Wide Band Gap Research Articles

Fundamentals and emerging optical applications of hexagonal boron nitride: a tutorial

Hexagonal boron nitride (hBN), also known as white graphite, is a transparent layered crystal with a wide bandgap. Its crystal structure resembles graphite, featuring layers composed of honeycomb lattices held together through van der Waals forces. The layered crystal structure of hBN facilitates exfoliation into thinner flakes and makes it highly anisotropic in in-plane and out-of-plane directions. Unlike graphite, hBN is both insulating and transparent, making it an ideal material for isolating devices from the environment and acting as a waveguide. As a result, hBN has found extensive applications in optical devices, electronic devices, and quantum photonic devices. This comprehensive tutorial aims to provide readers with a thorough understanding of hBN, covering its synthesis, lattice and spectroscopic characterization, and various applications in optoelectronic and quantum photonic devices. This tutorial is designed for both readers without prior experience in hBN and those with expertise in specific fields seeking to understand its relevance and connections to others.

Investigation of crystalline and band alignment properties of NiO/GaN and Ni0.5Co0.5O/GaN heterojunctions using synchrotron radiation based techniques

Epitaxial layers of NiO and Ni0.5Co0.5O have been deposited on GaN templates using RF magnetron sputtering deposition technique. The epitaxial relationship in out-of-plane and in-plane directions for NiO and Ni0.5Co0.5O layers with respect to GaN is [111]NiO(Ni0.5Co0.5O)∥[0001]GaN and [1̅10]NiO(Ni0.5Co0.5O)∥[1̅21̅0]GaN, respectively. The epi-layers are found to have two triangular domain structures oriented along [111] direction with an in-plane rotation of 60° with respect to each other. Photoelectron spectroscopy (PES) has been used to determine the valence band offset (∆EV) and the band alignment at the heterojunctions (HJs). The ∆EV at NiO/GaN and Ni0.5Co0.5O/GaN HJs are 1.2 ± 0.2 eV and 1.8 ± 0.2 eV, respectively. The conduction band offsets (∆EC) have also been estimated using the determined values of optical band gap (NiO: Eg = ∼ 3.5 eV; Ni0.5Co0.5O: Eg = ∼ 3.3 eV) from optical transmission measurements. The values of ∆EC are found to be 1.5 ± 0.2 eV and 1.9 ± 0.2 eV at NiO/GaN and Ni0.5Co0.5O/GaN HJs, respectively. A type-II band alignment is observed at both the HJs. Higher values of valence band and conduction band offsets are observed at Ni0.5Co0.5O/GaN HJ in comparison to NiO/GaN. These experimentally determined values of band offsets can be used to design wide band gap HJ based devices like photodetectors, where higher values of band off-set at the Ni0.5Co0.5O/GaN HJ can be utilized to confine the carriers.

Nanodiamond Coating in Energy and Engineering Fields: Synthesis Methods, Characteristics, and Applications.

Nanodiamonds are metastable allotropes of carbon. Based on their high hardness, chemical inertness, high thermal conductivity, and wide bandgap, nanodiamonds are widely used in energy and engineering applications in the form of coatings, such as mechanical processing, nuclear engineering, semiconductors, etc., particularly focusing on the reinforcement in mechanical performance, corrosion resistance, heat transfer, and electrical behavior. In mechanical performance, nanodiamond coatings can elevate hardness and wear resistance, improve the efficiency of mechanical components, and concomitantly reduce friction, diminish maintenance costs, particularly under high-load conditions. Concerning chemical inertness and corrosion resistance, nanodiamond coatings are gradually becoming the preferred manufacturing material or surface modification material for equipment in harsh environments. As for heat transfer, the extremely high coefficient of thermal conductivity of nanodiamond coatings makes them one of the main surface modification materials for heat exchange equipment. The increase of nucleation sites results in excellent performance of nanodiamond coatings during the boiling heat transfer stage. Additionally, concerning electrical properties, nanodiamond coatings elevate the efficiency of solar cells and fuel cells, and great performance in electrochemical and electrocatalytic is found. This article will briefly describe the application and mechanism analysis of nanodiamonds in the above-mentionedfields.

