Boron Nitride Films Research Articles

(Invited) Integrated Crystal Growth of 2D Materials

In order to serve the industrial demand for “electronic-grade” 2D materials, we focus on chemical vapour deposition (CVD), and in this talk I will review our recent progress in scalable CVD [1] and device integration approaches of highly crystalline graphene and hexagonal boron nitride (h-BN) films. The systematic use of in-situ metrology, ranging from high-pressure XPS to environmental electron microscopy, allows us to reveal some of the key mechanisms that dictate crystal phase, structural, defect, interfacial and heterogeneous integration control at industrially relevant conditions [2,3]. We systematically also explored the parameter space of atomic layer deposition (ALD) of oxides on graphene to achieve ultrathin continuous AlOx films directly on graphene [4]. These results have model system character for rational 2D/non-2D material process integration [5]. We highlight how the interplay between the 2D material and the catalyst is not only important for growth [6] but also decisive for transfer processes and show how intercalation processes allow for instance local Cu oxidation at the interface followed by selective oxide dissolution, which gently releases the 2D material [7]. We developed direct and transfer 2D material integration approaches for a large number of diverse applications, ranging from spintronics [8], to nanopore sensing [9] and THz devices [10]. Crystal growth and processing of these 2D materials reached a level where detailed, adequate characterisation over large areas has become a key challenge. Hence we also study new non-contact characterisation methods for rapid in-line monitoring [11]. References Hofmann et al., J. Phys. Chem. Lett. 6, 2714 (2015).Weatherup et al., Nano Lett. 16, 6196 (2016).Caneva et al. Nano Lett. 16, 1250 (2016).Aria et al. ACS Appl. Mater. Interfaces 8, 30564 (2016).Alexander-Webber et al., 2D Mater. 4, 011008 (2016).Braeuninger-Weimar et al., Chem. Mater. 28, 8905 (2016).Wang et al., ACS Appl. Mater. Interfaces 8, 33072 (2016).Piquemal-Banci et al., Appl. Phys. Lett. 108, 102404 (2016).Walker et al., ACS Nano 11, 1340 (2017).Degl’Innocenti et al., ACS Photonics 3, 1747 (2016).Lin et al. Sci. Rep. 7, 10625 (2017).

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.

Interface amorphization in hexagonal boron nitride films on sapphire substrate grown by metalorganic vapor phase epitaxy

Hexagonal boron nitride (h-BN) films directly grown on c-plane sapphire substrates by pulsed-mode metalorganic vapor phase epitaxy exhibit an interlayer for growth temperatures above 1200 °C. Cross-sectional transmission electron microscopy shows that this interlayer is amorphous, while the crystalline h-BN layer above has a distinct orientational relationship with the sapphire substrate. Electron energy loss spectroscopy shows the energy-loss peaks of B and N in both the amorphous interlayer and the overlying crystalline h-BN layer, while Al and O signals are also seen in the amorphous interlayer. Thus, the interlayer forms during h-BN growth through the decomposition of the sapphire at elevated temperatures.

Deposition of cubic boron nitride films by anode layer linear ion source assisted radio frequency magnetron sputtering

Anode Layer Linear Ion Source (ALLIS) can improve the ionization rate of working gases and enhance the activity of gas molecules. In the present work, the ALLIS assisted Radio Frequency Magnetron Sputtering (RFMS) method was proposed to deposit cubic boron nitride (cBN) films, and the effect of ALLIS on the microstructure and mechanical properties of the deposited cBN films was investigated. The phase component, chemical composition, bonding states, surface topography, deposition rate and nanohardness of the cBN films were characterized respectively by Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and nanoindentation. The results indicated that the sp3 phase content in cBN films increased at first and then decreased with the increase of ALLIS power, and the maximum value was 83% when the ALLIS power was 200 W, the surface quality and growth rate of cBN films were enhanced markedly. The hardness and elastic modulus exhibited the same trend to variation of cubic phase content. Finally, the effect of ALLIS on the cBN synthesis was discussed systematically.

