Fowler-Nordheim Tunneling Research Articles

Realizing improved performance of metal-insulator-semiconductor diodes with high-k MgO/SiOx stack

With the development of metal-insulator-semiconductor (MIS) field-effect transistors, the thickness of silicon dioxide (SiO2) must be scaled down; however, it causes a large gate leakage current and a low on/off current ratio. Magnesium oxide (MgO) exhibits a dielectric constant as high as 11.2, which is about three times larger than that (3.9) of SiO2; thus the thickness of MgO can be three times greater than that of SiO2 and significantly reduces the gate leakage current. Nevertheless, a large lattice mismatch (22.5%) exists between MgO and Si materials. In the study, the MgO/SiOx stack dielectrics are employed to suppress the large lattice mismatch and improve the performance of MIS diodes. The MgO/SiOx stack largely reduces the oxygen vacancy of MgO from 75% to 52.3%; also the fixed oxide charge and interface trap charge density are drastically decreased from 1.3 × 1012 and 1.8 × 1013 to 8.2 × 1011 cm−2 and 7.8 × 1012 cm−2eV−1, respectively. Compared to the MIS diodes with a single MgO, the MgO/SiOx stack dielectrics suppress the gate leakage current by about two orders in MIS diodes; hence the rectification ratio is enhanced from 114 to 1032 at ±2V. Band-diagram shows that Fowler-Nordheim tunneling occurs at a high reverse-bias voltage, and the barrier height increases from 0.18 to 0.42 eV in the MIS diodes with single MgO and MgO/SiOx stack dielectrics, respectively. Nevertheless, the MIS diodes operate through a direct tunneling at low reverse-bias voltages.

Interface engineering by inserting Al2O3 tunneling layer to enhance the performance of graphene/GaAs heterojunction photodetector

Interface engineering is an effective way to improve the performance of graphene-based photodetectors. Here, a graphene/GaAs heterojunction photodetector is fabricated, and when inserting an Al2O3 tunneling layer, its performance is improved through direct tunneling (DT) and Fowler-Nordheim tunneling (FNT). According to the experimental results, it is found that the thickness of the tunneling layer has a great influence on the performance of the photodetector. Compared with graphene/GaAs photodetector, the performance of graphene/Al2O3(2 nm)/GaAs photodetector is significantly improved with the responsivity, detectivity, and external quantum efficiency value of 0.80 A/W, 3.02 × 1011 Jones and 306% under 1 mW/cm2 light intensity at 2 V bias. Meanwhile, fast response is also observed (rise/decay time of 3 ms/8.6 ms). The improvement of the photodetector's performance in this work is mainly attributed to the effective modification of the interface state by the Al2O3 tunneling layer and the effect of the two tunneling mechanisms based on DT and FNT.

Flat band voltage (VFB) drift by integrated reference electrode work function in Electrolyte Insulator Semiconductor (EIS) devices

Electrolyte-insulator-semiconductor (EIS) devices were fabricated using titanium nitride (TiN), gold (Au) and platinum (Pt) as integrated reference electrodes. Here, Au and Pt were deposited by RF sputtering and TiN was deposited by DC sputtering. The devices were developed with different geometries and using silicon (Si) n and p type wafers. A titanium oxide (TiO2) film was used as a pH sensitive membrane in all EIS devices. Capacitance versus voltage (C-V) measurements were used to determine the sensitivity of the devices from their interaction with solutions of pH: 1, 3, 4, 7, 9, 10 and 12. From the analysis of the C-V curves for the EIS capacitors with integrated Pt and Au reference electrodes, two well-defined sensitivity regions are observed: one for acidic and neutral pH solutions, and one for basic pH solutions. This type of behavior is due to the difference between the values of the work functions of Au and Pt, in relation to that of Si. This difference between the work functions causes an increase in the curvature of the energy bands, as the pH value of the solution to be analyzed increases. As a result, leakage currents are generated by Schottky emission and Fowler-Nordheim (F-N) tunneling, causing incorrect displacements in C-V responses when interacting with different pH solutions, and with this, the formation of two regions of sensitivity. For EIS capacitors with TiN as an integrated reference electrode, this effect is not observed because the value of its work function is remarkably close to that of Si.

