Growth Of Hexagonal Boron Nitride Research Articles

Homoepitaxy of Boron Nitride on Exfoliated Hexagonal Boron Nitride Flakes.

Although large efforts have been made to improve the growth of hexagonal boron nitride (hBN) by heteroepitaxy, the non-native substrates remain a fundamental factor that limits the quality. This problem can be solved by homoepitaxy, which is the growth of hBN on hBN substrates. In this report, we demonstrate the homoepitaxial growth of triangular BN grains on exfoliated hBN flakes by Metal-Organic Vapor Phase Epitaxy and show by atomic force microscopy and photoluminescence that the stacking of these triangular islands can deviate from the AA' stacking of hBN. We show that the stacking order is enforced by the crystallographic direction of the edge of the exfoliated hBN flakes, with armchair edges allowing for centrosymmetric stacking, whereas zigzag edges lead to the growth of noncentrosymmetric BN polytypes. Our results indicate pathways to grow homoepitaxial BN with tunable layer stacking, which is required to induce piezoelectricity or ferroelectricity.

(Invited) Growth of Hexagonal Boron Nitride by MOCVD for Electronic and Photonic Applications

Hexagonal boron nitride (h-BN) is a 2-dimensional (2D) layered insulating material. Thanks to its wide bandgap of ~6 eV and high thermal conductivity, h-BN has been widely utilized as an ideal substrate, gate dielectric material, passivation layer, and atomic tunnelling layer for 2D electronic devices. Recently, it has gained more attention as active material for photonics applications due to its potential for stable quantum emitters in the visible spectral range, as well as deep-ultraviolet emitters and detectors due to its exceptionally high internal quantum efficiency despite its indirect bandgap. However, mechanically exfoliated bulk h-BN and h-BN films grown on catalytic metal substrates by conventional chemical vapor deposition (CVD) systems have been mainly used to study the fundamental properties, lacking in scalability for practical implementation of h-BN.Here, we exploit the scalable approach to grow h-BN on various substrates such as sapphire, Si, and GaN, by using metal-organic chemical vapor deposition (MOCVD). A few-layer h-BN films were successfully grown on those substrates. In particular, we successfully accomplished the conformal growth of few-layer h-BN over an array of Si-based nanotrenches with 45 nm pitch and the aspect ratio of ~ 7:1. Moreover, it was found that at a specific MOCVD growth condition, a very unique h-BN film can be grown on GaN substrates, in which few-layer h-BN film is suspended on GaN nanoneedles. The combination of state-of-the-art microscopic and spectroscopic analyses revealed that the suspended h-BN films exhibit unprecedented deep ultraviolet photoluminescence spectra with local variation in band-edge emission. In addition, the h-BN films show unprecedented atomic stacking configuration, the mechanism of which will be discussed with optical and structural characterizations and theoretical calculations.

Amorphous and hexagonal boron nitride growth using bromide chemistry

Amorphous and hexagonal boron nitride growth using bromide chemistry

Growth of Hexagonal Boron Nitride (hBN) on Silicon Carbide Substrates by the Physical Vapor Transport Method

Growth of Hexagonal Boron Nitride (hBN) on Silicon Carbide Substrates by the Physical Vapor Transport Method

Recent progress on chemical vapor deposition growth of 2D materials

Remarkable progress has been made to understand the chemical vapor deposition (CVD) of two-dimensional (2D) materials over the last two decades. The review summarized the state-of-the-art experimental synthesis and modelling and simulation on 2D materials CVD growth. Firstly, the family of 2D materials, and their CVD growth processes are introduced. Secondly, the experimental synthesis and modelling and simulation on graphene growth are discussed. In particular, the applications of reactive molecular dynamics methods, kinetic Monte Carlo and density-functional theory in 2D material growth are addressed. Then, the CVD growth of hexagonal boron nitride and transition metal dichalcogenides are further discussed, focusing on the effects of reaction conditions (growth temperature, pressure, vapour-phase composition, etc.) on the domain morphologies, edge structures and grain boundaries of 2D materials. Last, conclusions and outlooks are presented.

Interplay of Kinetic Limitations and Disintegration: Selective Growth of Hexagonal Boron Nitride and Borophene Monolayers on Metal Substrates.

