The role of pressure on thin amorphous carbon films deposited using microwave surface wave plasma CVD

We report the effects of gas composition pressure (GCP) on the optical, structural and electrical properties of thin amorphous carbon (a-C) films grown on p-type silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition (MW SWP CVD). The films, deposited at various GCPs ranging from 50 to 110 Pa, were studied by UV/VIS/NIR spectroscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and current–voltage characteristics. The optical band gap of the a-C film was tailored to a relatively high range, 2.3–2.6 eV by manipulating GCPs from 50 to 110 Pa. Also, spin density strongly depended on the band gap of the a-C films. Raman spectra showed qualitative structured changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm). The photovoltaic measurements under light illumination (AM 1.5, 100 mW/cm²) show that short-circuit current density, open-circuit voltage, fill factor and photo-conversion efficiency of the film deposited at 50 Pa were 6.4 μA/cm², 126 mV, 0.164 and 1.4 × 10^− 4% respectively.     

Synthesis of nitrogen incorporated diamond-like carbon thin films using microwave surface-wave plasma CVD

Nitrogenated diamond-like (DLC:N) carbon thin films have been deposited by microwave surface wave plasma chemical vapor deposition on silicon and quartz substrates, using argon gas, camphor dissolved in ethyl alcohol composition and nitrogen as plasma source. The deposited DLC:N films were characterized for their chemical, optical, structural and electrical properties through X-ray photoelectron spectroscopy, UV/VIS/NIR spectroscopy, Raman spectroscopy, atomic force microscope and current–voltage characteristics. Optical band gap decreased (2.7 to 2.4 eV) with increasing Ar gas flow rate. The photovoltaic measurements of DLC:N / p-Si structure show that the open-circuit voltage (V_oc) of 168.8 mV and a short-circuit current density (J_sc) of 8.4 μA/cm² under light illumination (AM 1.5 100 mW/cm²). The energy conversion efficiency and fill factor were found to be 3.4 × 10^{− 4}% and 0.238 respectively.     

Characteristics of nitrogen doped diamond-like carbon thin films grown by microwave surface-wave plasma CVD

Nitrogen doped diamond-like carbon (DLC:N) thin films were deposited on p-type silicon (p-Si) and quartz substrates by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD) at low temperature (< 100 °C). For films deposition, argon (Ar: 200 sccm), acetylene (C₂H₂:10 sccm) and nitrogen (N: 5 sccm) were used as carrier, source and doping gases respectively. DLC:N thin films were deposited at 1000 W microwave power where as gas composition pressures were ranged from 110 Pa to 50 Pa. Analytical methods such as X-ray photoelectron spectroscopy (XPS), UV-visible spectroscopy, FTIR and Raman spectroscopy were employed to investigate the chemical, optical and structural properties of the DLC:N films respectively. The lowest optical gap of the film was found to be 1.6 eV at 50 Pa gas composition pressure.     

Etching process of hydrogenated amorphous carbon (a-C:H) thin films in a dual ECR–r.f. nitrogen plasma

Hydrogenated amorphous carbon (a-C:H) films deposited from CH₄ in a dual electron cyclotron resonance (ECR)–r.f. plasma were treated in N₂ plasma at different r.f. substrate bias voltages after deposition. The etching process of a-C:H films in N₂ plasma was observed by in situ kinetic ellipsometry, mass spectroscopy (MS), and optical emission spectroscopy (OES). Ex situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the etched film surface. XPS analysis proves that the nitrogen treatment on the a-C:H film, induced by r.f. substrate bias, causes a direct nitrogen incorporation in the film surface up to 15–17 at.% to a depth of about 20–40 Å depending on the r.f. bias. Various bonding states between carbon and nitrogen, such as tetrahedral sp³ C–N, and trigonal sp² C–N were confirmed by the deconvolution analysis of C 1s and N 1s core level spectra. The evolution of etching rate and the surface roughness in the film measured by AFM exhibit a clear dependence on the applied r.f. bias. MS and OES show the various neutral species in the N₂ plasma such as HCN, CN, and C₂N₂, which may be considered as the chemical etching products during the N₂ plasma treatment of a-C:H film.     