Microscopic Quantum Transport Processes of Out‐of‐Plane Charge Flow in 2D Semiconductors Analyzed by a Fowler–Nordheim Tunneling Probe

AbstractWeak interlayer couplings at 2D van der Waals (vdW) interfaces fundamentally distinguish out‐of‐plane charge flow, the information carrier in vdW‐assembled vertical electronic and optical devices, from the in‐plane band transport processes. Here, the out‐of‐plane charge transport behavior in 2D vdW semiconducting transition metal dichalcogenides (SCTMD) is reported. The measurements demonstrate that, in the high electric field regime, especially at low temperatures, either electron or hole carrier Fowler–Nordheim (FN) tunneling becomes the dominant quantum transport process in ultrathin SCTMDs, down to monolayers. For few‐layer SCTMDs, sequential layer‐by‐layer FN tunneling is observed to dominate the charge flow, thus serving as a material characterization probe for addressing the Fermi level positions and the layer numbers of the SCTMD films. Furthermore, it is shown that the physical confinement of the electron or hole carrier wave packets inside the sub‐nm thick semiconducting layers reduces the vertical quantum tunneling probability, leading to an enhanced effective mass of tunneling carriers.

Generalizing Fowler–Nordheim Tunneling Theory for an Arbitrary Power Law Barrier

Herein, the canonical Fowler–Nordheim theory is extended by computing the zero‐temperature transmission probability for the more general case of a barrier described by a fractional power law. An exact analytical formula is derived, written in terms of Gauss hypergeometric functions, that fully capture the transmission probability for this generalized problem, including screened interaction with the image potential. First, the quality of approximation against the so far most advanced formulation of Fowler–Nordheim, where the transmission is given in terms of elliptic integrals, is benchmarked. In the following, as the barrier is given by a power law, in detail, the dependence of the transmission probability on the exponent of the power law is analyzed. The formalism is compared with results of numerical calculations and its possible experimental relevance is discussed. Finally, it is discussed how the presented solution can be linked in some specific cases with an exact quantum‐mechanical solution of the quantum well problem.

Gate leakage mechanisms of the AlGaN/GaN HEMT with fluorinated graphene passivation

The AlGaN/GaN high-electron-mobility transistor (HEMT) passivated by fluorinated graphene (FG) has been investigated. The performances of the HEMTs without passivation before and after F plasma treatment, with FG, Silicon nitride (SiNx), FG/SiNx passivation were contrasted. Compared with the HEMT without F plasma treatment and passivation, the gate leakage current decreases one and a half orders of magnitude after F plasma treatment and SiNx passivated, and four orders of magnitude after FG and FG/SiNx passivated. To study the leakage mechanisms, the temperature dependences of gate leakages have been tested at 323 K–473 K. Through analysis and fitting, the leakages are composed of three components, which are thermionic emission (TE) current, two-dimensional variable range hopping (2D-VRH) current, and Fowler–Nordheim tunneling (FNT) current. TE dominates in the high forward bias region. 2D-VRH dominates in low forward bias. And the chief leakage mechanisms at reverse bias are 2D-VRH and FNT. At 473 K, Schottky barrier height of the HEMT with FG passivation is 1.21 eV. The activation energy and the effective barrier height on the HEMT with FG passivation are 0.64 eV and 0.33 eV respectively. Furthermore, the reasons for the decrement of the gate leakage on the HEMT with FG and FG/SiNx passivation are the enhancement of Schottky barrier heights and augmentation of activation energy. And the improvement in gate leakages for the HEMT passivated by FG/SiNx mainly results from the effect of FG.

