Support−Catalyst−Gas Interactions during Carbon Nanotube Growth

ARTICLE pubs.acs.org/JPCC

Support-Catalyst-Gas Interactions during Carbon Nanotube Growth on Metallic Ta Films B. C. Bayer,*,† S. Hofmann,† C. Castellarin-Cudia,‡,§ R. Blume,|| C. Baehtz,^ S. Esconjauregui,† C. T. Wirth,† R. A. Oliver,# C. Ducati,# A. Knop-Gericke,|| R. Schl€ogl,|| A. Goldoni,§ C. Cepek,‡ and J. Robertson† †

Department of Engineering, University of Cambridge, Cambridge, CB3 0FA, United Kingdom Istituto Officina dei Materiali-CNR, Laboratorio TASC, s.s. 14 km 163.5, I-34149 Trieste, Italy § Sincrotrone Trieste SCpA, s.s. 14 km 163.5, I-34012 Trieste, Italy Fritz-Haber-Institut der Max-Planck-Gesellschaft, D-14195 Berlin-Dahlem, Germany ^ Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany # Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom

)

‡

bS Supporting Information ABSTRACT: We present a detailed study of processes and interactions occurring during the Fe-catalyzed chemical vapor deposition of carbon nanotubes on metallic Ta supports. In situ X-ray photoemission spectroscopy and X-ray diffraction show that the Fe catalyst increases the reactivity of Ta toward oxidation and carbide formation, whereas Ta promotes the reduction of Fe. This causes an unusual temperature dependence of carbon nanotube growth, where at low temperatures (∼550 C) vertically aligned forests of carbon nanotubes with ohmic contacts grow readily on metallic Ta, whereas at high temperatures (>600 C) nanotube growth is sparse because of the diffusion of Fe away from the surface through grain boundaries of in situ formed polycrystalline Ta2O5. The FeTa model system highlights general material selection criteria for nanotube applications that require a conductive support.

1. INTRODUCTION Carbon nanotubes (CNTs) have many potential applications, including interconnects for integrated circuits,1-7 supercapacitors,8,9 or field-emission devices,10 which require growth of vertically aligned CNTs on electrically conductive substrates. Chemical vapor deposition (CVD)11,12 is a well-established route to grow such CNT “forests” on insulating oxide supports such as SiO2 and Al2O3,13-15 but the growth on conductive substrates such as metals, metal-nitrides, and metal-silicides is more difficult and less well studied. In particular, there have been only a few studies of the structure, composition, and properties of nominally conductive supports during and after nanotube growth. One must realize that it is the properties of the support after CVD that are critical because the support material can react with the catalyst, process gases, or residual gases at the elevated CVD temperatures (by, e.g., oxidation, silicidation, alloying, or carbide-formation). Such reactions can have a large detrimental impact on support properties such as electrical conductivity or mechanical stability and are therefore mostly undesirable. To assess the effect of support-catalyst-gas interactions, we grow CNTs on thin films of the refractory metal tantalum16 over a wide pressure and temperature range and carry out extensive ex situ and in situ characterization of the support, catalyst, and r 2011 American Chemical Society

CNTs before, during and after growth. Ta is widely used in microelectronics as a diffusion barrier for Cu on Si17,18 and was used as a support layer in recent efforts to grow nanotubes for interconnects.19,20 Ta is also interesting because of its reactivity with C, O, H, and N,16 which are the elements comprising most CNT CVD process and contamination gases. The high heats of formation of the oxides, carbides, nitrides, and silicides of Ta21,22 (Figure 1) even suggest the metal as an extreme model system for metal-supported CNT growth. Previous literature on Ta-supported CNT growth shows varying results.1,3,19,20,23-37 Some groups report increased CNT growth on Ta compared with insulating supports, which they attribute to the favorable surface energetics of the Ta with the catalyst metal.28,29 Other groups found CNT growth to have lower yield on Ta than on oxide supports and attribute this to material interactions.23,30-32,35 However, details of these interactions are unclear, or the findings are limited to particular points in CVD parameter space. Possible changes in chemistry or structure of the films after CVD are often inferred rather than directly measured. Our study aims to Received: April 2, 2010 Revised: February 1, 2011 Published: March 03, 2011 4359

dx.doi.org/10.1021/jp102986f | J. Phys. Chem. C 2011, 115, 4359–4369

The Journal of Physical Chemistry C overcome this gap by focusing on the chemical (surface) and structural (“bulk”) properties of the Ta support films during and after various CVD conditions. We elucidate what metallurgical processes cause changes in the support material and how to minimize them. Overall, we show that good quality, wellgraphitized CNT forests grow on metallic Ta under certain process conditions, allowing low resistance and ohmic conduction through the Ta film and the nanotubes, as required for applications. Outside of these conditions however, there is a strong tendency to oxide and carbide formation of the Ta and alloying of Ta with the catalyst, which, in turn, leads to strongly reduced CNT growth and a loss of electrical conduction in the Ta. Our results provide conclusions for CNT growth on conductive supports in general.

