Amorphous Diamond Films Research Articles

Effects of the interface and surface nanostructures on field emission of amorphous diamond film

Details are given of a systematic experimental study into the effect of interface structure on the field emission properties of nanostructures using amorphous diamond (a-D) films. The field electron emission experiments indicates a better field emission characteristic after predepositing a metal (Al or Ti) interlayer. The effect of chemical etching of silicon substrate on the field emission of a-D film/silicon has been studied systematically. In addition, surface features of the film considered to be responsible for low field emission are discovered. Based on this latter finding, a new mechanism named field-induced transform step enhanced emission mechanism, which is responsible for electron emission from a-D film, is presented.

Compatibility of the totally replaced hip. Reduction of wear by amorphous diamond coating.

Particulate wear debris in totally replaced hips causes adverse local host reactions. The extreme form of such a reaction, aggressive granulomatosis, was found to be a distinct condition and different from simple aseptic loosening. Reactive and adaptive tissues around the totally replaced hip were made of proliferation of local fibroblast like cells and activated macrophages. Methylmethacrylate and high-molecular-weight polyethylene were shown to be essentially immunologically inert implant materials, but in small particulate form functioned as cellular irritants initiating local biological reactions leading to loosening of the implants. Chromium-cobalt-molybdenum is the most popular metallic implant material; it is hard and tough, and the bearings of this metal are partially self-polishing. In total hip implants, prerequisites for longevity of the replaced hip are good biocompatibility of the materials and sufficient tribological properties of the bearings. The third key issue is that the bearing must minimize frictional shear at the prosthetic bone-implant interface to be compatible with long-term survival. Some of the approaches to meet these demands are alumina-on-alumina and metal-on-metal designs, as well as the use of highly crosslinked polyethylene for the acetabular component. In order to avoid the wear-based deleterious properties of the conventional total hip prosthesis materials or coatings, the present work included biological and tribological testing of amorphous diamond. Previous experiments had demonstrated that a high adhesion of tetrahedral amorphous carbon coatings to a substrate can be achieved by using mixing layers or interlayers. Amorphous diamond was found to be biologically inert, and simulator testing indicated excellent wear properties for conventional total hip prostheses, in which either the ball or both bearing surfaces were coated with hydrogen-free tetrahedral amorphous diamond films. Simulator testing with such total hip prostheses showed no measurable wear or detectable delamination after 15,000,000 test cycles corresponding to 15 years of clinical use. The present work clearly shows that wear is one of the basic problems with totally replaced hips. Diamond coating of the bearing surfaces appears to be an attractive solution to improve longevity of the totally replaced hip.

Diamond and hard carbon films for microelectromechanical systems (MEMS)—a nanotribological study

Previously unreported amorphous carbon and nanocrystalline diamond films have been prepared by microwave plasma-enhanced chemical vapour deposition (MPCVD) in two novel reactor designs. Ar–CH 4 gas mixtures were used in a 1 kW coaxial bladed reactor design, and Ar–H 2–CH 4 gas mixtures were used in a 6 kW reactor with a dual mode applicator. Films were examined by ultra-violet and visible Raman spectroscopy, and subjected to nanotribological investigation by scanning force microscopy (SFM). The results are compared with those from randomly orientated and (100) microcrystalline MPCVD diamond, pulsed laser deposited (PLD) and filtered cathode vacuum arc (FCVA) carbon films, Si wafer and electroformed Ni. A consideration of the relative merits of the various film types for microelectromechanical systems (MEMS) is presented.

Synthesis and characterization of single-wall carbon nanotube–amorphous diamond thin-film composites

Thin-film single-wall carbon nanotube (SWNT) composites synthesized by pulsed laser deposition (PLD) are reported. Ultrahard, transparent, pure-carbon, electrically insulating, amorphous diamond thin films were deposited by PLD as scratch-resistant, encapsulating matrices for disperse, electrically conductive mats of SWNT bundles. In situ resistance measurements of the mats during PLD, as well as ex situ Raman spectroscopy, current–voltage measurements, spectroscopic ellipsometry, and field-emission scanning electron microscopy, are used to understand the interaction between the SWNT and the highly energetic (∼100 eV) carbon species responsible for the formation of the amorphous diamond thin film. The results indicate that a large fraction of SWNT within the bundles survive the energetic bombardment from the PLD plume, preserving the metallic behavior of the interconnected nanotube mat, although with higher resistance. Amorphous diamond film thicknesses of only 50 nm protect the SWNT against wear, providing scratch hardness up to 25 GPa in an optically transmissive, all-carbon thin-film composite.

