Growth and Electrical Characteristics of Platinum-Nanoparticle-Catalyzed Silicon Nanowires

Silicon nanowires (Si NWs) will likely revolutionize a wide variety of applications ranging from field-effect transistors (FETs) and other nanoelectronics to chemical and biological sensing, and even solar cells. These nanowire devices must be integrated with more traditional electronic or optical components to make a complete usable system, which will probably require standard silicon clean-room processing. Nearly all Si NWs are made using a gold (or gold-based) catalyst and the well-known vapor–liquid–solid (VLS) growth mechanism first discovered by Wagner and Ellis. Because Au creates mid-gap trap states in silicon, it poisons device performance and typically is not allowed for use in electronicsfabrication labs and clean rooms. Therefore a new, electronics-friendly catalyst is critical not only for nanowire electronics, but also for integrated devices incorporating Si NWs in any capacity. Several groups have successfully grown Si NWs with alternative catalyst thin films such as Ti, Al, Pt, and PtSi but extensive electrical characterization that is very important for many device applications has not been conducted. In this report, Pt was chosen as a catalyst because it has a high melting point, can be made into nanoparticles with a tight size distribution and shows orders-of-magnitudelower leakage current when incorporated into silicon diodes compared to gold. We have developed a chemical-vapor-deposition (CVD) synthesis based on our previous experience with gold catalysts to grow high-quality single-crystalline size-controlled epitaxial Si NWs from various sized Pt nanoparticles. The nanowires were characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to determine their size distribution, growth direction, and alignment, whereas their electrical properties were tested by making planar FETs. Unlike the Au–Si system, Pt does not form a simple eutectic with Si; rather, there are several stable platinum silicide compounds in the 800–1000 °C temperature range where Si-NW growth occurs. There is a eutectic formed between PtSi and Si at 979 °C, so at temperatures above this point and at high Si concentrations, it is thermodynamically favorable to precipitate pure Si. Si-NW growth below 979 °C can be explained by two possible mechanisms. First, because the Pt nanoparticles begin melting (at least surface melting) around 600 °C, which is about 1000 °C lower than the bulk melting point, the bulk phase diagram may not accurately represent the phase transitions occurring in the catalyst nanoparticle tip. In a very simplistic view, all the phase boundaries should shift down in temperature, with the Pt-rich phases being affected more strongly than the Si-rich phases. With a shift of over 1000 °C at the pure platinum side of the phase diagram, a 180 °C shift for the PtSi–Si eutectic down to 799 °C at 67 % Si seems likely. Several reports also show that Pt nanoparticles annealed in a hydrogen atmosphere at temperatures as low as 600 °C on silica substrates form PtxSiy [21,22] Additionally, Wagner and Ellis found that even Pt thin films as thick as 100 nm on Si formed a liquid surface layer at temperatures as low as 850 °C, further supporting a significant temperature decrease of the PtSi eutectic point from the bulk value. The second possible explanation is that the Pt nanoparticles do not completely melt and instead act as an active site for rapid SiCl4 decomposition and diffusion, leading to a vapor– solid–solid (VSS) rather than VLS growth mechanism. The VSS mechanism has been proposed to explain the growth of several other semiconducting nanowires, particularly III–V compounds, that were originally thought to grow according to the VLS mechanism. In a recent report, Pt thin films deposited on Si were annealed at 800 °C in a hydrogen atmosphere to form PtSi islands which in turn were used to catalyze Si-NW growth at temperatures between 500 and 700 °C through a proposed VSS mechanism. Considering the strong in situ TEM evidence from the literature mentioned above that the Pt nanoparticles begin melting well below their reaction temperatures, the island formation observed for Pt/Si films near 800 °C, and the evidence of strong eutectic-point depression seen for Pt thin films on Si, the Si NWs in this study most likely grow via the VLS mechanism. However, the VSS mechanism cannot be ruled out without in situ TEM evidence. Pt nanoparticle catalysts with average diameters of (9.3± 1.2) nm were used to synthesize Si NWs with average diameters of (11.3± 1.6) nm (Fig. 1). The standard deviations of the starting colloid and the resulting wire diameters were C O M M U N IC A TI O N