2. EXPERIMENTAL METHODS The substrates are polished Si(1 0 0) wafers covered with 200 nm thermally grown SiO2. Onto these, Ta films of ∼100 nm thickness are sputter deposited (dc, sputtering gas: Ar). The Ta microstructure was optimized by varying the sputtering power from 30 to 150 W, as described below. The sputtering power of the Ta films is indicated in parentheses, for example, Ta(30W). Care was taken to keep the film thickness of the Ta resulting from the different sputtering powers constant. As reference, 10 nm of alumina (Al2O3) is deposited onto the same Si/SiO2 substrates. After transfer in air, Fe catalyst films (1-6 nm) are evaporated onto the samples. Samples are then loaded (after transfer in air) into different CVD systems operating at different pressure regimes (Table 1). Atmospheric pressure (AP) CVD with a C2H2/H2/Ar atmosphere in a 2 in diameter quartz tube furnace is used for high-yield CNT growth. AP-CVD samples and selected samples from the other CVD routes are analyzed by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), Raman spectroscopy (514.5 nm excitation), and ex situ X-ray diffractometry (XRD, symmetric theta-theta scans, Cu KR1 X-ray source: 1.541 Å). Current-voltage (I-V) measurements are taken by a conductive tip AFM setup (I-V AFM). The structural and chemical properties of the catalyst and support during CNT growth are studied by in situ XRD measurements and in situ X-ray photoemission spectroscopy (XPS), respectively. In situ (grazing incidence) XRD measurements are performed at the ESRF synchrotron in the BM20 end station of the Helmholtz-Zentrum Rossendorf. A hemispherical Be-dome (base pressure 4 nm rms roughness for 750 C). This onset of roughening correlates with the appearance of polycrystalline Ta2O5 (i.e., also oxide grain boundary formation) and the onset of decreasing CNT yield, confirming that strong restructuring of the film accompanies the decreasing CNT yield in higher temperature CVD. As a third cross-check, we preoxidized Ta(30W) films at 750 C, then evaporated Fe and subsequently performed APCVD at 550 C (Figure 7). We find only sparse growth on the preoxidized samples after CVD at 550 C, resembling the 750 C yield on as-deposited Ta (whereas as-deposited Ta allows high density forest growth at 550 C). This is fully consistent with oxide grain boundaries in the preoxidized film offering fast diffusion paths for Fe depletion, as inferred above. We also note that as-deposited and preoxidized Ta show a similar sparse, Fe-catalyzed CNT yield after AP-CVD at 750 C (not shown). However, there we detect no Ta-carbide for the preoxidized Ta, implying that oxidation and carbide formation are competing processes. As a final cross-check, we directly observe Fe depletion from the surface region by process-step-resolved, in situ XRD of Fe-covered Ta(30W) during RP-CVD (information depth ∼50 nm, 700 C, at ∼150 mbar C2H2/H2/Ar, Figure 8). The higher resolution and intensity scans of the synchrotron measurements confirm that as-deposited Ta(30W) is textured β-Ta.42 First, no Fe is visible because of the insufficient crystallinity of the as-deposited Fe film. While heating to 700 C in H2/Ar, we find an instant oxidation of the Ta into crystalline Ta2O5. We again ascribe this to residual oxygen in the chamber or gas lines. During annealing, the Fe becomes visible as it crystallizes into metallic Fe.42 (The two visible Fe reflections are consistent with a bcc phase.) This shows that there is sufficient oxygen to oxidize Ta, but Fe is still reduced. This implies that Ta promotes Fe reduction, which is complementary to the Fe-catalyzed oxidation of Ta noted above and is also corroborated by XPS below. With the introduction of C2H2, we find both the onset of low-yield CNT growth and formation of TaCx.42 The key observation from in situ XRD is that the intensity of the Fe reflections decreases with processing time, implying that Fe is diffusing through the grain boundaries of the polycrystalline Ta2O5 beyond the information depth of our scans (top ∼50 nm of 100 nm film) into the support. We conclude from our XRD data and the cross-checks that at lower CVD temperatures CNT forest growth occurs on metallic Ta or amorphous Ta-oxide (depending on sputter conditions), whereas the onset of formation of crystalline Ta2O5 at higher 4364

dx.doi.org/10.1021/jp102986f |J. Phys. Chem. C 2011, 115, 4359–4369

The Journal of Physical Chemistry C

ARTICLE

Figure 9. Schematic temperature dependence of CNT yield on and bulk phases formed in optimized and nonoptimized Ta films during CNT-CVD. In addition, Ta-Fe surface alloying is occurring; see the XPS results section. Successful forest deposition conditions for ohmic contacts to metallic Ta support are indicated.

Figure 11. Process-step-resolved (bottom to top) in situ XPS scans of Ta4f and Fe2p regions during MP-CVD of Fe-covered Ta(30W) (Fit colors for Ta4f: green, Ta-Fe-alloy; purple, Ta-carbide; and gray and dotted, oxides. Note that the numbers in brackets indicate multipliers for intensities of separate scans).

