Hexagonal Boron Nitride Films Research Articles

Passivation of InP solar cells using large area hexagonal-BN layers

Surface passivation is crucial for many high-performance solid-state devices, especially solar cells. It has been proposed that 2D hexagonal boron nitride (hBN) films can provide near-ideal passivation due to their wide bandgap, lack of dangling bonds, high dielectric constant, and easy transferability to a range of substrates without disturbing their bulk properties. However, so far, the passivation of hBN has been studied for small areas, mainly because of its small sizes. Here, we report the passivation characteristics of wafer-scale, few monolayers thick, hBN grown by metalorganic chemical vapor deposition. Using a recently reported ITO/i-InP/p+-InP solar cell structure, we show a significant improvement in solar cell performance utilizing a few monolayers of hBN as the passivation layer. Interface defect density (at the hBN/i-InP) calculated using C–V measurement was 2 × 1012 eV−1cm−2 and was found comparable to several previously reported passivation layers. Thus, hBN may, in the future, be a possible candidate to achieve high-quality passivation. hBN-based passivation layers can mainly be useful in cases where the growth of lattice-matched passivation layers is complicated, as in the case of thin-film vapor–liquid–solid and close-spaced vapor transport-based III–V semiconductor growth techniques.

Growth of hexagonal boron nitride films on silicon substrates by low-pressure chemical vapor deposition

By respectively using Boron trichloride and ammonia as the boron source and the nitrogen source, and nitrogen as the carrier gas, hexagonal boron nitride films were grown on (100) silicon substrates via a low-pressure chemical vapor deposition method in a deposition temperature range of 900–1300 °C with a total pressure of 100 Pa. The effect of temperature on crystalline quality of the hexagonal boron nitride films was investigated using X-ray diffraction and Raman spectra. It was demonstrated that stoichiometric polycrystalline hexagonal boron nitride films were synthesized at 1200 °C. A deep-ultraviolet photodetector based on a 2.3 µm hBN film exhibited a photo-to-dark-current ratio of ~ 312 at 20 V bias under the irradiation of a UV-enhanced xenon lamp. It indicates that hBN films are promising functional materials for high-performance solar-blind photodetectors.

Synthesis of centimeter-scale high-quality polycrystalline hexagonal boron nitride films from Fe fluxes.

High-quality hexagonal BN (hBN) crystals, owing to their irreplaceable roles in new functional devices such as universal substrates and excellent layered insulators are exceedingly required in the field of two-dimensional (2D) materials. Although large-scale monolayer hBN crystals have been successfully grown on catalytic metals, the synthesis of large-area continuous hBN films with thickness in microns is challenging, hindering their applications at the mesoscopic level. Herein, we report the single-metal flux growth of centimeter-large, micron-thick, and high-quality continuous hBN films by balancing the grain size and coverage. The as-grown films can be readily exfoliated and transferred onto arbitrary substrates. Isotopically engineered hBN crystals can be obtained as well by the method. The narrow Raman line widths of the intralayer E2g mode peak (2.9 cm-1 for h11BN, 3.3 cm-1 for h10BN, and 7.9 cm-1 for hNaBN) and ultrahigh thermal conductivity (830 W m-1 K-1 for 4L h11BN) demonstrate high crystal quality and low defect density. Our results provide the foundation for the cost-efficient and lab-achievable synthesis of high-quality hBN films aimed at its mesoscopic applications.

Deep-UV hexagonal boron nitride (hBN)/BAlN distributed Bragg reflectors fabricated by RF-sputtering

The hexagonal boron nitride (hBN) and BAlN films were prepared by RF-sputtering, which were used as the low and high refractive index layers. A series of hBN/BAlN distributed Bragg reflectors (DBRs) were prepared on sapphire substrate. The reflectivity of 9-pair hBN/BAlN (39 nm/33 nm) DBR reached 90% at 300 nm with a bandwidth of 45 nm, and which of 6-pair hBN/BAlN (35 nm/29 nm) reached 52% at 280 nm. The hBN/BAlN DBRs can be used to achieve higher reflectivity in shorter UV bands with the improvement of BAlN material quality through the growth condition optimization.

