Vertical Tunnel Junctions Research Articles

Optical response of WSe2-based vertical tunneling junction

Layered materials have attracted significant interest because of their unique properties. Van der Waals heterostructures based on transition-metal dichalcogenides have been extensively studied because of potential optoelectronic applications. We investigate the optical response of a light-emitting tunneling structure based on a WSe2 monolayer as an active emission material using the photoluminescence (PL) and electroluminescence (EL) experiments performed at low temperature of 5 K. We found that the application of the bias voltage allows us to change both a sign and a value of free carriers concentrations. Consequently, we address the several excitonic complexes emerging in PL spectra under applied bias voltage. The EL signal was also detected and ascribed to the emission in a high-carrier-concentration regime. The results show that the excitation mechanisms in the PL and EL are different, resulting in various emissions in both types of experimental techniques.

Resonant Tunneling-Enhanced Photoresponsivity in a Twisted Graphene van der Waals Heterostructure.

Leveraging its ultrahigh carrier mobility, zero-bandgap linear dispersion, and extremely short response time, graphene exhibits remarkable potential in ultrafast broad-band photodetection. Nonetheless, the inherently low responsivity of graphene photodetectors, due to the low photogenerated carrier density, significantly impedes the development of practical devices. In this study, we present an improved photoresponse within a graphene-hexagonal boron nitride-graphene vertical tunnel junction device, where the crystallographic orientation of the two graphene electrodes is aligned. Through meticulous device structure design and the adjustment of bias and gate voltages, we observe a 2 orders of magnitude increase in tunneling photocurrent, which is attributed to the momentum-conserving resonant electron tunneling. The enhanced external photoresponsivity is evident across a wide temperature and spectral range and achieves 0.7 A/W for visible light excitation. This characteristic, coupled with the device's negative differential conductance, suggests a novel avenue for highly efficient photodetection and high-frequency, logic-based optoelectronics using van der Waals heterostructures.

Probing the Inelastic Electron Tunneling via the Photocurrent in a Vertical Graphene van der Waals Heterostructure.

Inelastic electron tunneling (IET), accompanied by energy transfer between the tunneling charge carriers and other elementary excitations, is widely used to investigate the collective modes and quasiparticles in solid-state materials. In general, the inelastic contribution to the tunneling current is small compared to the elastic part and is therefore only prominent in the second derivative of the tunneling current with respect to the bias voltage. Here we demonstrate a direct observation of the IET by measuring the photoresponse in a graphene-based vertical tunnel junction device. Characteristic peaks/valleys are observed in the bias-voltage-dependent tunneling photocurrent at low temperatures, which barely shift with the gate voltage applied to graphene and diminish gradually as the temperature increases. By comparing with the second-order differential conductance spectra, we establish that these features are associated with the phonon-assisted IET. A simple model based on the photoexcited hot-carrier tunneling in graphene qualitatively explains the response. Our study points to a promising means of probing the low-energy elementary excitations utilizing the graphene-based van der Waals (vdW) heterostructures.

Dependence of the vertical‐tunnel‐junction GaAs solar cell on concentration and temperature

Modelling the current–voltage (I–V) characteristics of photovoltaic (PV) devices is important for understanding their behaviour and potential. Typically, the single exponential model (SEM) is used because of its simplicity and accuracy. In this work, we validate the SEM for a gallium arsenide (GaAs) vertical-tunnel-junction (VTJ) using TCAD software. This cell is the key to overcoming the series resistance limitations of current concentrator solar cells, which can lead to the development of ultra-high concentrator photovoltaic systems (UHCPV) with concentrations (Cratio) greater than 1000 suns. The results indicate that the SEM is a suitable tool to model the I–V characteristics of the VTJ cell with mean errors lower than 1%. Moreover, the solar cell did not show any limitations regarding the Cratio and temperatures under investigation. In addition, the analysis of the temperature coefficients of the cell demonstrates that their dependency on temperature reduces as Cratio increases. The results of this work suggest that VTJ is a promising solution to produce a new generation of high-efficiency and low-cost UHCPV systems.

