Carbon nanotube field-effect transistors and gaseous interactions.
Date of Issue2009
School of Electrical and Electronic Engineering
This thesis presents the major findings achieved in my Ph.D project on carbon nanotube (CNT) field-effect transistors (FETs) and gaseous interactions. It consists of four main parts: (1) AC dielectrophoretic (DEP) manipulation of CNTs; (2) studies of gaseous interaction in CNTFETs through selective Si3N4 passivation; (3) real-time detection of chemical gases using CNTFETs and (4) differentiating the sensing mechanisms in CNTFET-based NH3 detectors. The motion of single-walled CNTs (SWNTs) in suspension under the influence of an applied electric field is analyzed in terms of induced DEP torque and force. The SWNTs are found to rotate to the field direction in a much shorter time than that needed for a translational motion along the field gradient. As a result, SWNTs are well placed normal to the electrode edges using the ac DEP technique. Due to different dielectric properties, metallic SWNTs and semiconducting SWNTs could be separated from each other by tuning the frequency of the applied ac electric field. Both theoretical and experimental results show that perpendicular electrodes have higher controllability of the SWNTs location than parallel electrodes. Our as-prepared CNTFETs are typically p-type Schottky barrier (SB) FETs in ambient. A selective Si3N4 passivation technique is developed to protect the CNT/metal contacts and/or CNT channel. The devices with passivated source and drain contacts and uncovered CNT channel show n-type characteristics in air, suggesting that a dominant influence of environmental oxygen is to modulate the SB height at the CNT/metal contacts. Moreover, by passivating only the source contact, a tunable Schottky diode involving the drain contact is obtained. A novel model is developed for CNT/metal contacts, in which the electrostatic charge balance across the contact and the dipole polarization along the CNT are appropriately taken into consideration. Using this model, the unique n-type characteristic of the CNTFET with passivated source contact is well interpreted by electron tunneling through VDS-dependant drain SB. By applying the model to Schottky diodes with asymmetric metal contacts, we show that only for those CNTFETs with one of the two CNT/metal contacts protected, the dipole polarization effect can be observed and becomes important to determine the conduction type of the devices. The CNTFETs are used as chemical gas sensors and their sensing performances are studied in terms of the applied gate voltages. The sensitivity and reversibility of the sensor is found to be significantly improved under appropriate positive gate voltages. Selective Si3N4 Passivation technique is emplyed to differentiate the sensing mechanism of CNTFET-based NH3 gas sensors. Our results clearly show that the SB modulation at the CNT/metal contacts dominates the sensing performance at room temperature. At temperature above 150oC, NH3 molecules start to adsorb on the CNT wall so that a charge transfer process contributes to the sensing signal. NH3 adsorption is confirmed to be facilitated by environmental oxygen.
DRNTU::Engineering::Electrical and electronic engineering::Microelectronics