High performances CNTFETs achieved using CNT networks for selective gas sensing

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1 High performances CNTFETs achieved using CNT networks for selective gas sensing Louis Gorintin, Paolo Bondavalli, Pierre Legagneux Nanocarb Laboratory, Thales Research and Technology, Palaiseau Didier Pribat, LPICM, Ecole Polytechnique, Palaiseau ABSTRACT Our study deals with the utilization of carbon nanotubes networks based transistors with different metal electrodes for highly selective gas sensing. Indeed, carbon nanotubes networks can be used as semi conducting materials to achieve good performances transistors. These devices are extremely sensitive to the change of the Schottky barrier heights between Single Wall Carbon Nanotubes (SWCNTs) and drain/source metal electrodes: the gas adsorption creates an interfacial dipole that modifies the metal work function and so the bending and the height of the Schottky barrier at the contacts. Moreover each gas interacts specifically with each metal identifying a sort of electronic fingerprinting. Using airbrush technique for deposition, we have been able to achieve uniform random networks of carbon nanotubes suitable for large area applications and mass production such as fabrication of CNT based gas sensors. These networks enable us to achieve transistors with on/off ratio of more than 5 orders of magnitude. To reach these characteristics, the density of the CNT network has been adjusted in order to reach the percolation threshold only for semi-conducting nanotubes. These optimized devices have allowed us to tune the sensitivity (improving it) of our sensors for highly selective detection of DiMethyl-Methyl-Phosphonate (DMMP, a sarin stimulant), and even volatile drug precursors using Pd, Au and Mo electrodes. Keyword list: Carbon nanotubes, Networks, Transistors, Gas sensors, Selectivity, Airbrush, Work function, percolation. INTRODUCTION The Carbon Nanotubes (CNT) is one of the most popular material know in nanotechnology and has been used either for its high electrical conductivity and mechanical properties in diverse applications such as field emission devices for composites, in order to improve electrical as well as mechanical resistance for electronic devices, to replace silicon based transistors and finally, for chemical and biological sensors. These sensors are developed because they can potentially identify a large number of gases at a very low concentration. They could also be ultra compact, effective at room temperature, with low power consumption, a very fast response time, a low recovery time (few seconds) and CMOS compatible. We present in this article, the main principle of the gas sensors based on carbon nanotube transistors, the technological challenges which need to be overcome and explain Thales choice in front of them. STORY AND WORKING PRINCIPLE 1. Carbon nanotube transistor as gas sensor As SWCNT can be either metallic or semiconducting, they could be used as electrodes or as channel in transistor. For sensing applications, they act as channel in the simplest transistor configuration: a single SWCNT is located between two metallic electrodes, on a SiO 2 /Si substrate acting as a bottom gate. Fig 1. single wall carbon nanotube transistor lateral configuration using a back gate Carbon Nanotubes, Graphene, and Associated Devices II, edited by Manijeh Razeghi, Didier Pribat, Young-Hee Lee, Proc. of SPIE Vol. 7399, SPIE CCC code: X/09/$18 doi: / Proc. of SPIE Vol

2 The first paper about this kind of device has been published by Kong et al. at Stanford University[1]. In this article, this transistor obtained with gold electrodes interacted with gas molecules, changing the CNTFET transfer characteristics (source-to-drain current as a function of the gate voltage). NH3 and NO2 gases have been used to perform this test with concentration from 2 ppm (particle per millions) to ppm. The reason why these gases were chosen is that they have two opposite electronics behaviors: NO2 act as an electron-acceptor whereas NH3 is an electron-donor gas. Indeed, NH3 decrease the activation bias of the CNTFET (from 2 to 2V) whereas NO2 increased it (from 2 to 6V). Fig 2. Transfer characteristic change after exposure to NO2 and NH3 of the first CNTFET gas sensor fabricated at Stanford in 2000 by Kong et al. [1] Several interpretations of these results should be done. Gas Molecules should: 1. Has a doping effect on the body of the CNT. 2. Be adsorbed at the oxide surface. 3. Interact at the contact between CNT and metal. 2. Gas-induced doping or Schottky barrier modulation? Liu et al., at the University of Southern California (USC) [2] performed in 2005 measurements on the same type of devices, covering alternatively the metal/swcnt contacts and the center of the channel with polymethylmethacrylate (PMMA) resin (Fig.3), to validate one of those hypotheses. They found that, even if metal/swcnt contacts where covered, the threshold voltage change. Their conclusion was that the metal/swcnt contacts were not responsible for this change. Unfortunately USC team, after each chemical exposure, only allowed the device to recover for 30 min in air whereas the Stanford team let its device without any resin, desorbs during 1 hour at 200 C in air or by exposing the sample to pure Ar (at room temperature) for around 12 hours. Therefore their study is not conclusively, because gas molecules are, very probably, not able to be sufficiently desorbed after 30 minutes without heating or UV treatment (known to be the most effective techniques to accelerate desorbtion gas molecules) and because gases could still be adsorbed in PMMA resin. Fig 3. Configurations of CNTFETs for the tests performed at the University of South California by Liu et al. in Schematics showing the CNTFET zones exposed to gas species : a) center-covered configuration, b) contact-covered configuration [2] USC measurement results were in opposition to results obtained by Georgestown University [3] a few months later. Actually, Georgetown team performed measurements using Pd electrodes covered both by 200 nm-thick layer of PMMA, and a 2 µm-thick layer of SU8, an epoxy-based photoresist. When the CNTFET is totally covered and exposed to 200 ppm of NO2 the transfer characteristic does not change after 30 minutes. Proc. of SPIE Vol

