Microwave and Millimeter Wave Properties of Vertically-AlignedSingle Wall Carbon Nanotubes Films

K. HADDADI,1,6 C. TRIPON-CANSELIET,2 Q. HIVIN,1 G. DUCOURNAU,1

E. TEO,3 P. COQUET,4 B.K. TAY,3 S. LEPILLIET,1 V. AVRAMOVIC,1

J. CHAZELAS,5 and D. DECOSTER1

1.—Institute of Electronics, Microelectronics and Nanotechnology, University Lille 1, AvenuePoincare CS, 60069 – 59652 Villeneuve d’Ascq Cedex, France. 2.—ESPCI ParisTech, 10 rueVauquelin, 75231 Paris Cedex 5, France. 3.—Nanyang Technological University, Nanyang Drive,Singapore, Singapore. 4.—CINTRA UMI 3288, Research Techno Plaza, NTU, Singapore, Singa-pore. 5.—Thales Systemes Aeroportes, Elancourt, France. 6.—e-mail: kamel.haddadi@iemn.univ-lille1.fr

We present the experimental determination of the complex permittivity ofvertically aligned single wall carbon nanotubes (SWCNTs) films grown onquartz substrates in the microwave regime from 10 MHz up to 67 GHz, withthe electrical field perpendicular to the main axis of the carbon nanotubes(CNTs), based on coplanar waveguide transmission line approach togetherwith the measurement of the microwave impedance of top metalized verti-cally—aligned SWCNTs grown on conductive silicon substrates up to 26 GHz.From coplanar waveguide measurements, we obtain a real part of the per-mittivity almost equal to unity, which is interpreted in terms of low carbonatom density (3 9 1019 at/cm3) associated with a very low imaginary part ofpermittivity (<10�3) in the frequency range considered due to a very smallperpendicular conductivity. The microwave impedance of a vertically alignedCNTs bundle equivalent to a low resistance reveals a good conductivity (3 S/cm) parallel to the CNTs axis. From these two kinds of data, we experimen-tally demonstrate the tensor nature of the vertically grown CNTs bundles.

Key words: Single wall carbon nanotubes (SWCNT), microwavecharacterization, nanomaterials, dielectric properties

INTRODUCTION

Due to their exceptional electrical, thermal, andmechanical properties, carbon nanotubes (CNTs)were extensively studied during the past recentyears.1 The electrical characterization of CNTs froma few MHz to hundreds of GHz is required tounderstand and design new components to beimplemented in the next generation of microwave,millimeter wave, and sub-millimeter wave sys-tems.2 The purpose of this paper is to present theexperimental determination of the dielectric prop-erties of vertically aligned single wall carbon nan-otubes (SWCNTs) films grown on quartz substrates

in the microwave and millimeter wave frequencyrange, from 10 MHz to 67 GHz, together with themeasurement of the microwave impedance of topmetalized vertically aligned SWCNTs grown onconductive silicon substrates up to 26 GHz. From10 MHz to 67 GHz, permittivity experiments werecarried out by the coplanar waveguide (CPW)transmission line approach.3–5 The extraction ofthe complex permittivity is based on the measure-ment of the magnitude and phase shift of thetransmission coefficient change considering teststructures with and without SWCNTs films, withthe electrical field perpendicular to the main axis ofthe CNTs. Microwave impedance measurementswere performed up to 26 GHz based on microwavereflection coefficient changes with CNTs films. Wefirst focus on the main material aspects. In a second(Received October 14, 2015; accepted January 18, 2016;

published online February 17, 2016)

Journal of ELECTRONIC MATERIALS, Vol. 45, No. 5, 2016

DOI: 10.1007/s11664-016-4362-3� 2016 The Minerals, Metals & Materials Society

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step, we detail the CPW transmission line micro-wave experiments. The fourth section is devoted tothe extraction of the complex permittivity of theCNTs film. We dedicate the fifth section to the CNTsmicrowave impedance measurements. In the lastsection we discuss and interpret quantitatively ourexperimental results.

