Sp2-bonded BN Research Articles

X-ray absorption studies on cubic boron nitride thin films

Cubic boron nitride (c-BN) films synthesized by various energetic species assisted physical vapor deposition and chemical vapor deposition techniques on Si and diamond-coated Si substrates have been investigated by boron and nitrogen K-edge angle-resolved x-ray absorption near-edge structure in both total electron yield and fluorescence yield modes. X-ray absorption spectrum has been developed to study the film structure, the quantity and distribution of the partially ordered turbostratic (t-BN) and amorphous (a-BN) sp2-hybridized BN phases, and the t-BN∕a-BN ratios. The preferred direction of the t-BN basal planes at the interface between c-BN and substrate is found to be normal or nearly normal to the substrate. The content of the sp2-bonded BN in the c-BN films deposited on diamond-coated Si substrates reduces remarkably. The modifications of the electronic structure of the c-BN films with respect to bulk hexagonal BN and c-BN have been investigated and the crystallinity of c-BN films has also been evaluated from the x-ray absorption near edge structure results.

Structural evolution of boron nitride films grown on diamond buffer-layers

Boron nitride films on diamond buffer layers of varying grain size, surface roughness and crystallinity are deposited by the reaction of B 2H 6 and NH 3 in a mixture of H 2 and Ar via microwave plasma-assisted chemical vapor deposition. Various forms of boron nitride, including amorphous α-BN, hexagonal h-BN, turbostratic t-BN, rhombohedral r-BN, explosion E-BN, wurzitic w-BN and cubic c-BN, are detected in the BN films grown on different diamond buffer layers at varying distances from the interface of diamond and BN layers. The c-BN content in the BN films is inversely proportional to the surface roughness of the diamond buffer layers. Cubic boron nitride can directly grow on smooth nanocrystalline diamond films, while precursor layers consisting of various sp 2-bonded BN phases are formed prior to the growth of c-BN film on rough microcrystalline diamond films.

Cubic Phase Content and Structure of BN Films from an X-ray Absorption Study

X-ray absorption near-edge structure (XANES) was used to study the cubic boron nitride (c-BN) content in the BN films deposited on various substrates by different physical vapor deposition or plasma-enhanced chemical vapor deposition methods. By fitting the XANES curves of thin-film samples using standard spectra of pure c-BN and sp(2)-bonded BN in the films with suitable weight factors, the c-BN contents at the film's surface region and across the film's thickness have been determined quantitatively. The results agree well with the previous transmission electron microscopic observations. The method is proved to be independent of the optical properties of thin film and provides a possibility to evaluate the cubic content of BN films accurately.

Effect of substrate surface on the structure and electronic properties of cubic boron nitride films

Cubic boron nitride (c-BN) films were prepared by mass-selected ion beam deposition (MSIBD) technique. The effects of substrate surface roughness were investigated by boron and nitrogen k-edge x-ray absorption near-edge structure, x-ray diffraction, and atomic force microscopy. All the films are a mixture of nanocrystalline sp3-bonded c-BN and sp2-bonded BN phases. The substrate with a rough surface causes a decrease of the c-BN phase content of the film on it. A significant large lattice contraction of the c-BN crystallites in the films, relative to the bulk, is observed. It is also found that the electronic structure of the nanocrystalline c-BN films by MSIBD technique is somewhat different from that of microcrystalline c-BN∕h-BN references. We attribute the effect of the nature of the substrate on the morphology and structure of the c-BN films to the orientation of sp2-bonded graphitic BN basal plane on the top surface of the films during their growth, and the lattice contraction and energy band structure modification of c-BN films to the large compressive stress, respectively.

