ARTICLE pubs.acs.org/crystal
Control of Seed Detachment in Au-Assisted GaN Nanowire Growths Wen-Chi Hou,† Liang-Yih Chen,^ Wei-Che Tang,† and Franklin C. N. Hong*,†,‡,§ †
Department of Chemical Engineering, ‡Center for Micro/Nano Science and Technology, and §Advanced Optoelectronic Technology Center, National Cheng Kung University, 1 University Road, Tainan 70101, Taiwan ^ Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan
bS Supporting Information ABSTRACT: The detachment and migration of Au catalyst seeds in the Au-assisted growth of GaN nanowires are observed to be controlled by the partial pressure of gallium and nitrogen radicals in the plasma-enhanced chemical vapor deposition system. The migration rate of Au catalyst increases with increasing the gallium vapor pressure during the growth. With the increasing gallium partial pressure, the compositions of gallium in the Au-Ga seeds of nanowires increase, which dramatically enhances the instability and the detaching of Au seeds during the growth. Besides, the gallium atoms adsorbed on the nanowire surface may act as a surfactant to facilitate the Au migration. Therefore, the increase of gallium partial pressure increases the amount of gallium atoms adsorbed on the nanowire surface and further enhances the rate of Au migration along the nanowire. On the other hand, with the increase of gas phase nitrogen radical concentration, an increasing amount of nitrogen atoms can be adsorbed on the nanowire surface to hinder Au migration along the nanowire. In addition to the vaporliquid-solid (VLS) growth process observed, the growth of GaN nanowires via the vapor-solid-solid (VSS) process is also observed. In VSS growth, the detachment of solid Au seeds was inhibited dramatically.
1. INTRODUCTION Semiconducting nanowires are promising candidates for applications in nanoelectronics, optoelectronics, and sensors.1-3 Most of the proposed electronic and photonic applications require nanowires to have controllable structure and desired physical properties. The catalyst-assisted vapor-liquid-solid (VLS) growth is still the most widely exploited approach for fabricating a variety of one-dimensional nanowires.4 Similar to the VLS mechanism, a vapor-solid-solid (VSS) process has been proposed and demonstrated to proceed for many nanowire materials, such as Si, Ge, GaAs, InAs, ZnO, and GaN etc.5-10 In the VSS process, the top-seed of the nanowire remains at solid state during the growth, with the precursors dissolving in the solid seed and migrating to the seed-nanowire interface through solid-state diffusion. In the last few decades, researchers used to accept that the seed droplets remained unchanged and the nanowire sidewalls were composed of clean surface during the growth. Recent studies indicated that the Au seeds on the top of silicon nanowires did not maintain the same diameter during the growth.11 Au in the seeds seemed to migrate to the substrate or other nanowires through surface diffusion on the nanowire sidewall and the substrate surface during the growth. The growth of nanowire may thus be terminated because of the exhaustion of the gold seed. The mechanism for the migration of Au seeds was proposed to be induced by the Ostwald ripening process, which was influenced by many factors during the growth, including the substrate type, r 2011 American Chemical Society
the growth temperature, the seed diameter, the seed composition, the nanowire sidewall composition, etc.12-15 Understanding the mechanism of the seed migration becomes essential in controlling the morphology of nanowires over the whole substrate, which is desired for the fabrication of novel nanowire devices. In this paper we report on the control of the migration and detaching of Au seeds during the growth of GaN nanowires in the plasma-enhanced chemical vapor deposition system. During the growth of nanowires, the gallium partial pressure in the gas significantly influences the final lengths and shapes of GaN nanowires, attributing to the effect of Au seed migration, which is greatly enhanced by the increasing gallium partial pressure. Our results are contrary to the previous reports, which indicated that the lower precursor supply would enhance the detaching rate of Au seed during the silicon nanowire growth.13,14 In this study the increase of gallium partial pressure is proposed to result in the increase of gallium composition in the Au seed, then inducing a higher detachment rate of the seed due to its higher excess energy, the energy in excess of the seed energy at the melting point. Besides, the increase of gallium partial pressure increases the adsorption of gallium atoms on the sidewalls of nanowires, facilitating the migration of Au atoms on the nanowire surface due to the surfactant effect. On the other hand, the Au migration Received: July 2, 2010 Revised: February 1, 2011 Published: February 28, 2011 990
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along the nanowire is hindered by the increasing amount of nitrogen adsorbed on the nanowire surface. We have further observed that, at the low gallium partial pressure, GaN nanowire growth is driven by a vapor-solid-solid (VSS) process, with the Au migration rate dramatically reduced due to the slow diffusion of Au in the solid Au-Ga alloy.