Feasibility studies in applying cork to pile-type two-layered seismic metamaterials

Seismic metamaterials, a novel seismic engineering technology composed of multiple layers of materials, can form barriers by being periodically arranged to protect properties in a region from earthquake disasters. Parametric analysis of pile-type two-layered seismic metamaterials reveals that the outer layer material with low Poisson's ratio, low Young's modulus, and low density can achieve low and wide frequency band gap, aligning with the design objectives of seismic metamaterials. Since cork, a readily available material, meets the above characteristics, this study aims to investigate its feasibility in being applied to pile-type two-layered seismic metamaterials. Furthermore, the effect of hollows within pile-type seismic metamaterials on lowering the bound frequency of the band gaps is explored. Through laboratory-scale tests at one-fifth scale and numerical simulation, it is demonstrated that cork pile-type seismic metamaterials without hollows can generate a low and wide frequency band gap and effectively attenuate waves at frequencies ranging from 5.6 Hz to 10 Hz. In addition, employing hollows in the cork outer layer can further decrease the lower bound frequency of the band gap by 25 % and result in an effective attenuation range of 4.6 Hz–9.8 Hz while slightly reducing the width of the band gap.

Mechanical and vibrational behaviors of bilayer hexagonal boron nitride in different stacking modes

Hexagonal boron nitride (h-BN) is a semiconductor material with a wide band gap, which has great potential to serve as a nanoresonators in microelectronics and mass and force sensing fields. This paper investigates the mechanical properties and natural frequencies of bilayer h-BN nanosheets under five different stacking modes, which have been rarely studied, using molecular dynamics simulations. The mechanical properties, including Young’s modulus, the ultimate stress, ultimate strain, Poisson’s ratio and shear modulus, are studied for all five stacking modes. And the effects of strain rate, crystal orientation and temperature to bilayer h-BN nanosheets’ tensile properties have also been studied. Our findings suggest that bilayer h-BN nanosheets are basically an anisotropic material whose tensile properties vary substantially with stacking modes and temperature. Moreover, the natural frequencies are proposed in an explicit form based on the nonlocal theory. The differences of the fundamental natural frequencies among different stacking modes are affected by the constraint condition of bilayer h-BN sheet. The theory results match well with the simulation results. These findings establish elementary understandings of the mechanical behavior and vibration character of bilayer h-BN nanosheets under five different stacking modes, which could benefit its application in advanced nanodevices.

Macroscale Superlubricity with High Pressure Enabled by Partially Oxidized Violet Phosphorus for Engineering Steel

AbstractAs a novel cross‐structured 2D material, violet phosphorus (VP) promises to further develop the dynamic performance, energy efficiency, and service lifetime of mechanical components owing to its high strength and toughness, large carrier mobility, wide bandgap, and good friction‐reducing properties. In this work, the partially oxidized violet phosphorus nanosheets (oVP) are synthesized and employed as the lubricating additives in the poly alpha olefin (PAO) oil environment with less oleic acid (OA) improver, which can trigger the macroscale superlubricity on the diamond‐like carbon (DLC) film deposited steel surface at high loading pressures (>800 MPa) and sliding speeds (>0.1 m s−1). The friction coefficient (COF) of the steel‐DLC tribopair lubricated by the oVP‐PAO oil can reduce down to 0.0064 with little wear. At the high‐pressure sliding interface, the force‐thermal coupling action during the running‐in process promotes the cleavage and recombination of oVP nanosheets and OA molecules to form a reliable chemical adsorption tribofilm mainly composed of phosphorus oxides and amorphous carbon, which prevents the original asperities from direct contact and provides an ultralow shear strength. These findings suggest a new method to achieve macroscale oil‐based high‐pressure superlubricity for engineering steel through the lubrication and catalyzation effects of oVP.

