BN Interface Research Articles

Optically Active Defect Engineering via Plasma Treatment in a MIS‐Type 2D Heterostructure

AbstractAt the interface of 2D heterostructures, the presence of defects and their manipulation play a crucial role in the interfacial charge transfer behavior, further influencing the device functionality and performance. In this study, the impact of deliberately introduced photo‐active defects in the h‐BN layer on the interfacial charge transfer and photoresponse performance of a metal‐insulator‐semiconductor type heterostructure device is explored. The formation and concentration of defects are qualitatively controlled using an inductive coupled plasma treatment method, as evidenced by enhanced h‐BN defect emission and more efficient optically induced doping of graphene at the graphene/h‐BN interface. Besides, the use of the h‐BN layer between graphene and WS2 not only suppresses charge carriers in the dark state, but also promotes the separation of photo‐generated electron‐hole pairs and interfacial charge transfer due to the existence of defect levels, leading to orders of magnitude improvement in the light on/off ratio and self‐driving performance of the heterostructure photodetector. This strategy of controlling defect states in the insulating layer provides a new approach to optimize the charge transfer processes at the 2D interfaces, so as to expand its potential applications in the fields of electronic and optoelectronic devices.

Ablative and mechanical properties of C/C‐SiC‐ZrC composites modified with PyC/SiC and PyC/BN interface layers

AbstractTwo kinds of novel modified C/C‐SiC‐ZrC composites were prepared via precursor infiltration and pyrolysis, as pyrocarbon (PyC)/silicon carbide (SiC) and PyC/boron nitride (BN) dual‐layer interphases were separately structured on the fibers by means of chemical vapor infiltration. Data analysis and conclusions are served for investigating the effects of these two interface layers on mechanical and anti‐ablative properties. On the mechanical property hand, both PyC/BN and PyC/SiC interphase layers play positive roles, resting with the reduction of fiber damage during the fabrication process. Compared with BN, SiC shows better enhancement as the flexural strength of PyC/BN and PyC/SiC interphase‐modified composites are 214.9 and 229.2 MPa, respectively. On the ablative property hand, after oxyacetylene flame ablation for 60 s, the mass and linear ablation rates of the composites modified by PyC/SiC interface were 2.2 mg/s and 9.7 μm/s, which is much lower than that modified by PyC/BN. The inferior ablation properties of PyC/BN‐CSZ were attributed to the vaporization of the B2O3 gas that destroys the integrity of the oxide film and oxygen erosion on the substrate through the damaged BN interface.

Microstructure control of BN interface and its effect on mechanical behavior of SiCf/SiC composites

Chemical vapor infiltration (CVI) of BCl3 and NH3 at low temperature was used to synthesize the BN interface of SiCf/SiC composites. The structure of the BN interface was adjusted by changing the deposition pressure, and its effect on the mechanical properties of SiCf/SiC composites was studied. The BN interface showed a quasi-isotropic microstructure when the deposition pressure was low at 500 Pa. After increasing the deposition pressure to 1500Pa, the crystallinity of BN improves and some high-crystallized domains appear. The later tensile test illustrated that increasing the deposition pressure of BN interface could enhance the matrix cracking stress but decrease the fracture toughness, indicating that the BN interface deposited at high pressure has higher bonding strength. It can be attributed to that the high-crystallized BN layers vertical to the SiC fibers suffering in-plane shear stress, which impedes the deformation and cracking of the BN interface. Our research provides a deep insight into the relationship between the structure and performance of composites, and a guiding significance in the preparation of high-strength and high-toughness SiCf/SiC composites.

