Magnesium oxide nanowires synthesized by pulsed

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magnetic tunnel memories.14 The synthesis of MgO nano- wires has been achieved mainly by chemical vapor deposi- tion (CVD) and by pulsed laser ...

APPLIED PHYSICS LETTERS 94, 022904 共2009兲

Magnesium oxide nanowires synthesized by pulsed liquid-injection metal organic chemical vapor deposition Y. F. Lai,1,a兲 P. Chaudouët,1 F. Charlot,2 I. Matko,1 and C. Dubourdieu1,b兲 1

Laboratoire des Matériaux et du Génie Physique, CNRS, Grenoble INP, 3 Parvis Louis Néel, 38016 Grenoble, France 2 CMTC-INPG, 1260 Rue de la Piscine, Domaine Universitaire, BP 75, 38402 Saint Martin d’Hères, France

共Received 17 October 2008; accepted 12 December 2008; published online 14 January 2009兲 Vertically aligned MgO nanowires were epitaxially grown at 600 ° C on Au-coated MgO 共001兲 substrates by metal organic chemical vapor deposition using Mg共tmhd兲2 precursor. Discrete Au particles were embedded in the wires and distributed along the central axis. Scanning and transmission electron microscopy show that the orientation, diameter, and length of the wires strongly depend on the processing conditions such as oxygen partial pressure and reactive species flow rate as well as the starting Au thickness. Diameters down to 15–20 nm were obtained. The growth can be switched from vertical to horizontal wires by decreasing the period at which reactants are supplied. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3064134兴 In order to address beyond Moore’s nanoelectronics, radically new approaches and devices are needed. Nanowires are one of the building blocks that could be used in future electronic devices. In this perspective, IV/IV as well as III/V semiconductor nanowires are extensively studied,1–5 and one-dimensional 共1D兲 growth has been extended to oxides, selenides, and tellurides.6,7 Integrating functional oxides in the form of nanowires could open up the route to new functionalities. Magnesium oxide MgO has been recently the subject of attention.8–13 Besides a wide-band gap insulating material, MgO is also used as a buffer layer for the growth of ferroelectric or magnetic oxides, or as a tunneling barrier in magnetic tunnel memories.14 The synthesis of MgO nanowires has been achieved mainly by chemical vapor deposition 共CVD兲 and by pulsed laser deposition 共PLD兲. CVD allows a flexible change in composition compared to physical vapor deposition techniques, and it is compatible with future device fabrication technologies. One major difficulty in the CVD of complex oxides is the control of the composition; the metal organic precursors are indeed highly thermally unstable and cannot sustain a prolonged heating. A liquidinjection delivery scheme was proposed in order to solve such issues.15 It allows for the growth of oxides at nanometer scale16 and has been used for the epitaxial growth of a variety of complex functional oxides such as ferroelectrics, magnetic oxides, or multiferroics.16–18 The sequential use of several injectors offers a large flexibility for film doping or for superlattice growth. The extension of the method to the growth of oxide nanowires would be extremely promising; indeed composition of nanowires could be, for example, graded or modulated at nanometer scale using several injectors. So far, MgO nanowires have been synthesized by CVD mainly through the thermal evaporation of MgB2 共Refs. 8 and 10兲 or Mg3N2 共Refs. 9 and 13兲 powders and the condensation of MgO from the vapors 共transported by a carrier gas兲 on a substrate placed in the furnace, next to the evaporation a兲

On leave from Department of Electronic Science and Applied Physics, Fuzhou University, Fuzhou 350002, People’s Republic of China. Author to whom correspondence should be addressed. Electronic mail: [email protected]

