Adhesion properties of functionally gradient diamond composite films on medical and tool alloys

We have investigated the adhesion properties of microcrystalline diamond thin films on Ti-Al-V alloy, Co-Cr-Mo alloy and steel. Microcrystalline diamond possesses high hardness, a low coefficient of friction, extreme chemical inertness and biocompatibility; these properties can enhance the performance of metal alloys used in medical implants and in machine tools. We have adopted three methods for improving the adhesion of microcrystalline diamond to commonly used metal alloys: (1) by alloying the substrate surface to minimize graphitization; (2) by employing appropriate buffer layers between the diamond film and the substrate; and (3) by creating functionally gradient diamond-(titanium carbide, tungsten carbide, titanium nitride and aluminum nitride) composites. We have demonstrated that functionally gradient discontinuous buffer layers of titanium carbide, titanium nitride, aluminum nitride and tungsten carbide are able to control stress and graphitization in microcrystalline diamond thin films. This work on buffer layers and functionally gradient coatings should allow the development of more adherent crystalline diamond films for medical and tribological applications.     

Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate

Zinc oxide thin films have been obtained on bare and GaN buffer layer decorated Si (111) substrates by pulsed laser deposition (PLD), respectively. GaN buffer layer was achieved by a two-step method. The structure, surface morphology, composition, and optical properties of these thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, infrared absorption spectra, and photoluminiscence (PL) spectra, respectively. Scanning electron microscopy images indicate that the flower-like grains were presented on the surface of ZnO thin films grown on GaN/Si (111) substrate, while the ZnO thin films grown on Si (111) substrate show the morphology of inclination column. PL spectrum reveals that the ultraviolet emission efficiency of ZnO thin film on GaN buffer layer is high, and the defect emission of ZnO thin film derived from Zn_i and V_o is low. The results demonstrate that the existence of GaN buffer layer can greatly improve the ZnO thin film on the Si (111) substrate by PLD techniques.     

Raman and photoluminescence spectra of indented cubic boron nitride and polycrystalline cubic boron nitride

The use of Raman spectroscopy, and in particular Raman line shifts, to measure stress in diamond and nitrides such as gallium nitride (GaN), is well known. In both diamond and GaN the application is principally to study stresses in thin films and at the substrate–thin film interface. Stresses in polycrystalline diamond composites have also been measured by this method. Typically stresses of the order of GPa can be determined with a spatial resolution of a few micrometers. In this paper, Raman spectra of indentations on cubic boron nitride (cBN) crystals and polycrystalline cubic boron nitride (PcBN) composites are presented. Shifts of the cBN Raman lines from their unstressed positions quantify the residual stresses in the boron nitride due to the deformation brought about by the indentation. Making use of the measured coefficient of shift of 3.39 cm⁻¹/GPa for the transverse optical Raman peak, these are of the order of 1 GPa. These measurements illustrate, for the first time, the use of Raman spectroscopy to study residual stresses in boron nitride. Plastic deformation is usually associated with the creation of vacancies. To investigate the possible presence of vacancy defects and vacancy-related defects, the indented boron nitride samples were also studied with photoluminescence spectroscopy.     

Fabrication of aluminium nitride/diamond and gallium nitride/diamond SAW devices

Surface acoustic wave (SAW) devices have been fabricated from thin films of gallium nitride and aluminium nitride deposited on a range of chemical vapour deposition (CVD) diamond substrates. The growth of aluminium nitride and gallium nitride layers on diamond by chemical beam epitaxy (CBE) is reported for the first time. Triethyl gallium and ethyldimethylamine alane precursors were used in conjunction with nitrogen from an RF atom source to deposit the gallium nitride and aluminium nitride layers at substrate temperatures in the range 540 to 575 °C. These layers have been characterised by Raman spectroscopy and atomic force microscopy. The SAW structures were completed by the deposition of gold or aluminium interdigitated electrode structures on the as-deposited nitride surfaces. Preliminary testing indicates that these devices operate as bandpass filters with characteristics consistent with the propagation of acoustic waves at very high phase velocities within the nitride–diamond multi-layer substrate.     