Heat treatment effects on microstructure and properties of CVI SiC/SiC composites with Sylramic™-iBN SiC fibers

Chemical-vapor-infiltrated (CVI) SiC/SiC composites with Sylramic™-iBN SiC fibers and CVI carbon, BN, and a combination of BN/C interface coating were heat treated in 0.1-MPa argon or 6.9-MPa N2 at temperatures to 1800 °C for exposure times up to 100 hr. The effects of thermal treatment on constituent microstructures, in-plane tensile properties, in-plane and through-the-thickness thermal conductivities, and creep behavior of the composites were investigated. Results indicate that heat treatment affected stoichiometry of the CVI SiC matrix and interface coating microstructure, depending on the interface coating composition and heat treatment conditions. Heat treatment of the composites with CVI BN interface in argon caused some degradation of in-plane properties due to the decrease in interface shear strength, but it improved creep resistance significantly. In-plane tensile property loss in the composites can be avoided by modifying the interface composition and heat treatment conditions.

Edge magnetism of triangular graphene nanoflakes embedded in hexagonal boron nitride

Graphene nanoflakes (GNFs), with intriguing zigzag edge magnetism, have great potential for the application in high-speed and low-power nanoelectronics and spintronics. However, scattering from edge defects can substantially degrade this novel edge magnetism. Precisely controlled growth of GNFs with zigzag edges on dielectric substrates to suppress edge defect scattering is still challenging. Herein, we report the successful synthesis of triangular zigzag-edged GNFs via epitaxy on hexagonal boron nitride (h-BN) templates. Further magnetic measurement results indicate the triangular zigzag-edged GNFs embedded in h-BN can induce the magnetization with a magnitude of 1e-4 emu/g at room temperature. Additionally, density functional theory calculations address such magnetic orderings originating from the superexchange interactions among local “unpaired” electrons located in the zigzag C−BN interface. Our in-plane hetero-integration approach to the template-epitaxial growth of GNFs on the pre-etched h-BN provides a promising platform for experimentally achieving GNFs with high electron spin states, which are highly desired for next-generation spintronics and nanoelectronics.

Fabrication and properties of SiCnw‐reinforced SiC composites by reactive melt infiltration with BN and PyC interface

AbstractTo tailor the fiber–matrix interface of SiC nanowires‐reinforced SiC (SiCnw/SiC) ceramic matrix composites (CMCs) for improved mechanical properties, SiC nanowires were coated with BN and pyrolytic carbon (PyC) compound coatings prepared by the dip‐coating process in boric acid and urea solution and the pyrolysis of phenolic resin. SiCnw/SiC CMC with PyC/BN interfaces were fabricated by reactive melt infiltration (RMI) at 1680°C for 1 h. The influences of phenolic resin content on the microstructure and mechanical properties of the CMC were investigated. The results showed that the flexural strength and fracture toughness reach the maximum values of 294 MPa and 4.74 MPa m1/2 as the phenolic resin content was 16 and 12 wt%, respectively. The displacement–load curve of the sample exhibited a gradient drop with increasing phenolic resin content up to 12 wt%. The results demonstrated that the PyC/BN compound coatings could play the role of protecting the SiCnw from degradation as well as improving the more moderate interfacial bonding strengths during the RMI.

Electrical characteristics of WSe2 transistor with amorphous BN capping layer

Back-gated WSe2 field effect transistor (FET) with amorphous BN (a-BN) capping layer was fabricated by a direct chemical vapor deposition (CVD) assemble technique, and the electrical performance was investigated. The field effect mobility and the on/off current ratio of the a-BN capped WSe2 FET both improved by one order of magnitude due to the clean and conformal contact between the BN and WSe2 interface that further reduced the extrinsic scattering. This technique could be further applied for other 2D material based devices.

