Hexagonal Boron Nitride Films Research Articles

Thickness-Controlled Synthesis of Uniform Hexagonal Boron Nitride Films on Metal Alloys by Chemical Vapor Deposition

Two-dimensional (2D) multilayer hexagonal boron nitride (hBN) is a superior substrate for other 2D materials due to its insulating property, atomic-scale smoothness, and absence of dangling bonds, enhancing electrical or optical properties in high-performance 2D material-based devices. In most fabrication processes for 2D heterostructure devices, thick hBN flakes exfoliated from bulk have been used because of their high quality and simplicity in the process. However, exfoliated hBN flakes are limited in size and thickness control, impeding scale-up production for industrialization. Therefore, new synthesis strategies are required to overcome the drawback of the exfoliation method. In this work, we investigated the synthesis of large-area, multilayer hBN via atmospheric pressure chemical vapor deposition using single crystalline metal and single crystal alloy substrates. The synthesized results are characterized by optical microscopy, scanning electron microscopy, Raman spectroscopy, atomic force microscopy, etc. These studies provide a more comprehensive understanding of the growth mechanism of hexagonal boron nitride.

Enhanced intensity of Raman signals from hexagonal boron nitride films

Optical spectroscopy is commonly used to study the properties of 2D materials. In order to obtain the best signal-to-noise ratio, it is important to optimize the incoupling of the excitation laser and, at the same time, reduce spurious light reflection. We performed Raman spectroscopy on exfoliated hexagonal boron nitride (hBN) flakes of different thicknesses, placed on a 300 nm SiO2 on Si substrate. By changing the hBN layer thickness, we found a specific thickness, where the Raman signals from the substrate and the hBN showed maximum intensity, whereas the backscattered laser light was suppressed. To explain the increased emission, we calculated the reflectivity and transmissivity of the full layer system (air, hBN, SiO2, and Si) as a function of hBN layer thicknesses for different excitation wavelengths (457, 532, and 633 nm), using the transfer-matrix algorithm. To compare theory with the experiment, we performed Raman measurements with these three different excitation wavelengths on different flakes and determined their thicknesses with AFM measurements. The experimental results are in good agreement with the calculations, which shows the importance of thin film interference to obtain optimum spectroscopic conditions. Since interference colors are easily visible in an optical microscope, this facilitates the choice of optimum flakes for a wide range of optical characterization techniques, including Raman, photoluminescence, and single defect spectroscopy.

Electronic Structure and Surface Chemistry of Hexagonal Boron Nitride on HOPG and Nickel Substrates.

The effect of point defects and interactions with the substrate are shown by density functional theory calculations to be of significant importance for the structure and functional properties of hexagonal boron nitride (h-BN) films on highly ordered pyrolytic graphite (HOPG) and Ni(111) substrates. The structure, surface chemistry, and electronic properties are calculated for h-BN systems with selected intrinsic, oxygen, and carbon defects and with graphene hybrid structures. The electronic structure of a pristine monolayer of h-BN is dependent on the type of substrate, as h-BN is decoupled electronically from the HOPG surface and acts as bulk-like h-BN, whereas on a Ni(111) substrate, metallic-like behavior is predicted. These different film/substrate systems therefore show different reactivities and defect chemistries. The formation energies for substitutional defects are significantly lower than for intrinsic defects regardless of the substrate, and vacancies formed during film deposition are expected to be filled by either ambient oxygen or carbon from impurities. Significantly lower formation energies for intrinsic and oxygen and carbon substitutional defects were predicted for h-BN on Ni(111). In-plane h-BCN hybrid structures were predicted to be terminated by N-C bonding. Substitutional carbon on the boron site imposes n-type semiconductivity in h-BN, and the n-type character increases significantly for h-BN on HOPG. The h-BN film surface becomes electronically decoupled from the substrate when exceeding monolayer thickness, showing that the surface electronic properties and point defect chemistry for multilayer h-BN films should be comparable to those of a freestanding h-BN layer.

Synthesis of hexagonal boron nitride nanocoatings for corrosion prevention of iron substrates

Iron and its alloys have applications in diverse fields such as civil infrastructure, aerospace, and defense. Corrosion of these alloys in saline environments is a significant concern that causes huge environmental losses. The current study explores pulsed laser deposition technique to synthesize hexagonal boron nitride (hBN) films as protective coatings, directly on iron (Fe), to mitigate the corrosion. Microstructural, mechanical, wetting, and corrosion properties of hBN-coated Fe substrates were investigated at different deposition temperatures (25–800 °C) and varying thicknesses (35–115 nm). Raman spectra and transmission electron microscopy confirmed the presence of hBN in as-deposited films. Crystallinity and surface roughness of hBN coatings increased as deposition temperature increased. Electrochemical studies performed in 3.5 wt% NaCl solution showed that the 115 nm thick hBN coatings deposited at 600 °C resulted in lower corrosion rates and has ~6-fold higher corrosion resistance than bare Fe. Also, the corrosion rates decreased with an increase in hBN coating thickness. Overall results suggest that hBN nanocoatings reduced the corrosion activity and can potentially serve as a corrosion-resistant barrier coatings in saline environments.

