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Topic Name: nanowire-based nanotechnology: sublithographic programmable logic arrays

Category: Electronics

Research persons: Lincoln J. Lauhon, Ph.D.
Department of Materials Science & Engineering
Robert R. McCormick School of Engineering and Applied Science
Northwestern University

Location: Evanston, IL,, United States

Details

1d nanostructures such as semiconductor nanowires; recently developed synthetic methods suggest ways to engineer the electronic, optical, and magnetic properties of semiconductor nanowires by varying composition on the nanometer scale. A few of the methods and objectives of the research are described below:
Nanostructure Synthesis
Chemical vapor deposition (CVD) and metal catalyzed vapor-liquid-solid (VLS) growth to define unique semiconductor nanowire heterostructures. Nanowires with diameters of 3-50 nm and lengths of several microns can be synthesized in sufficient quantities for fundamental characterization and device research. Currently,
II-VI nanowires are being synthesized in our own laboratory via thermal CVD.
Construction is underway on a thermal CVD reactor for group IV materials.
III-V nanowire materials (InAs, GaN) are being produced by metal-organic vapor phase epitaxy (MOVPE) in collaboration with Prof. Bruce Wessels (MSE and EE).
Nanoscale 3-D Composition MappingWe are using atom-probe tomography to map the composition of semiconductor nanowires in three dimensions with single-atom sensitivity and sub-nm resolution. We take advantage of the LEAP microscope housed at the Northwestern University Center for Atom Probe Tomography (NUCAPT); see the LEAP page for examples of recent data collected from individual semiconductor nanowires. LEAP microscopy can play an important role in the development of semiconductor nanostructure device technology by providing critical insight into the connection between synthesis schemes and nanoscale composition.

Nanoscale Property Characterization
A myriad of characterization methods based on the atomic force microscope (AFM) can be used to explore not only the size and shape of nanoscale objects but also their local electrical, optical, and magnetic properties. By combining electrical transport measurements on nanofabricated devices with simultaneous conductive scanning probe, for example, the electrical functionality of nanoscale devices such as intra-nanowire pn junctions can be revealed. Current efforts include using a Digital Instruments Nanoscope III and a JEOL 4210 SPM (both in NU facilities) to detect magnetization in ferromagnetic semiconductor nanowires via variable temperature magnetic force microscopy (MFM).
Device Fabrication
Measuring the electrical properties of individual nanowires requires connecting a nanoscale object with macroscale measurement tools like voltage sources and current meters. Electron beam lithography can be used to connect the macroscale with the nanoscale in a “top-down” approach to single nanowire measurement. The resulting “device” may act as a field effect transistor, a chemical sensing transistor, or a light emitting diode. The goals of our fabrication efforts are two-fold:
To enable the discovery of unique electronic, optical, and magnetic properties in semiconductor nanostructures that are fundamentally interesting.
To demonstrate superior performance with the nanowire realization of a conventional or unconventional device.
Electron beam lithography capabilities are provided by a Nabity-equipped FEI Quanta SEM housed in NUANCE, and additional microfabrication (including photolithography) is performed in the MRSEC cleanroom on the fourth floor of Cook Hall.

The develpoment expected after 5 or 10 years
The pace of development has been sufficiently rapid that it is difficult to pick just a few advances. Generally speaking, the breadth of applications being considered for nanowires represents a great advance in our understanding of their potential. More specifically, important advances in synthesis have been those which demonstrate some of the unique possibilities, including directed growth from selected sites and the low-temperature growth of heterostructures of dissimilar materials. With regards to electrical characterization, several groups have demonstrated that nanowires can behave close to the ideal one-dimensional electron boxes that physicists imagine, which is exciting from a fundamental perspective and from the perspective of nanotechnology development. Looking to the near future, there are manifold opportunities for advances in nanowire electromechanics and heat transport, and in the synthesis of nanowires with magnetic functionality or multiple functionalities, like multiferroics. Materials which exhibit large changes in properties of interest near phase transitions are good targets for new phenomena or enhanced properties in nanowire form. In five years, I think we might view this "field" of nanowires as part of a much larger exploration of multi-functional nanostructured materials. The idea is that you can design the response of a material from the bottom up by selecting appropriate building blocks and then assembling a material with the appropriate hierarchical structure. Nanowires are somewhat unique in that they bridge length scales by having nanoscale diameters but microscopic lengths. We are already seeing a good deal of work on carbon nanotubes in composites, and I expect the work in nanowires to follow. Ironically, semiconductor nanowires were discovered long before carbon nanotubes, and they were being considered for composites with high mechanical strength based on their degree of crystalline perfection. I’m not sure where the field will be in 10 years —if the funding agencies were giving 10-year grants I’d have a better answer.