Enhanced field emission from multi-walled carbon nanotubes grown on pure copper substrate

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Enhanced field emission from multi-walled carbon nanotubes grown on pure copper substrate Indranil Lahiri a, Raghunandan Seelaboyina a, Jun Y. Hwang b, Raj Banerjee b, Wonbong Choi a,* a

Nanomaterials and Devices Laboratory, Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA b Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA

A R T I C L E I N F O

A B S T R A C T

Article history:

An efficient CNT-based field emitter was prepared on a Cu substrate using thermal chem-

Received 13 May 2009

ical vapor deposition. Field emission characterization of the emitter showed a very low

Accepted 19 November 2009

turn-on field, high emission current, long time stability and good resistance to degradation

Available online 28 December 2009

in high-field, long-time exposure. These superior field emission characteristics are attributed to a lower contact barrier and higher conductivity of the substrate. High resolution transmission electron microscope analysis showed the presence of a conducting phase, TiC, at the metal-CNT interface, providing a low contact resistance barrier.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Cold field emitters have substantially replaced thermionic emitters in almost all vacuum electronic devices [1,2] like electron microscopes [3], X-ray tubes [4], microwave power amplifiers [5], field emission displays [6]. Among the field emitters, carbon nanotube (CNT) array emitter has attracted most attention due to its fine tips, high aspect ratio, good chemical resistance, efficient thermal conductivity, and exceptional mechanical strength [7–10]. Due to its potential applications in semiconductor industries, CNTs are conventionally grown on Si wafers. However, Si is not the best suitable substrate material for applications, where high electrical and thermal conductivity are required. Thus it is important to grow CNTs on suitable metallic substrates without compromising the properties required for the intended application. Moreover, minimization of the contact resistance between a thin metal film and the CNTs has remained a major issue in nano-electronics [11]. Apart from geometrical factors, contact resistance depends mostly on alignment of Fermi energy levels of CNT and the substrate. Since, MWCNTs

are predominantly metallic, thus a metallic substrate is expected to have least contact resistance. Talapatra et al. [12] have directly grown CNTs on Inconel substrate and stressed upon the immediate need of synthesizing CNTs on high conductivity metal surfaces, for applications in interconnects, super capacitors, and field emitters. The present paper focuses on the field emitter applications. The most important requirements of a commercial field emitter device are low turn-on field, high emission current, and good stability of emission. While an ohmic contact ensures easy electron transport and thus, low turn-on field, a well-developed CNT structure supports high emission current. However, emission stability is affected by many parameters, such as low substrate-CNT barrier (to ensure un-interrupted electron transport from substrate to CNT structure), high thermal conductivity of the substrate to extract any heat that is generated due to contact resistance (to minimize de-bonding of CNTs from substrate), presence of gas contaminants on CNT surface (most of which increases work function, hinders continuous tunneling of electrons from the emitter surface, thus reducing emissivity), poor

* Corresponding author: Fax: +1 305 348 1932. E-mail address: [email protected] (W.B. Choi). 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.11.064

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vacuum level etc. Thus, the research efforts in field emission from CNTs are mostly concentrated on addressing these issues. The recent trend has been towards application of doping [13–15], pre-treatments like ultrasonic scratching, laser grooving etc. [16] and post-treatments like ion irradiation [17], hydrogen etching [18], metal coating [19]. However, an intelligent selection of substrate, barrier layer and catalyst is expected to enhance field emission from CNT based devices, by reducing contact resistance, enhancing heat removal from contacts and a good bonding between substrate and CNTs. Though the need of growing vertically aligned CNTs has been stressed upon for a good field emission response [20], reports of field emission from randomly oriented CNTs are also available [21]. The present study concentrates on growing multiwalled CNTs (MWCNT) on a pure Cu substrate, characterizing its field emission response and correlates the properties with the structure.

2.