Study of digital and analog resistive switching memories based on methylammonium lead iodide (MAPbI3) perovskite by experiments and DFT calculations

In this study, a thin film of methylammonium lead iodide (MAPbI3) was employed as the switching layer in the metal/MAPbI3/FTO devices. Two metals, Ag and Cr, were used as active and inert top electrodes to govern the hysteresis effect of memory devices, respectively. While the Cr/MAPbI3/FTO device displayed an analog resistive switching (RS) behavior and a comparatively low ON/OFF ratio of 10, the Ag/MAPbI3/FTO structure displayed digital bipolar RS and a high ON/OFF ratio of 102. The density functional theory simulations suggest that these various behaviors may be caused by variations in the mutual interaction between the iodine vacancy defect and the metal contact properties. The SET process involved switching both devices from a high-resistance state to a low-resistance one using the space charge-limited current mechanism. In the RESET process, the Ag-electrode device is allocated the Poole–Frenkel emission mechanism, and the Cr-electrode device is followed the Fowler–Nordheim tunneling mechanism. The formation and dissociation of iodine vacancy filaments via the dielectric layer were identified as the RS mechanism in both devices. The findings show that organic–inorganic hybrid perovskite has a strong potential for data storage.

Regulating optoelectronics of carbon dots with redox-active dopamine

The electron transfer (ET) processes in carbon-based quantum dots (CDs) have laid to extensive research endeavors due to their tunable optoelectronic properties with alteration in surface functionalization, doping, surface charge, etc. Here, we proffer shreds of evidence on pH-dependent electron transfer and transport properties of undoped CDs (UCDs) and amine-doped CDs (ACDs) with a redox-active neurotransmitter, dopamine (DA). A pronounced pH-dependent photoluminescence (PL) quenching is depicted for both the CDs with DA, where the quenching efficiency substantially increases in alkaline solvents. Taking advantage of the structural transformation of DA from hydroquinone to quinone, the electron accepting capacity can be improved for alkaline pH, which enhances the ET efficiency. The pH-regulated conductance measurements across the metal-CD-metal junction reveal a hike in the conductivity for acidic and alkaline pH relative to the neutral one. The I-V traces of UCDs and ACDs are contributed by an initial linear current rise and non-linear growth at higher bias explicating both direct and Fowler-Nordheim (F-N) tunneling mechanisms. In addition, the incorporation of DA into these CDs, not only significantly increases conductance in all pH media but also enables them to tunnel the potential barrier directly in acidic media, reducing F-N tunneling. These results could provide new avenues for developing reliable optoelectronic devices and sensing probes for a wide range of applications.

Exploring optoelectronic properties of undoped and amine-doped carbon dots: Impact of surface functionalization and pH

Tuning optoelectronic properties through surface modifications and doping in low dimensional carbon dots (CDs) has been an asset in designing them for versatile applications. In this article, we explore the modulations in photophysical and charge transport characteristics of doped and undoped CDs functionalized with redox-active dopamine (DA). The pH-dependent DA chemistry on the surface of CDs is observed to have deterring effect on excited state dynamics with superior charge transfer efficiency for pristine CDs compared with the doped one. Charge transport studies through conducting measurements across electrode-CDs-electrode junction report a pH-induced current enhancement with DA functionalization resulting from a shifted equilibrium among the reduced (hydroquinone) and oxidized (quinone) isomers of DA. While the changes in the optical and electronic features are significantly perturbed with pH for the DA-treated undoped CDs, nominal alterations in spectral features are reported for the doped ones. The current-voltage conductance across the junctions is influenced by surface-treated CDs for acidic pH, with signatures of direct tunneling for a low bias regime, curtailing the Fowler-Nordheim (FN) tunneling. These features can have immediate optoelectronic-based applications in pH-based sensors and molecular electronics.

Comprehensive analysis of current leakage at individual screw and mixed threading dislocations in freestanding GaN substrates

The electrical characteristics of Schottky contacts on individual threading dislocations (TDs) with a screw-component in GaN substrates and the structures of these TDs were investigated to assess the effects of such defects on reverse leakage currents. Micrometer-scale platinum/GaN Schottky contacts were selectively fabricated on screw- and mixed-TD-related etch pits classified based on the pit size. Current–voltage (I–V) data acquired using conductive atomic force microscopy showed that very few of the screw TDs generated anomalously large reverse leakage currents. An analysis of the temperature dependence of the I–V characteristics established that the leakage current conduction mechanisms for the leaky screw TDs differed from those for the other screw and mixed TDs. Specifically, anomalous current leakage was generated by Poole–Frenkel emission and trap-assisted tunneling via distinctive trap states together with Fowler–Nordheim tunneling, with the mechanism changing according to variations in temperature and applied voltage. The leaky TDs were identified as Burgers vector b = 1c closed-core screw TDs having a helical morphology similar to that of other screw TDs generating small leakage currents. Based on the results, we proposed that the atomic-scale modification of the dislocation core structure related to interactions with point defects via dislocation climbing caused different leakage characteristics of the TDs.