Thermal stability of CVI and MI SiC/SiC composites with Hi-Nicalon™-S fibers

The influence of high-temperature argon heat-treatment on the microstructure and room- temperature in-plane tensile properties of 2D woven CVI and 2D unidirectional MI SiC/SiC composites with Hi-Nicalon™-S SiC fibers was investigated. The 2D woven CVI SiC/SiC composites were heat-treated between 1200 and 1600 °C for 1- and 100-hr, and the 2D unidirectional MI SiC/SiC composites between 1315 and 1400 °C for up to 2000 hr. In addition, the influence of temperature on fast fracture tensile strengths of these composites was also measured in air. Both composites exhibited different degradation behaviors. In 2D woven CVI SiC/SiC composites, the CVI BN interface coating reacted with Hi-Nicalon™-S SiC fibers causing a loss in fast fracture ultimate tensile strengths between 1200 and 1600 °C as well as after 100-hr isothermal heat treatment at temperatures > 1100 °C. In contrast, 2D unidirectional MI SiC/SiC composites showed no significant loss in in-plane tensile properties after the fast fracture tensile tests at 1315 °C. However, after isothermal exposure conditions from 1315° to 1400°C, the in-plane proportional limit stress decreased, and the ultimate tensile fracture strain increased with an increase in exposure time. The mechanisms of strength degradation in both composites are discussed.

Nano‐scale BN interface for ultra‐stable and wide temperature range tolerable Zn anode

AbstractAqueous Zn‐based energy storage device (ZESD) is a promising candidate for large‐scale energy storage applications due to its significant merits like low cost, inherent safety, and environmental benignity. However, one shortcoming of ZESDs is the performance deficiency of pristine Zn anode caused by detrimental dendrite formation and side reactions. In this work, a novel boron nitride nano‐scale interface was established for ultra‐stable and wide temperature range tolerable anode (BN@Zn) by a scalable magnetron sputtering technique. The as‐introduced BN layers afford enhanced Zn deposition kinetics for a wide temperature application range from −20 to 60°C and effectively mitigated dendritic growth, which were ascribed to the strong interlayer bonds and uniform active sites as demonstrated by both experimental and density functional theory research results. Thus, the ultra‐thin BN interface could significantly improve the reaction kinetics and electrochemical stability of Zn anode, providing a new perspective towards the advanced ZESDs.image

Effect of interface types on the heat-resistance of Cansas-II SiCf /SiC composites

The heat-resistance of the Cansas-II SiC/CVI-SiC mini-composites with a PyC and BN interface was studied in detail. The interfacial shear strength of the SiC/PyC/SiC mini-composites decreased from 15 MPa to 3 MPa after the heat treatment at 1500 °C for 50 h, while that of the SiC/BN/SiC mini-composites decreased from 248 MPa to 1 MPa, which could be mainly attributed to the improvement of the crystallization degree of the interface and the decomposition of the matrix. Aside from the above reasons, the larger declined fraction of the interfacial shear strength of the SiC/BN/SiC mini-composites might also be related to the gaps in the BN interface induced by the volatilization of B 2 O 3 ·SiO 2 phase, leading to a significant larger declined fraction of the tensile strength of the SiC/BN/SiC mini-composites due to the obvious expansion of the critical flaws on the fiber surface. Therefore, compared with the CVI BN interface, the CVI PyC interface has better heat-resistance at high temperatures up to 1500 °C due to the fewer impurities in PyC.

Optoelectronic Synapses Based on Photo‐Induced Doping in MoS2/h‐BN Field‐Effect Transistors

AbstractSynaptic device is an important component in artificial neural networks. Electrically‐controlled long‐term depression (LTD) is demonstrated in three‐terminal photonic synaptic devices, which is useful for improving the efficiency of artificial neural networks. The mechanism of the three‐terminal photonic synaptic device is similar to that of the photo‐induced doping effect. Photo‐assisted carrier transport and recombination are expected to accelerate the LTD process. However, the effects of photon illumination on LTD are not investigated. Here, the effects of electrical stimulation and photo‐stimulation on the long‐term plasticity (LTP) and LTD process of photonic synaptic devices based on MoS2/h‐BN field‐effect transistors (FETs) are systemically investigated. The influences of gate dielectric layer on the photo‐induced doping are explored. The non‐volatile and electrically‐erasable properties of photo‐induced doping in MoS2/h‐BN FETs are due to the van der Waals energy barrier at the MoS2/h‐BN interface. The synaptic functions of LTP and LTD can be mimicked by the photo‐induced doping effect. The LTD process will be accelerated with the applications of positive gate voltage and laser illumination, which improves the speed of information processing. These results presented in this paper are useful for realizing high‐performance synaptic devices based on the photo‐induced doping effect.