MATERIALS

A plasma-enhanced chemical vapor deposition(PECVD) system from Aixtron (2—inch BlackMagic) has been used for the vertical growth of

SWCNTs under the Thermal Chemical Vapor Depo-sition mode. Growth has been realized on doped Sisubstrates with a previously Ti/Pt back side metal-lized, and with quartz substrates. It was observedthat vertical alignment SWCNT can only beachieved for a temperature range between 600�Cand 670�C. The catalyst used for the growth ofSWCNT is Fe and the barrier layer is Al/Al2O3 for aconductive layer on doped silicon substrate withtypical thickness 8 nm/2 nm or only Al2O3 for a non-conductive layer on quartz substrate with typicalthickness 5 nm. The Fe thickness is 2 nm on dopedsilicon substrate and 0.5 nm on quartz substrate.

Fig. 1. Example of SWCNTs grown on a bare quartz substrate. (a) Scanning electron microscope picture of SWCNTs forest grown on quartzsubstrate (CNTs length 210 lm, diameter #1.5 nm, density 1010–1011 CNTs/cm2). (b) Measured Raman spectra after 60 min grown.

Haddadi, Tripon-Canseliet, Hivin, Ducournau, Teo, Coquet, Tay, Lepilliet, Avramovic, Chazelas and Decoster2434

The growth and the quality of SWCNTs arestrongly dependent on the growth parameters,such as pre-treatment time, growth temperature,and sequence of gas input, but also on the qualityand on the control of the thickness of the catalystlayer. This last part is challenging and needsvery precise deposition equipment, as the thick-ness of the catalyst is in the order of onenanometer. Figure 1 shows the scanning electronmicroscope (SEM) picture of a typical SWCNTsforest grown on a quartz substrate. The CNTslength is about 200 lm, the distribution diameterextends from 0.5 nm to 6 nm with a maximumat 1.5 nm. The density has been evaluated to1010–1011 CNTs/cm2 that correspond to 100–1000CNT/lm2.

The Raman spectra6,7 of our samples are system-atically measured, to check the single wall structureof the CNTs after the growth. Figure 2 gives atypical Raman spectra8,9 obtained with our samples.We can note the existence of the radial breathingmode (RBM) at 226 cm�1, which is a proof of thesingle wall structure of the CNTs.

MICROWAVE COPLANAR WAVEGUIDETRANSMISSION LINE MEASUREMENTS

The experiments were carried out with a vectornetwork analyzer (VNA) Agilent E8361A operatingin the frequency range 10 MHz–67 GHz. The CNTscharacterization is based on the scattering param-eters Sij measurements of a coplanar line covered ornot with CNTs vertically grown on a quartz sub-

strate. The choice of quartz is motivated by its lowabsorption of this dielectric material in the micro-wave regime. The change in magnitude and phase-shift of the transmission coefficient S21 without andwith CNTs is related to the complex permittivity ofCNTs thin films. Figure 2 describes the experimen-tal process by a flip-chip procedure. Under theseconditions, the propagation direction of the micro-wave signal is perpendicular to the CNTs axis.

The CPW transmission lines are impedancematched to 50 X and are fabricated with a deposi-tion of metal (Ti, Au) onto a semi-insulating GaAssubstrate. Coplanar lines were designed, fabricated,and are given in Fig. 3. To keep a monomodepropagation in the millimeter wave domain, a smallslit (S = 25 lm) was chosen for our experiments.The Sij parameters of the coplanar lines are mea-sured via a VNA associated with ground-signal-ground (GSG) probes. The same measurements areperformed with the CNT covering the coplanarlines; then the CNT sample is up and down withthe CNT top onto the coplanar line and the quartzsubstrate over the CNTs far from the line. Asillustration, we show in Fig. 4, the picture of a set ofcoplanar lines with a SWCNT sample under micro-wave probes of the Agilent E8361A VNA. TheSWCNTs are vertically grown on a quartz substratededicated to microwave applications. We preparedand measured a sample whose size is 1.25 91.25 cm2 with 180 lm long SWCNTs. We expectedthe microwave electric field saw only the CNTs,provided that the CNTs were longer than the electricfield region over the coplanar line. We estimated it isthe case for 180 lm long CNTs samples given thenarrow slit of the coplanar transmission line (25 lm).

