Hexagonal Boron Nitride Thin Films Research Articles

Low‐Pressure Chemical Vapor Deposition of Hexagonal Boron Nitride on a‐Plane Sapphire Using BCl3 as a Boron Source

Hexagonal boron nitride (h‐BN) thin films are grown on a‐plane sapphire substrates at different temperatures, Tg, by low‐pressure chemical vapor deposition with BCl3 and NH3 as boron and nitrogen sources, respectively, and their crystallographic and luminescence properties are compared with those grown on c‐plane sapphire. A notable difference from the c‐plane sapphire is that the substrate surface is significantly nitrided during film growth at Tg higher than 1200 °C, whereas no such nitridation is observed up to 1500 °C for the growth on c‐plane sapphire. The nitridation of the substrate surface is found to cause the degradation of the film quality. However, the properties of the films grown on a‐plane sapphire at Tg lower than 1200 °C are comparable to or better than those grown on c‐plane sapphire. The h‐BN epitaxial film is grown at 1150 °C epitaxially on the a‐plane sapphire substrate with the out‐of‐plane and in‐plane relationships, h‐BN//sapphire andh‐BN//sapphire, respectively. This study suggests that a‐plane sapphire is suitable for growing high‐quality h‐BN films at relatively low Tg.

P-type hexagonal boron nitride films with bis(cyclopentadienyl) magnesium as a doping gas in halide vapor phase epitaxy

We report an in situ carbon doping method for fabricating p-type hexagonal boron nitride thin films with a halide vapor phase epitaxy system by introducing bis(cyclopentadienyl) magnesium as a doping gas. The hBN films exhibited a growth rate of 3 μm/h, while the doped hBN films showed a considerable reduction in resistivity by 8 orders of magnitude. Hall measurements demonstrated that the doped hBN films were p-type conductive. At room temperature, the doped hBN films exhibited a free hole concentration of ∼1015 cm−3 and a resistivity of about 1000 Ω cm. X-ray photoelectron spectroscopy demonstrated the doping of carbon impurities into the hBN films and the formation of chemical bonds with B by mainly replacing nitrogen. Temperature-dependent I–V properties indicated that the ionization energy of the carbon impurities was about 320 meV.

Non-Linear Optics at Twist Interfaces in h-BN/SiC Heterostructures.

Understanding the emergent electronic structure in twisted atomically thin layers has led to the exciting field of twistronics. However, practical applications of such systems are challenging since the specific angular correlations between the layers must be precisely controlled and the layers have to be single crystalline with uniform atomic ordering. Here, an alternative, simple, and scalable approach is suggested, where nanocrystallinetwo-dimensional (2D) film on 3D substrates yields twisted-interface-dependent properties. Ultrawide-bandgap hexagonal boron nitride (h-BN) thin films are directly grown on high in-plane lattice mismatched wide-bandgap silicon carbide (4H-SiC) substrates to explore the twist-dependent structure-property correlations. Concurrently, nanocrystalline h-BN thin film shows strong non-linear second-harmonic generation and ultra-low cross-plane thermal conductivity at room temperature, which are attributed to the twisted domain edges between van der Waals stacked nanocrystals with random in-plane orientations. First-principles calculations based on time-dependent density functional theory manifest strong even-order optical nonlinearity in twisted h-BN layers. This work unveils that directly deposited 2D nanocrystalline thin film on 3D substrates could provide easily accessible twist-interfaces, therefore enabling a simple and scalable approach to utilize the 2D-twistronics integrated in 3D material devices for next-generation nanotechnology.

