3236 Chem. Mater. 2010, 22, 3236–3240 DOI:10.1021/cm1004483
Liquid-Phase Synthesis of Uniform Cube-Shaped GeTe Microcrystals Matthew R. Buck, Ian T. Sines, and Raymond E. Schaak* Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 Received February 12, 2010. Revised Manuscript Received March 30, 2010
Many Ge-based chalcogenide alloys, including GeTe, exhibit a reversible amorphous-to-crystalline phase change that is the basis for a wide range of current and next-generation technologies. Solution routes are attractive alternative strategies for synthesizing these materials, because they have the potential to impart morphology control on the crystallites and permit liquid-based processing of films and patterned structures. This paper describes a liquid-phase route to crystalline rhombohedral GeTe crystallites with cube-shaped morphologies and edge lengths of 1.0 ( 0.2 μm. The microcrystallites can be deposited onto planar substrates to produce highly textured (002) oriented films. During TEM imaging, the particles undergo electron beam induced fragmentation and, in some cases, partial amorphization. The GeTe crystallites are characterized by XRD, SEM, EDS (including element mapping), DSC, TEM, and electron diffraction. Introduction Chalcogenide alloys exhibiting a thermally induced, reversible, amorphous-to-crystalline phase change are important for a wide range of technologies.1,2 The large difference in reflectivity between the amorphous and crystalline states enables the operation of common optical data storage media, such as CDs and DVDs.3-5 The amorphous-tocrystalline transition is also accompanied by a significant decrease in resistivity, which is the basis of emerging and next-generation technologies such as phase change random access memory (PCRAM),6,7 phase change-based logic,8 and cognitive computing.9 Widespread use of phase change technologies that operate via differential resistivity, however, still relies critically on optimizing material parameters that govern the speed, cyclability, and power consumption of devices.7,10,11 Many technologically relevant phase-change materials are located on the ternary Ge-Sb-Te phase diagram, including alloys along the pseudobinary GeTe-Sb2Te3 tie line, Ge15Te85, and doped Sb2Te.11 Recent synthetic *Corresponding author. E-mail:
[email protected].
(1) Raoux, S. Annu. Rev. Mater. Res. 2009, 39, 25–48. (2) Raoux, S.; Welnic, W.; Ielmini, D. Chem. Rev. 2010, 110, 240–267. (3) Yamada, N.; Ohno, E.; Nishiuchi, K.; Akahira, N.; Takao, M. J. Appl. Phys. 1991, 69, 2849–2856. (4) Yamada, N. MRS Bull. 1996, 21, 48–50. (5) Wuttig, M.; Yamada, N. Nat. Mater. 2007, 6, 824–832. (6) Hudgens, S.; Johnson, B. MRS Bull. 2004, 29, 829–832. (7) Lankhorst, M. H. R.; Ketelaars, B. W. S. M. M.; Wolters, R. A. M. Nat. Mater. 2005, 4, 347–352. (8) Lyke, J. In Phase Change Materials: Science and Applications; Raoux, S., Wuttig, M., Eds.; Springer: Berlin; Chapter 18, pp 409-430. (9) Ovshinsky, S. R. Jpn. J. Appl. Phys. 2004, 43, 4695–4699. (10) Wuttig, M. Nat. Mater. 2005, 4, 265–266. (11) Lencer, D.; Salinga, M.; Grabowski, B.; Hickel, T.; Neugebauer, J.; Wuttig, M. Nat. Mater. 2008, 7, 972–977. (12) Chen, Y. C.; Rettner, C. T.; Raoux, S.; Burr, G. W.; Chen, S. H.; Shelby, R. M.; Salinga, M.; Risk, W. P.; Happ, T. D.; McClelland, G. M., et al. 52nd International Electron Devices Meeting; San Francisco, Dec 11-13, 2006; IEEE: Piscataway, NJ, 2006; pp 1-4.
