Carbon Nanotube Network Transistors Research Articles

Gate Capacitance Coupling of Double-Gate Carbon Nanotube Network Transistors.

Carbon nanotube (CNT) network channels constructed using a high-purity CNT solution for use in CNT thin-film transistors have the advantages of the possibility of requiring a low-temperature process and needing no special equipment. However, there are empty spaces between individual CNTs, resulting in unexpected effects. In this study, double-gate (DG) CNT network transistors were fabricated and measured in four different configurations to observe the capacitive coupling effects between the top gate (TG) and bottom gate (BG) in the DG structure. As a result, the electrical characteristics measured with the BG with a thicker gate oxide while floating the TG were similar to those measured with the TG with a thinner gate oxide. A comparison of the measured transfer curves showed that TG and BG were strongly coupled through the empty spaces in the channels. In addition, we evaluated the capacitance coupling effect due to changes in the CNT density, which is closely related to the empty space of the network channel. Finally, we proposed a method to determine the effective gate capacitance by considering the empty spaces between CNTs, which enabled the accurate evaluation of mobility. The effects of these materials were demonstrated by fabricating transistors using Al2O3, HfO2, and ZrO2 as TG oxide materials. By focusing on considerations based on the properties of CNT materials, our study provides valuable insights into accurate electrical modeling and potential advancements in CNT-based devices.

Cost-effective method for fabricating carbon nanotube network transistors by reusing a 99% semiconducting carbon nanotube solution

Carbon nanotubes (CNTs) are one-dimensional materials that have been proposed to replace silicon semiconductors and have been actively studied due to their high carrier mobility, high current density, and high mechanical flexibility. Specifically, highly purified, pre-separated, and solution-processed semiconducting CNTs are suitable for mass production. These CNTs have advantages, such as room-temperature processing compatibility, while enabling a fast and straightforward manufacturing process. In this paper, CNT network transistors were fabricated on a total of five 8 inch wafers by reusing a highly purified and pre-separated 99% semiconductor-enriched CNT solution. The results confirmed that the density of semiconducting CNTs deposited on the five selected wafers was notably uniform, even though the CNT solution was reused up to four times after the initial CNT deposition. Moreover, there was no significant degradation in the key CNT network transistor metrics. Therefore, we believe that our findings regarding this CNT reuse method may provide additional guidance in the field of wafer-scale CNT electronics and may contribute strongly to the development of practical device applications at an ultralow cost.

Ferroelectric polarization effect on hysteresis behaviors of single-walled carbon nanotube network field-effect transistors with lead zirconate-titanate gating

We report the fabrication of single-walled carbon nanotube (SWCNT) network transistors by ferroelectric Pb(Zr0.4Ti0.6)O3 (PZT) bottom-gating and investigate the polarization effects of PZT on the transport properties of the transistor device. Our devices exhibit typical p-channel transistor characteristics and a large hysteresis loop with high ON/OFF current ratio and large ON current as well as memory window (MW) measured up to 5.2 V. The origin of clockwise hysteresis is attributed to ferroelectric polarization modulated charge trapping/de-trapping process in the interface states between SWCNT networks and PZT. The retention time about 104s with two high stable current states preliminarily demonstrates great potential for future non-volatile memory applications based on such SWCNT/PZT hybrid systems.

Solution-processed polymer-sorted semiconducting carbon nanotube network transistors with low-k /high-k bilayer polymer dielectrics

Solution-processed semiconducting carbon nanotube transistors with a high mobility and an ON/OFF ratio are the most promising for use in flexible electronics. In this paper, we report low-k/high-k bilayer polymer dielectrics for solution-processed semiconducting single-walled carbon nanotube (s-SWNT) field-effect transistors (s-SWNT-FETs) with efficient charge transport and operation at low voltage. Thin low-k polystyrene (10 nm) is used for the first contact insulator with a channel in order to passivate the dipolar disorder induced by high-k insulators. The second gate insulator for low voltage operation is cyanoethyl pullulan (CEP), which is an environmentally friendly high-k insulator based on cellulose. Moreover, poly[(vinylidenefluoride-co-trifluoroethylene) is chosen as a single layer dielectric for comparison. A reasonably low operational voltage (<10 V) and high operational stability are achieved by the s-SWNT-FETs with polystyrene/CEP bilayer gate dielectrics. In addition, this indicates that the interface between the s-SWNTs and the low-k insulator is of critical importance for efficient charge transport.

