FeiGe Catalyzed Carbon Nanotube Growth onHf02 for Nano-Sensor Applications

T. Uchino', G. N. Ayre", D. C. Smitlr', J. L. Hutchison", C. H. de Groot', and P. Ashburn''School of Electronics and Computer Science, University of Southampton, Southampton Sal? IBJ, UK

2School of Physics and Astronomy, University of Southampton, Southampton Sal? IBJ, UK3Department of Materials, University of Oxford, Parks Road, Oxford, OXI 3PH , UK

Abstract-A carbon nanotube (CNT) growth process on Hf02is reported for the first time for application in nano-sensors, Theprocess uses a combination of Ge nanoparticles and ferric nitratedispersion and achieves an increase in CNT density from 0.15 to6.2 11m length/um" compared with the use of ferric nitratedispersion alone. The growth process is validated by thefabrication of back-gate CNT field-effect transistors (CNTFETs)using AI source/drain (SID) contacts and a "2 anneal at 400°C.The transistors exhibit p-FET behavior with an Iml1offratio of 105

and a steep sub-threshold slope of 130 mY/dec. These results arerather sur prising, as earlier research in the literature onCNTFETs with AI SID electrodes showed n-FET behavior. Thep-FET behavior is shown to be due to the H2 anneal, which weascribe to the smaller electron affinity of hydrogenised CNTs .Measurements of the temperature dependence of the draincurrent show low Schottky barrier height AI SID contacts after a"2 anneal, which tends to confirm this explanation.

(!) @ Ge e e (l)

(a)

PECVD Si0 2Hf02

Ge nanoparticle formation

Ferric nitrate dispersion

CNT growth at 850 °C for 20 min

Photolithography for SID contacts

AI sputtering (150 nm)

Lift-off

H2 anneal at 400 °C for 30 min

(b)

I. INTRODUCTION

Recently, carbon nanotubes (CNTs) are gammg muchattention for bio-sensing [1] because they offer the prospectreal-time, label-free sensing for point-of-care diagnosis . Themain advantage of CNTs for this application is a very highsensitivity due to the large surface to volume ratio of a carbonnanotube. The use of a high-x dielectric as a gate insulator for aCNT field-effect transistor (CNTFET) is of interest because itdelivers improved performance due to an increased l orlloffratio.CNTFETs with a HfDz gate dielectric have also recently beenresearched for application in high-speed CNT memories and astrong hysteresis effect has been observed [2]. CNTs can beintroduced onto HfOz using dispersion techniques, but CNTgrowth by chemical vapor deposition (CYD) would be morecompatible with mainstream silicon technology. However,CYD growth of CNTs on HfDz appears to be very difficult andto our knowledge no work has been reported on this topic todate.

In this paper, a CNT growth process on HfOz is reported forthe first time and this growth process is used to produce backgate CNTFETs with Al sourceldrain (SID) contacts. The novelgrowth process uses a combination of Ge nanoparticles andferric nitrate dispersion to achieve a dramatic increase in CNTyield compared with the use of ferric nitrate dispersion alone.Electrical measurements on completed CNTFETs show p-FETbehavior, an excellent l orlloff ratio of 105, and a steepsub-threshold slope of 130 mYIdec.

978-1-4 244-4353-6 /09 /$ 25 .00 ©2 0 0 9 IEEE

Fig. I. (a) Schematic cross-section illustrations of the Ge nanopartic1efabrication process. (b) Main CNTFET fabrication process flow after Genanoparti c1e formation.

II. E XPERIMENTAL

A p" Si substrate (0.005 Q·cm) was employed as a back gateand a passivating SiOz layer was thermally grown, followed bythe deposition of a HfOz layer by atomic-layer deposition, asshown in Fig.l . A 30nm SiOz layer was then deposited byplasma enhanced chemical vapor deposition (PECYD) on topof the HfOz and densified at 950 °C. The SiOz layer was thenimplanted with s-ro" em", 20 keY Ge and annealed in Nz at600 °C for 40 min to create Ge nanoparticles. The SiOz layerwas then removed using a HF vapor etch to expose the Genanoparticles on top ofthe HfOz layer. Then the HfOzsubstratewas dipped in ferric nitrate solution for 1 min and rinsed withhexane. The CNT growth was performed using chemical vapordeposition in a hot-wall reactor at atmospheric pressure. CNTswere grown at 850 °C for 20 min using a mixture of methane(1000 seem) and Hz(300 seem) immediately after a pre-annealin Hz (1000 seem) at 900 DC. For comparison, CNT growth onHfDz without Ge particles was also carried out.