Dynamical conductivity of lithium-intercalated hexagonal boron nitride films: A memory function approach

This work is a study of dynamical conductivity of a quasi-two-dimensional heterostructure Li(BN)8. The conducting electrons have a free-electron-like parabolic dispersion and are assumed to scatter only on acoustic phonons. The approach used to derive the generalized Drude relation with frequency and temperature dependent relaxation consists of defining the induced current density as a function of the electron-hole operator. This element is proportional to the perturbating electric field and is decomposed into the zeroth and second order of the electron-phonon interaction. The electron-hole operator is then evaluated using the Heisenberg equation. The connection between the electron-hole operator and the memory function is shown. The analytical properties of the imaginary and real part of the frequency and temperature dependent memory function, that describe electron scattering on acoustic and optical phonons, are examined in detail. Finally, the dynamical conductivity of Li(BN)8 is calculated, with the explanation of all relevant features originating from the memory function.

Vacancy-Assisted Growth Mechanism of Multilayer Hexagonal Boron Nitride on a Fe2B Substrate.

The controllable synthesis of large-area and uniform hexagonal boron nitride (h-BN) films has been recently achieved on metal-boron alloy catalysts with the use of N2 feedstock, representing important progress in an economic and environmentally friendly process. However, the systematic investigation of the growth mechanism is still lacking, which impedes the further development of this method. In this work, on the basis of density functional theory (DFT) calculations and experiments, we reveal the vacancy-assisted growth mechanism of h-BN on Fe2B substrate. It is found that B vacancies created by the formation of BN dimers play important roles in the migration of B and N atoms near the catalyst surface. The diffusions of B and N atoms in the Fe2B substrate need to overcome energy barriers of only less than 1.5 eV, which enables abundant dissolution of N atoms near the catalytic surface. Moreover, we found the critical barrier for h-BN growth is in the nucleation stage, which is ∼2 eV. These advantages enable the synthesis of h-BN at a low temperature of 700 K in our experiments. This vacancy-assisted growth of h-BN films on Fe2B substrates is beneficial to the wafer-scale fabrication of multilayer materials, paving the way to potential applications in two-dimensional electronic and optoelectronic devices.

Interlayer Bound Wannier Excitons in Germanium Sulfide.

We report a cathodoluminescence (CL) study of layered germanium sulfide (GeS) where we observe a sharp emission peak from flakes covered with a thin hexagonal boron nitride film. GeS is a material that has recently attracted considerable interest due to its emission in the visible region and its strong anisotropy. The measured CL peak is at ~1.69 eV for samples ranging in thickness from 97 nm to 45 nm, where quantum-confinement effects can be excluded. By performing ab initio ground- and excited-state simulations for the bulk compound, we show that the measured optical peak can be unambiguously explained by radiative recombination of the first free bright bound exciton, which is due to a mixing of direct transitions near the -point of the Brillouin Zone and it is associated to a very large optical anisotropy. The analysis of the corresponding excitonic wave function shows a Wannier–Mott interlayer character, being spread not only in-plane but also out-of-plane.

Thermal conductivity of monolayer hexagonal boron nitride: From defective to amorphous

Through extensive molecular dynamics simulations, we completed the thermal transport properties study of the monolayer hexagonal boron nitride (h-BN) films from the defect state to the amorphous state. To this end, a defective h-BN model construction program has been developed, which can customize the nature of the defects and realize the transition from the defect state to the amorphous state. By performing homogeneous non-equilibrium molecular dynamics simulations based on Tersoff multi-body potential, the thermal conductivity results of large-size h-BN films have been achieved. For the defect state h-BN, we studied the effect of different defect types and concentrations on its thermal conductivity and verified it from the phonon mechanism. After the transition to two-dimensional amorphous BN, we discussed and analyzed the influence of amorphous concentration, amorphous defect ratio and temperature on its thermal transport properties. On this basis, the restriction relationship between phonon-phonon scattering term and phonon-defect scattering term was also proposed from the atomic level mechanisms. Our results constitute a step in the deterministic engineering of thermal management devices in 2D materials, and hold great promise for the application of h-BN defect engineering in the field of nanomaterial thermal design.