Internal stress of a-C:H(N) films deposited by radio frequency plasma enhanced chemical vapor deposition

Amorphous hydrogenated carbon nitride [a-C:H(N)] films were deposited from the mixture of C₂H₂ and N₂ using the radio frequency plasma enhanced chemical vapor deposition technique. The films were characterized by X-ray photon spectroscopy, infrared, and positron annihilation spectroscopy. The internal stress was measured by substrate bending method. Up to 9.09 at% N was incorporated in the films as the N₂ content in the feed gas was increased from 0 to 75%. N atoms are chemically bonded to C as C–N, CN and CN bond. Positron annihilation spectra shows that density of voids increases with the incorporation of nitrogen in the films. With rising nitrogen content the internal stress in the a-C:H(N) films decrease monotonically, and the rate of decrease in internal stress increase rapidly. The reduction of the average coordination number and the relax of films structure due to the decrease of H content and sp³/sp² ratio in the films, the incorporation of nitrogen atoms, and the increases of void density in a-C:H(N) films are the main factors that induce the reduction of internal stress.     

Cathodoluminescence and electron field emission of boron-doped a-C:N films

The optical and electrical properties of so-called carbon nitride films (a-C:N) and boron doped so-called carbon nitride films (a-C:N:B) are studied with cathodoluminescence (CL) spectroscopy and electron field emission measurement. The a-C:N films were first deposited on Si by a filtered cathodic arc plasma system, and then boron ions (∼1 × 10¹⁶ cm⁻²) were implanted into the a-C:N films to form a-C:N:B films by a medium current implanter. The structural and morphological properties of a-C:N and a-C:N:B films were then analyzed using secondary ion mass spectrometer, X-ray photoelectron spectroscopy, FT-IR spectra, Raman spectroscopy and atomic force microscopy. The a-C:N film exhibits luminescence of blue light (∼2.67 eV) and red light (∼1.91 eV), and the a-C:N:B film displays luminescence of blue light (∼2.67 eV) in CL spectra measured at 300 K. Furthermore, the incorporated boron atoms change the electron field emission property, which shows a higher turn on field for the a-C:N:B film (3.6 V/μm) than that for the a-C:N film (2.8 V/μm).     

Semiconductor properties and redox responses at a-C:N thin film electrochemical electrodes

The semiconductor capacitances of the nitrogen-doped amorphous carbon (a-C:N) materials with different sp³/sp² C ratios were studied as a function of electrode potential in a-C:N/aqueous electrolyte systems. This dependence of capacitance on electrode potential in aqueous 0.1 M NaOH shows that the investigated a-C:N materials are intrinsic semiconductors. The space-charge layers inside the a-C:N electrodes behave similar to a Helmholtz layer because of the presence of surface states when the electrolytes contain O₂ or anions other than OH⁻. The lower density and mobility of carriers of materials with a higher sp³ C fraction within the a-C:N material causes a suppression of redox reactions, and the lower density of carriers contributes to a lower capacitance.     

Improving the corrosion resistance of a-C:H film by a top a-C:H:Si:O layer

During the deposition of a-C:H film, defects (pinholes or discontinuities) caused by excessive stress will inevitably appear, which will reduce the corrosion resistance of the a-C:H film. In this study, top a-C:H:Si:O layers (thickness of approximately 0.3 μm) on the surface of a-C:H films were deposited on a large scale by PACVD technology using acetylene (C₂H₂) and/or hexamethyldisiloxane (HMDSO) as reactants, to improve the corrosion resistance of a-C:H films while ensuring the appropriate overall hardness of the films. The corrosion behaviors of the films were studied by electrochemical impedance spectroscopy (EIS) and Tafel polarization. We found that the a-C:H/a-C:H:Si:O films possess a lower electrolyte penetration rate due to their stronger capacitance characteristics. In addition, the corrosion current density of the a-C:H/a-C:H:Si:O films (10^?10 A cm^?2) were reduced by 2 orders of magnitude compared to the a-C:H film (10^?8 A cm^?2), and by 3 orders of magnitude compared to 316 stainless steel (10^?7 A cm^?2). The impedance results obtained by EIS were simulated using appropriate equivalent circuits, and the corresponding electrical parameters were used to further verify the electrochemical protection behavior of the top a-C:H:Si:O layer.     

Enhanced performance of HFCVD nanocrystalline diamond self-mated tribosystems by plasma pretreatments on silicon nitride substrates

Nanocrystalline diamond (NCD) coatings were grown by the hot-filament chemical vapour deposition (HFCVD) method on hydrogen plasma pretreated silicon nitride (Si₃N₄) substrates. The friction and wear behaviour of self-mated NCD films, submitted to unlubricated sliding and high applied loads (up to 90 N), was assessed using an oscillating ball-on-flat configuration in ambient atmosphere. The reciprocating tests revealed an initially high friction coefficient peak, associated to the starting surface roughness of NCD coatings (R_q = 50 nm). Subsequently, a steady-state regime with low friction coefficient values (0.01–0.04) sets in, related to a smoother (R_q = 17 nm) tribologically modified surface. A polishing wear mechanism governing the material loss was responsible for mild wear coefficients (k  10^− 7 mm³ N^− 1 m^− 1). The hydrogen etching procedure notably increased the film adhesion with respect to untreated surfaces as demonstrated by the high threshold loads (60 N; 3.5 GPa) prior to film delamination.     

