arXiv subjects

Filippo Giubileo

Publications and source records attributed to Filippo Giubileo.

At least 19 recordsLinked to original sources

arXiv2021

Memory effects in black phosphorus field effect transistors

We report the fabrication and the electrical characterization of back-gated field effect transistors with black phosphorus channel. We show that the hysteresis of the transfer characteristic, due to intrinsic defects, can be exploited to realize non-volatile memories. We demonstrate that gate voltage pulses allow to trap and store charge inside the defect states, which enable memory devices with endurance over 200 cycles and retention longer than 30 minutes. We show that the use of a protective poly (methyl methacrylate) layer, positioned on top of the black phosphorus channel, does not affect the electrical properties of the device but avoids the degradation caused by the exposure to air.

cond-mat.mes-hall↗

arXiv2021

Coexistence of negative and positive photoconductivity in few-layer PtSe2 field-effect transistors

Platinum diselenide (PtSe_2) field-effect transistors with ultrathin channel regions exhibit p-type electrical conductivity that is sensitive to temperature and environmental pressure. Exposure to a supercontinuum white light source reveals that positive and negative photoconductivity coexists in the same device. The dominance of one type of photoconductivity over the other is controlled by environmental pressure. Indeed, positive photoconductivity observed in high vacuum converts to negative photoconductivity when the pressure is rised. Density functional theory calculations confirm that physisorbed oxygen molecules on the PtSe_2 surface act as acceptors. The desorption of oxygen molecules from the surface, caused by light irradiation, leads to decreased carrier concentration in the channel conductivity. The understanding of the charge transfer occurring between the physisorbed oxygen molecules and the PtSe_2 film provides an effective route for modulating the density of carriers and the optical properties of the material.

cond-mat.mtrl-sci↗

arXiv2020

Gate-controlled field emission current from MoS$_2$ nanosheets

Monolayer molybdenum disulfide (MoS$_2$) nanosheets, obtained via chemical vapor deposition onto SiO$_2$/Si substrates, are exploited to fabricate field-effect transistors with n-type conduction, high on/off ratio, steep subthreshold slope and good mobility. The transistor channel conductance increases with the reducing air pressure due to oxygen and water desorption. Local field emission measurements from the edges of the MoS$_2$ nanosheets are performed in high vacuum using a tip-shaped anode. It is demonstrated that the voltage applied to the Si substrate back-gate modulates the field emission current. Such a finding, that we attribute to gate-bias lowering of the MoS$_2$ electron affinity, enables a new field-effect transistor based on field emission.

physics.app-ph↗

arXiv2020

Field emission from AlGaN nanowires with low turn-on field

We 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 1$μm^2$. The field emission characteristics are analyzed 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/$μ$m 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/$μ$m 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.

physics.app-ph↗

arXiv2020

Field emission from two-dimensional GeAs

GeAs is a layered material of the IV-V groups that is attracting growing attention for possible applications in electronic and optoelectronic devices. In this study, exfoliated multilayer GeAs nanoflakes are structurally characterized and used as the channel of back-gate field-effect transistors. It is shown that their gate-modulated p-type conduction is decreased by exposure to light or electron beam. Moreover, the observation of a field emission current demonstrates the suitability of GeAs nanoflakes as cold cathodes for electron emission and opens up new perspective applications of two-dimensional GeAs in vacuum electronics. Field emission occurs with a turn-on field of ~80 V/μm and attains a current density higher than 10 A/cm^2, following the general Fowler-Nordheim model with high reproducibility.

physics.app-ph↗

arXiv2020

Field emission characteristics of InSb patterned nanowires

InSb nanowire arrays with different geometrical parameters, diameter and pitch, are fabricated by top-down etching process on Si(100) substrates. Field emission properties of InSb nanowires are investigated by using a nano-manipulated tungsten probe-tip as anode inside the vacuum chamber of a scanning electron microscope. Stable field emission current is reported, with a maximum intensity extracted from a single nanowire of about 1$μA$, corresponding to a current density as high as 10$^4$ A/cm$^2$. Stability and robustness of nanowire is probed by monitoring field emission current for about three hours. By tuning the cathode-anode separation distance in the range 500nm - 1300nm, the field enhancement factor and the turn-on field exhibit a non-monotonic dependence, with a maximum enhancement $β\simeq $ 78 and a minimum turn-on field $E_{ON} \simeq$ 0.033 V/nm for a separation d =900nm. The reduction of spatial separation between nanowires and the increase of diameter cause the reduction of the field emission performance, with reduced field enhancement ($β<$ 60) and increased turn-on field ($E_{ON} \simeq $ 0.050 V/nm). Finally, finite element simulation of the electric field distribution in the system demonstrates that emission is limited to an effective area near the border of the nanowire top surface, with annular shape and maximum width of 10 nm.