Cross-overlapped flat-silver/hexagonal boron nitride for translucent heat-reflective coatings

Multifunctional heat-reflective coatings are increasingly in demand for use in energy-efficient buildings and vehicles, as well as for optoelectronic devices. Conventional heat-reflective coatings (e.g. dielectric/metal/dielectric (DMD)) exhibit poor structural flexibility and combine complex fabrication process of multilayered structures, which hinders their scalable production. Herein, a low-cost translucent heat reflective mechanism is presented using single-coat composite film of flat-silver (flat-Ag) and hexagonal boron nitride (hBN) as an alternative to the multilayered DMD structure. The conversion of grey Ag-nanoparticles into shiny flat-Ag results in significant enhancement (~7 times) in infrared (IR) reflection. The dielectric-hBN layers integrated with flat-Ag improves the visible transmittance and heat-dissipating properties of coatings. The flat-Ag/hBN ink sprayed onto a flexible, transparent, and adhesive substrate of polydimethylsiloxane (PDMS) exhibits excellent IR-reflection (~70%) with ~57% visible transmittance. When irradiated by an IR heat source, the film displays 8.9 ± 0.5 °C lower temperature than those of uncoated surfaces, that results in reduction of the interior air-temperature by 5.4 ± 0.5 °C compared to the uncoated control. This newfabrication process of heat-reflective coatings with structural flexibility and optical transparency has great potentials to be used for thermal management in building and transportation sectors.

Tunable plasmon-phonon polaritons in anisotropic 2D materials on hexagonal boron nitride

Abstract Mid-infrared (MIR) plasmon-phonon features of heterostructures composing of a plasmonic anisotropic two-dimensional material (A2DM) on a hexagonal boron nitride (hBN) film are analyzed. We derive the exact dispersion relations of plasmon-phonons supported by the heterostructures and demonstrate the possibility of topological transitions of these modes within the second Reststrahlen band of hBN. The topological transitions lead to enhanced local density of plasmon-phonon states, which intensifies the spontaneous emission rate, if the thickness of the hBN layer is appropriately chosen. We also investigate a lateral junction formed by A2DM/hBN and A2DM, demonstrating that one can realize asymmetric guiding, beaming, and unidirectionality of the hybrid guided modes. Our findings demonstrate potential capabilities of the A2DM/hBN heterostructures for active tunable light–matter interactions and asymmetric in-plane polariton nanophotonics in the MIR range.

On the recovery of 2DEG properties in vertically ordered h-BN deposited AlGaN/GaN heterostructures on Si substrate

Vertically ordered hexagonal boron nitride (h-BN) films were successfully sputtered on AlGaN/GaN heterostructure (HS) using high power impulse magnetron sputtering at room temperature. The h-BNs vertical ordering along the (0002) plane was confirmed using high-resolution transmission electron microscopy. After the h-BN deposition, degradation of two-dimensional electron gas (2DEG) properties was observed in AlGaN/GaN HS. Full recovery of 2DEG mobility, along with an improvement in sheet resistance and an increase in sheet carrier concentration was obtained after rapid thermal annealing at 500 °C for 300 s in a N2 atmosphere, which is due to the reduction of sputtering related structural damage.

Synthesis of High-Quality Multilayer Hexagonal Boron Nitride Films on Au Foils for Ultrahigh Rejection Ratio Solar-Blind Photodetection.

Solar-blind photodetectors have widespread applications due to the unique merit of a "black background" on the earth. However, most solar-blind photodetectors reported previously exhibited quite low rejection ratios (R200nm/R280nm < 103) and were interfered with by light longer than 280 nm. Herein, by an ambient pressure chemical vapor deposition (CVD) method, large-area, clean, and uniform two-dimensional (2D) multilayer h-BN films with different thicknesses have been successfully synthesized on Au foils. The synthesized multilayer h-BN film is transparent to light longer than 280 nm, showing excellent optical and optoelectronic properties to weak solar-blind light (μW/cm2). This sensitive solar-blind h-BN photodetector exhibits ultrahigh rejection ratios (R220nm/R280nm > 103 and R220nm/R290nm > 104), a low dark current (102 fA), and a large detectivity (3.9 × 1010 Jones). It is noteworthy that the rejection ratio (R220nm/R290nm) here is superior to most of those previously reported based on traditional semiconductors. This large-scale, clean, and uniform multilayer h-BN film will contribute to the progress of next-generation optoelectronic devices.

