Charge storage and transport in single-walled carbon nanotube field-effect transistors
Date of Issue2008
School of Electrical and Electronic Engineering
This thesis presents the main findings achieved during my PhD project on electron transport properties of carbnno nanotues (CNTs) and CNT based electronic devices. It consists of two parts: (1) hysteresis effects in CNT field-effect transistors (CNTFETs) and (2) single-electron/hole transport properties in CNTFETs. Hysteresis in our CNTFETs is caused by charge trapping around the single-walled CNT (SWNT)-channel. First of all, the thermally activated charge trapping/detrapping processes are studied through the temperature dependence of hysteresis window. The hysteresis window size is reduced by > 50% when temperature is decreased from 295 to 16 K because thermally activated charge hopping into and out of the trapping centers is suppressed at low temperatures. Secondly, the sources of trapping centers are found to be the foreign species absorbed around the SWNT-channel, such as surfactants, impurities, traps in bottom gate oxide, water molecules and even ammonia molecules, etc. Thirdly, a technique for eliminating hysteresis has been developed. Annealing process (300 °C) in vacuum followed by Si3N4 and SiO2 passivation is found to be an efficient way. Hysteresis-free devices are obtained after passivation at 300 oC because the surface water and surfactant molecules desorb at this temperature in vacuum. However, new traps may locate at the dielectric/SWNT interface and the bulk of the low-density dielectrics (deposited by a PECVD). Lastly, the influences of trapped charges in Si3N4 have been investigated. Through applying voltage pulses to a global back-gate and a local top-gate, “globally” and “locally” distributed pre-trapped charges near the CNT-channel can be induced, respectively. The locally pre-trapped holes and electrons keep the CNT channel at “ON” and “OFF” states for the top-gate sweeping, respectively. Therefore, the trapped charges in the source/drain contact regions and SWNT-body could effectively influence the channel conductance through the “Schottky barrier modulation” and “bulk switching”, respectively. Single-electron/hole transports with the Coulomb blockade effects are studied using three types of CNT-based electronic devices. In the first type, significant Coulomb oscillations are found in specially constructed CNTFETs that consist of long SWNTs bridging the source/drain electrodes and short SWNTs attached to the source/drain contacts. The oscillation peaks fade away gradually with increasing temperatures up to 70 K. The IDS contour plot in the VDS-VGS plane shows “diamond”-shaped current forbidden regions at 15 K. In the second type of the devices, short-SWNT channel (< 100 nm) FETs with asymmetric drain and source contacts are fabricated with a self-alignment technique. The Coulomb oscillation peaks superimposed on the transfer curves of an n-type CNTFET (on/off ratio ~ 107) are observed up to 100 K. The activation energy at the Al/SWNT contact is found to be smaller than those at the Au/SWNT contact, by a factor of > 2 for all VGS. The third type of the devices has single, double or multiple SWNT gates that control SWNT-channel conductance. Significant Coulomb blockade characteristic are observed in the double-SWNT-gated CNTFETs. A pair of potential barriers is induced by the double-SWNT-gates so that a single-electron/hole transistor working in the classic regime with the Coulomb blockade effects is formed.
DRNTU::Engineering::Electrical and electronic engineering::Nanoelectronics