Bias dependent NDR in TiO2/NiO heterojunction diodes

TiO2/NiO thin film heterojunction diodes are fabricated using electron beam evaporation and DC sputtering techniques. Three different thicknesses of NiO (58, 82, and 160 nm) are combined with a TiO2 film of about 80 nm thick. The common device configuration is FTO/TiO2/NiO/Au with a 4 mm sq. device area. The thickness dependent diode I-V characteristics were analyzed; with the increase in thickness, the diode knee voltage, series resistance, and ideality factor have reduced. All the devices exhibit a high diode ideality factor, which means the diode currents are not limited by drift, diffusion, or recombination at the space charge region. The high ideality factor points to the presence of interface states, spatial inhomogeneities, or tunneling in the devices. Further, the bias dependent negative differential resistance (NDR) in I-V characteristics and the presence of an anomalous peak in C-V characteristics were analyzed, which claimed the presence of interfacial trap states, defects, or tunneling in the diodes. The Fowler-Nordheim tunnel model was applied to the devices and the field enhancement factors were calculated as −14.3, −9.3, and −9.2 for the diodes in the increasing order of thickness. The tunneling phenomenon was confirmed as trap-assisted indirect tunneling. Also, the temperature dependence of NDR was studied.

Ultrahigh Rectification Ratio in an Asymmetric Metal/Semiconductor/Metal Nanoscale Tunneling Junction: Implications for High-Frequency Rectifiers

The ultrafast quantum-based electron-transport mechanism with a typical tunneling time of femtoseconds is appealing to fabricate high-frequency rectifier diode devices for applications such as solar energy harvesting, THz mixers, and infrared detectors. Here, we demonstrate an ultrahigh rectification of more than 104 and a large forward current density of 13.8 A/cm2 at −2 V in the structure of a metal/semiconductor/metal (MSM, Pt/ZnO/TiN) nanoscale tunneling junction. Technology computer-aided design (TCAD) simulations demonstrate that high rectification originates from the switching of the electron-transport mechanism between direct tunneling (DT) and Fowler–Nordheim tunneling (FNT) under positive and negative voltage biases, respectively. The quantum-based FNT mechanism gives very slight temperature dependence of currents in a wide temperature range of 100–400 K. This work opens an avenue for high-frequency rectifier diodes based on MSM nanoscale tunneling junctions.

An in-depth investigation of gate leakage current degradation mechanisms in 1.2 kV 4H-SiC power MOSFETs

In this paper, a comprehensive study on the forward and reverse current degradation mechanisms in 1.2 kV planar SiC MOSFETs are investigated. With the help of Sentaurus TCAD simulations, numerical fitting method, step-bias stress, current-voltage (I-V) and capacitance-voltage (CV) experiments, we proposed defect-related transport models to describe the progression of gate leakage currents under forward and reverse high biases. It is found that, (1) trap-assisted tunneling (TAT) and Fowler-Nordheim (FN) tunneling dominate the forward low and high current transport process respectively. Moreover, the holes trapping is mainly responsible for the degradation of gate oxide and premature breakdown of SiC MOSFETs; (2) the reverse leakage current under high electric field is mainly carried by FN tunneling electrons, which lead to an increase of the intermediate localized states distributed in the SiC/SiO2 interface.

Highly Sensitive MoS2 Photodetectors Enabled with a Dry-Transferred Transparent Carbon Nanotube Electrode.