A study of diamond film deposition on WC–Co inserts for graphite machining: Effectiveness of SiC interlayers prepared by HFCVD

Thin silicon carbide (SiC) films were deposited from tetramethylsilane/hydrogen gas mixture on Co-cemented tungsten carbide (WC–Co) inserts by using Hot-Filament Chemical Vapour Deposition (HFCVD) technique. Grazing incidence X-Ray Diffraction (XRD) confirmed that the films were composed of cubic silicon carbide (β-SiC) and that small amounts of dicobalt silicide (Co₂Si) were formed. These films were used as interlayers for subsequent CVD of diamond films. XRD and combined Scanning and Transmission Electron Microscopies showed that the binder phase reacted during CVD to form cobalt silicides. However, these intermetallic compounds did not have bad effects on diamond adhesion. Dry turning of graphite was chosen to check the multilayer (SiC + diamond) film performance. For the sake of comparison, machining tests were also carried out under identical conditions using commercial sintered diamond (PCD) inserts and WC–Co diamond coated inserts with no interlayer. The wear mechanism of the tools has been identified and correlated with the criterion used to evaluate the tool life. The results showed that multilayer (SiC + diamond) coatings exhibited the longest tool lives. Therefore, thin SiC interlayers proved to be a new viable alternative and a suitable option for adherent diamond coatings on cemented carbide components and cutting tools.     

Formation and characterization of 4-inch GaN-on-diamond substrates

This paper reports on the first demonstration of four-inch gallium nitride (GaN) on 100-micron CVD diamond substrates and the characterization of the interface between the GaN and the diamond. Currently, gallium nitride devices are used for microwave power amplification at frequencies of up to 100 GHz. The very high thermal conductivity of diamond enables the increase in power and improvement in lifetime and reliability of the amplifiers by efficiently removing the heat from the active region of devices fabricated on GaN-on-diamond substrates. While we have previously demonstrated and currently are producing 2-inch GaN-on-diamond wafers. Increasing the diameter of GaN-on-diamond substrate is both non-trivial and essential for entry into high-volume GaN electronics manufacturing. Since the primary significance of the GaN-on-diamond structure lies in its ability to efficiently remove the heat from the active regions, the state and quality of the bond between the GaN, the diamond, and any enabling adhesion layers are critical in the transmission of heat through the interface and the reliability of the completed devices. In this paper, in addition to the discussion of challenges associated with the scale-up, we characterize the interfacial bonding between the critical layers using a picosecond ultrasonic measurement technique. The measurements indicate excellent adhesion of the interlayer to both the GaN and to the diamond. The qualified substrates from this exercise were used in fabrication of devices that have demonstrated transition frequencies of up to 85 GHz. These findings should help to further the development of GaN-on-diamond technology on the path to commercialization for high-power, high-frequency amplifiers.     

Catalytic growth of semiconductor micro- and nano-crystals using transition metal catalysts

The catalytic reaction concept was introduced in the growth of semiconductor micro- and nano-crystals. It was found that gallium nitride (GaN) micro- and nano-crystal structures, carbon nanaotubes, and silicon carbide (SiC) nanostructures could be efficiently grown using transition metal catalysts. The use of Ni catalyst enhanced the growth rate and crystallinity of GaN micro-crystals. At 1,100 ‡C, the growth rate of GaN micro-crystals grown in the presence of Ni catalyst was over nine times higher than that in the absence of the catalyst. The crystal quality of the GaN microcrystals was almost comparable to that of bulk GaN. Good quality GaN nanowires was also grown over Ni catalyst loaded on Si wafer. The nanowires had 6H hexagonal structure and their diameter was in the range of 30–50 nm. Multiwall nanotubes (MWNTs) were grown over 20Fe : 20Ni : 60Al₂O₃ catalyst. However, single wall nanotubes (SWNTs) were grown over 15Co : 15Mo : 70MgO catalyst. This result showed that the structure of CNTs could be controlled by the selection of catalysts. The average diameters of MWNTs and SWNTs were 20 and 10 nm, respectively. SiC nanorod crystals were prepared by the reaction of catalytically grown CNTs with tetrametysilane. Structural and optical properties of the catalytically grown semiconductor micro- and nano-crystals were characterized using various analytic techniques. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Tribological properties and cutting performance of boron and silicon doped diamond films on Co-cemented tungsten carbide inserts

