Boron Nitride Thin Films Research Articles

Encapsulation of metallic nanoclusters in carbon and boron nitride thin films prepared by ion-beam sputtering

Dual ion-beam sputtering has been used to achieve the growth of silver and iron nanoclusters embedded within graphite-like carbon and boron nitride thin films. Encapsulation of the nanoclusters in carbon nanocages has been realized when the deposition was performed at 500 °C whereas a temperature of 200 °C was sufficient to form hexagonal BN layers surrounding the nanoclusters. We show that, thanks to the enhanced surface diffusion of the deposited species, self-organized arrays of nanoclusters elongated along the growth direction can be produced by argon-assisted co-deposition of the metal and the matrix material. Vertical ordering of the nanoclusters, which develops from a topology-induced self-organization, can also be obtained by alternate deposition of very thin metallic layers separated by BN layers, as shown by high-resolution transmission electron microscopy and grazing incidence small-angle X-ray scattering.

Deposición y caracterización de recubrimientos de Nitruro de Boro producidos por pulverización catódica

El nitruro de boro (BN) es un material de gran importancia tecnológica, con propiedades térmicas, eléctricas y mecánicas que se aproximan a las del diamante. El nitruro de boro se puede formar principalmente en tres estructuras cristalinas: cúbico (c-BN). wurtzite (w-BN) y hexagonal (h-BN). El c-BN es la estructura más interesante, debido a su conjunto único de propiedades. Tres parámetros son comunes a todos los experimentos PVD para la producción exitosa de c-BN: Bombardeo de la película en crecimiento con iones energéticos (generalmente 100-500eV), elevada temperatura del substrato (300-400°C) y la correcta estequiometria en la película. En este trabajo se revisó la literatura de deposición física de vapor y caracterización de películas delgadas de c-BN, poniendo especial énfasis en el voltaje de polarización del substrato (d.c. y r.f.) como efecto dominante en el contenido de c-BN dentro de la película. Se prepararon películas delgadas de BN usando voltaje de polarización del substrato d.c. y r.f. Se identificó el voltaje crítico para la formación de la fase c-BN en ambos casos. Las películas se caracterizaron por medio de espectroscopia de Infrarrojo con Transformada de Fourier (FTIR), usada para la identificación de fase, Microscopia de Fuerza Atómica (AFM) y Microscopia Electrónica de Barrido (SEM) para la caracterización morfológica. La caracterización de las películas obtenidas este de acuerdo con el modelo estructural de capas propuesto en la literatura.

Review of synthesis and properties of cubic boron nitride (c-BN) thin films

Cubic boron nitride (c-BN) thin films are of significant interest because of their diamond like structure and properties. c-BN shows high thermal conductivity, chemical inertness against ferrous metals even at high temperatures, wide band gap, and good transmittance over a wide spectral range from UV to visible. Applications of c-BN include hard protective coatings for cutting tools, in optical instruments as UV detectors, as emitters, and in high temperature electronic devices. However, synthesis of phase pure c-BN thin films continues to be very challenging. The present study reviews the current status of the synthesis, characterisation, mechanical, electrical, and optical properties of c-BN thin films. Both physical and chemical vapour deposition methods used for the preparation of the c-BN films are covered. In addition, different nucleation and growth models of c-BN formation and growth on different substrates are described. The influence of process parameters such as ion energy, growth temperatures, chemical precursors, bias, and impurities on the nucleation and growth is reviewed. Mechanical properties including hardness, elastic modulus, and stiffness of c-BN films are discussed. The latest developments in electrical properties of the c-BN films based on metal–insulator–semiconductor (MIS) hetero-structures, interface states, impurity states, conduction mechanism, field emission properties, and negative electron affinity (NEA) are presented. The optical properties and cathodoluminescence characteristics of BN films are also discussed.

Surface properties of cubic boron nitride thin films

Studying the surface properties of cubic boron nitride (c-BN) thin films is very important to making it clear that its formation mechanism and application. In this paper, c-BN thin films were deposited on Si substrates by radio frequency sputter. The influence of working gas pressure on the formation of cBN thin film was studied. The surface of c-BN films was analyzed by X-ray photoelectron spectroscopy (XPS), and the results showed that the surface of c-BN thin films contained C and O elements besides B and N. Value of N/B of c-BN thin films that contained cubic phase of boron nitride was very close to 1. The calculation based on XPS showed that the thickness of hexagonal boron nitride (h-BN) on the surface of c-BN films is approximately 0.8 nm.

