Fowler Nordheim Research Articles

Comparison of electrochemical response and electric field emission characteristics of pristine La2NiO4 and La2NiO4/CNT composites: Origin of multi-functionality with theoretical penetration by density functional theory

Electrochemical performances of as-prepared La2NiO4 and La2NiO4@CNT based electrode are investigated in a 1M KOH solution using a three-electrode configuration. The La2NiO4 electrode delivers the specific capacity (Csp) 284 mAh g−1 at a current density 1.5 A g−1 with rate capability 74.5% at a current density 12 A g−1 and its value intensified up to 426 mAh g−1 with capability 82.1% at identical current density due to incorporation of CNT. Long-range stability and Nyquist plot (Z′vs.Z″) of the La2NiO4@CNT electrode exhibit better cyclic performances with capacity retentivity 93% after 3000 cycles and excellent charge transfer characteristics than pristine La2NiO4 electrode (capacity retentivity 85.6%). Fowler–Nordheim tunneling in electric field emission of CNT modified La2NiO4 exhibits superior current density (J~1705 µA/cm2) with inferior turn-on@1 µA/cm2 and threshold field@10µA/cm2 1.92 V/µm and 2.15 V/µm respectively. The enhanced electronic states near Fermi level for hybrid La2NiO4/CNT leading to improved conducitivity, increase mobility of electrolytic ions in the open space of CNTs and higher quantum capacitance support the improved charge storage performance. There is charge transfer from La2NiO4 to CNT due to interaction between d orbitals of Ni and La with 2p orbital of C. Also, reduction in work function for hybrid La2NiO4/CNT compared to pristine La2NiO4, introduction of defects near the boundary and improved conductivity attribute towards higher emission current observed in the experiment.

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 (FE) 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. FE occurs with a turn-on field of ∼80 and attains a current density higher than 10 , following the general Fowler–Nordheim model with high reproducibility.

Composite Metallic Nano Emitters Coated with a Layer of Insulator Covered by Au Layer

Abstract: In this work, the differences in the behavior and properties of the emitted electron beam from tungsten (W) tips were studied before and after coating these tips with a thin layer of dielectric material followed by a thin layer of gold, to improve the emission current density, stability and emission current pattern concentration. The core of the composite cathode is made of high-purity tungsten (W). Measurements have been made with clean W emitters before and after coating these tips with two types of epoxy resins (epoxy 478 resins or epoxy UPR- 4 resins) followed by a thin layer of gold. For critical comparison, several tungsten tips with various apex-radii have been prepared using electrochemical etching techniques. The emitters have been coated by dielectric thin films of various thicknesses and the layer of Au used for coating the Epoxy layer has the same thicknesses. Their behavior has been recorded before and after the process of coating. These measurements include the current-voltage (I-V) characteristics and Fowler-Nordheim (F-N) plots. Imaging has also been done using a visible light microscope (VLM), along with a scanning electron microscope (SEM) to help in characterizing the epoxy layer thickness on the tip surface after coating. Besides, the emission patterns have been recorded from the phosphorescent screen of a field electron emission microscope (FEM). Having two types of composite systems tested under similar conditions provided several advantages. These measurements helped in producing a new type of emitters that have more suitable features with each of the two resins. Keywords: Cold field emission, Nano emitter, Dielectric coating, Au layer.

Analysis of the Various Effects of Coating W Tips with Dielectric Epoxylite 478 Resin or UPR-4 Resin Coatings under Similar Operational Conditions

