Single-walled Carbon Nanotube Transistors Research Articles

Ultra-short suspended single-wall carbon nanotube transistors

We describe a method to fabricate clean suspended single-wall carbon nanotube (SWCNT) transistors hosting a single quantum dot ranging in length from a few 10 s of nm down to ≈3 nm. We first align narrow gold bow-tie junctions on top of individual SWCNTs and suspend the devices. We then use a feedback-controlled electromigration to break the gold junctions and expose nm-sized sections of SWCNTs. We measure electron transport in these devices at low temperature and show that they form clean and tunable single-electron transistors. These ultra-short suspended transistors offer the prospect of studying THz oscillators with strong electron-vibron coupling.

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.

Optimized photolithographic fabrication process for carbon nanotube devices

We have developed a photolithographic process for the fabrication of large arrays of single walled carbon nanotube transistors with high quality electronic properties that rival those of transistors fabricated by electron beam lithography. A buffer layer is used to prevent direct contact between the nanotube and the novolac-based photoresist, and a cleaning bake at 300C effectively removes residues that bind to the nanotube sidewall during processing. In situ electrical measurement of a nanotube transistor during a temperature ramp reveals sharp decreases in the ON-state resistance that we associate with the vaporization of components of the photoresist. Data from nearly 2000 measured nanotube transistors show an average ON-state resistance of 250 ± 100 kΩ. This new process represents significant progress towards the goal of high-yield production of large arrays of nanotube transistors for applications including chemical sensors and transducers, as well as integrated circuit components.

Dielectrophoretic assembly of semiconducting single-walled carbon nanotube transistor

A novel burning technique for making a semiconducting single-walled carbon nanotubes (SWNTs) transistor assembled by the dielectrophoretic force was suggested. The fabrication process consisted of two steps. First, to align and attach a bundle of SWNTs between the source and drain, the alternating (AC) voltage was applied to the electrodes. When a bundle of SWNTs was connected between two electrodes, some of metallic nanotubes and semi-conducing nanotubes existed together. The second step is to burn the metallic SWNTS by applying the voltage between two electrodes. With increasing the voltage, more current flowed through the metallic SWNTs, thus, the metallic SWNTs burnt earlier than the semiconducting one. This technique enables to obtain only semi-conducting SWNTs connection in the transistor. Through the I-V characteristic graph, the moment of metallic SWNTs burning and the characteristic of semi-conducing nanotubes were verified.

Gate-Induced Cross-Over between Fabry–Perot Interference and Coulomb Blockade in a Single-Walled Carbon Nanotube Transistor with Double-Gate Structure

The gate-induced cross-over between Fabry–Perot interference and Coulomb blockade characteristics in a single-walled carbon nanotube (SWNT) transistor are observed. The behaviors of Fabry–Perot interference and Coulomb blockade are controlled by the control-gate and observed by the sweep-gate. When a negatively high voltage is applied to the control-gate electrode, the SWNT transistor showed a resonant tunneling transistor (RTT) characteristic, e.g., a Fabry–Perot quantum interference pattern. However, when a negatively low voltage is applied to the control gate electrode, the SWNT transistor showed a single-hole transistor (SHT) characteristic, e.g., a Coulomb diamond characteristic. The transition between RTT and SHT is achieved by modulating the coupling strength between the source and drain electrodes and the quantum island using control-gate voltage change. Schottky barriers at the contact between the SWNT and the source and drain electrodes act as tunneling barriers. The thicknesses of the tunneling barriers are modulated by the control-gate voltage change, and the strength of the coupling between the source and drain electrode and the quantum island is also modulated.

