1,721,012 research outputs found
Graphene-Silicon Schottky diodes for photodetection
No full textWe present the optoelectronic characterization of graphene/silicon Schottky junctions, fabricated by transferring CVD-graphene on flat and nanotip-patterned n- and p-type Si substrates. We show that medium n-type doping results the in highest rectification. We demonstrate high photoresponsivity, exceeding 2.5 A/W under white light, which we attribute to the contribution of charges photogenerated in the surrounding region of the flat junction or to the internal gain by impact ionization caused by the enhanced field on the nanotips
Persistent Photoconductivity, Hysteresis and Field Emission in MoS2 Back-Gate Field-Effect Transistors
No full textWe present the electrical characterization of mono and bilayer MoS2 back-gate field-effect transistors with ohmic or Schottky contacts. We investigate features such as persistent photoconductivity, hysteresis and field emission
MoS2 and WSe2 in field effect transistors
No full textTransition-metal dichalcogenides, such as MoS2 or WSe2, have recently become very popularfornext-generation electronic devices and sensors as alternative or complement to graphene. Such materials offer remarkable properties, which include layer-dependent bandgap, intrinsic n- or p-type conduction, strong light interaction, good mechanical strength, etc. Monolayer MoS2 and WSe2 have direct bandgap and enable field-effect transistor with high On/Off current ratio and strong photoresponse. A drawback is the low carrier mobility, in the order of few tens cm2V-1s-1 on substrate, and the sensitivity to oxygen, water or other adsorbates, which make unprotected devices rather unstable. Here, we discuss the current-voltage (I-V) characteristics at high drain bias of monolayer MoS2 transistors with Schottky contacts [1]. We show that oxidized Titanium contacts, due to a long air exposure, form rectifying junctions on MoS2 and cause asymmetric output characteristics, which we explain in terms of two slightly asymmetric back-to-back Schottky barriers. We show that, the highest current arises from image-force barrier lowering at the electrically forced junction, while the reverse current is due to Schottky-barrier limited injection at the grounded junction. We demonstrate that features commonly observed in MoS2 transistors, such as persistent photoconductivity and hysteresis in the transfer characteristic, are peculiarities of the MoS2 channel rather than effects of the contacts. We use transistors with ohmic contacts, at low drain bias, to deeply investigate the photoconductive and photogating effects [2]. We point out that the photoconductivity can persist with a decay time longer than 104 s, due to photo-charge trapping in extrinsic and intrinsic defects, which are also the cause of hysteresis. We highlight the important role of intrinsic donor-like defects.Finally, we demonstrate n-type conduction in WSe2 transistors that, combined with the low workfunction of WSe2, we exploit for field emission applications [4]. We show that the field emission current from a WSe2 flake can be modulated by a back gate, thus enabling a new field emission transistor
MoS2 transistors with ohmic or Schottky contacts
No full textWe discuss several features of MoS2 back-gate transistors with ohmic or Schottky contacts. We investigate important phenomena such as hysteresis, persistent conductivity and field emission of mono or bilayer MoS2
Graphene-silicon Schottky heterojunctions for optoelectronic applications
No full textThe graphene/silicon (Gr/Si) junction has been the subject of an intense research activity both for the easy fabrication and for the variety of phenomena that it allows studying. It offers the opportunity to investigate new fundamental physics at the interface between a 2D semimetal and a 3D semiconductor, and holds promises for a new generation of graphene-based devices such as photodetectors, solar cells and chemical-biological sensors. A Gr/Si junction with defect-free interface exhibits rectifying current-voltage (I-V) characteristics, which are the result of the formation of a Schottky barrier, as in traditional metal-semiconductor (M/S) Schottky diodes. The vanishing density of states at the graphene Dirac point enables Fermi level tuning and hence Schottky barrier height modulation by a single anode-cathode bias. When the Gr/Si junction is used as a photodiode, graphene acts not only as anti-reflecting and transparent conductive layer for charge transport to the external circuit, but it functions also as active material for light absorption and electron-hole generation and separation. Although most of the incident light is converted to photocharge into Si, the absorbance in graphene enables detection of photons with Si sub-bandgap energy through internal photoemission over the Schottky barrier. Photo charges injected over the Schottky barrier, under high reverse bias, can be accelerated by the electric field in the depletion region of the diode and cause avalanche multiplication by scattering with the Si lattice, thus enabling internal gain. The Gr/Si junction forms the ultimate ultra-shallow junction, which is ideal to detect light absorbed very close to the Si surface, such as near- and mid-ultraviolet. In this talk, we present the electrical characterization and the photoresponse of two types of Gr/Si devices, shown in figures 1 (b) and (c). Although due to different mechanisms, on both devices we demonstrate photo-responsivity exceeding 2.5 A/W that is competitive with present solid-state devices. We attribute it to the contribution of charges photogenerated in the surrounding region of the flat junction or to the internal gain by impact ionization caused by the enhanced field on the nano tips
Field emission from mono and two-dimensional nanostructures
No full textIn recent years, nanostructured materials have been deeply investigated for field emission applications. The exploitation of different geometries and morphologies is a key ingredient for the realization of proficient field emitter devices. In this work, we highlight the features of two differently shaped nanostructured materials such as molybdenum disulphide and gallium oxide. We show that nanoflowers and β-Ga2O3 nanopillars can achieve different field emission performance in terms of maximum current density, turn-on field and field enhancement factor
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
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
Leakage and field emission in side-gate graphene field effect transistors
No full textWe fabricate planar graphene field-effect transistors with self-aligned side-gate, using a single lithographic step. We demonstrate side-gating below 1V with conductance modulation of 35% and transconductance up to 0.5 mS/mm at 10 mV drain bias. We measure the planar leakage along the SiO2/vacuum gate dielectric over a wide voltage range, reporting rapidly growing current above 15 V. We unveil the microscopic mechanisms driving the leakage, as Frenkel-Poole transport through SiO2 up to the activation of Fowler-Nordheim tunneling in vacuum, which becomes dominant at higher voltages. We report a field-emission current density as high as 1μA/μm between graphene flakes. These findings are important for the miniaturization of atomically thin devices
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