The influence of nitrogen on the dielectric constant and surface hardness in diamond-like carbon (DLC) films

In this work a carbon target was sputtered by a methane/argon/nitrogen plasma in order to produce nitrogenated diamond-like carbon films (a-C:H:N). As the N₂ content in the sputtering gas was increased, the deposition rate increased markedly. Rutherford backscattering spectrometry (RBS) was used to investigate the chemical composition of the films. This nitrogen incorporation modifies the chemical bonding structure of the films, as shown by the analysis of the Raman spectra, including the occurrence of two extra peaks at approximately 2200 and 690 cm⁻¹. Electrical properties were measured through capacitance–voltage (C–V) curves. The hardness of the films decreased with the N content as shown by measurements performed by indentation method. A correlation among the Raman studies, the N content in the films, the dielectric constant and the surface hardness is presented.     

Fourier transform infrared spectroscopic study of nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite films prepared by pulsed laser deposition

Nitrogen-doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films, which possess n-type conduction with enhanced electrical conductivities, were prepared by pulsed laser deposition and they were structurally studied by Fourier transform infrared (FTIR) spectroscopy. The film with a nitrogen content of 7.9 at.% possessed n-type condition with an electrical conductivity of 18 S/cm at 300 K. The FTIR spectra revealed peaks due to nitrogen impurities, C = N, C-N, and CH_n (n = 1, 2, 3) bands. The sp²-CH_n/(sp²-CH_n + sp³-CH_n), estimated from the area-integration of decomposed peaks, were 24.5 and 19.4% for undoped and 7.9 at.% doped films, respectively. The nitrogen-doping not only form the chemical bonds between carbon and nitrogen atoms such as C = N and C-N bonds but also facilitate the formation of both sp² and sp³ bonds, in particular, the sp³-CH_n bond is preferentially formed. From the analysis of the FTIR spectra, it was found that the hydrogen content in the film is increased with an increase in the nitrogen content. The increased hydrogen content might be owing to the enhanced volume of grain boundaries (GBs) between UNCD grains, and those between UNCD grains and an a-C:H matrix, which is caused by a reduction in the UNCD grain size. The CH_n peaks predominantly come from an a-C:H matrix and GBs. Since the nitrogen-doping for a-C:H has been known to be hardly effective, the n-type conduction with the enhanced electrical conductivities might be attributed to the sp²-CH_n formation at the GBs.     

Carbon spheres generated in ‘dusty plasmas’

The formation of spherical hydrogenated amorphous carbon (a-C:H) particulates generated in a radio frequency plasma enhanced chemical vapour deposition (rf-PECVD) system is reported. These particulates appear as a white powder—no other contaminants appear to be present. electron energy loss spectroscopy (EELS) shows characteristics typical of a-C:H with a reduced plasmon energy due to hydrogen incorporation. Raman spectroscopy however revealed a 1456 cm⁻¹ line which was previously not reported on a-C:H films deposited using similar processes. Infrared (IR) spectroscopy shows that these spheres are mainly partially oxidised, methyl-rich, aliphatic hydrocarbons.     

Structure,electrochemical properties and functionalization of amorphous CN films deposited by femtosecond pulsed laser ablation

Amorphous carbon nitride (a-C:N) material has attracted much attention in research and development. Recently, it has become a more promising electrode material than conventional carbon based electrodes in electrochemical and biosensor applications. Nitrogen containing amorphous carbon (a-C:N) thin films have been synthesized by femtosecond pulsed laser deposition (fs-PLD) coupled with plasma assistance through Direct Current (DC) bias power supply. During the deposition process, various nitrogen pressures (0 to 10 Pa) and DC bias (0 to − 350 V) were used in order to explore a wide range of nitrogen content into the films. The structure and chemical composition of the films have been studied by using Raman spectroscopy, electron energy-loss spectroscopy (EELS) and high-resolution transmission electron microscopy (HRTEM). Increasing the nitrogen pressure or adding a DC bias induced an increase of the N content, up to 21 at.%. Nitrogen content increase induces a higher sp² character of the film. However DC bias has been found to increase the film structural disorder, which was detrimental to the electrochemical properties. Indeed the electrochemical measurements, investigated by cyclic voltammetry (CV), demonstrated that a-C:N film with moderate nitrogen content (10 at.%) exhibited the best behavior, in terms of reversibility and electron transfer kinetics. Electrochemical grafting from diazonium salts was successfully achieved on this film, with a surface coverage of covalently bonded molecules close to the dense packed monolayer of ferrocene molecules. Such a film may be a promising electrode material in electrochemical detection of electroactive pollutants on bare film, and of biopathogen molecules after surface grafting of the specific affinity receptor.     

