Characterization of Contact mechanisms and Effect of Electron Irradiation on Conductance Mechanisms in Carbon Nanotube Field Effect Transistors

We have fabricated electrical devices based on thermal chemical vapor deposition (TCVD) grown single walled carbon nanotubes (SWCNTs). Long SWCNT are utilized to analyze electrical transport properties and extract contact data including Schottky Barrier heights (SBHs) and contact resistance. For all studies performed, multiple contact metals were used, and tens to hundreds of devices were fabricated on single CNT. This mass production method allows comparison of results, as well as greater options in device analysis.To analyze SBHs at carbon nanotube - metal contacts, field effect transistor (FET) devices were examined using AFM, low temperature measurements in closed - cycle refrigerator (CCR), and electrical characterization. SBH is measured on carbon nanotubes with multiple metal contacts for comparison purposes, with barriers extracted via low temperature activation energy measurements and nonlinear curve fitting using the program Origin. Two methods were utilized in the fabrication of carbon nanotube devices for the SB study. The first incorporated both electron beam lithography (EBL) for exposures and focused ion beam (FIB) for deposition of lead lines between CNT contacts and large probe pads. The second method used only EBL to prevent the ionic exposure common in FIB.The effect of using EBL with devices incorporating CNT has also been investigated. The effect on metallic and semiconducting CNT exposure in the channel of the transistor devices was examined and a physical mechanism for the variations discussed. We show that the subsequent generation of trap states along the CNT channel varies the conduction mechanism of the nanotube and has a significant effect on device performance. Metallic and Semiconducting CNT react very differently, with an apparent increased localization in the metallic tubes responsible for dramatic decreases in conductance.