cond-mat.mes-hall↗

arXiv2019

Field Emission Characterization of MoS2 Nanoflowers

Nanostructured materials have wide potential applicability as field emitters due to their high aspect ratio. We hydrothermally synthesized MoS2 nanoflowers on copper foil and characterized their field emission properties, by applying a tip-anode configuration in which a tungsten tip with curvature radius down to 30-100nm has been used as the anode to measure local properties from small areas down to 1-100um2. We demonstrate that MoS2 nanoflowers can be competitive with other well-established field emitters. Indeed, we show that a stable field emission current can be measured with a turn-on field as low as 12 V um-1 and a field enhancement factor up to 880 at 600nm cathode-anode separation distance.

cond-mat.mes-hall↗

arXiv2019

Pressure-Tunable Ambipolar Conduction and Hysteresis in Ultrathin Palladium Diselenide Field Effect Transistors

A few-layer palladium diselenide (PdSe2) field effect transistor is studied under external stimuli such as electrical and optical fields, electron irradiation and gas pressure. We observe ambipolar conduction and hysteresis in the transfer curves of the PdSe2 material unprotected and as-exfoliated. We tune the ambipolar conduction and its hysteretic behavior in the air and pure nitrogen environments. The prevailing p-type transport observed at room pressure is reversibly turned into dominant n-type conduction by reducing the pressure, which can simultaneously suppress the hysteresis. The pressure control can be exploited to symmetrize and stabilize the transfer characteristic of the device as required in high-performance logic circuits. The transistor is immune from short channel effects but is affected by trap states with characteristic times in the order of minutes. The channel conductance, dramatically reduced by the electron irradiation during scanning electron microscope imaging, is restored after several minutes anneal at room temperature. The work paves the way toward the exploitation of PdSe2 in electronic devices by providing an experiment-based and deeper understanding of charge transport in PdSe2 transistors subjected to electrical stress and other external agents.

cond-mat.mes-hall↗

arXiv2018

A WSe2 vertical field emission transistor

We report the first observation of gate-controlled field emission current from a tungsten diselenide (WSe2) monolayer, synthesized by chemical-vapour deposition on SiO2/Si substrate. Ni contacted WSe2 monolayer back-gated transistors, under high vacuum, exhibit n-type conduction and drain-bias dependent transfer characteristics, which are attributed to oxygen/water desorption and drain induced Schottky barrier lowering, respectively. The gate-tuned n-type conduction enables field emission, i.e. the extraction of electrons by quantum tunnelling, even from the flat part of the WSe2 monolayers. Electron emission occurs under an electric field ~100 V μm^(-1) and exhibit good time stability. Remarkably, the field emission current can be modulated by the back-gate voltage. The first field-emission vertical transistor based on WSe2 monolayer is thus demonstrated and can pave the way to further optimize new WSe2 based devices for use in vacuum electronics.

cond-mat.mes-hall↗

arXiv2018

Asymmetric Schottky Contacts in Bilayer MoS2 Field Effect Transistors

We discuss the high-bias electrical characteristics of back-gated field-effect transistors with CVD-synthesized bilayer MoS2 channel and Ti Schottky contacts. We find that oxidized Ti contacts on MoS2 form rectifying junctions with ~0.3 to 0.5 eV Schottky barrier height. To explain the rectifying output characteristics of the transistors, we propose a model based on two slightly asymmetric back-to-back Schottky barriers, where 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. The device achieves a photo responsivity greater than 2.5 AW-1 under 5 mWcm-2 white-LED light. By comparing two- and four-probe measurements, we demonstrate that the hysteresis and persistent photoconductivity exhibited by the transistor are peculiarities of the MoS2 channel rather than effects of the Ti/MoS2 interface.

cond-mat.mes-hall↗

arXiv2018

Effect of Electron Irradiation on the Transport and Field Emission Properties of Few-Layer MoS2 Field Effect Transistors

Electrical 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 is 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 three order of magnitudes after the exposure to an irradiation dose of 100e-/nm2. Finally, a complete field emission characterization of the MoS2 flake, achieving emission stability for several hours and a minimum turn-on field of about 20 V/um with a field enhancement factor of about 500 at anode-cathode distance of 1.5um, demonstrates the suitability of few-layer MoS2 as two-dimensional emitting surface for cold-cathode applications.