Figure 10. (a) SEM image of PPT-CVD forest with Fe-covered Ta(75W). (b) Corresponding XRD scan of the sample, showing only reflections of metallic Ta (labeled). The unlabeled reflections are Sisubstrate related.

temperatures correlates with the decrease in CNT yield. The oxygen for the Ta oxidation comes from residual oxygen or water contaminations. The decrease in growth yield is ascribed to catalyst depletion by Fe diffusion through Ta-oxide grain boundaries. (Additionally, the formation of the crystalline carbide might also add to the Fe depletion in a similar way.) Ta is more susceptible toward oxidation and carbide formation in the presence of Fe. The degree of Ta oxidation is controllable by changing the sputter deposition conditions of the support films. This is similar to other work where the support deposition conditions strongly influenced CNT growth results.48,49 In our case, for sputtered Ta, an intermediate sputtering power gives the best results for retaining a metallic structure. This is analogous to CNT growth on TiN films, where an intermediate bias was identified as the optimal condition to suppress diffusion and

enhance CNT growth.50 As there, an optimum ion energy during sputtering is necessary to compact the Ta film. Below this optimum ion energy, compactness of the film is too low, making it more reactive. However, too high an ion energy creates defects and resputtering in the Ta film, which also increases its chemical reactivity. Hence, the Ta(75W) is the optimum film, whereas Ta(30W) is less compact, and Ta(120-150W) is more defective and hence reactive. In Figure 9, our findings are schematically summarized. With the optimized conditions, we find that we grow well-graphitized CNT forests with ohmic contact to the metallic support. 3.3.2. PPT-CVD. We emphasize that our optimization is valid beyond one singular set of CVD conditions. We also tested plasma assistant pretreatment (PPT-CVD) on the different Ta supports. We have recently shown that PPT can significantly ease forest growth on TiN supports.39 Indeed, CNT forests were obtained with PPT-CVD on all Fe-covered Ta samples. However, in agreement with our results above, nonoptimized Ta(30W) films showed oxide and carbide formation after PPTCVD with stronger interactions with increasing temperatures (not shown). Only the optimized Ta(75W) films showed forest growth and an exclusively metallic post-CVD XRD signature (Figure 10). 3.4. In Situ XPS. The characterization data presented so far have studied the evolution of the “bulk” structure of the Ta films but, because of the comparably large information depth of XRD, did not resolve surface effects that actually govern CNT catalysis. Therefore, we finally measure complementary in situ XPS data with an information depth of ∼1 to 2 nm to study the Ta-Fe system in terms of the chemical support-catalyst-gas 4365

dx.doi.org/10.1021/jp102986f |J. Phys. Chem. C 2011, 115, 4359–4369

The Journal of Physical Chemistry C

ARTICLE

Figure 13. Post-CVD Ta-4f region XPS scans for effects of LP-CVD on Ta(30W) with increasing Fe thickness (bottom to top), showing formation of only Ta-carbide (23.2 eV) for bare Ta films but increasing Fe-Ta alloying (22.7 eV) with increasing Fe thickness. Metallic Ta is at 22.0 eV (Fit colors: red, metal; green, Ta-Fe-alloy; purple, Ta-carbide; black, TaOx contribution; gray, fit result; dotted, Shirley background). Figure 12. Time-resolved (bottom to top) in situ XPS scans of C1s region during MP-CVD of Fe-covered Ta(30W) showing Ta-carbide (282.8 eV), Fe-carbide (283.7 eV), and sp2 (284.3 eV) and sp3 (285 eV) C-bond formation. (Note that the numbers in brackets indicate multipliers for intensities of separate scans.)

interactions. This will elucidate what influence the Ta support has on CNT nucleation from the Fe catalyst. The MP- and LPCVD routes for in situ XPS are based on cold-wall systems with base pressures below 10-7 mbar and process pressures below 1 mbar, which reduce residual gas contamination compared with RP- and AP-CVD. 3.4.1. MP-CVD. Figure 11 shows process-step-resolved and Figure 12 shows time-resolved XPS during MP-CVD at ∼650 C. (See Table 1.) After transfer of a Fe-covered Ta(30W) sample in air, the Ta is in a mixed metallic (4f7/2 at 22.0 eV) and oxide surface state (Figure 11), which is expected as Ta forms a thin oxide passivation layer in air.16 The Fe is fully oxidized. Note that these samples show a bulk metallic Ta XRD structure. During heating to 600 C in vacuum, the chemical surface states of both the Fe and Ta change: Vacuum annealing of a reference sample of bare Ta(30W) shows a reduction of the Ta to almost exclusively metallic Ta (22.0 eV, not shown). In comparison, for Fe-covered Ta samples, the Ta 4f7/2 peak shows a 0.3 eV shift to 22.3 eV relative to pure metallic Ta. This shift is due to a partial alloying between the Fe and Ta in the surface region.51 Some Ta also remains in various (sub)oxide states. The Fe on Ta is reduced on heat treatment in vacuum (2p3/2 at 706.9 eV), unlike for Fe on Al2O3 or SiO2, where a reducing gas is needed to reduce Fe.52 Upon H2 introduction, we find that residual water/O2 contamination in the H2 (confirmed by RGA) causes a partial reoxidation of the Ta53,54