The poly (arylene ether urea) double interface layer formed in PEEU@HfO2/PEI nanocomposites enables enhanced dielectric and energy storage performance

Polyetherimide (PEI) has great potential as a dielectric because of its high-temperature resistance, but its low energy storage density limits practical applications. Incorporating hafnium dioxide (HfO2) with a moderate dielectric constant (k) and wide bandgap can simultaneously enhance the polarization intensity of PEI and potential barriers for charge injection. Nevertheless, the poor compatibility between HfO2 and PEI inevitably introduces defects, thereby weakening the inhibitory effect on dielectric loss. To tackle this issue, an organic double interface layer is introduced to enhance dielectric performance, where the polar poly(arylene ether urea) (PEEU) containing hydrogen bonds can ensure the layer's compactness. Notably, its non-bonding urea groups serve as deep traps to capture migrating carriers, and the gradient k values of HfO2, PEEU, and PEI help mitigate electric field distortions at the interface, therefore enhancing the suppression of conduction loss. The polarity of PEEU and the introduction of multiple interfaces significantly contribute to an increased k (maximum 9.7, at 1 kHz). At 300 kV mm−1, 5 vol% PEEU@HfO2/PEI achieves a discharged energy density of 3.3 J cm−3 and a high charge-discharge efficiency of 83 %, surpassing 5 vol% HfO2/PEI by 1.95 and 2.44 times, respectively. The further COMSOL simulation verifies and proves this work as a novel design strategy for nanocomposite dielectrics.

Improved Sequentially Processed Cu(In,Ga)(S,Se)2 by Ag Alloying

Alloying small quantities of silver into Cu(In,Ga)Se2 is shown to improve the efficiency for wide and low bandgap solar cells. Low bandgap industrial Cu(In,Ga)(S,Se)2 absorbers are studied, substituting less than 10% of the copper with silver, using absolute photoluminescence and cathodoluminescence spectroscopy. Silver improves the grain size and promotes the interdiffusion of Ga and In across the depth of the absorber, resulting in a smoother bandgap gradient. However, a certain lateral inhomogeneity is observed near the front and back sides. The nonradiative losses in the bare absorbers are reduced by up to 30 meV.

Modeling from the first principles of the electronic properties of compositions of REE as precursors of high-temperature superconductors

Modeling of the first principles of the electronic properties of complex oxides of rare earth elements (BaY2O4, BaGd2O4, BaLu2O4) as precursors of high-temperature superconductors has been performed. The VASP software package was used as a modeling environment, in particular the method of coupled plane waves (PAW method), which allows us to obtain fairly accurate results for calculating the electron density and band structure. From the analysis of the obtained band energy structure, it follows that the studied REE oxides have a band gap width Eg = 3.29–3.84 eV, which is characteristic for dielectric materials. The studied compounds based on these rare earth elements selected from the yttrium (Y, La, Gd–Lu) and cerium (Ce–Eu) groups are characterized by an increase in Fermi energy and a decrease in the band gap as the atomic number (39, 64, 71) of the element in the periodic table increases. A method for modeling the quantum layers of the studied materials by simulating the restriction of the crystal structure along one of the coordinate axes is proposed. This representation approximates the model of the crystal lattice of REE oxides to the situation of analyzing a quantum layer whose thickness is equal to the size of the crystal cell along the specified axis. The rupture of atomic bonds in a crystal is simulated by increasing the distance between atomic layers along this axis to values at which the value of free energy is stabilized. In the quantum layer of rare earth element oxide (with its thickness close to 1 nm), a wider range of energy values is formed in which electrons are distributed than is observed in the continuous version, and the expansion of the electron distribution area extends to the energy levels of the band gap. This is explained by the fact that the geometric discretization of nanoscale structures determines the discreteness of the quantum-dimensional energy spectrum.

N-doped CoNiOOH with optimized band-structure for photo-assisted electrocatalytic overall seawater splitting

In this work, an efficient and stable photo-assisted electrocatalyst of N-doped CoNiOOH for overall seawater splitting are proposed. The doped N atoms triggered local contractions of lattice, thereby, promoted the morphologies of products to evolve from nanoflowers to nanowires (NWs), and the obvious electrocatalytic enhancements. Especially under the illumination of AM 1.5 G, the U30-NWs exhibited a photo-assisted cathodic current density from −2.0 to −4.0 mA cm−2 at −0.05 V vs RHE, and also an anodic stability for more than 48 hours in seawater. Our studies reveal that the doped N atom could accumulate more electrons than O atom, leading to the degradation of Co and Ni 3d and O 2p states on either side of Fermi level, thus a p-type feature with the widened band gap and improved UV–visible light absorption. Meanwhile, the corrosion resistance was thus improved due to the chemical stable N-Co(Ni)-N bonds. This work provides a practical strategy towards the H2 production via photo-assisted electrocatalytic seawater splitting.