High detectivity solar blind photodetector based on mechanical exfoliated hexagonal boron nitride films

As an ultra-wide bandgap semiconductor, hexagonal boron nitride (h-BN) has drawn great attention in solar-blind photodetection owing to its wide bandgap and high thermal conductivity. In this work, a metal-semiconductor-metal structural two-dimensional h-BN photodetector was fabricated by using mechanically exfoliated h-BN flakes. The device achieved an ultra-low dark current (16.4 fA), high rejection ratio (R 205nm/R 280nm = 235) and high detectivity up to 1.28 × 1011 Jones at room temperature. Moreover, due to the wide bandgap and high thermal conductivity, the h-BN photodetector showed good thermal stability up to 300 °C, which is hard to realize for common semiconductor materials. The high detectivity and thermal stability of h-BN photodetector in this work showed the potential applications of h-BN photodetectors working in solar-blind region at high temperature.

Controllable Synthesis of High-Quality Hexagonal Boron Nitride Films on Ni-B Alloy

Controllable Synthesis of High-Quality Hexagonal Boron Nitride Films on Ni-B Alloy

Protected ultrathin cuprous oxide film for photocatalysis: Excitation and relaxation dynamics

In the search for cost-effective ways to produce green hydrogen, harnessing the most abundant renewable energy source---sunlight---directly through photoelectrochemical water splitting is highly desirable. Cuprous oxide (${\mathrm{Cu}}_{2}\mathrm{O}$) has proven itself as a material with great potential for the hydrogen evolution reaction (HER) and stands out by its Earth abundance and low processing cost. In this photoelectron spectroscopy study we investigated the electron dynamics in a heterostructure comprised of a ${\mathrm{Cu}}_{2}\mathrm{O}$ surface oxide grown on Cu(111) underneath a hexagonal boron nitride ($h$-BN) film, which replaces a conventional capping layer for corrosion protection. Our results show that it can be a viable cuprous oxide-based photoelectrode material. The $h$-BN film stays intact during the oxidation, and the oxide layer forms an ordered structure with defined valence and conduction bands. The conduction band is at a suitable energy to drive the HER for water splitting, but the photoexcited electrons display short lifetimes. Electrons are mainly excited in the copper substrate and are then captured in long-lived defect states in the ${\mathrm{Cu}}_{2}\mathrm{O}$ layer. The water splitting efficiency of this photocathode may still be improved by reducing the defect density.

Substrate effects on the near-field radiative heat transfer between two hBN films

Near-field radiative heat transfer (NFRHT) could surpass the blackbody limit defined by Stefan-Bolzmann’s law by several orders of magnitude, which has potential applications in thermal switching, thermal management, and photovoltaics. To further develop the NFRHT from theory to application, the substrate, which could enhance the stability of the structure, is a critical factor not to be ignored. However, the substrate effect on the NFRHT is still rarely discussed. In this work, we investigate the NFRHT between hexagonal boron nitride (hBN) films with different permittivities of the substrate. Results demonstrate that when the thickness of the film is 1 nm, increasing the permittivity of the substrate will suppress the NFRHT. In contrast, when the thickness of the film is larger (>2 nm), the high-permittivity substrate could enhance the NFRHT. The spectral heat flux (SHF) corresponding to substrates with different permittivities was investigated. The SHF in Type I hyperbolic band of hBN increases with the increase in the permittivity of the substrate, while that in Type II hyperbolic band is completely opposite. This competitive relationship leads to the above-mentioned phenomenon of NFRHT. The underlying physics mechanism can also be explained by the hyperbolic phonon polaritons (HPPs), which are analyzed by the energy transmission coefficients and dispersion relations. The findings in this work will deepen the understanding of the substrate on HPPs and pave a novel way for near-field radiation devices based on hyperbolic materials.

Mechanism of Transverse Charge Transfer in Thin Films of Hexagonal Boron Nitride

A mechanism of transverse charge transfer through hexagonal boron nitride (h-BN) in a MIS structure has been studied. Experimental data for charge transfer have been analyzed in terms of different models of charge transfer in insulators. It has been shown that charge transfer in h-BN is described by the model of phonon-assisted tunneling between neutral traps. The thermal and optical energies of phonon-coupled traps in h-BN have been determined. Based on charge transfer measurements, XPS spectra, and the ab initio electronic structure of intrinsic defects in h-BN it has been found that boron–nitrogen divacancies are most probably responsible for charge transfer in h-BN and transfer is provided by electrons.

