Wurtzite Boron Nitride Research Articles

Pressure dependence of electronic, vibrational and optical properties of wurtzite-boron nitride

Wurtzite Boron Nitride ($w$BN) is a wide band gap BN polymorph with peculiar mechanical properties (hardness and stiffness). After its first synthesis in 1963 as a transformation of hexagonal BN ($h$BN) under high temperature and pressure conditions, a lot of progress have been made in order to stabilize wurtzite phase at atmospheric pressure. Today the crystallization of good quality samples is finally possible. This fact motivates our first principles study of the electronic, vibrational and light absorption and emission properties of $w$BN over a wide range of pressures. Our findings are important in view of the potential use of $w$BN as a dielectric for integration in BN-based technologies in optoelectronics and harsh environment applications.

РАЗРАБОТКА МАТЕМАТИЧЕСКОЙ МОДЕЛИ ПРОЦЕССА СИНТЕЗА ПОЛИКРИСТАЛЛИЧЕСКОГО СВЕРХТВЕРДОГО МАТЕРИАЛА НА ОСНОВЕ КУБИЧЕСКОГО НИТРИДА БОРА ИЗ МОДИФИЦИРОВАННОГО АЛЮМИНИЕМ ВЮРЦИТНОГО НИТРИДА БОРА

A mathematical model is developed for the technological process of synthesis under conditions of high pressures P and temperatures T of a polycrystalline superhard material based on cubic boron nitride (cBN). The superhard material is obtained from a wurtzite boron nitride (wBN) powder modified with aluminum Al, which is the initiator of the wBN→cBN phase transformation process. As a result of modelling, the boundary parameters Р and Т of the superhard material synthesis and the required amount of aluminum addition are calculated. It has been established that the synthesis of a material with a hardness of 28–30 GPa and crack resistance in the range of 7–10 MPa·m1/2 is carried out in the pressure range of 5–7 GPa at temperatures of 2.100–2.250 °C, and the aluminum additive content should be 7.5–10.0 wt.%.

Strain Effects in Wurtzite Boron Nitride: Elastic Constants, Internal Strain, and Deformation Potentials from Hybrid Functional Density Functional Theory

Boron‐containing III‐nitride heterostructures have recently attracted significant attention for improving the efficiency of visible and UV light emitters. However, the fundamental material properties of wurtzite (WZ) boron nitride (BN) are largely unexplored. Here, highly accurate first‐principles calculations are used to gain insight into internal strain, elastic constants, and electronic band structure deformation potentials. These parameters are key ingredients for simulating, and thus predicting, electronic and optical properties of boron‐containing III‐nitride‐based light emitters. The ab initio calculations show, for instance, that the quasi‐cubic approximation for deformation potentials is a poor approximation for WZ BN.

Probing the physical properties of Boron Nitride with randomly distributed vacancies: A promising semiconductor for optoelectronics

Boron Nitride (BN) ceramics have good mechanical-strength, optical properties and excellent thermal stability. Due to its advantageous properties, BN has broad applications in ultra-hard semiconductor-based optoelectronic technologies. Herein, the effect of mono- and divacancies on the structural, electronic, magnetic, optical, and mechanical properties of BN crystallizing in hexagonal (h-BN), zinc blende (c-BN), and wurtzite Boron Nitride has been examined in detail using first principles-based calculations. We find that VB and VN vacancies change the electronic structure, shown by localized states near the Fermi level. While the divacancy produces more new states above the Fermi level. We also notice that BN system with VN had an n-type semiconductor while VB vacancies have an opposite trend that could stabilize BN well and reflect a p-type semiconductor with a largest hole mobility and a good electrical conductivity. Besides, mono- and divacancies induce ferromagnetism in the BN. The maximum total magnetic moment for the BN vacant system is 2.00 μB in case of 2-Boron vacancy configuration. This finding opens also the doors for spintronic applications. Optical and mechanical properties reveal that BN systems are promising materials for the next-generation electronics. Finally, this study paves the way for BN nanoribbon production and its use as functional semiconductors with a wide range of applications in optoelectronics and spintronics.