2. EXPERIMENTAL SECTION A novel method for the growth of GaN nanowires in a horizontal furnace was proposed using a dielectric-barrier discharge (DBD) to generate nitrogen plasma.16 Metallic gallium was placed in the upstream of the furnace, Ga source zone, to produce vapor species as the precursor for GaN growth. To ensure a high vaporization rate, gallium droplets were sealed in flowing argon inside a 15-mm-diameter quartz tube to avoid the reaction of gallium with nitrogen forming a surface layer of GaN on the Ga droplet. Gallium vapor was carried by argon of 250 sccm (standard cubic centimeters per minute) to mix with N2 of 200 sccm right before the substrate. A Si (100) wafer coated with a 5-nm-thick layer of gold (Au) is employed as the substrate and placed in the down stream of the furnace at the substrate zone. During the growth of GaN nanowires, N2 plasma was generated in the substrate zone by dielectric barrier discharge (DBD) using a 25 kHz power supply (Creating Nanotechnology Inc.) to produce highly reactive nitrogen radicals and ions. The substrate was immersed in N2 plasma at a total pressure of 2 Torr. The purity of N2 gas employed in this study was 99.9999%. The Ga source temperature was varied from 800 to 900 C to study the effect of gallium partial pressure on the growth of GaN nanowires by fixing the temperature of substrate at 900 C. The Ga vapor pressure at equilibrium for the Ga source at 800, 870, and 900 C was 3.7 10-5, 2.3 10-4, and 4.6 10-4 Torr, respectively. The saturated gallium vapor pressure at 870 C is only 50% of that at 900 C, and the value at 800 C is only 8% of that at 900 C. A large range of gallium partial pressure could be varied to investigate its influence on the migration of Au seeds. The growth periods ranged from 10 to 80 min. The growth was stopped by turning off the plasma source and cooling the gallium source simultaneously and rapidly to stop further supply of gallium in the gas. Substrates were cooled in Ar to room temperature. The above procedures were to ensure that the compositions of Au-Ga seeds at room temperature were very much close to those during the high-temperature growths. The surface morphology and length of nanowires were analyzed using scanning electron microscopy (SEM) (Philips XL-40FEG). The crystal structures of the nanowires were characterized by transmission electron microscope (TEM) (FEI, Tecnai G2 FE-TEM), and the elemental compositions were analyzed by energy-dispersive X-ray spectroscopy (EDS) installed in TEM.
Figure 1. Average length of GaN nanowires versus the growth time under low, intermediate, and high gallium partial pressures of growth.