Gate-Induced Cross-Over between Fabry–Perot Interference and Coulomb Blockade in a Single-Walled Carbon Nanotube Transistor with Double-Gate Structure

The gate-induced cross-over between Fabry–Perot interference and Coulomb blockade characteristics in a single-walled carbon nanotube (SWNT) transistor are observed. The behaviors of Fabry–Perot interference and Coulomb blockade are controlled by the control-gate and observed by the sweep-gate. When a negatively high voltage is applied to the control-gate electrode, the SWNT transistor showed a resonant tunneling transistor (RTT) characteristic, e.g., a Fabry–Perot quantum interference pattern. However, when a negatively low voltage is applied to the control gate electrode, the SWNT transistor showed a single-hole transistor (SHT) characteristic, e.g., a Coulomb diamond characteristic. The transition between RTT and SHT is achieved by modulating the coupling strength between the source and drain electrodes and the quantum island using control-gate voltage change. Schottky barriers at the contact between the SWNT and the source and drain electrodes act as tunneling barriers. The thicknesses of the tunneling barriers are modulated by the control-gate voltage change, and the strength of the coupling between the source and drain electrode and the quantum island is also modulated.

Influence of Electrolyte Composition on Liquid-Gated Carbon Nanotube and Graphene Transistors

Field-effect transistors based on single-walled carbon nanotubes (SWNTs) and graphene can function as highly sensitive nanoscale (bio)sensors in solution. Here, we compare experimentally how SWNT and graphene transistors respond to changes in the composition of the aqueous electrolyte in which they are immersed. We show that the conductance of SWNTs and graphene is strongly affected by changes in the ionic strength, the pH, and the type of ions present, in a manner that can be qualitatively different for graphene and SWNT devices. We show that this sensitivity to electrolyte composition results from a combination of different mechanisms including electrostatic gating, Schottky-barrier modifications, and changes in gate capacitance. Interestingly, we find strong evidence that the sensor response to changes in electrolyte composition is affected by a high density of ionizable groups on both the underlying substrate and the carbon surfaces. We present a model based on the (regulated) surface charge associated with these ionizable groups that explains the majority of our data. Our findings have significant implications for interpreting and optimizing sensing experiments with nanocarbon transistors. This is particularly true for complex biological samples such as cell extracts, growth media, or bodily fluids, for which the complete composition of the solution needs to be considered.

Flexible, transparent single-walled carbon nanotube transistors with grapheneelectrodes

This paper reports a mechanically flexible, transparent thin film transistor that usesgraphene as a conducting electrode and single-walled carbon nanotubes (SWNTs) as asemiconducting channel. These SWNTs and graphene films were printed on flexible plasticsubstrates using a printing method. The resulting devices exhibited a mobility of ∼ 2 cm2 V − 1 s − 1,On/Off ratio of ∼ 102, transmittanceof ∼ 81% and excellent mechanical bendability.

Record Endurance for Single-Walled Carbon Nanotube-Based Memory Cell.

We study memory devices consisting of single-walled carbon nanotube transistors with charge storage at the SiO2/nanotube interface. We show that this type of memory device is robust, withstanding over 105 operating cycles, with a current drive capability up to 10−6 A at 20 mV drain bias, thus competing with state-of-the-art Si-devices. We find that the device performance depends on temperature and pressure, while both endurance and data retention are improved in vacuum.

Hygroscopic Effects on AuCl3-Doped Carbon Nanotubes

Hygroscopic effects on AuCl3-doped single-walled carbon nanotubes (SWCNTs) were investigated. Various concentrations of AuCl3 solution were dropped on the random network SWCNT transistors, where the doping effect was confirmed by I−Vg characteristics and a blue shift in the Raman G band. The AuCl3-doped SWCNT transistor was highly sensitive to water vapor in the I−Vg curve, especially in the positive gate bias region, compared with the undoped SWCNT transistors. We also found the selective response of water from ambient gases, such as O2 and N2, in AuCl3-doped SWCNTs. This investigation could be beneficial for high-performance water sensor applications.

Transport properties of single-walled carbon nanotube transistors after gamma radiation treatment

Single-walled carbon nanotube field-effect transistors (CNT-FETs) were characterized before and after gamma radiation treatment using noise spectroscopy. The results obtained demonstrate that in long channel CNT-FETs with a length of 10 μm the contribution of contact regions can be neglected. Moreover, radiation treatment with doses of 1×106 and 2×106 rad allows a considerable decrease parallel to the nanotube parasitic conductivity and even the shift region with maximal conductivity to the voltage range of nearly zero gate voltage that improves the working point of the FETs. The Hooge parameters obtained before and after gamma radiation treatment with a dose of 1×106 rad are found to be about 5×10−3. The parameters are comparable with typical values for conventional semiconductors.