b兲

0003-6951/2009/94共2兲/022904/3/$23.00

zone. This method requires a quite high processing temperature of 700– 900 ° C for MgB2 and 900– 925 ° C for Mg3N2. Even higher temperatures 共1200 ° C兲 are required when a mixture of MgO and carbon powder is used.19 Here we demonstrate the growth of epitaxial vertical MgO nanowires on MgO substrates by metal organic CVD 共MOCVD兲, using a ␤-diketonate molecule as a precursor 共evaporation temperature of 220 ° C兲 and a growth temperature of 600 ° C. Gold was used as a catalyst. We discuss the growth mode and the dimensions of the wires as a function of processing conditions. ␤-diketonate Mg共tmhd兲2 共tmhd= 2 , 2 , 6 , The 6-tetramethyl-3,5-heptanedionate, with formula C11H19O2兲 was dissolved in 1,2-dimethoxyethane solvent at 0.02 mol/l. The synthesis was carried out on MgO 共001兲 substrates at 600 ° C. Argon was used as a carrier gas and O2 as an oxidizing agent. The total pressure was 10 Torr. The oxygen partial pressure was of 2.4 Torr. Before the synthesis, a thin gold layer, typically of 2 nm thickness, was sputtered onto the substrates. Prior to deposition, the coated substrates were heated at 600 ° C for 10 min under 10 Torr under the deposition atmosphere 共mixture of oxygen and argon兲. In order to vary the reactive species flow rate impinging the substrate, the injection period was changed in the range of 0.1–5 s. After the synthesis, the samples were cooled under 1 bar O2. The morphology and the dimensions of the nanowires were studied by field emission scanning electron microscopy 共SEM兲 and transmission electron microscopy 共TEM兲, averaging data from several images. The crystallinity of the wires was studied by x-ray diffraction and electron diffraction. Figure 1 shows SEM images of nanowires grown on ⬃2 nm gold-coated MgO 共001兲 and, for comparison, on Si 共001兲 substrates. On MgO, the wires are vertically aligned with the 关001兴 direction perpendicular to the substrate plane, while on Si 共001兲, various orientations are observed, as shown by x-ray diffraction 共Si wafers were prepared by HF—last in order to remove the SiO2兲. On Si, TEM shows, however, that the growth direction within the wire is 关001兴 as well. The simultaneous deposition on bare substrates leads to the formation of continuous MgO films. The gold acts clearly as a catalyst. In the following, we focus on the growth on MgO

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FIG. 1. SEM images 共tilt of 30°兲 of MgO nanowires grown by MOCVD at 600 ° C on ⬃2 nm Au-coated substrates: 共a兲 on MgO 共001兲 and 共b兲 on Si 共001兲.

substrates. The formation of MgO cubic structure is confirmed by electron diffraction and high-resolution TEM images, as shown in Fig. 2. An interplane d002 spacing of 0.21 nm is measured, in good agreement with the expected value 共aMgO = 0.421 nm兲. The growth direction along the 关001兴 direction is also confirmed. The wires exhibit a squarerod shape, with gradually narrowing size from bottom to top. The bottom “diameter” is typically 15–20 nm, while the top diameter is 4–5 nm. The length is 700 nm for 85 min deposition using an injection period of 3 s. A round-shaped gold nanoparticle is found at the tip of the wires 关see Fig. 2共a兲兴, indicating the role of gold as catalyst and pointing out to a vapor-liquid-solid 共VLS兲 growth mechanism.20 In most of the wires, discrete gold nanoparticles with ovoidal shape are also observed inside the wires, distributed along the central axis and with a decreasing diameter from bottom to top, as shown in Fig. 2共b兲. Few reports also mention the possibility to embed Au nanoparticles in oxide nanowires, such as SiO2 or Ga2O3.21,22 It was proposed that the Au-in-Ga2O3 peapods spontaneously crystallize under phase separation induced by the formation of twin boundaries in Ga2O3.22 In our case, no extended defects are observed inside the wires. As seen in Fig. 2共c兲, facets of 兵001其-type are observed on the sides. The clean 共001兲 MgO surface has the lowest surface energy.23 Density functional theory simulations of the growth of 共MgO兲n islands on MgO 共001兲 terraces have shown that the most stable islands expose nonpolar steps rather than polar ones.24 These energetic considerations are in favor of an epitaxial growth along the 关001兴 axis and of 兵001其 facets, as obtained. The tapered shape of the wires was also reported by other groups, for CVD as well as PVD growth.8,11 In the

FIG. 2. TEM images of MgO nanowires epitaxially grown at 600 ° C by MOCVD on ⬃2 nm Au-coated MgO 共001兲 substrate. 共a兲 High-resolution image showing the tip of a wire with Au particle. 共b兲 Nanowire with Au particles distributed along its axial axis. 共c兲 High-resolution image showing facetted sides.

FIG. 3. SEM images of MgO nanowires epitaxially grown by MOCVD at 600 ° C on Au-coated MgO 共001兲 substrate with different starting Au catalyst thicknesses of 共a兲 ⬃2 nm and 共b兲 ⬃4 nm.