Synaptic properties of GaOx-based memristor with amorphous GaOx deposited by RF magnetic sputtering

GaOx devices have been extensively explored for applications such as power devices and solar blind detectors, based on their wide bandgap. In this study, we investigated the synaptic properties of the amorphous gallium oxide (a-GaOx)- based memristor with a W/WOx/a-GaOx/ITO structure, in which a-GaOx are deposited by RF magnetic sputtering at ambient temperature. The structure and components of a-GaOx are characterized by XRD, XPS, SEM, and EDS. The electrical test indicates that W/WOx/a-GaOx is ohmic due to the thin WOx layer with a high concentration of oxygen vacancies. Consequently, the synaptic characteristics of the W/WOx/a-GaOx/ITO memristor depend on both the a-GaOx layer itself and the a-GaOx/ITO junction. The fitting results indicate that the a-GaOx/ITO junction is Schottky with unidirectional conductive properties. However, the elevated defect density results in a larger current for the reverse-biased a-GaOx/ITO junction. Moreover, adjusting the thickness of a-GaOx allows the device to achieve almost symmetrical forward and reverse currents. We have successfully observed typical synaptic characteristics in W/WOx/a-GaOx/ITO when stimulated by consecutive spike signals. Clearly, through careful design considerations regarding the structure and parameters, we have realized superior synaptic performance in a-GaOx-based memristors. This achievement shows that amorphous GaOx has great potential applications in neuromorphic computation chips for artificial intelligence or the Internet of Things in the future.

Surface Modification of TiO2 for High Performance in all Inorganic Perovskite Solar Cells

Titanium dioxide (TiO[Formula: see text] is a commonly employed electron transport layer in perovskite solar cells (PSCs) due to its exceptional attributes such as high mobility, a wide bandgap and remarkable stability against moisture and high temperatures. This holds particularly true for inorganic PSCs. Nevertheless, the substantial hysteresis and elevated trap defect levels following annealing present challenges to enhance the efficiency of the PSCs. Herein, we proposed a chemical bath deposition (CBD) method to fabricate the TiO2 layer, serving as the electron transport layer in CsPbBr3 solar cells. This method is coupled with KCl, which supplies an ample quantity of [Formula: see text] ions capable of diffusing into the CsPbBr3 perovskite film during the annealing process. The inclusion of [Formula: see text] ions significantly diminishes the solar cell hysteresis and enhances the crystalline quality of the perovskite material, thereby enhancing the performance of CsPbBr3 solar cells. By implementing the TiO2 and KCl treatments, we achieved an impressive efficiency of 9.49%. Furthermore, the device also exhibited excellent stability in air for 30 days.

An n-type host with a high triplet energy level and wide bandgap for RGB phosphorescent OLEDs

We applied 4,6-Bis(3,5-di(pyridin-2-yl)phenyl)-2-methylpyrimidine (B2PyMPM), a material with a wide bandgap and excellent electron mobility, as the host for the emissive layer in red, green, and blue organic phosphorescence-emitting devices. In the case of red emission, it exhibited a stable and outstanding external quantum efficiency (EQE) of 10.2% along with color coordinates (0.68, 0.31), indicating its potential as a candidate material for smartphone displays. For green and blue emissions, relatively low electroluminescence characteristics were observed due to insufficient energy transfer. With a high triplet energy level (T1) of 2.89 eV, B2PyMPM holds promise for use with other phosphorescent dopants in the future.