Nanoscale Molecular Building Blocks for Layer‐by‐Layer Assembly

AbstractUltrathin molecular films can be used as building blocks to enable rational design of functionalities at the molecular level. A versatile and scalable approach to fabricate transfer‐compatible ultrathin molecular films with a broad range of physiochemical properties is reported. Different molecules in layers of uniform nanoscale thickness are deposited on monolayer hexagonal boron nitride (ML h‐BN) film. Initially, the deposited molecules form islands due to limited molecular diffusion during deposition on ML h‐BN, but postannealing effectively transforms them to the thermodynamically preferred film structures. Ultrathin building blocks composed of ML h‐BN and different molecules (C60, pentacene, C44H32N2) are realized, then transferred layer‐by‐layer (L‐by‐L) to form materials with precisely controlled composition and thickness, including donor/acceptor superlattices. This approach can facilitate the integration of nanoscale molecules with different material platforms for advanced functionalities.

Epitaxially Grown Hexagonal Boron Nitride Films on Sapphire and Silicon Substrates

Hexagonal boron nitride (hBN) films were epitaxially grown on (100)-Oriented silicon and c-plane sapphire (α-Al2O3) substrates via a low-pressure chemical vapor deposition (LPCVD) method with boron trichloride (BCl3) and ammonia (NH3) as the boron source and nitrogen source. Crystalline quality differences between hBN films grown on different substrates are studied and discussed by XPS, Raman spectroscopy, XRD and SEM characterizations. All the characterization results indicate that the sapphire substrate is more suitable for epitaxial growth of hBN films than silicon substrates.

Thermal History-Dependent Current Relaxation in hBN/MoS2 van der Waals Dimers.

Combining atomically thin layers of van der Waals (vdW) materials in a chosen vertical sequence is an emerging route to create devices with desired functionalities. While this method aims to exploit the individual properties of partnering layers, strong interlayer coupling can significantly alter their electronic and optical properties. Here we explored the impact of the vdW epitaxy on electrical transport in atomically thin molybdenum disulfide (MoS2) when it forms a vdW dimer with crystalline films of hexagonal boron nitride (hBN). We observe a thermal history-dependent long-term (over ∼40 h) current relaxation in the overlap region of MoS2/hBN heterostructures, which is absent in bare MoS2 layers (or homoepitaxial MoS2/MoS2 dimers) on the same substrate. Concurrent relaxation in the low-frequency Raman modes in MoS2 in the heterostructure region suggests a slow structural relaxation between trigonal and octahedral polymorphs of MoS2 as a likely driving mechanism that also results in inhomogeneous charge distribution in the MoS2 layer. Our experiment yields an aspect of vdW heteroepitaxy that can be generic to electrical devices with atomically thin transition-metal dichalcogenides.

Mechanical properties of [formula omitted]-bonded carbon and boron nitride 2D membranes: A first principles study

Transverse compression and chemical passivation of few graphene or hexagonal boron nitride layers have been shown to be viable means to form ultrathin membranes rich in sp3 bonds exhibiting intriguing mechanical behaviors. Here, density functional theory calculations are used to calculate second- and third-order elastic constants, as well as ideal breaking strengths of both layered and sp3-bonded arbon and boron nitride films. The sp3-bonded membranes consist of two or three planes of both existing and new in silico-designed bulk structures of carbon and boron nitride, and have either both, one, or none of the surfaces passivated with hydrogen. This study shows that carbon and boron nitride membranes rich in sp3 bonds exhibit mechanical properties and ideal breaking strengths that compete with those of sp2-bonded layered films, and that there is a plethora of plausible stable structures of such sp3-bonded membranes, showing diverse electronic properties, and longitudinal and transverse mechanical behaviors. This study also suggests that by controlling thickness of the film and by tuning chemistry of the supporting substrate, transverse compression of layered carbon or hexagonal boron nitride films could lead to the formation of stable sp3-bonded 2D materials exposing clean surfaces, and therefore transferable, and exhibiting intriguing anisotropic mechanical and electronic properties.

