G. Timp

The electronic structure at the atomic scale of ultrathin gate oxides

The narrowest feature on present-day integrated circuits is the gate oxide—the thin dielectric layer that forms the basis of field-effect device structures. Silicon dioxide is the dielectric of choice and, if present miniaturization trends continue, the projected oxide thickness by 2012 will be less than one nanometre, or about five silicon atoms across. At least two of those five atoms will be at the silicon–oxide interfaces, and so will have very different electrical and optical properties from the desired bulk oxide, while constituting a significant fraction of the dielectric layer. Here we use electron-energy-loss spectroscopy in a scanning transmission electron microscope to measure the chemical composition and electronic structure, at the atomic scale, across gate oxides as thin as one nanometre. We are able to resolve the interfacial states that result from the spillover of the silicon conduction-band wavefunctions into the oxide. The spatial extent of these states places a fundamental limit of 0.7 nm (four silicon atoms across) on the thinnest usable silicon dioxide gate dielectric. And for present-day oxide growth techniques, interface roughness will raise this limit to 1.2 nm.

Ultra-thin gate dielectrics: they break down, but do they fail?

We study breakdown in high-quality 2-7 nm gate dielectrics, and find that soft breakdown becomes more likely for thinner oxides and for oxides stressed at lower voltages. For 2 nm oxides, an increase in gate noise is the only precise indication of soft breakdown. For many applications, devices should remain functional with the level of gate noise we have observed, after soft breakdown.

Simulation of the electric response of DNA translocation through a semiconductor nanopore-capacitor

A multi-scale/multi-material computational model for simulation of the electric signal detected on the electrodes of a metal–oxide–semiconductor (MOS) capacitor forming a nanoscale artificial membrane, and containing a nanopore with translocating DNA, is presented. The multi-scale approach is based on the incorporation of a molecular dynamics description of a translocating DNA molecule in the nanopore within a three-dimensional Poisson equation self-consistent scheme involving electrolytic and semiconductor charges for the electrostatic potential calculation. The voltage signal obtained from the simulation supports the possibility for single nucleotide resolution with a nanopore device. The electric signal predicted on the capacitor electrodes complements ongoing experiments exploring the use of nanopores in a MOS capacitor membrane for DNA sequencing.

Nanopore Sequencing: Electrical Measurements of the Code of Life

Sequencing a single molecule of deoxyribonucleic acid (DNA) using a nanopore is a revolutionary concept because it combines the potential for long read lengths (>5 kbp) with high speed (1 bp/10 ns), while obviating the need for costly amplification procedures due to the exquisite single molecule sensitivity. The prospects for implementing this concept seem bright. The cost savings from the removal of required reagents, coupled with the speed of nanopore sequencing places the

Low leakage, ultra-thin gate oxides for extremely high performance sub-100 nm nMOSFETs

1000 genome within grasp. However, challenges remain: high fidelity reads demand stringent control over both the molecular configuration in the pore and the translocation kinetics. The molecular configuration determines how the ions passing through the pore come into contact with the nucleotides, while the translocation kinetics affect the time interval in which the same nucleotides are held in the constriction as the data is acquired. Proteins like ¿-hemolysin and its mutants offer exquisitely precise self-assembled nanopores and have demonstrated the facility for discriminating individual nucleotides, but it is currently difficult to design protein structure ab initio, which frustrates tailoring a pore for sequencing genomic DNA. Nanopores in solid-state membranes have been proposed as an alternative because of the flexibility in fabrication and ease of integration into a sequencing platform. Preliminary results have shown that with careful control of the dimensions of the pore and the shape of the electric field, control of DNA translocation through the pore is possible. Furthermore, discrimination between different base pairs of DNA may be feasible. Thus, a nanopore promises inexpensive, reliable, high-throughput sequencing, which could thrust genomic science into personal medicine.

