Boron Nitride Films Research Articles

Oligomeric aminoborane precursors for the chemical vapour deposition growth of few-layer hexagonal boron nitride

We explore the use of stable, pre-formed, oligomeric aminoboranes as precursors for the chemical vapour deposition growth of few-layered hexagonal boron nitride (h-BN) films on Cu foils under atmospheric pressure conditions. Dimeric diborazane H3B·NH2BH2·NH3 (DAB), and trimeric triborazane H3B·(NH2BH2)2·NH3 (TAB), derivatives of ammonia borane, H3B·NH3 (AB), are compared with AB, a commonly used precursor for the CVD growth of h-BN. Both DAB and TAB show similar effectiveness to AB in growing h-BN few layered films. Using DAB as the precursor instead of AB leads to fully continuous h-BN films in a shorter period of time. Analysis of the surface of the h-BN films reveals that DAB and TAB precursors deposit more nanoparticles on the surface of the h-BN films during their CVD growth within the same time period as when using AB. The viability of these two new h-BN precursors (DAB and TAB), opens up a wider range of solid-state sources for growing wide band gap h-BN films using CVD techniques.

Deposition and Characterization of Multilayer DLC/BN Films

In this article, the results obtained from a study on multilayer diamond-like carbon and boron nitride (DLC/BN) films are reported. The microstructure, atomic concentration, hardness and friction coefficient of the films were characterized using transmission electron microscopy, auger electron microscopy, nano-indentation measurements and ball-on-disk friction testing. The effects of bilayer thickness and substrate bias on film growth were investigated. All multilayer films showed alternate DLC and BN layers, except the 2- and 4-nm bilayer of multilayer DLC/BN films deposited without substrate bias. Although the layers were very thin, each layer was distinguishable. This was confirmed by the use of TEM imaging and AES measurements. The hardness values of all the multilayer films were lower than those measured for the monolayer DLC and BN films. However, the hardness can be altered with a change in the bilayer thickness. Furthermore, in the case of the films deposited with substrate bias, multilayer DLC/BN films showed an improvement in wear resistance compared to monolayer DLC and BN films. Thus, the deposition of multilayer DLC/BN films can be considered to be beneficial in prolonging the service life of the surface.

Effects of sapphire nitridation and growth temperature on the epitaxial growth of hexagonal boron nitride on sapphire

This paper reports on the epitaxial growth of hexagonal boron nitride (hBN) films on sapphire substrates in a cold wall chemical vapor deposition (CVD) system where different sapphire nitridation and hBN growth temperatures were employed. A thin and amorphous nitridated layer was formed at a low temperature (850 °C), which enabled subsequent epitaxial hBN growth at 1350 °C. The influences of the sapphire nitridation temperature and the growth temperature on the film quality were analyzed by x-ray diffraction (XRD) measurements. Higher than optimum nitridation and growth temperatures improve the crystalline quality of the nitridated layer, but does not favor the epitaxial growth of hBN. hBN films grown at the optimum conditions exhibit the c-lattice constant of 6.66 Å from the XRD θ–2θ scan and the characteristic in plane stretching vibration at 1370.5 cm−1 from Raman spectroscopy. X-ray photoelectron spectroscopy analysis confirmed the formation of stoichiometric hBN films with excellent uniformity.

Comparison of device structures for the dielectric breakdown measurement of hexagonal boron nitride

Improving the film quality in the synthesis of large-area hexagonal boron nitride films (h-BN) for two-dimensional material devices remains a great challenge. The measurement of electrical breakdown dielectric strength (EBD) is one of the most important methods to elucidate the insulating quality of h-BN. In this work, the EBD of high quality exfoliated single-crystal h-BN was investigated using three different electrode structures under different environmental conditions to determine the ideal electrode structure and environment for EBD measurement. A systematic investigation revealed that EBD is not sensitive to contact force or electrode area but strongly depends on the relative humidity during measurement. Once the measurement environment is properly managed, it was found that the EBD values are consistent within experimental error regardless of the electrode structure, which enables the evaluation of the crystallinity of synthesized h-BN at the microscopic and macroscopic level by utilizing the three different electrode structures properly for different purposes.