Unidirectional domain growth of hexagonal boron nitride thin films

Two-dimensional van der Waals (2D-vdW) layered hexagonal boron nitride (h-BN) has gained tremendous research interest over recent years due to its unconventional domain growth morphology, fascinating properties and application potentials as an excellent dielectric layer for 2D-based nano-electronics. However, the unidirectional domain growth of h-BN thin films directly on insulating substrates remains significantly challenging because of high-bonding anisotropicity and complex growth kinetics than the conventional thin films growth, thus resulting in the formation of randomly oriented domains morphology, and hindering its usefulness in integrated nano-devices. Here, ultra-wide bandgap h-BN thin films are grown directly on low-miscut atomically smooth highly insulating c-plane sapphire substrates (without using any metal catalytic layer) by pulsed laser deposition, showing remarkable unidirectional triangular-shape domains morphology. This unidirectional domain growth is attributed to the step-edge guided nucleation caused by reducing the film-substrate interfacial symmetry and energy, thereby breaking the degeneracy of nucleation sites of random domains, as revealed by the density functional theory (DFT) calculations. Through extensive characterizations, we further demonstrate the excellent single crystal-like functional properties of films. Our findings might pave the way for feasible large-area direct growth of electronic-quality h-BN thin films on insulating substrates for high-performance 2D-electronics, and in addition would be beneficial for hetero engineering of 2D-vdW materials with emergent phenomena.

Reduction in Residual Impurities in Chemical Vapor Deposition–Grown Hexagonal Boron Nitride Thin Films

The residual impurities in chemical vapor deposition (CVD)‐grown hexagonal boron nitride (h‐BN) on Al2O3 substrate grown by CVD using BN‐molded susceptor with B2H6 as a boron precursor are investigated. The Si, C, and O residual impurities of 2.6 × 1016, 3.4 × 1018, and 2.0 × 1017 cm−3 are achieved. These concentrations are two orders of magnitude lower than those when grown on the SiC‐coated graphite susceptor using trimethylboron (TMB) and the lowest values in the CVD‐grown h‐BN thin films. The cathodoluminescence spectrum has 5.5 eV as a broad peak and 4 eV as several peaks. The free excitonic recombination at the higher energy emission (5.79 eV) is owing to the reduction in residual impurities. The absorption coefficient of 1 × 104 cm−1 is observed in the energy range below 5 eV only when using the SiC‐coated graphite susceptor, which is most probably associated with Si contamination in the BN layer. The negligible absorption coefficient when using the BN‐molded susceptor presents high‐purity h‐BN signature.

Study of dielectric parameters of hexagonal boron nitride thin film fabricated by RF magnetron sputtering method

We report the fabrication of hexagonal boron nitride (hBN) thin films over a sizable area (4 cm2) on a glass substrate by adjusting the deposition time during the radio frequency magnetron sputtering process. As-prepared hBN films were characterized using Raman spectroscopy, and we found the distinctive E2g vibration band at 1366 cm-1, which ensures the presence of hBN in its bulk form. AFM was used to examine how the deposition time affected the surface morphology. It has been found that the sputtered film is made up of large (about 1 μm) hBN flakes. Impedance measurements were made to determine the dielectric parameters of the hBN films. The specific capacitance of the hBN films is found to be 0.04 μF/cm2, and their resistances have been found to be as high as 105 Ω. This study shows that for an hBN film to have a high specific capacitance, less deposition time is needed during the sputtering process.

Ti3C2Tx supercapacitors with a hexagonal boron nitride separator manufactured by spray coating

Mobile electronic devices need electrochemical energy storage units to deliver high energy and power densities. Current efforts mostly focus on enhancing the performance of the electrode active materials. However, reducing the weight/volume fraction occupied by the passive components such as electrolyte, separator, and current collector can also dramatically improve the performance of the energy storage devices. Hexagonal boron nitride (h-BN) thin films have shown promising performance as separators in supercapacitor devices due to their electrically insulative character and high ionic conductivity. In this work, h-BN was used as an efficient separator in symmetric titanium carbide (Ti3C2Tx) MXene microsupercapacitors. The devices were fabricated using layer-by-layer deposition of h-BN nanosheets and Ti3C2Tx via the spray coating method. The total thickness of the symmetric Ti3C2Tx device with the h-BN separator was 20 times smaller than the control device with the Celgard separator. Significantly increased volumetric energy density, together with the facile fabrication procedure reported herein indicates that the sprayed all-2D symmetric supercapacitor devices are promising for microscale and flexible energy storage.Graphical abstract

Boron Nitride Thin Films with Anisotropic Optical Properties from Microscale Particle Density Distributions

Unusual optical anisotropy was experimentally observed in hexagonal boron nitride thin films produced from bulk boron nitride via ultrasonication. Both the linear and circular polarisation demonstrated a well-defined single axis of anisotropy over a large sample area. To understand this phenomenon, we employed statistical analysis of optical microscopy images and atomic force microscopy to reveal an ordered particle density distribution at the microscopic level corresponding to the optical axis observed in the polarisation data. The direction of the observed ordering matched the axis of anisotropy. Hence, we attribute the measured optical anisotropy of the thin films to microscopic variations in the particle density distribution.

