Metal-containing amorphous carbon film development using electron cyclotron resonance CVD

A technique for depositing metal–carbon (Me-C:H) thin films is demonstrated based on two metal screen grids embedded within an electron cyclotron resonance chemical vapour deposition (ECR-CVD) system. The grids are negatively biased and supported at adjustable distances above the substrate holder in the deposition chamber. With source gases of methane and argon, sputtering of the metal grids by Ar⁺ results in the incorporation of metal in the growing carbon films. The amount of metal in the films can be very well controlled over a wide range by varying the bias voltage at the grids, the separation of the grids from the substrate holder and the ratio of CH₄/Ar. Furthermore, by separately biasing the substrate holder, the properties of the films can be varied resulting in the formation of a great variety of Me-C:H films with very different mechanical and structural properties. Tungsten (W-C:H) and molybdenum (Mo-C:H) incorporated carbon films were deposited using this technique, with the metal fractions controlled by varying the flow ratio of CH₄/Ar and the bias at the substrates. The films were characterised using Rutherford backscattering, X-ray diffraction and Raman scattering measurements, and also in terms of their conductivity, optical absorption and hardness. Large changes are observed in the conductivity and optical gap of the films even at low fraction of metal incorporated. Metal carbides formation was observed for films deposited under bias. The results suggest that the substrate bias has a crucial effect on the incorporation of metal into the a-C:H films and their resulting microstructures.     

Molybdenum-containing carbon films deposited using the screen grid technique in an electron cyclotron resonance chemical vapor deposition system

A novel technique involving the incorporation of two molybdenum (Mo) screen grids embedded in an electron cyclotron resonance chemical vapor deposition (ECR–CVD) system is presented in this paper. A comprehensive set of film deposition experiments based on this screen grid sputtering technique has been carried out. The Mo-containing carbon films were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). The film resistivity, optical bandgap and hardness were evaluated as a function of the gas flow ratio (CH₄/Ar). XPS analysis showed that the fraction of Mo incorporated in the carbon film decreased drastically from 15.11 to 0.32% following an increase in the CH₄/Ar flow ratio. The optical absorption also decreases strongly and the film with the lowest Mo fraction has a bandgap of 2.0 eV. The film resistivity was found to increase by 11 orders of magnitude following the decrease in the metal fraction. It is found that Mo can exist in the forms of MoC, Mo₂C, Mo, and even MoO₃ in the films, the last being mainly due to air exposure. The results showed that our ECR-based screen grid technique for Me-C:H deposition is highly effective and flexible with good control over the amount of metal incorporated.     

Effects of nitrogen addition on morphology and mechanical property of DC arc plasma jet CVD diamond films

Nitrogen-doped diamond films have been synthesized by 100 KW DC arc plasma jet chemical vapor deposition using a CH₄/Ar/H₂ gas mixture. The effect of nitrogen addition into the feed gases on the growth and surface morphology and mechanical property of diamond film was investigated. The reactant gas composition was determined by the gas flow rates. At a constant flow rate of hydrogen (5000 sccm) and methane (100 sccm), the nitrogen to carbon ratio (N/C) were varied from 0.06 to 0.68. The films were grown under a constant pressure (4 KPa) and a constant substrate temperature (1073 K). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy and X-ray diffraction. The fracture strength of diamond films was tested by three point bending method. The results have shown that nitrogen addition to CH₄/H₂/Ar mixtures had led to a significant change of film morphology, growth rate, crystalline orientation, nucleation density and fracture strength for free-standing diamond films prepared by DC arc plasma jet.     