Enhanced mechanical property and tunable dielectric property of SiCf/SiC-SiBCN composites by CVI combined with PIP

The SiBCN matrix via chemical vapor infiltration (CVI) or/and polymer infiltration pyrolysis (PIP) technologies was orderly introduced to SiCf/SiC composites to optimize the mechanical property and electromagnetic (EM) shielding effectiveness simultaneously. The BN interface with the thickness of 350 nm was designed to obtain a little stronger interface bonding. The flexural strength of SiCf/SiC-SiBCN composites reached 545.45±29.59 MPa thanks to the crack deflection between the CVI SiC and CVI SiBCN, as well as CVI SiBCN and PIP SiBCN matrix because of the modulus difference between them. The fracture toughness (KIC) with the value of 16.02±0.94 MPa·m1/2 was obtained owing to the extension of crack propagation path. The adverse effect of stronger interface bonding was eliminated by the design of matrix microstructure for SiCf/SiC-SiBCN composites. The thermal conductivity in the thickness direction was 7.64 W·(m·K)−1 at 1200 °C and the electric resistivity decreased to 1.53×103 Ω·m. The tunable dielectric property was obtained with the coordination of wave-absorption CVI SiBCN matrix and impedance matching PIP SiBCN matrix, and the total shielding effectiveness (SET) attained 30.01 dB. It indicates that the SiCf/SiC-SiBCN composites have great potential to be applied as structural and functional materials.

Stressed-oxidation behavior of 2D CVI SiC/BN/SiC at elevated temperature in air

The stressed-oxidation behaviors of 2D woven SiCf/BN/SiC composites were investigated at 950 °C and 1100 °C in air. The different proportions (60%–90%) of the ultimate tensile strength (UTS) at corresponding temperatures were chosen as constant stress. The stressed-oxidation experiments were taken to failure or interrupted (240h). The UTS decreases by 20.75% at 950 °C and 30.71% at 1100 °C. The composites did not fail during stressed oxidation when subjected to constant stress corresponding to the initial linear and the beginning of nonlinear segments of the tensile curve, above which the composites failed with a maximum failure life of about 10h. Fiber degradation due to the thermal exposure and the fiber cracks caused by the oxidation of BN interface coating and SiC fiber could be responsible for the strength degradation and failure of the composites during stressed oxidation.

The tensile damage behavior of SiCf/SiC–B4C after oxidation in wet atmosphere based on acoustic emission pattern recognition

AbstractIn this research, SiCf/SiC–B4C composites were prepared through slurry impregnation and reaction melting infiltration (RMI). To explore the damage evolution of this composites after oxidation in wet atmosphere, the changes of damage behavior of composites before and after oxidation were studied using acoustic emission (AE) pattern recognition (PR) technology. Results show that since the damage behavior of the composites changes after oxidation, the optimal number of clusters transforms from 3 to 5. In addition, as the oxidation time increases, the area of concentration of damage events is gradually broadened towards lower stress direction due to the decrease of damage threshold of composites. Meanwhile, because the BN interface is gradually oxidized, the density of C3‐cracks (axial yarns cracks) near the fracture and the proportion of Cluster‐4 (the signals of transverse yarns cracks) will increase, for which the rising of interfacial shear stress should be responsible. Through this research, not only can the damage evolution of SiCf/SiC–B4C composites after oxidation be understood, but also the method employed in this research can be as guide for studying other similar materials.