Low-Energy Ion-Assisted Deposition of Boron Nitride Films in Surface-Wave Plasma

A surface-wave plasma operated at 2.45-GHz microwave (MW) is used to deposit hexagonal boron nitride (hBN) films by applying a negative or positive bias voltage to a substrate. Ion energy and flux onto the substrate are examined in terms of MW power, pressure, substrate location, and substrate bias. The mean ion energy is estimated from the sheath potential measured with a Langmuir probe and controlled between a few electronvolts and ~170 eV. hBN films are deposited on silicon in a gas mixture of He, N₂, H₂, and BFN₃ by varying either the ion energy or substrate temperature under a high ion flux condition (1017 cm−2s−1). The results show that the crystallinity of sp2-bonding network in the films depends upon ion energy more than substrate temperature.

Catalyst Interface Engineering for Improved 2D Film Lift-Off and Transfer.

The mechanisms by which chemical vapor deposited (CVD) graphene and hexagonal boron nitride (h-BN) films can be released from a growth catalyst, such as widely used copper (Cu) foil, are systematically explored as a basis for an improved lift-off transfer. We show how intercalation processes allow the local Cu oxidation at the interface followed by selective oxide dissolution, which gently releases the 2D material (2DM) film. Interfacial composition change and selective dissolution can thereby be achieved in a single step or split into two individual process steps. We demonstrate that this method is not only highly versatile but also yields graphene and h-BN films of high quality regarding surface contamination, layer coherence, defects, and electronic properties, without requiring additional post-transfer annealing. We highlight how such transfers rely on targeted corrosion at the catalyst interface and discuss this in context of the wider CVD growth and 2DM transfer literature, thereby fostering an improved general understanding of widely used transfer processes, which is essential to numerous other applications.

Perfect mid-infrared absorption by hybrid phonon-plasmon polaritons in hBN/metal-grating anisotropic structures

A nanostructure based on periodic metal gratings and hexagonal boron nitride (hBN) films is proposed here. As a phononic two-dimensional (2D) material, hBN has two Reststrahlen bands with natural hyperbolic responses. Magnetic polaritons (MPs) in metal gratings can couple with hyperbolic phonon polaritons (HPPs) in hBN to create hybrid hyperbolic phonon-plasmon polaritons, resulting in perfect or near-perfect absorption. It is found that the majority of the power absorbed by the hybrid polaritons is dissipated in the hBN film with a unique tunable location-dependent absorption profile. Meanwhile, out of the hyperbolic region, optical phonons in hBN can couple with MPs in gratings to yield resonance absorption. The radiative properties and dissipation profiles are calculated based on rigorous coupled-wave analysis, which has been extended to include anisotropic materials, to elucidate the mechanisms of the perfect and near-perfect absorption. This work reveals the possibility of using 2D materials and nanostructures to achieve unique radiative properties for applications such as energy harvesting, radiative cooling, and photodetection.

Surface-confined edge phonon polaritons in hexagonal boron nitride thin films and nanoribbons.

In this paper, we point out that the phonon polaritons in hexagonal boron nitride may not have to be volume-confined. Surface-confined phonon polaritons can occur at the edges of ultra-thin boron nitride films. Based on numerical calculations, we have investigated the effective indexes of such phonon polariton modes, and found that these modes normally have much lower losses than the volume-confined ones. Surface-confined phonon polaritons in boron nitride nanoribbons are also investigated that can be seen as the coupling of the phonon polariton modes at two edges of the nanoribbon, and the corresponding losses can be further reduced. Then, we give a deep discussion on the volume-confined and surface-confined phonon polaritons. In the end, the effects of defects and substrates on such modes are presented. The proposed low-loss surface-confined phonon polaritons may be useful in phonon polariton based photonic devices.

Role of Low-Energy Ion Impact in Plasma Deposition of Cubic Boron Nitride Films

Cubic boron nitride (cBN) is potentially expected as the semiconductor for high temperature condition due to the large band gap, high breakdown field, and high resistance to oxidation. cBN films have been synthesized from vapor phase using a variety of physical vapor deposition such as sputtering and ion beam deposition rather than plasma-enhanced chemical vapor deposition. However, high-energy (>50 eV) ion bombardment for cBN formation results in low-quality cBN films and thus obstructs their electrical applications. In our previous studies, high-quality cBN films were deposited under low-energy ion bombardment (<40 eV) by plasma-enhanced chemical vapor deposition using the chemistry of fluorine. The mean ion energy for cBN formation was reduced greatly, then the resulting films consisted of micron-sized grains with crystallographic morphology. In this study, the deposition mechanism of our cBN films is examined in terms of ion energy, ion flux, and ion to depositing boron flux ratio onto the substrate. The ion energy and the ion to boron flux ratio are determined from the sheath potential and the ratio of incident ion flux to net deposited boron flux, respectively. For negative substrate biases where sp2-bonded BN phase only or no deposit is formed, both the ion energy and the ion to boron flux ratio are high. For positive substrate biases where cBN phase is formed, the ion energy and the ion to boron flux ratio are estimated in the range of a few eV to 35 eV and 100 to 130, respectively. The impact of negative ions is presumed to be negligible due to their low kinetic energy. The impact of positive ions with high ion to boron flux ratios is primarily responsible for reduction of the ion energy for cBN film deposition. [1] K. Teii and S. Matsumoto, Thin Solid Films 576, 50-54 (2015).

