Amorphous Hydrogenated Carbon Films Research Articles

Effects of Deposition Time Duration on Thermal Diffusivity of Hydrogenated Amorphous Carbon Films

In the present investigation we study the effects of film-deposition time duration on thermal diffusivity (α) of hydrogenated amorphous carbon (a-C:H) thin-films grown in a radio-frequency (RF) plasma enhanced chemical vapor deposition system. A set of films was deposited at 50 W RF power for 40, 60, 80, and 100 min. Film characteristics were determined from the optical transmission spectroscopy, Fourier Transform Infrared spectroscopy, and Raman spectroscopy. Thermal diffusivity of a-C:H films was evaluated using the optical pump-and-probe technique on the aluminum-coated samples. Results show a trend of increase in α as the deposition time increases due to the microstructural changes associated with longer exposure to ion bombardment effects on the growth surface of the films.

Surface modifications of amorphous hydrogenated carbon films submitted to carbon tetrafluorine plasma treatment

Amorphous hydrogenated carbon films deposited by plasma enhanced chemical vapor deposition (PECVD) in a methane atmosphere were exposed to carbon tetrafluorine plasma. The r.f.-plasma treatments were performed with a chamber pressure of 8Pa and a self-bias voltage, Vb, between −100 and −600V. The treatment time varied from 5 to 160min. A contact angle of approximately 135° was obtained after 10min of CF4 r.f.-plasma treatment at a self-bias voltage of −350 or −600V and after 160min of CF4 r.f.-plasma treatment at self-bias of −100V. The modification of the surface roughness was assessed by atomic force microscopy, which showed an increase of up to a few nanometers for the longer treatment times. The surface layer was analyzed by low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). The chemical bonds were probed using XPS and two chemical ambients were dominant: pairs of bonded carbon atoms and carbon bonded to one fluorine atom. The content of fluorine measured by LEIS at the sample surface increased with the treatment time. When the contact angle reached its maximum value, LEIS results were nearly independent of Vb, indicating that the chemical composition of the outermost layer of the sample plays an important role in the control of the surface wettability.

Controllable Electrochemical Activities by Oxidative Treatment toward Inner-Sphere Redox Systems at N-Doped Hydrogenated Amorphous Carbon Films

The electrochemical activity of the surface of Nitrogen-doped hydrogenated amorphous carbon thin films (a-CNH, N-doped DLC) toward the inner sphere redox species is controllable by modifying the surface termination. At the oxygen plasma treated N-doped DLC surface (O-DLC), the surface functional groups containing carbon doubly bonded to oxygen (C=O), which improves adsorption of polar molecules, were generated. By oxidative treatment, the electron-transfer rate for dopamine (DA) positively charged inner-sphere redox analyte could be improved at the N-doped DLC surface. For redox reaction of 2,4-dichlorophenol, which induces an inevitable fouling of the anode surface by forming passivating films, the DLC surfaces exhibited remarkably higher stability and reproducibility of the electrode performance. This is due to the electrochemical decomposition of the passive films without the interference of oxygen evolution by applying higher potential. The N-doped DLC film can offer benefits as the polarizable electrode surface with the higher reactivity and higher stability toward inner-sphere redox species. By making use of these controllable electrochemical reactivity at the O-DLC surface, the selective detection of DA in the mixed solution of DA and uric acid could be achieved.

Carrier gas and ion beam parameter effects on the structure and properties of a-C:H/SiOx films deposited employing closed drift ion beam source

In the present study closed drift ion beam source was used to deposit SiOx containing amorphous hydrogenated carbon films (a-C:H/SiOx) employing hexamethyldisiloxane and H2 or He carrier gas. The structure, optical and mechanical properties of a-C:H/SiOx films deposited using different ion beam energies (300–800eV) and ion beam current densities (20–80μA/cm2) were analyzed. Raman spectroscopy has shown that the structure of a-C:H/SiOx films deposited using different carrier gas differs. In the case of H2 carrier gas I(D)/I(G) ratio decreased from 1.1 to 1 with the increase of ion beam energy from 300eV to 500eV. It is shown that the increase tendency observed for I(D)/I(G) ratio dependence on the ion beam current density was influenced by the structural changes (Si–H bonds formation) observed in FTIR analysis. The films with maximum hardness (12.8GPa for He and 11.9GPa for H2 carrier gas) were formed at 500eV ion beam energy for both carrier gas. The band gap and B parameter of the films (formed at 500eV with H2 carrier gas) increase almost linearly with the ion beam current density.