physics.app-ph↗

arXiv2017

Graphene-Silicon Schottky diodes for photodetection

We present the optoelectronic characterization of two graphene/silicon Schottky junctions, fabricated by transferring CVD-graphene on flat and nanotip-patterned n-Si substrates, respectively. We demonstrate record photo responsivity, 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.

cond-mat.mes-hall↗

arXiv2017

Field emission from self-catalyzed GaAs nanowires

We report observation of field emission from self-catalyzed GaAs nanowires grown on Si (111). The measurements are realized inside a scanning electron microscope chamber with nano-controlled tungsten tip functioning as anode. Experimental data are analyzed in the framework of Fowler-Nordheim theory. We demonstrate stable current up to 10$^{-7}$ A emitted from the tip of single nanowire, with field enhancement factor $β$ up to 112 at anode-cathode distance d=350 nm. A linear dependence of $β$ on the anode-cathode distance is experimentally found. We also show that the presence of a Ga catalyst droplet suppresses the emission of current from the nanowire tip. This allows detection of field emission from the nanowire sidewalls, which occurs with reduced field enhancement factor and stability. This study further extends the GaAs technology to vacuum electronics applications.

cond-mat.mes-hall↗

arXiv2017

Hysteresis in the transfer characteristics of MoS2 transistors

We investigate the origin of the hysteresis observed in the transfer characteristics of back-gated field-effect transistors with an exfoliated MoS2 channel. We find that the hysteresis is strongly enhanced by increasing either gate voltage, pressure, temperature or light intensity. Our measurements reveal a step-like behavior of the hysteresis around room temperature, which we explain as water-facilitated charge trapping at the MoS2/SiO2 interface. We conclude that intrinsic defects in MoS2, such as S vacancies, which result in effective positive charge trapping, play an important role, besides H2O and O2 adsorbates on the unpassivated device surface. We show that the bistability associated to the hysteresis can be exploited in memory devices.

cond-mat.mes-hall↗

arXiv2017

The role of contact resistance in graphene field-effect devices

The 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.

cond-mat.mes-hall↗

arXiv2017

Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors

We study electrical transport properties in exfoliated molybdenum disulfide (MoS2) back-gated field effect transistors at low drain bias and under different illumination intensities. It is found that photoconductive and photogating effect as well as space charge limited conduction can simultaneously occur. We point out that the photoconductivity increases logarithmically with the light intensity and can persist with a decay time longer than 10^4 s, due to photo-charge trapping at the MoS2/SiO2 interface and in MoS2 defects. The transfer characteristics present hysteresis that is enhanced by illumination. At low drain bias, the devices feature low contact resistance of 1.4 kΩ/μm, ON current as high as 1.25 nA/μm, 10^5 ON-OFF ratio, mobility of 1 cm^2/Vs and photoresponsivity R=1 A/W.

cond-mat.mes-hall↗

arXiv2017

Hybrid Graphene/Silicon Schottky photodiode with intrinsic gating effect

We propose a hybrid device consisting of a graphene/silicon (Gr/Si) Schottky diode in parallel with a Gr/SiO2/Si capacitor for high-performance photodetection. The device, fabricated by transfer of commercial graphene on low-doped n-type Si substrate, achieves a photoresponse as high as 3 AW^(-1) and a normalized detectivity higher than 3.5 10^12 cmHz^(1/2) W^(-1) in the visible range. The device exhibits a photocurrent exceeding the forward current, because photo-generated minority carriers, accumulated at Si/SiO2 interface of the Gr/SiO2/Si capacitor, diffuse to the Gr/Si junction. We show that the same mechanism, when due to thermally generated carriers, although usually neglected or disregarded, causes the increased leakage often measured in Gr/Si heterojunctions. At room temperature, we measure a zero-bias Schottky barrier height of 0.52 eV, as well as an effective Richardson constant A**=4 10^(-5) Acm^(-2) K^(-2) and an ideality factor n=3.6, explained by a thin (< 1nm) oxide layer at the Gr/Si interface.

cond-mat.mes-hall↗

arXiv2016

Observation of field emission from GeSn nanoparticles epitaxially grown on silicon nanopillar arrays

We apply molecular beam epitaxy to grow GeSn-nanoparticles on top of Si-nanopillars patterned onto p-type Si wafers. We use X-ray photoelectron spectroscopy to confirm a metallic behavior of the nanoparticle surface due to partial Sn segregation as well as the presence of a superficial Ge oxide. We report the observation of stable field emission current from the GeSn-nanoparticles. We prove that field emission can be enhanced by preventing GeSn nanoparticles oxidation or by breaking the oxide layer through electrical stress. Finally, we show that GeSn/p-Si junctions have a rectifying behavior.

cond-mat.mes-hall↗