Diffusion Monte Carlo Study on Relative Stabilities of Boron Nitride Polymorphs

Although Boron nitride (BN) is a well-known compound widely used for engineering and scientific purposes, the phase stability of its polymorphs, one of its most fundamental properties, is still under debate. The ab initio determination of the ground state of the BN polymorphs, such as hexagonal and zinc-blende, is difficult because of the elusive Van der Waals interaction, which plays a decisive role in some of the polymorphs, making quantitative prediction highly challenging. Hence, despite multiple theoretical studies, there has been no consensus on the ground state yet, primarily due to contradicting reports. In this study, we apply a state-of-the-art ab initio framework - fixed-node diffusion Monte Carlo (FNDMC), to four well known BN polymorphs, namely hexagonal, rhombohedral, wurtzite, and zinc-blende BNs. Our FNDMC calculations show that hBN is thermodynamically the most stable among the four polymorphs at 0 K as well as at 300K. This result agrees with the experimental data of Corrigan~{\it et al.} and Fukunaga. The conclusions are consistent with those obtained using other high-level methods, such as coupled cluster. We demonstrate that the FNDMC is a powerful method to address polymorphs that exhibit bonds of various forms. It also provides valuable information, like reliable reference energies, when reliable experimental data are missing or difficult to access. Our findings should promote the application of FNDMC for other van der Waals materials.

Super-hardening and localized plastic deformation behaviors in ZrB2 –TaВ2 ceramics

A systematic study of the Zr-Ta-B system was performed using thermodynamic and ab initio modeling, nanoindentation, and high-resolution TEM and XRD experiments. The phase diagram (convex hull) of the Zr-Ta-B system was constructed at 0 K, and phase stability, lattice parameters, and mechanical properties of ZrB2-TaB2 solid solutions were ascertained in silico. Dense ceramics were manufactured by the combination of combustion synthesis and hot pressing techniques. As opposed to theoretically predicted complete solubility in the ZrB2-TaB2 solid solution series, the highest experimentally attained Ta content in the ZrB2-based solution was 25–27 at%, suggesting that at higher Ta contents a non-equilibrium phase composition is formed during combustion synthesis and inherited during hot pressing. Single-phase ZrB2-TaB2 solid solutions displayed a super-hardness up to 70 GPa and coefficient of elastic recovery as high as 99.7%, which surpasses wurtzite and cubic boron nitride and rivals polycrystalline diamonds. In the non-equilibrium samples with multiple solid solutions, a “pop-in” (localized plastic deformation) effect was discovered and attributed to the formation of shearing bands, microtwins, and dislocation loops. The composition-dependent switching between super-hardened and locally plastic mechanical behavior allows the creation of a new generation of ultra-high temperature ceramic composites.

Bias-Enhanced Formation of Metastable and Multiphase Boron Nitride Coating in Microwave Plasma Chemical Vapor Deposition.

Boron nitride (BN) is primarily a synthetically produced advanced ceramic material. It is isoelectronic to carbon and, like carbon, can exist as several polymorphic modifications. Microwave plasma chemical vapor deposition (MPCVD) of metastable wurtzite boron nitride is reported for the first time and found to be facilitated by the application of direct current (DC) bias to the substrate. The applied negative DC bias was found to yield a higher content of sp3 bonded BN in both cubic and metastable wurtzite structural forms. This is confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Nano-indentation measurements reveal an average coating hardness of 25 GPa with some measurements as high as 31 GPa, consistent with a substantial fraction of sp3 bonding mixed with the hexagonal sp2 bonded BN phase.

Bandgap engineering in III-nitrides with boron and group V elements: Toward applications in ultraviolet emitters

A key material system for opto- and high-power electronics are III-nitrides. Their functionality can be expanded when bandgap engineering is extended beyond common materials such as AlN, GaN, and InN. Combining these three compounds with boron nitride and other III–V compounds (GaP, GaAs, GaSb, InP, etc.) is an intuitive method of expanding bandgap engineering in semiconductor devices. This may allow improvement of current devices for which performances are limited by the intrinsic properties of common III-nitride alloys, as well as the creation of novel devices. A comprehensive review of this activity is presented in this article, including an up-to-date compilation of material parameters for wurtzite boron nitride; its alloying with other III-nitrides, including structural and optical characterization; the band anticrossing model for III-nitrides diluted with group V atoms; their synthesis and structural and optical characterization; and examples of applications of III-nitrides containing boron and group V atoms in semiconductor devices. It is shown to be very beneficial for ultraviolet emitters to incorporate alloying of III-nitrides with BN, as these compounds have lattice constants much smaller than that of AlN, offering unique possibilities in strain engineering. It is shown that the incorporation of P, As, Sb, and Bi in GaN is low when the material is deposited at this temperature, which is optimal for the host. Lowering the growth temperature significantly enhances the incorporation of isovalent dopants, but deteriorates the optical quality of the material. The obtained changes in the electronic band structure can be beneficial in many applications, including water splitting or shifting emission toward longer wavelengths.