VLS mechanism proposed for the growth of Si nanowires,20 the liquid Au–Si droplet acts as an energetically favorable site for the adsorption of the gas phase species. Si vapor atoms enter the droplet and supersaturate it, leading to a condensation of Si at the interface between solid Si and the liquid alloy. In our case, the gaseous reactive species arriving on the droplets are bulky Mg共tmhd兲2 molecules. They decompose at the surface of the droplets and release Mg or Mg–O gaseous species to enter the droplets. These chemical reactions involved in the thermal decomposition of tmhd precursors are more complex than those occurring for solid precursors such as MgB2. The CVD process involves the diffusion out of the droplet’s surface of C- and H-containing species, which are gaseous byproducts of the chemical reactions taking place at the surface. The diffusion of Mg atoms 共or Mg–O dimers兲 at the catalyst droplet’s surface is certainly impeded by the large amount of in- and outdiffusing species at this surface. At the same time, heterogeneous reactions involving the Mg共tmhd兲2 precursor will also take place on the MgO substrate surface and at the sidewalls of the growing wires. Thus it is probable that the growth from the side of the wires is considerably favored. Moreover, the supersaturation of the liquid droplets may not be uniform as C, H, and O species are probably also incorporated. This may explain the presence of gold inside the wires, and this is consistent with the facets and tapered shape of the wires. The phase diagram of the Au–Mg system shows that a Au-rich eutectic 共Au: Mg= 67.5: 32.5兲 appears at 872 ° C, while Mgrich eutectic 共Au: Mg= 7 : 93兲 is formed at 576 ° C, which is similar to the growing temperature of the wires. Thus contribution from a solid/vapor mechanism cannot be precluded.25 In order to possibly eliminate the gold particles from the wires—for applications where pure wires are required— other less bulky precursors should be tested, such as Mg共acac兲2 or, more interestingly, Mg共Cp兲2. Catalyst starting thickness is one of the parameters determining the growth and dimensions of the wires. As observed after heating of a coated substrate, the continuous gold layer transforms into islands. The size of the islands increases with increasing gold thickness. A gold layer with thickness of 2–3 nm leads to vertically aligned nanowires with a bottom diameter of ⬃15–20 nm 共top diameter of 4–5 nm兲. A thicker catalyst of ⬃4 nm hampers the vertical growth, as can be seen on Fig. 3, and produces wires with a

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FIG. 4. Tilted views 共top images兲 and top views 共bottom images兲 by SEM of MgO nanowires grown by MOCVD on ⬃2 nm Au-coated MgO 共001兲 substrate at 600 ° C for different precursor injection periods: 关共a兲 and 共c兲兴 3 s and 关共b兲 and 共d兲兴 0.1 s.

much larger difference between bottom and top dimensions and with a larger bottom diameter 共⬃35–40 nm兲. It has been reported that ambient pressure plays an important role in the PLD growth of MgO nanowires and that increasing the ambient pressure led to an enhancement of the wire growth even under Ar atmosphere, while the O2 partial pressure impacted the oxide crystallinity rather than the wire morphology.12 In CVD, when the gas phase is generated by the thermal evaporation of powders such as MgB2 or Mg3N2, only trace of oxygen is used during deposition. In our case, the situation is quite different. Although oxygen is present in the precursor Mg共tmhd兲2 共four atoms of O for one atom of Mg兲, a rather high oxygen partial pressure is needed in order to avoid carbon contamination; the cracking of the solvent produces indeed a lot of C and H species, and oxygen is required to burn out these species. Contrary to the PLD growth, the oxygen partial pressure 共at constant total working pressure of 10 Torr兲 is found to strongly affect the wire morphology. Increasing the oxygen flow rate hampers the vertical epitaxial growth and leads to the growth of randomly distributed wires. Finally, a clear switch from vertical to horizontal growth is observed when changing the time period at which the gas reactants are injected to the reaction zone. For injection periods of 2 s or larger, the nanowires grow vertically with a bottom diameter of 15–20 nm, while for smaller periods 共0.1–1 s兲, they grow along the substrate surface and the tapered shape is enhanced, as shown by the top views in Fig. 4. For a small injection period, which means a high injection rate 共a period of 0.1 s corresponds to ten precursor droplets injected per second兲, the diffusion along the vertical direc-

