Device and circuit-level performance of carbon nanotube field-effect transistor with benchmarking against a nano-MOSFET

ABSTRACT: The performance of a semiconducting carbon nanotube (CNT) is assessed and tabulated for parameters against those of a metal-oxide-semiconductor field-effect transistor (MOSFET). Both CNT and MOSFET models considered agree well with the trends in the available experimental data. The results obtained show that nanotubes can significantly reduce the drain-induced barrier lowering effect and subthreshold swing in silicon channel replacement while sustaining smaller channel area at higher current density. Performance metrics of both devices such as current drive strength, current on-off ratio (Ion/Ioff), energy-delay product, and power-delay product for logic gates, namely NAND and NOR, are presented. Design rules used for carbon nanotube field-effect transistors (CNTFETs) are compatible with the 45-nm MOSFET technology. The parasitics associated with interconnects are also incorporated in the model. Interconnects can affect the propagation delay in a CNTFET. Smaller length interconnects result in higher cutoff frequency.     

Real time protein recognition in a liquid-gated carbon nanotube field-effect transistor modified with aptamers

The combination of optimized and passivated Field Effect Transistors (FETs) based on carbon nanotubes (CNTs) together with the appropriate choice and immobilization strategy of aptamer receptors and buffer concentration have allowed the highly sensitive and real time biorecognition of proteins in a liquid-gated configuration. Specifically we have followed the biorecognition process of thrombin by its specific aptamer. The aptamer modified device is sensitive enough to capture a change in the electronic detection mechanism, one operating at low protein concentrations and the other in a higher target concentration range. The high sensitivity of the device is also sustained by the very low detection limits achieved (20 pM) and their high selectivity when other target proteins are used. Moreover, the experimental results have allowed us to quantify the equilibrium constant of the protein-aptamer binding and confirm its high affinity by using the Langmuir equation.     

Synthesis of close-packed multi-walled carbon nanotube bundles using Mo as catalyst

A simple mixture of porous magnesium oxide and commercial molybdenum oxide shows high efficiency for the synthesis of carbon nanotubes through the catalytic decomposition of methane at 900 °C. Field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and transmission electron microscopy (TEM) were used to characterize the products. The results indicate that close-packed multi-walled carbon nanotube (MWCNT) bundles were synthesized and the carbon nanotubes restricted within the bundles were about 5-7 nm in diameter. A growth mechanism for the bundles was suggested based on the FE-SEM images of bundles produced using different reaction times, and the X-ray diffractions of the raw products grown at the initial stage. Raman spectroscopy and FE-SEM results on the bundles grown using different methane flow rates confirmed the growth mechanism of close-packed MWCNT bundles.     

High density carbon nanotube growth using a plasma pretreated catalyst

We have developed a direct current plasma pretreatment of the Fe catalyst to increase the area density of vertically-aligned carbon nanotube forests. The carbon wall density of the double- and multi-walled nanotubes reaches 4.8 × 10¹² cm⁻², with a 40% volume occupancy and a mass density of ∼0.4 g cm⁻³. The plasma pretreatment works by reducing the sintering of the catalyst nanoparticles during growth. This treatment increases the forest density by 8 times compared to the standard growth conditions.     

Coal as a carbon source for carbon nanotube synthesis

This article reviews the recent advances on the various processes used in the synthesis of carbon nanotubes (CNTs) from different types of coal (anthracite, bituminous, etc.) and on the role played by coal as carbon source in the production of CNTs. The molecular solid coal is inexpensive and widely available in comparison to the most widely used solid carbon precursor, graphite (a lattice solid) and high purity hydrocarbon gas sources. An account is given on the different processes involved in the synthesis of various CNTs (single and multi-walled, bamboo-shaped, branched, etc.) from different types of coal (anthracite, bituminous, etc.). Both arc-discharge and thermal plasma jet produce high quality CNTs but fundamental disadvantages limit their use as large-scale synthesis routes. Chemical vapour deposition appears to be promising but further experimental work is necessary in order to develop an understanding of the complex factors governing the formation of different carbon nanomaterials from coal. Successful utilization of CNTs in various applications is strongly dependent on the development of simple, efficient and inexpensive technology for mass production and coal as a carbon source has the potential to meet the needs.     