3 But thanks to diffusion through PMMA/SU8 it reacts after 88 minutes. If only the contact is covered the transistors behavior does not change after exposure to 200 ppm of NO2: no change for 30 min and a reaction after 144 minutes. Furthermore, this team has demonstrated a recovery time for this device without cover was about one day in ambient air. The two main conclusions of this study is that the NO2 could diffuse slowly in PMMA/SU8 and that the metal/swcnt junctions are the key players in the sensing mechanism. Id (na) Id (µa) Fig 4. Results of the tests performed at Georgestown University by Zhang et al. in Transfer characteristics change for a CNTFET in contact-covered configuration (a) and fully covered (b) for different exposure times to NO2[3] Focusing on the physics of the junctions, CNTFET are known to operate as Schottky-barrier transistors so each metal /semiconductor contact acts as a Schottky barrier done by: Φ Bh = E G + φ s - χ m (1) Electrode SWCNT Fig 5. Band diagram at metal/swcnt (semiconducting) junctions in vacuum using the Schottky model Where Φ Bh is the Schottky barriers, φ s the SWCNT electron affinities, E G is the SWCNT band gap and χ m, the metal work function.thus the gas effect on metal work function can explain the shift of the transfer characteristic. But, as it has been shown by an IBM team led by Ph. Avouris[4], [5], and modeled by T. Yamada of NASA Ames Research Center [6], SWCNT / Metal contact doesn t act as a traditional Schottky barrier which does not consider the electrostatic charge balance inside the Schottky junction. Indeed, for a gold electrode CNTFET, the traditional theory cannot explain that the adsorption of oxygen can change the behavior of the transistor from n type to p type: the adsorption of electronegative oxygen molecules at the material surface acts as negative charge and so should increase the electron affinity of CNT; This interaction should have left to a shift to an higher n type behavior. Proc. of SPIE Vol

4 Fig 6. Tests and simulations performed at IBM by Heinze et al. in (a) Transfer characteristic change as a function of oxygen concentration. (b) Simulation of the transfer characteristic change as a function of the metal work function change. This is the reason why Yamada propose a model which takes into account electrostatic phenomena at electrode/swcnt junction. It creates a transition region between the metal and the SWCNTs, where the gases molecules can be adsorbed and thus led to a potential drop ΔU. And this potential barrier can be modulated by charge introduction (coming from gas adsorption) in the transition region. Fig 7. On the left representation of the transition region between AU and SWCNT, on the right Band diagram at metal/swcnt (semiconducting) junctions in vacuum (left hand side) and exposed to air (right hand side) using the modified Schottky model This can be mathematically explained by: Φ Bh = E G + φ s - χ m - ΔU (2) To conclude this part, CNTFET can be used as sensors measuring the adsorption of different gas molecules at the contact between the electrodes and the SWCNT: a sandwich structures appears, where the Schottky junction is modulated by the gas concentration. For further explanation please report to the critical review on the subject fabricated by P. Bondavalli[7]. LARGE SCALE PRODUCTION OF CNTFET These studies have been realised using a single SWCNT between two electrodes to better understand the physics of this new sensor. Unfortunately it is hard to reproduce this kind of device to larger scale for two reasons: - First, the production of semi conducting SWCNT is not well controlled. And even in this case, we are not able to control the SWCNT gap and so the CNTFET behaviour. Proc. of SPIE Vol

