Search

We report field emission (FE) properties of cold cathodes made by a scalable chemical vapor deposition synthesis of three-dimensional graphene (3DG) from a cast catalyst followed by cold rolling. This process leads to an increase in mechanical strength and electrical conductivity of the tested material. For a given distance between the tip of the cathode and the anode, it is found that the FE current from the edge of a single sheet of cold-rolled 3DG-based cathode can be increased by over one order of magnitude when rolling the 3DG sheet in the shape of a cylinder with several turns. A FE current in the order of 4.5 mA was measured from a 3 mm diameter cold-rolled 3DG cylinder with six turns at a bias of 2400 V for a separation of 0.5 mm between the tip of the cylindrical cathode and the anode. The FE data of all cold-rolled 3DG-based cathodes are well fitted by the expression proposed by Abbot, Henderson, Forbes, and Popov [Filippov et al., R. Soc. Open Sci. 9, 220748 (2022)], I m = C V m κ exp (− B / V m) , where I m is the FE current, V m is the bias applied between the cathode and anode, and B and C are fitting parameters. It is found that κ = 1 and 3 / 2 for FE from the surface and edge of the cold-rolled based cathodes, respectively. [ABSTRACT FROM AUTHOR]

A scattering matrix technique is used to calculate the longitudinal and transverse energy dependence of the transmission probability through various heterostructures using both the BenDaniel–Duke (BD) and the lesser known Zhu–Kroemer (ZK) boundary conditions to take into account the spatial dependence of the effective mass. We first illustrate the large difference in the transmission probabilities calculated using both boundary conditions for the simple problems of tunneling through a potential step, a single rectangular barrier, and a resonant tunneling device. Then, we present numerical calculations of the external electric field dependence of the field emission (FE) current from a n-doped GaAs semiconductor/vacuum interface using both boundary conditions, showing that the BD boundary conditions underestimate the FE current for large values of the applied external electrostatic field. A comparison of calculated FE characteristics with FE data may be a way to determine the appropriate boundary conditions to solve tunneling problems through heterostructures with spatially varying effective mass. [ABSTRACT FROM AUTHOR]

We report the design, fabrication, and measurement of the field emission (FE) characteristics of AlGaN/GaN nanoscale lateral vacuum diodes with triangular cathodes and cathode to anode spacings from 50 to 600 nm. The FE characteristics of the AlGaN/GaN diodes with metallic or AlGaN/GaN anodes show successful rectification with forward bias FE current in the range of microamperes or milliamperes, respectively, when biased within a maximum range varying from 10 to 30 V. In the forward bias mode, the measured current I m vs applied anode to cathode bias V m are well fitted to Murphy–Good profiles associated with FE at higher biases, and an Ohmic leakage profile below the threshold for FE. Our results are the first successful demonstration of FE of electrons between the two two-dimensional electron gases (2DEGs) present on both sides of a nanogap formed by electron lithography through an AlGaN/GaN heterojunction. A qualitative explanation of the loop-type FE characteristics of both AlGaN/GaN vacuum diodes, with either metallic or AlGaN/GaN anodes, is presented. [ABSTRACT FROM AUTHOR]

Ferroelectricity in hafnium zirconium oxide (Hf1 − xZrxO2) and the factors that impact it have been a popular research topic since its discovery in 2011. Although the general trends are known, the interactions between fabrication parameters and their effect on the ferroelectricity of Hf1 − xZrxO2 require further investigation. In this paper, we present a statistical study and a model that relates Zr concentration (x), film thickness (tf), and annealing temperature (Ta) with the remanent polarization (Pr) in tungsten (W)-capped Hf1 − xZrxO2. This work involved the fabrication and characterization of 36 samples containing multiple sets of metal-ferroelectric-metal capacitors while varying x (0.26, 0.48, and 0.57), tf (10 and 19 nm), and Ta (300, 400, 500, and 600 ° C). In addition to the well-understood effects of x and Ta on the ferroelectricity of Hf1 − xZrxO2, the statistical analysis showed that thicker Hf1 − xZrxO2 films or films with higher x require lower Ta to crystallize and demonstrated that there is no statistical difference between samples annealed to 500 and 600 ° C, thus suggesting that most films fully crystallize with Ta ∼ 500 ° C for 60 s. Our model explains 95% of the variability in the Pr data for the films fabricated, presents the estimates of the phase composition of the film, and provides a starting point for selecting fabrication parameters when a specific Pr is desired. [ABSTRACT FROM AUTHOR]