Synthesis of hexagonal boron nitride thin film on Pt substrates for resistive switching memory applications

Hexagonal boron nitride (hBN), due to its high reliability as a two-dimensional (2D) dielectric material, has attracted much attention for its potential applications in nanoelectronic devices. Here, the use of radio frequency (RF) magnetron sputtering-grown hBN films to construct hBN-based resistive switching (RS) memory device is reported, and the RS mechanism is deduced. The hBN-based RS memory shows low operating voltage, reproducible write cycles, and long retention time. First-principles simulations further confirm the resistive switching. This work provides an important case to facilitate the future applications of 2D materials in the RS memory.

Effect of buffer layer and substrate growth temperature on the microstructural evolution of hexagonal boron nitride thin films

Hexagonal boron nitride (hBN) thin films have excellent optical, electrical, and corrosion resistance properties that can enable a wide range of applications. The current study explores pulsed laser deposition technique for synthesizing hBN thin films. The microstructure, mechanical, and wetting properties of hBN thin films were analyzed as a function of substrate growth temperature (25 °C, 300 °C, 500 °C, and 800 °C) and buffer layer addition (Cu or Ti). The microstructure of hBN thin films was found to be influenced by the deposition temperature. This study revealed that the layered growth (for samples deposited at 25 °C) and layer plus island growth (for samples deposited at ≥300 °C) to be the primary growth modes of the thin films regardless of the buffer layer. The roughness of hBN thin films with the Cu buffer layer, in general, increased with the increase in deposition temperature, whereas no significant differences in roughness were observed for films deposited on Si and Ti buffer layer. Grazing Incidence X-ray diffraction results showed the presence of hexagonal close packed crystal structure for all the deposited thin films. These results were corroborated with Raman spectroscopy. The hBN thin films exhibited the hardness and modulus values of ~10–14 GPa and ~120–180 GPa, respectively. No considerable differences in mechanical properties were observed for hBN thin films deposited at various temperatures except for the films deposited with the Cu buffer layer, which showed lower mechanical properties. The wettability of hBN thin films was analyzed using contact angle measurements. The wettability was found to be directly influenced by the surface roughness of hBN thin films. Characterization of hBN thin films indicates that the deposition temperature and buffer layer addition had an influence on the growth and microstructural evolution of hBN thin films.

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.

Plasmonic enhancement in deep ultraviolet photoresponse of hexagonal boron nitride thin films

Deep-ultraviolet (DUV) photodetectors based on hexagonal boron nitride (h-BN) have demonstrated great potentials for various commercial and military applications; however, to date, most studies show that the h-BN photodetectors suffer from poor performance, such as low responsivity and large dark current. Herein, we report the dramatic enhancement of photoresponse in the DUV region of a h-BN device coupled with plasmonic nanostructures of either h-BN nanosheets (BNNSs) or Au nanoparticles (NPs). Large-area h-BN thin films that have been directly grown on quartz substrates using the ion beam assistant deposition method exhibit a uniform thickness of ∼200 nm, an ultrawide bandgap (∼ 6 eV), and an excellent light transparency in the visible region. Based on the vertical charge transfer integrated with plasmonic nanoarrays, the photocurrent of the h-BN device can be greatly enhanced by up to about 7–9 times under the illumination of 205 nm by showing a cutoff wavelength at ∼220 nm. Owing to the retained low dark current and large photo-gain induced by localized plasmonic resonances, this hybrid photodetector exhibits 32- and 57-fold improvement in responsivity at a 205 nm wavelength by BNNSs and Au NPs, respectively. This work demonstrates plasmonic enhancement on optoelectronic properties of h-BN based on not only metallic but also semiconducting nanostructures and provides alternative pathways for the development of low-cost, large-area, high-performance, DUV photodetectors for various optoelectronic devices and security applications.