The CVD growth of bielemental 2D-materials by using molecular precursors involves complex formation kinetics taking place at the surface and sometimes also subsurface regions of the substrate. Competing microscopic processes fundamentally limit the parameter space for optimal growth of the desired material. Kinetic limitations for diffusion and nucleation cause a high density of small domains and grain boundaries. These are usually overcome by increasing the growth temperature and decreasing the growth rate. In contrast, the nature of molecular precursors with limited thermal stability can result in dissociation and preferential desorption, leading to an undesired or ill-defined composition of the 2D-material. Here we demonstrate these constraints in a combined low-energy electron diffraction and low-energy electron microscopy study by examining the selective formation of single-layer hexagonal boron nitride (hBN) and borophene on Ir(111) using a borazine precursor. We derive a temperature-pressure phase diagram and apply classical nucleation theory to describe our results. By considering the competing processes, we find an optimum growth temperature for hBN of 950 °C. At lower temperatures, the hBN island density is increased, while at higher temperatures the precursor disintegrates and borophene is formed. Our results introduce an additional aspect that must be considered in any high-temperature growth of bielemental 2D-materials from single molecular precursors.

Chemically detaching hBN crystals grown at atmospheric pressure and high temperature for high-performance graphene devices

In this work, we report on the growth of hexagonal boron nitride (hBN) crystals from an iron flux at atmospheric pressure and high temperature and demonstrate that (i) the entire sheet of hBN crystals can be detached from the metal in a single step using hydrochloric acid and that (ii) these hBN crystals allow to fabricate high carrier mobility graphene-hBN devices. By combining spatially-resolved confocal Raman spectroscopy and electrical transport measurements, we confirm the excellent quality of these crystals for high-performance hBN-graphene-based van der Waals heterostructures. The full width at half maximum of the graphene Raman 2D peak is as low as 16 cm−1, and the room temperature charge carrier mobilitiy is around 80 000 cm2/(Vs) at a carrier density 1 × 1012 cm−12. This is fully comparable with devices of similar dimensions fabricated using crystalline hBN synthesized by the high pressure and high temperature method. Finally, we show that for exfoliated high-quality hBN flakes with a thickness between 20 and 40 nm the line width of the hBN Raman peak, in contrast to the graphene 2D line width, is not useful for benchmarking hBN in high mobility graphene devices.

Scaling up of Growth, Fabrication, and Device Transfer Process for GaN‐based LEDs on H‐BN Templates to 6‐inch Sapphire Substrates

AbstractThe growth of hexagonal boron nitride (h‐BN) and van der Waals (vdW) epitaxy of blue multi‐quantum well (MQW) GaN‐based LED heterostructures on 6‐inch sapphire substrates using metal‐organic chemical vapour deposition (MOCVD) is demonstrated. Challenges associated with the growth of large surface h‐BN and the subsequent vdW epitaxy of GaN‐based LED heterostructures are discussed. To overcome these challenges, the spatial uniformity is controlled of the growth temperature, optimizes the slope of temperature variations during the growth and cooling process, and manages the surface temperature during switching of gas flows. With these adaptations, high quality GaN‐based LED heterostructures are grown on h‐BN without any spontaneous delamination. The GaN‐based LED devices are then fabricated on a 6‐inch sapphire wafer, which are lifted off as a membrane and transferred to a flexible copper support. These GaN‐based LED devices emitt bright blue illumination with an electroluminescence peak at 437 nm. This scaling up of growth, lift‐off, and transfer can lead to the commercialization of GaN‐based LEDs on h‐BN template on 6‐inch sapphire substrates, with a process compatible with current modern equipment for the fabrication of LEDs and electronic devices.

Growth and pre-cooling doping of semiconducting C-doped hBN as a sensitive thermistor

This letter describes the growth of hexagonal boron nitride (hBN) on plateau and terrace regions of nickel foil by tuning the V/III ratio of NH3 and B2H6, followed by pre-cooling carbon doping with CH4. The V/III ratio = 120 with a growth duration of 30 min is good to grow ∼ 14 nm-thick hBN for the application requiring it as an insulator. Pre-cooling doping by CH4 is possible because crystallizing BN prevents carbon from diffusing into bulk nickel and allows the substitution of nitrogen atoms. The C-doped hBN is having a lowered bandgap of 5.79/5.47 eV, sub-bandgap of 2.86 eV, narrowed Raman E2g peak on plateau and terrace regions, a slight decrease in nitrogen atomic%, and reduced thickness. These convert hBN from an insulator into a semiconductor. The C-doped hBN is a very sensitive thermistor with a temperature coefficient of resistance up to –0.037 K−1.

Kinetics effect of hydrogen passivation on the zigzag edge growth of h-BN

Edge kinetics in 2D structures has been a key to understanding the growth. In this paper, the effect of hydrogen passivation on the growth of hexagonal boron nitride (h-BN) was studied. Without hydrogen, the filling process of the gap on bare edges of h-BN is difficult because of the formation of dimers that distorts the edge. With hydrogen passivation, such difficulty can be largely reduced. In addition, hydrogen passivation can reduce the edge bending to the substrate. In summary, the amount of hydrogen passivation during the growth is the long-ignored parameter and can be the key to a good crystal quality.

