Properties of short channel ballistic carbon nanotube transistors with ohmic contacts

Advances and Frontiers in Single-Walled Carbon Nanotube Electronics

Single-walled carbon nanotubes (SWCNTs) have been considered as one of the most promising electronic materials for the next-generation electronics in the more Moore era. Sub-10 nm SWCNT-field effect transistors (FETs) have been realized with several performances exceeding those of Si-based FETs at the same feature size. Several industrial initiatives have attempted to implement SWCNT electronics in integrated circuit (IC) chips. Here, the recent advances in SWCNT electronics are reviewed from in-depth understanding of the fundamental electronic structures, the carrier transport mechanisms, and the metal/SWCNT contact properties. In particular, the subthreshold switching properties are highlighted for low-power, energy-efficient device operations. State-of-the-art low-power SWCNT-based electronics and the key strategies to realize low-voltage and low-power operations are outlined. Finally, the essential challenges and prospects from the material preparation, device fabrication, and large-scale ICs integration for future SWCNT-based electronics are foregrounded.

Can Carbon Nanotube Transistors Be Scaled Down to the Sub-5 nm Gate Length?

Single-walled carbon nanotubes (CNTs) have been considered as a promising semiconductor to construct transistors and integrated circuits in the future owing to their ultrathin channel thickness and ultrahigh injection velocity. Although a 5 nm gate-length CNT field-effect transistor (FET) has already been experimentally fabricated and demonstrates excellent device performance, the potential or constraint factors on performance have not been explored or revealed. Based on the benchmark of the device performance between the experimental and simulated 5 nm gate-length CNT FETs, we use the first-principles quantum transport approach to explore the performance limit of CNT FETs based on the gate-all-around (GAA) device geometry for the first time. It is found that the GAA CNT FETs can fulfill the ITRS 2028 high-performance target in the 2 nm gate-length node in terms of the on-state current, delay time, and power consumption. We also find that the energy-delay product of the CNT FETs is superior to those of the high-performance 2D materials and Si Fin FETs at the sub-5 nm gate length due to its unique electrical property. Though theoretically the gate length of CNT FETs can be potentially scaled to 2 nm, considering the tradeoff between the performance and power consumption, 5 nm is the ultimate scaled limit.

One-Dimensional van der Waals Heterojunction Diode

The synthesis of one-dimensional van der Waals heterostructures was realized recently, which offers alternative possibilities for prospective applications in electronics and optoelectronics. The even reduced dimension will enable different properties and further miniaturization beyond the capabilities of their two-dimensional counterparts. The natural doping results in p-type electrical characteristics for semiconducting single-walled carbon nanotubes and n-type for molybdenum disulfide with conventional noble metal contacts. Therefore, we demonstrate here a one-dimensional heterostructure nanotube, 11 nm wide, with the coaxial assembly of a semiconducting single-walled carbon nanotube, insulating boron nitride nanotube, and semiconducting molybdenum disulfide nanotube, which induces a radial semiconductor-insulator-semiconductor heterojunction. When opposite potential polarity was applied on a semiconducting single-walled carbon nanotube and molybdenum disulfide nanotube, respectively, the rectifying effect was materialized.

Horizontal Single-Walled Carbon Nanotube Arrays: Controlled Synthesis, Characterizations, and Applications

Single-walled carbon nanotubes (SWNTs) emerge as a promising material to advance carbon nanoelectronics. However, synthesizing or assembling pure metallic/semiconducting SWNTs required for interconnects/integrated circuits, respectively, by a conventional chemical vapor deposition method or by an assembly technique remains challenging. Recent studies have shown significant scientific breakthroughs in controlled SWNT synthesis/assembly and applications in scaled field effect transistors, which are a critical component in functional nanodevices, thereby rendering the horizontal SWNT array an important candidate for innovating nanotechnology. This review provides a comprehensive analysis of the controlled synthesis, surface assembly, characterization techniques, and potential applications of horizontally aligned SWNT arrays. This review begins with the discussion of synthesis of horizontally aligned SWNTs with regulated direction, density, structure, and theoretical models applied to understand the growth results. Several traditional procedures applied for assembling SWNTs on target surface are also briefly discussed. It then discusses the techniques adopted to characterize SWNTs, ranging from electron/probe microscopy to various optical spectroscopy methods. Prototype applications based on the horizontally aligned SWNTs, such as interconnects, field effect transistors, integrated circuits, and even computers, are subsequently described. Finally, this review concludes with challenges and a brief outlook of the future development in this research field.