Gate induced crossover between Fabry–Pérot and quantum dot behavior in a single walled carbon nanotube hole transistor

Gate induced crossover between Fabry–Pérot and quantum dot behavior in a single walled carbon nanotube (SWCNT) hole transistor is observed. The SWCNT transistor that can operate as a resonant tunneling transistor (RTT) and also as a single-hole transistor (SHT) using SWCNT as channel is fabricated. When negatively high voltage is applied to the gate electrode, the SWCNT transistor shows RTT characteristic, e.g., Fabry–Pérot quantum interference pattern. While, when negatively low voltage is applied to the gate electrode, the SWCNT transistor shows SHT characteristic, e.g., Coulomb diamond characteristic. The transition between RTT and SHT is achieved by modulating the coupling strength between the electrode and the quantum island using gate voltage change. Schottky barriers at the contact between the SWCNT and the electrodes act as tunneling barriers. The thicknesses of the tunneling barriers are modulated by the gate voltage change, and the strength of the coupling between the electrode and quantum island is also changed. The characteristics of the convertible transistor can be observed up to 100K. Moreover, the channel length dependence is observed.

Terahertz response in single-walled carbon nanotube transistor: a real-time quantumdynamics simulation

We use time-dependent quantum wavepacket methods to simulate ballistic electron transportin a single-walled carbon nanotube field-effect transistor at terahertz frequencies (∼100 GHz–10 THz). We observe an electron resonance phenomenon in a sub-picosecond-scaletime domain. Our simulation results clearly show that the electron resonance correspondsto the formation of the resonance cavity and the interference of the electron wavepackets,which is directly supported by recent experimental measurements (Zhong et al 2008 Nat.Nanotechnol. 3 201).

Illumination-Enhanced Hysteresis of Transistors Based on Carbon Nanotube Networks

The hysteresis in single-walled carbon nanotube (SWNT) transistors comprising Si backgate (SiO2 on doped Si) is normally attributed to either carrier injections from SWNTs to their surroundings or the presence of charge traps at a Si−SiO2 interface. We show that the hysteresis in SWNT transistors with a nearly trap-free Si backgate is thermally activated (activation energy Ea ∼ 129−184 meV) in a dark ambient condition, and it is attributed to hole trappings at the SiO2 surfaces proximate to SWNTs. Photon-illumination on the SWNT transistor devices with thin SiO2 dielectrics (80 nm) results in the ON-current increase due to the effective gating from the photovoltage generated at the Si−SiO2 interface. The light-induced simultaneous enhancement of ON-current and hysteresis suggests that the illumination-enhanced hysteresis is due to the photovoltage-activated hole trapping process on SiO2 surfaces.

Measuring Frequency Response of a Single-Walled Carbon Nanotube Common-Source Amplifier

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Frequency response function (FRF) showing ac gain from a single-walled carbon nanotube transistor is presented. A top-gated carbon nanotube FET (CNFET) is configured as a common-source amplifier and the FRF of the amplifier is measured. Evidence of unambiguous signal amplification is observed in time domain as well as frequency domain up to a unity voltage gain frequency of approximately 560 kHz. The observed roll-off in frequency is solely due to the <emphasis emphasistype="italic">RC</emphasis> time constant of the measurement apparatus. A specifically designed circuit-compatible SPICE model for the CNFET is used to model both dc and ac characteristic with the same set of physical parameters. Good agreement between measurement and simulation is obtained. For a device without the parasitic load capacitance, we predict an intrinsic unity voltage gain frequency of 29 GHz and a cutoff frequency of <formula formulatype="inline"><tex Notation="TeX">$\sim {50}$</tex></formula> GHz. </para>

