hBN Film Research Articles

Polymer transfer technique for strain-activated emission in hexagonal boron nitride.

We present a hexagonal boron nitride (hBN) polymer-assisted transfer technique and discuss subtleties about the process. We then demonstrate localized emission from strained regions of the film draped over features on a prepatterned substrate. Notably, we provide insight into the brightness distribution of these emitters and show that the brightest emission is clearly localized to the underlyin-g substrate features rather than unintentional wrinkles present in the hBN film. Our results aide in the current discussion surrounding scalability of single photon emitter arrays.

Toward non-gas-permeable hBN film growth on smooth Fe surface

Flexible, transparent, and thermally stable gas barrier films are required to seal organic-based ultra-thin, flexible, and transparent electronic devices against moisture. Thermally stable, two-dimensional hexagonal boron nitride (hBN) is an ideal non-gas-permeable material with high transparency and flexibility. Nevertheless, the polycrystalline multilayer hBN (m-hBN) grown on a rough Fe foil by chemical vapor deposition is not sufficient for use as a gas barrier due to the non-uniformity and discontinuity of the film. Here, we report a novel method for synthesizing highly uniform and continuous m-hBN films on smooth Fe foil on a wafer scale via deposition of an amorphous Fe layer on a rough Fe foil. The amorphous Fe layer on a unary Fe foil is effectively recrystallized to become a smooth surface via post-thermal annealing treatment at 1100 °C. The smoothed surface allows for the uniform precipitation of B and N atoms to form a highly continuous m-hBN film, as confirmed by cross-sectional transmission electron microscopy. m-hBN/graphene heterostructure on polyethylene terephthalate further demonstrates the significant improvement of gas barrier performance; a water vapor transmission rate of 0.01 g m−2 day is achieved, which is seven times lower than the previously reported value, while retaining a high transparency of 96.4% at a wavelength of 550 nm.

Effect of employing chromium as a buffer layer on the crystallinity of hexagonal boron nitride films grown by LPCVD

Using chromium (Cr), which was deposited by radio frequency magnetron sputtering, as a buffer layer for synthesizing high-quality hexagonal boron nitride (hBN) films by low-pressure chemical vapor deposition (LPCVD) was demonstrated. The effect of growth temperature and annealing process on the quality of the Cr buffer layer was investigated. The characterization of the dependence of hBN film quality on growth temperature, substrate, and annealing process was discussed. All evidence shows that using a Cr buffer layer can significantly improve the crystalline quality of hBN. A DUVPD based on hBN film using Cr as the bottom electrode was fabricated with a small leakage current. The photocurrent is 3.5 nA at a bias of 500 V, and it exhibits good switching characteristics.

Large Area Few-Layer Hexagonal Boron Nitride as a Raman Enhancement Material.

Increasingly, two-dimensional (2D) materials are being investigated for their potential use as surface-enhanced Raman spectroscopy (SERS) active substrates. Hexagonal Boron Nitride (hBN), a layered 2D material analogous to graphene, is mostly used as a passivation layer/dielectric substrate for nanoelectronics application. We have investigated the SERS activity of few-layer hBN film synthesized on copper foil using atmospheric pressure chemical vapor deposition. We have drop casted the probe molecules onto the hBN substrate and measured the enhancement effect due to the substrate using a 532 nm excitation laser. We observed an enhancement of ≈103 for malachite green and ≈104 for methylene blue and rhodamine 6G dyes, respectively. The observed enhancement factors are consistent with the theoretically calculated interaction energies of MB > R6G > MG with a single layer of hBN. We also observed that the enhancement is independent of the film thickness and surface morphology. We demonstrate that the hBN films are highly stable, and even for older hBN films prepared 7 months earlier, we were able to achieve similar enhancements when compared to freshly prepared films. Our detailed results and analyses demonstrate the versatility and durability of hBN films for SERS applications.

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.

Two-dimensional hexagonal boron nitride as saturable absorber for a 1.5 µm passively Q-switched erbium-doped fiber laser.

