Session W37: Focus Session: Carbons Nanotubes: Towards More Complex Circuits

Carbon Nanotube Computer: Transforming Scientific Discoveries into Working Systems

The miniaturization of electronic devices has been the principal driving force behind the semiconductor industry, and has brought about major improvements in computational power and energy efficiency. Although advances with silicon-based electronics continue to be made, alternative technologies are being explored. Digital circuits based on transistors fabricated from carbon nanotubes (CNTs) have the potential to outperform silicon by improving the energy-- delay product, a metric of energy efficiency, by more than an order of magnitude. Hence, CNTs are an exciting complement to existing semiconductor technologies. However, carbon nanotubes (CNTs) are subject to substantial inherent imperfections that pose major obstacles to the design of robust and very large-scale CNFET digital systems: \begin{itemize} \item It is nearly impossible to guarantee perfect alignment and positioning of all CNTs. This limitation introduces stray conducting paths, resulting in incorrect circuit functionality. \item CNTs can be metallic or semiconducting depending on chirality. Metallic CNTs cause shorts resulting in excessive leakage and incorrect circuit functionality. \end{itemize} A combination of design and processing technique overcomes these challenges by creating robust CNFET digital circuits that are immune to these inherent imperfections. This imperfection-immune design paradigm enables the first experimental demonstration of the carbon nanotube computer, and, more generally, arbitrary digital systems that can be built using CNFETs. The CNT computer is capable of performing multitasking: as a demonstration, we perform counting and integer-sorting simultaneously. In addition, we emulate 20 different instructions from the commercial MIPS instruction set to demonstrate the generality of our CNT computer. This is the most complex carbon-based electronic system yet demonstrated. It is a considerable advance because CNTs are prominent among a variety of emerging technologies that are being considered for the next generation of highly energy-efficient electronic systems.   

Carbon Nanotube Network Anti-fuses

Copper interconnects are known to fail from electromigration at $\sim$10$^{6}$~A/cm$^{2}$. However, to continue aggressive scaling in integrated circuits (ICs), new materials that can carry much higher current densities will be required. For this reason, Carbon nanotubes (CNTs), which can carry up to 10$^{9}$ A/cm$^{2}$, are a promising replacement. We discover that after Joule breakdown, CNTs can be healed by applying a healing voltage $V_{H}$. Such a technology could be useful to repair failed interconnects, creating anti-fuses for field programmable gate array (FPGA) or memory technology. We fabricate a CNT network using a dip-coating method and then we sweep a voltage across the device until the CNT network undergoes Joule breakdown, creating a physical gap (of $\sim$ 10-40~nm) within the network. Making hysteretic $I-V$ sweeps, we observe a sudden increase in current at a voltage $\sim$50 -- 80~{\%} of the breakdown voltage $V_{BD}$. We can reliably break and heal the device multiple times. We also observe the Raman coalescence induced mode (CIM), which is characteristic of \textit{sp} hybridized carbon chains, after the breaking and healing process. According to our analysis, we conclude that the formation of carbon chains is key to promote the electrical restoration of the broken CNT network.   

Fabrication and electrical properties of single wall carbon nanotube channel and graphene electrode based transistors; Toward all carbon electronics

A transistor structure composed of an individual single-walled carbon nanotube (SWNT) channel with a graphene electrode was demonstrated. The integrated arrays of transistor devices were prepared by transferring patterned graphene electrode array on top of the pre-deposited SWNTs which were aligned along one direction. Aligned arrays of SWNTs were synthesized by thermal chemical vapor deposition (CVD) method on quartz substrate. The micro scale contact electrodes and following circuit structures were defined by photo lithography on the large area graphene produced by CVD. Both of the single and multi layer graphene were used for the electrode materials. In this presentation, the device fabrication procedure, the contact properties, and the transistor performances of the device structures were discussed.   

Capacitance of nanostructures

Modeling capacitance of nanostructures in nanoscale circuits presents particular challenge because of contribution from electrodes, which can usually be neglected in modeling conductance, or even capacitance of macrodevices. We present a model of capacitance of a nano-gap configuration and applied to calculate capacitance of a carbon nanotube nano-gap and effective capacitance of a buckyball inside the nano-gap. The capacitance of the carbon nanotube nano-gap increases with length of electrodes, which shows the important contribution from the electrodes in dynamic transport properties of nanoscale circuits.   

Self-aligned T-gate Nanotube Radio Frequency Transistors and Circuits Application

We applied self-aligned T-gate design to aligned carbon nanotube array transistors and achieved an extrinsic current-gain cut-off frequency (ft) of 25 GHz, which is the best on-chip performance for nanotube RF transistors reported to date. Meanwhile, an intrinsic current-gain cut-off frequency up to 102 GHz is obtained, comparable to the best value reported for nanotube RF transistors. Armed with the excellent extrinsic RF performance, we performed both single-tone and two-tone measurements for aligned nanotube transistors at a frequency up to 8 GHz. Furthermore, we utilized T-gate aligned nanotube transistors to construct mixing and frequency doubling analog circuits operated in gigahertz frequency regime. Our results confirm the great potential of nanotube-based circuit application and indicate that nanotube transistors are promising building blocks in high-frequency electronics..   