Large-Area Orientation-Controlled Growth of Hexagonal Boron Nitride on Liquid Copper

Two-dimensional semiconductors have great potential in applications of high-performance electrical and optoelectrical devices. Many efforts have been made to meet the challenge of reducing charge scattering between semiconducting and dielectric layers. Hexagonal boron nitride (h-BN) with a large band gap and a flat surface is thus an ideal dielectric layer for addressing this particular requirement. Therefore, preparing high-quality h-BN is an important topic in the electronic industry. In this work, two-opposite orientation growth modes of h-BN domains on liquid Cu are discovered. Comprehensive characterizations verify this anti-orientation feature of the as-grown domains and the grain boundary-free nature in the merged domains with the same orientation. The orientation-controlled h-BN domain growth is achieved by tuning the growth temperature and H2 and Ar flow rates, where the proportion of 0°-orientation domains is modulated in a wide range from ∼54.4 to 75%. Furthermore, based on the controllable growth of oriented h-BN, centimeter-sized h-BN films with high quality are produced.

Edge-controlled growth of single-crystal hexagonal boron nitride domains by chemical vapor deposition

Understanding the role of the edge during the growth of hexagonal boron nitride (hBN) by chemical vapor deposition (CVD) is essential for its controllable synthesis and applications. We demonstrated two kinds of hydrogen-induced edge evolution of single-crystal hBN domains over time on liquid Cu via CVD. The morphology and edge evolution of the hBN domains in growth were well explained by the kinetic Wulff construction theory. This work provides a detailed understanding of the role of the edge during the growth progress of hBN, which will be helpful for the production of wafer-scale single-crystalline hBN.

Towards the Growth of Hexagonal Boron Nitride on Ge(001)/Si Substrates by Chemical Vapor Deposition.

The growth of hexagonal boron nitride (hBN) on epitaxial Ge(001)/Si substrates via high-vacuum chemical vapor deposition from borazine is investigated for the first time in a systematic manner. The influences of the process pressure and growth temperature in the range of 10−7–10−3 mbar and 900–980 °C, respectively, are evaluated with respect to morphology, growth rate, and crystalline quality of the hBN films. At 900 °C, nanocrystalline hBN films with a lateral crystallite size of ~2–3 nm are obtained and confirmed by high-resolution transmission electron microscopy images. X-ray photoelectron spectroscopy confirms an atomic N:B ratio of 1 ± 0.1. A three-dimensional growth mode is observed by atomic force microscopy. Increasing the process pressure in the reactor mainly affects the growth rate, with only slight effects on crystalline quality and none on the principle growth mode. Growth of hBN at 980 °C increases the average crystallite size and leads to the formation of 3–10 well-oriented, vertically stacked layers of hBN on the Ge surface. Exploratory ab initio density functional theory simulations indicate that hBN edges are saturated by hydrogen, and it is proposed that partial de-saturation by H radicals produced on hot parts of the set-up is responsible for the growth.

Neural network approach to diffusion of B and N adatoms on the Pt(111) surface

Artificial neural network boasts an outstanding class in terms of its performance and efficiency and is being spread widely in most fields of physics. We report our investigation of the diffusions of boron and nitrogen adatoms on the Pt(111) surface by performing molecular dynamics simulations equipped with machine-learned potentials. Platinum is commonly used as a substrate for the growth of hexagonal boron nitride (h-BN) thin films, and the diffusion of B and N atoms on the substrate, which are decomposed from the precursor molecules, plays important roles in the initial stages of h-BN growth. The two-dimensional potential energy surfaces and the trajectories of the B and N adatoms are consistent with the DFT calculation. The Arrhenius plots of the diffusion coefficients produce the diffusion barriers of the B and N adatoms on Pt(111), which agree well with the DFT barriers.

Mechanistic insights into supersaturation mediated large area growth of hexagonal boron nitride for graphene electronics

Supersaturation mediated large area growth of hexagonal boron nitride (h-BN) for graphene electronics.

Growth of hexagonal boron nitride (hBN) on silicon carbide substrates by the physical vapor transport method

The possibility of growing hexagonal boron nitride (hBN) of high structural perfection on hexagonal silicon carbide (SiC) substrates using the physical vapor transport method is demonstrated. The results obtained indicate that this method is promising for the formation of large-area high-quality hBN/SiC heterostructures in the course of one technological process, which have a high potential for device applications. Keywords: hexagonal boron nitride, high-temperature sublimation from the gas phase, Raman spectroscopy, photoluminescence.

