Danvers E. Johnston

Controlled fabrication of nanogaps in ambient environment for molecular electronics

We have developed a controlled and highly reproducible method of making nanometer-spaced electrodes using electromigration in ambient lab conditions. This advance will make feasible single molecule measurements of macromolecules with tertiary and quaternary structures that do not survive the liquid-helium temperatures at which electromigration is typically performed. A second advance is that it yields gaps of desired tunneling resistance, as opposed to the random formation at liquid-helium temperatures. Nanogap formation occurs through three regimes: First it evolves through a bulk-neck regime where electromigration is triggered at constant temperature, then to a few-atom regime characterized by conductance quantum plateaus and jumps, and finally to a tunneling regime across the nanogap once the conductance falls below the conductance quantum.

Electronic devices based on purified carbon nanotubes grown by high-pressure decomposition of carbon monoxide

The excellent properties of transistors, wires and sensors made from single-walled carbon nanotubes (SWNTs) make them promising candidates for use in advanced nanoelectronic systems1. Gas-phase growth procedures such as the high-pressure decomposition of carbon monoxide (HiPCO) method2,3 yield large quantities of small-diameter semiconducting SWNTs, which are ideal for use in nanoelectronic circuits. As-grown HiPCO material, however, commonly contains a large fraction of carbonaceous impurities that degrade the properties of SWNT devices4. Here we demonstrate a purification, deposition and fabrication process that yields devices consisting of metallic and semiconducting nanotubes with electronic characteristics vastly superior to those of circuits made from raw HiPCO. Source–drain current measurements on the circuits as a function of temperature and backgate voltage are used to quantify the energy gap of semiconducting nanotubes in a field-effect transistor geometry. This work demonstrates significant progress towards the goal of producing complex integrated circuits from bulk-grown SWNT material.

Image quality and pattern transfer in directed self assembly with block-selective atomic layer deposition

Self-assembled block copolymer patterns may render more robust masks for plasma etch transfer through block-selective infiltration with metal oxides, affording opportunities for improved high contrast, high fidelity pattern transfer for sub-15 nm lithography in wafer-scale processes. However, block selective infiltration alters the self-assembled block copolymer latent image by changing feature size, duty cycle, and sidewall profile. The authors systematically investigate the effects of aluminum oxide infiltration of 27 and 41 nm pitch line/space patterns formed using polystyrene-b-poly(methyl methacrylate) block copolymers and evaluate the process compatibility with directed self assembly. The degree of image distortion depends on the amount of infiltrated material, with smaller amounts resulting in complete mask hardening and larger amounts shifting and collapsing pattern features. An attractive feature of the resulting oxide mask is the relatively smooth line edge roughness of the final transferred fea...

Nanostructured surfaces frustrate polymer semiconductor molecular orientation.

Nanostructured grating surfaces with groove widths less than 200 nm impose boundary conditions that frustrate the natural molecular orientational ordering within thin films of blended polymer semiconductor poly(3-hexlythiophene) and phenyl-C61-butyric acid methyl ester, as revealed by grazing incidence X-ray scattering measurements. Polymer interactions with the grating sidewall strongly inhibit the polymer lamellar alignment parallel to the substrate typically found in planar films, in favor of alignment perpendicular to this orientation, resulting in a preferred equilibrium molecular configuration difficult to achieve by other means. Grating surfaces reduce the relative population of the parallel orientation from 30% to less than 5% in a 400 nm thick film. Analysis of in-plane X-ray scattering with respect to grating orientation shows polymer backbones highly oriented to within 10 degrees of parallel to the groove direction.

One-volt operation of high-current vertical channel polymer semiconductor field-effect transistors.

We realize a vertical channel polymer semiconductor field effect transistor architecture by confining the organic material within gratings of interdigitated trenches. The geometric space savings of a perpendicular channel orientation results in devices sourcing areal current densities in excess of 40 mA/cm(2), using a one-volt supply voltage, and maintaining near-ideal device operating characteristics. Vertical channel transistors have a similar electronic mobility to that of planar devices using the same polymer semiconductor, consistent with a molecular reorientation within confining trenches we understand through X-ray scattering measurements.

Grazing-incidence transmission X-ray scattering: surface scattering in the Born approximation

Determination of the three-dimensional order in thin nanostructured films remains challenging. Real-space imaging methods, including electron microscopies and scanning-probe methods, have difficulty reconstructing the depth of a film and suffer from limited statistical sampling. X-ray and neutron scattering have emerged as powerful complementary techniques but have substantial data collection and analysis challenges. This article describes a new method, grazing-incidence transmission small-angle X-ray scattering, which allows for fast scattering measurements that are not burdened by the refraction and reflection effects that have to date plagued grazing-incidence X-ray scattering. In particular, by arranging a sample/beam geometry wherein the scattering exits through the edge of the substrate, it is possible to record scattering images that are well described by straightforward (Born approximation) scattering models.

Plasma etch transfer of self-assembled polymer patterns

Abstract. Self-organizing block copolymer thin films hold promise as a photolithography enhancement material for the 22-nm microelectronics technology generation and beyond, primarily because of their ability to form highly uniform patterns at the relevant nm scale dimensions. Importantly, the materials are chemically similar to photoresist and can be implemented in synergy with photolithography. Beyond the challenges of achieving sufficient control of self-assembled pattern defects and feature roughness, block copolymer-based patterning requires creation of robust processes for transferring the polymer patterns into underlying electronic materials. Here, we describe research efforts in hardening block copolymer resist patterns using inorganic materials and high aspect ratio plasma etch transfer of self-assembled patterns to silicon using fluorine-based etch chemistries.