Showing papers by "Ali Javey published in 2004"

Carbon Nanotube Field-Effect Transistors with Integrated Ohmic Contacts and High-κ Gate Dielectrics

Abstract: High-performance enhancement-mode semiconducting carbon nanotube field-effect transistors (CNTFETs) are obtained by combining ohmic metal-tube contacts, high-dielectric-constant HfO2 films as gate insulators, and electrostatically doped nanotube segments as source/drain electrodes. The combination of these elements affords high ON currents and subthreshold swings of 70-80 mV/decade and allows for low OFF currents and suppressed ambipolar conduction. The doped source and drain approach resembles that of MOSFETs and can impart excellent OFF states to nanotube FETs under aggressive vertical scaling. This presents an important advantage over devices with a metal source/drain, or devices commonly referred to as Schottky barrier FETs.

High-field quasiballistic transport in short carbon nanotubes.

Abstract: Single walled carbon nanotubes with Pd Ohmic contacts and lengths ranging from several microns down to 10 nm are investigated by electron transport experiments and theory. The mean-free path (MFP) for acoustic phonon scattering is estimated to be ${l}_{\mathrm{ap}}\ensuremath{\sim}300\text{ }\text{ }\mathrm{nm}$, and that for optical phonon scattering is ${l}_{\mathrm{op}}\ensuremath{\sim}15\text{ }\text{ }\mathrm{nm}$. Transport through very short ($\ensuremath{\sim}10\text{ }\text{ }\mathrm{nm}$) nanotubes is free of significant acoustic and optical phonon scattering and thus ballistic and quasiballistic at the low- and high-bias voltage limits, respectively. High currents of up to $70\text{ }\ensuremath{\mu}\mathrm{A}$ can flow through a short nanotube. Possible mechanisms for the eventual electrical breakdown of short nanotubes at high fields are discussed. The results presented here have important implications to high performance nanotube transistors and interconnects.

Self-Aligned Ballistic Molecular Transistors and Electrically Parallel Nanotube Arrays

Abstract: Carbon nanotube field-effect transistors with structures and properties near the scaling limit with short (down to 50 nm) channels, self-aligned geometries, palladium electrodes with low contact resistance, and high-K dielectric gate insulators are realized. Electrical transport in these miniature transistors is nearly ballistic up to high biases at both room and low temperatures. Atomic-layer-deposited (ALD) high-K films interact with nanotube sidewalls via van der Waals interactions without causing weak localization at 4 K. New fundamental understanding of ballistic transport, optical phonon scattering, and potential interfacial scattering mechanisms in nanotubes is obtained. Also, parallel arrays of such molecular transistors are enabled to deliver macroscopic currents-an important milestone for future circuit applications.

Preferential Growth of Semiconducting Single-Walled Carbon Nanotubes by a Plasma Enhanced CVD Method

Abstract: Single-walled carbon nanotubes (SWNT) are grown by a plasma enhanced chemical vapor deposition (PECVD) method at 600 °C. The nanotubes are of high quality as characterized by microscopy, Raman spectroscopy, and electrical transport measurements. High performance field effect transistors are obtained with the PECVD nanotubes. Interestingly, electrical characterization reveals that nearly 90% of the nanotubes are semiconductors and thus highly preferential growth of semiconducting over metallic tubes in the PECVD process. Control experiments with other nanotube materials find that HiPco nanotubes consist of ∼61% semiconductors, while laser ablation preferentially grows metallic SWNTs (∼70%). The characterization method used here should also be applicable to assessing the degree of chemical separation of metallic and semiconducting nanotubes.

Self-Aligned Ballistic Molecular Transistors and Electrically Parallel Nanotube Arrays

Abstract: Carbon nanotube field-effect transistors with structures and properties near the scaling limit with short (down to 50 nm) channels, self aligned geometries, palladium electrodes with low contact resistance and high-k dielectric gate insulators are realized. Electrical transport in these miniature transistors is near ballistic up to high biases at both room and low temperatures. Atomic layer deposited (ALD) high-k films interact with nanotube sidewalls via van der Waals interactions without causing weak localization at 4 K. New fundamental understanding of ballistic transport, optical phonon scattering and potential interfacial scattering mechanisms in nanotubes are obtained.