We present a study of Ge segregation at the surface of highly germanium-doped gallium oxide (2.5 × 1020 cm−3 nominal doping level) grown by molecular beam epitaxy. We probed the dopant concentration as a function of depth by hard x-ray photoelectron spectroscopy and standard laboratory photoemission spectroscopy. We notably found that there is germanium segregation within the top 2 nm where its concentration is 3 times the nominal doping level. This increased dopant concentration leads to a threefold enhancement of surface conductivity. The results suggest a reliable method for delta doping for power electronics applications. [ABSTRACT FROM AUTHOR]

We demonstrate field emission characteristics of β-Ga2O3 nano-pillar arrays fabricated using a damage-free etching technique. The technique utilizes Ga flux in an ultra-high vacuum environment (molecular beam epitaxy) to form high aspect ratio Ga2O3 nano-pillars with atomic-scale etching precision. Electrically conductive Ga2O3 nano-pillars with uniform widths of ∼200–300 nm were realized without the use of e-beam lithography. Furthermore, field emission characteristics on the nano-pillars displayed an emission current of 19 nA per tip at an electric field of 385 kV/cm. The field emission characteristics were modeled using the Murphy–Good model, and the measurements were validated with a field emission orthodoxy test. [ABSTRACT FROM AUTHOR]

Rapidly developing electronics industry is striving for higher energy‐storage capability dielectric capacitors for pulsed power electronic devices. Both high dielectric permittivity and high dielectric breakdown strength are essential properties to meet the desired device performance. However, due to materials limitations and their preparation requirements, there are significant challenges which limit the use of current dielectrics in high‐energy storage capacitors. In addition material limitations such as, low dielectric permittivity, low breakdown strength, and high hysteresis loss decrease these materials' energy density and efficiency, restricting potential applications. Thus, a thorough understanding of the implementation, optimization and limitations of ferroelectric, relaxor‐ferroelectric, and anti‐ferroelectric thin films in high‐energy storage dielectric capacitors is an essential and important research topic for the incorporation of these materials in near future applications. [ABSTRACT FROM AUTHOR]

(Ba0.955Ca0.045)(Zr0.17Ti0.83)O3 [BCZT] epitaxial were grown on La0.67Sr0.33MnO3 (LSMO)‐coated (100)‐oriented MgO single crystal substrates by the pulsed laser deposition (PLD) technique. Herein, we report crystal structure, ferroelectric, piezoresponse force microscopy (PFM), and energy storage properties near the morphotropic phase boundary (MPB) composition of [Ba0.850Ca0.15Zr0.1Ti0.90O3] solid solution. Epitaxial growth of the films was confirmed using X‐ray diffraction (XRD) spectra. Room‐temperature Raman spectroscopy confirms perovskite phase of BCZT films. Room‐temperature polarization‐electric field (P‐E) loops confirm the ferroelectric nature of BCZT films. Ferroelectric hysteresis loops demonstrate high saturation polarization (Pmax ~ 97.96 μC/cm2) and remanant polarization (Pr ~ 70.1 μC/cm2) at an applied maximum electric field ~2.77 MV/cm. The optimized BCZT epitaxial thin films have shown moderate dielectric properties at different measured frequencies (10‐100 kHz). Nanoscale piezoresponse force microscopy (PFM) images demonstrate switchable ferroelectric polarization of these thin films above ±9 V of dc voltage applied. Energy storage properties measured from ferroelectric loops revealed a high discharge curve energy density ~27.5 J/cm3 at a maximum electric field of 2.77 MV/cm. Epitaxial‐grown BCZT films are suitable candidate materials for capacitor applications. [ABSTRACT FROM AUTHOR]