pubs.acs.org/cm
routes to these materials have largely focused on sputter deposition and lithographical patterning of phase-change films.7,12-16 Nanoscale phase change materials have been important targets, particularly as platforms for investigating the influence of reduced dimensionality on crystallization properties,15-18 melting temperature,19,20 thermal bistability,21,22 and material degradation.23 Among the nanoscale materials that have been made, single-crystalline phase change nanowires fabricated by chemical vapor deposition (CVD) via vapor-liquid-solid (VLS) growth have received the most attention.24-30 Electrical contacts (13) Kim, S. M.; Shin, M. J.; Choi, D. J.; Lee, K. N.; Hong, S. K.; Park, Y. J. Thin Solid Films 2004, 469, 322–326. (14) Zhang, Y.; Wong, H-S.P.; Raoux, S.; Cha, J. N.; Rettner, C. T.; Krupp, L. E.; Topuria, T.; Milliron, D. J.; Rice, P. M.; Jordan-Sweet, J. L. Appl. Phys. Lett. 2007, 91, 013104. (15) Raoux, S.; Rettner, C. T.; Jordan-Sweet, J. L.; Kellock, A. J.; Topuria, T.; Rice, P. M.; Miller, D. C. J. Appl. Phys. 2007, 102, 094305. (16) Zhang, Y.; Raoux, S.; Krebs, D.; Krupp, L. E.; Topuria, T.; Caldwell, M. A.; Milliron, D. J.; Kellock, A.; Rice, P. M.; Jordan-Sweet, J. L.; Wong, H-S.P. J. Appl. Phys. 2008, 104, 074312. (17) Martens, H. C. F.; Vlutters, R.; Prangsma, J. C. J. Appl. Phys. 2004, 95, 3977–3983. (18) Wang, W. J.; Shi, L. P.; Zhao, R.; Lim, K. G.; Lee, H. K.; Chong, T. C.; Wu, Y. H. Appl. Phys. Lett. 2008, 93, 043121. (19) Raoux, S.; Shelby, R. M.; Jordan-Sweet, J. L.; Munoz, B.; Salinga, M.; Chen, Y.-C.; Shih, Y.-H.; Lai, E.-K.; Lee, M.-H. Microelectron. Eng. 2008, 85, 2330–2333. (20) Sun, X.; Yu, B.; Ng, G.; Meyyappan, M. J. Phys. Chem. C 2007, 111, 2421–2425. (21) Gotoh, T; Sugawara, K.; Tanaka, K. Jpn. J. Appl. Phys. 2004, 43, 818–821. (22) Lee, S.-H.; Jung, Y.; Agarwal, R. Nano Lett. 2008, 8, 3303–3309. (23) Lai, S. Electron Devices Meeting 2003 Technical Digest; IEEE: Piscatway, NJ, 2003; 10.1.1-10.1.4. (24) Meister, S.; Peng, H.; McIlwrath, K.; Jarausch, K.; Zhang, X. F.; Cui, Y. Nano Lett. 2006, 6, 1514–1517. (25) Yu, D.; Wu, J.; Gu, Q.; Park, H. J. Am. Chem. Soc. 2006, 128, 8148– 8149. (26) Jung, Y.; Lee, S.-H.; Ko, D.-K.; Agarwal, R. J. Am. Chem. Soc. 2006, 128, 14026–14027. (27) Lee, S.-H.; Ko, D.-K.; Jung, Y.; Agarwal, R. Appl. Phys. Lett. 2006, 89, 223116. (28) Lee, S.-H.; Jung, Y.; Agarwal, R. Nat. Nanotechnol. 2007, 2, 626–630. (29) Lee, J. S.; Brittman, S.; Yu, D.; Park, H. J. Am. Chem. Soc. 2008, 130, 6252–6258.