The return loss S11 and S22 are almost (with andwithout CNTs) lower than �10 dB, showing a goodimpedance match (50 X) to the measurement portsof the vector network analyzer. We show in Fig. 5 a

Quartz

GaAsGaAsQuartz

CNTs film

CNT sample CPW TLCPW TL loaded with CNTs

Fig. 2. Experimental flip-chip process for the report of CNTs films onGaAs CPW transmission lines.

GW W GWS S

AsGa (εr=12.9)

center line ground planesFig. 3. Geometry of the CPW line. Dimensions are GW = 300 lm,W = 30 lm and S = 25 lm.

Fig. 4. Photograph of the experimental set-up showing two micro-wave ground-signal-ground (GSG) probes contacting a CPW trans-mission line loaded with a CNTs film.

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comparison between the magnitude |S21| and thephase-shift arg(S21) with and without the CNTs upto 67 GHz. From these data, we clearly see a slightdecrease in the magnitude of the transmissioncoefficient S21 magnitude due to the CNTs roughlyproportional to the frequency and almost no effect ofCNTs on the measured phase-shift. Measurementshave been performed on three different CPW trans-mission lines with the same geometry, and theyhave shown negligible dispersion. Therefore, thevariation in the measured responses is attributed tothe presence of the CNTs.

COMPLEX PERMITTIVITY EXTRACTION

From the measurement of the transmissioncoefficients S21 considering the reference line(without CNTs) and the loaded line (with CNTs),we present in Fig. 6 the phase-shifts difference and

the attenuation coefficient of the CNTs obtainedfrom the measured data. We observe a lineardependency of the phase-shifts difference withfrequency. A variation around only 10 degrees isnoticed at 67 GHz. The attenuation coefficient a ofthe CNTs, defined as the ratio between the mea-sured magnitudes of S21 with and without CNTs,increases almost linearly with frequency to reach avalue lower than 0.5 dB at 67 GHz. From thesedata, we can extract the complex permittivity ofthe CNTs bundle using conventional CPW trans-mission line formalism.3,10–12

Given the fact that matching impedance proper-ties to 50 X of the CPW transmission lines both withand without the CNTs bundle are fulfilled, the realpart of the relative permittivity e0CNT can be derivedeasily from the measured difference of the phase-shift DArg(S21) by the following expression

Fig. 5. Measurements of the main S-parameters considering (a) unloaded and (b) loaded with a CNTs film CPW transmission lines. Presenteddata correspond to measurements performed on three different transmission lines.

Haddadi, Tripon-Canseliet, Hivin, Ducournau, Teo, Coquet, Tay, Lepilliet, Avramovic, Chazelas and Decoster2436

e0CNT ¼ 2cDArgðS21Þ

2pfLþ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1 þ eGaAsð Þ=2p

� �2

�eGaAs;

ð1Þ

where L = 1.25 cm denotes the length of the quartzsubstrate.

In the same manner, the imaginary part of therelative permittivity e00CNT can be derived by thefollowing form10

e00CNT ¼ak0ðe0CNT � 1Þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

e0CNT þ eGaAs

� ��

2q

p e0CNT þ eGaAs

� �

=2 � 1� � : ð2Þ

The extracted real and imaginary parts of therelative complex permittivity are depicted in Fig. 7.The relative permittivity of the CNTs bundle isaround unity, whereas very low dielectric losseswith an imaginary part of the relative permittivityunder 10�3 are found.