of 120 min with a low growth rate of 31 nm/min and long nanowires, >7 μm. By increasing the Ga partial pressure (from 4 10-5 to 5 10-4 Torr), the nanowire growth termination time was significantly reduced from >120 to 30 min, and the nanowire growth rate slightly increased from 31 nm/min to 52 nm/min. The termination time and the final lengths of nanowires could be controlled by varying the Ga partial pressure. TEM and EDS analyses were further employed to investigate this phenomenon. (The growth rate and termination time in Figure 1 are also dependent on the plasma power in generating nitrogen radicals, which strongly influence the nanowire growth. The results will be discussed later.) Figures 2a, c, and e show the SEM images of the nanowires grown under various Ga partial pressures at a plasma power of 25 W (to dissociate N2) after a growth period of 80 min. Under the high gallium partial pressure, the nanowires after growth had an average length of 1.5 μm (Figure 2a). The high-resolution transmission electron microscopy (HRTEM) image in Figure 2b shows that the nanowires had tapered tips consisting of perfect GaN single-crystals. All the Au seeds at the tips of nanowires had already disappeared, suggesting the termination of growth within 80 min. The results are consistent with Figure 1, showing that the nanowire growth at the high Ga partial pressure was terminated at 30 min. Under the intermediate Ga partial pressure, the nanowires after a growth period of 80 min had an average length of 4.5 μm (Figure 2c), much longer than 1.5 μm under a high Ga pressure in Figure 2a. A typical HRTEM image in Figure 2d shows that all Au seeds were consumed after 80 min of growth at intermediate Ga pressure, consistent with Figure 1 showing the termination of the nanowire growth after 60 min. Under low Ga pressure, the nanowires after the same growth period of 80 min had the highest average length of 5 μm (Figure 2e). A typical Au seed of 30 nm was observed at the tip of a nanowire by HRTEM (Figure 2f). Figures 2e and f are consistent with Figure 1 showing the continuous growth of nanowires at a low Ga pressure even after 120 min, indicating no disappearance of Au seeds. As a result, the long nanowires at the low Ga pressure were due to the high stability and thus long lifetime of Au seeds. The mechanism of Au diffusion was further investigated by HRTEM characterization. After the growth under intermediate Ga pressure for 20 min, the nanowires were found to have taper ends with a Au seed on the tip of each nanowire. And the Au-Ga
3. RESULTS AND DISCUSSION Figure 1 shows the growth behavior of nanowires by depicting their lengths versus the growth times under the high, intermediate, and low gallium vapor pressures controlled by varying the Ga source temperature. Each point of data in Figure 1 represents the average length of more than 50 nanowires as measured by crosssectional SEM. Under the high Ga partial pressure (Ga source at 900 C), the growth of nanowires was spontaneously terminated after a short period of 30 min with a high growth rate of 52 nm/ min and a short final nanowire length of 1.5 μm on average. Under the intermediate Ga partial pressure (Ga source at 870 C), the growth of nanowires was spontaneously terminated after a period of 60 min with a medium growth rate of 46 nm/min and an intermediate final nanowire length of 4.5 μm on average. Under the low Ga partial pressure (Ga source at 800 C), the growth of nanowires was not terminated, even after a long period 991
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Figure 3. High resolution TEM image of a Au-Ga alloy nanoparticle observed on the sidewall of a nanowire grown under the intermediate gallium partial pressure for a period of 20 min. The inset is a low magnification TEM image showing the Au-Ga seed on the tip of the same nanowire (scale bar =10 nm).
diagram (see Figure 4), the top-seeds should be in the liquid phase state at the growth temperature of 900 C.17 The results indicated that the growths under intermediate Ga pressure followed a typical vapor-liquid-solid (VLS) process, which is the most common GaN nanowire growth mechanism in seedassisted growths. A detailed description of the growth mechanism is supplied in the Supporting Information. To further clarify the Au diffusion mechanism, we also investigated the gallium contents of the seed on the top and of the nanoparticles on the nanowire sidewall grown under high Ga pressure. The seeds at high Ga pressure had a much higher gallium content of around 20-35 atom % than those (12-18 atom %) at intermediate Ga pressure. According to the Au-Ga binary phase diagram in Figure 4, the growth under high Ga pressure followed the VLS process. The high gallium content in the alloy seed and on the nanowire sidewall surface, under high Ga pressure, was suspected to induce the high shrinking rate of Au seeds. A probable mechanism is discussed below. According to the phase diagram, the seed with a high gallium content (2035 atom %) has a lower melting point than that with an intermediate gallium content (12-18 atom %). The growth temperature, 900 C, is well above the melting points of the seeds at the high and intermediate gallium contents. Since both seeds were at the same growth temperature, the seed with a lower melting point should have a higher excess energy (the energy in excess of the melting point) than that with a higher melting point. Thus, a higher excess energy would induce a higher migration rate of Au in the Au-Ga seed. And, at the same growth temperature, the seed with higher gallium content would have a higher Au migration rate. Therefore, the phase diagram explains why the Au seed at high Ga pressure migrates faster than that at intermediate Ga pressure. Furthermore, the gallium atoms adsorbed on the nanowire surface may significantly influence the migration rate of Au atoms across the nanowire surface. As shown in Figure 3, an amorphous layer consisting of Ga and N atoms with 1-2 nm thickness was actually observed on the nanowire sidewall. The surface condition has been shown to play an important
Figure 2. Under the (a and b) high, (c and d) intermediate, and (e and f) low gallium partial pressures as described in Figure 1, the Au-assisted GaN nanowire growths were conducted for a growth period of 80 min. For each pair of (a-b), (c-d), and (e-f), on the left are SEM images of the nanowires taken at an inclined angle of 45 to the normal of the substrate and on the right are high-resolution transmission electron microscopy (HRTEM) images of the tips of the nanowires shown on the left.