We report on two-dimensional (2D) hexagonal boron nitride (hBN) as saturable absorber (SA) material in a passively Q-switched erbium-doped fiber laser (EDFL) operating at 1.5 µm. The 2D hBN film as an SA is fabricated and transferred onto the optical fiber tip by natural deposition technology. In the Q-switched operation, we obtain stable Q-switched laser operation with a maximum average 10% output power of 2.25 mW, corresponding to a repetition frequency of 55.5 kHz, shortest pulse width of 6.77 µs, and single pulse energy of 40.49 nJ. The achieved PQS at 1.5 µm EDFL with 2D hBN as an SA may have potential applications in many novel 2D materials and all-fiber lasers.

Resistance switching behaviors of continuous-thick hBN films fabricated by radio-frequency-sputtering

Abstract

Strong hyperbolic-magnetic polaritons coupling in an hBN/Ag-grating heterostructure.

Strong coupling between hyperbolic phonon-polaritons (HP) and magnetic polaritons (MP) is theoretically studied in a hexagonal boron nitride (hBN) covered deep silver grating structure. It is found that MP in grating trenches strongly interacts with HP in an anisotropic hBN thin film, leading to a large Rabi splitting with near-perfect dual band light absorption. Numerical results indicate that MP-HP coupling can be tuned by geometric parameters of the structure. More intriguingly, the resonantly enhanced fields for two branches of the hybrid mode demonstrate unusually different field patterns. One exhibits a volume-confined Zigzag propagation pattern in the hBN film, while the other shows a field-localization near the grating corners. Furthermore, resonance frequencies of these strongly coupled modes are very robust over a wide-angle range. The angle-insensitive strong interaction of hyperbolic-magnetic polaritons with dual band intense light absorption in this hybrid system offers a new paradigm for the development of various optical detecting, sensing and thermal emitting devices.

Synthesis of Wafer-Scale Monolayer Pyrenyl Graphdiyne on Ultrathin Hexagonal Boron Nitride for Multibit Optoelectronic Memory.

Graphdiyne is a new two-dimensional carbon allotrope with many attractive properties and has been widely used in various applications. However, the synthesis of large-area, high-quality, and ultrathin (especially monolayer) graphdiyne and its analogues remains a challenge, hindering its application in optoelectronic devices. Here, a wafer-scale monolayer pyrenyl graphdiyne (Pyr-GDY) film is obtained on hexagonal boron nitride (hBN) via a van der Waals epitaxial strategy, and top-floating-gated multibit nonvolatile optoelectronic memory based on Pyr-GDY/hBN/graphene is constructed, using Pyr-GDY as a photoresponsive top-floating gate. Benefiting from the excellent charge trapping capability and strong absorption of the graphdiyne film, as well as the top-floating-gated structure and the ultrathin hBN film used in the device, the optoelectronic memory exhibits high storage performance and robust reliability. A huge difference in the current between the programmed and erased states (>26 μA μm-1 at Vds = 0.1 V) and a prolonged retention time (>105 s) enable the device to achieve multibit storage, for which eight and nine distinct storage levels (3-bit) are obtained by applying periodic gate voltages and optical pulses in the programming and erasing processes, respectively. This work provides an important step toward realizing versatile graphdiyne-based optoelectronic devices in the future.

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.

Microcavity-coupled emitters in hexagonal boron nitride

Abstract Integration of quantum emitters in photonic structures is an important step in the broader quest to generate and manipulate on-demand single photons via compact solid-state devices. Unfortunately, implementations relying on material platforms that also serve as the emitter host often suffer from a tradeoff between the desired emitter properties and the photonic system practicality and performance. Here, we demonstrate “pick and place” integration of a Si3N4 microdisk optical resonator with a bright emitter host in the form of ∼20-nm-thick hexagonal boron nitride (hBN). The film folds around the microdisk maximizing contact to ultimately form a hybrid hBN/Si3N4 structure. The local strain that develops in the hBN film at the resonator circumference deterministically activates a low density of defect emitters within the whispering gallery mode volume of the microdisk. These conditions allow us to demonstrate cavity-mediated out-coupling of emission from defect states in hBN through the microdisk cavity modes. Our results pave the route toward the development of chip-scale quantum photonic circuits with independent emitter/resonator optimization for active and passive functionalities.

