH4 within the wide range of precursor partial pressures and growth temperatures of 10–100 torr and 250–600 °C, the reaction produces either Ni-silicide thin films or nanowires of various phases. Figure 1 illustrates the evolution of the morphology and the phases of Ni-silicides, grown at 250, 400, and 600 °C at 50 torr of SiH4, with the corresponding scanning electron microscopy (SEM) images, X-ray diffraction (XRD), and element depth profiling by Auger spectrometry. Within the entire growth range, Ni-silicide thin films have been commonly formed on the substrates; however, the surface morphology of the Ni-silicide thin films has been modified from planar sheets to nanowires, as well as phases in Ni-to-Si stoichiometry. The observations from Figure 1 are summarized in the following within the framework of SiH4 vapor–Ni solid reactions, where SiH4 decomposition and solid Ni diffusion are both thermally activated. First, at 250 °C, the catalyzed decomposition of SiH4 and the Ni out-diffusion from the pre-deposited Ni thin films are not sufficient to turn the Ni film into a Ni-silicide thin film during the reaction, and C O M M U N IC A IO N