Electron-Enhanced Atomic Layer Deposition of Boron Nitride Thin Films at Room Temperature and 100 °C

Electron-enhanced atomic layer deposition (EE-ALD) was used to deposit boron nitride (BN) thin films at room temperature and 100 °C using sequential exposures of borazine (B3N3H6) and electrons. Electron-stimulated desorption (ESD) of hydrogen surface species and the corresponding creation of reactive dangling bonds are believed to facilitate borazine adsorption and reduce the temperature required for BN film deposition. In situ ellipsometry measurements showed that the BN film thickness increased linearly versus the number of EE-ALD cycles at room temperature. Maximum growth rates of ~3.2 Å/cycle were measured at electron energies of 80-160 eV. BN film growth was self-limiting versus borazine and electron exposures, as expected for an ALD process. The calculated average hydrogen ESD cross section was σ = 4.2 × 10-17 cm2. Ex situ spectroscopic ellipsometry measurements across the ~1 cm2 area of the BN film defined by the electron beam displayed good uniformity in thickness. Ex situ X-ray photoelectron spectroscopy and in situ Auger spectroscopy revealed high purity, slightly boron-rich BN films with C and O impurity levels <3 at. %. High-resolution transmission electron microscopy (HR-TEM) imaging revealed polycrystalline hexagonal and turbostratic BN with the basal planes approximately parallel to the substrate surface. Ex situ grazing incidence X-ray diffraction measurements observed peaks consistent with hexagonal BN with domain sizes of 1-2 nm. The BN EE-ALD growth rate of ~3.2 Å/cycle is close to the distance of 3.3 Å between BN planes in hexagonal BN. The growth rate and HR-TEM images suggest that approximately one monolayer of BN is deposited for every BN EE-ALD cycle. TEM and scanning TEM/electron energy loss spectroscopy measurements of BN EE-ALD on trenched wafers also showed preferential BN EE-ALD on the horizontal surfaces. This selective deposition on the horizontal surfaces suggests that EE-ALD may enable bottom-up filling of vias and trenches.

Review Article: Challenge in determining the crystal structure of epitaxial 0001 oriented sp2-BN films

Boron nitride (BN) as a thin film is promising for many future electronic applications. On 0001 α-Al2O3 and 0001 4H/6H-SiC substrates, chemical vapor deposition yields epitaxial sp2-hybridized BN (sp2-BN) films oriented around the c-axis. Here, the authors seek to point out that sp2-BN can form two different polytypes; hexagonal BN (h-BN) and rhombohedral BN (r-BN), only differing in the stacking of the basal planes but with the identical distance between the basal planes and in-plane lattice parameters. This makes structural identification challenging in c-axis oriented films. The authors suggest the use of a combination of high-resolution electron microscopy with careful sample preparation and thin film x-ray diffraction techniques like pole figure measurements and glancing incidence (in-plane) diffraction to fully distinguish h-BN from r-BN.

Multi c-BN Coatings by r.f Diode Sputtering and Investigation of Wear Behavior

Cubic boron nitride (c-BN) films on tool substrates are tendency to delaminate. Therefore, many research groups have studied improvement of c-BN synthesis method and deposition processes due to many potential applications. In this paper, we show that the adhesion property of c-BN layer system can be improved by deposing multi c-BN layers. The multi c-BN layers were deposited by r.f (13.56 MHz) diode sputtering apparatus on cemented carbide tool substrates with a TiAlN adhesion layer. For industrial applications, we performed turning and milling experiments under dry and high speed cutting conditions. In this multi c-BN layer system, the lifetime of the tool is affected by the physical properties of the substrate and coated layers such as substrate grain size, thickness of the TiAlN and first c-BN layer and the total number of c-BN layers in this system. Mostly, fine grain size substrates showed longer lifetimes of over 4 times than raw one. In the turning performance, mono TiAlN layer systems were about two times lower lifetime than mono and multi c-BN layer system, moreover, we could be improved adhesion property for milling performance on tool substrates with binary multi c-BN layer systems under dry and high speed cutting conditions. The new application results of the multi c-BN layer system confirm that the high potential of c-BN coatings on cutting tools.

RETRACTED: Exploration of Growth Window for Phase-Pure Cubic Boron Nitride Films Prepared in a Pure N2 Plasma

Cubic boron nitride (c-BN) films were prepared via radio frequency (RF) magnetron sputtering from a hexagonal boron nitride (h-BN) target in a pure N2 plasma. The composition and microstructure morphology of the BN films with different deposition times under pure N2 plasma or mixed Ar/N2 plasma were investigated with respect to the nucleation and growth processes. The pure-phase c-BN growth window was obtained using pure N2 gas. The effects of pure N2 gas on the growth mechanism, structural morphology, and internal compressive stress of the as-synthesized c-BN films were studied. Using pure N2 gas instead of additional Ar resulted in improved microstructure quality and much reduced compressive stress, suggesting a fundamental strategy for achieving high-quality c-BN films.