Abstract: The objective of this work is to study the differences that occur in behavior and properties of the emitted electron beam from tungsten (W) tips before and after coating these tips with a thin layer of a good proven dielectric material. Core metallic tips have been prepared from a polycrystalline (99.995% purity) tungsten (W) wire. Analysis has been carried out for clean W emitters before and after coating these tips with two differences types of epoxy resins; namely: (Epoxylite 478 and UPR-4). For critical comparison and analysis, several tungsten tips with various apex- radii (very sharp) have been prepared with the use of electrochemical etching techniques. The tips have been coated by dielectric thin films of various thicknesses. Their characteristics have been recorded before and after the process of coating. These measurements have included the current-voltage (I-V) characteristics, Fowler-Nordheim (F-N) plots, visible light microscope (VLM) image and scanning electron microscope (SEM) micrographs to measure the influence of the Epoxylite resin coating’s thickness on the tips after coating. Special distributions have been recorded from the phosphorescent screen of a field electron emission microscope as well. Comparing the two sets of composite systems tested under similar conditions has provided several advantages. Recording highly interesting phenomena has produced a wide opportunity to develop a new type of emitter that includes the most beneficial features of both types. Keywords: Cold field emission, Epoxylite 478, Epoxylite UPR-4.

Field Emission Air-Channel Devices as a Voltage Adder.

Field emission air-channel (FEAC) devices can work under atmospheric pressure with a low operation voltage when the electron channel is far less than the mean free path (MFP) in the air, thereby making them a practical component in circuits. Forward and reverse electron emissions of the current FEAC devices demonstrated symmetric Fowler–Nordheim (F–N) plots owing to the symmetric cathode and anode electrodes. This research aimed to demonstrate the arithmetic application of the FEAC devices, their substrate effect, and reliability. A voltage adder was composed of two FEAC devices whose two inputs were connected to two separate function generators, and one output was wire-connected to an oscilloscope. The devices were on a thin dielectric film and low-resistivity silicon substrate to evaluate the parasitic components and substrate effect, resulting in frequency-dependent impedance. The results show that the FEAC devices possessed arithmetic function, but the output voltage decreased. The FEAC devices were still capable of serving as a voltage adder after the reliability test, but electric current leakage increased. Finite element analysis indicated that the highest electrical fields and electron trajectories occur at the apices where the electrons travel with the shortest route less than the MFP in the air, thereby meeting the FEAC devices’ design. The modeling also showed that a sharp apex would generate a high electric field at the tip-gap-tip, enhancing the tunneling current.

Theoretical analysis of the transition from field emission to space-charge-limited emission in liquids and gases

Discharge formation and breakdown in liquids have critical implications for water purification, medicine, and combustion, thereby motivating characterization of liquid behavior under high voltages. While several experiments with dielectric liquids have demonstrated that current transitions from Fowler–Nordheim (FN) scaling for field emission to Mott–Gurney (MG) scaling for space-charge-limited emission (SCLE) with increasing voltage, there is no common model linking electron emission from the liquid to gas phase. We show that applying a theory unifying FN, MG, and the Child–Langmuir law (CL) for SCLE at vacuum as a function of electron mobility and gap distance [Darr et al., Appl. Phys. Lett. 114, 014103 (2019)] yields excellent agreement with dielectric liquid emission experiments [Dotoku et al., J. Chem. Phys. 69, 1121 (1978)]. Specifically, current follows FN scaling at lower voltages before space charge contributions dominate, although none of the dielectric liquids considered achieve MG scaling in the experimental regime of interest. Considering a higher mobility representative of a vapor in the theory demonstrates the feasibility of achieving CL scaling for the gaps of the size considered experimentally at reasonably achievable applied voltages. Increasing the gap distance by an order of magnitude eliminates the contribution of space charge; decreasing gap distance by an order of magnitude causes a transition to MG. The implications of these results on electron emission in liquids and during a phase change to vapor and gas will be discussed.

Regression Models for the Field Electron Emission Signal

The field electron emission signal is described using the regression approach with three models. Each regression function has its own theoretical justification. The three-parameter models make it possible to take into account the deviations of the I‒V characteristics from a straight in the Fowler–Nordheim coordinates in both the high- and low-voltage region. For clear comparison, investigated data samples are obtained in the numerical experiment with specified signal parameters. A special noise model is considered, in which the use of conventional linearization over the parameters seems fair. Attention is paid to checking the regression residuals for normality of their distribution. The parameters of the proposed models are cross estimated and their statistical significance with a confidence level of 95% is verified. It is shown that, for each set of experimental data, a separate model may be necessary. The adjusted coefficient of determination is used as a measure of the dependence of the response on the factor.