Experimental

A 500 lm thick pure Cu sheets (with surface roughness <10 lm) were chosen as the substrate material. Before inserting in a thermal chemical vapor deposition chamber (CVD), Cu samples were sputtered with thin layers of Ti and Ni. CVD was performed at a temperature of 973 K using H2 + C2H4 gas mixture, for CNT growth. At the end of the growth period, samples were slowly cooled, within the furnace, under an inert (Ar) gas envelope. The detailed experimental condition of CNT growth can be found in our early publications [22,23]. Field emission characteristics of the samples were tested at a vacuum level of 10 7 torr. In the diode configuration, used for field emission tests, CNT grown on Cu was made cathode and a flattened Cu sheet was used as anode. For direct current (DC) voltage and current measurements, Keithley 248 high voltage supply and 2010 digital multimeter, respectively, were used. In the pulsed mode, field was generated by an Agilent function generator (model 33220A), coupled with a Trek high voltage amplifier (model 20/20C) and the current was monitored through a PEARSON current monitor (model 4100, having 1.0 V/A), attached with an Agilent oscilloscope (model MSO6034A). Frequency of the pulsed signal was kept at 400 Hz and a sine-wave function was applied through the function generator. Structure of the CNTs was characterized by field emission SEM (JEOL, JSM7000F) and Raman spectrometer (Ar+ laser with k = 514 nm, 15 mW power). To observe the interface between Cu substrate and CNTs under transmission electron microscope (TEM), site-specific sample preparation technique, using FEI Nova 200 NanoLab dual-beam FIB, has been used. The area of interest has been protected by Pt layer deposition, in order to minimize the gallium damage during sample preparation. The interest region was sectioned and milled using gallium ion beam and lifted out. The sample, with dimensions 10 · 5 · 2 lm, was attached to a copper-TEM grid. Additional thinning and cleaning using FIB, till 60 nm thickness,

1 2

was consequently performed at 30 keV and 5 keV, respectively, to remove the redeposition and ion beam damage. The site-specific sample has been characterized using FEI TECHNAI F20 field emission TEM, operating at 200 kV.

3.

Results and discussion

With the aim to grow CNTs on highly conductive substrate, Cu automatically becomes the first choice, due to its high conductivity (among metallic conductors, Cu (5.800E+07 Siemens/m) is second to Ag (6.287E+07 Siemens/m) only, in electrical conductivity)1 and easy availability. In the present study, selection of Ti and Ni as diffusion barrier layer and catalyst layer, respectively, was guided by the importance of conductivity of the chosen material and adhesion of CNTs to the substrate. Deposition of highly conductive metals as diffusion barrier layer and catalyst layer ensures a lower contact barrier, thus minimum resistive heating during electron transmission and thus, a better life for the field emitter. TiN has been used as a diffusion barrier in semiconducting devices due to its higher electrical conductivity [24]. Since, Ti has similar electrical conductivity as TiN, but is much more abundant than TiN, Ti is preferable as a barrier layer. Ti is known to have very low contact resistance with CNT [25]. During initial stage of CNT growth, Ti is expected to form a very thin (few nm thick) TiC film, which, being conductive, reduces the interface barrier and helps in electron channeling through it [26]. Ni also has better conductivity as compared to other commonly used catalysts, e.g., Fe, Cr, Pd (electrical conductivity of Ni, Fe, Pd, and Cr are 0.143, 0.0993, 0.095, and 0.0774, 106/cm-X, respectively).2 Thus, from the point of view of conductivity and reducing interface barrier, Ti and Ni seem to be the best choices for barrier and catalyst materials, respectively. Further, it has been reported that poor adherence of CNTs to the substrate is known to affect field emission under an intense local electric field, often leading to its failure [27]. Good adhesion strength results in lowering of contact resistance and a high, sustained emission current [28]. Cu is known to react with Ti, above 873 K, to form CuTi [29] and thus a good adhesion between them could be obtained. A thin catalytic film (5–10 nm) of Ni is also known to favor CNT growth, both thermodynamically and kinetically, at a temperature of 973 K [30]. Since, the growth temperature, in the present study, was 973 K, the selection of substrate, barrier layer and catalyst was expected to lead to an efficient CNT growth and good field emission from them. Fig. 1 shows the Raman spectra, obtained from the CNTs grown on Cu substrate. First-order Raman spectra of MWCNTs (actually, all graphitic materials) show a strong peak at 1580 cm 1 (‘G’ band, which is a high frequency E2g first-order mode from graphite like sp2 bonds), along with an additional peak at 1350 cm 1 (‘D’ band, which is from diamond like sp3 bonds) [31]. Since the origin of D band can be explained by double resonance theory, it is also indicated as A1gD mode – a band caused by defects and disorder of the graphitic material. In the present study, a shift of G band from

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Intensity (arb. unit)

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Fig. 1 – Raman spectrum of the MWCNT structure (using Ar+ laser, with an excitation wavelength of 514 nm, power 15 mW), showing D and G peaks at 1339.5 and 1592.4 cm 1, respectively.