Coherent dynamics and mapping of excitons in single-layer MoSe2 and WSe2 at the homogeneous limit

We perform coherent nonlinear spectroscopy of excitons in single-layers of MoSe$_2$ and WSe$_2$ encapsulated between thin films of hexagonal boron nitride. Employing four-wave mixing microscopy we identify virtually disorder free areas, generating exciton optical response at the homogeneous limit. Focussing on such areas, we measure exciton homogeneous broadening as a function of environmental factors, namely temperature and exciton density. Exploiting FWM imaging, we find that at such locations, nonlinear absorption of the exciton excited states and their coherent couplings can be observed. Using the WSe$_2$ heterostructure, we infer coherence and density dynamics of the exciton 2S state. Owing to its increased radiative lifetime, at low temperatures, the dephasing of the 2S state is longer than of the 1S transition. While scanning various heterostructures across tens of micrometers, we conclude that the disorder, principally induced by strain variations, remain to be present creating spatially varying inhomogeneous broadening.

Bandgap opening in MoTe2 thin flakes induced by surface oxidation

Recently, the layered transition metal dichalcogenide 1T′-MoTe2 has generated considerable interest due to their superconducting and non-trivial topological properties. Here, we present a systematic study on 1T′-MoTe2 single-crystal and exfoliated thin-flakes by means of electrical transport, scanning tunnelling microscope (STM) measurements and band structure calculations. For a bulk sample, it exhibits large magneto-resistance (MR) and Shubnikov–de Hass oscillations in ρxx and a series of Hall plateaus in ρxy at low temperatures. Meanwhile, the MoTe2 thin films were intensively investigated with thickness dependence. For samples, without encapsulation, an apparent transition from the intrinsic metallic to insulating state is observed by reducing thickness. In such thin films, we also observed a suppression of the MR and weak anti-localization (WAL) effects. We attributed these effects to disorders originated from the extrinsic surface chemical reaction, which is consistent with the density functional theory (DFT) calculations and in-situ STM results. In contrast to samples without encapsulated protection, we discovered an interesting superconducting transition for those samples with hexagonal Boron Nitride (h-BN) film protection. Our results indicate that the metallic or superconducting behavior is its intrinsic state, and the insulating behavior is likely caused by surface oxidation in few layer 1T’-MoTe2 flakes.

On the Limits of Scanning Thermal Microscopy of Ultrathin Films.

Heat transfer processes in micro- and nanoscale devices have become more and more important during the last decades. Scanning thermal microscopy (SThM) is an atomic force microscopy (AFM) based method for analyzing local thermal conductivities of layers with thicknesses in the range of several nm to µm. In this work, we investigate ultrathin films of hexagonal boron nitride (h-BN), copper iodide in zincblende structure (γ-CuI) and some test sample structures fabricated of silicon (Si) and silicon dioxide (SiO2) using SThM. Specifically, we analyze and discuss the influence of the sample topography, the touching angle between probe tip and sample, and the probe tip temperature on the acquired results. In essence, our findings indicate that SThM measurements include artefacts that are not associated with the thermal properties of the film under investigation. We discuss possible ways of influence, as well as the magnitudes involved. Furthermore, we suggest necessary measuring conditions that make qualitative SThM measurements of ultrathin films of h-BN with thicknesses at or below 23 nm possible.

Ultra-thin films of solution-exfoliated hexagonal boron nitride by Langmuir deposition

Langmuir films of pure, solution-exfoliated hexagonal boron nitride, transferable to arbitrary substrates, are demonstrated as promising dielectric layers suitable for transparent and flexible optoelectronics.