Substrate nanoprotrusions and their effect on field electron emission from amorphous-diamond films

Geometrical field-enhancement structures at the surface of a substrate have been expected to effectively lower fields required to inject electrons from the substrate into amorphous-diamond (a-D) films. Two techniques have been developed to create a high density of nanoprotrusions on substrate surfaces: (a) on silicon (Si) substrates using our patented chemical-etching technique, and (b) on aluminum (Al) film by following a special deposition procedure. The density of nanoprotrusions can be as high as 108/cm2, and the mean roughness of the substrates is 2–30 nm. From a-D emitters, substrate surfaces with such nanoprotrusions can lower the field for emission. The results are discussed in light of several existing theoretical models.

Band gap structure and electron emission property of chemical-vapor-deposited diamond films

The structures of the band gap and defect states of chemical-vapor-deposited diamond films were investigated by photoluminescence spectroscopy, covering wavelength from visible to vacuum ultraviolet (VUV). Band gaps ranging from 5.5 to 3.2 eV were measured for natural, polycrystalline, and amorphous diamond films. Low voltage field emissions were obtained from wide band gap films with band gap states distributed close to the conduction band and extended deeply into the band gap. Amorphous diamond film with a narrower band gap could not provide low field emission.

The intrinsic relation between field electron emission and structurecharacteristics of amorphous diamond film

Stable electron emission from amorphous diamond film at a lowthreshold field of 6-7 mV m-1 has been observed using atransparent anode technique. The electron emission characteristics arediscussed in terms of the structural properties of films. Using scanningelectron microscopy and Raman spectra, the emitting and the non-emittingregions of the amorphous diamond film are analysed. The emitting regionsexhibit rougher microstructure in morphology and a stronger Raman peak at1360 cm-1 than the non-emitting regions, a signature of having moremicro-sp2 components. It is suggested that field emission is prone totake place at the regions containing more sp2 nanoclusters ornanocrystalline graphite.

Physical origin of nonlinearity in the Fowler–Nordheim plot of field-induced emission from amorphous diamond films: Thermionic emission to field emission

Nonlinearity is observed in Fowler–Nordheim (FN) plots of field emission from nondoped and nitrogen-doped amorphous diamond films. Based on a unified electron emission equation a detailed analysis is carried out. The results from numerical calculation of the unified equation are consistent with the experimental data. It is shown that the nonlinearity in the FN plot originates from a transition from thermionic emission to field emission as the applied field increases. The electrical field ranges are derived in which the field emission and thermionic emission approximation applies.

Effect of magnetic field structure near cathode on the arc spot stability of filtered vacuum arc source of graphite

Amorphic diamond films deposited by the filtered vacuum arc (FVA) method have attracted much attention due to their superior mechanical and optical properties. However, the instability of the arc limits the continuous operation of the FVA source, resulting in a poor productivity. In the present work, we investigated the effects of the cathode shape and the structure of the magnetic field near the cathode on the erosion behavior by both computer simulations and experimental studies. Arc instability in the configuration of parallel magnetic polarities of the source magnet and the extraction could be suppressed by placing a permanent magnet of opposite polarity behind the cathode. We show further that oscillation of the current of the source magnet was effective in extending the area of the arc spot movement. A tapered cathode exhibited a more stable arc than a cylindrical cathode, as confirmed by the time variation of the beam current. By using the oscillating current of the source magnet and a tapered cathode of diameter 80 mm, a continuous operation for 2000 min with an arc current of 60 A was obtained, at which more than 90% of the cathode volume could be used. A stable beam current of about 350 mA was obtained under the present operating conditions.

