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NANO LETTERS Carbon Nanotube Schottky Diodes Using Ti Schottky and Pt Ohmic Contacts for High Frequency Applications 2005 Vol 5 No 7 1469 1474 Harish M Manohara Eric W Wong Erich Schlecht Brian D Hunt and Peter H Siegel Jet Propulsion Laboratory California Institute of Technology 4800 Oak GroVe DriVe Pasadena California 91109 Received May 4 2005 Revised Manuscript Received May 13 2005 ABSTRACT We have demonstrated Schottky diodes using semiconducting single walled nanotubes s SWNTs with titanium Schottky and platinum Ohmic contacts for high frequency applications The diodes are fabricated using angled evaporation of dissimilar metal contacts over an s SWNT The devices demonstrate rectifying behavior with large reverse bias breakdown voltages of greater than 15 V To decrease the series resistance multiple SWNTs are grown in parallel in a single device and the metallic tubes are burnt out selectively At low biases these diodes showed ideality factors in the range of 1 5 to 1 9 Modeling of these diodes as direct detectors at room temperature at 2 5 terahertz THz frequency indicates noise equivalent powers NEP potentially comparable to that of the state of the art gallium arsenide solid state Schottky diodes in the range of 10 13 W xHz For high frequency applications in the range of 30 GHz to 3 THz microwave to submillimeter wave regime diodes are of particular interest as detectors mixers and frequency multipliers 1 In particular solid state Schottky diodes rectifying metal semiconductor junctions are employed because of their higher switching speeds and their inherent suitability for low voltage high current applications The state of theart utilizes solid state Schottky diode detectors for room temperature sensor systems and Schottky diode multipliers for submillimeter wave power generation However above a few hundred GHz the inherent parasitic capacitance proportional to semiconductor junction area and resistance inversely proportional to electron mobility of these devices because of the limitations of the fabrication process and the material properties severely limits the achievable sensitivity for detection direct detector noise equivalent power or NEP 10 12 W xHz heterodyne NEP 10 17 W xHz for cooled operation at 4 K 1 and generated power at THz frequencies only microwatts of power up to 1 5 THz 1 From the material point of view carbon nanotubes2 offer an excellent alternative to their solid state counterparts because of their small junction areas due to their physical dimensions 1 to 2 nm diameter high electron mobilities up to 200 000 cm2 V s as reported by Durkop et al 3 and low estimated capacitances tens of aF m 4 5 leading to predicted cutoff frequencies in the THz range 5 10 1021 nl050829h CCC 30 25 Published on Web 05 27 2005 2005 American Chemical Society The electronic properties of single walled carbon nanotubes SWNTs have been studied in detail 6 10 The synthesis of SWNTs results in tubes that are either metallic mSWNTs or



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