Suspended Nanoscale Field Emitter Devices for High-Temperature Operation

In this work, we demonstrate suspended two- and four-terminal field emission devices for high-temperature operation. The planar structures were fabricated with tungsten on a 200-nm silicon nitride membrane. The insulator in the vicinity of the terminals was removed to minimize undesirable Frenkel–Poole emission and increase the resistance of leakage current pathways. The effects of temperatures up to 450 °C on Fowler–Nordheim emission characteristics and parasitic leakage resistance were studied. Turn-on voltages with magnitudes under 15 V that further decreased as a function of increasing temperature for the two-terminal device were reported. Gating at temperatures of 150 °C and 300 °C was shown for the four-terminal device, and corresponding transconductance and cutoff frequency values were computed.

Electrical characteristics, stability, electromigration, Joule heating, and reliability aspect of focused ion beam fabricated gold and copper nanobar interconnects on SiO2 and glass substrates

The electrical characteristics and stability of rectangular nanobar interconnects are investigated owing to their importance and reliability concern in electronic devices. One dimensional gold and copper nanobars (cross section 150–180 × 80–150 nm2 and length 3.0–5.0 μm), fabricated by milling of respective thin films with a 30 keV Ga+ ion probe (size 10–20 nm) at a current of ∼1 nA, are studied for their current bearing capacity and temperature profile caused by Joule heating. The temperature attained is shown to depend on the length with a maximum lying at the bar center. The electromigration of species (drift velocity for gold being ∼0.92 nm/s) forms void and induces breakage in the bar at a current density of ∼1011 A m−2. The phenomenon is governed by the bar length, prevailing temperature gradient, crystal defects, and grain boundaries. The thermo-migration process facilitates or impedes the electromigration effects depending upon the direction of the thermal gradient and electric field. The I–V characteristics of a gold bar with a gap of ∼44 nm under a vacuum of ∼10−6 mbar follow a classical Child–Langmuir V3/2 law in the voltage range of 10–45 V, but the copper electrodes with a large gap of ∼250 nm (created by ion milling) demonstrate V0.05-dependence up to 32 V, V1/2-law at 39–58 V, and Fowler–Nordheim emission [with an effective area of 1600 nm2 and a field enhancement factor of 8.1] above 66 V.

Electrical Properties of Al2O3/ZnO Metal–Insulator–Semiconductor Capacitors

An Al2O3 thin film is assembled on an unin- tentionally doped ZnO for metal–insulator–semiconductor (MIS) capacitors. Leakage current density ( ${J}$ ), capacitance–voltage ( ${C} - {V}$ ) properties, and their thermal stabilities are investigated. The ${J}$ -values for the MIS capacitors before and after annealing at 300 °C are lower than $2.0 \times 10^{-7}$ A/cm2 at a gate voltage of −10.0~10.0V. Frequency-dependent ${C}-{V}$ characteristic shows a stable maximum capacitance for the MIS capacitor before annealing. However, it decreases dramatically at 1 MHz for that after annealing at 300 C, which is attributed to the change of ZnO channel resistance. Flat band voltage shifts in the–curves after annealing at 300 C are one order lower than those before annealing, indicating the decrease of positive fixed charge density in the Al2O3 film. There are ledge phenomena for hysteresis characteristics in the curves, which result possibly from the delayed evacuation of minority carriers in deep depletion region. By considering the Fowler–Nordheim tunneling model, band configurations of the Al2O3/ZnO heterojunctions are clarified to be straddling gap structures with conduction band offsets larger than 1.3 eV.

Temperature dependent polarization-switching behavior in Hf0.5Zr0.5O2 ferroelectric film

Ferroelectricity and polarization-switching behavior were investigated in Hf0.5Zr0.5O2 films from 25°C to 150°C. It is found that a large remnant polarization (Pr) of about 30 μC/cm2 is achieved, and Hf0.5Zr0.5O2 films behave a good stability in a wide range of temperature. By analyzing the physical mechanism of leakage current, it indicates that electron conduction mechanism obeys Schottky emission at low voltages, and Fowler-Nordheim (F-N) tunneling mechanism along with space-charge-limited-conduction (SCLC) at high voltages. The temperature dependent polarization switching and related leakage current behaviors reveal a strong thermal activation of the oxygen vacancies, which can explain the varieties of ferroelectricity and leakage current under different temperatures.