but not of the (alloyed) Fe. These observations confirm that only small oxygen contaminations are sufficient to oxidize the Ta. They also again imply that not only does the presence of Fe enhance Ta oxidation but also the presence of Ta promotes Fe reduction (corroborating our XRD results). Time-resolved XPS monitoring of the C1s core level signature during C2H2 exposure (Figure 12) shows that in the first few seconds, a mixture of Ta-carbide (at 282.8 eV) and Fe-carbide (283.7 eV) as well as sp3 (285 eV) and sp2 (284.3 eV) C-bonding peaks appear.52,55 During continued exposure, the graphitic sp2 component grows significantly in intensity with only minor carbide and sp3 contributions remaining, implying the growth of CNTs (confirmed by postgrowth SEM analysis, showing a low CNT yield corresponding to our AP-CVD results at similar temperatures). The (alloyed) Ta transforms to Ta-carbide (23.5 eV)51,56 with some Ta-oxide remaining (Figure 11). The Fe exhibits a small shift to 707.2 eV, which points toward stronger interactions between the now carbidic Ta surface and the Fe. Furthermore, a small Fe carbide component becomes visible. MP-CVD of bare Ta showed a similar evolution, however, with the lack of Ta-Fe alloy shifts and a significantly smaller sp2 contribution (and no tubes), confirming our assignments. We also emphasize that the oxide component in the bare Ta after growth was significantly smaller, implying again that the presence of Fe promotes the oxidation of the Ta surface. We also cross-checked the MP-CVD in situ XPS surface information by ex situ XRD analysis of the samples and find corresponding mixed contributions of metallic, carbidic, and oxidized Ta in the bulk of the films (data not shown). 3.4.2. LP-CVD. To investigate the observed Fe-Ta alloying in more depth, we use in situ XPS during LP-CVD. The UHV 4366

dx.doi.org/10.1021/jp102986f |J. Phys. Chem. C 2011, 115, 4359–4369

The Journal of Physical Chemistry C environment in this CVD route is the cleanest CVD system in our study, minimizing residual contamination effects. The initial surface oxide on the Ta (from transport in air) is removed by heating of the films in UHV vacuum. During the LP-CVD, we find Ta-carbide formation and Ta-Fe alloying in Fe-covered Ta(30W) as in MP-CVD but no reoxidation of the Ta surface from process or residual gases. We find that the degree of alloy formation on Ta(30W) is dependent on the Fe film thickness, as seen in Figure 13, which shows post-LP-CVD XPS scans of Ta films with increasing Fe coverage. Without Fe, only Ta-carbide57 (TaCx, 23.2 eV) is formed during CVD, whereas with increasing Fe thickness, we increasingly observe Fe-Ta alloy formation (∼22.7 eV). The LP-CVD did not yield CNTs (because of too low C2H2 pressure), but the XPS data confirm that Fe-Ta alloying is an intrinsic process of Fe catalysts on Ta supports, irrespective of residual gas contamination and oxide formation.

4. DISCUSSION Our work, showing consistent trends in 5 CVD routes with processing pressures spread over seven orders of magnitude (from LP- to AP-CVD), highlights that a reactive metal support can have many possible undesirable material interactions compared with using inert oxide supports. This is an important but often overlooked aspect of CNT growth. Many proposed applications of CNTs will not work if the support does not retain its desired properties after CVD. This study used Ta as a metallic “model” material to study interaction effects, but we have found similar effects with other functional support films (e.g. Ti). We suggest that close control of post-CVD structure and chemical state of the support material is needed to retain the intended support material’s properties and functions. Similarly, close control of gas-phase contamination levels and avoiding exposure of support films to ambient conditions in between process steps (by, e.g., adding additional capping layers for oxygen sensitive materials58 or by cluster tool processing) could alleviate many materials interactions. Finally, metallic compounds with a lower carbon and oxygen affinity, such as carbides, nitrides,39 or silicides,59 may offer means to reduce support interactions during CNT growth on conductive substrates. 5. CONCLUSIONS In summary, we provide comprehensive in and ex situ evidence of temperature-dependent “bulk” and surface interactions of Ta, a conductive, ultralarge-scale-integration (ULSI) compatible support material, during CNT growth. Ta-oxide, Ta-carbide, and Ta-Fe alloy formation are identified. We find that it is possible to grow dense vertically aligned forests of well graphitized CNTs with good electrical properties on a metallic Ta support layer if the growth temperature is kept at ∼550 C. If the growth temperature exceeds 600 C, then the high affinity of Ta for oxygen, further catalyzed by Fe, causes the formation of polycrystalline Ta2O5 grains even from minor residual oxygen or water contamination. This allows the depletion of surface Fe into the Ta by grain boundary diffusion, leading to decreasing CNT growth with increasing temperature. Ta’s susceptibility toward material interactions depends strongly on the Ta deposition conditions. For optimized deposition conditions (intermediate sputtering power), we obtain CNT forest growth on metallic Ta at comparatively low temperatures (∼550 C) and ohmic I-V characteristics between support and nanotubes.