Carbon surface modifications during sliding test and friction behavior of carbon thin films against XC 48 steel

The anisotropic properties of carbon due to sp 2, sp 3, and π bonds can lead to different carbon surfaces with different chemical, mechanical and tribological properties. This paper reports on fundamental friction and wear mechanisms of graphite, diamond and carbon coatings: diamond-like carbon (DLC) with nanophase diamond structure, amorphous carbon (a-C) corresponding to graphite and diamond structure, amorphous hydrogenated carbon films (a-C:H) with more graphite, and amorphous diamond film (a-D) obtained by ablation laser of the diamond structure. The friction coefficients μ of carbon and carbon coatings can be varied by a factor of 50 ( μ=0.02–1) and the wear rate by factor of 10 2–10 4 according to the nature of the carbon film, the tribocontact environment, and the temperature. A compilation of the last 10 years' published studies on the mechanical and tribological behavior of different carbon coatings will be presented and discussed in order to specify the tribological film behavior according to its nature and mechanical properties. The transition phenomenon from mild wear to severe wear is investigated for each rubbing surface. Many surface structural modifications occur during friction. Scanning electron microscopy, X-ray analysis and diffraction electron microscopy analyses of the sliding surface will be presented and discussed.

Deposition of diamond-like films upon laser irradiation of the interface of a transparent substrate with liquid aromatic hydrocarbons

Amorphous diamond films are deposited on transparent substrates upon exposure of its interface with liquid hydrocarbons (C6H5CH3, C6H6, and C6H5CH(CH3)2) to pulsed visible radiation of a Cu vapor laser (λ=510.6 nm). The X-ray Auger electron spectroscopy (XAES), reflection high energy electron diffraction (RHEED), scanning electron microscopy (SEM), profilometry, optical spectroscopy, and Raman analysis at λ=632.8 nm excitation wavelength are employed to characterize the deposited films. The sp3 fraction in deposited films amounts to 60–70% and depends on the precursor. The films show excellent adherence, are transparent in the visible and have microhardness of 50–70 GPa, as measured with a nanoindentor. The sp3 bond fraction in deposited films increases with addition to the liquid hydrocarbon a suspension of diamond nanoparticles.

Effect of interface layers on electron field emission properties of amorphous diamond films

Hydrogen-free high sp3 content amorphous diamond (AD) films are deposited on three different substrates—Au-coated Si (Au/Si), Ti-coated Si (Ti/Si) and Si wafers. Electron field emission properties and fluorescent displays of the above AD films are studied by using a sample diode structure. The compositional profile of the interfaces of AD/Ti/Si and AD/Si is examined by using secondary ions mass spectroscopy (SIMS). Because of the reaction and interdiffusion between Ti and C, the formation of a thin TiC intermediate layer is possible between AD film and Ti/Si substrate. The field emission properties of AD/TI/Si are sufficiently improved, especially its uniformity. A field emission density of 0.352 mA/cm2 is obtained under an electric field of 19.7 V/μm. The value is much more than that of AD/Au/Si and AD/Si under the same electric field.

Effects of the surface treatment of silicon substrate on the field emission characteristic of a silicon and amorphous diamond cold cathode emitter

Details are given of an experimental study of the effects of surface treatment of silicon substrate on the field emission process of flat amorphous diamond (a-D) film field emitters. Using a filtered cathodic vacuum arc plasma deposition system (FCVAPD), the amorphous diamond (a-D) film was deposited on both non-treated and etched silicon wafers (n-type and p-type). The field electron emission characteristic was measured before and after depositing a-D film. The a-D film on etched silicon wafer shows distinct increase in emission current compared with that on non-treated silicon wafer. The phenomenon is attributed to two important reasons: the low or even negative surface electron affinity of a-D film and the local field enhancement at the Si-diamond interface.

Study of instability in the field electron emission from amorphous diamond films

Details are given of an experimental study of the current–voltage characteristics and instability of the field electron emission from amorphous diamond films prepared using a magnetic field-filtered ion deposition technique. A switch-on phenomenon is observed, and two types of instability are found, i.e., fluctuation at low currents and an abrupt change in emission associated with local breakdown events at high currents. Finally, it is found that, in order to establish stable emission with a current density approaching 50 mA/cm2, it is necessary to undergo the switch-on and local breakdown processes. These phenomena are characterized, and the physical mechanisms responsible for them are proposed.