Coexisting 1T/2H polymorphs, reentrant resistivity behavior, and charge distribution in MoS2−hBN 2D/2D composite thin films

In view of their immensely intriguing properties, two-dimensional (2D) materials are being intensely researched in search of novel phenomena and diverse application interests; however, studies on the realization of 2D/2D nanocomposites in the application-worthy thin-film platform are rare. Here we have grown $\mathrm{Mo}{\mathrm{S}}_{2}$-hexagonal boron nitride (hBN) 2D/2D composite thin films on different substrates by the pulsed laser deposition technique and made comparative studies with the pristine $\mathrm{Mo}{\mathrm{S}}_{2}$ and hBN films. The Raman and x-ray photoelectron spectroscopy techniques as well as high-resolution transmission electron microscopy confirm the concomitant presence of both the 1T (conducting) and 2H (semiconducting) polymorphs of $\mathrm{Mo}{\mathrm{S}}_{2}$ in the composite film. Interestingly, a peculiar reentrant semiconductor-metal-insulator transition is seen in the $\mathrm{Mo}{\mathrm{S}}_{2}\ensuremath{-}\mathrm{hBN}$ 2D/2D composite film which is absent in the $\mathrm{Mo}{\mathrm{S}}_{2}$ film, and it correlates well with the signatures of phonon softening seen in temperature-dependent Raman spectroscopy. Furthermore, electrostatic force microscopy reveals the presence of three distinct regions (metallic, semiconducting, and insulating) in the $\mathrm{Mo}{\mathrm{S}}_{2}\ensuremath{-}\mathrm{hBN}$ composite film with differing contact potentials and enhanced propensity for charge transfer with respect to pristine $\mathrm{Mo}{\mathrm{S}}_{2}$. A triboelectric nanogenerator device containing biphasic $\mathrm{Mo}{\mathrm{S}}_{2}\ensuremath{-}\mathrm{hBN}$ composite film as an electron acceptor exhibits more than twofold (sixfold) enhancement in peak-to-peak output voltage as compared to the pristine $\mathrm{Mo}{\mathrm{S}}_{2}$ (hBN) film. These observations bring out the potential of 2D/2D nanocomposite thin films for unfolding emergent phenomena and technological applications.

Low temperature wafer-scale synthesis of hexagonal boron nitride by microwave assisted surface wave plasma chemical vapour deposition

Here, we report on the large-area synthesis of hBN layer at a comparatively lower temperature using ammonia borane as precursor by microwave assisted surface wave plasma (MW-SWP) chemical vapour deposition (CVD). The solid precursor was sublimed inside the CVD chamber and decomposed to form plasma radicals, which allowed the growth of hBN layer at a lower temperature (∼500 °C). The growth of hBN on Cu catalyst and Si wafer was confirmed by X-ray photoelectron spectroscopy, ultraviolet absorption spectroscopy, Fourier-transform infrared spectroscopy and transmission electron microscopy analysis. The hBN film synthesized on Cu catalyst showed a sharp absorption peak at 276 nm wavelength corresponding to an optical band gap of ∼4.1 eV, owing to the incorporation of carbon and oxygen doping impurities. The reduction of optical band gap of the hBN film with impurity doping can be significant to tune its optoelectronic properties. Thus, the demonstrated MW-SWP-CVD process can be significant to synthesize hBN layers independent of the catalytic behaviour of the substrate, thereby opening enormous possibilities of transfer-free application for device fabrication and as transparent coating on various surfaces.

Nondestructive Thickness Mapping of Wafer-Scale Hexagonal Boron Nitride Down to a Monolayer.