Boron and silicon doped diamond films are deposited on the cobalt cemented tungsten carbide (WC-Co) substrate by using a bias-enhanced hot filament chemical vapor deposition (HFCVD) apparatus. Acetone, hydrogen gas, trimethyl borate (C₃H₉BO₃) and tetraethoxysilane (C₈H₂₀O₄Si) are used as source materials. The tribological properties of boron-doped (B-doped), silicon-doped (Si-doped) diamond films are examined by using a ball-on-plate type rotating tribometer with silicon nitride ceramic as the counterpart in ambient air. To evaluate the cutting performance, comparative cutting tests are conducted using as-received WC-Co, undoped and doped diamond coated inserts, with high silicon aluminum alloy materials as the workpiece. Friction tests suggest that the Si-doped diamond films present the lowest friction coefficient and wear rate among all tested diamond films because of its diamond grain refinement effect. The B-doped diamond films exhibit a larger grain size and a rougher surface but a lower friction coefficient than that of undoped ones. The average friction coefficient of Si-doped, B-doped and undoped diamond films in stable regime is 0.143, 0.193 and 0.233, respectively. The cutting results demonstrate that boron doping can improve the wear resistance of diamond films and the adhesive strength of diamond films to the substrates. Si-doped diamond coated inserts show relatively poor cutting performance than undoped ones due to its thinner film thickness. B-doped and Si-doped diamond films may have tremendous potential for mechanical application.     

Low temperature growth of gallium nitride

Radio frequency reactive sputtering was used to produce gallium nitride films on glass and silicon substrates at close to room temperature. The films were analysed by transmission electron microscopy and Rutherford backscattering spectroscopy. The films were found to consist of nanocrystalline wurtzite GaN with c-axis-oriented columnar grains growing perpendicular to the substrate. Varying the N₂:Ar sputtering gas ratio was found to have little effect on the grain size. Annealing the films at 400°C was found to increase the E₁(TO) signal observed by Fourier transform IR spectroscopy and to reduce the porosity of the columnar structure.     

Deposition of cubic boron nitride films on diamond-coated WC:Co inserts

Cubic boron nitride (cBN) thin films were deposited on diamond-coated tungsten carbide (WC) cutting inserts using electron cyclotron resonance (ECR) microwave plasma chemical vapor deposition (MPCVD). The effects of gas flow rate and substrate bias on the phase composition and structure of the BN films deposited on diamond surfaces were studied. It was revealed that both the cubic phase formation and the selective etching of hexagonal phase were controlled by modulating the hydrogen and boron trifluoride flow rate ratio. By the trial and error method the gas flow rate ratio and substrate bias voltage were optimized. Moreover the phase composition of the BN film was found to be affected by the thickness of diamond buffer layer and interrelated to the effective substrate bias. The hardness of the resulting cBN films reached the value of 70 GPa. In the synthesized coatings, the diamond beneath renders the best mechanical supporting capacity while the top cBN provides the superior chemical resistance and extreme hardness. The cBN/diamond bilayers deposited on WC inserts may serve as universal tool coatings for machining steels and other ferrous metals.     

AlN films grown on diamond and silicon carbide

Structural changes in surface layers of CVD AlN films were studied by the reflection high-energy electron diffraction technique. Film thickness was varied from 0.2 to 20 μm. Single crystals of diamond, silicon carbide and also bilayered structures of natural diamond/CVD diamond films were used as substrates. On the (111) surfaces of natural diamond, (111) CVD diamond films and (00.1) 6H-SiC at AlN thickness up to 1 μm one can observe the epitaxial correspondence between the growing film and the substrate: (00.1) [11.0] AlN//(111) [110] C_α or (00.1) [10.0] AlN//(00.1) [10.0] 6H-SiC. At larger film thickness the epitaxial growth was replaced gradually by the formation of one of axial textures for which the planes (00.1), (10.3) or (11.4) of wurtzite-like aluminum nitride were parallel to the substrate. During the epitaxial growth the twinning structures with the twinning planes of types (10.1) and (11.1) were observed.     

