NANO LETTERS
In-Situ Contacted Single-Walled Carbon Nanotubes and Contact Improvement by Electroless Deposition
2003 Vol. 3, No. 7 965-968
Robert Seidel,* Maik Liebau, Georg S. Duesberg, Franz Kreupl, Eugen Unger, Andrew P. Graham, and Wolfgang Hoenlein Infineon Technologies AG, Corporate Research, 81730 Munich, Germany
Wolfgang Pompe Institut fu¨r Werkstoffwissenschaft, TU Dresden, 01062 Dresden, Germany Received April 15, 2003; Revised Manuscript Received May 7, 2003
ABSTRACT This work presents a simple and versatile approach for the growth of in-situ contacted single-walled carbon nanotubes (SWCNTs) and subsequent improvement of the contacts using a self-aligned process. We investigated the conditions for the thermal CVD growth of SWCNTs and developed a variety of multilayered metal systems for SWCNT growth at temperatures between 650 °C and 900 °C. The catalytically active layers were either an Fe/Mo bilayer or a Co layer. Further, we report that some of the multilayer systems presented allow lithography free contact improvement by electroless metal deposition.
To investigate the potential of single-walled carbon nanotubes (SWCNTs) for future nanoelectronic devices, it is necessary to develop methods that facilitate their electronic characterization. The first carbon nanotubes to be electronically characterized were fabricated using either laser ablation1 or arc discharge evaporation.2,3 To fabricate electronic devices, SWCNTs first had to be cleaned, deposited on a substrate, located via atomic force microscopy (AFM), and in some cases contacted via arduous e-beam lithography.4-6 Chemical vapor deposition (CVD) offers the new possibility to directly grow clean SWCNTs on a substrate. Initially, the grown SWCNTs still had to be located by AFM and contacted using e-beam lithography.7-9 The first in-situ contacted SWCNTs were produced by spin-on deposition of an alumina-supported iron catalyst on prepatterned Mo electrodes coated with a resist mask.10 However, this method requires two photolithographic steps. Rawlett et al.11 presented a similar method involving the spraying of a catalyst consisting of alumina nanoparticles impregnated with an Fe/ Mo salt solution onto Au electrodes. These two approaches rely on alumina nanoparticles as catalyst support and an application from solution onto the substrate. However, liquidbased deposition techniques might become inaccurate, once the size of the catalyst islands becomes very small. In a different approach, enhanced SWCNT growth in combination with Fe or Fe/Mo catalyst layers was reported from ionbeam deposited underlayers consisting of 10-20 nm Al.12 * Corresponding author. Phone:
[email protected]. 10.1021/nl034229z CCC: $25.00 Published on Web 05/20/2003
(+49) 89 234-52755, E-mail:
© 2003 American Chemical Society
We have developed a simpler method to reliably grow insitu contacted SWCNTs by using multilayered metal systems in which the catalyst is separated from the electrode material by a thin Al layer. Usually, however, the in-situ contacted SWCNTs display high contact resistances between the electrodes and the SWCNT. We have recently developed an electroless metal deposition process13 to simultaneously improve the contact resistances of a large number of CNT devices in parallel. Here we extend this method in order to improve the contacts to SWCNTs by using a post-growth electroless Ni deposition and subsequent annealing, which is a lithography-free and self-aligned process. The metal layers were deposited with a high precision ion beam deposition system, which allowed controlled deposition of layers as thin as 0.2 nm. The multilayer systems were deposited on Si wafers with 200 nm thermal oxide and a resist mask. The resist was patterned by conventional 350 nm photolithography. The multilayer system consists of the electrode metal, a 5-10 nm Al-separation layer, and the active catalyst layer on top. Ti or Ta layers were, when necessary, first deposited in order to improve the adhesion of the multilayers to the substrate. The schematic multilayer system is illustrated in Figure 1. The patterned electrodes were obtained using a lift-off process by dissolving the resist in acetone. The active catalyst layers were either Fe, Fe/ Mo, or Co. Table 1 shows the two catalyst systems investigated and the corresponding growth conditions. SWCNT growth was made in 4 in. quartz tube furnaces employing methane as the carbon feedstock. The samples were typically
Figure 1. Schematic illustration of the ion-beam deposited metal multilayer stack (not to scale). Table 1. Catalysts and Growth Systems Investigated for the Thermal CVD of SWCNTs catalyst
catalyst support
carbon supply
temp [°C]
Fe/Mo Co
Al Al
methane methane
900 650-800