Monolayer Hexagonal Boron Nitride: An Efficient Electron Blocking Layer in Organic Photovoltaics

AbstractIn this study, monolayer hexagonal boron nitride (h‐BN) grown via chemical vapor deposition (CVD) as an effective electron blocking layer (EBL) for the organic photovoltaics (OPVs) is proposed. Unexpectedly, it is found that h‐BN can replace the commonly used hole transport layers (HTLs), i.e., molybdenum trioxide (MoO3) and poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) in an inverted device architecture. Here, a wet‐transfer technique is employed and a single layer of h‐BN on top of the PV2000:PC60BM blend is successfully placed. Analysis of the bandgap diagram shows that the monolayer h‐BN makes smaller barrier for holes but significantly larger barrier for electrons. This makes the h‐BN effective in blocking electrons while creating a possible path for the holes through tunneling to the electrode, due to the low energy barrier at the PV2000/h‐BN interface. Using h‐BN as an EBL, efficient inverted OPVs are achieved with an average solar‐to‐power conversion efficiency of 6.13%, which is comparable with that of reference devices based on MoO3 (7.3%) and PEDOT:PSS (7.6%) as HTLs. Interestingly, the devices with h‐BN shows great light‐soak stability. The study reveals that the monolayer h‐BN grown by CVD could be an effective alternative EBL for the fabrication of efficient, lightweight, and stable OPVs.

Influences of the dip-coated BN interface on mechanical behavior of PIP-SiC/SiC minicomposites

SiC/SiC minicomposites with BN interfaces were fabricated by a two-step route comprising dip-coating and polymer impregnation and pyrolysis (PIP). The thickness of the dip-coated BN interface was in the range of 76–300 nm, which was adjusted by dipping times. The influences of BN interface thickness on the mechanical behavior of the as-fabricated SiC/SiC minicomposites were investigated. The mechanical properties of SiC/SiC composites were improved by introducing BN interfaces, which showed an optimized tensile strength distribution with a higher Weibull modulus m. The optimal mechanical properties could be obtained in SiC/BN/SiC-2 samples with the BN interface thickness of ~160 nm. The variation trend of tensile strengths caused by the increasing interface thickness was analyzed using a probabilistic-statistical model. Especially, both the interface delamination caused by the secondary crack and the fiber Weibull parameter mfiber influenced by the BN interface coating were considered in the probabilistic-statistical model, leading to a better matching between the experimental results and the model simulation. Microstructure characterization combined with numerical analysis showed that the dip-coated BN interface with appropriate thickness played an important role in improving the mechanical properties of SiC/SiC minicomposites. The results indicated that both the ability of crack deflection of interfaces and the fiber in-situ strengths should be considered in the interface design of SiC/SiC composites.

Experiment and simulation analysis on thermal shock resistance of laminated ceramics with graphite and boron nitride interfaces

Laminated ZrB2–SiC ceramics with graphite and BN as the interface layers were fabricated by tape casting and consequent hot pressing. The influences of interface materials on the thermal shock resistance of the laminated ceramics were studied by both experimental and theoretical analyses. Experimental results show that the critical temperature differences of G/LZS (laminated ZrB2–SiC/G) and BN/LZS (laminated ZrB2–SiC/BN) were 237 and 99 °C, respectively. The stress gradients of G/LZS and BN/LZS calculated via finite element analysis were 709 and 809 MPa/mm, respectively. Thus, the thermal shock resistance of G/LZS was higher than that of BN/LZS. Such merits of G/LZS can be explained by the following two reasons: one is the thermal conductivity of the graphite interface layer was higher than that of the BN interface layer; the other is the difference of the thermal expansion coefficient for the graphite interface layer and the matrix layer was lower than that for the BN interface layer and the matrix layer.