Silicon carbide films from polycarbosilane and their usage as buffer layers for diamond deposition

Thin films of polycarbosilane (PCS) were coated on a Si (100) wafer and converted to silicon carbide (SiC) by pyrolyzing them between 800 and 1150 °C. Granular SiC films were derived between 900 and 1100 °C whereas smooth SiC films were developed at 800 and 1150 °C. Enhancement of diamond nucleation was exhibited on the Si (100) wafer with the smooth SiC layer generated at 1150 °C, and a nucleation density of 2 × 10¹¹ cm^− 2 was obtained. Nucleation density reduced to 3 × 10¹⁰ cm^− 2 when a bias voltage of − 100 V was applied on the SiC-coated Si substrate. A uniform diamond film with random orientations was deposited to the PCS-derived SiC layer. Selective growth of diamond film on top of the SiC buffer layer was demonstrated.     

Plasma-enhanced atomic layer deposition of gallium nitride thin films on fluorine-doped tin oxide glass substrate for future photovoltaic application

To serve as an electron transport layer (ETL) or a buffer layer for the third-generation solar cells, a compact and uniform gallium nitride (GaN) thin layer with suitable energy level is needed. Meanwhile, it is also meaningful to explore its low-temperature deposition especially on transparent electrodes. In this work, GaN thin films have been deposited on fluorine-doped tin oxide (FTO) glass substrate for the first time by plasma-enhanced atomic layer deposition (PEALD) technology. 280–300 °C is identified as the optimized deposition temperature for forming a compact and uniform n-type GaN layer on FTO substrate. The 50–200 PEALD cycles of GaN layers show an amorphous structure, and their bandgap values ranging from 3.95 eV to 3.58 eV have been displayed. Interestingly, as the GaN thickness increases, Fermi level moves upward obviously along with a reduction of conduction band minimum (CBM) value as well as an increase of valence band maximum (VBM) value. The thickness-dependent band structure is preliminarily explained as the relaxation of compressive stress and increased carrier concentration for a thicker GaN layer. The above situation enables us to regulate the energy level of GaN layer via thickness control, and thus accelerates its future application in new generation solar cells.     

Chemical and structural transformations of silicon submitted to H2 or H2/CH4 microwave plasmas

Silicon substrates are often used to synthesize polycrystalline diamond films by microwave plasma assisted chemical vapour deposition technique (MPCVD). In the case of highly oriented diamond films, several steps are employed to carefully prepare the silicon surface (pre-treatment steps), to nucleate diamond crystals (nucleation step) and to thick the film (growth step). In this study, we characterize {100} silicon substrates and diamond released from its silicon substrate by electronic microscopies (TEM and SEM), by Atomic Force Microscopy (AFM) and by X-ray photoelectron spectroscopy (XPS), to follow the substrate transformations after each step, particularly the formation and the evolution of the silicon carbide and to characterise the diamond films grown on the carburised silicon. We show that according to the experimental conditions and the level of surface/gas contamination by carbon and silicon species, isolated islands or continuous β-SiC compound are formed over the silicon surface and can generate defects such as voids or strip structures that influence the subsequent diamond nucleation and growth.     

SMA-纳米SiC及SMA-纳米金刚石复合膜制备及其Cu2+离子吸附性能

以纳米SiC及纳米金刚石粉体为添加剂,通过苯乙烯与顺丁烯二酸酐的聚合,成功制备了苯乙烯-马来酸酐-纳米碳化硅(SMA-纳米SiC)及苯乙烯-马来酸酐-纳米金刚石(SMA-nano diamond)复合薄膜材料。通过热重分析(TG)、傅里叶红外光谱仪(FTIR)和扫描电子显微镜(SEM)对吸附复合膜的结构进行表征。研究了SMA-纳米SiC及SMA-纳米金刚石复合膜的吸水性及其对二价铜(Cu²⁺)离子的吸附特性。结果表明,无机纳米颗粒的添加均提高了复合薄膜的耐热特性,提高了热分解温度。当纳米粉体含量在0.8~1.2 g之间,可以获得较为均匀的泡沫状多孔复合薄膜。浸泡6 h,纳米SiC添加量为1.0 g的复合膜吸水率低至4.81%。纳米金刚石添加量为1.0 g,复合膜吸水率低至3.52%。适量的纳米SiC及纳米金刚石可以提高复合膜的耐水性能。SMA-纳米SiC及SMA-纳米金刚石复合膜材料均对Cu²⁺离子具有一定的吸附特性,泡沫状复合膜吸附性能最佳。纳米SiC及纳米金刚石能够有效改善SMA膜对重金属离子的吸附性能。     

