Contact resistivity and current flow path at metal/graphene contact

The contact properties between metal and graphene were examined. The electrical measurement on a multiprobe device with different contact areas revealed that the current flow preferentially entered graphene at the edge of the contact metal. The analysis using the cross-bridge Kelvin structure (CBK) suggested that a transition from the edge conduction to area conduction occurred for a contact length shorter than the transfer length of ~1 micron. The contact resistivity for Ni was measured as ~5*10-6 Ohmcm2 using the CBK. A simple calculation suggests that a contact resistivity less than 10-9 Ohmcm2 is required for miniaturized graphene field effect transistors

Estimation of residual carrier density near the Dirac point in graphene through quantum capacitance measurement

We discuss the residual carrier density (n*) near the Dirac point (DP) in graphene estimated by quantum capacitance (CQ) and conductivity measurements. The CQ at the DP has a finite value and is independent of the temperature. A similar behavior is also observed for the conductivity at the DP, because their origin is residual carriers induced externally by charged impurities. The n* extracted from CQ, however, is often smaller than that from the conductivity, suggesting that the mobility in the puddle region is lower than that in the linear region. The CQ measurement should be employed for estimating n* quantitatively.Comment: APL 201

Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: an Insight into Current Flow across Schottky Barriers

In this article, we study the properties of metal contacts to single-layer molybdenum disulfide (MoS2) crystals, revealing the nature of switching mechanism in MoS2 transistors. On investigating transistor behavior as contact length changes, we find that the contact resistivity for metal/MoS2 junctions is defined by contact area instead of contact width. The minimum gate dependent transfer length is ~0.63 {\mu}m in the on-state for metal (Ti) contacted single-layer MoS2. These results reveal that MoS2 transistors are Schottky barrier transistors, where the on/off states are switched by the tuning the Schottky barriers at contacts. The effective barrier heights for source and drain barriers are primarily controlled by gate and drain biases, respectively. We discuss the drain induced barrier narrowing effect for short channel devices, which may reduce the influence of large contact resistance for MoS2 Schottky barrier transistors at the channel length scaling limit.Comment: ACS Nano, ASAP (2013

Effective electro-optical modulation with high extinction ratio by a graphene-silicon microring resonator

Graphene opens up for novel optoelectronic applications thanks to its high carrier mobility, ultra-large absorption bandwidth, and extremely fast material response. In particular, the opportunity to control optoelectronic properties through tuning of Fermi level enables electro-optical modulation, optical-optical switching, and other optoelectronics applications. However, achieving a high modulation depth remains a challenge because of the modest graphene-light interaction in the graphene-silicon devices, typically, utilizing only a monolayer or few layers of graphene. Here, we comprehensively study the interaction between graphene and a microring resonator, and its influence on the optical modulation depth. We demonstrate graphene-silicon microring devices showing a high modulation depth of 12.5 dB with a relatively low bias voltage of 8.8 V. On-off electro-optical switching with an extinction ratio of 3.8 dB is successfully demonstrated by applying a square-waveform with a 4 V peak-to-peak voltage.Comment: 12 pages, including 7 figure