Spectroscopic infrared scanning near-field optical microscopy (IR-SNOM)

Scanning near-field optical microscopy (SNOM or NSOM) is the technique with the highest lateral optical resolution available today, while infrared (IR) spectroscopy has a high chemical specificity. Combining SNOM with a tunable IR source produces a unique tool, IR-SNOM, capable of imaging distributions of chemical species with a 100 nm spatial resolution. We present in this paper boron nitride (BN) thin film images, where IR-SNOM shows the distribution of hexagonal and cubic phases within the sample. Exciting potential applications in biophysics and medical sciences are illustrated with SNOM images of the distribution of different chemical species within cells. We present in this article images with resolutions of the order of λ/60 with SNOM working with infrared light. With our SNOM setup, we routinely get optical resolutions between 50 and 150 nm, regardless of the wavelength of the light used to illuminate the sample.

Cubic boron nitride thin film growth by boron and nitrogen ion implantation

Cubic boron nitride $(c\text{\ensuremath{-}}\mathrm{B}\mathrm{N})$ thin films were deposited on silicon substrates using mass separated ion beam deposition (MSIBD). In order to investigate the influence of the ion energy on the growth of $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$ films, $^{11}\mathrm{B}^{+}$ and $^{14}\mathrm{N}^{+}$ ions were implanted into $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$ with ion energies ranging from $5\phantom{\rule{0.3em}{0ex}}\mathrm{keV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}43\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ and substrate temperatures $({T}_{S})$ from room temperature (RT) to $250\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. A systematic study on the interplay of ${E}_{\mathit{\text{ion}}}$ and ${T}_{S}$ has revealed a characteristic energy-dependent temperature threshold for $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$ growth. This behavior is explained by dynamic annealing of defects caused by a penetrating ion in a collison cascade. In this picture, the suppression of defect accumulation that is crucial for maintaining cubic phase formation is attributed to temperature-driven back diffusion and subsequent annihilation of B and N interstitial recoils. The model is confirmed by analyzing the depth profile of implanted, isotopically pure $^{10}\mathrm{B}$, and its application for both $c\text{\ensuremath{-}}\mathrm{B}\mathrm{N}$ nucleation and growth is discussed.

Defect-induced electronic conduction of tBN thin films

Two kinds of turbostratic boron nitride (tBN) thin films were deposited by sputtering with pure Ar or N 2 plasma in an ultraclean vacuum chamber. Current–voltage (I–V) characteristics of Ti/tBN/Ti structures revealed that films deposited in Ar showed at least three-orders higher conductivity with strong temperature and electric field dependence compared with those deposited in pure N 2. The origin of the higher conductivity was confirmed to be the nonstoichiometry and higher defect density of the tBN films. On the other hand, films deposited in pure N 2 plasma showed the characteristics of intrinsic insulator even though the basal plane of tBN was oriented normal to the substrate, namely, parallel to the direction of electronic conduction.

PECVD of h-BN and c-BN films from boranedimethylamine as a single source precursor

Boron nitride (BN) thin films have been successfully synthesised via low pressure plasma enhanced chemical vapour deposition (PECVD) by using boranedimethylamine, BH 3NH(CH 3) 2, as a single source precursor in the temperature range 280–550 °C in a nitrogen–argon atmosphere. The plasma power was optimised with the aim of obtaining suitable cubic/hexagonal phase ratios. The annealing of the h-BN films at temperatures up to 1000 °C in a nitrogen atmosphere, at normal pressure, gave rise to a complete transformation into the cubic phase. FTIR measurements provided a suitable method for identifying the structure of BN films. UV–vis spectroscopy was carried out in order to investigate the optical behaviour of the films.

Cubic and hexagonal boron‐nitride (c‐BN/h‐BN) thin films deposited in situ by r.f. magnetron sputtering

