Graphitic interfacial layer to carbon nanotube for low electrical contact resistance
ABSTRACT Graphitic interfacial layer is used to wet the surface of carbon nanotube and dramatically lower the contact resistance of metal to metallic single-wall carbon nanotube (m-CNT). Using Ni-catalyzed graphitization of amorphous carbon (a-C), the average resistance of metal/m-CNT is reduced by 7X compared to the same contact without the graphitic layer. Small-signal conductance measurements from 77K to 300K reveal the effective contact improvement.
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ABSTRACT: Rhodium (Rh) is found similar to Palladium (Pd) in making near-ohmic electrical contacts to single-walled carbon nanotubes (SWNTs) with diameters d > ~ 1.6 nm. Non-negligible positive Schottky barriers (SBs) exist between Rh or Pd and semiconducting SWNTs (S-SWNTs) with d < ~ 1.6 nm. With Rh and Pd contacts, the characteristics of SWNT field-effect transistors (FETs) and SB heights at the contacts are largely predictable based on the SWNT diameters, without random variations among devices. Surprisingly, electrical contacts to metallic SWNTs (M-SWNTs) also appear to be diameter dependent especially for small SWNTs. Ohmic contacts are difficult for M-SWNTs with diameters < ~ 1.0 nm possibly due to tunnel barriers.Applied Physics Letters 06/2005; · 3.79 Impact Factor
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ABSTRACT: We have demonstrated the fabrication of horizontally aligned carbon nanotube (CNT) bundles on Si substrate for interconnect line application. By controlling the catalyst thickness, we fabricated multi-walled CNT and few-walled CNT bundles with different diameters. We measured the resistances of the CNTs as a function of the length and the diameter. The dependence of the contact resistance between the CNT and the metal on the CNT diameter was extracted from the resistance plots. We investigated and experimentally validated the relationship between the diameter and the mean free path of the CNT.Nanotechnology 06/2010; 21(23):235705. · 3.84 Impact Factor
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ABSTRACT: Multi-walled carbon nanotubes (CNTs), either on an SiO(2) substrate or suspended above the substrate, were contacted to W, Au and Pt tips using a nanoprobe system, and current-voltage (I-V) characteristics were measured inside a scanning electron microscope. Linear I-V curves were obtained when Ohmic contacts were established to metallic CNTs. Methods for establishing Ohmic contacts on a CNT have been developed using the Joule heating effect when the tips are clean and e-beam exposing the contacting area of the tip when the tips are covered by a very thin contamination layer. When the contact is not good, non-linear I-V curves are obtained even though the CNTs that have been contacted are metallic. The resistance measured from the metal tip-CNT-metal tip system ranges from 14 to 200 k Ω. When the CNT was contacted via with Ohmic contacts the total resistance of the CNT was found to change roughly linearly with the length of the CNTs between the two tips. Field effect measurements were also carried out using a third probe as the gate, and field effects were found on certain CNTs with non-linear I-V characteristics.Nanotechnology 02/2006; 17(4):1087-98. · 3.84 Impact Factor
Graphitic Interfacial Layer to Carbon Nanotube for Low Electrical Contact
Yang Chai†‡, Arash Hazeghi †, Kuniharu Takei*, Hong-Yu Chen†, Philip C. H. Chan
H. -S. Philip Wong†
‡§, Ali Javey* and
†Department of Electrical Engineering and Center for Integrated Systems, Stanford University, USA
‡Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong, China
*Department of Electrical Engineering and Computer Sciences, University of California at Berkeley, USA
§Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong, China
Tel.: (1)-650-283-0290, Email: firstname.lastname@example.org
Graphitic interfacial layer is used to wet the surface of carbon
nanotube and dramatically lower the contact resistance of
metal to metallic single-wall carbon nanotube (m-CNT).
Using Ni-catalyzed graphitization of amorphous carbon (a-C),
the average resistance of metal/m-CNT is reduced by 7X
compared to the same contact without the graphitic layer.
Small-signal conductance measurements from 77K to 300K
reveal the effective contact improvement.
CNT is a promising both as interconnect [1, 2] and transistor
. Electrical contact is an indispensable part in integrated
circuit. The small contact area makes the electrical coupling
between CNT and metal electrode extremely difficult.
Experimental results have shown that the contact resistance is
still large even for the CNT with metallic band-structure, and
the contact resistance is diameter-dependent [4, 5]. This
contact resistance possibly results from the non-wetting
tubular structure and non-clean interface between CNT and
metal (Fig. 1(a)). There is an atomic-level separation between
CNT and metal for non-wetting surfaces. In this work, we
used amorphous carbon (a-C) as an interfacial layer between
CNT and metal electrode. The a-C was graphitized at high
temperature catalyzed by carbon-soluble Ni [6, 7]. We studied
the electrical transport through the same CNT device with and
without graphitic interfacial layer using small-signal
conductance measurement at low temperature.