Hexagonal boron nitride as a low-loss dielectric for superconducting quantum circuits and qubits.

Dielectrics with low loss at microwave frequencies are imperative for high-coherence solid-state quantum computing platforms. Here we study the dielectric loss of hexagonal boron nitride (hBN) thin films in the microwave regime by measuring the quality factor of parallel-plate capacitors (PPCs) made of NbSe2-hBN-NbSe2 heterostructures integrated into superconducting circuits. The extracted microwave loss tangent of hBN is bounded to be at most in the mid-10-6 range in the low-temperature, single-photon regime. We integrate hBN PPCs with aluminium Josephson junctions to realize transmon qubits with coherence times reaching 25 μs, consistent with the hBN loss tangent inferred from resonator measurements. The hBN PPC reduces the qubit feature size by approximately two orders of magnitude compared with conventional all-aluminium coplanar transmons. Our results establish hBN as a promising dielectric for building high-coherence quantum circuits with substantially reduced footprint and with a high energy participation that helps to reduce unwanted qubit cross-talk.

Atomic Defect Induced Saturable Absorption of Hexagonal Boron Nitride in Near Infrared Band for Ultrafast Lasing Applications.

Defect-induced phenomena in 2D materials has received increasing interest among researchers due to the novel properties correlated with precise modification of materials. We performed a study of the nonlinear saturable absorption of the boron-atom-vacancy defective hexagonal boron nitride (h-BN) thin film at a wavelength of ~1 μm and its applications in ultrafast laser generation. The h-BN is with wide band gap of ~6 eV. Our investigation shows that the defective h-BN has a wide absorption band from visible to near infrared regimes. First-principle calculations based on density functional theory (DFT) indicate that optical property changes may be attributed to the boron-vacancy-related defects. The photoluminescence spectrum shows a strong emission peak at ~1.79 eV. The ultrafast Z-scan measurement shows saturable absorbance response has been detected for the defective h-BN with saturation intensity of ~1.03 GW/cm2 and modulation depth of 1.1%. In addition, the defective h-BN has been applied as a new saturable absorber (SA) to generate laser pulses through the passively Q-switched mode-locking configuration. Based on a Nd:YAG waveguide platform, 8.7 GHz repetition rate and 55 ps pulse duration of the waveguide laser have been achieved. Our results suggest potential applications of defective h-BN for ultrafast lasing and integrated photonics.

An investigation of the coupling of phonon-polaritons with plasmon-polaritons in hBN/nanopatterned Au layered devices

Thin hexagonal boron nitride layers have been shown to support highly confined hyperbolic phonon-polaritons, which are of interest for light guiding applications. Localized plasmon resonances in nanopatterned metal films can exhibit subwavelength-scale confinement as well as a high local field strength that is of importance to imaging and sensor applications. In this work, the interaction between hyperbolic phonon-polaritons in a hexagonal boron nitride thin film and plasmon-polaritons in a nanopatterned gold thin film is investigated by means of finite-difference time-domain simulations of a series of coupled and uncoupled layered devices. Both far-field and near-field properties are calculated and analyzed, enabling the features due to plasmon-polaritons and phonon-polaritons, individually, to be distinguished and the coupling between these excitations to be explored and characterized.

Thermodynamic calculations for the chemical vapor deposition of hexagonal boron nitride using triethylboron, ammonia, and hydrogen

The presence of C in B(C2H5)3 makes possible its incorporation into hexagonal boron nitride (h-BN) thin films grown during chemical vapor deposition (CVD) under select process conditions. A series of CVD phase diagrams that indicate the regions of stability of the phases of h-BN, C and B4C as a function of temperature and input mole ratio of reactants (N/B) under set pressures and H2 diluent-to-reactant mole ratios (H2/(B + N)) have been calculated and analyzed for the TEB/NH3/H2 system. The equilibrium calculations showed that within the total pressure range of 0.01-100 Torr, increasing the H2 content in the TEB/NH3/H2 gas mixture tends to suppress the stability of C throughout a 400–1600 °C temperature range, but it also suppresses the stability of h-BN at higher temperatures. Further, it was determined that increasing the total system pressure suppresses the occurrence of C as a second phase while expanding the stability of h-BN. The thermodynamic equilibrium results were compared to the results of experimental investigations involving the CVD synthesis of sp2-BN as a means to evaluate their usefulness as a guide for avoiding the co-deposition of C and B4C with h-BN.