Single-molecule magnet Mn12 on GaAs-supported graphene: Gate field effects from first principles

We study gate field effects on the heterostructure ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}{(\mathrm{COOH})}_{16}{({\mathrm{H}}_{2}\mathrm{O})}_{4}|\mathrm{graphene}|\mathrm{GaAs}$ via first-principles calculations. We find that under moderate doping levels, electrons can be added to but not taken from the single-molecule magnet ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}{(\mathrm{COOH})}_{16}{({\mathrm{H}}_{2}\mathrm{O})}_{4}$ (${\mathrm{Mn}}_{12}$). The magnetic anisotropy energy (MAE) of ${\mathrm{Mn}}_{12}$ decreases as the electron-doping level increases due to electron transfer from graphene to ${\mathrm{Mn}}_{12}$ and change in the band alignment between ${\mathrm{Mn}}_{12}$ and graphene. At an electron-doping level of $\ensuremath{-}5.00\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.16em}{0ex}}\text{cm}{}^{\ensuremath{-}2}$, the MAE decreases by about 18% compared with zero doping. The band alignment between graphene and GaAs is more sensitive to electron doping than to hole doping, since the valence band of GaAs is close to the Fermi level. The GaAs substrate induces a small band gap in the supported graphene under zero gate field and a nearly strain-free configuration. Finally, we propose a vertical tunnel junction for probing the gate dependence of MAE via electron transport measurements.

Room Temperature Synthesis of Colossal Magneto-Resistance of La2/3Ca1/3MnO3: Ag0.10 Composite

Rare-earth manganite-based perovskite has great potential as a promising material for spintronics and ferroelectromagnets. Herein, we have synthesized La2/3Ca1/3MnO3:Silverx (LCMO-Agx; where x = 0.00 and 0.10) composite using a standard solid-state reaction route. Their structural and physical properties have been investigated. Pristine LCMO and LCMO-Ag composite are crystallized in an orthorhombic structure, which is in a single-phase and has a space group of Pbnm. Pristine LCMO and LCMO-Ag composite’s structural analysis showed better grain connectivity in ferromagnetic domains of LCMO-Ag composite compared to pristine LCMO. Ag doping enhances the paramagnetic-ferromagnetic transition Tc (Curie temperature) to 277 K, which is 8 K higher than that of pristine LCMO (Tc = 269 K). Additionally, the magneto-resistance (MR) of LCMO-Ag composite was improved by ∼10% with Ag doping even at room temperature (RT), which is due to improved connectivity and grain size with Ag doping. Thus, the enhanced value of MR at RT may efficiently open up the possible use of LCMO-Ag composite as ferroelectromagnets and spintronics applications. Additionally, LCMO thin films can be useful in artificial planar junctions, vertical tunnel junctions, and sensing applications.

Microfabricated Nano-Gap Tunneling Current Zika Virus Sensors With Single Virus Detection Capabilities

Electron tunneling through thin ( ~ 1-50 nm), insulating materials can provide information regarding their band structure, density of states, surface and bulk defect densities and their energy levels and distributions. While electron tunneling has been used in thin inorganic insulating materials for nearly 7 decades, its applications in organic and biological materials, with exceptions of deoxyribonucleic acid and a few other materials, are not extensively explored. Here we report room temperature electron tunneling studies to differentiate between aptamers designed to attach to the capsid proteins on Zika viruses and the aptamer-Zika complexes. We show that the electron tunneling induced by the current versus voltage (I-V) measurements through these materials contain information that enable identification of the Zika virus without any need for DNA sequencing or additional molecular labels. The electron tunneling measurements were carried out using a conducting atomic force microscope, a home-made vertical tunneling junction and an electromigration-induced nanogap in a microfabricated 1 μm -wide gold line on silicon nitride bridges. The current through aptamers and aptamer-Zika complexes all have a threshold voltage just before the exponential increase in the junction current. Below this voltage, the leakage currents are different for the aptamer and aptamer-Zika samples. The tunnel nanogap device is suitable for detecting viruses that are nanometer-scale biomaterials.

Numerical optimisation and recombination effects on the vertical-tunnel-junction (VTJ) GaAs solar cell up to 10,000 suns

Ultra-high concentrator photovoltaic systems (UHCPV), usually referred to CPV systems exceeding 1000 suns, are signalled as one of the most promising research avenues to produce a new generation of high-efficiency and low-cost CPV systems. However, the structure of current concentrator solar cells prevents their development due to the unavoidable series resistance losses at such elevated concentration ratios. In this work, we investigate the performance of the so-called vertical-tunnel-junction (VTJ), recently introduced by the authors, by using advance TCAD. In particular, we carry out an optimisation procedure of the key parameters that affect its performance and conduct a deep investigation of the impact of the main recombination mechanisms and of sun concentration up to 10,000 suns. The results indicate that the performance of the novel structure is not significantly affected by these two factors. A record efficiency of 32.2% at 10,000 suns has been found. This represents a promising way to obtain state-of-the-art efficiencies above 30% for single-band-gap cells, and offers a new route towards the development of competitive CPV systems operating at ultra-high concentration fluxes.