The availability of an accurate, nondestructive method for measuring thickness and continuity of two-dimensional (2D) materials with monolayer sensitivity over large areas is of pivotal importance for the development of new applications based on these materials. While simple optical contrast methods and electrical measurements are sufficient for the case of metallic and semiconducting 2D materials, the low optical contrast and high electrical resistivity of wide band gap dielectric 2D materials such as hexagonal boron nitride (hBN) hamper their characterization. In this work, we demonstrate a nondestructive method to quantitatively map the thickness and continuity of hBN monolayers and bilayers over large areas. The proposed method is based on acquisition and subsequent fitting of ellipsometry spectra of hBN on Si/SiO2 substrates. Once a proper optical model is developed, it becomes possible to identify and map the commonly observed polymer residuals from the transfer process and obtain submonolayer thickness sensitivity for the hBN film. With some assumptions on the optical functions of hBN, the thickness of an as-transferred hBN monolayer on SiO2 is measured as 4.1 Å ± 0.1 Å, whereas the thickness of an air-annealed hBN monolayer on SiO2 is measured as 2.5 Å ± 0.1 Å. We argue that the difference in the two measured values is due to the presence of a water layer trapped between the SiO2 surface and the hBN layer in the latter case. The procedure can be fully automated to wafer scale and extended to other 2D materials transferred onto any polished substrate, as long as their optical functions are approximately known.

The near-field radiative heat transfer between graphene/SiC/hBN multilayer structures

We propose a multilayer structure alternately stacked by graphene, silicon carbide (SiC) film, and hexagonal boron nitride (hBN) bulk to theoretically study the near-field radiative heat transfer. Compared with blackbody result, the heat transfer coefficient (HTC) can be significantly enhanced due to the effect of surface plasmons (SPs), surface phonon polaritons (SPhPs), and hyperbolic modes (HMs) supported by graphene, SiC, and hBN, respectively. HTC between the proposed structures with thin SiC can even be larger than that of graphene-covered hBN. Since SPs could be coupled both with HMs supported by the hBN and with SPhPs supported by SiC, HTC can be flexibly modulated by chemical potential of graphene. In addition, HTC of another structure which is composed with graphene, hBN film, and SiC bulk in order is also investigated in detail. The impacts of the thickness of components and vacuum gap on HTCs for both configurations are also studied. The results in this study are helpful to control near-field radiative heat transfer.

Parallel boron nitride nanoribbons and etch tracks formed through catalytic etching

One-dimensional (1D) catalytic etching was investigated in few-layer hexagonal boron nitride (hBN) films. Etching of hBN was shown to share a number of similarities with that of graphitic films. As in graphitic films, etch tracks in hBN commenced at film edges and occurred predominantly along certain crystal directions of its lattice, though it was shown that the tracks were generally narrower than those of few-layer graphene under similar processing conditions. It was also shown that catalytic hydrogenation can occur completely through a few-layer hBN film, demonstrating that this process can be used in the formation of isolated low-dimensional nanoscale structures from other layered 2D materials beyond graphene. This ability for thin hBN films to be etched completely through allowed for a crystalline substrate to guide the etching process, which was demonstrated with the successful etch track formation of few-layer hBN on single-crystalline sapphire substrates. The substrate-guided etching resulted in parallel few-layer hBN nanoribbons having an average width of 32 nm and spacing of 13 nm.

Electrical transport and capacitance characteristics of metal-insulator-metal structures using hexagonal and cubic boron nitride films as dielectrics

Metal-insulator-metal capacitor structures using thick hexagonal and cubic boron nitride (hBN and cBN) films as dielectrics are produced by plasma jet-enhanced chemical vapor deposition, and their electrical transport and capacitance characteristics are studied in a temperature range of 298 to 473 K. The resistivity of the cBN film is of the order of 107 Ω cm at 298 K, which is lower than that of the hBN film by two orders of magnitude, while it becomes the same order as the hBN film above ∼423 K. The dominant current transport mechanism at high fields (≥1 × 104 V cm−1) is described by the Frenkel-Poole emission and thermionic emission models for the hBN and cBN films, respectively. The capacitance of the hBN film remains stable for a change in alternating-current frequency and temperature, while that of the cBN film has variations of at most 18%. The dissipation factor as a measure of energy loss is satisfactorily low (≤5%) for both films. The origin of leakage current and capacitance variation is attributed to a high defect density in the film and a transition interlayer between the substrate and the film, respectively. This suggests that cBN films with higher crystallinity, stoichiometry, and phase purity are potentially applicable for dielectrics like hBN films.