ARTICLE

’ ASSOCIATED CONTENT

bS

Supporting Information. More details of apparatus and methods and typical raw Raman spectroscopy data used for Figure 4b. 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 acknowledge funding from the EU Integrated Project “Technotubes” under grant agreement no. 226716. We acknowledge the Helmholtz-Zentrum-Berlin BESSY II synchrotron, and we thank the BESSY staff for continuous support. S.H. and C.D. acknowledge funding from the Royal Society. A.G. and C.C.C acknowledge the Friuli Venezia Giiulia Regional Funding “Ambiosen”. ’ REFERENCES (1) Kreupl, F.; Graham, A. P.; Duesberg, G. S.; Steinhoegl, W.; Liebau, M.; Unger, E.; Hoenlein, W. Carbon nanotubes in interconnect applications. Microelectron. Eng. 2002, 64, 399–408. (2) Nihei, M.; Horibe, M.; Kawabata, A.; Awano, Y. Simultaneous Formation of Multiwall Carbon Nanotubes and their End-Bonded Ohmic Contacts to Ti Electrodes for Future ULSI Interconnects. Jpn. J. Appl. Phys. 2004, 43, 1856–1859. (3) Horibe, M.; Nihei, M.; Kondo, D.; Kawabata, A.; Awano, Y. Carbon Nanotube Growth Technologies Using Tantalum Barrier Layer for Future ULSIs with Cu/low-k Interconnect Processes. Jpn. J. Appl. Phys. 2005, 44, 5309–5312. (4) Yamazaki, Y.; Katagiri, M.; Sakuma, N.; Suzuki, M.; Sato, S.; Nihei, M.; Wada, M.; Matsunaga, N.; Sakai, T.; Awano, Y. Synthesis of a Closely Packed Carbon Nanotube Forest by a Multi-Step Growth Method Using Plasma-Based Chemical Vapor Deposition. Appl. Phys. Express 2010, 3, 55002–55004. (5) Dijon, J.; Fournier, A.; Szkutnik, P. D.; Okuno, H.; Jayet, C.; Fayolle, F. Carbon Nanotubes for Interconnects in Future Integrated Circuits: The Challenge of the Density. Diamond Relat. Mater. 2010, 19, 382–388. (6) Dijon J.; Okuno H.; Fayolle M.; Vo T.; Pontcharra J.; Acquaviva D.; Bouvet D.; Ionescu A. M.; Esconjauregui C. S.; Capraro B.; Quesnel E.; Robertson J. Ultra-High Density Carbon Nanotubes on Al-Cu for Advanced Vias. Proceedings of the International Electron Devices Meeting, San Francisco, CA, Dec 6-8, 2010; no. 33.4 (7) Robertson, J.; Zhong, G.; Hofmann, S.; Bayer, B. C.; Esconjauregui, C. S.Telg, H.; Thomsen, C. Use of Carbon Nanotubes for VLSI Interconnects. Diamond Relat. Mater. 2009, 18, 957–962. (8) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845–854. (9) Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. ShapeEngineerable and Highly Densely Packed Single-Walled Carbon Nanotubes and Their Application As Super-Capacitor Electrodes. Nat. Mater. 2006, 5, 987–994. (10) Teo, K. B. K.; Minoux, E.; Hudanski, L.; Peauger, F.; Schnell, J.P.; Gangloff, L.; Legagneux, P.; Dieumegardt, D.; Amaratunga, G. A. J.; Milne, W. I. Microwave Devices: Carbon Nanotubes As Cold Cathodes. Nature 2005, 437, 968. (11) Dai, H. J. Carbon Nanotubes: Synthesis, Integration, and Properties. Acc. Chem. Res. 2002, 35, 1035–1044. 4367