A novel strategy to prepare BN interface and its effects on microstructure and properties of SiCw/BN/SiC composites

Based on the microstructure and composition design idea of continuous fiber toughened ceramic matrix composites (CFCCs), a BN interface layer was prepared on the surface of silicon carbide whiskers (SiCw) for the first time. And SiCw toughened SiC ceramic matrix composites with BN interface (SiCw/BN/SiC composites) were prepared by precursor infiltration and pyrolysis (PIP) combined with reactive melt infiltration (RMI). The as-prepared SiCw/BN/SiC composites possess high strength and high density, as well as the isotropic characteristics. By adjusting the thickness of the BN interface, the residual stress inside the composites is reduced and the bonding strength between the whisker and the matrix is optimized, thereby improving the toughness of the SiCw/BN/SiC composites. When the thickness of BN interface is about 3 nm, the flexural strength and the fracture toughness of the composites attain 425 MPa and 6.6 MPa m1/2, respectively. The bulk density of the SiCw/BN/SiC composites is about 2.87 g cm−3 and the open porosity is less than 4%, indicating the high densification and excellent gas tightness. The toughening mechanisms of SiCw/BN/SiC composites contain SiCw toughening and micron-sized SiC particles (produced by RMI process) toughening, which ensure a good toughness for the dense composites together with the positive effect of BN interface.

Simulation design of a high-breakdown-voltage p-GaN-gate GaN HEMT with a hybrid AlGaN buffer layer for power electronics applications

We propose a novel GaN high-breakdown-voltage high-electron-mobility transistor (HB-HEMT) with a p-GaN gate and hybrid AlGaN buffer to improve the breakdown voltage and Baliga’s figure of merit. The hybrid AlGaN buffer is composed of a horizontally arranged AlaGa1−aN zone and an AlbGa1−bN zone, each having different Al compositions a and b. The proposed HB-HEMT is simulated using the Silvaco technology computer-aided design (TCAD) tool ATLAS, considering the polarization model, low-field mobility, high-field mobility, and Selberherr’s impact ionization model to simulate the direct-current (DC), breakdown, and C–V properties of the proposed HB-HEMT. The breakdown voltage of the HB-HEMT is significantly improved by introducing the hybrid AlGaN buffer structure, which can effectively modulate the electric field distributions within the channel and the buffer. A high breakdown voltage (1450 V), a low specific on-state resistance (0.47 mΩ cm2), and a high Baliga’s figure of merit (4.47 GW/cm2) are obtained at the same time with an Lgd (gate-to-drain distance) of 6 µm, a distance from the gate to the AlaGa1−aN/AlbGa1−bN interface of 4 µm, and Al compositions of a = 0.25 and b = 0.1. The simulated C–V results also reveal that the GaN HB-HEMT shows better switching characteristics than the conventional GaN HEMT with a p-GaN gate.

Oxidation and microstructure of SiCf/SiC composites in moist air up to 1600°C by X‐ray tomographic characterization

AbstractSiCf/SiC composites that possess PyC or BN interface layers were fabricated and then oxidized in moist air at 1000, 1200, 1400, and 1600°C. High‐resolution CT was used for capturing 3D images and quantifying the SiC phase, mesophase, and voids. The oxidation behavior and microstructural evolution of SiCf/SiC with PyC or BN interface are discussed in this study. The microstructure of the SiCf/SiC with a PyC layer was seriously damaged in moist air at high temperature, whereas the BN interface layer enhanced the oxidation resistance of the SiCf/SiC. These results are also confirmed by using XRD, oxidation mass gain, tensile testing, and SEM measurements. The results of the oxidation behavior and microstructural evolution for SiCf/SiC oxidized in dry air are also compared with the results of this study. Comparing the SiCf/SiC with a PyC interface layer, the composite with a BN interface layer oxidized in moist air exhibits a high void growth rate and a low SiO2 grain growth rate from 1000 to 1600°C. This work will provide guidance for predicting the service life of SiCf/SiC for multiscale damage rate models of materials at a local scale and will also provide guidance on the life service design of SiCf/SiC materials.