AbstractCubic boron‐nitride (c‐BN)/hexagonal boron nitride (h‐BN) thin films were grown in situ on (100) oriented silicon substrates by r.f. (13.56 MHz) magnetron sputtering technique. In order to obtain the highest fraction of the c‐BN phase, a negative d.c bias voltage, varying from 0 to –200 V was applied to the substrate during deposition. Another set of boron nitride thin films was deposited in situ on (100) oriented silicon substrates under r.f. bias voltage. The substrate holder was biased from 0 to –350 V by connecting such to an auxiliary r.f. generator (operated at 13.56 MHz). Films were characterized by Fourier Transformed Infrared Spectroscopy (FTIR) and Atomic Force Microscope (AFM). Well‐defined peaks at 787 cm–1, 1100 cm–1 and 1387 cm–1, corresponding to the 2Au (out‐plane bending of B–N–B bond) h‐BN vibration mode, the F2 (stretching) c‐BN Transversal Optical (TO) mode and the E1u (in‐plane stretching of B–N bond) vibration mode of the h‐BN, respectively, were observed in the FTIR spectra. A maximal fraction of the c‐BN phase close to 85% was obtained under a bias voltage of –150 V at substrate temperature of 300 °C and a total pressure of 4 × 10–2 mbar. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of mechanical stress on the absorption band tail of cubic boron nitride thin films synthesized by inductively coupled radio-frequency plasma chemical vapour deposition

Thin films of cubic boron nitride were deposited on silica and Si substrates by inductively coupled radio-frequency plasma chemical vapour deposition (IPCVD) technique using a B2H6 + N2 + Ar gas mixture. Cubic phase formation was confirmed by glancing-angle X-ray diffraction studies, which showed reflections up to (311). Fourier-transform infrared (FTIR) spectra also indicated the predominantly cubic nature of the deposited films. The optical properties of the films were studied in the wavelength range 200–1000 nm. Both direct and indirect transitions were found to be present. Mechanical stress in the grain-boundary region of the films seems to contribute significantly to the optical absorption below the band gap. The intercrystalline barrier height (E b) and the trap state density (Q t ) were obtained from an analysis of the effects of grain boundaries on the optical properties of the samples.

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.

Resonant Raman scattering in cubic and hexagonal boron nitride

We measured first- and second-order Raman scattering in cubic and hexagonal boron nitride using excitation energies in the visible and in the UV. The nonresonant first-order Raman susceptibilities for cubic and hexagonal BN are 1 and $10\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{2}$, respectively. Raman scattering is thus very powerful in detecting the hexagonal phase in mixed thin boron nitride films. In cubic BN the constant Raman sucseptibility in the visible and the UV is due to its indirect band gap. For hexagonal BN a Raman enhancement is found at $5.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. It is well explained by the energy dependence of the dielectric function of hexagonal BN. The second-order spectrum of cubic boron nitride is in excellent agreement with first-principles calculations of the phonon density of states. In hexagonal BN the overbending of the LO phonon is $\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, five times larger than in graphite.

Ion implantation induced phase transformation in carbon and boron nitride thin films

The mechanism behind energetic ion impact induced stress reduction in highly stressed tetrahedral amorphous carbon and cubic boron nitride thin films is investigated by real time in situ spectroscopic ellipsometry and ex situ electron microscopy. Highly stressed carbon and boron nitride films were grown by filtered cathodic vacuum arc and RF magnetron sputtering, respectively. The films were then implanted by 5–10 keV argon ions and the film optical properties and thickness monitored in situ by spectroscopic ellipsometry. In both cases the films were observed to expand due to a reduction in the density of the ion-modified layer. Cross-sectional transmission electron microscopy and electron energy loss spectroscopy of the carbon films showed that this reduction in density is associated with a conversion of diamond-like bonding to graphite-like bonding. In situ stress measurements performed on the boron nitride films revealed a simultaneous reduction in stress with expansion of the material.

A New Method to Prepare Boron Nitride Thin Films

We report a new method to prepare boron nitride (BN)thin films on Si (100) substrates in anAr–N2-BCl3-H2 gas system by magnetron arc enhancedplasma chemical vapour deposition. Fourier transform infraredspectroscopy (FTIR) and x-ray diffraction (XRD) are used tocharacterize the films. The FTIR spectra show that the deposited boronnitride films experienced a transition from pure h-BN phase to acubic-containing phase with the variation of arc current ranging from10 A to 18 A. The BN film with 42% c-BN was obtained without substratebias voltage. In the gas system of Ar–N2–BCl3–H2,h-BN can be preferentially etched by chlorine. The chemical etchingeffect of chlorine allows the formation of c-BN without substrate biasvoltage, which may develop a new perspective for the deposition of highquality c-BN film with low stress.