Boron doped amorphous carbon thin films grown by r.f. PECVD under different partial pressure

Boron doped hydrogenated amorphous carbon (a-C) thin films have been deposited by r.f.-plasma CVD with a frequency of 13.56 MHz at room temperature using pure methane as a precursor of carbon source mixed with hydrogen (H₂) as a carrier gas. The films were prepared by varying the r.f. power, different flow rates of CH₄, and partial pressure of mixed gas (CH₄/H₂) using solid boron as a target. The thickness, structural, bonding and optical properties of the as-deposited films were studied by Alpha step surface profiler, Raman, FT-IR, XPS and UV–visible spectroscopy. It was found that changing the deposition pressure in presence of solid boron dopant in the r.f. PECVD process has a profound effect on the properties of the deposited films, as evidenced from their Raman scattering and optical results. The grown p-C: B films were found very smooth and thickness in the range of 240 to 360 nm for 1 h deposition. Films deposited at lower pressure appear brownish color whereas those deposited at higher pressure appear pale yellowish. The as-deposited film is found to be dominated by sp² rather than sp³, which might be due to the formation of small crystallites. The optical band gap is found to be reduced from 2.601.58 eV as the partial pressure of CH₄/H₂ gas is reduced.     

Dielectric and optical properties of nitrogen-incorporated polyethylene films fabricated by plasma polymerization

The dielectric and optical properties of nitrogen-incorporated polyethylene films fabricated by radio-frequency glow discharge in ethylene-nitrogen (C₂H₄ ? N₂) gas mixtures have been measured for various nitrogen concentrations. The results show that the plasma-polymerized films have distinct properties from the ones based on conventional chemical polymerization. The incorporation of nitrogen in polyethylene results in an increase in breakdown strength, dissipation factor, and dielectric constant at frequencies below the optical frequency range, and in a decrease in dark conductivity, and in photoconductivity and extinction coefficient in the optical frequency range. However, the incorporation of nitrogen does not cause significant change in refractive index. The effects of the incorporation of nitrogen in polyethylene are attributed to the shallow acceptor-like traps introduced by incorporated nitrogen.     

Bonding defects and optical band gaps of DLC films deposited by microwave surface-wave plasma CVD

The effects of CH₄ / C₂H₄ flow ratio and annealing temperature on the defect states and optical properties of diamond-like carbon (DLC) films deposited by novel microwave surface-wave plasma chemical vapour deposition (MW SWP CVD) are studied through UV/VIS/NIR measurements, atomic force microscopy, Raman spectroscopy and electron spin resonance analysis. The optical band gap of DLC has been tailored between a relatively narrow range, 2.65–2.5 eV by manipulating CH₄ / C₂H₄ flow ratio and a wide range, 2.5–0.95 by thermal annealing. The ESR spin density varied between 10¹⁹ to 10¹⁷ spins/cm³ depending on the CH₄ / C₂H₄ flow ratio (1 : 3 to 3 : 1). The defect density increased with increasing annealing temperature. Also, there is a strong dependence of spin density on the optical band gap of the annealed-DLC films, and this dependency has been qualitatively understood from Raman spectra of the films as a result of structural 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).     

Formation of ultrananocrystalline diamond films with nitrogen addition

Nitrogen-doped ultrananocrystalline diamond (UNCD) films have been prepared by the microwave plasma jet chemical vapor deposition system (MPJCVD) using a gas mixture of Ar-1%CH₄-10%H₂ and addition of 0.5-7% nitrogen. This growth process by MPJCVD with 10% hydrogen addition that yields UNCD films compared with those UNCD films produced by MPCVD with a high Ar/CH₄ ratio due to the focused microwave plasma jet greatly enhanced the enough dissociation of react gases and formed C₂ species with an energetic state at lower argon concentration. The surface morphologies were changed drastically from continuous to rough granular surface with increasing the nitrogen content due to the great rise of CN species in the plasma. The width of grain boundaries composed of sp²-bonded carbon increased with increasing nitrogen content in the films. Moreover, the seldom defects in the UNCD films induced by the addition of nitrogen in the plasma were identified and investigated by using a scanning transmission electron microscope (STEM). The highest nitrogen-doped benefit with a N/C atomic ratio of 3.25% in UNCD films was reached by addition of only 3% N₂ in plasma (Ar-1%CH₄-10%H₂-3%N₂), showing the MPJCVD can greatly reduce the used amount of nitrogen in the synthesis of nitrogen-doped UNCD films.     