Prospects for Bioinspired Single-Photon Detection Using Nanotube-Chromophore Hybrids

The human eye is an exquisite photodetection system with the ability to detect single photons. The process of vision is initiated by single-photon absorption in the molecule retinal, triggering a cascade of complex chemical processes that eventually lead to the generation of an electrical impulse. Here, we analyze the single-photon detection prospects for an architecture inspired by the human eye: field-effect transistors employing carbon nanotubes functionalized with chromophores. We employ non-equilibrium quantum transport simulations of realistic devices to reveal device response upon absorption of a single photon. We establish the parameters that determine the strength of the response such as the magnitude and orientation of molecular dipole(s), as well as the arrangements of chromophores on carbon nanotubes. Moreover, we show that functionalization of a single nanotube with multiple chromophores allows for number resolution, whereby the number of photons in an incoming light packet can be determined. Finally, we assess the performance prospects by calculating the dark count rate, and we identify the most promising architectures and regimes of operation.

Room-Temperature Phototransistor with Negative Photoresponsivity of 108 A W-1 Using Fullerene-Sensitized Aligned Carbon Nanotubes

Detection of low intensity light down to a few photons requires photodetectors with high gain. A new photodetector is reported based on C₆₀ -sensitized aligned carbon nanotube (CNT) transistors with an extremely high responsivity of 10⁸ A W^-1 (gain > 10⁸ ) in the ultraviolet and visible range, and 720 A W^-1 (gain = 940) in the infrared range. In contrast to most sensitized phototransistors that operate on the photogating effect, the new photodetector operates on the modulation of the electrons scattering in the CNTs, leading to negative photoconductivity. Comparison with similar photodetectors using random CNT networks shows the benefit of using aligned CNTs. At room temperature, the aligned CNT photodetectors are demonstrated to detect a few tens of photons per CNT.

Dynamic Wavelength-Tunable Photodetector Using Subwavelength Graphene Field-Effect Transistors

Dynamic wavelength tunability has long been the holy grail of photodetector technology. Because of its atomic thickness and unique properties, graphene opens up new paradigms to realize this concept, but so far this has been elusive experimentally. Here we employ detailed quantum transport modeling of photocurrent in graphene field-effect transistors (including realistic electromagnetic fields) to show that wavelength tunability is possible by dynamically changing the gate voltage. We reveal the phenomena that govern the behavior of this type of device and show significant departure from the simple expectations based on vertical transitions. We find strong focusing of the electromagnetic fields at the contact edges over the same length scale as the band-bending. Both of these spatially-varying potentials lead to an enhancement of non-vertical optical transitions, which dominate even in the absence of phonon or impurity scattering. We also show that the vanishing density of states near the Dirac point leads to contact blocking and a gate-dependent modulation of the photocurrent. Several of the effects discussed here should be applicable to a broad range of one- and two-dimensional materials and devices.

Toward high-performance digital logic technology with carbon nanotubes

The slow-down in traditional silicon complementary metal-oxide-semiconductor (CMOS) scaling (Moore's law) has created an opportunity for a disruptive innovation to bring the semiconductor industry into a postsilicon era. Due to their ultrathin body and ballistic transport, carbon nanotubes (CNTs) have the intrinsic transport and scaling properties to usher in this new era. The remaining challenges are largely materials-related and include obtaining purity levels suitable for logic technology, placement of CNTs at very tight (∼5 nm) pitch to allow for density scaling and source/drain contact scaling. This review examines the potential performance advantages of a CNT-based computing technology, outlines the remaining challenges, and describes the recent progress on these fronts. Although overcoming these issues will be challenging and will require a large, sustained effort from both industry and academia, the recent progress in the field is a cause for optimism that these materials can have an impact on future technologies.

Single-walled carbon nanotubes for high-performance electronics

Single-walled carbon nanotubes (SWNT) could replace silicon in high-performance electronics with their exceptional electrical properties and intrinsic ultra-thin body. During the past five years, the major focus of this field is gradually shifting from proof-of-concept prototyping in academia to technology development in industry with emphasis on manufacturability and integration issues. This article reviews recent advances, starting with experimental and modeling works that evaluate the potential of adopting SWNTs in ultimately scaled transistors. Techniques to separate nanotubes according to their electronic types and assemble them into aligned arrays are then discussed, followed by a description of the engineering aspects in their implementation in integrated circuits and systems. A concluding discussion provides some perspectives on future challenges and research opportunities.

