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

As materials and device architectures shrink to the nanoscale, the underlying device principles will cross over from classical to quantum mechanical regime, which could lead to peculiar behavior of the devices and open up new opportunities. In this thesis, I discuss some of the most fundamental nanoscale electronic/optoelectronic elements, including p-n junction, Schottky junction and tunneling junction based on carbon nanotubes and graphene. By characterizing these nanoscale junctions with different electrical as well as optical spectroscopy, unconventional device operation principles were unveiled. More importantly, these fundamental understandings combined with novel design of device structures provided us the capability to tailor material properties and engineer novel carbon based optoelectronics.
Firstly, we demonstrate a tunable diode based on a fully suspended single-walled carbon nanotube structure. The diode's turn-on voltage 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 results suggest that a tunable nanotube diode can be a unique building block for developing next generation programmable nanoelectronic logic and integrated circuits.
Secondly, we present spatially and temporally photocurrent measurements of graphene p-n and graphene-metal junctions. The results explicitly confirm that hot carrier photoresponse of graphene is closely related to its doping level, mobility and optical excitation power. More importantly, our photocurrent measurements reveal the formation of 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. Optical illumination of this device causes hot carriers in graphene tunnel vertically to the nearby graphene layer and these pile-up photocarriers lead to 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.