nanoparticles of 5-25 nm in diameter were easily found on the sidewalls of nanowires, as evidenced by the TEM image (Figure S1) and EDS. A HRTEM image of one Au-Ga alloy nanoparticle found on the nanowire sidewall is shown in Figure 3. This provides direct evidence that Au atoms actually migrated along the nanowire surface, inducing the detachment or shrinkage of the seeds. As Au atoms diffused from the top seed along the Ga-adsorbed nanowire surface, a uniform Au-Ga alloy adlayer should be formed on the nanowire surface during the growth and shrank into Au-Ga nanoparticles by fast cooling after the growth. During the growth of nanowires, Au-Ga nanoparticles should have not been formed yet on the nanowire surface, since no nanowire side-branch was observed by TEM after the growth. The TEM image in the inset of Figure 3 shows that the nanowire followed the seed-assisted growth. EDS analysis showed that both the seeds at the tip and the nanoparticles on the sidewall were composed of Au-Ga alloy with similar gallium compositions, around 12-18 atom %. The EDS data are supplied in the Supporting Information. According to the Au-Ga alloy phase 992
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nitrogen gas radicals while maintaining the same Ga pressure during the growth. Our TEM results showed that the gallium contents of the Au-Ga seeds on the tips of nanowires were not affected by increasing the nitrogen radical concentrations, consistent with our observation that nitrogen was not dissolved in Au seed and the nitrogen radical concentration should not influence the gallium content. The reduced shrinkage rate of Au-Ga seed with the increase of nitrogen radical concentration was thus due to the slow migration of Au along the nanowire surface adsorbing a high concentration of nitrogen atoms. As a result, the Au migration rate along the nanowire surface was enhanced by increasing the gallium adlayers and reduced by increasing the nitrogen adlayers. The above results further imply that the evaporation of Au in Au-Ga seed cannot be the dominant mechanism for the detachment of Au seed, since the evaporation mechanism could not explain the reduced seed migration by increasing nitrogen radical concentration. The evaporation of Au from the seed should not be affected by the amount of nitrogen radicals, as the composition of the Au-Ga seed was measured by HRTEM and found to be the same for low and high nitrogen radical concentrations. Au migration along the nanowire surface must be the main mechanism for the detachment of Au seed. The average nanowire diameters were larger in high and intermediate Ga pressure than those in low Ga pressure. During the nucleation stage in VLS or VSS growth, Au atoms on the substrate would react with Ga vapor and form Au-Ga alloy nanoparticles due to Ostwald ripening. The nanowire diameter increased with the gallium partial pressure due to the enhanced surface mobility of Au alloy with its increasing gallium contents, resulting in the increase of Au-Ga seed sizes. The crystal quality of GaN nanowires grown under low Ga pressure was slightly worse than those under high and intermediate Ga pressures due to the fast migration on the gallium-rich surface under high Ga pressure.