Effect of Polymer Gate Dielectrics on Charge Transport in Carbon Nanotube Network Transistors: Low-k Insulator for Favorable Active Interface.

Charge transport in carbon nanotube network transistors strongly depends on the properties of the gate dielectric that is in direct contact with the semiconducting carbon nanotubes. In this work, we investigate the dielectric effects on charge transport in polymer-sorted semiconducting single-walled carbon nanotube field-effect transistors (s-SWNT-FETs) by using three different polymer insulators: A low-permittivity (εr) fluoropolymer (CYTOP, εr = 1.8), poly(methyl methacrylate) (PMMA, εr = 3.3), and a high-εr ferroelectric relaxor [P(VDF-TrFE-CTFE), εr = 14.2]. The s-SWNT-FETs with polymer dielectrics show typical ambipolar charge transport with high ON/OFF ratios (up to ∼105) and mobilities (hole mobility up to 6.77 cm2 V-1 s-1 for CYTOP). The s-SWNT-FET with the lowest-k dielectric, CYTOP, exhibits the highest mobility owing to formation of a favorable interface for charge transport, which is confirmed by the lowest activation energies, evaluated by the fluctuation-induced tunneling model (FIT) and the traditional Arrhenius model (EaFIT = 60.2 meV and EaArr = 10 meV). The operational stability of the devices showed a good agreement with the activation energies trend (drain current decay ∼14%, threshold voltage shift ∼0.26 V in p-type regime of CYTOP devices). The poor performance in high-εr devices is accounted for by a large energetic disorder caused by the randomly oriented dipoles in high-k dielectrics. In conclusion, the low-k dielectric forms a favorable interface with s-SWNTs for efficient charge transport in s-SWNT-FETs.

Three-Dimensional Fin-Structured Semiconducting Carbon Nanotube Network Transistor.

Three-dimensional (3-D) fin-structured carbon nanotube field-effect transistors (CNT-FETs) with purified 99.9% semiconducting CNTs were demonstrated on a large scale 8 in. silicon wafer. The fabricated 3-D CNT-FETs take advantage of the 3-D geometry and exhibit enhanced electrostatic gate controllability and superior charge transport. A trigated structure surrounding the randomly networked single-walled CNT channel was formed on a fin-like 3-D silicon frame, and as a result, the effective packing density increased to almost 600 CNTs/μm. Additionally, highly sensitive controllability of the threshold voltage (VTH) was achieved using a thin back gate oxide in the same silicon frame to control power consumption and enhance performance. Our results are expected to broaden the design margin of CNT-based circuit architectures for versatile applications. The proposed 3-D CNT-FETs can potentially provide a desirable alternative to silicon based nanoelectronics and a blueprint for furthering the practical use of emerging low-dimensional materials other than CNTs.

Investigation of optimal hydrogen sensing performance in semiconducting carbon nanotube network transistors with palladium electrodes

The work function of palladium (Pd) is known to be sensitive to hydrogen (H2) via the formation of a surface dipole layer or Pd hydride. One approach to detect such a change in the work function is based on the formation of a Schottky barrier between Pd and a semiconductor. Here, we demonstrate a H2 sensor operable at room temperature by assembling solution-processed, pre-separated semiconducting single-walled carbon nanotube (SWNT) network bridged by Pd source/drain (S/D) electrodes in a configuration of field-effect transistors (FETs) with a local back-gate electrode. To begin with, we observed that the H2 response of the fabricated SWNT FETs can be enhanced in the linear operating regime, where the change in the work function of the Pd S/D electrodes by H2 can be effectively detected. We also explore the H2 responses in various SWNT FETs with different physical dimensions to optimize the sensing performance.

Enhancement of ambipolar characteristics in single-walled carbon nanotubes using C60 and fabrication of logic gates

We demonstrate a technique to convert p-type single-walled carbon nanotube (SWNT) network transistor into ambipolar transistor by thermally evaporating C60 on top. The addition of C60 was observed to have two effects in enhancing ambipolar characteristics. First, C60 served as an encapsulating layer that enhanced the ambipolar characteristics of SWNTs. Second, C60 itself served as an electron transporting layer that contributed to the n-type conduction. Such a dual effect enables effective conversion of p-type into ambipolar characteristics. We have fabricated inverters using our SWNT/C60 ambipolar transistors with gain as high as 24, along with adaptive NAND and NOR logic gates.