CVD synthesis of carbon nanotubes using a finely dispersed cobalt catalyst and their use in double layer electrochemical capacitors

Carbon nanotubes (CNT) were obtained by chemical vapour deposition (CVD), decomposing turpentine oil over finely dispersed Co metal as a catalyst at 675 °C. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images reveal that the nanotubes are densely packed and of 10-50 nm in diameter. The XRD pattern of purified CNT shows that they are graphitic in nature. Resistivity measurements of these CNT indicate that they are highly conducting. Hall measurements of CNT reveal that electrons are the majority carriers with a carrier concentration of 1.35×10²⁰ cm⁻³. Cyclic voltammetry (CV) and constant current charging/discharging was used to characterise the behaviour of electrochemical double layer capacitors of purified CNT with H₂SO₄. For CNT/2 M H₂SO₄/CNT, a capacitance of 12 F g⁻¹ (based on the weight of the active material) was obtained.     

Fabrication and characterization of carbon nanotube arrays using sandwich catalyst stacks

A novel synthesis of carbon nanotubes for field-emitter arrays with a uniform field emission current is reported. Microwave plasma chemical vapor deposition and a unique structure of a sandwich catalyst stack are used to grow vertically aligned carbon nanotubes with a high density, uniform length and diameter. After being etched in a H₂/N₂-microwave plasma, the overall field emission current density from the prepared emitter arrays is 1.2 A/cm² at an electric field of 6.5 V/μm with stable and uniform emission characteristics. The threshold field is 3.2 V/cm, defined at an emission current density of 10⁻⁶ A/cm².     

Noise spectroscopy of transport properties in carbon nanotube field-effect transistors

Transport properties of single-walled carbon nanotube (CNT) structures with Pd contacts were studied using noise spectroscopy. The high values of the mobility and low noise level are characteristic of high-quality CNT material. The detailed analysis of the transport and noise properties of the CNT structure with back gate topography allows us to study the transport determined by Schottky barriers and by pure CNT channel conductivity and to establish their separate contribution to the total conductivity of the structure. It was demonstrated that at small gate overdrive the main source of flicker noise is related to the Schottky barriers of the CNT–FETs. With increasing gate voltage, the magnitude of flicker noise decreases and at a certain gate voltage it is only determined by the transport properties of carbon nanotubes with a noise level lower by one order of magnitude. In contrast to previous studies where flicker noise determined the excess noise of CNT-based structures, we registered generation–recombination noise components in our structures and studied their behavior in a wide temperature range. This allowed us to investigate the origin of traps capturing the carriers, which considerably affects the noise and transport properties of CNT structures.     

Single-walled carbon nanotube growth using cobalt nanoparticles prepared by vacuum deposition on a surface-active liquid

We demonstrate that the Co nanoparticles prepared by vacuum deposition on a surface-active liquid can be used as catalysts of single-walled carbon nanotube (SWCNT) growth using alcohol catalytic chemical vapor deposition method. These Co nanoparticles are embedded in the y-type zeolite powder. The Co nanoparticles used in this study is the highly efficient catalysts for CNT growth, which is comparable to the commonly-used Co-Fe binary metal catalysts embedded in the y-type zeolite. The preparation method of the metal nanoparticles used in this study is very simple. Thus, such metal nanoparticles might be promising as catalysts of CNT growth.     

Growth of single-crystalline cobalt silicide nanowires and their field emission property

In this work, cobalt silicide nanowires were synthesized by chemical vapor deposition processes on Si (100) substrates with anhydrous cobalt chloride (CoCl₂) as precursors. Processing parameters, including the temperature of Si (100) substrates, the gas flow rate, and the pressure of reactions were varied and studied; additionally, the physical properties of the cobalt silicide nanowires were measured. It was found that single-crystal CoSi nanowires were grown at 850°C ~ 880°C and at a lower gas flow rate, while single-crystal Co₂Si nanowires were grown at 880°C ~ 900°C. The crystal structure and growth direction were identified, and the growth mechanism was proposed as well. This study with field emission measurements demonstrates that CoSi nanowires are attractive choices for future applications in field emitters.     

Liquid-phase alkali-doping of individual carbon nanotube field-effect transistors observed in real-time

The carbon nanotube (CNT) is known to be very sensitive to changes in its surrounding environment. Our study is on the effects of mild, liquid-phase alkali-doping on electronic transport in individual CNTs. We find clear and consistent reversal from p- to n-type behavior, with all seven investigated CNT field-effect transistors (FETs) retaining a similar ON/OFF ratio and subthreshold slope. We have also measured the realtime electronic response during liquid-phase doping, and demonstrate detection of alkali cations with a signal response that ranges over more than three orders of magnitude. The doping is fully reversible upon exposure to oxygen, and the doping cycle is repeatable. We also confirm an earlier finding that the redox chemistry involved can facilitate thermodynamically stable solutions of CNTs, and its potential for multi-purpose use in nanotechnology processing may be appealing for large-scale fabrication.     