Preferable orientation of turbostratic BN basal planes from an x-ray absorption study

Two cubic boron nitride (c-BN) thin films (thickness, 80 nm), which were grown on silicon by mass-selected ion beam deposition and thin diamond film-coated silicon by magnetron sputtering technique respectively, were investigated by x-ray absorption near-edge spectroscopy (XANES) at the B K-edge. The angular dependences of the XANES recorded in fluorescence yield (FY) were used to show that the preferable orientation of the sp2-bonded turbostratic BN (t-BN) basal planes at the interfacial layers between the top c-BN film and Si substrate is normal or nearly normal to the substrate, which is consistent with previous transmission electron microscope analysis. The angular dependence was also used to show that the film deposited on diamond-coated Si has a higher relative amount of ordered t-BN at its film-substrate interface than the film on Si substrate. This work that shows a technique to determine the thin film structure, especially the interfacial structure between the thin films and their substrates x-ray absorption fine structure is a powerful mode.

Effects of crystalline quality on the phase stability of cubic boron nitride thin films under medium-energy ion irradiation

To investigate the effect of radiation damage on the stability and the compressive stress of cubic boron nitride (c-BN) thin films, c-BN films with various crystalline qualities prepared by dual beam ion assisted deposition were irradiated at room temperature with 300 keV Ar+ ions over a large fluence range up to 2×1016 cm−2. Fourier transform infrared spectroscopy (FTIR) data were taken before and after each irradiation step. The results show that the c-BN films with high crystallinity are significantly more resistant against medium-energy bombardment than those of lower crystalline quality. However, even for pure c-BN films without any sp2-bonded BN, there is a mechanism present, which causes the transformation from pure c-BN to h-BN or to an amorphous BN phase. Additional high resolution transmission electron microscopy (HRTEM) results support the conclusion from the FTIR data. For c-BN films with thickness smaller than the projected range of the bombarding Ar ions, complete stress relaxation was found for ion fluences approaching 4×1015 cm−2. This relaxation is accompanied, however, by a significant increase of the width of c-BN FTIR TO-line. This observation points to a build-up of disorder and/or a decreasing average grain size due to the bombardment.

Electron Field Emission from Self-Organized Micro-Emitters of sp3-Bonded 5H Boron Nitride with Very High Current Density at Low Electric Field

Electron field emission with high current density ∼0.9 A/cm2 at a low electric field of 8.6 V/μm was achieved by self-organized cone-shaped emitters with dimensions on the order of ∼10 μm made of sp3-bonded 5H boron nitride, which was grown by plasma-assisted chemical vapor deposition with the assistance of 193 nm laser irradiation of the surface. The work function of this material proved to be ∼5 eV, whereas the geometrical field enhancement factor amounts to ∼106 cm-1. The known robustness of sp3-bonded BN with its excellent electron emission characteristics and the self-organization of emitter shaped structures may provide new applications for electron emitting devices.

High-resolution transmission electron microscopy of as-deposited boron nitride on the edge of ultrathin Si flake

Boron nitride was deposited on the edge of ultrathin Si flake substrates by low-pressure inductively coupled plasma-enhanced chemical vapor deposition. Since the as-deposited boron nitride was sufficiently thin for cross-sectional high-resolution transmission electron microscopy, the postdeposition thinning process was omitted. The morphology of the boron nitride was revealed to vary from an amorphous structure to a three-layer structure, that is, amorphous/turbostratic/cubic boron nitride, upon increasing the thickness of the edge of Si flake substrates from a few to 50 nm. The critical thickness of the edge of Si flakes required for the nucleation of cubic boron nitride was found and estimated to be approximately 40 nm. The surface of cubic boron nitride nuclei was composed of {111} nanofacets, and no evidence of a thin sp2-bonded BN structure was observed at the top surface. This clearly reveals that cubic boron nitride is nucleated and grows on the top surface, not in the subsurface of the turbostratic boron nitride layer, during chemical vapor deposition.

Homogeneous and amorphous sputtered sp 3-bonded BN films at RT: a stress, spectroscopic ellipsometry and XPS study

A study of amorphous and homogeneous BN (a-BN) films produced by r.f. magnetron sputtering on c-Si(1 0 0) substrates at room temperature is presented. a-BN films with mixed sp 3- and sp 2-bonded BN, were studied in terms of internal stresses, bonding configuration and composition. The optical properties and the composition of BN films were examined by spectroscopic ellipsometry based on specific criteria such as optical gap; while the presence of sp 3-bonded BN in the films was also identified by high resolution X-ray photoelectron spectroscopy. It was found that the growth mechanism of sputtered BN films is sensitive to bias voltage ( V b). There is an ion energy range (40 V<| V b|<80 V), where sp 3-bonded BN is predominant, providing films with internal compressive stresses approximately 8 GPa. A threshold stress value of 3.5 GPa, the same as for the formation of the c-BN phase, was measured for the formation of the sp 3 bonding in the amorphous network of a-BN films.