Accurate extraction of mobility in carbon nanotube network transistors using C-V and I-V measurements

The mobility of single-walled carbon nanotube (SWNT) network thin-film transistors (TFTs) is an essential parameter. Previous extraction methods for mobility encountered problems in extracting accurate intrinsic mobility due to the uncertainty of the SWNT density in the network channel and the existence of contact resistance at the source/drain electrodes. As a result, efficient and accurate extraction of the mobility in SWNT TFTs is challenging using previous methods. We propose a direct method of extracting accurate intrinsic mobility in SWNT TFTs by employing capacitance-voltage and current-voltage measurements. Consequently, we simply obtain accurate intrinsic mobility within the ink-jet printed SWNT TFTs without any complicated calculations.

Recent Advances on Electrically Induced Light Emission from Carbon Nanotubes

Light emission from individual (or small bundle) and network carbon transistors based on single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) is here reviewed in relation with the different mechanisms for light emission: electroluminescence [1,2,3] and incandescence or thermal radiation [4]. These studies guide our ongoing efforts towards making efficient light sources from nanotube emitters. The approach consists of operating the nanotubes transistors in different conditions of thermalisation and of exploring for each condition the different signatures in the emission, either thermal or electroluminescent, using an analysis of the resulting near-IR spectrum. The spectra were acquired in the energy range between 0.5 and 1.3 eV using the Spectrometer Infrared of Montreal (SIMON) mounted with a HgCdTe detector and on a probe station. Three different device configurations were measured. The first is a sub-monolayer network of nanotubes interconnected together to form a percolating semiconducting layer. [5] The numerous tube-tube junctions in the networks limit the current and prevent temperature raise by joule heating at higher bias. Electroluminescence from exciton recombination is found to dominate in all conditions, evidenced by well-defined emission peaks (~200meV in width) in the near-IR (Figure 1). The emission is ascribed to a radiative relaxation of the first excitonic transition. Even at very large biases, the main component of the emission from network transistors is electroluminescence, not incandescence. The second device geometry consists of a thick (~250 nm) and suspended carbon nanotube film having low resistance and reduced thermalization. This configuration favors joule heating when operating at large source-drain voltage and allows to significantly increase the nanotube temperature. The resulting incandescence was measured and used to calibrate SIMON for blackbody emission from a thermal source of pure nanotubes. A third set of devices was finally prepared with individual SWNT and DWNT transistors operating in vacuum, which conditions is used to prevent convection and for reducing the cooling capability of the substrate. The spectra were then compared with the different spectral signatures ascribed either to electroluminescence or thermal radiation depending on the biasing conditions. The experiments show a transition from electro-luminescence at low source-drain voltage to incandescence at higher voltage, with a regime of both in-between. These results will be discussed in terms of the dominant dissipation channels and carrier scatterings in nanotubes. Last, the best conditions to maximize light emission while avoiding heating and progress towards our effort to build electroluminescent transistors at 1,55 μm wavelength using sorted SWNTs will be discussed. Figure 1 Left: SEM and infrared micrographs of a large SWNT network transistor. Right: A carbon nanotube network transistor with the electroluminescence spectra at different biases. [Adapted from Ref. 5] REFERENCES [1] J. A. Misewich, R. Martel, Ph. Avouris, J. C. Tsang, S. Heinze, and J. Tersoff, Science, 300, 783 (2003). [2] J. Chen, V. Perebeinos, M. Freitag, J. Tsang, Q. Fu, J. Liu, and P. Avouris, Science, 310, 1171 (2005). [3] L. Marty, E. Adam, L. Albert, R. Doyon, D. Ménard and R. Martel, Phys. Rev. Lett. 96, 136803 (2006). [4] D. Mann, K. K., A. Kinkhabwala, E. Pop, J. Cao, X. Wang, L. Zhang, Q. Wang, J. Guo, and H. Dai, Nat. Nano, 2, 33 (2006). [5] E. Adam, C. M. Aguirre, L. Marty, B. C. St-Antoine, F. Meunier, P. Desjardins, D. Menard, and R. Martel, Nano Lett. 8, 2351 (2008).This work was done in collaboration with : Elyse Adam1, Pierre Lévesques2, Étienne Gaufrès2, Vincent Aymong2, David Ménard1 : RQMP and 1École Polytechnique de Montréal, Département de génie physique, Montréal, Québec H3C 3A7, Canada 2Université de Montréal, Département de chimie, Montréal, Québec H3T1J4, Canada.