loaded into the furnace at operating temperature. Prior to growth, the samples were pretreated with hydrogen at a pressure of 2.5 Torr. The growth was initiated by charging the furnace with methane or a methane/hydrogen mixture and proceeded at constant pressure (approximately 0.5 atm) without gas flow. Thus, the amount of hydrocarbon gas consumed is reduced significantly. The growth time was between 2 and 15 min. The samples were finally cooled in hydrogen to prevent oxidation of the electrodes. The cobalt electrodes enabled the electroless deposition of Ni using an ammonia buffered solution containing hypophosphite and NiCl2 at a bath temperature of 80 °C.13 In addition, a second electroless deposition of Au on electroless Ni could be performed in a boronhydride-based solution at 70 °C. Gate dependent electrical measurements at room temperature in air were performed to verify that SWCNTs were obtained. In those measurements the highly doped Si substrate was used as a backgate. The semiconducting fraction of the SWCNTs exhibited p-type semiconducting behavior. The observed p-type behavior is most likely related to adsorbed oxygen as experimentally investigated by Avouris et al.14 Further, we performed Raman spectroscopy on SWCNTs grown under equivalent conditions, where a catalyst-Al bilayer was deposited on a quartz substrate. The diameters deduced from the Raman measurements were found to be between 1 and 2 nm. Scanning electron microscopy (SEM) studies were carried out using a LEO 1560 at 5 kV. We investigated Mo, Ta, W, Au, Cu, Co, TaN, and TiN as electrode materials. By introducing a 5-10 nm Al separation layer, as schematically shown in Figure 1, we achieved growth on all electrodes except of W. Typical results are presented in Figure 2a and 2b, which show bundles of SWCNTs that were grown on Ta and TaN electrodes, respectively. This is in contrast to the findings of Franklin et al.15 who could achieve growth only on Mo using an alumina supported iron catalyst. We observed that the growth of SWCNTs is entirely inhibited only on W 966
Figure 2. SEM images of different growth experiments (900 °C, methane) on various substrates. (a) bundles of SWCNTs: Ta (25 nm)/Al (10 nm)/Fe (1 nm); (b) bundles of SWCNTs: TaN (50 nm)/ Al (10 nm)/Fe (1 nm); (c) no SWCNT growth due to large coalesced Fe particles: TiN (100 nm)/W (250 nm)/Al (10 nm)/Fe (1 nm); (d) no SWCNT growth due to large coalesced Fe particles: TaN (50 nm)/Fe (1 nm).
electrodes (Figure 2c). This might be caused by poisoning or alloying of the catalyst with W. The multilayer experiments indicate that the Al separation layer is crucial for a high yield of SWCNTs. The catalyst is probably in contact with alumina rather than with metallic Al, because the samples are loaded into the hot furnace causing at least a partial oxidation of the Al layer. Upon annealing, the Al layer will be further oxidized by extracting oxygen from the surrounding layers. However, if the catalyst is in direct contact with metallic Al, a catalyst-Al alloy with no appropriately sized catalyst particles would form which would not support SWCNT growth. By conducting a comparative SEM study we found that the Al layer prevents the catalyst from coalescence and, additionally, acts as a diffusion barrier. Without the Al separation layer, the catalyst either coalesces during growth to form 10-20 nm diameter particles, as observed on TaN (Figure 2d), or diffuses into the electrode material, as observed on Mo, Ta, Cu, and TiN. Interestingly, a sputtered alumina layer did not prevent the catalyst from coalescing and, therefore, did not support SWCNT growth. Thus, we tentatively propose that the Al separation layer promotes the formation of sufficiently small catalyst particles that are able to catalyze SWCNT growth, as visible in Figures 2a and 2b. These particles result from the direct interaction of the catalyst with the Al layer at the beginning of the growth process. After the formation of the particles, further coalescence is inhibited by the Al layer. The size and density of the catalyst particles on top of the Al depends on the thickness of the catalyst system and the growth regime. If the catalyst layer is too thick (>1.2 nm), the resulting catalyst particles will be to large to promote CVD growth of SWCNTs. Precise control of the diameter of the particles is, however, not yet possible since they form by random break-up of the catalyst system after initiating the growth process. Nano Lett., Vol. 3, No. 7, 2003
Figure 4. SEM images of bundles of SWCNTs grown at 750 °C on Co contact electrodes and coated by electroless Ni deposition Co (50 nm)/Al (10 nm)/Co (∼0.8 nm).
Figure 3. SEM images of SWCNTs grown between Mo pads. (a) Individual SWCNT: Mo (50 nm)/Al (10 nm)/Mo (∼0.2 nm)/Fe (0.8 nm). (b) Several SWCNTs obtained with a slightly thicker catalyst layer: Mo (50 nm)/Al (10 nm)/Mo (∼0.3 nm)/Fe (1 nm).