1580 to 1592.4 cm 1 indicates presence of significant amount of nanocrystalline graphitic phase in the material. Comparatively wider peaks of D and G bands indicate presence of disorder induced features in the graphite like (sp2) material and predominance of tubular structures in the CNTs, respectively. Moreover, high ratio of ID/IG peaks (=0.996) shows that the multi-walled CNT structure has very high defect density [16]. Raman spectra do not show any appreciable RBM peak, indicating absence of SWNTs in the structure. SEM images of the MWCNTs, as presented in Fig. 2, show the randomly oriented MWCNTs, whose diameter measured to be 70– 100 nm. It may be mentioned at this point that the thermal CVD process is known to produce randomly oriented, thicker CNTs [32]. Vertical alignment of CNTs is possible in plasma enhanced CVD (PECVD) process or on materials like porous Si, anodic aluminum oxide (AAO) and pre-patterned substrates. CVD-grown CNTs are conventionally found to have less crystalline structure i.e. higher fraction of defects and larger diameters, as compared to CNTs synthesized through arc discharge method [33]. Hence, CNT structure observed in the present study is in agreement with the expectation.

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Field emission characterization of the MWCNT structure initiated with the current–voltage (I–V) measurement, under DC bias, as shown in Fig. 3. Turn-on voltage was found to be relatively low (600 V) for this emitter structure. In fact, the turn-on field was calculated as 0.6 V lm 1, which is one of the lowest values reported so far. Total emission current (under DC bias) was recorded as 1.5 mA, with a field enhancement factor (b) of 11,300. In spite of speculation that random CNTs may add to enhancement of field emission when they stand erected during application of bias, it has been shown by Ikuno et al. [32] that field enhancement due to standing-up of randomly oriented CNTs is almost negligible. Hence, structural advantages of our emitter are solely responsible for the excellent field emission properties shown by it. Inset in Fig. 3 shows the Fowler–Nordheim plot and its straight line behavior confirms that the source of the current is field electron emission only. The field emission images, in the inset, were captured using an indium tin oxide (ITO) glass anode, screen printed with phosphor. The images show that a good fraction of the CNTs contribute to the electron emission process. The bottom inset, which is the emission image at lower applied field (0.8 V lm 1), shows clearly that a good fraction of the CNTs, 30% of the emitters, are participating in the emission process, from the onset of electron emission. However, under DC applied field, the emitters are damaged during field emission test, probably due to continuous ion bombardment. This can be reduced by application of pulsed (AC) electric field. Thus, further field emission studies were continued under an AC applied bias. Fig. 4 shows the I–V plot, as generated from the AC field emission test. The maximum current recorded was 9.6 mA (peak current), at a peak excitation field of 2.3 V lm 1. However, RMS values of a sinusoidal wave are comparable to DC conditions. The sample showed RMS current of 6.7 mA, at an excitation RMS field of 1.6 V lm 1. Straight line behavior of the Fowler–Nordheim plot, in the inset, proves that the current is due to field electron emission only. At higher operating current levels, a deteriorating CNTsubstrate electrical contact poses an extra resistance in series, causing a small voltage drop and saturation of emission current. Under such circumstances, the field emission re-

Fig. 2 – SEM images of the MWCNT structure, showing randomly oriented CNTs.

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For all practical applications, stability of the emission current is one of the main selection criteria for the field emitters. Emission stability of the CNT emitter was measured in a diode set-up, applying a fixed AC bias to it and measuring the emission current as a function of time. Fig. 5 shows the stability behavior of the CNT emitter, under an AC field (RMS) of 1.5 V lm 1 (sine wave, 400 Hz), with an average RMS current of 4.6 mA. The curve clearly indicates good stability of high emission current for 5 h, showing a very low variation of ±5%. Thus, the emitter shows efficiency to be used in industrial field emitters. Moon et al. [35] has shown the importance of adhesion between nanotube emitter and the substrate, to achieve an acceptable life-time of the CNT emitters. Our results from field emission studies indirectly indicate that the adhesion between the substrate and the CNTs is good. Long exposure at higher voltage could damage the CNT structure and hence, the field emission behavior may also deteriorate. Chen et al. [28] have shown that CNTs are destroyed when exposed to higher electric field. To verify the

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Fig. 3 – (a) Applied field vs. emission current for the MWCNT emitter in DC bias, showing appreciable field emission. The upper left inset shows the Fowler–Nordheim plot, while the upper right and bottom insets present the field emission image at different applied field levels (by using a phosphor screen-printed ITO-glass, as anode). (b) Schematic diagram of the field emission test set-up.

