arXiv subjects

Kosuke Nagashio

Publications and source records attributed to Kosuke Nagashio.

24 records · Page 2Linked to original sources

arXiv2015

Gap state analysis in electric-field-induced band gap for bilayer graphene

The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3*10^3 is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 10^12 to 10^13 eV^-1cm^-2. Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~10^11 eV^-1cm^-2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications.

cond-mat.mtrl-sci↗

arXiv2014

Large Fermi energy modulation in graphene transistors with high-pressure O2-annealed Y2O3 topgate insulators

We demonstrate a considerable suppression of the low-field leakage through a Y2O3 topgate insulator on graphene by applying high-pressure O2 at 100 atm during post-deposition annealing (HP-PDA). Consequently, the quantum capacitance measurement for the monolayer graphene reveals the largest Fermi energy modulation (EF = ~0.52 eV, i.e., the carrier density of ~2*10^13 cm^-2) in the solid-state topgate insulators reported so far. HP-PDA is the robust method to improve the electrical quality of high-k insulators on graphene.

cond-mat.mtrl-sci↗

arXiv2013

The density of states of graphene underneath a metal electrode and its correlation with the contact resistivity

The density of states (DOS) of graphene underneath a metal is estimated through a quantum capacitance measurement of the metal/graphene/SiO2/n+-Si contact structure fabricated by a resist-free metal deposition process. Graphene underneath Au maintains a linear DOS - energy relationship except near the Dirac point, whereas the DOS of graphene underneath Ni is broken and largely enhanced around the Dirac point, resulting in only a slight modulation of the Fermi energy. Moreover, the DOS of graphene in the contact structure is correlated with the contact resistivity measured using devices fabricated by the resist-free process.

cond-mat.mtrl-sci↗

arXiv2011

DOS-limited contact resistance in graphene FETs

Graphene has attracted much attention as one of promising candidates of future high-speed transistor materials because of its high carrier mobility of more than 10,000 cm2 V-1 s-1. Up to this point, we have focused on the contact properties as performance killers, as a very small density of states in graphene might suppress the current injection from metal to graphene. This paper systematically reviews the metal/graphene contact properties and discusses the present status and future requirements of the specific contact resistivity.

cond-mat.mtrl-sci↗

arXiv2010

Systematic Investigation of the Intrinsic Channel Properties and Contact Resistance of Monolayer and Multilayer Graphene FET

The intrinsic channel properties of monolayer and multilayer graphene were systematically investigated as a function of layer number by the exclusion of contact resistance using four-probe measurements. We show that the continuous change in normalized sheet resistivity from graphite to a bilayer graphene is governed by one unique property, i.e., the band overlap, which markedly increases from 1 meV for a bilayer graphene to 11 meV for eight layers and eventually reaches 40 meV for graphite. The monolayer graphene, however, showed a deviation in temperature dependence due to a peculiar linear dispersion. Additionally, contact resistivity was extracted for the case of typical Cr/Au electrodes. The observed high contact resistivity, which varies by three orders of magnitude (from ~103 to 106 Ohm micron), might significantly mask the outstanding performance of the monolayer graphene channel, suggesting its importance in future research.

cond-mat.mtrl-sci↗

arXiv2008

Mobility variations in mono- and multi-layer graphene films

The electric properties of mono- and multi-layer graphene films were systematically studied with the layer number determined by their optical contrast. The current modulation increased monotonically with a decrease in the layer number due to the reduction of the interlayer scattering. Carrier mobility in the monolayer was significantly greater than that in the multilayer due to linear dispersion relation. On the other hand, in the monolayer, carrier transport was extremely sensitive to charged impurity density due to the reduction in screening effect, which causes larger mobility variation. Reduction of the charged impurity density is thus key for high mobility.

cond-mat.mtrl-sci↗