Growth with Fe or Fe/Mo catalyst was optimum at temperatures of approximately 900 °C. Figure 3a shows a SEM image of a SWCNT between two Mo electrodes. Here the multilayer system consisted of 10 nm Ti, 50 nm Mo, 6 nm Al, ∼0.1 nm Mo, and ∼0.5 nm Fe. In the SEM images the individual SWCNTs become visible due to charging effects of the insulating oxide around a SWCNT.16 The yield of SWCNTs can be easily controlled by changing the thickness of the catalyst system. Figure 3b shows a higher yield with a multilayer system consisting of 10 nm Ti, 50 nm Co, 6 nm Al, ∼0.2 nm Mo, and ∼0.6 nm Fe. There was no clear indication of the extent to which Mo enhances SWCNT growth since the most important criterion for the yield of SWCNTs seems to be the total thickness of the catalyst system. CVD synthesis of SWCNTs based on Fe or Fe/Mo catalyst succeeded only at rather high temperatures (∼900 °C). Thin layers of metals with rather low melting points (1000-1500 °C) will coalesce at those temperatures. To ensure the integrity of Co, Cu, and Au electrodes, which were the investigated metals with the lowest melting points, it was essential to reduce the growth temperature below 800 °C. This could be achieved by replacing the Fe/Mo catalyst bilayer by a thin Co layer which is known to catalyze SWCNT synthesis at relatively low temperatures.17,18 Again it was found that an underlayer of 5-10 nm Al is necessary Nano Lett., Vol. 3, No. 7, 2003
to prevent the coalescence of the catalyst and its diffusion into the electrode beneath. We could successfully grow SWCNTs on Co, Cu, and Au electrodes at temperatures up to 750 °C. The highest yield for Co-mediated SWCNT growth was obtained at a growth temperature of 750 °C and pure methane with a pressure between 0.5 and 1 atm. SWCNTs could also be synthesized at temperatures as low as 650 °C with a rather small yield. Changing the thickness of the Co catalyst was found again to determine the yield of SWCNTs. Growing the SWCNTs on Al layers will yield in-situ contacted SWCNTs with rather high contact resistances due to the oxidized Al. To improve the contact resistance we have developed a process to encapsulate the SWCNTs by electroless deposition of Ni followed by an annealing step. This process is self-aligned and does not require additional lithography. Figure 4 shows SWCNTs grown on a multilayer system of Co (50 nm)/Al (5 nm)/Co (∼0.5 nm) at 750 °C and electroless plated with 50-200 nm Ni. The Ni had to be soldered to the SWCNTs by annealing at 400 °C in nitrogen after electroless metal deposition. Figure 5 shows a gate dependent electrical transport measurement illustrating that the on-conductance of electroless plated p-type SWCNTs is of the order of 10 µS-clearly better than the conductance of the as grown SWCNTs (∼1 µS). Furthermore, we could coat these Ni electrodes with an additional electroless deposited Au layer. Such an Au layer facilitates direct wire bonding to the electrodes. The encapsulation process works selectively on the electrodes and does not influence the intrinsic properties of the SWCNTs as evidenced by the conductivity modulation of more than 5 orders of magnitude (Figure 5). The process of embedding SWCNTs using electroless deposition and subsequent annealing presented here might also enable ultrashort (∼10 nm) SWCNT field effect transistors to be obtained. Those devices will help to investigate the ultimate scaling limits of SWCNTs. In summary, we have developed a very simple method to grow in-situ contacted SWCNTs by means of a metal multilayer system consisting of an electrode metal, an Al 967
References
Figure 5. Conductance of SWCNTs grown on Co electrodes as a function of the gate voltage (multilayer: Co (50 nm)/Al (10 nm)/ Co (∼0.5 nm) before and after contact improvement by electroless Ni deposition and subsequent annealing (N2, 400 °C). The gateoxide thickness was 200 nm.
separation layer, and the active catalyst. A multilayer system with a thin catalyst layer of Fe/Mo or Co yields SWCNTs at 900 °C or 650-800 °C, respectively. The Co catalyst enables the synthesis of SWCNTs at rather low temperatures, broadening the range of possible electrode materials. Growth on Co electrodes even offers the possibility of post-growth electroless plating in order to improve the contact resistances. This presents an easy method for the fabrication of large numbers of simultaneously contacted SWCNTs without e-beam lithography, a prerequisite for the integration of SWCNTs into nanoelectronic devices or sensors.
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