Thick c-BN films deposited by radio frequency magnetron sputtering in argon/nitrogen gas mixture with additional hydrogen gas**Project supported by the National Natural Science Foundation of China (Grant Nos. 51572105, 61504046, and 51272224), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, China, the Development and Reform Commission of Jilin Province, China (Grant No. 2015Y050), and the

The excellent physical and chemical properties of cubic boron nitride (c-BN) film make it a promising candidate for various industry applications. However, the c-BN film thickness restricts its practical applications in many cases. Thus, it is indispensable to develop an economic, simple and environment-friend way to synthesize high-quality thick, stable c-BN films. High-cubic-content BN films are prepared on silicon (100) substrates by radio frequency (RF) magnetron sputtering from an h-BN target at low substrate temperature. Adhesions of the c-BN films are greatly improved by adding hydrogen to the argon/nitrogen gas mixture, allowing the deposition of a film up to 5-μm thick. The compositions and the microstructure morphologies of the c-BN films grown at different substrate temperatures are systematically investigated with respect to the ratio of H2 gas content to total working gas. In addition, a primary mechanism for the deposition of thick c-BN film is proposed.

Adhesion mechanics of graphene on textured substrates

Graphene, the two dimensional form of carbon, has excellent mechanical, electrical and thermal properties and a variety of potential applications including nano-electro-mechanical systems, protective coatings, transparent electrodes in display devices and biological applications. Adhesion plays a key role in many of these applications. In addition, it has been proposed that the electronic properties of graphene can be affected by elastic deformation caused by adhesion of graphene to its substrate. In light of this, we present here a continuum mechanics based theoretical framework to understand the effect of nanoscale morphology of substrates on adhesion and mechanics of graphene. In the first part, we analyzed the adhesion mechanics of graphene on one and two dimensional periodic corrugations. We carried out molecular statics simulations and found the results to be in good agreement with our theory. We modeled adhesive interactions as surface forces due to a Lennard–Jones 6–12 potential in both our analysis and simulations and in principle any other interaction potential can be used with our methodology. The results show that graphene adheres conformally to substrates with large curvatures. We showed in principle that the theory developed here can be extended to substrates with arbitrary textures that can be represented by a Fourier series.In the second part, we study the mechanics of peeling of graphene ribbons from one dimensional sinusoidally textured substrates. In the molecular statics simulations, we observed two key features in the peel mechanics of the ribbons - the ribbons slide over the substrate and undergo adhesion and peeling near the crack front in an oscillatory manner, the frequency of which reveals the wavelength of the underlying substrate. Our theory qualitatively captures these features of the peel mechanics and is general enough that it can be extended to other two dimensional materials like Molybdenum Disulphide (MoS2), Boron Nitride (BN) or other thin films and different kinds of interaction potentials.

Synthesis and Characterization of Hexagonal Boron Nitride as a Gate Dielectric.

Two different growth modes of large-area hexagonal boron nitride (h-BN) film, a conventional chemical vapor deposition (CVD) growth mode and a high-pressure CVD growth mode, were compared as a function of the precursor partial pressure. Conventional self-limited CVD growth was obtained below a critical partial pressure of the borazine precursor, whereas a thick h-BN layer (thicker than a critical thickness of 10 nm) was grown beyond a critical partial pressure. An interesting coincidence of a critical thickness of 10 nm was identified in both the CVD growth behavior and in the breakdown electric field strength and leakage current mechanism, indicating that the electrical properties of the CVD h-BN film depended significantly on the film growth mode and the resultant film quality.