5 - Second, to fabricate this kind of device you need to precisely position your SWCNT between at least two electrodes. And if laboratory methods, as positioning using an Atomic Force microscope Probe or electron beam lithography, have been developed to fabricate punctual CNTFET, they are not suitable for industrial application and sensors mass production. 1. The use of SWCNT Random networks as semiconducting materials To overtake this issue, some team has proposed to use random networks of carbon nanotubes[8], [9], [10]. Indeed, in general when SWCNT are produced one third are metallic and two third are semiconducting and so with a carefully controlled areal densities, you can achieve a global semiconducting behavior: as you have twice more semiconducting SWCNT than metallic ones, according to the percolation theory, there is a domain of SWCNT density where semiconducting ones are connected to make a path between electrodes while metallic do not. Moreover, the distance between the electrodes should be larger than the nanotube length in order to avoid short-circuit. If the SWCNT are consider as sticks the percolation threshold [11] is given by: ρ th = (3) 2 π. LSWCNT where L SWCNT is the nanotube length. So for a length around 1µm the percolation threshold is around 6 SWCNT/µm². 2. What kind of technique to fabricate SWCNT mats? To fabricate this kind of networks several techniques has been explored: Snow and al.[8] proposed to grow SWCNT by CVD techniques directly on the electrodes. But with this method it is hard to have a good density control, and the metallic catalyst particles are hard to be erased: transistor performances are decreased and it is hard to perform homogenous grown over large areas. Therefore we first choose to make some works using drop casting deposition: Using commercially available powder of SWCNT with an enhanced concentration of semiconducting ones, we achieved SWCNT solutions with different good nanotube solvent[12] as N,N-diméthylformamide (DMF), N-Méthyl-2-pyrrolidone (NMP), 1,2-dichloroéthane (DCE). These solution were debundleised for 3 hours using a sonicator finger and the purified with centrifugation to finally deposit droplets with a micro pipette on each transistor. If several good transistors can be obtained with these techniques, the results are not reproducible and only a part (around 60%) of the transistors works. The main problem with drop casting is that the mats are not homogeneous due to the so-called coffee ring effect [13]. Fig 8. On the left a dried coffee drop. On the right particles movement during the droplet drying[13] To avoid this problem you need to reduce the convection which is represented by the dimensionless Rayleigh number: 3 ρ. g. α. ΔT. d Ra = (4) κ. η Proc. of SPIE Vol

6 Where ρ is the density, g the gravity force, α the coefficient of thermal expansion, ΔT the temperature difference across the medium, d the diameter of the droplet, κ thermal diffusivity and η the dynamic viscosity. If Ra is bigger than 1000 the convection phenomena start, which is the case for drop casting. To reduce it the easier way is to reduce the drop size. This is the reason why two deposition techniques using small droplets to make SWCNT mats deposit have focused the scientists interest: - Ink jet printing, which can make droplet down to 3 µm o f diameter. - Spray technique, which can go down to an average 10µm droplet(figure 9.) Fig 9. AFM image of PMMA resin after2 second exposure under sprayed NMP. The droplet size is not bigger than 20µm and the average size is about 10µm With the inkjet printer to be able to go down to 3µm you need to use fluid containing small particles but as carbon nanotubes commercially available have a length of about 1µm, they obstruct inkjet printer nozzle if it is not enlarged up to several tenth micro meter. This is the reason why we have chosen in Thales spray technique. Using these techniques we have been able to realise homogeneous mats on large surface (up to 6 inches) with scalable SWCNT areal density, using SWCNT solution in NMP as describe previously. For this we have developed an automatic moving spray gun in collaboration with Dosage 2000 which can be updated to perform deposition up to 12 inches. This automatic spray gun enables to make deposition of random size droplet containing SWCNT with random position. But to not recreate bigger droplets during the deposition, the substrate is heated over the boiling point of the NMP (202 C): all the solvent is instantaneously evaporate avoiding every coffee ring effect. Like this the nanotubes deposit on the substrate in the same direction as they were in the solvent. Controlling the spraying time we are able to perfectly control the SWCNT density. These mats have been used to realised large number (here 8 of each) of reproducible transistors for different channel length (2, 5, 10, 15). Fig 10. SEM image of a) interdigited CNT transistors fabricated at Thales Research and Technology in b)detail of carbon nanotubes random network between two electrodes Proc. of SPIE Vol