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Double-Gate TFET With Vertical Channel Sandwiched by Lightly Doped Si

This paper examines a tunnel field-effect transistor (TFET) as a promising device for achieving steeper switching and better electrical performances in low-power operation. It features a double-gate TFET with vertical channel sandwiched by lightly doped Si (VS-TFET). The vertical tunnel junction is employed on the source side for the steeper subthreshold swing (SS) and for the higher ON-current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ) by restricting tunnel barrier width. The VS-TFET shows 17-mV/dec minimum SS and ${10}^{ {4}}~{ \mathrm{\scriptstyle ON}}/{ \mathrm{\scriptstyle OFF}}$ current ratio ( ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ) for sub-0.7-V gate overdrive. In addition, the VS-TFET shows sub-60-mV/dec SS in a wide range of ${I}_{D}$ regardless of sweep directions. In conclusion, the work presented here demonstrates that the VS-TFET will be one of the most promising candidates for a next-generation low-power device.

Investigation of Electrical Characteristics of Vertical Junction Si n-Type Tunnel FET

In this paper, Si tunnel FETs (TFETs) with the vertical tunneling junction (nVTFETs) were demonstrated by utilizing fabrication processes compatible to those used for conventional Si MOSFETs. The vertical tunneling junction was realized by forming n-type surface pocket regions under gate overlap regions of p-type source extensions only using the ion implantation (I/I). The impacts of carbon (C) coimplantation and formation of offset spacer (OSS) on the TFET performance were examined. As a result, the performance of the fabricated nVTFETs was enhanced by applying the C coimplantation and the OSS formation before the I/I for the vertical tunneling junctions. It has been found, on the other hand, that nVTETs exhibit a reverse short-channel effect, where the threshold voltage increases with decreasing the channel length, and that this effect is correlated with the enhanced temperature dependence of subthreshold slope. These phenomena are attributable to the existence of a thermionic barrier between the source junction and the channel, formed by boron diffusion from the source region.

Vertical Tunnel Junction Embedding a Spin Crossover Molecular Film

AbstractThin films of a molecular spin crossover (SCO) Iron(II) complex featuring a high transition temperature are grown by sublimation in high vacuum on TSAu and investigated by X‐ray and UV photoelectron spectroscopies. Temperature‐dependent studies demonstrate that the thermally induced spin crossover behavior is preserved in thin films. A large‐area ultrathin switchable spin crossover molecular vertical tunnel junction with top electrodes of the liquid eutectic of gallium and indium, for which the spin‐state switching of the films induces a two orders of magnitude change in the tunneling current density flowing through the junction, is reported here. The results on large‐area junctions, rationalized by density functional theory calculations, demonstrate the high potential of SCO‐based switchable molecular junctions as functional devices.

High Quality Oxide Films Deposited at Room Temperature by Ion Beam Sputtering

ABSTRACTWe have deposited dense and pinhole-free thin films of SiO2, Al2O3 and ITO at room temperature via ion beam sputtering. The SiO2 films were found to be of similar quality as thermal oxide with a resistivity greater than 1015 Ω·cm and breakdown field in excess of 7 MV/cm. The Al2O3 films were part of a Pt- Al2O3-Pt vertical tunnel junction and were kept extremely thin, from 2 nm to 4 nm. The current-voltage characteristics of these junctions indicated a breakdown field in excess of 20 MV/cm, roughly twice that achieved by ALD films. This breakdown voltage was found to be independent of junction area, strongly suggesting the absence of pinholes in the film. The ITO films were 50 nm to 100 nm thick. As deposited, they are fully transparent with an electrical resistivity of 5x10-4 Ω·cm.

Spin filter effect in iron oxide nanocrystal arrays

Integrating nanocrystals (NCs) into magnetic tunnel structures is of considerable interest due to expectation of novel properties from their spin selective transport and single electron features. Superstructures by colloidal NCs having translational and orientational order and interesting collective magnetic properties can be prepd. by soln. casting through sensitive interparticle and particle-substrate interactions. In this work, we discuss the study on magnetic field induced assembly of mono-dispersed iron oxide NCs to obtain spin filter effect across the superlattice array, when sandwiched between gold electrodes. The deposition of mixed phase Fe3O4@γ-Fe2O3 NCs on SiO2/Au surface proceeds through slow solvent evapn. and are studied for controlled interparticle spacing. For specific NC concn., the ordering depends on the substrate chem. and the ligands passivating NC surface, which affects the concn. of cluster nuclei formed. In presence of a magnetic field, the tunnel structure exhibits enhanced pos. tunnel magnetoresistance at low temps., which could be related to their ferromagnetism and the attempts by electrons to percolate NC superlattice with preserved spin. A sign reversal for magnetoresistance is exhibited by the vertical tunnel junctions on raising the temp.