Back gate CNTFETs were fabricated with Al SID contacts.Al was deposited by sputtering and the SID electrodes wereformed using direct write optical lithography and lift-off. Theuse of Al instead of the more common Pd can both reduce the

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Fig. 2. SEM images after CNT growth on HID, substrates using (a) Fenanoparticles only and (b) a eombination of Ge and Fe nanoparticles. CNTarea densities are 0.15 and 6.2 11m length/urn' respectively.

cost and improve the yield, as Pd has poor adhesion to oxides.The gap between the SID electrodes was 2.0 urn and the widthwas 5.0 urn After AI patterning, the devices were annealed inH2 at 400 DC for 30 min. The area densities of CNTs wereevaluated using FE-SEM images and ImageJ was used todetermine the total contour length of CNTs [3]. Ten SEMimages taken from the same sample were used for quantitativeanalysis, with overlapping regions being discarded. Ramanspectra were obtained using a Renishaw micro-Raman systemwith He-Ne (Aexcilalion =632.8 nm) laser excitation with power of12mW.

III. R ESULTS AND DISCUSSION

The Ge nanoparticles were evaluated by means of atomicforce microscopy. These results showed a high density ofparticles (460 ± 30 particl es/urn"), with particle heightsbetween 1.3 and 2.9 nm. This result agrees well with previousresults by Min et at [4]. Fig. 2 shows FE-SEM images afterCNT growth for samples using Fe nanoparticles only (a) and a

G

-III~l:::::l

.ci100 200 300 400...

~>-~IIIl:C1l-l:

D M

1200 1400 1600 1800 2000

Wave number (cm-1)

Fig. 3. Raman spectra of CNTs grown from a combination of Ge and Fenanoparti cles on l lfO, substrate. The inset shows radial breathing mode(RBM), indicating that SWNTs are present,

combination of Ge and Fe nanoparticles (b). For Fenanoparticles only, very few CNTs are present and onlyisolated CNTs are occasionally seen as shown in inset ofFig.2(a). The area density of CNTs was estimated as 0.15 urnlength/urn", In contrast, the presence of the Ge nanoparticlesdramatically aids CNT growth, giving an area density of6.2 urnlength/urn", These results suggest that selective CNT growth ispossible by means of Ge ion implantation with photo resistmask patterns. The improvement of the CNT yield using Genanoparticles is consistent with the work of Segura et at. [5],who showed that Ge could suppre ss agglomeration of metalparticles during annealing.

The CNTs grown from combined FeiGe nanop articles werecharacterized by micro Raman spectroscopy (Fig. 3). Allsamples clearly showed the radial breathing mode (RBM) andM-band, indicating that single-wall ed CNTs (SWNTs) arepresent. The Raman intensity ratio (IllIG) of D-band to G-bandwas less than O. I, indicating that synthesized SWNTs have alow defect density. The diameter of the SWNTs is estimated atabout 1.5-2.0 nm from the RBM peaks, though thicker SWNTsmay also be present due to the cut-offofour Raman notch filter.

Fig. 4 shows an FE-SEM image of a back gate CNTF ET witha Si02/Hf02 gate insulator and AI SID contacts. The SWNTsbridge the 2.0 urn gap between the AI SID electrodes. We havesuccessfully fabricated in total more than 80 functional devices.

Fig. 5 shows one of the best electrical characteristics ofCNT FETs measured in ambient air and it can be clearly seenthat the devices exhibit p-FET behavior. The sub-thresholdcharacteristics show an excellent Imlloff ratio of 10

5 and areasonably steep sub-threshold slope of 130 mVIdec. Theoutput characteristic shows linear characteristics below

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Fig. 5. I-V characteristics of an AI contacted CNTFET with channel length Lg= 2.0 11m after H2 anneal. (a) Sub-threshold characteristics for Vd = -0.1 and-1.0 V and (b) Output characteristics for Vg = -1.0, -1.5, -2.0, -2.5 V.

o-0.5-1Vd (V)

-1.5

-710

-810

-910

~ 10-10

'0-11""j"

10

-1210

-1310

-1410

-4 -2 0 2 4

Vg (V)15

12

42oVg (V)

-2

10 -8

Vd=-1.0 V

10 -9

~ 10 -10"C-r

10 -11

10 -12 L---'-__---'-'-__-'--__-'-__---'_-'

42

-- in air- - - - - In vacuum

Vd =-1.0V

oVg (V)

-2

10 -13 L---'-__---'L.JL:....JL!....-'--'--__...L__--'-----l

230 dayslater

10 -10

10 -8 r-r-t-t-r-__--,-__---,-__---,,-__,.----,

10 -12

10 -11

Fig. 6. The sub-threshold characteristics of an AI contacted CNTFET (Lg = 2.011m) with a SiO,IHfD, gate insulator were measured under several cond itionsineluding just after H, anneal., in vacuum of I x I0-6 Torr, and in ambient airafter 230 days.

Fig. 8. A hysteresis of fd- Vg characteristics of the same device in Fig. 6.

ACKNOWLEDGMENT

The authors acknowledge EPSRC for supporting this work.

1.2

Vd = Vg -Vth = -1.0 V

1.0

•0.8 •