Influence of diamond component based on wurtzite boron nitride on wear resistance of cutting tool

The article is devoted to the study of the influence of the diamond component in a composite superhard material based on wurtzite boron nitride on the stability of the tool during turning of hardened steels in the mode of smooth turning and when processing intermittent surfaces. The aim of the work is to study the influence of the content of the diamond component in the composite superhard material based on wurtzite boron nitride on the stability of the tool. Based on the analysis of the main patterns of changes in the coefficient of friction depending on various factors, the evaluation of composites containing diamonds of different dispersion in comparison with other materials of this class. The results of the study of the technical level of composite polycrystalline superhard materials based on wurtzite boron nitride of different composition and determination of the efficiency of their use as a cutting tool in the machining of hardened steels are presented. The intensity of wear of composites in the cutting tool during processing of hardened steels is experimentally determined. It is established that the presence of a diamond component in the composite significantly affects the stability of the tool in the conditions of smooth turning due to heat dissipation in contact with the processed material. In the processing of hardened steels with the presence of shock loads, the phase state of the matrix component of the composite plays a decisive role. The obtained research results make it possible to determine the optimal composition and conditions for obtaining a composite material of the system “boron nitride – diamond”, which provides the most effective application of the cutting tool in practice.

Nonreversible Transition from the Hexagonal to Wurtzite Phase of Boron Nitride under High Pressure: Optical Properties of the Wurtzite Phase

We present an infrared reflectance and transmission study of a pressure-induced phase transition of boron nitride from the hexagonal layered structure to the wurtzitic phase. The transition is completed at about 13 GPa. The phase transition is nonreversible and the optical features of the metastable wurtzitic phase are retained after a pressure cycle from 20.5 GPa down to ambient pressure. This allows the infrared-active optical phonons and the dielectric properties of the cold-pressed wurtzitic boron nitride sample to be studied over the whole range of pressures. Experimental permittivity values of e0 = 6.65 ± 0.03 and e∞ = 4.50 ± 0.05 are determined from fits to the reflectance spectra at ambient pressure. Accurate values of the refractive index in the mid-infrared and visible–ultraviolet regions are evaluated from the interference patterns. Contrary to the h-BN case, the refractive index of w-BN decreases slightly with pressure, on account of the much lower compressibility of the close-packed structure. The pressure coefficients for the longitudinal optical and transverse optical modes are determined, and an overall good agreement with ab initio calculations is found.

Hardness of Polycrystalline Wurtzite Boron Nitride (wBN) Compacts

Wurtzite boron nitride (wBN), due to its superior properties and many potential practical and scientific applications, such as ideal machining/cutting/milling ferrous and carbide materials, especially as an ideal dielectric substrate material for optical, electronic, and 2-D graphene-based devices, has recently attracted much attention from both academic and industrial fields. Despite decades of research, there is an ongoing debate about if the single-phase wBN is harder than diamond because of the difficulty to make pure wBN material. Here we report the successful synthesis of pure single-phase polycrystalline wurtzite-type boron nitride (wBN) bulk material by using wBN powder as a starting material with a well-controlled process under ultra-high pressure and high temperature. The cubic boron nitride (cBN) was also successfully prepared for the first time from wBN starting material for comparison and verification. The X-ray diffraction (XRD) and TEM clearly confirmed that a pure single-phase wBN compact was produced. The microstructure and mechanical properties including Vickers hardness, fracture toughness, and thermal stability for the pure single-phase wBN was first evaluated.