Miniature Organic Transistors with Carbon Nanotubes as Quasi-One-Dimensional Electrodes

Abstract: As the dimensions of electronic devices approach those of molecules, the size, geometry, and chemical composition of the contact electrodes play increasingly dominant roles in device functions. It is shown here that single-walled carbon nanotubes (SWNT) can be used as quasi-one-dimensional (1D) electrodes to construct organic field effect transistors (FET) with molecular scale width (∼2 nm) and channel length (1−3 nm). An important feature owing to the quasi-1D electrode geometry is the favorable gate electrostatics that allows for efficient switching of ultra-short organic channels. This affords room temperature conductance modulation by orders of magnitude for organic transistors that are only several molecules in length, with switching characteristics superior to similar devices with lithographically patterned metal electrodes. With nanotubes, covalent carbon−carbon bonds could be utilized to form contacts to molecular materials. The unique geometrical, physical, and chemical properties of carbon nanot...

Monolithic Integration of Carbon Nanotube Devices with Silicon MOS Technology

Abstract: An integrated circuit combining single-walled carbon nanotube (SWNT) devices with n-channel metal oxide semiconductor (NMOS) field effect transistors was fabricated. SWNTs were grown, using chemical vapor deposition, from catalyst islands located on an NMOS decoder circuit. Massive arrays of nanotube devices, each addressed individually using the NMOS circuit, were rapidly characterized. The successful monolithic integration of nanotube devices and MOS transistors creates many possibilities, including electronically addressable nanotube chemical sensor arrays.

Performance analysis and design optimization of near ballistic carbon nanotube field-effect transistors

Abstract: A near ballistic carbon nanotube field-effect transistor (CNTFET) that integrates an ultra-short channel, low-barrier metal contacts, and a thin high-K gate insulator is modeled and analyzed using self-consistent quantum simulations. Numerical simulations, which solve a quantum transport equation self-consistently with a 3D Poisson equation using the non-equilibrium Green's function (NEGF) formalism, are used to understand the transistor physics and to suggest design optimization. Important device issues of: (1) how close the transistor operates to its ballistic limit; (2) what are the roles of phonon scattering and higher subband conduction; (3) how to further optimize the CNTFET; and (4) how the CNTFET compares to a state-of-the-art Si MOSFET, are explored and discussed.

Ten- to 50-nm-long quasi-ballistic carbon nanotube devices obtained without complex lithography

Abstract: A simple method combining photolithography and shadow (or angle) evaporation is developed to fabricate single-walled carbon nanotube (SWCNT) devices with tube lengths of ≈10–50 nm between metal contacts. Large numbers of such short devices are obtained without the need of complex tools such as electron beam lithography. Metallic SWCNTs with lengths of ≈10 nm, near the mean free path of lop ≈ 15 nm for optical phonon scattering, exhibit nearly ballistic transport at high biases and can carry unprecedented 100-μA currents per tube. Semiconducting SWCNT fieldeffect transistors with ≈50-nm channel lengths are routinely produced to achieve quasi-ballistic operations for molecular transistors. The results demonstrate highly length-scaled and high-performance interconnects and transistors realized with SWCNTs.

Advancements in Complementary Carbon Nanotube Field-Effect Transistors

Abstract: High performance p- and n-type single-walled carbon nanotube (SWNT) field-effect transistors (FETs) are obtained by using high and low work function metals, Pd and Al as source/drain (S/D) electrodes respectively. Ohmic contacts made to chemically intrinsic SWNTs, with no or small Schottky barriers (SB), afford high ON-state currents up to 20 uA per tube. The lack of significant Fermi-level pinning at the nanotube-metal interfaces allows for fine-tuning of the barrier heights for p-and n-channel conductions by changing the contact metals. The air-stable p- and n-FETs thus obtained can be used for complementary nanoelectronics, as demonstrated with the fabrication of an inverter. Other important issues regarding nanotube FETs including hysteresis, OFF-state leak currents, choice of nanotube diameter, and threshold voltage control are discussed.

High Performance N-Type Carbon Nanotube Field Effect Transistors with Chemically Doped Contacts

Abstract: Short channel (~80 nm) n-type single-walled carbon nanotube (SWNT) field-effect transistors (FETs) with potassium (K) doped source and drain regions and high-k gate dielectrics (ALD HfO2) are obtained. For nanotubes with diameter ~ 1.6 nm and bandgap ~ 0.55 eV, we obtain n-MOSFET-like devices exhibiting high on-currents due to chemically suppressed Schottky barriers at the contacts, subthreshold swing of 70mV/decade, negligible ambipolar conduction and high on/off ratios up to 10^6 at a bias voltage of 0.5V. The results compare favorably with the state-of-the-art silicon n-MOSFETs and demonstrate the potential of SWNTs for future complementary electronics. The effects of doping level on the electrical characteristics of the nanotube devices are discussed.