X-ray photoelectron study of the effect of the composition of the initial gas phase on changes in the electronic structure of hexagonal boron nitride films obtained by PECVD from borazine

Hexagonal boron nitride films are synthesized by plasma enhanced chemical vapor deposition (PECVD) from a gas mixture of borazine and ammonia or helium on Si(100) substrates. X-ray photoelectron spectroscopy is used to study changes in the electronic structure and chemical composition of the films depending on the composition of the initial gas mixture. It is found that the chemical composition of the samples depends on the gas used. The use of helium results in an excess of boron atoms on the film surface, the appearance of B–B bonds, and a decrease in the contribution of B–N bonds in the hexagonal structure. The preparation of h-BN films close to the stoichiometric composition by PECVD methods with the use of borazine is shown to be possible with the addition of ammonia. Based on the literature data, the binding energies in the B 1s XPS spectra are calculated for different boron environments in the hexagonal lattice.

Two-dimensional hexagonal boron nitride as lateral heat spreader in electrically insulating packaging

The need for electrically insulating materials with a high in-plane thermal conductivity for lateral heat spreading applications in electronic devices has intensified studies of layered hexagonal boron nitride (h-BN) films. Due to its physicochemical properties, h-BN can be utilised in power dissipating devices such as an electrically insulating heat spreader material for laterally redistributing the heat from hotspots caused by locally excessive heat flux densities. In this study, two types of boron nitride based heat spreader test structures have been assembled and evaluated for heat dissipation. The test structures separately utilised a few-layer h-BN film with and without graphene enhancement drop coated onto the hotspot test structure. The influence of the h-BN heat spreader films on the temperature distribution across the surface of the hotspot test structure was studied at a range of heat flux densities through the hotspot. It was found that the graphene-enhanced h-BN film reduced the hotspot temperature by about 8–10 °C at a 1000 W cm−2 heat flux density, a temperature decrease significantly larger than for h-BN film without graphene enhancement. Finite element simulations of the h-BN film predict that further improvements in heat spreading ability are possible if the thermal contact resistance between the film and test chip are minimised.

Anisotropic charge carrier transport in free-standing hexagonal boron nitride thin films

The in-plane and out-of-plane mobility–lifetime products of electrons and holes in free-standing hexagonal boron nitride (hBN) films are extracted from current–voltage characteristics of metal–hBN–metal structures measured under external excitations. The in-plane mobility–lifetime products for electrons and holes are ∼2.8 × 10−5 and ∼4.85 × 10−6 cm2/V, measured from lateral carrier collection, whereas the out-of-plane mobility–lifetime products for electrons and holes are ∼5.8 × 10−8 and ∼6.1 × 10−9 cm2/V, measured from vertical carrier collection, respectively. The mobility–lifetime product is a few orders of magnitude higher along the plane than along the out of plane in hBN films.

Thickness-controlled multilayer hexagonal boron nitride film prepared by plasma-enhanced chemical vapor deposition

Two-dimensional (2D) hexagonal boron nitride (h-BN) is a thin insulating material that can be used to enhance the electrical and optical properties of other 2D materials when used as a substrate or a capping layer, owing to its absence of dangling bonds on the surface. The use of multilayer h-BN films is often required in such applications to realize high material performance. However, previous works have focused mostly on the synthesis of monolayer or few-layer h-BN films. Herein we report a method to control the thickness of h-BN film up to the centimeter scale by means of plasma-enhanced chemical vapor deposition (PECVD). The thickness of the h-BN film is controlled by varying the deposition time of borazine precursor onto a monolayer h-BN film on a Pt foil substrate at room temperature. The resultant film is then annealed at high temperature (1050 °C) to increase the crystallinity of the h-BN. Monolayer h-BN film grown on Pt foil used as a buffer layer is of importance to improve uniformity and smooth surface of the multilayer h-BN film over the whole area. We further demonstrate that our multilayer h-BN film is very useful in graphene/h-BN/SiO2 heterostructures as a charge-blocking layer between graphene and SiO2.