dx.doi.org/10.1021/jp102986f |J. Phys. Chem. C 2011, 115, 4359–4369

The Journal of Physical Chemistry C (12) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes. Science 2004, 306, 1362–1364. (13) Cantoro, M.; Hofmann, S.; Pisana, S.; Scardaci, V.; Parvez, A.; Ducati, C.; Ferrari, A. C.; Blackburn, A. M.; Wang, K.-Y.; Robertson, J. Catalytic Chemical Vapor Deposition of Single-Wall Carbon Nanotubes at Low Temperatures. Nano Lett. 2006, 6, 1107–1112. (14) Wirth, C. T.; Zhang, C.; Zhong, G.; Hofmann, S.; Robertson, J. Diffusion- and Reaction-Limited Growth of Carbon Nanotube Forests. ACS Nano 2009, 3, 3560–3566. (15) Mattevi, C.; Wirth, C. T.; Hofmann, S.; Blume, R.; Cantoro, M.; Ducati, C.; Cepek, C.; Knop-Gericke, A.; Milne, S.; Castellarin-Cudia, C.; Dolafi, S.; Goldoni, A.; Schloegl, R.; Robertson, J. In-situ X-ray Photoelectron Spectroscopy Study of Catalyst-Support Interactions and Growth of Carbon Nanotube Forests. J. Phys. Chem. C 2008, 112, 12207–12213. (16) Cardonne, S. M.; Kumar, P.; Michaluk, C. A.; Schwartz, H. D. Tantalum and its Alloys. Int. J. Refract. Met. Hard Mater. 1995, 13, 187–194. (17) Holloway, K.; Fryer, P. M.; Cabral, C.; Harper, J. M. E.; Bailey, P. J.; Kelleher, K. H. Tantalum As a Diffusion Barrier between Copper and Silicon: Failure Mechanism and Effect of Nitrogen Additions. J. Appl. Phys. 1992, 71, 5433–5444. (18) Sze, S. M. Semiconductor Devices, Physics and Technology, 2nd ed.; John Wiley & Sons: New York, 2002. (19) Graham, A. P.; Duesberg, G. S.; Seidel, R.; Liebau, M.; Unger, E.; Kreupl, F.; Hoenlein, W. Towards the Integration of Carbon Nanotubes in Microelectronics. Diamond Relat. Mater. 2004, 13, 1296– 1300. (20) Nessim, G. D.; Seita, M.; O’Brien, K. P.; Hart, A. J.; Bonaparte, R. K.; Mitchell, R. R.; Thompson, C. V. Low Temperature Synthesis of Vertically Aligned Carbon Nanotubes with Electrical Contact to Metallic Substrates Enabled by Thermal Decomposition of the Carbon Feedstock. Nano Lett. 2009, 9, 3398–3405. (21) Robertson, J.; Sharia, O.; Demkov, A. A. Fermi Level Pinning by Defects in HfO2-Metal Gate Stacks. Appl. Phys. Lett. 2007, 91, 1329121–132912-3. (22) CRC Handbook of Chemisty and Physics, 67th ed.; Weast, R. C., Astle, M. J., Beyer, W. H., Eds.; CRC Press: Boca Raton, FL, 1986. (23) Zhang, H.; Cao, G.; Wang, Z.; Yang, Y.; Gu, Z. Electrochemical Capactive Properties of Carbon Nanotube Arrays Directly Grown on Glassy Carbon and Tantalum Foils. Carbon 2008, 46, 818–832. (24) Dang, C.; Wang, B. B.; Wang, F. Y. Study on Effect of Oxygen Adsorption on Characteristics of Field Electron Emission from Aligned Carbon Nanotubes Grown by Plasma-Enhanced Hot Filament Chemical Vapor Deposition. Vacuum 2009, 83, 1414–1418. (25) Ng, H. T.; Chen, B.; Koehne, J. E.; Cassell, A. M.; Li, J.; Han, J.; Meyyappan, M. Growth of Carbon Nanotubes: A Combinatorial Method to Study the Effects of Catalysts and Underlayers. J. Phys. Chem. B 2003, 107, 8484–8489. (26) Wang, B.; Liu, X.; Liu, H.; Wu, D.; Wang, H.; Jiang, J.; Wang, X.; Hu, P.; Liu, Y.; Zhu, D. Controllable Preparation of Patterns of Aligned Carbon Nanotubes on Metals and Metal-Coated Silicon Substrates. J. Mater. Chem. 2003, 13, 1124–1126. (27) Wang, T.; Wang, B. Study on Structure Change of Carbon Nanotubes Depending on Different Reaction Gases. Appl. Surf. Sci. 2006, 253, 1606–1610. (28) Wang, Y.; Li, B.; Ho, P. S.; Yao, Z.; Shi, L. Effect of Supporting Layer on Growth of Carbon Nanotubes by Thermal Chemical Vapor Deposition. Appl. Phys. Lett. 2006, 89, 183113-1–183113-3. (29) Wang, Y.; Luo, Z.; Li, B.; Ho, P. S.; Yao, Z.; Shi, L.; Bryan, E. N.; Nemanich, R. J. Comparison Study of Catalyst Nanoparticle Formation and Carbon Nanotube Growth: Support Effect. J. Appl. Phys. 2007, 101, 124310-1–124310-8. (30) Lee, K. M.; Han, H. J.; Choi, S.; Park, K. H.; Oh, S.; Lee, S.; Koh, K. H. Effects of Metal Buffer Layers on the Hot Filament Chemical Vapor Deposition of Nanostructured Carbon Films. J. Vac. Sci. Technol., B 2003, 21, 623–626.