Preparation and properties of BN/AlN nanolaminates

Nanolaminates and bilayers of BN/AlN on Si were deposited using a reactive DC magnetron sputtering technique in a dual-cathode system. In the BN/AlN bilayers, there was little effect of the AlN thickness as a buffer layer on the crystallinity of the BN layer. A series of BN/AlN nanolaminates with periods (Λ) from 2.2 to 80 nm was produced by varying the rotation speed of the sample holder disc. At the periods thicker than 20 nm, the nanolaminates primarily consisted of sp 3-bonded BN and w-AlN layers. For periods below 3.4 nm, the BN and AlN layers were intermixed and apparent wurtzite–AlBN phase was formed in the interfaces. Transparent and intact BN/AlN nanolaminates with thickness over 5 μm and with hardness over 33 GPa were obtained. The hardness of the nanolaminates was related to the structural characteristics obtained from XRD and FTIR results.

Mechanical performance and growth characteristics of boron nitride films with respect to their optical, compositional properties and density

We present a study of the growth processes of sputtered BN films, deposited on c-Si (001) substrates, at room temperature (RT) by rf magnetron sputtering. Nanoindentation (hardness ∼21 GPa), density (∼2.6 g/cm 3) and roughness by X-ray reflectivity and stress measurements indicated the existence of sp 3-bonded BN with properties similar to those of crystalline BN being superior for optical applications due to their homogeneity offering a single refractive index and smooth surfaces. The growth mechanism of sputtered BN films is sensitive to the Ar partial pressure and the bias voltage applied to the substrate during deposition inducing bombardment of the film with Ar + ions. There is a narrow ion energy window where sp 3-bonded BN is predominant providing dense, hard- and wear-resistant films. The fact that they are produced at RT and they are homogeneous with no gradient refractive index and smooth surfaces are correct for processes, such as optical applications on polymers or soft substrates.

The Effects of Hydrogen Etching on Different Carbon and Boron Nitride Phases

Communication: Understanding difficulties associated with depositing cubic boron nitride by CVD is essential.The activities of hydrogen atoms/ions as etching species are calculated here by applying an ab initio Hartree–Fock approach to clusters (see Figure) used to simulate BN phases. In marked contrast to the CVD of diamond, the difference in reactivity of hydrogen species with sp2- and sp3-bonded BN is small.

Crystallography and structural evolution of cubic boron nitride films during bias sputter deposition