Consideration on SiO2/4H-SiC Band Alignment Modulation by NO Annealing

Post oxidation annealing (POA) process in NO ambient is one of the standard techniques to control 4H-SiC MOS characteristics, which introduces nitrogen atoms to the surface of SiC by substituting the topmost carbon atoms [1]. Together with the reduction of interface state density, however, a significant negative shift of threshold voltage is often caused by this process [2]. In this study, we clarified that such negative shift is not only caused by fixed charge introduction but originates from an anomalous modulation of band alignment of SiO2/4H-SiC interface.First we systematically studied the flat-band voltage (VFB) of SiO2/4H-SiC (0001) n-type MOS capacitors with various oxide thickness, fabricated by conventional dry oxidation followed by the POA in NO/N2 = 1/2 ambient at 1150°C. From the oxide thickness dependence of the VFB we can deduce the expected value of VFB when the effects of fixed charges are removed. As a result we found an anomalous negative shift of VFB irrespective of oxide thickness, which was enlarged by extending NO annealing duration [3]. This phenomenon would be explainable by assuming a dipole layer formation at the interface by SiC surface nitridation.Next this phenomenon was investigated from the viewpoint of SiO2/4H-SiC band alignment change by two kinds of analyzing methods: the valence-band spectra analysis using x-ray photoelectron spectroscopy (XPS) and the energy barrier height characterization of Fowler-Nordheim (F-N) currents. By systematic comparisons of the valence-band XPS of SiO2/SiC stack with a few-nm-thick SiO2 and that of SiC substrate without SiO2, we could see a clear trend that the valence-band offset between SiC and SiO2 changes monotonically by the NO-POA durations. Since no change of bandgap of SiO2 was suggested from O1s loss spectrum, this phenomenon was regarded as the band alignment change by the nitridation of SiC surface. To our surprise, this trend appeared in opposite direction for 4H-SiC (0001) and (000-1) stacks: the NO-POA increases the conduction-band offset for a few hundreds of meV in case of 4H-SiC (0001) stack, but it decreases in the case of 4H-SiC (000-1) stack for a similar amount [3]. For the F-N current analysis, the I-V characteristics at -150°C were evaluated where we confirmed the leakage currents were perfectly dominated by F-N tunneling mechanism. We could clearly see the trend of decreasing of F-N tunneling current for 4H-SiC (0001) MOS capacitors by extending NO annealing duration which was attributable to the increase of conduction-band offset, while the opposite trend was observed for 4H-SiC (000-1) MOS capacitors. These trends are quite consistent with the results of XPS analysis. Thus we could conclude that the observed change of VFB shift by NO-POA as described above was determined by the band alignment change at SiO2/4H-SiC interface. A significant difference of the band alignment on (0001) and (000-1) was also clarified which would be the reason for the disagreement of VFB of the capacitors on those crystal faces.Since the nitrogen atoms replacing the topmost carbon atoms on SiC surface are considered to be coordinated by three Si atoms, this replacement may induce an additional polarization at the interface. Considering the fact that high density of nitrogen (~1014 atoms/cm2) are introduced [1] mostly in the top of SiC, a significant magnitude of polarization in the order of hundreds of meV would be achievable. We speculate this array of polarized structure at the SiC surface would be observed as a formation of interface dipole layer. With this model we can clearly explain the reason why 4H-SiC (0001) and (000-1) interfaces showed the dipole layer formation toward different direction by nitrogen introduction, taking account of the opposite Si-N bond arrangements on those interfaces. We consider that this phenomenon is one of the essential reasons to cause unexpected threshold voltage shifts often reported for SiC MOSFETs with NO-POA.[1] J. Rozen et al., J. Appl. Phys. 105, 124506 (2009).[2] G. Chung et al., Appl. Surf. Sci. 184, 399 (2001).[3] T. H. Kil and K. Kita, Appl. Phys. Lett. 116, 122103 (2020).

