1,721,095 research outputs found
The role of contact resistance in graphene field-effect devices
No full textThe extremely high carrier mobility and the unique band structure, make graphene very useful for field-effect transistor applications. According to several works, the primary limitation to graphene based transistor performance is not related to the material quality, but to extrinsic factors that affect the electronic transport properties. One of the most important parasitic element is the contact resistance appearing between graphene and the metal electrodes functioning as the source and the drain. Ohmic contacts to graphene, with low contact resistances, are necessary for injection and extraction of majority charge carriers to prevent transistor parameter fluctuations caused by variations of the contact resistance. The International Technology Roadmap for Semiconductors, toward integration and down-scaling of graphene electronic devices, identifies as a challenge the development of a CMOS compatible process that enables reproducible formation of low contact resistance. However, the contact resistance is still not well understood despite it is a crucial barrier towards further improvements. In this paper, we review the experimental and theoretical activity that in the last decade has been focusing on the reduction of the contact resistance in graphene transistors. We will summarize the specific properties of graphene-metal contacts with particular attention to the nature of metals, impact of fabrication process, Fermi level pinning, interface modifications induced through surface processes, charge transport mechanism, and edge contact formation
Double conductance minima in graphene field-effect transistors
Full text linkIn this study we present measurements on Cr/Au contacted long-channel (10 μm)
graphene transistors on Si-SiO2 substrate and we report the observation of hysteresis as well
as double dips in the transfer characteristics. Charge trapped in the surrounding dielectric and
in particular in silanol groups at the SiO2 surface is at the origin of the hysteresis; while, the
gradient of carriers along the channel caused by electron transfer from the graphene to the
Au/Cr contacts and the band shift induced by the backgate voltage and the SiO2-trapped
charge are proposed to account for the double dip feature. We show in particular that p–n
junctions are spontaneously formed by charge transfer between the graphene and the
electrodes and that a double Dirac point can be achieved when low-resistivity contacts are
fabricated. We further clarify the role of charge stored at the SiO2 interface in the formation
of the double dip and we propose partial charge pinning at the contacts to explain the current
saturation observed at high back-gate voltages. Accordingly, a phenomenological modeling of
experimental data is successfully implemented.
We finally show that the hysteresis, enhanced by a double dip, can conveniently be exploited
to build graphene-based memory devices
Focus on graphene and related materials
No full textThis focus collection, dedicated to graphene and other 2D materials, summarizes
some of the contributions presented at the International Conference GM-
2016 ‘Graphene and related materials: properties and applications’ held in Paestum,
Italy, in May 2016. It was an intense multidisciplinary meeting that brought together
about 150 physicists, chemists and engineers working on fundamental and applicative
aspects
Generalized Blonder-Tinkham-Klapwijk theory and conductance spectra with particle-hole mixing interface potential
Full text linkWe extend the Blonder-Tinkham-Klapwijk treatment including particle-hole mixing boundary conditions in the Bogoliubov-de Gennes scattering problem to describe anomalous conductance features often reported in normal-metal/superconductor junctions. We calculate the differential conductance spectra and show that conductance dips, not expected in the standard formulation, can be explained in terms of a phase π-shift between the bulk and the interface order parameter. A tight-binding model is also introduced to give a quantitative description of the phase-shift in terms of the transparency and polarization of the interface. We characterize the physics arising from particle-hole mixing boundary conditions at the interface and its effects on the conductance anomalies in superconductor-normal heterostructures
The Superconducting Mechanism in BiS2-Based Superconductors: A Comprehensive Review with Focus on Point-Contact Spectroscopy
No full textThe family of BiS2-based superconductors has attracted considerable attention since their discovery in 2012 due to the unique structural and electronic properties of these materials. Several experimental and theoretical studies have been performed to explore the basic properties and the underlying mechanism for superconductivity. In this review, we discuss the current understanding of pairing symmetry in BiS2-based superconductors and particularly the role of point-contact spectroscopy in unravelling the mechanism underlying the superconducting state. We also review experimental results obtained with different techniques including angle-resolved photoemission spectroscopy, scanning tunnelling spectroscopy, specific heat measurements, and nuclear magnetic resonance spectroscopy. The integration of experimental results and theoretical predictions sheds light on the complex interplay between electronic correlations, spin fluctuations, and Fermi surface topology in determining the coupling mechanism. Finally, we highlight recent advances and future directions in the field of BiS2-based superconductors, underlining the potential technological applications