Influences of growth parameters on the film formation of hexagonal boron nitride thin films grown on sapphire substrates by low-pressure chemical vapor deposition

Hexagonal boron nitride (h-BN) films were grown on c-plane sapphire substrates by low-pressure chemical vapor deposition with BCl3 and NH3 as the boron and nitrogen sources, respectively, and the influences of growth parameters on the film quality were investigated for samples with a thickness of about 1 µm. The dependence of X-ray diffraction on the growth temperature (Tg) indicated that the crystalline quality is most improved in the sample grown at 1200 °C, in which the epitaxial relationship of {100}h-BN ∥ {110}sapphire and {001}h-BN ∥ {001}sapphire was confirmed. This condition enhanced lateral growth, resulting in the formation of grains with flat top surfaces. The Tg dependence was discussed in relation to the amorphous AlN formed on the substrate surface and the reaction between BCl3 and NH3 in the vapor phase. The correlation between the structural and luminescent properties, which was found from the Tg dependence of CL, was also discussed.

Etching Studies of BCN Thin Films

The ever increasing advancements in semiconductor technology and continuous scaling of CMOS devices mandate the need for new dielectric materials with low-k values. The interconnect delay can be reduced not only by the resistance of the conductor but also by decreasing the capacitance of dielectric layer. Also cross-talk is a major issue faced by semiconductor industry due to high value of k of the inter-dielectric layer (IDL) in a multilevel wiring scheme in Si ultra large scale integrated circuit (ULSI) devices. Recently it has been found that Boron Nitride (BN) films can achieve low-k values around and hardness comparable to diamond [1][2]. BN is hygroscopic in nature. In this respect carbon can be added to the BN structure to produce Boron Carbon Nitride (BCN) films as the possible substitute for BN. The minimum dielectric constant achieved by plasma assisted chemical vapor deposition for a BCN thin film was reported to be 1.9. [2] The BCN films are also instrumental in suppressing the Cu migration [3]. The Young’s modulus of BCN films are reported well above 20 GPa making it one of the suitable options for low-k materials [4]. In the present work, BCN films were deposited by dual target sputtering of boron carbide (BC) target (DC), boron nitride (BN) target (RF) in nitrogen ambient with argon. The films were deposited under various N­2/Ar gas flow ratios and temperature. BCN is etched using an etchant that consists of 16 parts of Phosphoric acid (H3PO4), 1 part of Nitric acid (HNO3), 1 part of Acetic acid (CH3COOH), 20 parts of DI water. Various plots of etch rates with respect to deposition parameters and Arrhenius plots are studied. REFERENCES [1] Jun Liu, Kian Ping Loh, Ming Lin, Yong Lim Foo, Wei De Wang, and Dong Zhi Ch, Journal of Applied Physics, 96 (2004) 6679. [2] Riedel R., Advanced Materials, 4 (1992) 759. [3] M. K Mazumder, R. Moriyama, D. Watanabe, C. Kimura, H. Aoki, and T. Sugino: Jpn. J. Appl. Phys. 46 (2007) 2006. [4] C. Morant, D. Caceres, J. M. Sanz, and E. Elizalde: Diamond Relat. Mater. 16 (2007) 1441.

Trimethylamine Borane: A New Single-Source Precursor for Monolayer h-BN Single Crystals and h-BCN Thin Films

Due to their exceptional chemical and thermal stabilities as well as electrically insulating property, atomically thin hexagonal boron nitride (h-BN) films have been identified as a promising class of dielectric substrate and encapsulation material for high-performance two-dimensional (2D) heterostructure devices. Herein, we report a facile chemical vapor deposition synthesis of large-area atomically thin h-BN including monolayer single crystals and C-doped h-BN (h-BCN) films utilizing a relatively low-cost, commercially available trimethylamine borane (TMAB) as a single-source precursor. Importantly, pristine 2D h-BN films with a wide band gap of ∼6.1 eV can be achieved by limiting the sublimation temperature of TMAB at 40 °C, while C dopants are introduced to the h-BN films when the sublimation temperature is further increased. The h-BCN thin films displayed band gap narrowing effects as identified by an additional shoulder at 205 nm observed in their absorbance spectra. Presence of N–C bonds in the h-B...

Nonlinear absorption and nonlinear refraction in a chemical vapor deposition-grown, ultrathin hexagonal boron nitride film.

An ultrathin hexagonal boron nitride film is synthesized by a method of chemical vapor deposition. Irradiated by femtosecond laser pulses in the visible spectrum of 400-800 nm, it exhibits multiphoton absorption and positive nonlinear refraction properties. The two-photon and three-photon absorption coefficients are of the order of 10-5 cm W-1 and 10-14 cm3 W-2, respectively. The nonlinear refraction coefficient is as large as ∼10-8 cm2 W-1. These nonlinear coefficients lead to figures of merit that meet the material requirements for all-optical switching devices.