An analytical model of P+ SiC core-shell JLFETs to analyze the performance for higher breakdown voltages applications

In this article, we proposed a core-shell (CS) architecture that uses the silicon carbide (SiC) as a nanowire for higher breakdown voltages in a Junctionless field effect transistor (JLFET). The P+ core-shell improves the electrostatic integrity and reduces lateral band-to-band tunneling in SiC JLFET. Furthermore, the SiC is wide band gap material which helps to achieve the full volume depletion in JLFET at such short channel lengths. Thus, the OFF-state current in P+ SiC CS JLFET has reduced to 10−16 μA even at temperature 600 K and VDS = 2.0 V. The P+ SiC CS JLFET shows significant performance even at higher drain voltages. Hence, we investigate the P+ SiC CS JLFET for higher breakdown voltages with the impact of higher temperatures. In addition, the impact of higher drain voltages with higher temperatures is also examined. This paper presents an analytical model for P+ SiC CS JLFETs which considers Poisson's equation for nanowires as well as the surface potential at the threshold voltage (Vth) and electric field at the zero point (Ez). The model shows that the surface potential and the electric fields in nanowires are essential for an accurate estimation of the drain current. In addition to the numerical modeling results provided by SILVACO TCAD, the performance of the proposed analytical model was compared with simulation results. The results showed good agreement between the simulations and the proposed compact model. This indicates that the proposed model can be used to provide accurate drain current predictions in nanoscale transistors.

Synchrotron radiation x-ray topography applied to nitride semiconductor crystals

Gallium nitride (GaN) and aluminum nitride (AlN), as examples of third-generation semiconductors, have attracted significant interest due to their remarkable physical attributes, including a wide bandgap, high breakdown voltage, exceptional chemical stability, and high thermal conductivity. These characteristics render GaN and AlN highly promising for use in power and (opto)-electronic devices. Consequently, there is a growing demand for high-quality GaN and AlN crystals on the centimeter scale. As the dislocation density in these materials decreases, the need for a reliable method of dislocation characterization becomes more pressing. Synchrotron radiation x-ray topography (SR-XRT) has emerged as a superior, nondestructive technique for the precise characterization of crystal defects. This review briefly introduced the principle of XRT, and its application in the analysis of dislocations in GaN and AlN crystals is summarized. By examining the relationship between the SR-XRT image contrast and the Burgers vectors of dislocations, it is possible to categorize wafer dislocations and determine the magnitude and direction of Burgers vectors. Additionally, SR-XRT facilitates the analysis of interactions between dislocations in GaN and AlN crystals. These analyses are instrumental in advancing the development of superior crystals. This review concludes with a discussion of the current challenges faced by SR-XRT and a projection of its future applications in characterizing third-generation semiconductor crystal materials. This review offers significant guidance for the characterization of nitride crystal defects using SR-XRT.

Oxide Based Pentachromatic‐Vision Inspired Optoelectronic Synaptic Transistor with Large Conduction States Over 512

AbstractOptoelectronic neuromorphic devices based on oxide semiconductors have been potentially investigated to mimic the functions of human visual synapses. However, the challenge comes from the wide bandgap characteristics of numerous oxide semiconductors, which restricts the response range of the device under ultra‐violet (UV) region. Strategies for widening the response range are mostly focused on artificially generating the defect states, however, most of them results in mimicking the tetrachromatic visual system from UV to visible light range. To be used for industries such as robotics, or autonomous vehicles, mimicking the tetrachromatic vision system should be overcome up to near‐infrared (NIR) region. Here, a facile solution processed indium‐gallium‐zinc‐oxide and silver oxide structured optoelectronic synaptic transistor is fabricated not only to mimic the function of human synapses, but to overcome the tetrachromatic human visual system up to the NIR region. The device not only showed photoresponse characteristics under the entire 405 to 830 nm wavelength region, but also showed significant synaptic behaviors with over 512 conduction states under a reasonable incident light power density of 4.5 mW cm−2. The results will offer a useful facile method for fabricating optoelectronic synaptic transistors that can overcome the tetrachromatic vision systems.

