Formation of tunnel barriers and quantum dot structure in suspended carbon nanotubes by focused ion beam for single electron transistor

Abstract

Carbon nanotube (CNT) represents a new class of building blocks for quantum do (QD) based nanodevices and circuits due to its extremely small diameter of 10nm whereby the standard lithography technique cannot easily realize such dimension. In addition to their electrical and optical applications, their unique mechanical properties such as light mass and large stiffness are attractive for a high-frequency resonator with a possible ultrasensitive mass sensing capability. This research demonstrates for the first time a reliable process to fabricate tunnel barriers in the suspended multi-wall carbon nanotubes (MWCNTs) by the mechanical transfer technique. This technique may also open a way to fabricate nanotube based mechanical functional devices. The two-terminal resistance of each device was measured before the formation of tunnel barriers ranging from 10kΩ to 20kΩ by focused ion beam (FIB). The suspended MWCNTs was then single scanned using FIB. More than 5 samples were fabricated for each ion dose from 1.5 × 10¹⁶ ions/cm² to 6 × 10¹⁶ ions/cm² and the effect of the resistance changed after irradiation was studied. This result showed that the resistance increased with rising Ga ion dose for both nanotubes on a substrate and the suspended nanotubes. Interestingly, for the case of the suspended nanotubes a higher dose was necessary to increase the resistance compared to the nanotube on a substrate. To analyse the data further, the sample was cooled down in a liquid helium refrigerator from room temperature to the lowest temperature of 1.5 K. The estimation of barrier height by Arrhenius plot obtained was then plotted as a function of resistance change after irradiation. There was a tendency that the barrier height correlated with the increased resistance after irradiation that was controlled by the dose of ion beam and diameter of the nanotubes. The single electron transistor (SET) was fabricated in suspended MWCNT by forming two barriers. The regular Coulomb diamonds and Coulomb oscillations were observed and some sample showed spike-like noise, superimposed on the regular Coulomb oscillations. It can be concluded that the SET was successfully fabricated in suspended MWCNT using simple and low-cost technique wherein Coulomb diamond and Coulomb oscillation were successfully observed.