Contact resistance between carbon nanomaterials

Abstract

In the past decade, contact resistance problem have been regarded as a significant issue in electronics industry due to reducing electrical performance in transistor, logic circuit, sensor and so on. Recently, following the industrial needs for transparency and flexible devices, there have been many efforts to establish high performance electronic devices using carbon nanomaterials such as graphene and single-wall carbon nanotube (SWNT) which has superior electrical, mechanical and optical properties instead of silicon. However, recognition of the importance of the contact between electrodes and channel region in electronic devices based on the carbon nanomaterials is still insufficient to understand the basic science of the electrical contacts and to reduce the electrical contact resistance.

Carbon nanomaterials, for example, graphene and carbon nanotube have attracted the great interest in electrical devices. Field effect transistors based on carbon nanotube and graphene have been demonstrated to have several impressive characteristics such as ON current level, high ON/OFF ratio, high mobility and work function modulation with metal electrode (Au, Pd and Ni). However, as mentioned earlier, full transparency electronics and the realization of minimal contact resistance are required in future electronics. Consequently, we will need a deeper understanding of the junction properties of electrical contacts fabricated by carbon nanotubes and graphene. In this research, therefore clarifying the contact resistance and junction properties between carbon nanomaterials such as carbon nanotube and graphene was proposed for transparency and high performance thin film transistor. Moreover, we also provided the potential route to enhance electrical property of each carbon nanomaterials at the synthesis and patterning methods.

In chapter 3, first, by chemical vapor deposition, aligned single wall carbon nanotubes and a network of SWNTs are simultaneously grown as the channel and the source-drain electrodes of thin film transistors (TFTs). Despite of the channel conductance induced by the increase of aligned SWNTs, it shows a similar contact resistance. However, the increased density of network type SWNTs from 19 to 32.5 (SWNTs/μm) lead to five-fold lower contact resistance. The contact resistance of all-SWNT TFT is three times lower compared to that of a SWNT TFT using metal electrodes. Moreover, the all-SWNT TFTs transferred on PET show a transparency of > 80% in the visible range of wavelengths.

In chapter 4, the analysis of out of plane resistance in graphene interlayer made of CVD graphene using the conformal contact of PDMS was reported for the first time. We developed a circuit model that can be used to obtain some key parameters of contact, such as graphene conductivity, interlayer distance, total resistance and transfer length. Using conformal contact property of PDMS, the graphene/PDMS layer was attached to bottom graphene transferred on Si/$SiO_2$ substrate with controlling the contact area. Modified transmission line method (TLM) was used to contact resistance between Au/graphene interlayer. According to several measurements, we confirm the interlayer graphene conductivity of ~2.5x$10^{-2}$ S/m, a value from $10^4$ to $10^5$ times lower than the graphene inplane conductivity. However, if the graphene presents different sheet resistance or workfunction induced by various doping method, the interlayer conductivity can be changed. Consequently, the numerical results related to workfunction, defect density of graphene and domain boundary is still necessary to form the conductivity map as following contacted materials and further works is needed.

In chapter 5, for high performance graphene in thin film transistor, we present the growth dynamics and recrystallization of 2D material graphene under a mobile hot-wire assisted chemical vapor deposition (MHW-CVD) system. The local but sequential endowing thermal energy to nano-crystalline graphenes enabled us to simultaneously reveal the recrystallization and healing dynamics in graphene growth, which suggests an alternative route to synthesize a highly crystalline and large domain size graphene. Also, this recrystallization and healing of 2D nano-crystalline graphenes offers an interesting insight on the growth mechanism of 2D materials.

In chapter 6 for the application possibility as graphene-based electronic device, we propose the nano-patterning method with size-controlled fabrication of uniform graphene quantum dots using self-assembled block copolymer (BCP) as an etch mask on graphene films grown by chemical vapor deposition (CVD). In the measured AFM and Raman spectra, the defect density in graphene surface is shown quite low compared to general fabrication method using graphene oxide. Moreover in chapter 7, we demonstrate a new materials design in the form of metal (Cu, Ni) ？graphene alternating nanolayered composites which indicate ultra-high strengths of 1.5 GPa and 4.0 GPa for Cu- and Ni-graphene nanolayers with 100nm repeat layer spacing, respectively. The ultra-high strengths of metal-graphene nanolayered structures indicate the effectiveness of graphene in blocking dislocation propagations across the metal-graphene interface, and the electron microscopy of the deformed nanopillar shows a build-up of dislocations and therefore a strain fringe at the interface. The molecular dynamic simulations confirm that the graphene can efficiently block dislocations and that the piled-up dislocations can result in “self-healing” that is aided by the thermal fluctuation of graphene.