Atomic bonds in boron carbon nitride films synthesized by remote plasma-assisted chemical vapor deposition

Boron carbon nitride (BCN) films are synthesized by remote plasma-assisted chemical vapor deposition (RPCVD) method. The present experimental apparatus is featured by introducing BCl₃ gas near the substrate without mixing to plasma consisting of N₂ and CH₄ gases. Two sample groups of the BCN films are prepared. One is grown with various CH₄ flow rates, and another is grown with various BCl₃ flow rates. The composition ratio of the constituent atoms, atomic bonds and optical bandgap are investigated. C composition ratio of the BCN film increases with increasing CH₄ flow rate, leading to a reduction in the optical bandgap with increasing C composition ratio. On the other hand, it is found that no significant variation in the composition ratio occurs for the BCN films grown with various BCl₃ flow rates and that the optical bandgap decreases with increasing BCl₃ flow rate. This behavior of the optical bandgap is related to a change of the atomic bonds in the BCN film grown with various BCl₃ flow rates.     

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.     

A novel “coral” carbon microprobe with and without N2 incorporation

“Coral”-type microstructure carbon films, with and without N₂ incorporation, were grown on sharpened tungsten microprobes by plasma enhanced chemical vapor deposition (PECVD) using H₂/CH₄/N₂ and H₂/CH₄ gas mixtures, respectively. The electrochemical behaviors of the coral-type carbon coated tungsten microprobe, characterized by various concentrations of ferrocyanide in a background of 0.1 M KCl, show excellent structural stability with similar microstructure before and after prolonged analysis without the need of surface pretreatment. The microprobes exhibit quasi-reversible kinetics with high signal-to-noise S/B ratio. The N₂ incorporated microprobe shows a slightly wider potential window, no surface adsorption of the analyte and higher sensitivity as compared to the sample without nitrogen incorporation. Furthermore, the wide potential window of  3 V is very good as compared to boron-doped diamond electrodes which are  3.5 V. This well behaved; broad electrochemical behavior and the simple fabrication method make the “coral” carbon film microprobe an excellent candidate for electrochemical sensing.     

Surface and optical analysis of SiCx films prepared by RF-RMS technique

Silicon Carbide thin films have been prepared by RF reactive magnetron sputtering of a silicon target in a mixture of Ar and CH₄. Surface analysis was performed by X-ray photoelectron spectroscopy (XPS) to examine the elemental bonding at the surface and in bulk of the material. Optical analysis was carried out by ellipsometry to study the optical constants (n and k) and band gap of the films. Transmission and scanning electron microscopy, FTIR and X-ray diffraction, were employed to supplement our results. The near surface of SiC exposed to atmosphere was primarily composed of SiO₂ along with amorphous carbon while the bulk of the material was SiC. At higher plasma power and lower CH₄ concentration, the graphitic phase in the surface decreases and the refractive index increases while surface oxide layer remains present.     

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.     

The Synthesis and Photocatalytic Properties of Nitrogen Doped TiO<Subscript>2</Subscript> Films Prepared Using the AC-PLD Method

Synthesis of N doped TiO₂ films were conducted by the atmospheric controlled pulsed laser deposition (AC-PLD) method to generate visible light active photocatalytic films. In this method, the anion doped TiO₂ films were synthesized on a quartz substrate by the irradiation of a pulsed Nd:YAG laser on a TiO₂ target in the presence of gaseous nitrogen containing reagents at reduced pressure. For nitrogen doping, the use of CH₃CN was found to be more effective than the use of NH₃. The visible light absorption properties of the films were very sensitive to the CH₃CN partial pressure during ablation. When using CH₃CN, nitrogen and an equal quantity of carbon was uniformly doped into the TiO₂ films. The resultant films showed better catalytic performance than those which were either un-doped or doped using NH₃. The formation of nitrogen doped TiO₂ is discussed by relating experimental results to thermodynamic considerations. It is also suggested that stronger reducing agents such as carbon are required for doping nitrogen into TiO₂ films.     