Mechanism of Transverse Charge Transfer in Thin Films of Hexagonal Boron Nitride

Mechanism of Transverse Charge Transfer in Thin Films of Hexagonal Boron Nitride

High-Throughput Growth of Hexagonal Boron Nitride Film Using Porous-Structure Isolation Layer

Chemical vapor deposition is considered as the most hopeful method for the synthesis of large-area high-quality hexagonal boron nitride on the substrate of catalytic metal. However, the size the hexagonal boron nitride films are limited to the size of growth chamber, which indicates a lower production efficiency. In this paper, the utilization efficiency of growth chamber is highly improved by alternately stacking multiple pieces of Cu foils and carbon fiber surface felt with porous structure. Uniform and continuous hexagonal boron nitride films are prepared on Cu foils through chemical vapor deposition utilizing ammonia borane as the precursor. This work develops a simple and practicable method for high-throughput preparation of hexagonal boron nitride films, which could contribute to the industrial application of hexagonal boron nitride.

High responsivity and broadband photodetector based on SnS2/Ag2S heterojunction

• SnS 2 /Ag 2 S heterojunctions were designed on hexagonal boron nitride films. • The responsivity of SnS 2 /Ag 2 S heterojunctions higher than Ag 2 S or SnS 2 devices. • The responsivity of SnS 2 /Ag 2 S heterojunctions under 405 nm is 4.52 A/W. Photodetectors made of single Selenium sulfide (SnS 2 ) or Silver sulfide (Ag 2 S) are difficult to achieve superiority in performance. Therefore, it is a good idea to prepare SnS 2 /Ag 2 S heterojunctions on hexagonal boron nitride (hBN). The responsivity ( R ) of SnS 2 /Ag 2 S photodetector under 405 nm laser is 4.52 A/W, the specific detectivity ( D* ) is 3.53×10 11 Jones and the response speed reaches 37/50 ms. Ag 2 S quantum dots (QDs) overcome the problem of easy recombination of photogenerated carriers in SnS 2 . This makes the R of SnS 2 /Ag 2 S photodetectors about three orders of magnitude higher than that of Ag 2 S or SnS 2 devices.

Realizing avalanche criticality in neuromorphic networks on a 2D hBN platform.

Networks and systems which exhibit brain-like behavior can analyze information from intrinsically noisy and unstructured data with very low power consumption. Such characteristics arise due to the critical nature and complex interconnectivity of the brain and its neuronal network. We demonstrate a system comprising of multilayer hexagonal boron nitride (hBN) films contacted with silver (Ag), which can uniquely host two different self-assembled networks, which are self-organized at criticality (SOC). This system shows bipolar resistive switching between the high resistance state (HRS) and the low resistance state (LRS). In the HRS, Ag clusters (nodes) intercalate in the van der Waals gaps of hBN forming a network of tunnel junctions, whereas the LRS contains a network of Ag filaments. The temporal avalanche dynamics in both these states exhibit power-law scaling, long-range temporal correlation, and SOC. These networks can be tuned from one to another with voltage as a control parameter. For the first time, two different neural networks are realized in a single CMOS compatible, 2D material platform.

Influence of sapphire substrate with miscut angles on hexagonal boron nitride films grown by halide vapor phase epitaxy

(002) hexagonal boron nitride (h-BN) micron films were epitaxially grown on c-plane sapphire substrates with miscut angles to m-plane (m-miscut angle) by high-temperature and low-pressure halide vapor phase epitaxy.

Post-growth enhancement of CVD-grown hexagonal boron nitride films on sapphire

This study explores the effect of post-growth annealing on hexagonal boron nitride (hBN) films grown on c-sapphire substrates by chemical vapor deposition as post-growth annealing proves to overcome the equipment limitations of commercial thermal CVD systems and offers a simplistic way to enhance the properties of the as-grown hBN films useful for multiple-wafer scalability, thereby making it a practical approach for industrial manufacturing of high-quality hBN films. hBN films are grown at 1100 °C by CVD process and are annealed up to 1300 °C under nitrogen atmosphere in an annealing furnace system. It is observed that the annealed hBN films are continuous and have a pronounced granular structure as compared to the as-grown hBN films. The annealed BN films retain the 1:1 B:N stoichiometry and the hexagonal phase as observed in the as-grown hBN films. The optical bandgap of the annealed hBN films is augmented from 5.70 eV to 5.82 eV as a result of the annealing treatment, indicating enhanced optical properties.