Carbon nanotube complementary wrap-gate transistors

Among the challenges hindering the integration of carbon nanotube (CNT) transistors in digital technology are the lack of a scalable self-aligned gate and complementary n- and p-type devices. We report CNT transistors with self-aligned gates scaled down to 20 nm in the ideal gate-all-around geometry. Uniformity of the gate wrapping the nanotube channels is confirmed, and the process is shown not to damage the CNTs. Further, both n- and p-type transistors were realized by using the appropriate gate dielectric-HfO2 yielded n-type and Al2O3 yielded p-type-with quantum simulations used to explore the impact of important device parameters on performance. These discoveries not only provide a promising platform for further research into gate-all-around CNT devices but also demonstrate that scalable digital switches with realistic technological potential can be achieved with carbon nanotubes.

Sub-10 nm carbon nanotube transistor

Although carbon nanotube (CNT) transistors have been promoted for years as a replacement for silicon technology, there is limited theoretical work and no experimental reports on how nanotubes will perform at sub-10 nm channel lengths. In this manuscript, we demonstrate the first sub-10 nm CNT transistor, which is shown to outperform the best competing silicon devices with more than four times the diameter-normalized current density (2.41 mA/μm) at a low operating voltage of 0.5 V. The nanotube transistor exhibits an impressively small inverse subthreshold slope of 94 mV/decade-nearly half of the value expected from a previous theoretical study. Numerical simulations show the critical role of the metal-CNT contacts in determining the performance of sub-10 nm channel length transistors, signifying the need for more accurate theoretical modeling of transport between the metal and nanotube. The superior low-voltage performance of the sub-10 nm CNT transistor proves the viability of nanotubes for consideration in future aggressively scaled transistor technologies.

Length scaling of carbon nanotube transistors

Carbon nanotube field-effect transistors are strong candidates in replacing or supplementing silicon technology. Although theoretical studies have projected that nanotube transistors will perform well at nanoscale device dimensions, most experimental studies have been carried out on devices that are about ten times larger than current silicon transistors. Here, we show that nanotube transistors maintain their performance as their channel length is scaled from 3 µm to 15 nm, with an absence of so-called short-channel effects. The 15-nm device has the shortest channel length and highest room-temperature conductance (0.7G₀) and transconductance (40 µS) of any nanotube transistor reported to date. We also show the first experimental evidence that nanotube device performance depends significantly on contact length, in contrast to some previous reports. Data for both channel and contact length scaling were gathered by constructing multiple devices on a single carbon nanotube. Finally, we demonstrate the performance of a nanotube transistor with channel and contact lengths of 20 nm, an on-current of 10 µA, an on/off current ratio of 1 x 10⁵, and peak transconductance of 20 µS. These results provide an experimental forecast for carbon nanotube device performance at dimensions suitable for future transistor technology nodes.

Anomalous n-type electrical behaviour of Pd-contacted CNTFET fabricated on small-diameter nanotube

A Pd-contacted dopant-free CNTFET with small-diameter (0.57 nm) carbon nanotube showing an anomalous n-type electrical characteristic is reported for the first time. This observed behaviour is attributed to a carbon nanotube work function higher than (or close to) palladium as well as a large hole-to-electron effective mass ratio of approximately 2.5 predicted by hybridization in small-diameter nanotubes. A variation of the conduction type with temperature is also observed and is attributed to an increase of the palladium work function and decrease of the CNT work function with increasing temperature.

Terahertz response of carbon nanotube transistors

We present an approach for time-dependent quantum transport based on a self-consistent nonequilibrium Green function formalism. The technique is applied to a ballistic carbon nanotube transistor in the presence of a time-harmonic signal at the gate. In the on state, the dynamic conductance exhibits plasmonic resonant peaks at terahertz frequencies. These vanish in the off state, and the dynamic conductance displays smooth oscillations, a signature of single-particle quantum effects. We show that the nanotube kinetic inductance plays an essential role in the high-frequency behavior.

Optically modulated conduction in chromophore-functionalized single-wall carbon nanotubes

We demonstrate an optically active nanotube-hybrid material by functionalizing single-wall nanotubes with an azo-based chromophore. Upon UV illumination, the conjugated chromophore undergoes a cis-trans isomerization leading to a charge redistribution near the nanotube. This charge redistribution changes the local electrostatic environment, shifting the threshold voltage and increasing the conductivity of the nanotube transistor. For a approximately 1%-2% coverage, we measure a shift in the threshold voltage of up to 1.2 V. Further, the conductance change is reversible and repeatable over long periods of time, indicating that the chromophore-functionalized nanotubes are useful for integrated nanophotodetectors.