Improvement of Conductivity by Incorporation of Boron Atoms in Hydrogenated Amorphous Carbon Film Fabricated by Plasma CVD methods and Electrochemical Properties of the Film

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Deposition of amorphous hydrogenated carbon films on Si and PMMA by pulsed direct-current plasma CVD

A pulse-modulated direct-current methane plasma is used to deposit amorphous hydrogenated carbon (a-C:H) films on Si and polymethyl methacrylate (PMMA) substrates. The structure and mechanical properties of the films are examined by applying a negative pulse bias voltage of 0.5 to 3 kV to the substrate at a pulse bias period of 100 to 200 μs. The deposition rate on both Si and PMMA increases with increasing the net input power, independent of the pulse period. The Raman spectra demonstrate that the films on Si are diamond-like carbon (DLC), while those on PMMA are polymer-like or soft amorphous carbon because of higher crystallinity of the sp 2 phase and lower nanoscale hardness. The residual compressive stress of the films on PMMA is constantly low ranging from 0 to 2 GPa due exclusively to high flexibility of PMMA, which causes the easy relief of the stress and thus the density decrease in the films.

Effect of hydrogen flow on the properties of hydrogenated amorphous carbon films fabricated by electron cyclotron resonance plasma enhanced chemical vapor deposition

The hydrogenated amorphous carbon films (a-C:H, so-called diamond-like carbon, DLC) have exceptional physical and mechanical properties and have wide applications. In the present study, amorphous hydrogenated carbon films (a-C:H) have been deposited on a Si (100) substrate at different hydrogen flow using electron cyclotron resonance chemical vapor deposition (ECR-CVD). The flow of hydrogen changed from 10 sccm to 40 sccm and the flow of acetylene was fixed at 10 sccm. The microstructure and properties of the a-C:H were measured using visible Raman spectra, Fourier transform infrared (FTIR) spectroscopy, UV–VIS spectrometer,surface profilometer and nano-indentation. The results showed that the sp 3 content and sp 3–CH 2 structure in the amorphous hydrogenated carbon films increased with the hydrogen flow. The deposition rate decreased with the hydrogen flow. The residual stress and the nano-hardness of the amorphous hydrogenated carbon films increased with the hydrogen flow. Consequently, the a-C:H film become more diamond-like with the increase of hydrogen flow.

Ion beam deposition of amorphous hydrogenated carbon films on amorphous silicon interlayer: Experiment and simulation

A thick layer of amorphous silicon (a-Si) was deposited on industrial grade crystalline n-Si < 111 > substrate by means of electron beam evaporation. On top of a-Si layer, amorphous hydrogenated carbon (a-C:H) film was grown by direct ion beam deposition from acetylene precursor gas. In order to study on atomic level the a-C:H film growth on amorphous silicon, a theoretical model was developed in a form of reaction rate (kinetic) equations. Numerical simulation using this model has revealed that the ratio of sp 3/sp 2 content in the film is heavily influenced by relaxation rate of the carbon atoms in a sub-surface region of the film that were activated by ion irradiation. The final structure of a-C:H film does not depend much on elemental composition and structure of amorphous Si coating, provided that deposition procedure is not terminated at its initial stage but continues for more than 60 s. It became evident, therefore, that the use of a-Si interlayer with a-C:H films could be particularly beneficial when a need arises to minimize or eliminate the effect of the substrate. As one of such cases, a poor adhesion of amorphous carbon on steel and other ferrous alloys could be mentioned.