Laser deposition of amorphous diamond films from liquid aromatic hydrocarbons

H5CH3, C6H6, and C6H5CH(CH3)2) to pulsed visible laser radiation of a copper vapor laser (λ=510.6 nm). The X-ray Auger electron spectroscopy (XAES), reflection high energy electron diffraction (RHEED), and Raman analysis are employed to characterize the deposited films. The sp3 fraction in deposited films amounts to 60–70% and depends on the precursor. The average film thickness on a glass substrate is about 100 nm. The films show excellent adherence, are transparent in the visible and have microhardness of 50–70 GPa, as measured by nanoindentor.

High sp3 content hydrogen-free amorphous diamond: an excellent electron field emission material

Details are given of a study of the characteristics of field-induced electron emission from hydrogen-free high sp3 content (>90%) amorphous diamond (a-D) film deposited on heavily doped (ρ<0.01ω·cm) n-type monocrystalline Si (111) substrate. It is demonstrated that a-D film has excellent electron field emission properties. The emission current can reach 0.9 μA at applied field as low as 1 V/μm, and the emission current density can be about several mA/cm2 under 20 V/μm. The emission current is stable when the beginning current is at 50 μA within 72 h. Uniform fluorescence display of electron emission from the whole face of the a-D film under the electric field of 10–12 V/μm is also observed. The contribution of excellent electron emission property results from the smooth, uniform, amorphous surface and high sp3 content of the a-D film.

Structure and optical properties of amorphous diamond films prepared by ArF laser ablation as a function of carbon ion kinetic energy

Amorphous carbon films with variable sp3 content were produced by ArF (193 nm) pulsed laser deposition. Electron energy loss spectroscopy and spectroscopic ellipsometry were employed to systematically study changes in the bonding and optical properties of the carbon films as a function of the kinetic energy of ablated C ions, which was measured using an ion probe. The measurements reveal that the films with the most diamond-like properties are obtained at the C ion kinetic energy of ∼90 eV. In contrast to measurements made as a function of laser fluence, ion probe measurements of kinetic energy are a convenient as well as more accurate and fundamental method for monitoring deposition conditions, with the advantage of being readily transferable for interlaboratory comparisons.

Raman-based measurements of optical properties of thin solid films: application to amorphous diamond

The potentialities of Raman spectroscopy to measure optical properties of thin solid films are considered. A new approach based on the analysis of interference-induced modification of the Raman scattering from the substrate material together with the normal reflection coefficient is proposed. It is shown that the method provides the absorption coefficient and refractive index of the film bulk with high spatial resolution and accuracy, and it can be applied to various transparent thin films. The developed approach is used to measure optical properties of amorphous diamond films deposited onto crystalline silicon. In the case of the amorphous diamond films deposited with a mass-separated ion beam, a straightforward correlation between the absorption coefficient and Raman spectra is obtained, which can be also used to estimate the optical properties. For the films deposited with pulsed cathodic arc discharge, such a relation appears to be more complex, which possibly indicates extensive variations of the sp 2 clusterization in the latter deposition method.

Field emission from filtered arc deposited amorphous diamond

Abstract In this work, amorphous diamond (a-D) films have been prepared by filtered arc deposition (FAD). Field emission data from the a-D film were obtained using planar diode structure. Emission currents in the range of 20–40 μ A were detected at an electric field of 20 V μ m −1 . A mechanism for electron emission from the a-D film is discussed. FAD a-D films had a low effective work function (1.0 eV) and negative electron affinity (−0.2 eV). In addition, uniform and stable emission can be achieved for FAD a-D films.

Field emission from amorphous diamond films prepared by filtered arc deposition

Nonhydrogenated amorphous carbon films have been prepared by filtered arc deposition (FAD), which provides carbon ions with optimum kinetic energies at practical deposition rates. It was found that the carbon atoms were so highly tetrahedrally bonded that the films were referred to as amorphous diamond ( α-D). The field emission behaviour of the α-D films has been studied for the first time. The field emission current more that 10 μA was detected at an electric field intensity of 18 V/ μm. Such a low threshold field provides the designer of flat panel displays with an opportunity to develop a planar emitter, rather than the conventional Spindt-tip field emitter which requires lithographic and microstructural burdens.