Silicon–germanium nanowire tunnel-FETs with homo- and heterostructure tunnel junctions

Experimental results on tunneling field-effect transistors (TFETs) based on strained SiGe on SOI nanowire arrays are presented. A heterostructure SiGe/Si TFET with a vertical tunnel junction consisting of an in situ doped SiGe source and a Si channel with a minimum inverse subthreshold slope of 90mV/dec is demonstrated. An increase in tunneling area results in higher on-current. The in situ doped heterojunction TFET shows great improvement compared to a homojunction SiGe on SOI nanowire design with implanted junctions. Temperature dependent measurements and device simulations are performed in order to analyze the tunnel transport mechanism in the devices.

ROOM-TEMPERATURE ANTIFERROMAGNETIC TUNNELING ANISOTROPIC MAGNETORESISTANCE

We systematically investigate the room-temperature antiferromagnetic tunneling anisotropic magnetoresistance (TAMR) effect in [Pt/Co] / IrMn/AlO x/ Pt vertical tunnel junctions as a function of IrMn thicknesses and field directions. The experimental results indicate that a partial rotation of IrMn moments is triggered by ferromagnetic Co/Pt with the formation of exchange-spring in vertical fields, whereas two distinct modes of positive and negative spin-valve-like signals are detected with different in-plane fields due to the unidirectional anisotropy of IrMn . Electrical transport and magnetization reversal measurements confirm the crucial role of stable antiferromagnetic IrMn moments in the observed TAMR effect, which is profoundly affected by the operation temperature, IrMn thicknesses, magnetic fields and field directions.

Spintronics: perspectives for the half-metallic oxides

Due to their total spin polarization (∼100%), the half-metallic oxides, such as La0.7Sr0.3MnO3 (LSMO), CrO2, Fe3O4 and Sr2FeMoO6, are promising materials for producing very high magneto-resistive (MR) responses in magnetic devices. Extremely large MR values have been obtained by tunneling through vertical tunnel junctions, as well as through nano-interfaces or domain walls in planar devices. The state of the art of the spin-polarized half-metallic devices in a two-electrode configuration is described. Moreover, some recent results, that confirm the high spin polarization of these magnetic oxides, are detailed. In case of planar devices, nanofabrication is shown to allow transferring of nanopatterns in a sub-50 nm scale. The technical challenges to integrate these half-metallic oxides devices in the industrial context of magnetic random access memories (MRAMs) are finally discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ fabrication of vertical tunnel junctions for SET devices

A modified fabrication technique for vertical metallic tunnel junctions is presented. This approach for metallic single electron transistors (SETs) is based on the in situ growth of Ti–TiO X –Ti layer stacks embedded between chromium and gold contacts. The sequence of process steps as well as complementary nanoanalytic investigations are presented and discussed within the frame of data obtained by electrical characterization.

Surface-field-induced transverse and vertical tunnel junctions

We report on tunneling between a 2D electronic system at the semiconductor/insulator interface influenced by an electric surface field and a 3D counterelectrode with tunneling directions parallel and perpendicular to the surface. Junctions of Yb and Au to electron and hole inversion layers, respectively, show thermally activated tunneling where the tunneling probability is controlled by the electric surface field. Vertical junctions consist of an Yb/oxide/semiconductor tunnel junction on a degenerate p-type Si, InAs, and InSb substrate. The low work function leads to a high electric field that inverts the semiconductor surface and forms a Zener tunnel junction between the electron layer and the bulk valence states. Current-voltage characteristics of junctions on Si are similar to backward diodes with phonon-induced structures at low temperatures. InAs junctions show room-temperature negative differential conductance with peak-to-valley ratios up to 2.7 increasing up to 15 at low temperatures. InSb junctions, which are nearly ohmic at room temperature, yield typical current ratios of 11. Junctions on high-quality InAs substrates exhibit a resonance-like characteristic which is explained in terms of wave vector conservation law.