Mechanical properties of boron nitride films prepared by plasma-enhanced chemical vapor deposition

Two kinds of boron nitride thin films, cubic boron nitride film with a layered structure, and turbostratic boron nitride film with a c-axis parallel to the substrate surface, were prepared by inductively coupled plasma-enhanced chemical vapor deposition. Nanoindentation tests revealed that the hardness and elastic modulus of cubic boron nitride films increased with cubic phase content and approached approximately 50 and 250 GPa, respectively, for the case of 96% cubic phase, while those of turbostratic boron nitride films were load and loading rate dependent and as low as 1.5 and 30 GPa, respectively. Moreover, the hysteretic recovery of turbostratic boron nitride after indentation was confirmed, and approximately 50% of the energy dissipated during indentation was found to be released within 1 min.

RF plasma reactive pulsed laser deposition of boron nitride thin films

Thin films of boron nitride (BN) have been deposited on Si(1 0 0) substrates by reactive pulsed laser ablation (PLA) of a boron target in the presence of a 13.56 MHz radio frequency (RF) nitrogen plasma. The gaseous species have been deposited at several substrate temperatures, using the on-axis configuration. The film properties have been investigated by Scanning Electron Microscopy, Atomic Force Microscopy, Fourier Transformed Infrared Spectroscopy, and X-ray diffraction characterization techniques, and compared to those resulting from the conventional PLA method. The behavior of hexagonal-BN and cubic-BN phases grown by PLA as function of substrate temperature is also reported.

Improvement of Adhesion of Cubic Boron Nitride Films: Effect of Interlayer and Deposition Parameters

Diamond and B4C coatings were used as an interlayer for the growth of cubic boron nitride thin films on c-silicon. By employing a B-C-N gradient layer on top of the B4C interlayer, improved adhesion occured between BN and B4C. A multi-step process after the nucleation of c-BN was found very helpful for improving the adhesion of c-BN on silicon with interlayers. Residual stress of c-BN thin film was significantly decreased by using a new post-deposition annealing treatment.

Peculiar deformation characteristics of turbostratic boron nitride thin film

The deformation behavior of a textured turbostratic boron nitride thin film of 120 nm in thickness, with its c-axis parallel to the film surface, was investigated under nanoindentation. The load displacement response of the textured turbostratic boron nitride film was revealed to be loading rate dependent, and a creep-like phenomenon was observed under constant load. Moreover, the textured turbostratic boron nitride film was found to be able to recover with hysteresis, even when pressed to the maximum depth of approximately 20% of the film thickness. The apparent hardness and Young's modulus were estimated to be 1.5 and 30 GPa, respectively. The reversible hysteretic deformation, similar to that of a viscoelastic body, is clearly controlled by the bending of the sp 2-bonded turbostratic boron nitride basal planes.

Atomic Force Microscopy study of hexagonal boron nitride film growth on 6H‐SiC (0001)

AbstractBoron nitride thin films with their unique mechanical and electrical properties are of interest for various applications and have been grown on a number of different substrates. The cover picture from this issue's Editor's Choice [1] shows an Atomic Force Microscopy topography of a hexagonal BN film grown on 6H‐SiC substrate by Plasma Enhanced Chemical Vapor Deposition at 600 °C. Interesting stress‐relief features developed on the non‐reactive substrate. The corresponding author Kian Ping Loh is Assistant Professor at the National University of Singapore. His research is devoted to the growth of diamond and boron nitride thin films and the fabrication of nanocrystalline diamond and boron nitride nanotubes.As a follow‐up to the recent special issue on ‘Physics of Organic Semiconductors’ [2], Detlef Berner gives insights into organic light emitting diode functioning by coordinated experiment and simulation in his Review Article [3].The present issue also sees the start of our rapid research letters, the fastest peer‐reviewed publication medium in solid state physics. For more information see www.pss‐rapid.com and the Editorial by the Editor‐in‐Chief Martin Stutzmann on page 7 [4].

X-ray diffraction study of stress relaxation in cubic boron nitride films grown with simultaneous medium-energy ion bombardment

Relaxation of the intrinsic stress of cubic boron nitride (cBN) thin films has been studied by x-ray diffraction (XRD) using synchrotron light. The stress relaxation has been attained by simultaneous medium-energy ion bombardment (2–10 keV) during magnetron sputter deposition, and was confirmed macroscopically by substrate curvature measurements. In order to investigate the stress–release mechanisms, XRD measurements were performed in in-plane and out-of-plane geometry. The analysis shows a pronounced biaxial state of compressive stress in the cBN films grown without medium-energy ion bombardment. This stress is partially released during the medium-energy ion bombardment. It is suggested that the main path for stress relaxation is the elimination of strain within the cBN grains due to annealing of interstitials.