Fig. 2 shows the process flow for fabricating the test structure.
Horizontally aligned single walled CNTs (200 µm long) were
grown on quartz substrate using methane and Fe catalyst .
The diameter of the CNT is 1.2±0.3 nm. The CNTs were then
transferred to a Si/SiO2 substrate . Then a 2.5 nm-thick a-C
layer was deposited by e-beam evaporation on top of the CNT
and patterned by lithography. Au/Ni metal contact was
subsequently patterned on the a-C surface by liftoff process.
The channel length of the CNT device was defined by
standard photolithography. The CNT device length studied
here is 1 μm between the two metal electrodes. A control
structure without a-C interfacial layer was also fabricated on
the same CNT. Unwanted CNT on the substrate were etched
by oxygen plasma. The average density of CNT is 3–5
CNT/μm, thus there are ~ 1–3 CNTs per 1μm-width device.
The fabricated samples were annealed at 750 ˚C for 10 min in
hydrogen. After the anneal, the a-C interfacial layer is
graphitized, assisted by the carbon-soluble Ni (Fig. 1(b)).
Fig. 1. Schematic of the interface between CNT and metal electrode. (a) A
finite separation caused by non-wetting or non-clean metal/CNT interface. A
barrier forms between the metallic CNT and metal electrode. (b) The
graphitic interface has similar bonding to CNT, extending effective
wave-function overlap, improving the wetting and enlarging the contact area.
Fig. 2. Process flow for fabricating the test structure. The metal pads with
and without graphitic interfacial layer were fabricated on the same CNT (200
μm long). The a-C layer was transformed to graphitic carbon by a high
temperature annealing process catalyzed by Ni.
Annealing: Graphitic carbon
(a) Non-wetting interface (b) Graphitic interface
9.2.1IEDM10-210978-1-4244-7419-6/10/$26.00 ©2010 IEEE
The transformation from a-C to graphitic carbon, assisted by
the e-beam irradiation inside an SEM or TEM [9, 10], has
been used to improve the contact between multi-walled CNT
and metal. However this transformation involves the high
local energy and temperature. Ref.  shows that the
presence of a carbon-soluble catalyst (e.g. Ni) effectively
improves this transformation with an annealing process. This
work capitalizes on this observation for a wafer-scale contact
improvement technology. Fig. 3 shows the Raman spectra of
the as-deposited a-C layer and the Ni-catalyzed carbon layer
after the annealing process. The sharp G (1571 cm-1) peak
indicates the successful conversion from the a-C to the
graphitic carbon. This graphitic carbon has similar bonding as
the carbon of the CNT, extending effective wave function
overlap for conduction band electrons in the form of Pz-Pz
covalent bonding. The high temperature annealing process
also improves the metal wetting to CNT, and increases the
actual contact area.
Fig. 3. Raman spectra of (a) as-deposited a-C layer, showing broad peak
distribution. (b) a-C layer annealed at 750 ºC with hydrogen flow for 10 min.
The sharp G (1571 cm-1) peak indicates the successful transformation from
the a-C to the graphitic carbon. The Ni catalyst effectively improves the
Fig. 4 shows SEM images of the fabricated device. To ensure
a direct comparison, the same 200 µm long CNT was used for
both the devices with and without graphitic interfacial layer
metal electrode. The CNTs extend beyond the electrodes and
make side contacts to the electrodes.
Fig. 4. SEM images of (a) 200 μm long horizontally aligned CNT, (b) a test
structure on the same CNT, (c) the CNT with graphitic interfacial layer, and
(d) Au/Ni pad only without the graphitic interfacial layer.
Fig. 5 shows the typical I-V curves of the same CNT with and
without graphitic interfacial layer. The as-synthesized CNTs
are single-wall CNTs and consist of a mixture of
semiconducting CNT and metallic CNT. We selected the
CNT devices without back-gate modulation (inset of Fig. 5)
and measured the resistance for the different samples in a
two-point configuration. The drive current in the CNT device
increase dramatically after introducing the graphitic
Fig. 5. I-V curves of the same CNT w/ and w/o graphitic interfacial layer.
The CNT w/ graphitic layer shows more drive current than that w/o graphitic
layer. Inset shows the current as a function of the back-gate voltage. The
CNT has weak gate modulation over a large gate voltage scan, indicating the
Fig. 6 shows the histogram of the conductance distribution for
59 metallic CNT devices. The conductance is greatly
improved after the introduction of the graphitic layer. Since
the devices with and without graphitic interfacial contacts are
fabricated on the same CNT, the improvement is a direct
result of efficient bonding between the CNT and the graphitic
layer. It is well-known that traditional 4-terminal
Intensity (a. u.)