Microscopic Mechanics of Hairpin DNA Translocation through Synthetic Nanopores

Reports measurements of the DC characteristics of sub-100 nm nMOSFETs that employ low leakage ultra-thin gate oxides only 1-2 nm thick to achieve high current drive capability and transconductance. We demonstrate that I/sub Dsat//spl ap/1.8 mA//spl mu/m can be achieved with a 60 nm gate at 1.5 V using a 1.3-1.4 nm gate oxide with a gate leakage current less than 20 nA//spl mu/m/sup 2/. Furthermore, we find that I/sub Dsat/ deteriorates for gate oxides thicker or thinner than this.

Selectively δ‐doped AlxGa1−xAs/GaAs heterostructures with high two‐dimensional electron‐gas concentrations n2DEG≥1.5×1012 cm−2 for field‐effect transistors

Nanoscale pores have proved useful as a means to assay DNA and are actively being developed as the basis of genome sequencing methods. Hairpin DNA (hpDNA), having both double-helical and overhanging coil portions, can be trapped in a nanopore, giving ample time to execute a sequence measurement. In this article, we provide a detailed account of hpDNA interaction with a synthetic nanopore obtained through extensive all-atom molecular dynamics simulations. For synthetic pores with minimum diameters from 1.3 to 2.2 nm, we find that hpDNA can translocate by three modes: unzipping of the double helix and--in two distinct orientations--stretching/distortion of the double helix. Furthermore, each of these modes can be selected by an appropriate choice of the pore size and voltage applied transverse to the membrane. We demonstrate that the presence of hpDNA can dramatically alter the distribution of ions within the pore, substantially affecting the ionic current through it. In experiments and simulations, the ionic current relative to that in the absence of DNA can drop below 10% and rise beyond 200%. Simulations associate the former with the double helix occupying the constriction and the latter with accumulation of DNA that has passed through the constriction.

Slowing the translocation of double-stranded DNA using a nanopore smaller than the double helix

The δ‐doping concept is applied to selectively doped heterostructures in the AlxGa1−xAs/GaAs material system. High two‐dimensional electron‐gas concentrations ≥1.5×1012 cm−2 are obtained at T=300 K in such selectively δ‐doped heterostructures due to (i) size quantization in the AlxGa1−xAs and (ii) localization of donor impurities within one atomic monolayer. Shubnikov–de Haas measurements yield n2DEG =1.1×1012 cm−2 at 300 mK and at a spacer thickness of 25 A. Selectively δ‐doped heterostructure transistors (SΔDHT’s) are fabricated and have excellent characteristics due to the enhanced electron‐gas concentrations achieved. A very high transconductance of gm ≂360 mS/mm at a gate length of 1.2 μm is obtained in depletion‐mode SΔDHT’s at T=300 K.

The ballistic nano-transistor

It is now possible to slow and trap a single molecule of double-stranded DNA (dsDNA), by stretching it using a nanopore, smaller in diameter than the double helix, in a solid-state membrane. By applying an electric force larger than the threshold for stretching, dsDNA can be impelled through the pore. Once a current blockade associated with a translocating molecule is detected, the electric field in the pore is switched in an interval less than the translocation time to a value below the threshold for stretching. According to molecular dynamics (MD) simulations, this leaves the dsDNA stretched in the pore constriction with the base-pairs tilted, while the B-form canonical structure is preserved outside the pore. In this configuration, the translocation velocity is substantially reduced from 1 bp/10 ns to approximately 1 bp/2 ms in the extreme, potentially facilitating high fidelity reads for sequencing, precise sorting, and high resolution (force) spectroscopy.

Nanopores in solid-state membranes engineered for single molecule detection

We have achieved extremely high drive current performance and ballistic (T>0.8) transport using ultra-thin (<2 nm) gate oxides in sub-30 nm effective channel length nMOSFETs. The peak drive performance in an nMOSFET was observed at t/sub ox//spl ap/1.3 nm for a 1.5 V power supply voltage with T/sub n//spl ap/0.7, while the peak performance in a pMOSFET was observed at t/sub ox//spl ap/1.5 nm for a -1.5 V supply with T/sub p//spl ap/0.5. Since the carrier scattering in the channel is due predominately to interface roughness, reducing the transverse surface field, either by reducing the gate voltage or by increasing the oxide thickness, can be used to improve the transmittance T/sub n//spl rarr/0.85, T/sub p//spl rarr/0.6, while diminishing the drive current.