Interfacial Strength and Surface Damage Characteristics of Atomically Thin h-BN, MoS2, and Graphene.

Surface damage characteristics of single- and multilayer hexagonal boron nitride (h-BN), molybdenum disulfide (MoS2), and graphene films were systematically investigated via atomic force microscopy (AFM)-based progressive-force and constant-force scratch tests and Raman spectroscopy. The film-to-substrate interfacial strengths of these atomically thin films were assessed based on their critical forces (i.e., the normal force where the atomically thin film was delaminated from the underlying substrate), as determined from progressive-force scratch tests. The evolution of surface damage with respect to normal force was further investigated using constant-force tests. The results showed that single-layer h-BN, MoS2, and graphene strongly adhere to the SiO2 substrate, which significantly improves its tribological performance. Moreover, defect formation induced by scratch testing was found to affect the topography and friction force differently prior to failure, which points to distinct surface damage characteristics. Interestingly, the residual strains at scratched areas suggest that the scratch test-induced in-plane compressive strains were dominant over tensile strains, thereby leading to buckling in front of the scratching tip and eventually failure at sufficient strains. These trends represent the general failure mechanisms of atomically thin materials because of a scratch test. As the number of layers increased, the tribological performances of atomically thin h-BN, MoS2, and graphene were found to significantly improve because of an increase in the interfacial strengths and a decrease in the surface damage and friction force. In all, the findings on the distinctive surface damage characteristics and general failure mechanisms are useful for the design of reliable, protective and solid-lubricant coating layers based on these materials for nanoscale devices.

Conductive Atomic Force Microscope Study of Bipolar and Threshold Resistive Switching in 2D Hexagonal Boron Nitride Films

This study investigates the resistive switching characteristics and underlying mechanism in 2D layered hexagonal boron nitride (h-BN) dielectric films using conductive atomic force microscopy. A combination of bipolar and threshold resistive switching is observed consistently on multi-layer h-BN/Cu stacks in the low power regime with current compliance (Icomp) of less than 100 nA. Standard random telegraph noise signatures were observed in the low resistance state (LRS), similar to the trends in oxygen vacancy-based RRAM devices. While h-BN appears to be a good candidate in terms of switching performance and endurance, it performs poorly in terms of retention lifetime due to the self-recovery of LRS state (similar to recovery of soft breakdown in oxide-based dielectrics) that is consistently observed at all locations without requiring any change in the voltage polarity for Icomp ~1–100 nA.

Defect-selective dry etching for quick and easy probing of hexagonal boron nitride domains

In this study, we demonstrate a new method to selectively etch the point defects or the boundaries of as-grown hexagonal boron nitride (hBN) films and flakes in situ on copper substrates using hydrogen and argon gases. The initial quality of the chemical vapor deposition-grown hBN films and flakes was confirmed by UV–vis absorption spectroscopy, atomic force microscopy, and transmission electron microscopy. Different gas flow ratios of Ar/H2 were then employed to etch the same quality of samples and it was found that etching with hydrogen starts from the point defects and grows epitaxially, which helps in confirming crystalline orientations. However, etching with argon is sensitive to line defects (boundaries) and helps in visualizing the domain size. Finally, based on this defect-selective dry etching technique, it could be visualized that the domains of a polycrystalline hBN monolayer merged together with many parts, even with those that grew from a single nucleation seed.

Efficient exfoliation of layered materials by waste liquor

Based on their unique material properties, two-dimensional (2D) nanomaterials such as graphene, molybdenum disulfide (MoS2), and boron nitride (BN) have been attracting increased research interest. The potential of 2D materials, in the form of nanoplatelets that are used as new materials, will be important to both nanomaterials and advanced materials. Water is usually considered to be the ideal dispersed medium, and the essential hydrophobicity and limitations to mass production of 2D nanoplatelets have become quite serious obstacles to their usage in various fields. In this paper, pulping black liquor was used as dispersant, with high concentration of lignin to get single- and few-layered nanoplatelets. The whole process required only the high-shear mixing of 2D layered materials and pulping waste liquor. This method was not only simple and efficient but also environmentally friendly and resource-recycling. Moreover, the fabricated single- or few-layered nanoplatelets possessed good solubility in aqueous solution due to their edge functionalization, and could be well dispersed in water at concentrations (10 mg ml−1 for graphene, 6.3 mg ml−1 for MoS2, and 6.0 mg ml−1 for BN) which were much higher than that of other methods. The dispersions of graphene, MoS2, and BN nanosheets were highly stable over several months, which allowed us to easily prepare graphene, MoS2, and BN films through simple vacuum filtration or spraying. These results indicated that pulping black liquor can be used as a material or reagent, and the mass production of 2D material is possible in a simple and fast method.