Tribological properties of DLC films with different hydrogen contents in water environment

Diamond-like carbon (DLC) films were deposited on silicon wafers by thermal electron excited chemical vapor deposition (CVD). To change the hydrogen content in film, we used three types of carbon source gas (C₇H₈, CH₄, and a CH₄+H₂) and two substrate bias voltages. The hydrogen content in DLC films was analyzed using elastic recoil detection analysis (ERDA). Tribological tests were conducted using a ball-on-plate reciprocating friction tester. The friction surface morphology of DLC films and mating balls was observed using optical microscopy and laser Raman spectroscopy.Hydrogen content in DLC films ranged from 25 to 45 at.%. In a water environment, the friction coefficient and specific wear rate of DLC films were 0.07 and in the range of 10⁻⁸–10⁻⁹ mm³/Nm, respectively. The friction coefficient and specific wear rate of DLC film in water were hardly affected by hydrogen content. The specific wear rate of DLC film with higher hardness was lower than that of film with low hardness. Mating ball wear was negligible and the friction surface features on the mating ball differed clearly between water and air environments, i.e., the friction surface on mating balls in water was covered with more transferred material than that in air.     

Formation and characteristics of undoped and doped tetrahedral amorphous carbon films

Synthesis of undoped and doped tetrahedral amorphous carbon (ta-C) films has been achieved using magnetic field filtered plasma stream system in an ambient gas of pure Ar and Ar with N₂, respectively. The optical and electrical properties of these films as a function of the substrate bias voltages (V_b) or nitrogen partial pressures (P_N) have been studied using UV-visible optical absorption spectroscopy, Fourier-transform infra-red spectroscopy (FTIR) and measurements of electrical conductivity. The results show that ta-C films with a high sp³ fraction were formed when the V_b was in the range of −10 to −50 V. The optical band gap of such ta-C films was found to be larger than 3 eV. The incorporation of nitrogen into the ta-C films deposited at low P_N (P_N<25%), results in a slight drop in activation energy, which indicates that there is evidently some doping effect of nitrogen. The configurations of N atoms in ta-C network are identified and discussed.     

The use of aliphatic alcohol chain length to control the nitrogen type and content in nitrogen doped carbon nanotubes

Nitrogen doped carbon nanotubes (N-CNTs) were synthesized using acetonitrile/alcohol mixtures as the nitrogen, carbon and oxygen sources using the chemical vapor deposition (CVD) method. XPS analysis of the CNTs produced from an acetonitrile/ethanol mixture using different CVD temperatures (700–1000 °C), revealed that nitrogen incorporation in N-CNTs decreased with an increase in CVD temperature and that the type of nitrogen species incorporated also varied. Molecular nitrogen and a low content of pyridinic nitrogen was obtained in N-CNTs grown at 700 and 800 °C, while quaternary nitrogen was noted in all N-CNTs grown. Use of 20% acetonitrile/ROH (R = CH₃, C₂H₅, C₄H₉, C₅H₁₁, C₇H₁₅ and C₈H₁₇) mixtures allowed the C/O ratio to be changed whilst the N content in the precursor mixture was kept constant. The N content in the N-CNTs grown at 850 °C increased with the alcohol chain length and also controlled the nitrogen species incorporated, an effect related to the oxygen content of the reactant mixtures.     