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Fig. 5 – Normalized emission current as a function of time, at an applied AC field (peak) of 2.2 V lm 1 (400 Hz, sine wave, average peak current of 6.6 mA), showing excellent stability (within ±5%) of the emission current.

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Fig. 4 – Applied field vs. emission current (RMS values) for the MWCNT emitter in AC bias (400 Hz, sine wave), showing very high field emission.

sponse of the emitter shifts considerably from the Fowler– Nordheim behavior [34]. It may be noted that, in the present study, the CNT emitters have always shown a straight line behavior in Fowler–Nordheim plot. Thus, the contact voltage drop is expected to be negligible for these emitters. Total resistance of the emitter (Cu substrate to CNT tip), as measured by a two-probe method after a prolonged field emission test, is found to be 3.3 X, which is considered to be too low, to develop any such contact voltage drop.

Fig. 6 – Comparison of applied field vs. emission current for the MWCNT emitter, before and after the stability test, showing a small increase in the turn-on field and achieving comparable current.

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Fig. 7 – TEM and HRTEM images of the interface of Cu-CNT (from the sample prepared by a dual-beam FIB cut), clearly showing presence of (a) an interface between CNT and the substrate; (b) nanocrystalline grains in the interface region (taken from the circle-marked region of figure a); (c) diffraction spots of TiC in the selective area diffraction pattern, taken from the circle-marked area of figure a. (NC: nano crystals).

Another important issue is the superiority of the proposed CNT structure on Cu substrate, as compared to that grown on Si substrates. For the comparison study, CNTs were grown on a p-type Si substrate of same size, using the same barrier layer and catalyst and following the same processing steps. Field emission behavior of such a CNT emitter structure was performed under AC bias, using similar conditions. Fig. 8 shows the I–V responses of both the structures (CNT on Cu and Si, respectively). The figure clearly shows that the emission current from the Si structure is much less, as compared to the Cu structure. Field emission process consists of three steps – electron injection, electron transmission, and emission in vacuum [36]. For the case of CNT field emitters, populating the conduction band of the CNTs, with charge carriers, become important. The all-metallic bottom contact of the present structure (CNT on Cu) works manifold to (i) be available as a good source of electrons, (ii) support efficient electron transmission through them to the CNT root, (iii) dissipate heat, generated by resistive heating during field emission, much more efficiently – thus preventing degradation of the CNT-metal bond. Under an applied external electric field, the triangular

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6 Emission Current (mA)

possibility of CNT damage, in our structure, DC I–V test was repeated, after the stability test. Fig. 6 shows a comparison of the I–V behaviors of the samples, for the samples, before and after the stability test. Though the turn-on voltage field was found to increase to 1 V lm 1, possibly due to destruction of few longer CNTs, still its value is comparable to or even better than many reported CNT field emitters. Final emission current value and the field enhancement factor (10,700) were found to be nearly comparable with those shown by fresh sample. This indicates that the field emitter has good resistance against possible structural damage during highfield, long-time exposure. In order to investigate the structure of the interface region between the substrate and CNTs, site-specific sample has been prepared using the dual-beam FIB and subsequently investigated by TEM and HRTEM. A bright-field TEM image shows three distinct layers, the Cu substrate region, interface region and CNT region (Fig. 7a). The CNT region did not exhibit the characteristic structure of nanotubes, possibly due to ionbeam induced damage during the FIB sample preparation. The sample preparation was challenging, as in the process of achieving reasonable electron transparency at the Cu/ CNT interface region, the nanotubes get substantially damaged by the ion beam. The higher magnification HRTEM image, obtained from the interface region (circle-marked area in Fig. 7a), is shown in Fig. 7b. The image indicates presence of grains of nanocrystalline phases in the metal/CNT interface region. As evident from Fig. 7b, some regions of CNT are diffracting, while other regions are not. Fig. 7c shows a selected area electron diffraction pattern from the circlemarked region of Fig. 7a, clearly indicating presence of some diffraction spots that can be consistently indexed based on a face centered cubic structure, with a lattice parameter of 0.43 nm. While equilibrium phase diagrams of Cu–C and Ni– C do not exhibit any equilibrium phase, Ti–C system shows the equilibrium TiC phase, which matches well with structural prediction of the diffraction spots. TiC formation in this layer is likely to increase the bonding strength between metal substrate and CNTs and is also known to affect the field emission property beneficially.