Raman Shift (cm-1)
1000 1500 2000 2500 3000
Raman shift (cm-1)
Intensity (a. u.)
a-C annealed at
750 C assited by Ni
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
Current (μ μA)
W/ graphitic layer
W/O graphitic layer
-4 -2 024
Current (μ μA)
measurements for extracting contact resistance do not yield
accurate results for 1-D systems such as a single m-CNT with
small density of states and semi-ballistic transport. The metal
contacts disturb the local channel potential at the point of
contact with CNT and therefore an accurate voltage drop
cannot be measured . The CNT without graphitic
interfacial layer shows larger resistance and the resistance is
bias-dependent, indicating a contact barrier between the CNT
and metal (Fig. 7).
Fig. 6. Histogram of the conductance distribution from 59 working m-CNT
devices. The conductance of the CNT was extracted by two-point
measurement. The conductance of the CNT with graphitic layer was
normalized over that of the CNT without graphitic layer. The conductance
increase results from the electrical contact improvement by the introduction
of the graphitic interfacial layer.
Fig. 7. DC bias-dependent resistance of the same CNT with and without
graphitic interfacial layer. The large dependency of the resistance of the CNT
w/o graphitic layer suggests the existence of the contact barrier. Measured
lower resistance (20 KΩ) is close to but still higher than the quantum
conductance limit (6.5KΩ). This is due to scattering (diffusive transport)
inside the channel and residual barrier at the contacts.
A lock-in amplifier setup (Fig. 8) was used to measure
zero-bias small-signal conductance of the metal/m-CNT
contact more accurately. Fig. 9 shows the conductance
dispersion as a function of the signal amplitude. The
conductance of CNT device without graphitic interfacial
layers shows large dispersion as a function of the signal
amplitude. This non-linear effect is indicative of a transport
barrier between m-CNT and metal contact. In contrast, the
conductance of the CNT device with graphitic contact shows
little dependence on the signal amplitude, indicating
Ohmic-like contact. This provides additional evidence of the
CNT/metal contact improvement by graphitic interfacial layer.
Fig. 10 shows Arrehenius plot of the same CNT with and
without graphitic layer. The CNT device with the graphitic
interfacial layer shows less temperature dependence.
Fig. 8. Schematics of experimental setup for small-signal conductance
measurement using a lock-in amplifier and a cryogenic probe station. Signal
is sampled with switch S1 open and then closed. Conductance of the DUT
can be calculated using this difference and RREF.
Fig. 9. Small signal conductance of the same CNT as a function of the signal
amplitude. The same CNT (a) with and (b) without graphitic interfacial layer.
The CNT without graphitic interfacial layer shows large signal amplitude
dependency, indicating the large barrier between the CNT and metal
Fig. 10. Arrhenius plots of the same CNT (a) with and (b) without graphitic
interfacial layer at 5 different signal amplitudes. The CNT without graphitic
interfacial layer shows large temperature dependency, indicating the large
barrier between the CNT and metal electrode.
Fig. 11 shows the extracted effective activation energies for
the same CNT with (36meV) and without (204meV) graphitic
layer. Fig. 12 shows conductance improvement of the same
device over the one without graphitic interfacial layer as a
function of the signal amplitude and the temperature. The
conductance was greatly increased for all 6 measured devices
by the introduction of the graphitic interfacial layer.
Controller PC with GPIB
0510 1520 2530
G1: W/ graphitic layer
G2: W/O graphitic layer
20kΩ Ω > Quantum limit (6.5kΩ Ω)
Resistance (kΩ Ω)
W/O graphitic layer
W/ graphitic layer
Signal amplitude (V)
Signal amplititude (V)
Fig. 11. Effective activation energies as a function of square root of signal
amplitude for the same CNT device w/ and w/o graphitic interfacial layer.
The effective activation energies ware extracted according to the Arrhenius
plots. The activation energies for the CNT device w/ and w/o graphitic
interfacial layer are 36 meV and 204 meV at zero bias, respectively.
Fig. 12. Conductance improvement of the same device with graphitic
interfacial layer over the one without the layer as a function of (a) signal
amplitude and (b) temperature. Although the conductance varies for different
devices due to the variations from fabrication process and CNT diameter, the
conductance is noticeably improved for all measured devices.
Metal to metallic CNT contact resistance is dramatically
improved by a graphitic interfacial layer. The graphitic layer
is formed by Ni-catalyzed dissolution of a-C in the metal.
This opens up a new pathway toward Ohmic contact between
metal and metallic CNT and may point to a way to form
Ohmic contact for semiconducting CNT as well.
The authors thank Mr. Jason Matthew Parker for his help with
the annealing process. This work is supported in part by
FENA, one of six research centers funded under the Focus
Center Research Program (FCRP), a Semiconductor Research
Corporation subsidiary, the Research Grant Council of Hong
Kong Government under CERG grant HKUST 611307, and
the Berkeley Sensors and Actuators Center (BSAC).
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Signal amplitude (V)
G1: Conductance w/ graphitic layer
G2: Conductance w/o graphitic layer
0.003 0.006 0.009 0.012
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Without graphitic layer
With graphitic layer