ARTICLE

(31) Gao, Y.; Liu, J.; Elder, S. H.; Virden, J. W. Dense Arrays of WellAligned Carbon Nanotubes Completely Filled with Single Crystalline Titanium Carbide Wires on Titanium Substrates. Appl. Phys. Lett. 1999, 74, 3642–3644. (32) Harris, J. D.; Raffaelle, R. P.; Gennett, T.; Landi, B. J.; Hepp, A. F. Growth of Multi-Walled Carbon Nanotubes by Injection CVD Using Cyclo-pentadienyl-iron Dimer and Cyclo-octatetraene Iron Tricarbonyl. Mater. Sci. Eng., B 2005, 116, 369–374. (33) Dang, C.; Wang, T. Study on Effects of Substrate Temperature on Growth and Structure of Alignment Carbon Nanotubes in PlasmaEnhanced Hot Filament Chemical Vapor Deposition System. Appl. Surf. Sci. 2006, 253, 904–908. (34) Thakur, D. B.; Tiggelaar, R. M.; Gardeniers, J. G. E.; Lefferts, L.; Seshan, K. Growth of Carbon Nanofiber Coatings on Nickel Thin Films on Fused Silica by Catalytic Thermal Chemical Vapor Deposition: On the Use of Titanium, Titanium-Tungsten and Tantalum As Adhesion Layers. Surf. Coat. Technol. 2009, 203, 3435–3441. (35) Bouchet-Fabre, B.; Fadjie Djomkam, A.; Delmas, M.; Jin, C.; Antonin, O.; Hugon, M. C.; Mayne-L’Hermite, M. F.; Alvarez, F.; Minea, T. Tantalum Based Coated Substrates for Controlling the Diameter of Carbon Nanotubes. Carbon 2009, 47, 3424–3426. (36) Ting, J.; Chiu, C.; Huang, F. Carbon Nanotube Array Vias for Interconnect Applications. J. Vac. Sci. Technol., B 2009, 27, 1086–1092. (37) Nessim, G. D.; Acquaviva, D.; Seita, M.; O’Brien, K. P.; Thompson, C. V. The Critical Role of the Underlayer Material and Thickness in Growing Vertically Aligned Carbon Nanotubes and Nanofibres on Metallic Substrates by Chemical Vapour Deposition. Adv. Funct. Mater. 2010, 20, 1306–1312. (38) Knop-Gericke, A.; Kleimenov, E.; H€avecker, M.; Blume, R.; Teschner, D.; Zafeiratos, S.; Schl€ogl, R.; Bukhtiyarov, V. I.; Kaichev, V. V.; Prosvirin, I. P.; Nizovskii, A. I.; Bluhm, H.; Barinov, A.; Dudin, P.; Kiskinova, M. X-ray Photoelectron Spectroscopy for Investigation of Heterogeneous Catalytic Processes. Adv. Catal. 2009, 52, 213–272. (39) Esconjauregui, S.; Bayer, B. C.; Fouquet, M.; Wirth, C. T.; Ducati, C.; Hofmann, S.; Robertson, J. Growth of High-Density Vertically Aligned Arrays of Carbon Nanotubes by Plasma-Assisted Catalyst Pre-Treatment. Appl. Phys. Lett. 2009, 95, 1731151–173115-3. (40) Zhang, C.; Pisana, S.; Wirth, C. T.; Parvez, A.; Ducati, C.; Hofmann, S.; Robertson, J. Growth of Vertically Aligned MillimeterLong Carbon Nanotube by Chemical Vapor Deposition. Diamond Relat. Mater. 2008, 17, 1447–1451. (41) Cantoro, M.; Hofmann, S.; Mattevi, C.; Pisana, S.; Parvez, A.; Fasoli, A.; Ducati, C.; Scardaci, V.; Ferrari, A. C.; Robertson, J. Plasma Restructuring of Catalysts for Chemical Vapor Deposition of Carbon Nanotubes. J. Appl. Phys. 2009, 105, 064304-1–064304-10. (42) PCPDFWIN, version 2.4; 2003. JCPDS-ICDD, Ta: 89-1545, Ta2O5 89-2843, Fe: 89-7194, TaC 89-2870, Graphite: 75-2078, Ta2C: 73-1321; International Centre for Diffraction Data: Newtown Square, PA, USA. (43) Jiang, A.; Yohannan, A.; Nnolim, N. O.; Tyson, T. A.; Axe, L.; Lee, S. L.; Cote, P. Investigation of the Structure of β-Tantalum. Thin Solid Films 2003, 437, 116–122. (44) Lee, S. L.; Doxbeck, M.; Mueller, J.; Cipollo, M.; Cote, P. Texture, Structure and Phase Transformation in Sputter Beta Tantalum Coating. Surf. Coat. Technol. 2004, 177-178, 44–51. (45) Chaneliere, C.; Autran, J. L.; Devine, R. A. B.; Balland, B. Tantalum Pentoxide (Ta2O5) Thin Films for Advanced Dielectric Applications. Mater. Sci. Eng., R. 1998, R22, 269–322. (46) Rubinshtein, A.; Shneck, R.; Raveh, A.; Klemberg-Sapieha, J. E.; Martinu, L. Carburizing of Tantalum by Radio-Frequency Plasma Assisted Chemical Vapor Deposition. J. Vac. Sci. Technol., A 2000, 18, 2017–2022. (47) Charlesby, A.; Polling, J. J. The Optical Properties of Thin Oxide Films of Tantalum. Proc. R. Soc. London, Ser. A 1955, 227, 434–447. (48) Amama, P. B.; Pint, C. L.; Kim, S. M.; McJilton, L.; Eyink, K. G.; Stach, E. A.; Hauge, R. H.; Maruyama, B. Influence of Alumina Type on 4368