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Fig. 8 – Comparative field emission characteristics of CuCNT and Si-CNT samples, showing excellence of Cu-CNT sample.

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barrier between the Fermi level of the bottom contact and the conduction band of CNTs narrows down, facilitating efficient electron tunneling to the CNTs. Raman spectra of the CNT structure has shown presence both sp2 and sp3 bonds in the MWCNT structure. While sp2 C clusters provide sufficient conducting channels, improving electron transfer from the metallic substrate to the emitter tip [37], sp3 portions reduce the electron affinity and thus shorten the potential barrier to aid in an easy escape of electrons (emission) into vacuum [38]. Furthermore, all metallic nature of our proposed structure (CNT on Cu) aids in efficient electron movement, as compared to that of CNTs grown on Si, which imposes a definite energy barrier due to presence of Si. The main importance of the present study lies in its capacity to show a good combination of low turn-on field, high emission current, good emission stability, and high resistance to structural damage in long-time, high-field exposure. Novelty of the present study can best be appreciated from Fig. 9, which compares literature reported field emission data with those of the present study. Two important parameters of field emission, turn-on field, and field enhancement factor (b), were compared. The figure clearly shows the superiority of the MWCNT field emitter structure, produced in the present study, with the lowest turn-on field and highest b. However, few reports show almost comparable turn-on field [39,40] or high total emission current [28]. In all those cases, CNT field emitters were prepared using special techniques, including sandwich-growth technology, micro or nano patterning, microwave plasma enhanced CVD etc., to grow well-patterned and aligned CNTs. The present study uses a simple thermal CVD processing technique to grow randomly oriented CNTs, showing a very high field emission application potential. An intelligent choice of substrate, barrier layer, and catalyst has probably contributed most to the excellent contact degradation resistance of the structure. Efforts are on, in our lab, to understand and characterize the adhesion strength of the

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Fig. 9 – A comparison of the achievements of the present study with the literature reported values, for turn-on field (left half) and field enhancement factors (right half) for CNTbased field emitters. While the points denote literature values [5,12,13,16,21,39–60], the lines represent the status of the present study.

CNTs with the substrate, as well as to study field emission behavior from CNT based structures, grown on other metallic substrates. Another important issue of scientific interest would be the emission sites of this emitter structure. In spite of the concern that the actual emission sites of these randomly oriented CNTs could be different from CNT tips, sufficient proof is not available. Published reports [61–63] have not pointed out to any emission site, other than the CNT tips. High b values found in our study also indicate that the emission site is actually very fine and probably the CNT tip. However, this is an indirect support and a detailed study involving simulation of the emitter system or in situ field emission test within SEM [64] may throw some light on this issue.

4.

Conclusion

In the present study, CNT was grown on a pure Cu substrate, using Ti barrier layer and Ni catalyst. The MWCNT structure, thus prepared, showed very low turn-on field, high emission current and very good resistance to degradation with time. The excellent contact degradation resistance could probably be related to the good interface bonding of the CNTs with the substrate and high conductivity of the all-metallic contacts, below CNTs. Enhanced field emission properties were explained from the point of view of material selection and structure of the CNTs. This CNT-based field emitter, processed through a simple thermal CVD process, opens a way to prepare CNT-based field emitters on metallic substrates and makes it amenable to be used in those industrial applications, where high conductivity of the substrate is an important issue.

Acknowledgement This study was partly supported by AFOSR grant (#FA9550-05-10232). The authors would like to thank Dr. K.W. Jones, Director, AMERI, FIU, Mr. Neal Ricks, Manager, Motorola Nano-fabrication Research Facility, AMERI, FIU and Dr. S. Saxena, Director, CeSMEC, FIU for allowing to use their research facilities.

R E F E R E N C E S

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