Fabricating electronic and optoelectronic devices by transferring pre-deposited metal electrodes has attracted considerable attention, owing to the improved device performance. However, the pre-deposited metal electrode typically involves complex fabrication procedures. Here, we introduce our facile electrode fabrication process which is free of lithography, lift-off, and reactive ion etching by directly press-transferring a single-walled carbon nanotube (SWCNT) film. We fabricated Schottky diodes for photodetector applications using dry-transferred SWCNT films as the transparent electrode to increase light absorption in photoactive MoS2 channels. The MoS2 flake vertically stacked with an SWCNT electrode can exhibit excellent photodetection performance with a responsivity of ∼2.01 × 103 A/W and a detectivity of ∼3.2 × 1012 Jones. Additionally, we carried out temperature-dependent current-voltage measurement and Fowler-Nordheim (FN) plot analysis to explore the dominant charge transport mechanism. The enhanced photodetection in the vertical configuration is found to be attributed to the FN tunneling and internal photoemission of charge carriers excited from indium tin oxide across the MoS2 layer. Our study provides a novel concept of using a photoactive MoS2 layer as a tunneling layer itself with a dry-transferred transparent SWCNT electrode for high-performance and energy-efficient optoelectronic devices.

Crossover from direct to trap assisted Fowler Nordheim (FN) tunneling in CoFeB/MgO/CoFeB magnetic tunnel junctions

Electron conduction mechanisms in CoFeB (0.8–10 nm)/MgO (3 nm)/CoFeB (4.2 nm) magnetic tunnel junctions (MTJs) have been investigated in detail. A clear crossover from direct tunnelling to trap assisted Fowler Nordheim (FN) tunneling is observed with increasing bias voltage in all the pristine MTJs. In contrast, FN-like tunneling is completely suppressed in vacuum annealed MTJs, which plausibly indicates diffused interfaces and enhanced trap state density in the MgO barrier. In annealed MTJs, beyond the direct tunneling regime, bulk-limited Pool Frenkel emission followed by interface-limited Schottky emission are found to be the dominant transport mechanisms. Simulations of tunnel current density J(V) and differential conductance (dJ/dV) of MTJs have been carried out using Simmons (symmetric rectangular barrier) and Brinkman (asymmetric trapezoidal barrier) model, which provides valuable insights into the barrier height and interface property at the MgO/CoFeB interface. Results of this study might be helpful to further improve CoFeB/MgO based MTJs for efficient implementation in sensors and memory devices.

Anisotropic charge transfer and gate tuning for p-SnS/n-MoS2 vertical van der Waals diodes.

2D-material-based van der Waals heterostructures (vdWhs) have shown great potential in next-generation multi-functional microelectronic devices. Thanks to their sharp interface and ultrathin thickness, 2D p-n junctions with high rectification properties have been established by combining p-type monochalcogenides with n-type transition metal dichalcogenides. However, the anisotropic rectification together with the charge transfer and gate effect has not been clarified. Herein, the electrical anisotropy of p-SnS/n-MoS2 diodes was studied. Optimum ideality factors within 1.08-1.18 have been achieved for the diode with 6.6 nm thick SnS on monolayer MoS2, and a high rectification ratio of 3.1 × 104 with strong in-plane anisotropy is observed along the zigzag direction of SnS. A strong gate effect on the anisotropic series resistance has been verified and an effective tuning over the transport length of the SnS channel can be established through adjustment of the current orientation and gate voltage. A thickness-dependent minority carrier transport mechanism has also been demonstrated for the reverse drain current, and Fowler-Nordheim tunneling and direct tunneling are proposed for the increase of the reverse current of the thicker and thinner diodes, respectively. This work will provide another strategy for high-performance diodes based on vdWhs via the control of the current orientation and the gate effect.

Pseudo-flexible resistive switching characteristics of nano-bowl-like NiO arrays on mica substrates

Nano-bowl-like NiO (nb-NiO) arrays on flexible Pt-coated mica substrates have been fabricated by electro-deposition and magnetron sputtering with the assistance of the monolayer colloidal sphere template. The nano-bowl array makes the polycrystalline NiO film thickness change periodically in the range of 150–450 nm, and the HfO2 buffer layer effectively depresses the local leakage current. A voltage which is lower than ±0.4 V can make the Ag/HfO2/nb-NiO/Pt cell reversibly switch between the high and the low resistance states within 11 μs. This cell exhibits a ratio of the high to the low resistance (RH/RL) of 103 ∼ 104, a data retention time over 104 s and an endurance of >1200 cycles. Driven by the non-uniform electric field, oxygen ions accumulated at the HfO2/NiO interface prefer to occur the Fowler-Nordheim tunneling at the thinnest bowl bottom, which decreases switching voltages. The bent cell almost keeps initial resistive switching properties before the bending radius (r) is below 2.5 cm. Though its properties greatly degrade when the r further reduces below 1.5 cm, it can recover most of initial memory window after returning to the flat state again, showing potential applications in new types of pseudo-flexible memory devices.