The mechanism and the crystallography of the growth of cubic boron nitride (c-BN) films deposited on 〈100〉-oriented silicon substrates by radio-frequency bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy. Particular attention has been paid to the atomic structure of graphitic (sp 2-bonded) BN and grain boundaries in the c-BN films. The c-BN films grow in the sequence of amorphous boron nitride (a-BN), turbostratic boron nitride (t-BN) and c-BN layers, similar to previous results. The sp 2-bonded BN material consists of small regions, 1–5 nm thick, forming in a layered manner parallel to the substrate surface. Each region consists of parallel lamellae in both the hexagonal and rhombohedral (h-BN and r-BN, respectively) configurations. Three orientation relationships between h-BN and r-BN phases were determined: OR-1: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (0001) h-BN‖(0001) r-BN; OR-2: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (01 1 ̄ 0) h-BN ‖(01 1 ̄ 1 ̄ ) r-BN ; OR-3: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (01 1 ̄ 0) h-BN ‖(01 1 ̄ 2) r-BN . The r-BN crystallites within the sp 2-bonded BN grow preferentially in such a way that the (01 1 ̄ 2) r-BN ‖(01 1 ̄ 1 ̄ ) r−BN plane is almost parallel to the grain boundary. Twinning about both the basal planes and the {01 1 ̄ 1} r-BN planes is common within r-BN phase. The c-BN crystallites adjacent to the graphitic BN layer are highly twinned, the {111} c-BN twin planes being parallel to the basal planes of the sp 2-bonded BN. The c-BN phase nucleates on the oriented graphitic BN layer in a semicoherent manner and obeys specific orientation relationships with the hexagonal and rhombohedral phases, namely: OR-I: [ 2 ̄ 110] r-BN ‖[110] c-BN OR-II: [ 2 ̄ 110] r-BN ‖[ 1 ̄ 1 ̄ 0] c-BN (01 1 ̄ 1 ̄ ) r-BN ‖( 1 ̄ 11) c-BN ( interface plane) (01 1 ̄ 2) r-BN ‖(1 1 ̄ 1 ̄ ) c-BN ( interface plane) (0001) r-BN ≈‖(1 1 ̄ 1) c-BN (0001) r-BN ≈‖(1 1 ̄ 1) c-BN They are related by a rotation of 180° about their common axis, [0001] r-BN ‖[1 1 ̄ 1] c-BN , [2 1 ̄ 1 ̄ 0] h-BN ‖[110] c-BN (01 1 ̄ 2) h-BN ‖(001) c-BN (0001) h-BN ‖(1 1 ̄ 1) c-BN In the mainly c-BN region of the films, twins are present about more than one of the sets of {111} c-BN planes. The intrinsic microstructure of the c-BN films and the crystallography between the cubic and graphitic BN phases show that the structure of cubic boron nitride is directly related to the structure of the precursor phases. The atomic structure of an interface depends on the orientation relationship between adjacent c-BN grains and the boundary inclination. The grain boundaries consist of twin boundaries when two adjacent grains are oriented close to the [110] c-BN zone axis and the boundary plane is parallel to the {111} c-BN close-packed planes of both grains. However, a thin layer, 1–2 nm, of sp 2-bonded BN forms between the c-BN grains when the boundary plane inclines a few degrees from the {111} c-BN planes of the adjacent grains.

Mechanism of nucleation and growth of cubic boron nitride thin films

The mechanism and the crystallography of the nucleation and growth of cubic boron nitride (c-BN) films deposited on 〈100〉-oriented silicon substrate by RF bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Both methods provide experimental information showing no sp2-bonded BN layer formation in the subsurface region of c-BN phase. This is clear evidence for layer-by-layer homoepitaxial growth of cubic boron nitride without graphitic monolayers in the near-surface region of the film. The turbostratic boron nitride (t-BN) consists of thin sub-layers, 0.5–2 nm thick, growing in such a way that a sub-layer normal is almost parallel to the growth direction. t-BN also comprises a large volume fraction of the grain boundaries with high interface energies. The present result and the finding by Shtansky et al. [Acta Mater. 48, 3745 (2000)], who showed that an individual sub-layer consists of parallel lamellae in both the hexagonal +h-BN) and rhombohedral (r-BN) configurations, demonstrate that high intrinsic stress in the films is due to the complex structure of sp2-bonded BN. The crystallography of c-BN films indicates heteroepitaxial nucleation of cubic phase on the graphitic BN structural precursor. The present results are consistent with stress-induced c-BN formation.

New Phase of sp3-Bonded BN: The 5H Polytype

A new phase of sp3-bonded BN, that is, 5H polytype, has been found. The representative lattice parameters a and c were determined to be 2.528 and 10.407 A, respectively. The new BN phase was prepar...

