AFM-based Electrical Characterization of Nano-structures
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AFM-based Electrical Characterization of Nano-structures Sujit K. Biswas1, Sandra B. Schujman1, Robert Vajtai2, Bingqing Wei2, Allen Parker1, Leo J. Schowalter1 and Pulickel M. Ajayan2 1 Department of Physics, Applied Physics and Astronomy, 2 Department of Material Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, U.S.A. ABSTRACT Carbon nanotubes have the potential of being used as interconnects and active semiconducting material in future electronic circuits. It is necessary to study such nano-scale circuits with probes that can make measurements with molecular precision. We describe results using two nanoprobe techniques, namely scanning surface potential microscopy (SSPM), and conductive tip atomic force microscopy (CT-AFM), in the investigation of electrical properties of nanotube circuits. Vertical arrays of multi-walled nanotubes, grown in a porous alumina template with a metal back contact were analyzed. Current mapping confirmed that the nanotubes were electrically connected to the back contact. Isolated single-walled nanotube bundles deposited on an oxidized silicon wafer, and contacted electrically through chromium electrodes were also studied. Contact potential differences between the metal and nanotubes, and the current in some connected nanotubes were measured. Measurements of contact potential with different metals, and the nature of microscopic transport is crucial. Contact potential measurements can also provide fast and reliable characterization of junctions between metallic and semiconducting nanotubes and metals electrodes.
INTRODUCTION As electronic circuit dimensions are reduced, carbon nanotubes, as well as structures and devices based on them, may offer a possible way to overcome the limitations imposed by current procedures for circuit design and implementation [1]. With nano-scale circuits, local effects manifested in electrical transport can be significant. Conventional methods of electrical characterization produce only averaged information on the processes occurring within nanostructures. In order to gain insight into the microscopic nature of the electronic properties, atomic force microscopy (AFM) based methods are best suited to produce the desired resolution [2-4].
EXPERIMENTAL DETAILS Topographic information of the nano-structures is gathered with an AFM. Potential is measured with a scanning surface potential microscope (SSPM), also called the Kelvin force microscope [5]. Current measurement is carried out with a conductive tip AFM (CT-AFM) which offers the flexibility to scan over insulating and conductive surfaces alike. In both cases silicon tips with a conductive coating are used.
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An AFM tip with a conductive coating of tungsten carbide is used for potential measurement. The topography of the surface is recorded during the first scan with the AFM operating in tapping mode, with the tip electrically grounded. During this scan, the tip vibrates very close to the resonant frequency ω ,of the cantilever. The second sc
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