Figure 4. Au-Ga binary phase diagram in the Au-rich region. For the GaN nanowire growths at 900 C under the low, intermediate, and high gallium partial pressures, the ranges of “a”, “b”, and “c”, respectively, indicate the ranges of the compositions of Au-Ga seeds during growths. In the composition range “a”, the seed is in the solid state during the nanowire growth, indicating that the growth of nanowires follows the vapor-solid-solid (VSS) growth mechanism. In the composition ranges “b” and “c”, the seeds are in the liquid state, indicating the conventional vapor-liquid-solid (VLS) mechanism. The melting point of the Au-Ga alloy in the composition range “b” is obviously higher than that in the composition range “c”.
role in enhancing atomic migration on the surface. Upon increasing Ga pressure, the gallium atoms adsorbed on the (0001) GaN surface reconstruct to form a single adlayer, a bilayer, or even Ga droplets. And the Ga adlayers acting as a surfactant effectively enhance the diffusion of Ga and N atoms on the GaN surface.18,19 Similarly, during the GaN nanowire growth, the gallium adlayers on the nanowire surface might enhance the migration of Au atoms. Thus, a higher Ga pressure resulted in more gallium adlayers on the nanowire surface and induced much faster migration of Au atoms on the nanowire surface. As a result, the Au-Ga seed shrinking rate was significantly enhanced by the increase of Ga pressure during the nanowire growth. In nanowire growths under low Ga pressure, the seeds on the top of nanowires were found to exhibit a very low shrinking rate with a long lifetime over 120 min, significantly different from those under high and intermediate Ga pressures. EDS analysis in TEM indicated that the top-seed of the nanowire grown under the low Ga pressure had a very low gallium content of 1-2 atom %. The nanoparticles on the sidewall also had the same gallium composition as the seeds on the tips. According to the phase diagram, the Au-Ga seed with 1-2 atom % of gallium at the temperature of 900 C should be in the solid phase, with no formation of liquid phase. Actually, in the low Ga pressure case in Figure 2f, the seed crystallites with nonspherical shapes (i.e., sharp corners) were often observed on nanowires by HRTEM, suggesting no formation of liquid seeds during the growth. To our knowledge, this study is the first report on the Au-assisted GaN nanowire growths by the VSS mechanism. The ultralow shrinking rate of seed in the low Ga pressure sample is not only due to the formation of solid seeds but also due to a low concentration of Ga adatoms on the nanowire surface under Ga-poor conditions. To further clarify the migration mechanism of Au seed, the effect of nitrogen radical concentration on the detachment rate of the seed was also studied. Figure 1 shows that under high Ga pressure the lifetime of Au seeds increased from 27 to 40 min with the nanowire length from 1.5 to 3.8 μm by increasing the plasma power from 25 W to 65 W for increasing the concentration of nitrogen radicals. Evidently, the migration rate of Au seed was significantly reduced by increasing the concentration of
4. CONCLUSIONS In conclusion, we have shown that Au migration on the surface of GaN nanowires can be controlled by varying the gas compositions, such as gallium partial pressure and nitrogen radical concentration, during nanowire growths. For GaN nanowire growths, there are three aspects related with the migration of Au seeds. First, the increase of Ga partial pressure during the nanowire growth increases the gallium content in the Au-Ga seed, which induces a higher excess energy above the melting of the seed and increases the activity of the seed. As a result, the migration of Au atoms out of the Au-Ga seed is significantly enhanced. Second, the nanowire surface composition, which can be controlled by varying the concentration of gases, dramatically influences the migration rate of the Au seed. Under a high gallium partial pressure, a high concentration of gallium atoms will be adsorbed on the GaN nanowire surface to act like a slippery surfactant to enhance the Au migration on the nanowire surface. Similarly, the increase of nitrogen radicals during the growth can reduce the migration of Au seed along the nanowires by inhibiting the surfactant effect. Finally, the growth mechanism (VLS or VSS) also played an important role in this study. In addition to the VLS process, the VSS process has also been observed in the Au-seed assisted GaN nanowire growth. Furthermore, VSS growth of GaN nanowires provides significant resistance to the migration and detachment of the seeds due to difficult migration of Au atoms from the seed in the solid state. 993
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Controlling and understanding the surface migration of Au atoms are useful in tailoring the length, shape, and structure of nanowires for future applications.
