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2009, 113, 6898–6901 Published on Web 04/06/2009
Diameter-Controlled Synthesis of Phase-Change Germanium Telluride Nanowires via the Vapor-Liquid-Solid Mechanism Andrew T. Jennings, Yeonwoong Jung, Johanna Engel, and Ritesh Agarwal* Department of Materials Science and Engineering, UniVersity of PennsylVania, 3231 Walnut Street, Philadelphia, PennsylVania 19104 ReceiVed: February 27, 2009; ReVised Manuscript ReceiVed: March 29, 2009
The ability to control the size of nanostructures still presents one of the biggest challenges in nanosciences. While impressive progress has been made toward diameter-controlled synthesis of nanocrystals via solutionphase chemical techniques, control over nanowire diameters grown via the gas-phase vapor-liquid-solid mechanism is still challenging. Diameter-controlled growth of nanowires have been reported by controlling the size of the metal nanocatalysts, which is a general technique. However, the complex dynamics of gasphase reactants and their reaction with catalysts requires in-depth understanding of the effect of various growth parameters on the size of catalysts during growth, which makes diameter-controlled growth of nanowires challenging. Here, we report diameter-controlled growth of GeTe nanowires, which are important materials for phase-change memory devices. Recently, several groups have investigated phase-change nanowires for memory applications, but the ability to control their diameters has been lacking. This lack of nanowire size control has made investigation of phase-change memory switching difficult for both fundamental science and device applications. We find that by controlling the rate of supercooling and the reactant supply rate we can produce large quantities of nanowires with uniform, narrow diameter distributions. The effects of various growth parameters such as temperature, pressure, and reactant supply rate on nanowire morphologies are discussed. Introduction Phase-change materials (PCMs), which reversibly switch between crystalline and amorphous phases, have demonstrated considerable promise for next generation memory devices due to the materials’ nonvolatility, rapid switching times, and random access capability. Ever since the discovery of phase-change phenomena in 1968 by Ovshinsky,1 subsequent generations of research have developed faster PCMs with properties that are competitive with modern magnetic and DRAM memory technologies.2-4 One of the main challenges for next generation memory is the continued scalability of the technology to produce efficient, high-density devices. As device feature sizes decrease, bottomup-based technologies become attractive due to their ability to assemble materials at nanoscale dimensions with potentially atomic level control. PCM nanowires have shown considerable promise for memory devices,4-9 since the unique geometry of nanowires greatly improves device performance in comparison to the bulk due to heat and current localization within their structure.7-11 Also, it has been shown that, by combing different compositions of phase-change materials in heterostructured devices,12,13 significant increase in memory density can be achieved through multiple memory states within each cell.12 However, in order to fully study phase-change mechanisms and utilize them for memory applications, synthesis of PCM nanowires with controlled diameter distributions is paramount. * To whom correspondence
[email protected].
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E-mail:
10.1021/jp901845c CCC: $40.75
Better control of nanowire diameter has been a long-standing and important issue in realizing nanowires as building blocks for electronic and photonic devices.14,15 Size control becomes critical in PCM nanowires, since the lateral size of the nanowires effectively confines current and heat, thereby determining the power required for memory switching and data retention.5-13,16 Diameter control of vapor-liquid-solid (VLS)17-grown nanowires is typically achieved by controlling the catalyst size, which seeds the growth of nanowires. However, all previously reported syntheses of PCM nanowires based on the VLS process5-13 have produced nanowires with large diameters and poor control over their size distribution and morphologies including a variety of unwanted nano/microsctructures from side-reactions.5,6,8 For example, Yu et al. used colloidal Au nanoparticles of ∼5-10 nm, and found that the average diameters of the VLS-grown GeTe nanowires were 65 ( 20 nm along with nanoscale helices.9 This difficulty in morphological control is also apparent in the recently developed solutionbased synthetic approach for GeTe nanowires where Te nanoparticles are typically found attached on the surface of growing GeTe nanowires.18 The lack of control over nanowire diameters along with these undesired growths complicate fundamental investigations of phase-change properties and their use for future memory devices. Further complications arise from the fact that GeTe is a non-stoichiometric alloy, as it is a IV-VI materials system. The chemical composition of GeTe in the atomic ratio of 1:1 is possible only with the formation of a large number of vacancies. This leads to added complexity of 2009 American Chemical Society