Ultrafine Nickel Sulfide-Based Bipolar Resistive Switching Device as Artificial Synapses for Neuromorphic Application

Biologically inspired systems, particularly those that mimic the nervous system of living beings, are becoming more demanded due to their ability to solve ill-posed problems such as pattern recognition or communication with the external environment. Memristors are essential components to replicate high-density networks of biological synapses that control the effectiveness of communication among neurons and implement learning capability because of their tunable conductance. In this study, an organic–inorganic hybrid system of hexamethylenediamine-stabilized ultrafine nickel sulfide particles was synthesized by employing a complexation-mediated route. Here, we propose a nickel sulfide-based memristor as an artificial synapse for neuromorphic application. The current–voltage behavior of the device exhibited bipolar resistive switching with a stable ON and OFF state with an ON/OFF ratio value of 2.5 × 101. The high-conductance state of the device showed the Ohmic conduction mechanism, and the low-conductance state of the device exhibited the Fowler–Nordheim tunneling mechanism. Using identical and nonidentical pulses, the synaptic plasticity behavior of the device was investigated, which revealed the inverse-symmetric and mirror-symmetric patterns, respectively. The device mimicked the spike-time-dependent plasticity properties for Hebbian learning with a conductance value change from −86 to 91%. We also designed the spiking neural network, consisting of 18 synapses and 11 integrated firing neurons, based on the winner-take-all strategy for unsupervised feature learning.

Fundamental Mechanisms Behind the Reverse Characteristic of Cu(In,Ga)Se2 Solar Cells

Partial shading of PV modules can lead to degradation of the shaded cells. The degradation originates from a reverse bias voltage over the shaded cells. In order to mitigate reverse bias damage in Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) modules, a good understanding of the fundamental mechanisms governing the reverse characteristic is required. In this study, a model is introduced that describes this behavior for CIGS cells. In this model, the low and non-Ohmic leakage current is accounted for by the space charge limited current component. A sharp increase in current that is typically observed in the CIGS reverse characteristics can be described by Fowler–Nordheim tunneling. This model has been validated against measurements performed at different temperatures and illumination intensities, and is able to describe the dependencies of the reverse bias behavior on both temperature and illumination.

Self-rectifying and artificial synaptic characteristics of amorphous Ta2O5 thin film grown on two-dimensional metal-oxide nanosheet

Amorphous Ta2O5 (ATa2O5) films were grown on Sr2Nb3O10/TiN/SiO2/Si (SN-TSS) at room temperature. Sr2Nb3O10 (SN) metal-oxide nanosheets were used as a tunneling barrier to induce self-rectifying properties in ATa2O5 memristors. An ATa2O5 thin film grown on one SN monolayer exhibited a typical bipolar switching curve without any self-rectifying property, indicating that one SN monolayer cannot function as a tunneling barrier. This ATa2O5 thin film showed switching behavior owing to the formation and breakage of oxygen vacancy (OV) filaments. Self-rectifying characteristics were observed in an ATa2O5 thin film grown on two SN monolayers, which behaved as a tunneling barrier in the Pt/ATa2O5/SN-TSS memristor. The current conduction of this ATa2O5 thin film in the high-resistance state (HRS) is explained by Schottky emission, direct tunneling, and Fowler–Nordheim (FN) tunneling. In the low-resistance state (LRS), this ATa2O5 thin film shows insulating behavior, indicating that OV filaments were not formed. The current conduction of this ATa2O5 thin film in the LRS was attributed to direct tunneling and FN tunneling. Moreover, ATa2O5 thin films grown on two SN monolayers exhibited artificial synaptic properties. Therefore, the ATa2O5 thin film can be used as an artificial synapse with a cross-sectional array structure.