MICROWAVE IMPEDANCEMEASUREMENTS

To characterize CNTs impedance, two deviceswere prepared and tested. Figure 8 presents thefirst device for CNTs impedance characterization.The CNTs sample is introduced in a metallic sub-mount equipped with a K microwave connector.This sample is a growth of 80 lm long CNTs on asilicon-doped substrate with Ti/Pt back side layerdeposition prior to CNTs growth. A 1 9 1 mm2 goldsquare deposition on top CNTs is performed usingshadow mask technique. Then a metallic wire isglued between the microwave connector and theCNTs gold top contact. In this way, the CNTsbundle impedance located between silicon substrateand top gold contact are under our reach at themicrowave K connector level. The second device to

be used as a reference is the same as before whenCNTs on doped silicon substrate sample is removedto get an open circuit. Thanks to a comparison withand without CNTs sample, it is possible to extractthe impedance of a CNTs bundle only by subtractingconnector access impedance.

Experiments were carried out with a PNA-XNetwork Analyzer N5242A to measure the complexreflection coefficient S11. The network analyzer ispreliminary calibrated in the coaxial referenceplane. With the reflection coefficient S11, we areable to characterize the impedance of CNTs from10 MHz to 26.5 GHz. We present respectively inFigs. 9 and 10 the magnitude and phase-shift of thereflection coefficients with and without CNTs. Wecan clearly see that the magnitude of S11 is nearlyequal to �0.5 dB in the frequency range considered.It means that almost all the microwave energy isreflected by the CNTs in this frequency range. Wecan also deduce from these experiments the differ-ence of the measured phase-shifts of the reflectioncoefficients with and without CNTs.

This value is around 180� at the lowest frequen-cies, and decreases almost linearly down to 130� at26.5 GHz. This indicates the microwave equivalentcircuit of CNTs bundles appears as a low valueresistance in series with an inductor. We expect theresistance to be around 1.4 X and the inductancebetween 0.1 nH and 0.2 nH. This inductance could

Fig. 8. Schematic view of the CNTs film mounted onto a coaxial Ktest fixture and dedicated to localized microwave impedance mea-surement with contact area on top of the CNTs film.

Fig. 6. Measured phase-shifts difference and attenuation coefficientrelated to the CNTs film.

Fig. 7. Extracted relative complex permittivity e�CNT ¼ e0CNT � je00CNT asa function of the frequency.

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come together from the bundle and from the metal-lic connection between the CNTs bundles and the Kconnector.

DISCUSSION AND INTERPRETATION

Let us look first at the real part of the complexrelative permittivity for an electric field perpendic-ular to CNTs axis, which is almost equal to one. Thiscan easily be explained by the density of carbonatoms. If we start from the density of carbonnanotubes, which was estimated between 1010 and1011 CNTs/cm2, and if we introduce the size of acarbon–carbon bond, which is 154 pm in diamond, itturns out there is a carbon atom density which is, atmaximum, around 3 9 1019 at/cm3. This is threeorders of magnitude less than for diamond, which is1.77 9 1023 at/cm3 and for graphite, which is1.14 9 1023 at/cm3. Knowing that the relative per-mittivity is around 10 and 15, respectively, fordiamond and graphite, it obviously turns out thatthe permittivity of CNTs bundles can be 1 or only alittle bit higher than 1.

In a second step, we discuss the imaginary part ofthe relative complex permittivity for an electric fieldperpendicular to CNTs axis between 10 MHz and67 GHz, which was found lower than 0.001 corre-sponding to a small absorption of CNTs bundle inthis frequency band. Assuming the absorption

mechanism is mostly due to a conduction phe-nomenon perpendicular to the CNTs axis, theperpendicular conductivity should be written asr? ¼ 2pf e00CNTe0. Introducing a 67 GHz frequency fore00CNT = 0.001, we deduce a perpendicular conductiv-ity lower than 10�4 S/cm, which is low. Again, thissurprising result can be easily explained by thedensity of carbon nanotubes. Starting from a den-sity of 1011/cm2, which corresponds also to 1000/lm2, it means an average distance of roughly 32 nmbetween the two closest CNTs axis. Introducing themost likely CNT diameter, namely 1.5 nm, it comesout for 30 nm as the average distance between twonearby CNTs. This is too far to allow current flowingperpendicular to CNTs, since the tunneling effectshould be efficient for shorter distances. Conse-quently, the conductivity for current flowing per-pendicular to CNTs axis is surely very low.