An infrared energy harvesting device using planar cross bowtie nanoantenna arrays and diode-less rectification based on electron field emission

ABSTRACT In this paper, we present a rectification process through electron emission from sharp tips under a high electric field of 109–1010 V/m based on the Fowler–Nordheim theory. A large planar array of 104–105 nanoantennas is proposed to achieve the required electric field for electron emission in the nanoantenna gap. Spatial coherence of the incident wave is a prohibitive issue in using large antenna arrays, and it is overcome by choosing proper size for the array. The design starts with a single-element nanoantenna, for which, five different types are considered, namely dipole, spiral, cross bowtie, sinuses and square spiral. Full-wave simulations show that cross bowtie has the highest field enhancement. Moreover, two array configurations, i.e. series and parallel feed networks, are considered and their field enhancement are compared. Moreover, field enhancement in large array configurations is predicted using proposed equations based on curve fitting.

Iron oxide-coated MWCNTs nanohybrid field emitters: a potential cold cathode for next-generation electron sources

In this paper, we have reported that the field emission properties of multiwall carbon nanotubes (MWCNTs) were significantly increased by decorating their surface by iron oxide nanoparticles. MWCNTs were prepared on silicon substrate through low-pressure chemical vapour deposition using acetylene as source gas. The iron oxide nanoparticles were grown on the surface of MWCNTs by thermal evaporation technique. Modified surface morphologies of the prepared films were characterized through field emission scanning electron microscope (FESEM), Raman spectroscopy and X-ray diffraction. A significant change in current density, stability and turn-on voltage has been observed in iron oxide-coated MWCNT films. Decoration of iron oxide nanoparticles reduces turn-on voltage from 4 to 3.4 V/μm, while current density increases from 10.15 to 33.26 mA/cm2. For practical MWCNT-based field emission devices, it is necessary to improve the emission current density and stability. The excellent field emission parameters are obtained and calculated which make them useful for high-performance field emission-based devices. The mechanism of emission of electrons is well described by the Fowler Nordheim theory.

Control of the Schottky Barrier and Contact Resistance at Metal–WSe2 Interfaces by Polymeric Doping

AbstractTungsten diselenide (WSe2) is attracting attention because of its superior electronic and optoelectronic properties. In recent years, the number of research works related to the WSe2‐based field‐effect transistors (FETs) has increased dramatically. Nonetheless, the performance of 2D WSe2 is influenced sensitively by metal–semiconductor (MS) interface states, where Fermi‐level pinning is substantial. This research explores Fermi‐level depinning by doping with an n‐type polymer. In this work, spin‐coated polyvinyl alcohol (PVA) is used as an n‐type dopant for achieving low‐contact‐resistance WSe2 FETs in cases of both high‐work‐function (Pd) and low‐work‐function (In) metals. Interestingly, the increase in the Schottky barrier height resulting from the application of PVA gives rise to Fowler–Nordheim tunneling for a doped Pd‐WSe2 contact. By contrast, only direct tunneling is observed for an In‐WSe2 contact irrespective of whether the dopant is used. The barrier‐height modification after doping reveals that the improvement of the contact resistance is correlated to the enhancement of tunneling current after doping, which is consistent with the measurement results. This work suggests a practical direction for contact engineering of future WSe2‐based electronic devices and expands the current understanding of charge transport at the MS contact when a polymeric n‐type dopant is applied.

