| Early studies of electron transport in single-wall carbon nanotubes (SWNTs) have been hindered by large resistance of the nanotube-contact interface and weak electrostatic coupling to the gate electrode. Several groups have shown that the contact resistance can be reduced to kΩ range by growing nanotubes directly on the substrate using chemical vapor deposition (CVD).; In our work we implement this growth technique by fabricating electrodes from cobalt, which creates perfectly transparent nanotube-electrode contacts, = 0.98. In addition, we use high temperature anneal in hydrogen atmosphere to reduce defect density in the dielectric layer and enhance gate effectiveness. At low temperatures we reduce the amount of gate voltage needed to add a single electron to 0.8 μm long nanotube by an order of magnitude to 15 mV. Using these improvements, we demonstrate ambipolar transport in nanotube-based field effect transistor (TubeFET), and use it to develop a novel type of memory device for data storage. In a metallic nanotube, we show that gate voltage can be used to tune the reflection probability of defect-induced scattering centers. Furthermore, total scattering probability increases with device length, suggesting weak scattering from molecular adsorbates. Studies of resistance in metallic nanotubes reveal both metallic and insulating temperature behaviors, depending on the total reflection probability in the device. Recently, it has been observed that metallic nanotubes can carry up to 25 μA of current at high bias. Our work indicates that the observed current saturation phenomena are not a contact-related issue, but rather due to electron-phonon scattering. Moreover, we demonstrate a tunable change in current carrying capacity, up to 40 μA, and functional form of the current-voltage (I − V) characteristic. These changes can be understood quantitatively within a model predicting a transition between stimulated and spontaneous phonon emission as a function of the nanotube Fermi energy. Finally, we present the first observation of current saturation in semiconducting nanotubes which are in qualitative agreement with the phonon emission model. |