Correlation between film structures and potential limits for hydrogen and oxygen evolutions at a-C:N film electrochemical electrodes

A correlation between film structures and the width of the potential windows defined by the dihydrogen and dioxygen evolutions in aqueous media at nitrogen-doped amorphous carbon thin film electrodes prepared using a filtered cathodic vacuum arc system is reported. A range of film structures were obtained by changing the nitrogen mass flow rate during deposition, and the film structures were determined using electron energy-loss spectroscopy. For the film electrodes, the width of the potential windows in 0.1 M NaOH and in 0.1 M H₂SO₄ at a current density of 100 μA/cm² exceed those for glassy carbon electrodes, and increase with an increase in the sp³ C fraction in the a-C:N materials. A film electrode with a rich sp² C content, has a lower electrical resistance, but still possesses a relatively large potential window. These features combined allow the materials to be tailored for particular electroanalysis.     

Residence time distributions and reaction kinetics in a fuel cell anode

A method which treats the fuel cell anode as a chemical reactor is developed to predict fuel cell performance. The method is based on experimentally measured residence time distribution parameters and differential cell kinetic data. The apparatus and experimental technique used to obtain the gas-phase residence time distributions are described. Kinetic data obtained from differential cell tests of the electrodes are used to evaluate an empirical rate expression.Axial dispersion model solutions for flow with volume change are obtained, based on the measured Peclet numbers and empirical rate expressions, and compared with experimental data from operating large high-temperature molten carbonate fuel cells. Agreement between the model and the experimentally determined data is very good, but only for low conversions of the fuel.Notation A cross-sectional area, cm² - C concentration of hydrogen. (g mole/cm³) - c=C/C _o dimensionless concentration of hydrogen - D dispersion coefficient cm²/s - d _e equivalent diameter, cm - F Faraday's constant - I total current, A - J current density, mA/cm² - k reaction rate constant, appropriate units - L length, cm - M number of moles - N =D/UL dispersion number - n order of reaction - n _e number of electrons transferred - –r rate of reaction based on volume of fluid, moles of reactant reacted/ cm³ s - S _e surface of electrode, cm² - T absolute temperature, °K - mean residence time, s - U velocity component in Z direction, cm/s - u = U/U ₀ dimensionless velocity - V _a volume of system, cm³ - V operating voltage, V - v volumetric flow rate, cm²/s - fractional conversion, degree of conversion of hydrogen - y mole fraction of hydrogen - Z space coordinate, cm - z =Z/L fractional length Greek letters coefficient of expansion - _m molar density of fuel, g mole/cm³ - overvoltage, V - dimensional variance, s² - ² dimensionless variance - =V_a/v ₀ space time, s     

Diamond-like carbon thin films by microwave surface-wave plasma CVD aimed for the application of photovoltaic solar cells

The nitrogen doped diamond-like carbon (DLC) thin films were deposited on quartz and silicon substrates by a newly developed microwave surface-wave plasma chemical vapor deposition, aiming the application of the films for photovoltaic solar cells. For film deposition, we used argon as carrier gas, nitrogen as dopant and hydrocarbon source gases, such as camphor (C₁₀H₁₆O) dissolved with ethyl alcohol (C₂H₅OH), methane (CH₄), ethylene (C₂H₄) and acetylene (C₂H₂). The optical and electrical properties of the films were studied using X-ray photoelectron spectroscopy, Nanopics 2100/NPX200 surface profiler, UV/VIS/NIR spectroscopy, atomic force microscope, electrical conductivity and solar simulator measurements. The optical band gap of the films has been lowered from 3.1 to 2.4 eV by nitrogen doping, and from 2.65 to 1.9 eV by experimenting with different hydrocarbon source gases. The nitrogen doped (flow rate: 5 sccm; atomic fraction: 5.16%) film shows semiconducting properties in dark (i.e. 8.1 × 10^{− 4} Ω^{− 1} cm^{− 1}) and under the light illumination (i.e. 9.9 × 10^{− 4} Ω^{− 1} cm^{− 1}). The surface morphology of the both undoped and nitrogen doped films are found to be very smooth (RMS roughness ≤ 0.5 nm). The preliminary investigation on photovoltaic properties of DLC (nitrogen doped)/p-Si structure show that open-circuit voltage of 223 mV and short-circuit current density of 8.3 × 10^{− 3} mA/cm². The power conversion efficiency and fill factor of this structure were found to be 3.6 × 10^{− 4}% and 17.9%, respectively. The use of DLC in photovoltaic solar cells is still in its infancy due to the complicated microstructure of carbon bondings, high defect density, low photoconductivity and difficulties in controlling conduction type. Our research work is in progress to realize cheap, reasonably high efficiency and environmental friendly DLC-based photovoltaic solar cells in the future.     