7 5µm channel length 1E-4 2µm channel length 1E-5 I DS (A) 1E-5 I DS (A) 1E-6 a) b) V GS (Volt) 10µm channel length 1E V GS (Volt) 15µm channel length 1E-6 1E-7 1E-8 I DS (A) 1E-7 I DS (A) 1E-9 1E-8 c) 1E-10 1E-11 d) 1E V GS (Volt) 1E V GS (Volt) Fig 11. Ids as a function of Vgs for CNTFET obtained using Spray-gin technique with go and back transfer characteristics between -10V and 10V for a), b), c) and d) respectively 2, 5, 10, 15 channel lengths. these transfer characteristics show hysteresis The larger Ion/Ioff ratio is obtained for 15µm channel length transistor up to In this way, we are able to obtain a large number of cost effective transistor compatible with CMOS technology and flexible electronics. 3. How to perform gas sensor selectivity? We have described the working principle of CNTFET gas sensor and our approach to fabricate CNTFETs. Actually, one of the main issue for these sensors is how to perform a selective sensing. Our original approach is to achieve an array of CNTFETs using different metal as electrodes and so discriminate gases. In fact, as the adsorption of a gas on a surface depends on their own affinity each gas metal couple will induce a different modulation of the barrier at the metal/swcnt junction. For an CNTFETs array with different metal we obtain a specific gas fingerprint. This concept has been patented by Thales[14] and has been demonstrated using CNTFETs achieved by drop casting technique. Using Pd, Au, Mo metals we have observed a change of the transfer characteristics which is different for each transistor after exposure to of 1ppm of Dimethyl methylphosphonate (DMMP is a simulant of Sarin nerve gas) and 10ppm of NH3. The carrier gas was the ambient air. We measured the change of IDS (VGS) for each transistors using specific point probes. The measurements have been achieved after an exposure of half an hour to the targeted gases. We observed that the change of the transfer characteristics was different for the CNTFETs fabricated using different metals: the IDS for Pd, Au, and Mo is reduced respectively of 90%, 80% and 65% of their initial values measured in air (for a VDS=0.5Volt) for 1ppm of DMMP and respectively 50%, 65% and 50% for 10 ppm of Proc. of SPIE Vol

8 NH3. These reductions are reached for a value of the gate voltage of 15 Volts. In this work we have demonstrated that CNTFET based array could be potentially used to obtain an electronic fingerprinting of specific gases. Actually, we have observed that the ratio of the IDS current before and after gas exposure is different for 1 ppm of DMMP and 10 ppm of NH3. 1E-8 (a) (b) 1E-9 I DS (A) 1E Volt AIR 0.5 Volt NH3 10ppm 30min 0.5 Volt DMMP 1ppm 30min 1E V GS (Volt) (c) (d) Fig 12. Ids (as a function of Vgs) change after NH3 and DMMP for CNTFETs achieved using Au (a) and Pd (b) for metal electrodes. And the ratio Ids before exposure and Ids after exposure for Au (c) and Pd (d) The ratio changes in a different way for each gas as a function of the metal electrodes. The results are summarized in Table I. The sensing mechanism does not seem to depend on the initial electrode work function(same work function for Au and Pd) seems to be related to specific chemical adsorption. Ti/Pd Ti/Au Ti/Mo DMMP 90% 80% 65% NH3 65% 45% 50% Table I. Relative change of Ids as a function of the metal electrodes after gas exposure for DMMP and NH3. Even with a matrix of 3 different CNTFETs, and only measurement of few we are able to realise a differentiation between NH3 and DMMP, and so detect specifically each gas. Proc. of SPIE Vol

9 4. Other Field of application We have shown in the last paragraph that CNTFET can be used to selectively detect nerve agent as DMMP. But this device can also been used for other security application. For example, we perform measurement on Heroin precursor acetic anhydride at saturation pressure in air Fig 13. Ids (as a function of Vgs) change after exposure after exposure to Acetic anhydride for CNTFETs achieved using Pd for metal electrodes. The results show, for example, a sudden change of IDS as a function of the gate voltage, after 10secs of around 30% of the initial current using Pd metal electrodes (Fig.12). In this case we observed a recovery time of around 30mins (see inset Fig.12). CONCLUSION After have explained the sensing mechanism of CNTFET based on modulation of the Schottky barrier by the gas, we present our approach which has led us to chose spray techniques to fabricate a large number and reproducible CNTFET; we demonstrate that our approach was good showing realization of transistor with state of the art behavior and reproducible for low cost to large scale. We finally introduce our idea to obtain selectivity for this kind of sensors using a matrix of transistor fabricated with different metals and show that the main principle works: we discriminate the detection of DMMP from the NH3. An other application, drug precursor detection, has been shown with a good reactivity and a reasonable recovery time in ambient air. ACKNOWLEDGMENT. This study has been performed in the frame of the project NANOSENSOFIN (Carbon NANOtubes based SENSOrs for gas electronic FINgerprinting) funded by French National Research Agency (ANR). REFERENCES [1] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, et H. Dai, Nanotube Molecular Wires as Chemical Sensors, Science, vol. 287, 2000, pp [2] X. Liu, Z. Luo, S. Han, T. Tang, D. Zhang, et C. Zhou, Band engineering of carbon nanotube field-effect transistors via selected area chemical gating, Applied Physics Letters, vol. 86, 2005, p [3] J. Zhang, A. Boyd, A. Tselev, M. Paranjape, et P. Barbara, Mechanism of NO[sub 2] detection in carbon nanotube field effect transistor chemical sensors, Applied Physics Letters, vol. 88, 2006, p Proc. of SPIE Vol

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