Smoothening of wrinkles in CVD-grown hexagonal boron nitride films.

Hexagonal boron nitride (h-BN) is an ideal substrate for two-dimensional (2D) materials because of its unique electrically insulating nature, atomic smoothness and low density of dangling bonds. Although mechanical exfoliation from bulk crystals produces the most pristine flakes, scalable fabrication of devices is still dependent on other more direct synthetic routes. To date, the most utilized method to synthesize large-area h-BN films is by chemical vapor deposition (CVD) using catalytic metal substrates. However, a major drawback for such synthetic films is the manifestation of thermally-induced wrinkles, which severely disrupt the smoothness of the h-BN films. Here, we provide a detailed characterization study of the microstructure of h-BN wrinkles and demonstrate an effective post-synthesis smoothening route by thermal annealing in air. The smoothened h-BN film showed an improved surface smoothness by up to 66% and resulted in a much cleaner surface due to the elimination of polymer residues with no substantial oxidative damage to the film. The unwrinkling effect is attributed to the hydroxylation of the h-BN film as well as the substrate surface, resulting in a reduction in adhesion energy at the interface. Dehydroxylation occurs over time under ambient conditions at room temperature and the smoothened film can be restored back with the intrinsic properties of h-BN. This work provides an efficient route to achieve smoother h-BN films, which are beneficial for high-performance 2D heterostructure devices.

Measuring the dielectric and optical response of millimeter-scale amorphous and hexagonal boron nitride films grown on epitaxial graphene

Monolayer epitaxial graphene (EG), grown on the Si face of SiC, is an advantageous material for a variety of electronic and optical applications. EG forms as a single crystal over millimeter-scale areas and consequently, the large scale single crystal can be utilized as a template for growth of other materials. In this work, we present the use of EG as a template to form millimeter-scale amorphous and hexagonal boron nitride (a-BN and h-BN) films. The a-BN is formed with pulsed laser deposition and the h-BN is grown with triethylboron (TEB) and NH3 precursors, making it the first metal organic chemical vapor deposition (MOCVD) process of this growth type performed on epitaxial graphene. A variety of optical and non-optical characterization methods are used to determine the optical absorption and dielectric functions of the EG, a-BN, and h-BN within the energy range of 1 eV–8.5 eV. Furthermore, we report the first ellipsometric observation of high-energy resonant excitons in EG from the 4H polytype of SiC and an analysis on the interactions within the EG and h-BN heterostructure.

Leakage current characteristics of thick cubic boron nitride films deposited on titanium

Thick cubic and sp2-bonded boron nitride (cBN and sp2BN) films are deposited on Ti substrates by plasma jet enhanced chemical vapor deposition using the chemistry of fluorine, and their direct current-voltage characteristics are studied for Ni-BN-Ti capacitor structures. The resistivity of the cBN film measured at room temperature is of the order of 108 Ωcm, which is three to four orders of magnitude lower than that of the sp2BN film. At high electric fields, Frenkel-Poole emission dominates the conduction of the sp2BN film, while thermionic emission is better able to describe the conduction of the cBN film at temperatures up to 473 K. A lower leakage current indicates lower densities of carriers and trap sites associated with defects, suggesting that a higher-quality cBN film with higher crystallinity and stoichiometry is potentially promising as an ultrahard dielectric material in high temperature condition.

Nearly perfect resonant absorption and coherent thermal emission by hBN-based photonic crystals.

In this paper, we numerically demonstrate mid-IR nearly perfect resonant absorption and coherent thermal emission for both polarizations and wide angular region using multilayer designs of unpatterned films of hexagonal boron nitride (hBN). In these optimized structures, the films of hBN are transferred onto a Ge spacer layer on top of a one-dimensional photonic crystal (1D PC) composed of alternating layers of KBr and Ge. According to the perfect agreements between our analytical and numerical results, we discover that the mentioned optical characteristic of the hBN-based 1D PCs is due to a strong coupling between localized photonic modes supported by the PC and the phononic modes of hBN films. These coupled modes are referred as Tamm phonons. Moreover, our findings prove that the resonant absorptions can be red- or blue-shifted by changing the thickness of hBN and the spacer layer. The obtained results in this paper are beneficial for designing coherent thermal sources, light absorbers, and sensors operating within 6.2 μm to 7.3 μm in a wide angular range and both polarizations. The planar and lithography free nature of this multilayer design is a prominent factor that makes it a large scale compatible design.