High-performance carbon nanotube network transistors fabricated using a hole punching technique

We develop a unique approach for fabricating high-performance carbon nanotube network transistors based on lithographic density control via a hole punching technique. In an as-grown dense network of single-walled carbon nanotubes (SWNTs), approximately two-thirds of the tubes are semiconducting and one-third are metallic, leading to a poor on/off ratio ( 10 000, with a high yield (>85%) and a large carrier mobility (20 cm2 V−1 s−1). The on current degradation is not severe, unlike the degradation in networks prepared with a homogeneous density. The hole punching technique introduced here may be applied to network SWNT field effect transistor applications and provide new opportunities for controlling the properties of one-dimensional nanostructured percolating systems.

Impact of thermal boundary conductances on power dissipation and electrical breakdown of carbon nanotube network transistors

We study the impact of thermal boundary conductance (TBC) at carbon nanotube (CNT)-substrate interfaces and CNT junctions on power dissipation and breakdown in CNT network based thin film transistors (CN-TFTs). Comparison of our results from an electro-thermal transport model of CN-TFTs to experimental measurements of power dissipation and temperature profiles allows us to estimate the average CNT-SiO2 TBC as g ∼ 0.16 Wm−1 K−1 and the TBC at CNT junctions as GC ∼ 2.4 pWK−1. We find the peak power dissipation in CN-TFTs is more strongly correlated to the TBC of the CNT-substrate interface than to the TBC at CNT junctions. Molecular dynamics simulations of crossed CNT junctions also reveal that the top CNT is buckled over ∼30 nm lengths, losing direct contact with the substrate and creating highly localized hot-spots. Our results provide new insights into CNT network properties which can be engineered to enhance performance of CN-TFTs for macro and flexible electronics applications.

Complementary Logic Gate Arrays Based on Carbon Nanotube Network Transistors

An efficient technique of fabricating high performance n- and p- type single-walled carbon nanotube (SWNT) network field-effect transistors (NET-FETs) is successfully demonstrated. Complementary inverters, NOR, NAND, OR, AND logic gates have been achieved from integrating these p- and n-type SWNT-NET-FETs. The processing technique described here is fully compatible with conventional silicon microelectronic technologies and it is readily suitable for scalable integration.

Improvement in the I–V characteristics of carbon nanotube network transistors using microwave treatment

This work describes the effects of microwave irradiation on the transfer characteristics of single-walled carbon-nanotube network transistors. The microwave treatment dramatically increases the on-off ratio of the electrical currents by reducing the off-state current. Detailed analyses of Raman spectroscopy data, the thermal effect, and a direct image comparison suggest preferential removal of the metallic paths formed in the transistor channel. This treatment can be utilized as method to repair for electrically failed transistors even after completion of device fabrication.

Comparative study of solution-processed carbon nanotube network transistors

Carbon nanotube networks in thin-film type transistors were studied experimentally, comparing the use of pre-separated semiconducting enriched nanotubes (90% and 99% purity) to examine how topology affects the properties of the devices. Measurements are reported for two deposition methods used for network formation: random and spin-aligned deposition methods. The results show that the thin-film transistors fabricated via spin-aligned deposition demonstrate better electrical uniformity and performance than those produced by the random network deposition method. Our results imply that coverage and alignment are strongly correlated with the properties of the devices and should therefore be simultaneously optimized for improved electrical uniformity and performance.

Effect of the metal work function on the electrical properties of carbon nanotube network transistors

A nearly perfect semiconducting single-walled carbon nanotube random network thin film transistor array was fabricated, and its reproducible transport properties were investigated. The effects of the metal work function for both the source and the drain on the electrical properties of the transistors were systematically investigated. Three different metal electrodes, Al, Ti, and Pd, were employed. As the metal work function increased, p-type behavior became dominant, and the fieldeffect hole mobility dramatically increased. Also, the Schottky barrier of the Ti-nanotube contact was invariant to the molecular adsorption of species in air.