Epitaxial Intercalation Growth of Scalable Hexagonal Boron Nitride/Graphene Bilayer Moiré Materials with Highly Convergent Interlayer Angles.

Vertically stacked two-dimensional van der Waals (vdW) heterostructures with specific interlayer angles exhibit peculiar physical properties. Nowadays, most of the stacked layers are fabricated by mechanical exfoliation followed by precise transfer and alignment with micrometer spatial accuracy. This stringent ingredient of sample preparation limits the productivity of device fabrication and the reproducibility of device performance. Here, we demonstrate the one-pot chemical vapor deposition growth of hexagonal boron nitride (hBN)/graphene bilayers with a high-purity moiré phase. The epitaxial intercalation of graphene under a hydrogen-terminated hBN template leads to convergent interlayer angles of less than 0.5°. The near 0° stacking angle shows almost 2 orders of magnitude higher likelihood of occurrence compared with angles larger than 0.5°. The bilayers show a substantial enhancement of carrier mobility compared with monolayer graphene owing to protection from the top hBN layer. Our work proposes a large-scale fabrication method of hBN/graphene bilayers with a high uniformity and controlled interlayer rotation and will promote the production development for high-quality vdW heterostructures.

Synthesis of Three-Dimensionally Interconnected Hexagonal Boron Nitride Networked Cu-Ni Composite

A three-dimensionally interconnected hexagonal boron nitride (3Di-hBN) networked Cu-Ni (3DihBN-Cu-Ni) composite was successfully synthesized in situ using a simple two-step process which involved the compaction of mixed Cu-Ni powders (70 wt.% Cu and 30 wt.% Ni) into a disc followed by metal-organic chemical vapor deposition (MOCVD) at 1000 <sup>o</sup>C. During MOCVD, the Cu-Ni alloy grains acted as a template for the growth of hexagonal boron nitride (hBN) while decaborane and ammonia were used as precursors for boron and nitrogen, respectively. Boron and nitrogen atoms diffused into the Cu-Ni solution during the MOCVD process and precipitated out along the Cu-Ni interfaces upon cooling, resulting in the formation of the 3Di hBN-Cu-Ni composite. Energy-dispersive spectroscopic analysis confirmed the presence of boron and nitrogen atoms at the interfaces of Cu-Ni alloy grains. Optical microscopy examination indicated that there was a minimum amount of bulk hBN at a certain compaction pressure (280 MPa) and sintering time (30 min). Scanning electron microscopy and transmission electron microscopy revealed that an interconnected network of hBN layers surrounding the Cu-Ni grains developed in the 3Di-hBN-Cu-Ni composite. This 3Di-hBN network is expected to enhance the mechanical, thermal, and chemical properties of the 3Di-hBN-Cu-Ni composite. Moreover, the foam-like 3Di-hBN extracted from 3Di-hBN-Cu-Ni composite could have further applications in the fields of biomedicine and energy storage.

Direct Growth of Hexagonal Boron Nitride on Photonic Chips for High-Throughput Characterization

Adapting optical microscopy methods for nanoscale characterization of defects in two-dimensional (2D) materials is a vital step for photonic on-chip devices. To increase the analysis throughput, waveguide-based on-chip imaging platforms have been recently developed. Their inherent disadvantage, however, is the necessity to transfer the 2D material from the growth substrate to the imaging chip, which introduces nonuniform material coverage and contamination, potentially altering the characterization results. Here we present a unique approach to circumvent these shortfalls by directly growing a widely used 2D material (hexagonal boron nitride, hBN) on silicon nitride chips and optically characterizing the defects in the intact as-grown material. We compare the direct growth approach to the standard PMMA-assisted wet transfer method and confirm the clear advantages of the direct growth. While demonstrated with hBN in the current work, the method can be extended to other 2D materials.

Silicon-assisted growth of hexagonal boron nitride to improve oxidation resistance of germanium

Hexagonal boron nitride (h-BN) is an exciting two dimensional dielectric due to its atomic flatness, free of dangling bonds, exceptional thermal and chemical stabilities. Here we report a method of silicon-assisted synthesis of monolayer h-BN on germanium (Ge) by chemical vapor deposition. The silicon atoms dissolve into Ge (110) and promote the growth of h-BN domains by formation of Si–N bonds. The oxidation dynamic shows that monolayer h-BN film is a high-performance passivation layer, preserving Ge from oxidation in air at high temperature. This work sheds lights on the outstanding oxidation resistance of h-BN for further Ge-based electronic devices.