Band Engineering of Partially Exposed Carbon Nanotube Field-Effect Transistors
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Band Engineering of Partially Exposed Carbon Nanotube Field-Effect Transistors Xiaolei Liu, Zhicheng Luo, Song Han, Tao Tang, Daihua Zhang, and Chongwu. Zhou Dept. of E.E.-Electrophysics, University of Southern California, Los Angeles, CA 90089 ABSTRACT We present a new approach to engineer the band structure of carbon nanotube field-effect transistors via selected area chemical gating. By exposing the center part or the contacts of the nanotube devices to oxidizing or reducing gases, a good control over the threshold voltage and subthreshold swing has been achieved. Our experiments reveal that NO2 shifts the threshold voltage higher while NH3 shifts it lower for both center-exposed and contact-exposed devices. However, modulations to the subthreshold swing are in opposite directions for center-exposed and contact-exposed devices: NO2 lowers the subthreshold swing of the contact-exposed devices, but increases that of the center-exposed devices; In contrast, NH3 reduces the subthreshold swing of the center-exposed devices, but increases that of the contact-exposed devices. A model has been developed based on Langmuir isotherm, and the experimental results can be well explained. INTRODUCTION As individual molecular wires, the lack of traps and scattering centers allows carriers to ballistically transport through the carbon nanotubes with mean free path on the order of a micrometer, which makes carbon nanotubes an excellent candidate material for high performance field-effect transistors. Great effort has been devoted to understanding the operation of these transistors[1, 2] and improving their performance[3-7]. For example, metals, such as Pd, were demonstrated to form ohmic contacts[5]; materials with high dielectric constants[3] and liquid/polymer electrolytes have been used to enhance the gate effect[4,6-7]; and nonuniform electrical doping using split gates has been employed to tailor the band structure of the CNTFETs[8]. Here we present an interesting approach of using selected area chemical gating to tune the electrical properties of nanotube transistors. CNT-FETs are also very sensitive to the ambient environment[9], which makes them very good sensors[10-12]. Early work on controlling the CNT-FET characteristics focused on the attachment of oxidizing and reducing chemicals, such as potassium and bromine, to the nanotubes to modulate the Fermi level[14,15]. Later on, nonuniform chemical doping on carbon nanotubes was performed to create intramolecular p-n junctions and p-n-p junctions[16,17]. These studies represented significant advances; however, an important parameter widely used to characterize transistor performance, the subthreshold swing, received only limited attention at that time. Using nonuniform doping to control both the threshold voltage and the subthreshold swing of nanotube transistors has not been demonstrated so far. We report for the first time selected-area chemical gating can be employed to tune the contact transparency relative to the bulk nanotube, which leads to intriguing influe
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