Letters controlling the exact chemical composition of GeTe nanowires along with control over their diameters. The biggest challenge to grow nanowires with uniform diameters is to make sure that the metal catalyst remains stable at its initial size throughout the growth process.19 There are two main sources for an increase in catalyst size; the catalyst particles may agglomerate through Ostwald ripening,20,21 or the vaporized GeTe precursor increases the catalyst size prior to nanowire growth. It is known that nanometer-sized metals in the liquid phase possess a significantly higher solubility limit for solutes than their bulk counterparts, visible in a noticeable suppression of the phase diagram’s liquidus line, for example, as known for the Au-Ge binary system.22 As a result, at temperatures above the system’s eutectic temperature, where the bulk phase diagram would predict a small volume increase of Au-Ge droplets with increasing temperature, a substantial size increase of the droplets may occur for nanoscale systems. The catalyst expansion leads to an equivalent expansion of the nanowire’s diameter, therefore rendering high-temperature strategies for diameter control ineffective.22 Although low temperatures are preferred to maintain the catalyst’s initial size, they also increase the degree of supercooling, resulting in a large driving force for the formation of large GeTe microcrystals and other undesirable nucleation events. Therefore, the competition between different growth parameters establishes requirements to carefully optimize growth conditions for diameter control. In this Letter, we report uniform, controlled distributions of germanium telluride (GeTe) nanowires with diameters between 20 and 100 nm via the VLS process at lower temperatures and controlled reactant delivery. The mechanisms for the optimized nanowire growth are discussed in detail and contrasted with nonideal conditions. Therefore, the ability to control the chemical composition of a non-stoichiometric phase-change alloy along with the nanowire diameters down to 20 nm presents a significant advance toward assembly of phase-change memory devices with controlled properties. Experimental Section Silicon wafers (Silicon Sense) were used as the growth substrates for GeTe nanowire growth. Poly-L-lysine solution (Ted Pella) was subsequently applied to the cleaned substrates to act as a binding agent to promote the adhesion of gold colloids on the silicon substrate. Size-controlled gold colloids (Ted Pella), with diameters of 20 ( 1.6, 50 ( 4, 80 ( 6.4, and 100 ( 8 nm, were first diluted in a 1:5 solution of acetone and then spread uniformly on the substrate surface. The colloidal solution was rinsed off after a few seconds with acetone, and the substrate was gently dried. GeTe nanowires were grown in a horizontal tube furnace with 0.7 mg of GeTe (Alfa Aesar) placed 12 cm upstream (∼450 °C) from the thermocouple in an alumina boat, and the Au-covered substrate placed 17 cm downstream (∼275 °C) from the thermocouple. The furnace was evacuated to 0.01 Torr before filling with Ar gas (flow rate, 15 SCCM) to a base pressure of 120 Torr. The furnace was then heated to 720 °C with reaction times varied between 15 min and 2 h to control the length of the nanowires. The furnace was slowly cooled to 300 °C before rapidly cooling to room temperature. Different vapor concentrations were obtained by varying the evaporation temperature as well as by placing different amounts of powder (precursor) in the furnace. More powder in the furnace at a particular temperature will increase the absolute vapor concentration of GeTe. Furthermore, at constant mass, an increased temperature will dramatically increase the vapor pressure. Nanowire morphologies and their diameters were determined
J. Phys. Chem. C, Vol. 113, No. 17, 2009 6899
Figure 1. SEM images of GeTe nanowire morphology under optimal growth conditions. (A and B) 80 nm GeTe wires grown for 15 min. Both parts A and B are images of nanowires on the growth substrate. (C) A large group of 40 nm nanowires grown for 2 h. (D) A 60 nm nanowire grown for 2 h. The inset is a magnified image of the same wire. Parts C and D are images taken of wires transferred to a second substrate.
using an FEI Strata DB235 scanning electron microscope (SEM). Accurate measurement of the diameters was further performed using a JEOL 2010F transmission electron microscope (TEM), and the chemical composition of individual nanowires was characterized by energy dispersive X-ray spectroscopy (EDS) in scanning TEM (STEM) mode. Results and Discussion Nanowires grown for only 15 min under growth conditions as mentioned above (Figure 1A,B) are typically 200-500 nm long and grow initially perpendicular to the substrate with Au catalysts clearly visible at their ends. Some nanowires (Figure 1A) subsequently change their directions and grow at varying angles. Nanowires grown under the above conditions but for 2 h are shown in Figure 1C,D. These wires were mechanically transferred to a second substrate for imaging. The nanowires show substantially increased lengths (>10 µm), while maintaining uniform diameters along their entire length (Figure 1C,D). Nanowire diameters were measured at multiple positions along the lengths of the wires to verify morphological integrity: at the catalyst/nanowire interface, at the nanowire/substrate interface, and along the length of the nanowires. At the nanowire/ substrate interface, nanowires typically exhibit a slight broadening, as seen in Figure 1B. Comparing diameters at different positions, it is observed that the taper region in GeTe nanowires is localized only at the interface with the substrate. It should be noted that the tapered region is extremely small,