(18) Neugebauer, J.; Zywietz, T. K.; Scheffler, M.; Northrup, J. E.; Chen, H.; Feenstra, R. M. Phys. Rev. Lett. 2003, 90, 056101. (19) Takeuchi, N.; Selloni, A.; Myers, T. H.; Doolittle, A. Phys. Rev. B 2005, 72, 115307.
’ ASSOCIATED CONTENT
bS Supporting Information. TEM images of the Au-Ga seeds diffusing along a GaN nanowire surface as the nanowire was grown under intermediate gallium partial pressure after a growth period of 20 min; EDS results of Au-Ga seeds and GaN nanowires in Figure 3 of the manuscript; and a schematic illustration of a Au-assisted VLS (or VSS) mechanism for the growth of GaN nanowires. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge support for this work from the National Science Council of Taiwan under Grant NSC-99-2221E-006-197-MY3 and the Ministry of Economic Affairs, Taiwan, through Projects 96-EC-17-A-07-S1-0018 and 99-D0204-2. ’ REFERENCES (1) Li, Y.; Xiang, J.; Qian, F.; Gradecak, S.; Wu, Y.; Yan, H.; Blom, D. A.; Lieber, C. M. Nano Lett. 2006, 6, 1468–1473. (2) Kim, H. M.; Kang, T. W.; Chung, K. S. Adv. Mater. 2003, 15, 567–569. (3) Lim, W.; Wright, J. S.; Gila, B. P.; Johnson, J. L.; Ural, A.; Anderson, T.; Ren, F.; Pearton, S. J. Appl. Phys. Lett. 2008, 93, 072109. (4) Cheyssac, P.; Sacilotti, M.; Patriarche, G. J. Appl. Phys. 2006, 100, 044315. (5) Wang, Y. W.; Schmidt, V.; Senz, S.; Gosele, U. Nat. Nanotechnol. 2006, 1, 186–189. (6) Kang, K.; Kim, D. A.; Lee, H. S.; Kim, C. J.; Yang, J. E.; Jo, M. H. Adv. Mater. 2008, 20, 4684. (7) Persson, A. I.; Larsson, M. W.; Stenstrom, S.; Ohlsson, B. J.; Samuelson, L.; Wallenberg, L. R. Nat. Mater. 2004, 3, 677–681. (8) Dick, K. A.; Deppert, K.; Martensson, T.; Mandl, B.; Samuelson, L.; Seifert, W. Nano Lett. 2005, 5, 761–764. (9) Campos, L. C.; Tonezzer, M.; Ferlauto, A. S.; Grillo, V.; Magalhaes-Paniago, R.; Oliveira, S.; Ladeira, L. O.; Lacerda, R. G. Adv. Mater. 2008, 20, 1499. (10) Weng, X.; Burke, R. A.; Redwing, J. M. Nanotechnology 2009, 20, 085610. (11) Hannon, J. B.; Kodambaka, S.; Ross, F. M.; Tromp, R. M. Nature 2006, 440, 69–71. (12) Kodambaka, S.; Hannon, J. B.; Tromp, R. M.; Ross, F. M. Nano Lett. 2006, 6, 1292–1296. (13) Kawashima, T.; Mizutani, T.; Nakagawa, T.; Torii, H.; Saitoh, T.; Komori, K.; Fujii, M. Nano Lett. 2008, 8, 362–368. (14) Den Hertog, M. I.; Rouviere, J.-L.; Dhalluin, F.; Desre, P. J.; Gentile, P.; Ferret, P.; Oehler, F.; Baron, T. Nano Lett. 2008, 8, 1544–1550. (15) Dick, K. A.; Deppert, K.; Samuelson, L.; Wallenberg, L. R.; Ross, F. M. Nano Lett. 2008, 8, 4087–4091. (16) Hou, W.-C.; Chen, L.-Y.; Hong, F. C.-N. Diamond Relat. Mater. 2008, 17, 1780–1784. (17) Massalski, T. B.; Murray, J. L.; Bennett, L. H.; Baker, H., Binary Alloy Phase Diagrams, 2nd ed; American Society for Metals: Metals Park, OH, 1986; Vol. 1. 994
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