Non-thermal and thermal effects on mechanical strain in substrate-transferred wafer-scale hBN films

Wafer-scale thin films of hexagonal boron nitride have exceptional thermal and mechanical properties, which harness the potential use of these materials in two-dimensional electronic, device applications. Along with unavoidable defects, grains, and wrinkles, which develop during the growth process, underlying substrates influence the physical and mechanical properties of these films. Understanding the interactions of these large-scale films with different substrates is, thus, important for the implementation of this 2D system in device fabrication. MOVPE-grown 2 and 30 nm hBN/sapphire films of size 2 in. diameter are delaminated chemically and transferred on quartz, SiO2/Si, and sapphire substrates. The structural characteristics of these films are investigated by employing Raman spectroscopy. Our results suggest that not only the roughness but also the height modulation at the surface of the substrates play a pivotal role in determining substrate-mediated mechanical strain inhomogeneity in these films. The statistical analysis of the spectral parameters provides us with the overall characteristics of the films. Furthermore, a Stark difference in the thermal evolution of strain in these films depending on substrate materials is observed. It has been demonstrated that not only the differential thermal expansion coefficient of the substrates and the films, but also slippage of the latter during the thermal treatment determines the net strain in the films. The role of the slippage is significantly higher in 2 nm films than in 30 nm films. We believe that the observations provide crucial information on the structural characteristics of the substrate-coupled wafer-scale hBN films for their future use in technology.

Synthesis of Graphene and Related Materials by Microwave-Excited Surface Wave Plasma CVD Methods

Several kinds of chemical vapor deposition (CVD) methods have been extensively used in the semiconductor industries for bulk crystal growth, thin film deposition, and nanomaterials synthesis. In this article, we focus on the microwave-excited surface wave plasma CVD (MW-SWP CVD) method for growth of graphene and related materials. The MW-SWP CVD system consisting of waveguide, slot antenna, and dielectric windows is significant for generating high density plasma with low electron temperature, enabling low temperature growth of materials without damaging the surface of base substrates. The synthesis of graphene and hexagonal boron nitride (hBN) films has been achieved on metals, semiconductors, insulators, and dielectric substrates for application in photovoltaics, sensors, batteries, supercapacitors, fuel cells, and various other electronic devices. The details of the synthesis process for graphene films, vertically-oriented graphene, doped-graphene, and hBN films by the MW-SWP CVD method are summarized to understand the growth mechanism, which will enable further development of the plasma CVD process for material synthesis at a low temperature for industrial applications.

Tuning infrared absorption in hyperbolic polaritons coated silk fibril composite

Advanced textiles for thermal management give rise to many functional applications and unveil a new frontier for the study of human thermal comfort. Manipulating the coated quasi-particles between the composite components offers a platform to study the advanced thermoregulatory textiles. Here, we propose that coating the hyperbolic polariton can be an effective tool to tune infrared absorption in hexagonal boron nitride-coated silk composite. Remarkably, we achieve significant tuning of the infrared absorption efficiency of silk fibrils through the designed hexagonal boron nitride film. The underlying mechanism is related to resonance coupling between hyperbolic phonon polaritons. We find a notably high infrared absorption efficiency, nearly 3 orders larger than that without hBN coating, which can be achieved in our composite system. Our results indicate the promising future of advanced polariton-coated textiles and open a pathway to guide the artificial-intelligence design of advanced functional textiles.

Chemical vapor deposition growth of magnesium-doped hexagonal boron nitride films via in situ doping

Chemical vapor deposition growth of magnesium-doped hexagonal boron nitride films via in situ doping

Enhancement of n-type conductivity of hexagonal boron nitride films by in-situ co-doping of silicon and oxygen

Effective doping of ultra-wide band gap semiconductors is of crucial importance, yet, remains challenging. Here, we report the enhancement of n-type conductivity of nanocrystalline hexagonal boron nitride (h-BN) films with simultaneous incorporation of Si and O while deposition by radio frequency magnetron sputtering method. The resultant h-BN films are of ∼50 nm in thickness, containing nitrogen vacancy (VN) defects. Incorporation of O together with Si results in effective healing of VN defects and significantly reduces electric resistivity in h-BN thin films. X-ray photoelectron spectroscopy results reveal that under B-rich condition, the substitutional O in VN bonding with B leads to the formation of Si–N, which thus plays an important role to the n-type conductivity in h-BN films. The temperature dependent electrical resistivity measurements of the Si/O co-doped h-BN films reveal two donor levels of 130 and 520 meV at room temperature and higher temperatures, respectively. The n-h-BN/p-Si heterojunctions demonstrate apparent rectification characteristics at room temperature, where the tunneling behavior dominates throughout the injection regimes due to the effective carrier doping. This work proposes an effective approach to enhance the n-type conductivity of h-BN thin films for future applications in electronics, optoelectronics and photovoltaics.