BN Nanosheet/Polymer Films with Highly Anisotropic Thermal Conductivity for Thermal Management Applications.

The development of advanced thermal transport materials is a global challenge. Two-dimensional nanomaterials have been demonstrated as promising candidates for thermal management applications. Here, we report a boron nitride (BN) nanosheet/polymer composite film with excellent flexibility and toughness prepared by vacuum-assisted filtration. The mechanical performance of the composite film is highly flexible and robust. It is noteworthy that the film exhibits highly anisotropic properties, with superior in-plane thermal conductivity of around 200 W m-1 K-1 and extremely low through-plane thermal conductivity of 1.0 W m-1 K-1, making this material an excellent candidate for thermal management in electronics. Importantly, the composite film shows fire-resistant properties. The newly developed unconventional flexible, tough, and refractory BN films are also promising for heat dissipation in a variety of applications.

Enhanced Crystallinity of h‐BN Films Induced by Substrate Bias During Magnetron Sputtering

The synthesis of uniform, highly crystalline thin films of hexagonal boron nitride (h‐BN) is an important step for its applications in multilayered devices with other emerging 2D materials. We report on the growth of high purity h‐BN by reactive magnetron sputtering from a pure B target in an Ar/N2 plasma. Enhanced h‐BN crystallinity on Ni substrates was achieved using a negative DC substrate bias during deposition, which served to increase the energy of incident ions, causing higher adatom mobility and re‐sputtering of weakly bound species on the growth surface. Growth rates between 0.002 to 1.1 Å s−1 were observed as the power was varied from 10 to 200 W and the growth temperature was either room temperature or 850 °C. BN films grown on a r‐sapphire substrate or a silicon (100) wafer with native oxide were smooth but amorphous under all conditions. Crystalline growth was observed on unbiased Ni foil substrates heated to 850 °C provided the growth rate was below 0.01 Å s−1, but the films were a non‐uniform mixture of h‐BN crystals and a disordered phase. Using a −50 V substrate bias, a high degree of h‐BN crystallinity and film homogeneity over large areas on the Ni substrate was achieved.

Oxygen-assisted synthesis of hexagonal boron nitride films for graphene transistors

We grow high-quality two-dimensional hexagonal boron nitride (h-BN) films on copper pockets by chemical vapor deposition. A piece of sapphire embedded in the pocket serves as an oxygen supply during the growth process. To obtain clean h-BN films, source powders are placed in a U-shaped quartz tube and heated up in a water bath without the carrier-gas flow. These films are characterized by using SEM, Raman, XPS, and selected area electron diffraction analyses. As dielectric substrates, h-BN films significantly enhance the charge-carrier mobility of graphene transistors. This facile and robust method can be a scalable approach to synthesize large-area high-quality h-BN films for related electronic applications.

Mapping the layer count of few-layer hexagonal boron nitride at high lateral spatial resolutions

Layer count control and uniformity of two dimensional (2D) layered materials are critical to the investigation of their properties and to their electronic device applications, but methods to map 2D material layer count at nanometer-level lateral spatial resolutions have been lacking. Here, we demonstrate a method based on two complementary techniques widely available in transmission electron microscopes (TEMs) to map the layer count of multilayer hexagonal boron nitride (h-BN) films. The mass-thickness contrast in high-angle annular dark-field (HAADF) imaging in the scanning transmission electron microscope (STEM) mode allows for thickness determination in atomically clean regions with high spatial resolution (sub-nanometer), but is limited by surface contamination. To complement, another technique based on the boron K ionization edge in the electron energy loss spectroscopy spectrum (EELS) of h-BN is developed to quantify the layer count so that surface contamination does not cause an overestimate, albeit at a lower spatial resolution (nanometers). The two techniques agree remarkably well in atomically clean regions with discrepancies within ±1 layer. For the first time, the layer count uniformity on the scale of nanometers is quantified for a 2D material. The methodology is applicable to layer count mapping of other 2D layered materials, paving the way toward the synthesis of multilayer 2D materials with homogeneous layer count.