State of the catalyst during carbon nanotube growth

We review our in-situ X-ray photoemission (XPS) and in-situ transmission electron microscopy studies which determined that the catalyst is in the metallic state for Fe, Co and Ni catalysts. We show that the existence of surface carbide phases in related catalytic reactions could account for the observation of carbide peaks in XPS. The observed catalytic activity of gold is discussed in terms of carbon solubility, reaction rates, and surface coordination numbers.     

The use of a carbon nanotube layer on a polyurethane multifilament substrate for monitoring strains as large as 400%

A noninvasive approach is used to fabricate electronically conductive and flexible polymer fibers by fixing carbon nanotube (CNT) networks as a thin layer on thermoplastic polyurethane (TPU) multifilaments. The anchoring of the CNT layer is achieved by partially embedding or penetrating CNTs from the dispersion into the swollen multifilament surface. Thus a stable and high conductivity (up to 10² S/m at 10 wt.% CNT loading) of the resulting CNTs–TPU fibers is realized while the mechanical properties of the TPU multifilament, especially the strain to failure of >1500%, are not affected by increasing the thickness of the CNT layer. Real time analysis of the resistance of the CNTs–TPU fibers during incremental tensile loading tests reveal that the increase of resistance as a function of the strain is attributed to stretching-induced deformation, alignment, and, at high strains, destruction of the conducting network. Moreover, the changes in resistance are highly reversible under cyclic stretching up to a strain deformation of 400%.     

Charge-induced conductance modulation of carbon nanotube field effect transistor memory devices

We effectively tailored the charge trapping and transport behavior of a carbon nanotube field effect transistor memory device using charge interaction with underlying Ge nanoparticles in a HfO₂ high-κ dielectric. We also suggest a new route for modulating the Schottky barrier at the nanotube-electrode interface with localized charge trapping in discrete nanoparticles. This modification leads to an effective increase in the read-out conductance ratio of two to three orders magnitude under low voltage operation, associated with a large memory window of ∼5.3 V. Furthermore, we achieved a more controllable and reliable memory effect due to stable charge storage in deep nanoparticle traps, as compared to shallow HfO₂ defect states.     

High-field electrical transport and breakdown behavior of double-walled carbon nanotube field-effect transistors

Double-walled carbon nanotube (DWCNT) field-effect transistors have been fabricated, and their high-field transport and breakdown behavior investigated, both at room temperature and temperatures down to 4.2 K. In some cases controlled shell-by-shell breakdown of the DWCNT is realized, and field-effect measurements before and after breakdown reveal the nature of the two shells of the DWCNT and their relationship to the field-effect characteristics of the device. The breakdown of the DWCNT is found typically to occur within a few ms, opening up a gap of typically a few tens of nanometers.     

A single poly-Si gate-all-around junctionless fin field-effect transistor for use in one-time programming nonvolatile memory

This work demonstrates a feasible single poly-Si gate-all-around (GAA) junctionless fin field-effect transistor (JL-FinFET) for use in one-time programming (OTP) nonvolatile memory (NVM) applications. The advantages of this device include the simplicity of its use and the ease with which it can be embedded in Si wafer, glass, and flexible substrates. This device exhibits excellent retention, with a memory window maintained 2 V after 10⁴ s. By extrapolation, 95% of the original charge can be stored for 10 years. In the future, this device will be applied to multi-layer Si ICs in fully functional systems on panels, active-matrix liquid-crystal displays, and three-dimensional (3D) stacked flash memory.     

Narrow-chirality distributed single-walled carbon nanotube synthesis by remote plasma enhanced ethanol deposition on cobalt incorporated MCM-41 catalyst

We demonstrated the synthesis of single-walled carbon nanotubes (SWCNTs) with narrow chiral distribution on cobalt incorporated MCM-41 catalyst using ethanol as carbon feedstock by remote plasma enhanced chemical vapor deposition. The use of remote plasma enables the decomposition of the nontoxic ethanol to occur away from the site of SWCNT growth. This allows separate manipulation of carbon radical generation and SWCNT growth. A series of syntheses were carried out at plasma power from 0 to 250 W and growth temperature from 625 to 875 °C. Results have revealed that the plasma power and growth temperature affect carbon radical generation and recombination, as well as the reduction and nucleation of cobalt species. In addition, the remote ethanol plasma etching does not damage the grown SWCNTs. The chirality of resulting SWCNTs shows minor changes under different plasma power. On the other hand, the diameter of SWCNTs can be adjusted (from 0.7 to 1 nm) by changing the growth temperature. At the optimum condition of 200 W plasma power and 775 °C growth temperature, (7,5) and (8,4) nanotubes account for more than 50% of all semiconducting nanotube species. These results demonstrate the potential of utilizing plasma process in chiral selective growth of SWCNTs.