Selective synthesis of diamond and CNT nanostructures directly on stainless steel substrates

Without surface pretreatment or applying additional interlayer, diamond films have been directly synthesized on an Fe-25Cr-5Al steel substrate by a hot filament chemical vapor deposition method from an H₂-1vol.% CH₄ gas mixture. Due to an effective removal of intermediate graphite phase from the diamond-steel interface, the coated diamond films were continuous and adherent well to the steel substrate. Aligned conical diamond structures were also achieved on this steel substrate by negatively biasing the substrate holder and inducing a glow discharge. The deposition behavior of carbon on Fe-Cr-Ni steel substrate was different. A graphite-rich carbon film incorporated with diamond particles grew in the absence of biasing, then aligned carbon nanotube bundles were formed in the presence of negative biasing and glow discharge. The different deposition behavior of carbon on the two kinds of steel substrates was addressed in terms of the effect of their chemical compositions.     

Study the effect of O2 addition on hydrogen incorporation in CVD diamond

The effect of a small amount of O₂ addition on film quality and hydrogen incorporation in chemical vapour deposition (CVD) diamond films was investigated and the films were grown using a 5-kW microwave plasma CVD reactor. Film quality and bonded hydrogen were characterized using micro-Raman and Fourier transform infrared (FTIR) spectroscopy, respectively. It was found that in general for films grown using CH₄/H₂ plasma both without and with O₂ addition, the hydrogen incorporation increases with increasing substrate temperature, while a small amount of O₂ addition (O₂/CH₄=0.1) into CH₄/H₂ (4%) plasma strongly suppresses the incorporation of hydrogen into the film. Raman spectra show that the added oxygen improved film quality by etching and suppressing the amorphous carbon component formed in the film. The above effect of oxygen addition on hydrogen incorporation and film quality is discussed according to the growth mechanism of CVD diamond. The CVD diamond specific hydrogen related IR vibration at 2828 cm⁻¹ appears as a sharp and strong peak only in the FTIR spectra of poor quality films grown at high temperature both without and with O₂ addition, but it appears much stronger in the film grown without O₂ addition. This result experimentally excludes the assignment of the 2828 cm⁻¹ peak arises from hydrogen bonded to oxygen related defect in the literature.     

Structures and luminescence properties of polymer-like a-CNx:H films

Polymer-like hydrogenated amorphous carbon nitride (a-CNx:H) films were prepared in CH₄/N₂ r.f. plasma with different CH₄/N₂ mixing ratio and r.f. power input, and the structural and luminescence properties of these films were examined. Both optical band gap and PL peak energies of the a-CNx:H films were shifted with a variation of nitrogen content in the film. The theoretical PL efficiency described well the experimental data, and it was suggested that the PL of the a-CNx:H films arise from tail-to-tail electron-hole recombination in the localized π and π* states. From the FT-IR measurements, it was found that the films with lower nitrogen content included a large amount of sp³C–H bond, while in the film with higher nitrogen content, N–H, C=N and CN bonds were predominantly formed. From the EL devices fabricated using both a-CNx:H film and thin organic layers, EL spectra with a broad band of 350–700 nm were observed. Both EL and PL from the a-CNx:H film indicated almost the same behavior, and the components observed in the PL spectrum of each organic material were not included in the EL spectrum.     

Electrical contacts to nitrogen incorporated nanocrystalline diamond films

The effect of surface plasma treatment on the nature of the electrical contact to the nitrogen incorporated nanocrystalline diamond (n-NCD) films is reported. Nitrogen incorporated NCD films were grown in a microwave plasma enhanced chemical vapor deposition (MPECVD) reactor using CH₄ (1%)/N₂ (20%)/Ar (79%) gas chemistry. Raman spectra of the films showed features at ∼ 1140 cm^− 1, 1350 cm^− 1(D-band) and 1560 cm^− 1(G-band) respectively with changes in the bonding configuration of G-band after the plasma treatment. Electrical contacts to both untreated and surface plasma treated films are formed by sputtering and patterning Ti/Au metal electrodes. Ohmic nature of these contacts on the untreated films has changed to non-ohmic type after the hydrogen plasma treatment. The linear current–voltage characteristics could not be obtained even after annealing the contacts. The nature of the electrical contacts to these films depends on the surface conditions and the presence of defects and sp² carbon.