High throughput characterization and processing techniques are becoming increasingly necessary to navigate multivariable, data-driven design challenges for sensors and electronic devices. For two-dimensional materials, device performance is highly dependent upon a vast array of material properties including the number of layers, lattice strain, carrier concentration, defect density, and grain structure. In this work, laser crystallization was used to locally pattern and transform hundreds of regions of amorphous MoS2 thin films into 2D 2H-MoS2. A high throughput Raman spectroscopy approach was subsequently used to assess the process-dependent structural and compositional variations for each illuminated region, yielding over 6000 distinct nonresonant, resonant, and polarized Raman spectra. The rapid generation of a comprehensive library of structural and compositional data elucidated important trends between structure–property processing relationships involving laser-crystallized MoS2, including the relationships between grain size, grain orientation, and intrinsic strain. Moreover, extensive analysis of structure/property relationships allowed for intelligent design and evaluation of major contributions to device performance in MoS2 chemical sensors. In particular, it is found that NO2 sensor performance is strongly dependent on the orientation of the MoS2 grains relative to the crystal plane. [ABSTRACT FROM AUTHOR]

Electron emission characteristic, electrical conductivity of polycrystalline mayenite (12CaO·7Al2O3) electride, formation of [Ca24Al28O64]4+(e−)4 framework as a function of phase content, and microstructure have been investigated. The mayenite microstructure was investigated using high‐resolution transmission microscopy which revealed the type cage structure of 12CaO·7Al2O3 partially filled by extra‐framework oxygen ions. Incorporation of electrons by means of carbon C22‐ ion template 12CaO·7Al2O3 produces complex structure, and an incomplete C22‐ ion template 12CaO·7Al2O3 structure consisting of mixture of a [Ca24Al28O64]4+(e−)4 and [Ca24Al28O64]4+(O2−)2 framework had a direct effect on the electron emission. Surface chemistry and stability of the 12CaO·7Al2O3 electride have been studied using x‐ray photoelectron spectroscopy. The work function of phase pure 12CaO·7Al2O3 electride was determined from direct thermionic emission data and compared to the measurement from ultraviolet photoelectron spectroscopy (UPS). Depending on the extent of C22‐ ion template of 12CaO·7Al2O3 structure, a work function of 0.9–1.2 eV and 2.1–2.4 eV has been measured and thermionic emission initiating at 600°C. [ABSTRACT FROM AUTHOR]

Chemical and electrical measurements of Ti/(010) β-Ga2O3 and Ti/(001) β-Ga2O3 interfaces were conducted as a function of annealing temperature using x-ray photoelectron spectroscopy (XPS), current density–voltage (J–V), and capacitance–voltage (C–V) measurements. XPS revealed partial Ti oxidation at both interfaces in the as-deposited condition, with more Ti oxidation on the (001) β-Ga2O3 epilayer surface than the (010) β-Ga2O3 substrate surface. The amount of oxidized Ti increased with annealing temperature. The Schottky barrier heights for as-deposited (unannealed) Au/Ti/(010) β-Ga2O3 and Au/Ti/(001) β-Ga2O3 contacts as determined from J–V and C–V measurements were between 0.64 and 0.83 eV. Shifts in XPS core level peaks for Ti/(010) β-Ga2O3 suggest that the Schottky barrier height decreases with temperature up to 350 °C for 10-min anneals and increases for 10-min anneals ≥460 °C. Taken together, the results suggest a strong dependence of Ti reactivity on the β-Ga2O3 surface, which can affect the electrical performance and stability of Ti/β-Ga2O3 ohmic contacts at elevated temperatures. [ABSTRACT FROM AUTHOR]

This work investigates secondary electron yield (SEY) mitigation from a metal surface with a microporous array fabricated using the laser drilling technique. We propose a general empirical model to fit the experimentally measured SEY of a flat gold surface for normal and oblique incidences of primary electrons. Using this empirical model, we develop a two-dimensional Monte Carlo (MC) simulation scheme to determine the effective SEY of a microporous array. It is found that the SEY from a porous surface is significantly reduced compared to that of the flat surface. By taking into account all the generations of secondary electrons inside a well, our MC results are found to be in very good agreement with the experimental data. The dependence of the SEY on the aspect ratio of the micropores and porosity of the surface is examined. A simple empirical formula has been proposed to evaluate the effective SEY of the gold microporous array for pores of arbitrary aspect ratios. [ABSTRACT FROM AUTHOR]