In a third step, we examine the microwaveimpedance determined in Sect. 5. It is obtainedwhen the electrical field and the flowing current areparallel to the CNTs axis. The result is a 1.4 Xresistor in series with a 0.2 nH inductance. It isdifficult to give a clear interpretation for theinductance, which includes the parasitic connectionto the microwave connector; consequently, theCNTs bundle’s inductance is difficult to estimate.But it is very interesting to extract, from theresistance value, the parallel conductivity r//. Fromthe CNTs length (80 lm), the gold contact paddimensions (1 9 1 mm2), and the resistance value(1.4 X) it comes out to r//# 3S/cm, which is muchhigher than the perpendicular conductivity. This isconsistent with the 1/3 metallic (2/3 semiconductor)CNTs rate, which induces an easy conduction alongCNTs axis.

COMPARISON WITH DC EXPERIMENTS

We would think of geting parallel and perpendic-ular conductivity of CNTs forest easily in a firstprocedure by using direct current (DC) experimentson dedicated test samples. We demonstrate exper-imentally in this section it is very difficult and quasiimpossible to reach parallel and perpendicularconductivity with accuracy from only DCexperiments.

We start with experiments carried out on samplealready presented in Sect. 5 for microwave impe-dance experiments. Briefly, the sample is thenconstituted of CNTs vertically grown on a dopedsilicon substrate back side metallized prior to CNTsgrowth, and gold top metallized using the shadowmask technique (Fig. 11). The size of the top goldcontact pad is 2 9 2 mm2. Two samples with CNTslengths, namely, 80 lm and 180 lm, were preparedand checked. The measured I-V characteristics withthese two samples are given in Figs. 12 and 13. Weclearly see no significant difference from these twoexperiments. For these two samples, the CNTsdevices exhibit almost the same ohmic nature with

Fig. 9. Measured magnitude of the reflection coefficient S11 with andwithout CNTs.

Fig. 10. Measured phase-shift of the reflection coefficient S11 withand without CNTs.

Haddadi, Tripon-Canseliet, Hivin, Ducournau, Teo, Coquet, Tay, Lepilliet, Avramovic, Chazelas and Decoster2438

the same resistance value (# 150 X), which meanswe do not see the resistance of CNTs at all, which isscreened by the parasitic contact resistances of thedevices. This can be easily controlled from the valueof the parallel conductivity of the CNTs bundle,which leads to a resistance value for the CNTSbundles of the devices under test, respectively,0.06 X and 0.13 X for the 80 lm and 180 lm longCNTs. This is much lower than the measuredresistance values, explaining why these CNTs resis-tances cannot be extract from these DC experimentswithin the frame of our experimental data accuracy.It turns out the parallel DC conductivity is verydifficult to get from these kinds of experiments.Because the parasitic contact resistance shouldincrease together with the CNTs resistance whendiminishing the size of the top contact pad, the onlyway would be to take advantage of much longerCNTs, almost one order of magnitude, which is,from up-to-date growth conditions, impossible toobtain. The advantage of impedance characteriza-tion in the microwave regime is coming from thecapacitance nature of contact parasitic impedance.From low frequency experiments, we do not detailhere, the contact parasitic impedance is equivalentto a capacitance (around 250 pF for 1 9 1 mm2 sizepad) in parallel to a 150 X resistance (almost themeasured DC resistance). This can be easilyexplained by the structure of the device: first atthe bottom, a dielectric Al2O3 layer is grownbeneath the CNTs in sandwich between two metallayers before CNTs growth, this looks like a capac-itance, and on top CNTs, all CNTs have not thesame length, which introduces for a part of them anair gap between the end and the contact gold, whichis also like a capacitance. As a consequence, forfrequencies high enough, the contact resistance isshortcut by the low impedance associated with thecapacitance. This this way, it is possible to overcomethe strong limitation coming from parasitic contactresistances without the need to change the CNTstechnology.