Thermo-mechanical characterization of carbon fiber reinforced silicon carbide composites having boron nitride as an interphase material

The carbon fiber reinforced silicon carbide composites were prepared by an isothermal chemical vapour infiltration process. In order to achieve the required density, the carbon fiber preforms in the form of rectangular panels were infiltrated by silicon carbide (SiC) matrix. Prior to the matrix infiltration, a thin coating of boron nitride, as an interphase, was applied on the fiber preform. The test samples were subjected to seal coating of silicon carbide by chemical vapour deposition process. The effect of protective SiC seal coating was examined by testing (3-point bend test) the uncoated and the seal coated samples at different temperatures. Higher value of the flexural strength was observed for the seal coated samples as compared to the uncoated samples, when got tested at high temperature (up to 1400?°C). The detailed analysis of the fractured surfaces of the tested samples was carried out.     

Effects of plasma hydrogenation on low temperature growth of nanocrystalline cubic SiC thin films

The effects of plasma hydrogenation on the fabrication of nanocrystalline cubic silicon carbide (SiC) thin films on Si (100) are investigated. Our results indicate that plasma hydrogenation is an effective method to reduce the deposition temperature and to improve the composition and microstructure of the cubic SiC (β-SiC) thin films. In particular, the crystal particle size and the oxygen diffusion can be controlled. The mechanism is discussed.     

The wetting behaviour and reaction kinetics in diamond–silicon carbide systems

The wetting behaviour of silicon on diamond and the interaction of diamond with molten silicon were investigated. It was found that diamond is well wetted by molten silicon reaching a contact angle of about 20° after melting. The wetting is caused by the rapid formation of a SiC interlayer by nucleation of silicon carbide grains on the surface of the diamond. Investigations of the interaction of silicon with CVD diamond, using SEM, showed that the initial rate of SiC formation is very fast and is significantly reduced after the formation of a 4–6 μm thick dense SiC interlayer. At that stage further growth is likely to be controlled by the diffusion of Si and C through the grain boundaries of the silicon carbide interlayer. The results were compared with the interaction of silicon with glassy carbon.     

The Manipulation and Alignment of Silicon Carbide Whiskers for Gallium Nitride Epitaxial Growth

As a substrate candidate for low‐cost III‐nitride thin film growth, 3C–SiC whiskers are employed and manipulated in this work. The alignment of the whiskers is achieved on a patterned 3M Vikuiti^? Brightness Enhancement Film surface. The degree of whisker alignment using this approach is higher than the whiskers lined up by extrusion methods according to X‐ray diffraction (XRD) analysis. The aligned whiskers are transferred from the 3M film and embedded into an alumina matrix by tape casting. A self‐regulating sintering technique for SiC whiskers is used to protect the whiskers from being oxidized in air during sintering at 1600°C. The aligned whiskers are rigidly embedded in the alumina matrix as shown in scanning electron microscopy (SEM) images and energy‐dispersive X‐ray spectrometry energy mapping images. GaN thin films grown by a low‐cost sputtering process on Alumina/SiC as well as Si and SiC as reference materials are characterized by XRD and SEM.     

Structural characteristics of single crystalline GaN films grown on (111) diamond with AlN buffer

Hexagonal GaN films with the [0001] direction parallel to the surface normal were grown on (111) oriented single crystalline diamond substrates by plasma-assisted molecular beam epitaxy. Pre-treatments of the diamond surface with the nitrogen plasma beam, prior the nucleation of a thin AlN layer, eliminated the inversion domains and reduced the density of threading dislocations in the GaN epilayers. The films have an in-plane epitaxial relationship [1010]GaN//[110]diamond. Thus GaN (0001) thin films of single epitaxial relationship and of single polarity were realised on diamond with AlN buffer.