Microstructure and nanomechanical properties of cubic boron nitride films prepared by bias sputter deposition

Nanoindentations, using a triangular based cube-corner shaped diamond tip, were tried to study the mechanical properties of cubic boron nitride ( cBN) films. Since the cBN film has a layered structure consisting of an initial sp 2-bonded BN (amorphous BN and turbostratic BN) layer and a sp 3-bonded BN ( cBN) layer, which was previously studied by cross-sectional transmission electron microscopy (TEM), cBN films in different growth stages were prepared for the measurements. Hardness and elastic modulus were evaluated as a function of the penetration depth from the obtained loading-unloading curves using a method described by Oliver et al. The maximum hardness and elastic modulus of a 110-nm thick cBN film (including an approximately 50-nm thick initial layer) were approximately 10 and 3 times, respectively, larger than the values evaluated for a silicon wafer substrate. The dependence of the evaluated hardness and the elastic modulus on film thickness and penetration depth can be explained in terms of the layered structure of the cBN film, i.e. soft initial sp 2-bonded BN layer with low elastic modulus, and hard cBN layer with high elastic modulus.

Review of advances in cubic boron nitride film synthesis

Abstract Cubic boron nitride (cBN) has a number of highly desirable mechanical, thermal, electrical, and optical properties. Because of this, there has been an extensive worldwide effort to synthesize thin films of cBN. Film synthesis is difficult in that without significant levels of ion bombardment during growth, only sp2-bonded BN forms, not sp3-bonded cBN. Recently there has been considerable progress in improving the deposition techniques and cBN film quality. In addition, progress has been made in understanding how energetic deposition conditions can lead to cBN formation. However, unanswered questions remain and process improvements are still needed. In this paper we critically and comprehensively review recent developments in cBN film synthesis and characterization. First, the structures and stability of the BN phases and characterization techniques are described. Next, the key experimental parameters controlling cBN film formation and synthesis techniques are discussed. Following a review of microstructure, the proposed mechanisms of cBN formation and the observed mechanical and electrical properties of cBN films are analyzed. We conclude by highlighting the current impediments to the practical realization of cBN-film technology.

Low-Temperature Heat Capacity of Wurtzite-Type Boron Nitride*1

The heat capacity for wurtzite-type BN has been determined between 50 and 300 K. The difference from that of zinc blende-type BN, which was also measured between 15 and 300 K, indicates that the interatomic force constants of the wurtzite-type lattice are generally 4% weaker than those of the zinc blende-type one. This result is compatible with the fact that sp3-bonded BN is elongated along a unique axis to form the hexagonal lattice.

On the low-temperature threshold for cubic boron nitride formation in energetic film deposition

The sharp threshold in substrate temperature below which cubic boron nitride (cBN) cannot be formed in energetic film-deposition processes was investigated. We found that cBN could be synthesized below the threshold temperature on top of cBN that had been previously formed above the threshold temperature. That the initial nucleation of cBN is more strongly dependent on temperature than its subsequent growth is suggested. How the structure of the sp 2-bonded BN that accompanied cBN growth changed with temperature was also investigated. Lowering the substrate temperature decreased the local ordering within the graphitic planes, and below the threshold temperature the separation of the graphitic planes increased dramatically. How these structural changes may influence the nucleation of cBN is discussed.

Analysis of residual stress in cubic boron nitride thin films using micromachined cantilever beams

We have investigated the residual stress of cubic boron nitride (cBN) thin films using a cantilever-beam deflection method. The residual stresses were determined by measuring the deflection of cantilever beams micromachined from (100)-oriented silicon, and the hardness and elastic modulus of sp 2-bonded BN (sp 2-BN) were determined from nanoindentation experiments. Residual stresses were compressive, and varied from approximately 0.15 GPa to 1.2 GPa for sp 2-BN films and from 0.5 to 5.5 GPa for cBN-containing films; the lower stresses are below a threshold stress level for cBN formation previously suggested by McKenzie et al. A correlation between the relative cBN content and the compressive film stress was also established: larger residual stresses occurred in the films containing a greater percentage of the cubic phase. We find that both sp 2-BN and predominantly cBN films have approximately the same elastic strain. Thus, assuming that the strain is independent of composition, we develop a quantitative model that well describes the observed correlation of compressive film stress with cBN content. Within the model, the film stress increases with increasing cBN content simply because the elastic modulus of cBN is substantially larger than that of sp 2-BN. The relevance of this analysis to the mechanism of cBN formation is discussed.