Atomic Layer Deposition of Aluminum Oxide Films for Carbon Nanotube Network Transistor Passivation

Ultra-thin (2-5 nm thick) aluminum oxide layers were grown on non-functionalized individual single walled carbon nanotubes (SWCNT) and their bundles by atomic layer deposition (ALD) technique in order to investigate the mechanism of the coating process. Transmission electron microscopy (TEM) was used to examine the uniformity and conformality of the coatings grown at different temperatures (80 degrees C or 220 degrees C) and with different precursors for oxidation (water and ozone). We found that bundles of SWCNTs were coated continuously, but at the same time, bare individual nanotubes remained uncoated. The successful coating of bundles was explained by the formation of interstitial pores between the individual SWCNTs constituting the bundle, where the precursor molecules can adhere, initiating the layer growth. Thicker alumina layers (20-35 nm thick) were used for the coating of bottom-gated SWCNT-network based field effect transistors (FETs). ALD layers, grown at different conditions, were found to influence the performance of the SWCNT-network FETs: low temperature ALD layers caused the ambipolarity of the channel and pronounced n-type conduction, whereas high temperature ALD processes resulted in hysteresis suppression in the transfer characteristics of the SWCNT transistors and preserved p-type conduction. Fixed charges in the ALD layer have been considered as the main factor influencing the conduction change of the SWCNT network based transistors.

Post Annealing Effect on the Electrical Properties of Top-Gate SWNT Network Transistors

High performance top-gate single walled carbon nanotube network transistors are fabricated with aluminum oxide (Al2O3) layer as a gate dielectric by atomic layer deposition. It exhibits large on/off ratio (>10(4)) due to selective growth of semiconducting tubes by the plasma enhanced chemical vapor deposition. I-V characteristics show p-type or n-type depending on the deposition temperature. We investigate the type dependent characteristics for the carrier polarities with the post annealing effect on the top-gate SWNT network transistors. The dramatic change in the polarity of the top-gate SWNT network transistors, from n-type to p-type due to conversion of I-V characteristics is observed by post-annealing at 350 degrees C for 30 minutes under vacuum. Our observation suggests that competition between electron transfer from the Al2O3 layers to the SWNT surface and electron capture by oxygen molecules adsorbed on the tube walls seems to be the key point for the V(th) change as a function of Al2O3 deposition temperature.

The Geometrical Effect of Single Walled Carbon Nanotube Network Transistors on Low Frequency Noise Characteristics

The noise characteristics of randomly networked single walled carbon nanotubes grown directly by plasma enhanced chemical vapor deposition with field effect transistor. Geometrical complexity due to the large number of tube-tube junctions in the nanotube network is expected to be one of the key factors for the noise power of 1/f dependence. We investigated low frequency noise as a function of channel length (2-10 microm) and found that increased with longer channel length. Percolational behaviors of nanotube network that differs from ordinary semiconducting and metallic materials can be characterized by a geometrical picture with electrical homo- and hetero-junctions. Fixed nanotube density provides a test conditions to evaluate the contributions of junctions as a noise center. Hooge's empirical law is applied to investigate the low frequency noise characteristics of single walled carbon nanotube random network transistors. The noise power shows the dependence of the transistor channel length. It is understood that nanotube/nanotube junctions act as a noise center. However, the differences induced by channel length in the noise power are observed as not so significant. We conclude that tolerance of low frequency noise is important property for SWNT networks as an electronic device application.

Imaging conduction pathways in carbon nanotube network transistors by voltage-contrastscanning electron microscopy

The performance of field-effect transistors based on single-walled carbon nanotube(SWCNT) networks depends on the electrical percolation of semiconducting and metallicnanotube pathways within the network. We present voltage-contrast scanningelectron microscopy (VC-SEM) as a new tool for imaging percolation in a SWCNTnetwork with nano-scale resolution. Under external bias, the secondary-electroncontrast of SWCNTs depends on their conductivity, and therefore it is possibleto image the preferred conduction pathways within a network by following thecontrast evolution under bias in a scanning electron microscope. The experimentalVC-SEM results are correlated to percolation models of SWCNT-bundle networks.