Field emission from AlGaN nanowires with low turn-on field
No full textWe fabricate AlGaN nanowires by molecular beam epitaxy and we investigate their field emission properties by means of an experimental setup using nano-manipulated tungsten tips as electrodes, inside a scanning electron microscope. The tip-shaped anode gives access to local properties, and allows collecting electrons emitted from areas as small as 1μm2. The field emission characteristics are analysed in the framework of Fowler-Nordheim theory and we find a field enhancement factor as high as β = 556 and a minimum turn-on field E_(turn-on) = 17 V/μm for a cathode-anode separation distance d = 500 nm. We show that for increasing separation distance, E_(turn-on) increases up to about 35 V/μm and β decreases to 100 at d = 1600 nm. We also demonstrate the time stability of the field emission current from AlGaN nanowires for several minutes. Finally, we explain the observation of modified slope of the Fowler-Nordheim plots at low fields in terms of non-homogeneous field enhancement factors due to the presence of protruding emitters
Transfer characteristics and contact resistance in Ni- and Ti-contacted graphene-based field-effect transistors
No full textWe produced graphene-based field-effect transistors by contacting mono- and bi-layer graphene by sputtering Ni or Ti as metal electrodes. We performed electrical characterization of the devices by measuring their transfer and output characteristics. We clearly observed the presence of a double-dip feature in the conductance curve for Ni-contacted transistors, and we explain it in terms of charge transfer and graphene doping under the metal contacts. We also studied the contact resistance between the graphene and the metal electrodes with larger values of ~30 kΩμm2 recorded for Ti contacts. Importantly, we prove that the contact resistance is modulated by the back-gate voltage
Imaging the spontaneous formation of vortex-antivortex pairs in planar superconductor/ferromagnet hybrid structures.
No full textLow-temperature magnetic force microscopy has been used to visualize spontaneous formation of vortexantivortex
pairs in hybrid ferromagnet/superconductor systems. Vortex-antivortex pairs are induced by the periodic
stray field of the ferromagnet. We find general equilibrium conditions for which spontaneous vortex-antivortex
pairs are formed during zero-field cooling of the hybrid ferromagnet/superconductor bilayers. Vortices can be
generated by the ferromagnet domains in the absence of an external field and they are thermodynamically stable
for values of the stray field and the period of the stripe magnetic domains that exceed a certain threshold
Environmental effects on transport properties of PdSe2 field effect transistors
No full textWe study the effect of pressure, voltage sweeping rate and electron irradiation on the transfer characteristics of field effect transistors fabricated by using exfoliated palladium diselenide flake as transistor channel and Ti/Au metallic electrodes as source and drain. The silicon substrate is used as gate, the flake being transferred on Si/SiO2 substrate. We report ambipolar behavior for the devices under investigation and we demonstrate that external stimuli have dramatic effects on the transport properties. In particular, increasing the acquisition time (by using slower sweeping rate) we demonstrate that the hysteresis observed in the transfer characteristics is widened. Electron irradiation, necessary for SEM imaging of the device, also dramatically affects the characteristics due to induced defects and consequent charge trapping at PdSe2 and SiO2. Finally, we demonstrate that the device can be tuned from n-type conduction in vacuum, to p-type conduction in atmospheric pressure
Effect of Electron Irradiation on the Transport and Field Emission Properties of Few-Layer MoS2 Field-Effect Transistors
Full text linkElectrical characterization of few-layer MoS2-based field-effect transistors with Ti/Au electrodes is performed in the vacuum chamber of a scanning electron microscope in order to study the effects of electron-beam irradiation on the transport properties of the device. A negative threshold voltage shift and a carrier mobility enhancement are observed and explained in terms of positive charges trapped in the SiO2 gate oxide, during the irradiation. The transistor channel current is increased up to 3 orders of magnitudes after the exposure to an irradiation dose of 100 e−/nm2. Finally, a complete field emission characterization of the MoS2 flake, achieving emission stability for several hours and a minimum turn-on field of ≈20 V/μm with a field enhancement factor of about 500 at an anode−cathode distance of ∼1.5 μm, demonstrates the suitability of few-layer MoS2 as a two-dimensional emitting surface for cold-cathode applications
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