dx.doi.org/10.1021/jp102986f |J. Phys. Chem. C 2011, 115, 4359–4369

The Journal of Physical Chemistry C

ARTICLE

the Evolution and Activity of Alumina-Supported Fe Catalysts in SingleWalled Carbon Nanotube Carpet Growth. ACS Nano 2010, 4, 895–904. (49) Burt, D. P.; Whyte, W. M.; Weaver, J. M. R.; Glidle, A.; Edgeworth, J. P.; Macpherson, J. V.; Dobson, P. S. Effects of Metal Underlayer Grain Size on Carbon Nanotube Growth. J. Phys. Chem. C 2009, 113, 15133–15139. (50) Garcia-Cespedes, J.; Alvarez-Garcia, J.; Zhang, X.; Hampshire, J.; Bertran, E. Optimal Deposition Conditions of TiN Barrier Layers for the Growth of Vertically Aligned Carbon Nanotubes onto Metallic Substrates. J. Phys. D. 2009, 42, 104002-1–104002-9. (51) Yang, D.; Jiang, H.; Ott, R.; Minor, K.; Grant, J.; Varga, L.; Barnard, J. A.; Doyle, W. D. XPS Study of Polycrystalline and Epitaxial FeTaN Films Deposited by d.c. Reactive Magnetron Sputtering. Surf. Interface Anal. 1999, 27, 259–272. (52) Hofmann, S.; Blume, R.; Wirth, C. T.; Cantoro, M.; Sharma, R.; Ducati, C.; Haevecker, M.; Zafeiratos, S.; Schnoerch, P.; Oesterreich, A.; Teschner, D.; Albrecht, M.; Knop-Gericke, A.; Schloegl, R.; Robertson, J. State of Transition Metal Catalysts during Carbon Nanotubes Growth. J. Phys. Chem. C 2009, 113, 1648–1656. (53) Khanuja, M.; Sharma, H.; Mehta, B. R.; Shivaprasad, S. M. XPS Depth-Profile of the Suboxide Distribution at the Native Oxide/Ta Interface. J. Electron Spectrosc. Relat. Phenom. 2009, 169, 41–45. (54) Atanassova, E.; Tyuliev, G.; Paskaleva, A.; Spassov, D.; Kostov, K. XPS Study on the N2 Annealing Effect on Thermal Ta2O5 Layers on Si. Appl. Surf. Sci. 2004, 225, 86–99. (55) Hofmann, S.; Sharma, R.; Ducati, C.; Du, G.; Mattevi, C.; Cepek, C.; Cantoro, M.; Pisana, S.; Parvez, A.; Cervantes-Sodi, F.; Ferrari, A. C.; Dunin-Borkowski, R.; Lizzit, S.; Petaccia, L.; Goldoni, A.; Robertson, J. In situ Observation of Catalyst Dynamics during SurfaceBound Carbon Nanotube Nucleation. Nano Lett. 2007, 7, 602–608. (56) Itchkawitz, B. S.; Lyman, P. F.; Ownby, G. W.; Zehner, D. M. Monolayer Graphite on TaC (111): Electronic Band Structure. Surf. Sci. 1994, 318, 395–402. (57) Choi, J. The Influence of Surface Properties on Catalytic Activities of Tantalum Carbides. Appl. Catal., A 1999, 184, 189–201. (58) Chiodarelli, N.; Li, Y.; Cott, D. J.; Mertens, S.; Peys, N.; Heyns, M.; De Gendt, S.; Groeseneken, G.; Vereecken, P. M. Integration and Electrical Characterisation of Carbon Nanotube via Interconnects. Microelectron. Eng. 2011, in press doi:10.1016/j.mee.2010.06.017. (59) Zhang, C.; Yan, F.; Allen, C. S.; Bayer, B. C.; Hofmann, S.; Hickey, B. J.; Cott, D.; Zhong, G.; Robertson, J. Growth of Dense Vertically-Aligned Carbon Nanotube Mats on Conductive Cobalt Disilicide Support. J. Appl. Phys. 2010, 108, 024311–1-024311-6.

4369

dx.doi.org/10.1021/jp102986f |J. Phys. Chem. C 2011, 115, 4359–4369