Dielectric response and structure of amorphous hydrogenated carbon films with nitrogen admixture

The optical properties and structure of a-C:H films were modified by addition of nitrogen into the CH 4/H 2 deposition mixture. Three films prepared in capacitively coupled rf discharge were compared: (a) hydrogenated diamond like carbon film with hydrogen content of 34% and indentation hardness of 21.7 GPa, (b) hard a-C:H:N film with nitrogen content of 13% and indentation hardness of 18.5 GPa and (c) soft a-C:H:N film with nitrogen content of 10% and indentation hardness of 6.7 GPa. It is shown how the parametrized density of states model describing dielectric response of electronic interband transitions can be applied to modified a-C:H:N and how it can be combined with correct treatment of transmittance measured in infrared range using additional Gaussian peaks in joint density of phonon states. This analysis resulted in determination of film dielectric function in wide spectral range (0.045–30 eV) and provided also information about the density of states of valence and conduction bands and lattice vibrations.

Kinetic modeling of laser annealing processes in a-C:H films

In this research work, a nanosecond-pulsed YAG:Nd laser was used to modify the properties of an amorphous hydrogenated carbon film, which was deposited on c–Si substrate. Experimental examination has revealed that the primary effect of irradiation is diamond-to-graphite transformation or, more specifically, carbon sp 3-to-sp 2 hybridization transition. These findings were qualitatively verified by numerical simulation of kinetic processes that proceed in the film under laser irradiation.

Hydrogenated Amorphous Carbon Films Prepared by Filtered Vacuum Arc Method with Various C2H2 Pressures

Hydrogenated amorphous carbon films (a-C:H) were deposited on silicon (100) substrates using a filtered vacuum arc (FVA) method. A graphite cathode and acetylene (C2H2) at various flow rates were used to synthesize the carbon films. The deposition rate of the carbon films without C2H2 addition was 5 nm/min, whereas the deposition rate increased from 23 to 82 nm/min with increasing C2H2 flow rate from 2.5 to 20 sccm. The supply of C2H2 gas induces an increase in CH radical density near the substrate, resulting in a high deposition rate. The plasma diagnostics using optical emission spectroscopy showed that the emission peak intensity of the CH radicals (A 3Πg–X 3Πu, 431.26 nm) increased with increasing C2H2 flow rate. Raman spectroscopy revealed a change in the deposited films from nano-crystalline graphite to a-C:H as the C2H2 flow rate was increased.

Electrical, optical, and mechanical properties of amorphous hydrogenated carbon obtained under various deposition conditions

Activation energy of dc conductivity, refractive index, and microhardness of amorphous hydrogenated carbon films a-C:H have been studied. The films are produced from a methane-argon mixture under various deposition conditions on quartz and silicon substrates: E/p = 40–180 V/mPa (E is the electric-field strength in the space between the electrodes, and p is the gas mixture pressure in the chamber) and substrate temperature Ts = 50–300°C. In these deposition conditions, diamond-like, polymer-like, and graphite-like a-C:H films can be obtained with widely varying activation energy of conductivity, microhardness, and refractive index.

Hydrogenated Amorphous Carbon Films Prepared by Filtered Vacuum Arc Method with Various C2H2Pressures

Hydrogenated Amorphous Carbon Films Prepared by Filtered Vacuum Arc Method with Various C₂H₂Pressures

Deposition of WO 3 doped amorphous hydrogenated carbon film by using liquid phase electrodeposition technique and its mechanical properties

Tungsten trioxide nanoparticles doped amorphous hydrogenated carbon (WO 3/a-C:H) films were successfully fabricated on silicon substrates by electrochemical deposition technique under atmospheric pressure. The as-deposited films were characterized by X-ray photoelectron spectroscopy, Transmission electron microscopy, Fourier transform infrared spectroscopy and Raman spectroscopy, respectively. Results showed that nanocrystalline tungsten trioxide particles with a grain size in the range of 8–12 nm were homogeneously embedded in the amorphous carbon matrix, and the sp 3-hybridized carbon content in the a-C:H films increased. Compared with pure amorphous carbon film, the hardness and elastic recovery were significantly improved due to the doping of WO 3. At the end, an understandable model was proposed to interpret the growth mechanism of the WO 3/a-C:H composite films.