Sputter synthesis of wafer-scale hexagonal boron nitride films via interface segregation

We demonstrated the crystal growth of wafer-scale hexagonal boron nitride (hBN) films on AlN template substrates by pulsed sputtering deposition using Fe catalysts. It was found that hBN films with high crystalline quality were formed at the hetero-interfaces between the Fe catalytic layers and the AlN templates. The full width at half maximum value of the E2g Raman spectrum for the hBN film was as small as 5.1 cm−1. The hBN film showed a highly c-axis-oriented structure and a strong near-band-edge deep ultraviolet emission at room temperature. The present results indicate that the sputter synthesis of hBN films via interface segregation opens a new pathway for the wafer-scale production of high-quality hBN films, and we envisage its potential applications in the fabrication of prospective hBN-based optoelectronic devices.

Near-field heat transfer between graphene/hBN multilayers

We study the radiative heat transfer between multilayer structures made by a periodic repetition of a graphene sheet and a hexagonal boron nitride (hBN) slab. Surface plasmons in a monolayer graphene can couple with a hyperbolic phonon polaritons in a single hBN film to form hybrid polaritons that can assist photon tunneling. For periodic multilayer graphene/hBN structures, the stacked metallic/dielectric array can give rise to a further effective hyperbolic behavior, in addition to the intrinsic natural hyperbolic behavior of hBN. The effective hyperbolicity can enable more hyperbolic polaritons that enhance the photon tunneling and hence the near-field heat transfer. However, the hybrid polaritons on the surface, i.e. surface plasmon-phonon polaritons, dominate the near-field heat transfer between multilayer structures when the topmost layer is graphene. The effective hyperbolic regions can be well predicted by the effective medium theory (EMT), thought EMT fails to capture the hybrid surface polaritons and results in a heat transfer rate much lower compared to the exact calculation. The chemical potential of the graphene sheets can be tuned through electrical gating and results in an additional modulation of the heat transfer. We found that the near-field heat transfer between multilayer structure does not increase monotonously with the number of layer in the stack, which provides a way to control the heat transfer rate by the number of graphene layers in the multilayer structure. The results may benefit the applications of near-field energy harvesting and radiative cooling based on hybrid polaritons in two-dimensional materials.

Growth temperature impact on film quality of hBN grown on Al2O3 using non-catalyzed borazane CVD

Hexagonal boron nitride (hBN) films were directly grown on c-plane Al2O3 substrates by low pressure thermal chemical vapor deposition (CVD) with single precursor i.e. borazane (NH3-BH3) without employing any extra catalysis. The growth temperature influence on the growth and properties of BN films were investigated. It was found that with a suitable source supply rate, the growth temperature could obviously influence the growth rate, chemical state, crystal structure and optical properties of BN films. High growth temperature depicts low thickness film grown, even there was no BN film deposited when the temperature was higher than or equal to 1400 °C. The B–N bonding type in all the deposited BN films were conformed as sp2 bonding, and films were evidenced to be amorphous BN (aBN) at lower growth temperature (i.e. less than 1350 °C) and hBN in turbostratic phase (tBN) between 1350 and 1400 °C. All the grown BN films on sapphire substrate showed good ultraviolet (UV) absorption edge near 210 nm, according to transmittance spectra. This work presents a feasible route towards combination of large-scale CVD-grown hBN film with traditional III-N compounds for applications in deep UV optoelectronic devices.