Room-temperature intrinsic excitonic luminescence from a hexagonal boron nitride thin film grown on a sapphire substrate by low-pressure chemical vapor deposition using BCl3 as a boron source

A hexagonal boron nitride thin film was prepared on a c-plane sapphire substrate at a substrate temperature of 1200 °C and a reactor pressure of 5 kPa by chemical vapor deposition using BCl3 as a boron source. The film, consisting of columnar grains with flat top surfaces, grew epitaxially on the substrate. This sample showed pronounced intrinsic exciton cathodoluminescence (CL) at 215 nm at room temperature, although broad emission at around 350 nm was dominant. Note that the spectrum shape in the band edge region closely matched that observed for a high-quality bulk crystal in terms of peak wavelength and width. The CL images of the surface revealed the correlation between the film structure and the luminescence property, where the intrinsic excitonic luminescence was exhibited only from the columnar grains, whereas the impurity- and/or the defect-related emissions were observed mainly from surface defects with randomly oriented grains.

Optical Third-Harmonic Generation in Hexagonal Boron Nitride Thin Films

Hexagonal boron nitride (hBN) is a layered material that exhibits remarkable optical features in the UV, visible, and IR ranges, attractive for applications in modern photonics. Being transparent in a wide spectral range, hBN is now considered an important building block for novel integrated photonic platforms, thus requiring the study of its optical properties. In this work, we report on the measurements of hBN optical cubic nonlinearity χ(3) equal to 8.4 × 10-21 m2/V2 by observing the third-harmonic generation for 1080 nm pump wavelength from mechanically exfoliated hBN flakes with thicknesses varying from 5 to 170 nm. The third-order susceptibility of hBN is close to that of Si3N4 highlighting the potential of hBN for nonlinear applications.

The impact of ultraviolet laser excitation during Raman spectroscopy of hexagonal boron nitride thin films

AbstractWe utilized excitation in the ultraviolet (UV) spectral range for the study of hexagonal boron nitride (h‐BN) thin films on different substrates by Raman spectroscopy. Whereas UV excitation offers fundamental advantages for the investigation of h‐BN and heterostructures with graphene, the actual Raman spectra recorded under ambient conditions reveal a temporal decay of the signal intensity. The disappearance of the Raman signal is found to be induced by thermally activated chemical reactions with ambient molecules at the h‐BN surface. The chemical reactions could be strongly suppressed under vacuum conditions which, however, favor the formation of a carbonaceous surface contamination layer. For the improvement of the signal‐to‐noise ratio under ambient conditions, we propose a line‐scan method for the acquisition of UV Raman spectra in atomically thin h‐BN, a material which is expected to play a key role in future technologies based on 2D van der Waals heterostructures.

High repetition rate deposition of boron nitride films using femtosecond pulsed laser

Cubic (c-BN), and hexagonal (h-BN) boron nitride thin films are of interest in many applications and industries because of their unique mechanical, thermal and chemical properties. In this work, we investigate high repetition rate deposition of BN films using femtosecond pulsed laser deposition. Boron nitride (BN) films were deposited on silicon wafers using 800 nm, 100 fs Ti:sapphire femtosecond laser with 2.4 mJ pulse energy and high repetition rate of 1 kHz using a c-BN target. The deposited films were analyzed using transmission electron microscopy (TEM), scanning electron microscope (SEM), and optical profilometer. Nano-indentation tests were performed to measure the hardness of the adhered film. The results indicate the influence of the high repetition rate on the film growth, crystalline arrangement and adhesion. The experimental work is utilized to identify the process parameters that can be used in pulsed laser deposition (PLD) process to grow thick and adherent BN films.