Mid-frequency deposition of a-C:H films using five different precursors

The plasma-enhanced chemical vapour deposition (PECVD) of amorphous hydrogenated carbon films from pulsed discharges with frequencies in the range from 50 kHz to 250 kHz was investigated. Five different hydrocarbons (acetylene C 2H 2, isobutene C 4H 8, cyclopentene C 5H 8, toluene C 7H 8 and cycloheptatriene C 7H 8) were probed as film growth precursors. In addition, two types of pulse-generators with somewhat different waveforms were used to power the discharges in the so called mid-frequency range. The a-C:H films deposited in a parallel-plate reactor were characterised for their thickness/deposition rate, hardness and hydrogen content. The hydrogen concentration in the films varied between 19 at.-% and 37 at.-%. With the substrate temperature held constant, it is roughly in inverse proportion to the hardness. The film with the highest hardness of 25 GPa was formed at a deposition rate of 0.8 μm/h in the C 2H 2 discharge at the lowest investigated pressure of 2 Pa. With increasing molecular mass of the precursor mostly weaker films were deposited. Relatively high values of both deposition rate and hardness were achieved using the precursor isobutene: a hardness of 21 GPa combined with a deposition rate of 4.1 μm/h. From the probed precursors, isobutene is also most advantageous for a-C:H deposition at higher pressures (up to 50 Pa investigated). But, as an over-all trend, the a-C:H hardness decreases with increasing deposition rate.

Stages in the interaction of deuterium atoms with amorphous hydrogenated carbon films: Isotope exchange, soft-layer formation, and steady-state erosion

We report studies of the interactions of quantified deuterium (hydrogen) atom beams with hard amorphous hydrogenated carbon films at a substrate temperature of ∼330 K in an ultrahigh-vacuum chamber. The modification/erosion of a-C:H (a-C:D) films was monitored in situ by ellipsometry in real time. By interpreting the ellipsometric information and combining it with measurements of the absolute D areal density changes in the a-C:H (a-C:D) films by ion beam analysis as a function of D (H) atom fluence, we are able to distinguish three sequential stages of D interaction with hard a-C:H films. The first stage is replacement of bonded hydrogen by deuterium up to an areal density of ∼5×1015 D cm−2 to a depth of ∼1.4 nm from the surface. This phase is complete after a deuterium fluence of ≈2×1018 cm−2. The effective cross section for isotopic exchange of H with D atoms for the a-C:H layer is found to be σ=2.0×10−18 cm2, and is close to the cross section for H abstraction from a carbon surface. This may indicate that H abstraction by D from the a-C:H surface is the rate limiting step for isotope exchange in this situation. Hydrogen replacement is followed by creation of additional C–D bonds in the near-surface region and increases the D areal density by about 2.5×1015 D cm−2. By ellipsometry this process can be observed as the formation of a soft a-C:D layer on top of the hard a-C:H bulk film, with the soft layer extending about 1.4 nm from the surface. This stage is complete after a deuterium fluence of about 2×1019 cm−2. Subsequently, steady-state erosion of the a-C:H film takes place. Here, a soft a-C:D layer with roughly constant thickness (∼1.4 nm) remains on the hard a-C:H substrate and is dynamically reformed as the underlying hard a-C:H film becomes thinner. A similar sequence of processes takes place at a substrate temperature of 650 K, albeit at a much faster rate.