Nucleation and growth of amorphous carbon film on tungsten-implanted stainless steel substrates

Amorphous carbon (a-C) films were deposited on W-implanted (20 kV, 3 × 10¹⁷ ions cm^− 2) and un-implanted steel substrates by plasma immersion ion implantation and deposition (PIII&D). The W implantation pretreatment changes the surface structure and impacts film nucleation. Consequently, the growth mechanism of the a-C film is altered resulting in different surface morphologies and roughnesses even though the films deposited on the un-implanted steel substrates possess similar a-C structures as revealed by Raman spectroscopy. The structural differences are probed by X-ray photoelectron spectroscopy and X-ray diffraction. Moreover, microstructural observations were carried out by transmission electron microscopy. A model based on the statistical formation theory is proposed to explain the growth of the a-C films on the implanted and un-implanted substrates.     

Low-temperature thermionic emission from nitrogen-doped nanocrystalline diamond films on n-type Si grown by MPCVD

Temperature-dependent emission current–voltage measurements were carried out for nitrogen (N)-doped nanocrystalline diamond (NCD) films grown on n-type Si substrates by microwave plasma-assisted chemical vapor deposition (MP-CVD). Low threshold temperature (~ 260 °C) and low threshold electric field (~ 5 × 10^− 5 V/µm) were observed. Both the temperature dependence and the electric field dependence have shown that the obtained emission current was based on electron thermionic emission from N-doped NCD films. We have also studied the relation between nitrogen concentration and the saturation emission current. The saturation current obtained was as high as 1.4 mA at 5.6 × 10^− 3 V/µm at 670 °C when the nitrogen concentration was 2.4 × 10²⁰ cm^− 3. Low value of effective work function (1.99 eV) and relatively high value of Richardson constant (~ 70) were estimated by well fitting to Richardson–Dushman equation. The results of smaller φ and larger A′ suggest that N-doped NCD has great possibility of being a highly efficient thermionic emitter material.     

Electrochemical machining of refractory materials

The feasibility of the electrochemical machining (ECM) of pure TiC, ZrC, TiB₂ and ZrB₂ has been established. In addition, the ECM behaviour of a cemented TiC/10% Ni composite has been investigated and compared to that of its components, TiC and nickel. ECM was carried out in 2M KNO₃ and in 3 M NaCl at applied voltages of 10–31 V and current densities of 15–115 A cm^–2. Post-ECM surface studies on the TiC/Ni composite showed preferential dissolution of the TiC phase during machining.Nomenclature E ₀ thermodynamic equilibrium potential (V) - F Faraday's constant (96 500 Coul mol^–1) - toolpiece feed rate (cm s^–1 or mm min^–1) - I current (A) - i current density (A cm^–2) - k electrolyte conductivity (^–1 cm^–1) - l interelectrode gap (mm) - mass removal rate (g s^–1 or g min^–1) - M formula weight (g mol^–1) - Q electrolyte flow rate (l min^–1) - t electrolyte temperature (°C) - V applied voltage (V) - V _IR ohmic drop through electrolyte (V) - z apparent valence of dissolution (eq mol^–1) - _i overvoltages (V) - density of refractory materials (g cm^–3)     

Nitrogen incorporated diamond-like carbon films by microwave surface wave plasma CVD

Nitrogen incorporated diamond like carbon films have been deposited by microwave surface wave plasma chemical vapor deposition (MW-SWP-CVD), using methane (CH₄) as the source of carbon and with different nitrogen flow rates (N₂ / CH₄ flow ratios between 0 and 3). The influence of the nitrogen incorporation on the optical, structural properties and surface morphology of the carbon films were investigated using different spectroscopic techniques. The nitrogen has been incorporated into DLC:N films which was confirmed by the X-ray photoelectron spectroscopy (XPS) measurement. Moreover, the nitrogen incorporation was accompanied by a variation in the optical gap, which was attributed to the removal or creation of band tail states.