Low-cost O2 plasma activation assisted direct bonding of β-Ga2O3 and Si substrates in air

This study presents a novel technique for directly bonding β-Ga2O3 and Si substrates using O2 plasma activation at room temperature. The activation process resulted in excellent surface hydrophilicity and roughness. Annealing in an N2 atmosphere revealed a bonding strength of 6.23 MPa, with partial fracture of the Si substrate. The bonding interface was observed by TEM and EDS. A distorted amorphous intermediate layer with a thickness of 100 nm and amorphous bubbles were observed in the bonding interface. In addition, the Ga, O, and Si elements were diffused and enriched, forming approximately 30 nm nodules at the bonding interface. This method offers a promising approach for integrating wide bandgap semiconductor materials in devices, bypassing the need for vacuum treatment.

Parasitic-Based Model for Characterizing False Turn-On and Switching-Based Voltage Oscillation in Hybrid T-Type Converter

High frequency and high voltage switching converters utilizing wide bandgap semiconductors are gaining popularity thanks to their compactness and improved efficiency. However, the faster switching requirements gives rise to new challenges. A key issue is the increased oscillation of the drain–source voltage caused by the switching action of the complementary switch in the same phase or change of state of the other phase switches. The voltage stress caused by these oscillations can damage the switch. Furthermore, the high dv/dt during turning-on of one switch might result in false turn-on of the complementary switch due to the miller effect. In this paper, these issues are investigated in a T-type converter through analytical and experimental analysis. Based on the proposed analytical approach, simple and cost-wise solutions utilizing an optimum design of gate driver circuits and circuit layout modifications can be developed to cope with the aforementioned issues. A comprehensive analytical model of the converter with consideration of parasitic capacitances and inductances is developed. By performing sensitivity analysis on the model, the effect of the parasitic parameters on the drain–source voltage oscillation and gate–source voltage amplitude in case of false turn-on is studied. The validity of the model is then assessed through numerical simulations and experimental results.

High-Entropy Lithium Niobate Nanocubes for Photocatalytic Water Splitting under Visible Light.

The vast compositional space available in high-entropy oxide semiconductors offers unique opportunities for electronic band structure engineering in an unprecedented large room. In this work, with wide band gap semiconductor lithium niobate (LiNbO3) as a model system, we show that the substitutional addition of high-entropy metal cation mixtures within the Nb sublattice can lead to the formation of a single-phase solid solution featuring a substantially narrowed band gap and intense broadband visible light absorption. The resulting high-entropy LiNbO3 [denoted as Li(HE)O3] crystallizes as well-faceted nanocubes; atomic-resolution imaging and elemental mapping via transmission electron microscopy unveil a distinct local chemical complexity and lattice distortion, characteristics of high-entropy stabilized solid solution phases. Because of the presence of high-entropy stabilized Co2+ dopants that serve as active catalytic sites, Li(HE)O3 nanocubes can accomplish the visible light-driven photocatalytic water splitting in an aqueous solution containing methanol as a sacrificial electron donor without the need of any additional co-catalysts.

Synthesis of 5-(Aryl)amino-1,2,3-triazole-containing 2,1,3-Benzothiadiazoles via Azide-Nitrile Cycloaddition Followed by Buchwald-Hartwig Reaction.

An efficient access to the novel 5-(aryl)amino-1,2,3-triazole-containing 2,1,3-benzothiadiazole derivatives has been developed. The method is based on 1,3-dipolar azide-nitrile cycloaddition followed by Buchwald-Hartwig cross-coupling to afford the corresponding N-aryl and N,N-diaryl substituted 5-amino-1,2,3-triazolyl 2,1,3-benzothiadiazoles under NHC-Pd catalysis. The one-pot diarylative Pd-catalyzed heterocyclization opens the straightforward route to triazole-linked carbazole-benzothiadiazole D-A systems. The optical and electrochemical properties of the compound obtained were investigated to estimate their potential application as emissive layers in OLED devises. The quantum yield of photoluminescence (PLQY) of the synthesized D-A derivatives depends to a large extent on electron-donating strengths of donor (D) component, reaching in some cases the values closed to 100%. Based on the most photoactive derivative and wide bandgap host material mCP, a light-emitting layer of OLED was made. The device showed a maximum brightness of 8000 cd/m2 at an applied voltage of 18 V. The maximum current efficiency of the device reaches a value of 3.29 cd/A.