H-atom interaction with amorphous hydrocarbon films: Effect of surface temperature, H flux and exposure time

In the present paper, we study the interaction between atomic hydrogen generated in a microwave afterglow with amorphous hydrogenated carbon films. A simple surface model is described and compared with the experimental results. Erosion rate is time dependent and exhibits a transient regime before reaching a constant value. Estimate of the modified film thickness by ellipsometry shows that thickness increases with time and becomes constant and equal to 1.4 nm when reaching the permanent regime. In addition, this limit is independent on the conditions, e.g., on hydrogen flux and temperature. Erosion rate depends linearly on hydrogen flux arriving at the surface and shows an exponential increase with surface temperature. A simple model proposed in the paper is in good agreement with the experimental data and allows giving an estimate of the erosion activation energy Ea=0.2 eV. This value is in agreement with the energy involved in the reaction between hydrogen atom and carbon atom in sp3 hybridization.

Transformation of light paraffins in a microwave-induced plasma-based reactor at reduced pressure

In this work, the effects of the plasma chemistry of an argon microwave (2.45 GHz) discharge at reduced pressure on the conversion of three different alkanes ( n-pentane, n-hexane and n-heptane) have been studied. Optical emission spectroscopy has been used for identifying the species generated in the plasma and for estimating its gas temperature. Gas chromatography, mass spectrometry, X-ray diffraction and FTIR spectroscopy have been employed for identifying and analyzing all the compounds present as reaction products. Microwave power and hydrocarbon flow rate have been found critically to affect both conversion and selectivity. The main gas products have been hydrogen and ethylene. At low powers (100–150 W) the conversion to hydrogen has been quite selective. However, at high powers (>300 W) or slow hydrocarbon flow rate ethylene has resulted to be the major product. In most cases, an important fraction of a carbon deposit has been obtained which has been characterized as an amorphous hydrogenated carbon film. Some plausible mechanisms explaining the formation of the main reaction products have been discussed.

Mid-frequency deposition of a-C:H films using five different precursors

The plasma-enhanced chemical vapour deposition (PECVD) of amorphous hydrogenated carbon films from pulsed discharges with frequencies in the range from 50 kHz to 250 kHz was investigated. Five different hydrocarbons (acetylene C 2H 2, isobutene C 4H 8, cyclopentene C 5H 8, toluene C 7H 8 and cycloheptatriene C 7H 8) were probed as film growth precursors. In addition, two types of pulse-generators with somewhat different waveforms were used to power the discharges in the so called mid-frequency range. The a-C:H films deposited in a parallel-plate reactor were characterised for their thickness/deposition rate, hardness and hydrogen content. The hydrogen concentration in the films varied between 19 at.-% and 37 at.-%. With the substrate temperature held constant, it is roughly in inverse proportion to the hardness. The film with the highest hardness of 25 GPa was formed at a deposition rate of 0.8 μm/h in the C 2H 2 discharge at the lowest investigated pressure of 2 Pa. With increasing molecular mass of the precursor mostly weaker films were deposited. Relatively high values of both deposition rate and hardness were achieved using the precursor isobutene: a hardness of 21 GPa combined with a deposition rate of 4.1 μm/h. From the probed precursors, isobutene is also most advantageous for a-C:H deposition at higher pressures (up to 50 Pa investigated). But, as an over-all trend, the a-C:H hardness decreases with increasing deposition rate.

Hydrogenated amorphous carbon films deposited on 316L stainless steel

Research on hydrogen amorphous carbon films (a-C:H), which possess the diamond-like characteristic, has been stimulated for many years by need to simultaneously optimizing the mechanical, optical and biological properties, and by challenges related to the deposition of a-C:H films on medical implants. In the present work, we investigate the structure, optical and mechanical properties (hardness, elastic modulus and stress) of a-C:H films deposited on 316L stainless steel substrate by the radio frequency plasma enhanced chemical vapor deposition (RF PECVD). The negative self-bias voltages significantly influence on temperature of steel substrates during the deposition process and films properties. Specifically, the high energetic deposition leads also to stabilization of the sp 2 content and thermally-activated relaxation in the stress of a-C:H films. Presented correlation between the obtained results and literature analysis let deem the Raman spectra as a good tool to control the properties of implants made of 316L stainless steel with a-C:H film for general use.