In a second procedure, we tried to extract theperpendicular conductivity from DC experiments.We prepared specific samples based on the use ofquartz substrates on which were vertically grown15 lm long CNTs. On top of CNTs, we thendeposited three square gold 1 9 1 mm2 contact padsseparated by respectively 1 mm and 2 mm gaps (seeFig. 14). With such a device, we performed

IDC (mA)

VDC (V)

-30

-15

0

15

30

-4 -2 0 2 4

Fig. 12. Measured I–V characteristic for 80 lm long CNTs grown onsilicon substrate.

IDC (mA)

VDC (V)

-30

-15

0

15

30

-4 -2 0 2 4

Fig. 13. Measured I–V characteristic for 180 lm long CNTs grownon silicon substrate.

Fig. 14. Set-up for the measurement of the I–V characteristic (DCcurrent perpendicular to the CNTs axis).

Fig. 11. Set-up for the measurement of the I–V characteristic (DCcurrent parallel to the CNTs axis).

Microwave and Millimeter Wave Properties of Vertically-Aligned Single Wall CarbonNanotubes Films

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conventional transmission line measurements(TLM). The I–V characteristic (Fig. 15) was foundto be following the Ohm law, and the TLM mea-surements are given in Fig. 16.

We deduce a contact resistance of 170 X and aresistance between 1 mm electrode spacing equal to130 X leading to a conductivity of 5 S/cm. This is notin agreement with the perpendicular conductivityobtained from microwave experiments. Given thedata results obtained in the DC domain, i.e., we donot see a signature of the CNTs, it seems that theCNTs are shortcut at their bases. Among thepossible explanations, the iron film catalyst couldremain after the CNTs growth beneath the CNTs.The measured DC resistance is in agreement withthe calculation of the corresponding resistancebetween the two contact pads assuming a 0.5 nmFe film thickness and a current flowing directlyfrom the contact pad down to the iron film parallelto the CNTs axis with almost no resistance. Indeed,we find a resistance value around 200 X. In anyevent, we conclude that the perpendicular conduc-tivity cannot be determined from DC experiments.By using microwave experiments, with a

suitable test structure as we did, it is possible toput this conductive parasitic layer far away enoughfrom the electric field to get no effect on themeasurement from this metallic parasitic layer. Bydoing this, the microwave electric field sees only theCNTs. Therefore, measurements should then givethe right value of perpendicular conductivity.

CONCLUSION

In this paper, we detailed the microwave andmillimeter wave experiments we carried out onCNTs forest vertically grown on quartz substrates.The main results are the following.

The real part of the permittivity for an electricfield perpendicular to CNTs axis is nearly equal to 1over the entire frequency band (10 MHz to 67 GHz),due to a low density of carbon atoms (#1019 cm�3).

The imaginary part of the permittivity for anelectric field perpendicular to CNTs axis is very low(<10�3) for frequencies lower than 67 GHz, weexplained as a consequence of a very low conduc-tivity (<10�4 S/cm) perpendicular to the CNTs axis.

The microwave (up to 26.5 GHz) impedancebetween bottom and top CNTs bundles appears asa very low microwave resistance, due to a parallelconductivity of the CNTs bundle along the CNTsaxis quite high (3 S/cm) to its perpendicular coun-terpart. The tensor nature of the vertically grownCNTs bundle conductivity is here clearly experi-mentally demonstrated.

ACKNOWLEDGEMENTS

This work was performed under SAFARI French-Singapore joint defense contract. The authors wouldlike to warmly thank Rose Marie Sauvage andPascal Maignet from DGA (French Defense) fortheir strong support. We also would like to deeplythank Jean Paul Martinaud (Thales SystemesAeroportes), Claude Chekroun (SART), MathieuHalbwax, and Jean Francois Legier (IEMN) fortheir collaboration in this project.

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