Towards parallel, CMOS-compatible fabrication of carbon nanotube single electron transistors

We demonstrate an approach for the parallel fabrication of single electron transistor (SET) using single-walled carbon nanotube (SWNT). The approach is based on the integration of individual SWNT via dielectrophoresis (DEP) and deposition of metal top contact. We fabricate SWNT devices with a channel length of 100 nm and study their electron transport properties. We observe a connection between the SET performance and room temperature resistance ($R_{T})$ of the devices. Majority (90{\%}) of the devices with 100 K$\Omega $ \textless $R_{T}$ \textless 1 M$\Omega $, show periodic, well defined Coulomb diamonds with a charging energy around 15 meV, corresponding to transport through a single quantum dot (QD), defined by the top contact. For high $R_{T}$ (\textgreater 1M$\Omega )$, devices show multiple QD behaviors, while QD was not formed for low $R_{T}$ (\textless 100 K$\Omega )$ devices. This easy, simple and CMOS-compatible fabrication process will provide a much desired insight towards the wide spread application and commercialization of SWNT SET devices.   

Improving the conductivity of carbon nanotube wires through resonant momentum exchange

Carbon nanotubes (CNTs) have remarkable properties that make them excellent candidates for nano-electronic devices. Retaining these properties in CNT networks scalable for manufacture is a significant challenge. Experiment shows that conductivities of CNT networks are at least an order of magnitude lower than the theoretical maximum based on single CNT performance. In a CNT network, typically no single tube spans the device. As a result, electrons must travel between CNTs in order to contribute to the conductivity. Optimizing the conductivity of CNT networks, therefore, requires a detailed understanding of inter-tube electron transport. To this end, we present theoretical investigations of inter-tube conductivity of CNTs. We find, in agreement with previous studies, that conductivity between CNTs of different chirality is strongly suppressed as a consequence of the requirement for momentum conservation. We show that this problem can be overcome by providing a weak perturbation to the system, resulting in increases in inter-tube conductivity by over one order of magnitude. We will discuss practical realizations of the required perturbation and its experimental relevance for enhancing the conductivity of CNT networks.   

High Frequency Generation from Horizontally Aligned Carbon Nanotube Field-effect Transistors

Horizontally aligned carbon nanotubes grown on quartz substrates are used to fabricate top-gated field-effect transistors. Second, third and even higher order harmonic products are observed when high frequency signals are applied on gate side and detected from drain side. Measurements on control devices with identical geometry but without carbon nanotubes indicate all the harmonic generations are due to the carbon nanotubes in the channel. The second harmonic generation can be explained by following a traditional transistor model. The third and even higher harmonics observed are attributed to the Schottky barrier between the metal contacts and the carbon nanotubes. Devices with different metals as source and drain are fabricated to evaluate the effects of Schottky barrier. Taking the harmonic generation as the combination of a field-effect transistor and a Schottky diode, high frequency measurements and corresponding DC characterization data are combined to quantify the contribution of the non-linear elements on the measured output signal.   

Comparative study of gel-based separated arcdischarge, HiPCO, and CoMoCAT carbon nanotubes for macroelectronic applications

Due to their excellent electrical properties and compatibility with room-temperature deposition/printing processing, single-walled semiconducting carbon nanotubes (SWNTs) hold great potential for macroelectronic applications. However, the relative advantages and disadvantages of various SWNTs for macroelectronics remains an open issue, despite the great significance. Here we report a systematic study of three kinds of mainstream SWNTs (arc-discharge, HiPCO, CoMoCAT) separated using gel-based column chromatography for thin-film transistor applications, and high performance transistors---which satisfy the requirements for transistors used in active matrix organic light-emitting diode displays---have been achieved. We observe a trade-off between transistor mobility and on/off ratio depending on the SWNT diameter. While arc-discharge SWNTs with larger diameters lead to high device mobility, HiPCO and CoMoCAT SWNTs with smaller diameters can provide high on/off ratios (\textgreater 10$^{\mathrm{6}})$. Furthermore, we compare gel-based separated SWNTs with SWNTs separated by the density gradient ultracentrifuge (DGU) method, and find that gel-separated SWNTs can offer purity and thin-film transistor performance as good as DGU-separated SWNTs.   

In-plane Thermal and Electrical Transport Through Single-walled Carbon Nanotube Thin Films

Recent advances in both chemical processing and fabrication techniques have enabled the development of a variety of new nanostructured materials for energy conversion technologies. Single-walled carbon nanotube (SWNT) networks may enable a number of cost-effective energy technologies, including transparent conductors for photovoltaics and thermoelectric composites. For such applications, a fundamental understanding of the physics governing their thermal and electrical properties is needed. Transport in SWNT networks is highly anisotropic; therefore the ability to measure the in-plane transport, both thermal and electrical, for these systems is extremely important. In this talk, we discuss the dispersion of highly enriched semiconducting SWNTs in organic solvents and deposition techniques optimized to enable measurements of in-plane transport of uniform thin films. We present results from in-plane thermal and electrical measurements as well as optical properties of SWNT:polymer thin films. Finally, we discuss the application of these results to developing nanocomposite films optimized for thermoelectric applications.