tion is thus impeded. The driving force in the VLS mechanism is the supersaturation in the liquid catalyst droplet. The increase in precursor supply may lower the adsorption of reactants at the droplet’s surface, and thus leads to a lowering of the supersaturation. In summary, vertical epitaxial MgO nanowires with Au particles embedded along the radial axis were synthesized by MOCVD. Metal-dielectric 1D nanostructures are of interest for optoelectronics or optics applications such as ultrafast optical switching devices.21 Moreover, we showed that oxide nanowires can be grown using precursors, which are conventionally used for the MOCVD of complex functional oxides. This opens up the route to the design by CVD of new heterostructures such as superlattices or core-shell nanowires combining various oxides. A. M. Morales and C. M. Lieber, Science 279, 208 共1998兲. Y. Li, F. Qian, J. Xiang, and C. M. Lieber, Mater. Today 9, 18 共2006兲. 3 C. M. Lieber and Z. L. Wang, MRS Bull. 32, 99 共2007兲. 4 L. Samuelson, Mater. Today 6, 22 共2003兲. 5 C. Thelander, P. Agarwal, S. Brongersma, J. Eymery, L.-F. Feiner, A. Forchel, M. Scheffler, W. Riess, B. J. Ohlsson, U. Gösele, and L. Samuelson, Mater. Today 9, 28 共2006兲. 6 Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, and H. Yan, Adv. Mater. 共Weinheim, Ger.兲 15, 353 共2003兲. 7 J. Wang, S. Xie, and W. Zhou, MRS Bull. 32, 123 共2007兲. 8 Y. Yin, G. Zhang, and Y. Xia, Adv. Funct. Mater. 12, 293 共2002兲. 9 S. Han, C. Li, Z. Liu, B. Lei, D. Zhang, W. Jin, X. Liu, T. Tang, and C. Zhou, Nano Lett. 4, 1241 共2004兲. 10 H. W. Kim and S. H. Shim, Chem. Phys. Lett. 422, 165 共2006兲. 11 K. Nagashima, T. Yanagida, H. Tanaka, and T. Kawai, J. Appl. Phys. 101, 124304 共2007兲. 12 T. Yanagida, K. Nagashima, H. Tanaka, and T. Kawai, Appl. Phys. Lett. 91, 061502 共2007兲. 13 G. Kim, R. L. Martens, G. B. Thompson, B. C. Kim, and A. Gupta, J. Appl. Phys. 102, 104906 共2007兲. 14 J. Faure-Vincent, C. Tiusan, C. Bellouard, E. Popova, M. Hehn, F. Montaigne, and A. Schuhl, Phys. Rev. Lett. 89, 107206 共2002兲. 15 J. P. Sénateur, C. Dubourdieu, F. Weiss, M. Rosina, and A. Abrutis, Adv. Mater. Opt. Electron. 10, 155 共2000兲. 16 C. Dubourdieu, M. Rosina, H. Roussel, F. Weiss, J. P. Sénateur, and J. L. Hodeau, Appl. Phys. Lett. 79, 1246 共2001兲. 17 N. Lemée, C. Dubourdieu, G. Delabouglise, J. P. Sénateur, and F. Laroudie, J. Cryst. Growth 235, 347 共2002兲. 18 C. Dubourdieu, G. Huot, I. Gélard, H. Roussel, O. Lebedev, and G. Van Tendeloo, Philos. Mag. Lett. 87, 203 共2007兲. 19 P. Yang and C. M. Lieber, J. Mater. Res. 12, 2981 共1997兲. 20 R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 共1964兲. 21 M.-S. Hu, H.-L. Chen, C.-H. Shen, L.-S. Hong, B.-R. Huang, K.-H. Chen, and L.-C. Chen, Nature Mater. 5, 102 共2006兲. 22 C.-H. Hsieh, L.-J. Chou, G.-R. Lin, Y. Bando, and D. Goldberg, Nano Lett. 8, 3081 共2008兲. 23 F. Finocchi and J. Goniakowski, Surf. Sci. 601, 4144 共2007兲. 24 J. Goniakowski, F. Finocchi, and C. Noguera, Rep. Prog. Phys. 71, 016501 共2008兲. 25 V. Schmidt and U. Gösele, Science 316, 698 共2007兲. 1 2

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