Published on Web 04/15/2010
r 2010 American Chemical Society
Article
Chem. Mater., Vol. 22, No. 10, 2010
for interrogating the resistive phase change behavior can be attached directly to the ends of the nanowires, and the melting temperature required to achieve the crystallineto-amorphous transition has been shown to scale directly with nanowire diameter. Solution-based synthetic routes represent an attractive alternative for depositing films and generating dimensionally controlled nanoscale materials, including for quantum-confined binary antimonide and telluride semiconductors. However, there have been few reports describing liquid-phase routes to chalcogenide phase change materials. Milliron and co-workers deposited GeSbSe phase change films from solutions containing Ge-Se and Sb-Se precursors, which had been prepared by dissolving the bulk metal chalcogenides in hydrazine.31 Korgel and co-workers reacted diphenylgermane and a trioctylphosphine-tellurium complex (TOP-Te) in supercritical hexane, which generated micrometer-scale GeTe octahedra decorated with vertically aligned arrays of Te nanowires.32 GeTe nanowires were formed in high-boiling organic solvents via seeded growth from Bi nanoparticles, but the product contained large amounts of excess Te.33 More recently, a GeI2-TOP adduct and TOP-Te were reacted at 250 C in the presence of an activating alkylthiol to produce amorphous GeTe nanoparticles.34 Interestingly, in situ X-ray diffraction measurements while heating revealed a dramatic increase in crystallization temperature above the bulk value, which was strongly dependent on particle size. Here, we describe an important addition to the small number of reports on solution routes to Ge-based phase change materials: a benchtop liquid-phase synthesis of crystalline GeTe via reduction of GeI2 with tert-butylamine borane (TBAB) in the presence of TOP-Te at 180 C. The GeTe product consists of highly faceted microcrystals having a cube-shaped morphology with a narrow size distribution. These microcrystals can be deposited on a planar substrate to form highly textured films. The effects of exposure to an electron beam, including electron beam induced fragmentation and partial amorphization, are also reported. Experimental Section Materials. All chemicals were used as received. Germanium(II)iodide (GeI2, 99.99þ%), trioctylphosphine (TOP, tech. 90%), tertbutylamine borane complex (TBAB, powder 97%) and octyl ether (99%) were purchased from Aldrich. Tellurium powder (99.99%, -325 mesh) was obtained from Alfa Aesar. All syntheses were carried out under Ar using standard Schlenk techniques, and workups were performed in air. Synthesis of GeTe Microcrystallites. A clear, yellow stock solution 0.4 M in TOP-Te complex was first prepared by dissolving 512 mg of Te powder in 10 mL of TOP at 260 C. GeI2 (66 mg) was dissolved in 2.5 mL of TOP via ultrasonication and (30) Jennings, A. T.; Jung, Y.; Engel, J.; Agarwal, R. J. Phys. Chem. C 2009, 113, 6898–6901. (31) Milliron, D. J.; Raoux, S.; Shelby, R.; Jordan-Sweet, J. Nat. Mater. 2007, 6, 352–356. (32) Tuan, H.-Y.; Korgel, B. A. Cryst. Growth Des. 2008, 8, 2555–2561. (33) Lee, M.-K.; Kim, T. G.; Ju, B.-K.; Sung, Y.-M. Cryst. Growth Des. 2009, 9, 938–941. (34) Caldwell, M. A.; Raoux, S.; Wang, R. Y.; Wong, H-S. P.; Milliron, D. J. J. Mater. Chem. 2010, 20, 1285–1291.
3237
Figure 1. Experimental and simulated XRD patterns for dried powders and drop-cast films of rhombohedral GeTe. Drop-casting the microcrystallites produces preferred orientation along Æ002æ, indicating a faceted, nonspherical morphology.
mixed with 2.5 mL of TOP-Te solution in a 25 mL roundbottom flask equipped with a reflux condenser, thermometer adapter, rubber septum, and vacuum adapter. The yellow solution was heated to 120 C under a vacuum for 30 min to remove air and moisture, and then heated to 180 C under an Ar blanket. TBAB (52 mg), a mild reducing agent, was dissolved in 1 mL warmed (∼100 C) octyl ether, and swiftly injected into the reaction flask. The introduction of TBAB produced an immediate color change from yellow to brown-black, presumably correlated with the reduction of Ge2þ to Ge0. It is worth noting that the precursors do not react to form GeTe in the absence of reducing agent, suggesting that the reduction of Ge2þ is an essential step. After aging at 180 C for 90 min, the product mixture was cooled to room temperature. The precipitate was separated by centrifugation from a transparent, pale yellow supernatant, washed several times with 3:1 toluene/ethanol and dried under vacuum, yielding a gray powder. Characterization. Powder X-ray diffraction (XRD) data were collected using a Bruker D8 Advance X-ray diffractometer equipped with CuKR radiation, and lattice parameters were determined using Chekcell. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) data were acquired from a FEI Quanta 200 Environmental SEM operating in low vacuum mode. Differential scanning calorimetry (DSC) was performed on a TA Instruments Q600 SDT under flowing Ar at a heating rate of 10 C/min. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained from a JEOL 1200 EX II operating at 80 kV. Samples were prepared by suspending the washed crystallites in toluene and drop-casting onto Formvar-coated copper TEM grids.
Results and Discussion The product formed upon reducing GeI2 with TBAB in the presence of TOP-Te at 180 C is crystalline rhombohedral GeTe, as shown by XRD in Figure 1. This rhombohedral phase is slightly distorted from the rocksalt structure and is the GeTe polymorph that is typically observed at low temperatures (
