Characterization of Nanoscale Junctions in Carbon Nanotubes and Graphene for Novel Device Applications.

Abstract

As materials and device architectures shrink to the nanoscale, the underlying device principles will cross over from the classical to quantum mechanical regime, which could lead to novel device behavior and provide new opportunities. In this thesis, I discuss some of the most fundamental nanoscale electronic/optoelectronic elements, including p-n, Schottky and tunneling junctions based on carbon nanotubes and graphene. By characterizing these nanoscale junctions using electrical and optical spectroscopic techniques, unconventional device operation principles were unveiled. More importantly, these fundamental understandings combined with the novel design of the device structures provided us with the ability to tailor material properties and engineer novel carbon-based optoelectronics. First, we demonstrate a tunable diode based on a fully suspended single-walled carbon nanotube structure. The turn-on voltage of the diode under forward bias can be continuously and widely tuned by controlling gate voltages. Additionally, the same device design could be configured into a backward diode by tuning the band-to-band tunneling current in the reverse bias region. A nanotube backward diode is demonstrated for the first time with nonlinearity exceeding the ideal diode. These suggest that a tunable nanotube diode could be a unique building block for developing next generation programmable circuits. Second, we present spatio-temporal photocurrent measurements of graphene p-n and graphene-metal junctions. The results explicitly confirm that the hot carrier photoresponse of graphene is related to its doping level, mobility and optical excitation power. Furthermore, our photocurrent measurements reveal the formation of an ultrafast photo-Dember process in graphene. These results not only mark the first time lateral photo-Dember effect is observed in atomically thin 2D materials, but also hint at the possibility of efficient terahertz generation in graphene. Finally, we develop a graphene-based hot carrier photodetector, which consists of a pair of graphene monolayers separated by a thin tunnel barrier. The optical illumination of this device causes hot carriers in graphene tunnel vertically to the nearby graphene layer and these pile-up photocarriers induce a strong photogating effect on the graphene channel conductance. This novel device structure and sensing scheme provide a viable route for achieving ultra-broad spectral, room temperature and high photoresponsivity photodetection.