Effects of annealing on the interfacial properties and energy-band alignment of AlN dielectric on 4H–SiC

Interfacial properties and energy-band alignment of annealed AlN dielectric films on Si-face 4° off-axis 4H–SiC substrates were characterized and demonstrated by x-ray photoelectron spectroscopy (XPS) and current-electric field (I–E) measurements. The XPS results reveal that the Al–O bonds and the silicon suboxides can convert into more stable Al–N bonds and Al–O–Si bonds at the AlN/4H–SiC interface under 1000 °C annealing. The variations in both oxygen-rich composition and the crystallinity in AlN make annealing-dependent conduction band offsets of as-deposited and annealed AlN/4H–SiC to be 1.36 eV and 1.20 eV, respectively. Meanwhile, I–E measurements separately yield the occurrence of the Fowler–Nordheim (FN) tunneling and space-charge-limited conduction in as-deposited and annealed metal-insulator-semiconductor capacitors, corresponding to lower barrier heights of 0.92 eV and 0.54 eV, respectively. The reason for the energy-band shift between I–E derivations and the XPS results was analyzed and demonstrated together. These results can provide considerable insight into the energy-band alignment of AlN as gate dielectric or passivation layers on 4H–SiC based devices.

A Test for Compliance with the Cold Field Emission Regime Using the Elinson–Schrednik and Forbes–Deane Approximations (Murphy–Good Plot)

A test that allows one to determine the compliance of the experimental data with the classical (orthodox) field dependence is described. The test is presented in two versions: using the Elinson–Schrednik approximation of the current–voltage characteristics in the classical Fowler–Nordheim coordinates, as well as the modern Forbes–Deane approximation with modified Murphy–Good coordinates. The use of the test is demonstrated by the study of a multitip emitter based on carbon nanotubes. A technique of online (real-time) calculation of the effective parameters of the emitter and testing is presented.

3D AND-Type Stacked Array for Neuromorphic Systems.

NOR/AND flash memory was studied in neuromorphic systems to perform vector-by-matrix multiplication (VMM) by summing the current. Because the size of NOR/AND cells exceeds those of other memristor synaptic devices, we proposed a 3D AND-type stacked array to reduce the cell size. Through a tilted implantation method, the conformal sources and drains of each cell could be formed, with confirmation by a technology computer aided design (TCAD) simulation. In addition, the cell-to-cell variation due to the etch slope could be eliminated by controlling the deposition thickness of the cells. The suggested array can be beneficial in simple program/inhibit schemes given its use of Fowler–Nordheim (FN) tunneling because the drain lines and source lines are parallel. Therefore, the conductance of each synaptic device can be updated at low power level.

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 µm2. 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 = 17 V µm−1 for a cathode–anode separation distance = 500 nm. We show that for increasing separation distance, increases up to about 35 V µm−1 and β decreases to ∼100 at = 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.

Selective photosensitivity of metal–oxide–semiconductor structures with SiOx layer annealed at high temperature

SiOx layers with x = 1.15 and 1.3 and thicknesses of 50 and 100 nm were deposited on crystalline silicon wafers by thermal evaporation in vacuum. Part of the samples were annealed at 250 °C (control samples), while the rest were annealed at temperatures of 700, 800, and 1000 °C. The high-temperature annealing leads to phase separation and formation of Si nanoclusters in a dielectric matrix with higher x than the initial one. Metal–Oxide–Semiconductor structures were fabricated and characterized by current–voltage and capacitance–voltage measurements. The relative permittivity (er) and refractive index (n) of the layers depend on the annealing temperature and vary in the 5.4–6.9 (er) and 1.72–1.90 (n) ranges for x = 1.15. For x = 1.3, er and n vary in the 5.1–6.5 and 1.67–1.78 ranges, respectively. All annealed samples showed selective photosensitivity as the responsivity increases with the photon energy. The observed effect is explained by assuming that the blue and UV lights provide sufficient energy to photocarriers generated in the Si wafer to be injected into the SiOx layer at relatively low electric fields, ~ 0.3 MV/cm for layers annealed at 700 and 800 °C and ~ 1 MV/cm for those annealed at 1000 °C (x = 1.15). It was found that the current through the annealed layers under UV and blue light illumination is controlled by Fowler–Nordheim tunneling. The structures with x = 1.3 show higher selectivity to UV light but lower responsivity compared to structures with x = 1.15. No selective photosensitivity was observed in the control samples.