Thermodynamic stability of hexagonal and rhombohedral boron nitride under chemical vapor deposition conditions from van der Waals corrected first principles calculations

Thin films of boron nitride (BN), particularly the sp2-hybridized polytypes hexagonal BN (h-BN) and rhombohedral BN (r-BN), are interesting for several electronic applications, given the bandgaps in the UV. They are typically deposited close to thermal equilibrium by chemical vapor deposition (CVD) at temperatures and pressures in the regions 1400–1800 K and 1000–10 000 Pa, respectively. In this letter, the authors use the van der Waals corrected density functional theory and thermodynamic stability calculations to determine the stability of r-BN and compare it to that of h-BN as well as to cubic BN and wurtzitic BN. The authors find that r-BN is the stable sp2-hybridized phase at CVD conditions, while h-BN is metastable. Thus, their calculations suggest that thin films of h-BN must be deposited far from thermal equilibrium.

Stabilizing the metastable superhard material wurtzite boron nitride by three-dimensional networks of planar defects

Wurtzite boron nitride (w-BN) is a metastable superhard material that is a high-pressure polymorph of BN. Clarifying how the metastable high-pressure material can be stabilized at atmospheric pressure is a challenging issue of fundamental scientific importance and promising technological value. Here, we fabricate millimeter-size w-BN bulk crystals via the hexagonal-to-wurtzite phase transformation at high pressure and high temperature. By combining transmission electron microscopy and ab initio molecular dynamics simulations, we reveal a stabilization mechanism for w-BN, i.e., the metastable high-pressure phase can be stabilized by 3D networks of planar defects which are constructed by a high density of intersecting (0001) stacking faults and {10[Formula: see text]0} inversion domain boundaries. The 3D networks of planar defects segment the w-BN bulk crystal into numerous nanometer-size prismatic domains with the reverse crystallographic polarities. Our findings unambiguously demonstrate the retarding effect of crystal defects on the phase transformations of metastable materials, which is in contrast to the common knowledge that the crystal defects in materials will facilitate the occurrence of phase transformations.

Pressure dependent ultrasonic characterization of nano-structured w-BN

The work includes evaluation of elastic constants of wurtzite Boron nitride (w-BN) at different particle-size (5–40 nm) and pressure (0–60 GPa) at 300 K using potential model approach. Size and pressure dependent thermo-physical and ultrasonic parameters are also calculated using evaluated elastic constants. It is found that the elastic constants, ultrasonic velocities and Debye temperature of nanostructured w-BN enhance with increase in pressure and reduction in particle size. The size variation of thermal relaxation time resembles the dependency of thermal conductivity with size. The thermal conductivity of nanostructured w-BN is found to increase with reduction in nanoparticle size.

Structural Engineering of Zinc‐Blend/Wurtzite BN Superlattices

AbstractBy constructing Zinc‐Blende (ZB)/Wurtzite (WZ) boron nitride (BN) (ZB1‐x/WZx‐BN) superlattices (SLs), their electronic structure and mechanical properties were investigated by first‐principles techniques based on density functional theory. It is found that the band structure is significantly modulated by the ratio of WZ phase in ZB/WZ‐BN SLs. With the increase of WZ ratio, the band gap of SLs decreases and then increases dramatically. More importantly, transformation from type‐I to type‐II band alignment between ZB and WZ can be tuned by adjusting the ratio of WZ to be >0.6. In addition, a universal expression of the band gap is proposed to reveal a nonlinear dependence of the band gap within the structure of SLs as induced by from the interface effect achieved via structural transformation. The elastic constants results revealed that the change of ZB/WZ ratio poses no significant effect on the mechanical properties. The effect of structure modulation on band structure and mechanical properties is significantly important for future engineering of novel semiconductor SLs.

Thermally Induced Wurtzite to h‐BN Structural Transition

Wurtzite (WZ) and hexagonal boron nitride (h‐BN) are closely related crystal structures, both of which belong to the hexagonal crystal family. However, the WZ structure is commonly found in binary compounds such as ZnO and CdS, whereas crystals having a h‐BN structure are very rare. In this study, a WZ→h‐BN structural transition is predicted to take place at high temperatures using molecular dynamic simulations with a generic pair potential model. This transition is entropically driven, whereby cations in crystal show extremely high mobility and are able to jump between nearby lattice sites when the temperature is higher than the critical point. Density function theory calculations for 14 AB compounds show that ZnO is the best candidate material that will undergo the WZ→h‐BN transition at high temperatures. This study predicts a new ferroelectric–paraelectric phase transition in WZ crystals that will change the materials’ physical properties such as piezoelectricity and pyroelectricity.