Single-walled carbon nanotube transistors fabricated by advanced alignment techniques utilizing CVD growth and dielectrophoresis

Single-walled carbon nanotube field effect transistors (SWNT-FETs) are fabricated by two different alignment techniques. The first technique is based on direct synthesis of an aligned SWNTs array on quartz wafer using chemical vapor deposition. The transistor with three SWNTs and atomic layer deposited (ALD) Al2O3 gate oxide shows a contact resistance of 280KΩ, a maximum on-current of −7μA, and a high Ion/Ioff ratio (>103). The second technique is based on room temperature self-assembly of SWNT bundles using dielectrophoresis. By applying AC electric fields, we have aligned nanotube bundles between drain and source contact patterns of a transistor at room temperature. Transistors based on twisted bundle of SWNTs show high contact resistance (MΩ range) and low current drive in the order of tens of nA.

Noise characteristics of single-walled carbon nanotube network transistors

The noise characteristics of randomly networked single-walled carbon nanotubes grown directlyby plasma enhanced chemical vapor deposition (PECVD) are studied with field effecttransistors (FETs). Due to the geometrical complexity of nanotube networks in the channelarea and the large number of tube–tube/tube–metal junctions, the inverse frequency,1/f, dependence of the noise shows a similar level to that of a single single-walled carbonnanotube transistor. Detailed analysis is performed with the parameters of number ofmobile carriers and mobility in the different environment. This shows that the change inthe number of mobile carriers resulting in the mobility change due to adsorption anddesorption of gas molecules (mostly oxygen molecules) to the tube surface is a key factor in the1/f noise level for carbon nanotube network transistors.

Terahertz time-domain measurement of ballistic electron resonance in a single-walled carbon nanotube

Understanding the physics of low-dimensional systems and the operation of next-generation electronics will depend on our ability to measure the electrical properties of nanomaterials at terahertz frequencies ( approximately 100 GHz to 10 THz). Single-walled carbon nanotubes are prototypical one-dimensional nanomaterials because of their unique band structure and long carrier mean free path. Although nanotube transistors have been studied at microwave frequencies (100 MHz to 50 GHz), no techniques currently exist to probe their terahertz response. Here, we describe the first terahertz electrical measurements of single-walled carbon nanotube transistors performed in the time domain. We observe a ballistic electron resonance that corresponds to the round-trip transit of an electron along the nanotube with a picosecond-scale period. The electron velocity is found to be constant and equal to the Fermi velocity, showing that the high-frequency electron response is dominated by single-particle excitations rather than collective plasmon modes. These results demonstrate a powerful new tool for directly probing picosecond electron motion in nanostructures.

Low-Frequency Noise in Ambipolar Carbon Nanotube Transistors

Low-frequency noise measurements on individual single-walled carbon nanotube transistors exhibiting ambipolar characteristics have been carried out. With a polymer electrolyte as gate medium, low-frequency noise can be monitored in both p- and n-channel operation of the same nanotube under the same chemical environment. 1/ f noise in the p-channel of polymer electrolyte gated nanotube transistor is similar to that of back gate operation. However, most devices exhibit significantly larger noise amplitude in the n-channel operation that has a distinct dependence on the threshold voltage. A nonuniform energy distribution of carrier trapping/scattering sites is considered to explain these observations.

Charge Noise in Liquid-Gated Single-Wall Carbon Nanotube Transistors

The noise properties of single-walled carbon nanotube transistors (SWNT-FETs) are essential for the performance of electronic circuits and sensors. Here, we investigate the mechanism responsible for the low-frequency noise in liquid-gated SWNT-FETs and its scaling with the length of the nanotube channel down to the nanometer scale. We show that the gate dependence of the noise amplitude provides strong evidence for a recently proposed charge-noise model. We find that the power of the charge noise scales as the inverse of the channel length of the SWNT-FET. Our measurements also show that surprisingly the ionic strength of the surrounding electrolyte has a minimal effect on the noise magnitude in SWNT-FETs.