Pressure- and orientation-dependent elastic and ultrasonic characterisation of wurtzite boron nitride

The present study discloses the evaluation of second-order elastic constants of wurtzite boron nitride (w-BN) at room temperature and at different pressures using the many-body interaction potential model approach. Orientation- and pressure-dependent ultrasonic velocity, thermal relaxation time and other related thermophysical parameters (Debye temperature, Debye average velocity, specific heat and thermal energy density) are also calculated using the evaluated second-order elastic constants. The orientation-dependent thermal relaxation time of w-BN is predominantly affected by the Debye average velocity and is indirectly governed by second-order elastic constants. Thermal relaxation time of w-BN is found to decrease with pressure. Calculated elastic and ultrasonic properties of w-BN are compared with the properties of other wurtzite structured materials for a complete analysis and characterisation of the material.

Ab initiocalculations of carbon and boron nitride allotropes and their structural phase transitions using periodic coupled cluster theory

We present an ab-initio study of boron nitride as well as carbon allotropes. Their relative thermodynamic stabilities and structural phase transitions from low- to high-density phases are investigated. Pressure-temperature phase diagrams are calculated and compared to experimental findings. The calculations are performed using quantum chemical wavefunction based as well as density functional theories. Our findings reveal that predicted energy differences often depend significantly on the choice of the employed method. Comparison between calculated and experimental results allows for benchmarking the accuracy of various levels of theory. The produced results show that quantum chemical wavefunction based theories allow for achieving systematically improvable estimates. We find that on the level of coupled cluster theories the low- and high-density phases of boron nitride become thermodynamically degenerate at 0 K. This is in agreement with recent experimental findings, indicating that cubic boron nitride is not the thermodynamically stable allotrope at ambient conditions. Furthermore we employ the calculated results to assess transition probabilities from graphitic low-density to diamond-like high-density phases in an approximate manner. We conclude that the stacking order of the parent graphitic material is crucial for the possible formation of meta-stable wurtzite boron nitride and hexagonal carbon diamond also known as lonsdaleite.

Lattice thermal transport in superhard hexagonal diamond and wurtzite boron nitride: A comparative study with cubic diamond and cubic boron nitride

Hexagonal diamond (h-C) and wurtzite boron nitride (w-BN) are two superhard materials recently identified to be comparable to or even harder than their cubic counterparts, cubic diamond (c-C) and cubic boron nitride (c-BN). To understand the effect of lattice structure on thermal transport in these materials, we conduct first-principles calculations to investigate their harmonic and anharmonic lattice properties. Owing to the strong C-C or B-N bonds, h-C and w-BN are found to have a high lattice thermal conductivity (κL) exceeding the overall thermal conductivity of metals, albeit lower than that of their cubic counterparts. By analyzing the phonon band structure and volume of the 3-phonon scattering phase space, we attribute the lower κL of the hexagonal phases to their larger volume of 3-phonon scattering phase space than the cubic ones. Moreover, we reveal that a high pressure of 125 GPa leads to a two-to three-fold increase in the κL of these materials, because the pressure enlarges the optical-acoustic phonon bandgap and thus reduces the volume of the 3-phonon scattering phase space. This work uncovers the significant effect of lattice structure and pressure on phonon scattering and transport, which is crucial for the application of superhard materials.

Mechanical Properties of Cubic‐BN(111) Bulk Single Crystal Evaluated by Nanoindentation

We performed nanoindentation measurements on the (111) plane of a cubic boron nitride (c‐BN) bulk single crystal and a c‐BN sintered compact at room temperature to evaluate the intrinsic mechanical properties of c‐BN. Our load–penetration depth curves for bulk single crystal feature a low noise level and small variation in the data. These features reflect the perfect phase purity and high crystal coherency of the c‐BN bulk crystal. The hardness and Young's modulus are determined to be 61 ± 1 and 1070 ± 30 GPa, respectively. These values are larger than those corresponding to the c‐BN sintered compact. The obtained Young's modulus followed a similar trend to previous experimental and calculation studies. Hence, we demonstrate the effectiveness of this evaluation method based on high‐quality bulk single crystals. We also compared the mechanical properties with our results for wurtzite boron nitride (w‐BN) bulk crystal.