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).     

Low surface temperature synthesis and characterization of diamond thin films

Polycrystalline diamond films are deposited on p-type Si(100) and n-type SiC(6H) substrates at low surface deposition temperatures of 370–530 °C using a microwave plasma enhanced chemical vapor deposition (MPECVD) system. The surface temperature during deposition is monitored by an IR pyrometer capable of measuring temperature between 250 and 600 °C in a microwave environment. The lower deposition temperature is achieved by using an especially designed cooling stage. The influence of the deposition conditions on the growth rate and structure of the diamond film is investigated. A very high growth rate up to 1.3 μm/h on SiC substrate at 530 °C surface temperature is attributed to an optimized Ar-rich Ar/H₂/CH₄ gas composition, deposition pressure, and microwave power. The structure and microstructure of the films are characterized by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. A detailed stress analysis of the deposited diamond films of grain sizes between 2 and 7 μm showed a net tensile residual stress and predominantly sp³-bonded carbon in the deposited films.     

Hydrogenated amorphous carbon films deposited in an electron cyclotron resonance–chemical vapor deposition discharge reactor using acetylene

Hydrogenated amorphous carbon (a-C:H) films have been deposited from acetylene gas in a microwave electron cyclotron resonance (ECR) plasma reactor. The films were deposited at a pressure of 0.2 mTorr and at radio frequency (r.f.) induced substrate biases from 80–300 V. Selected film properties, including optical bandgap and bonded hydrogen content, were measured. At r.f. induced biases from 150 to 300 V, corresponding to ion energies for C₂H₂⁺ of approximately 150–300 eV, the hydrogen content remains constant and the optical bandgap peaks at a bias of 200 V, or approximately 100 eV per carbon in the C₂H₂⁺ ions. This ECR system result is in agreement with those observed by other researchers using different deposition methods where an optical bandgap maximum and an sp³ maximum occurs at ion energies of 90–100 eV per carbon atom. The discharge properties measured include a partial pressure analysis of the residual exit gas and the substrate current density.     

The introducing of fluorine into the deposition of BN: a successful method to obtain high-quality,thick cBN films with low residual stress

Cubic boron nitride films were synthesized on silicon substrates by DC-bias-assisted DC jet chemical vapor deposition in an Ar–N₂–BF₃–H₂ system. By this method, the deposition of cBN at high gas pressure of 50 torr became possible, and the conditions of cBN CVD approached to those of diamond CVD. cBN films with low residual stress (1–2 GPa) and with large crystal size of up to several hundred nanometers were obtained and clear Raman peaks of cBN appeared. Furthermore, the deposition rate was as high as 0.3 μm/min at the initial stage and over 20-μm-thick BN films were obtained for a 3-h deposition. These remarkable improvements are attributed to the preferential etching effect of fluorine to sp² bonds and the decrease of the bombarding energy of ions.     

Microstructure and optical band gap control of DLC film deposited by pulsed discharge plasma CVD

DLC films were deposited on silicon and quartz glass substrates by pulsed discharge plasma chemical vapor deposition (CVD), where the plasma was generated by pulsed DC discharge in H₂–CH₄ gas mixture at about 90 Torr in pressure, and the substrates were located near the plasma. The repetition frequency and duty ratio of the pulse were 800 Hz and 20%, respectively. When CH₄ / (CH₄ + H₂) ratio, i.e. methane concentration (Cm), increased from 3 to 40%, C₂ species in the plasma was increased, and corresponding to the increase of C₂, deposition rate of the film was increased from about 0.2 to 2.4 μm/h. The absorption peaks of sp³C–H and sp²C–H structures were observed in the FT-IR spectra, and the peak of sp²C–H structure was increased with increasing Cm, showing that sp² to sp³ bonding ratio was increased when Cm was increased. Corresponding to these structural changes due to the increase of Cm, optical band gap (Eg) was decreased from 3 to 0.5 eV continuously when Cm was increased from 3 to 40%.     

Diamond growth on WC-Co substrates by hot filament chemical vapor deposition: Effect of filament–substrate separation

Polycrystalline diamond films have been grown by hot filament (HF) chemical vapor deposition on WC-Co bar substrates using different CH₄/H₂ source gas mixing ratios and two different total gas pressures. Each substrate was mounted so as to span a range of HF-substrate separations, d_f, (and thus substrate temperatures) and therefore samples a spread of incident gas phase chemistry and compositions. Spatially resolved scanning electron microscopy and Raman analysis of the deposited material provides a detailed picture of the evolution of film morphology, growth rate, sp³/sp² content and stress with d_f in each deposited sample, and of how these properties vary with process conditions. The experimental study is complemented by two-dimensional model calculations of the HF-activated gas phase chemistry and composition, which succeeds in reproducing the measured growth rates well.     

Effect of deposition voltage on the microstructure of electrochemically deposited hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films were deposited on Si substrates by electrolysis in a methanol solution at ambient pressure and a low temperature (50 °C), using various deposition voltages. The influence of deposition voltage on the microstructure of the resulting films was analyzed by visible Raman spectroscopy at 514.5 nm and X-ray photoelectron spectroscopy (XPS). The contents of sp³ bonded carbon in the various films were obtained by the curve fitting technique to the C1s peak in the XPS spectra. The hardness and Young’s modulus of the a-C:H films were determined using a nanoindenter. The Raman characteristics suggest an increase of the ratio of sp³/sp² bonded carbon with increasing deposition voltage. The percentage of sp³-bonded carbon is determined as 33–55% obtained from XPS. Corresponding to the increase of sp³/sp², the hardness and Young’s modulus of the films both increase as the deposition voltage increases from 800 V to 1600 V.     

Diamond nucleation and growth on zeolites

In this work, we report the use of zeolites as substrates for the deposition of porous diamond films. Films were deposited in a hot-filament chemical vapor deposition (HFCVD) apparatus. The HFCVD system was fed with a mixture of methane (0.8%) with the balance being hydrogen. A series of depositions were done in the pressure range 20–120 Torr and at substrate temperature 880 °C. The morphologies of the as-deposited films were analyzed by scanning electron microscopy and show isolated diamond grains in the initial nucleation stages, which develop into a microporous film in the next stage and form a continuous film after long time deposition. Raman spectroscopy was used to investigate the crystal morphology, structure and non-diamond impurities in the films deposited at various growth conditions. The nature of the hydrogen bonding with sp³ and sp² network and the quantitative analysis were done by Fourier transform infrared spectroscopy.     

Low temperature growth of highly transparent nanocrystalline diamond films on quartz glass by hot filament chemical vapor deposition

Highly transparent and hard nanocrystalline diamond (NCD) films were prepared on quartz glass by hot filament chemical vapor deposition (HFCVD). The effects of total gas pressure, substrate's temperature, and concentration of CH₄ on the grain size, surface's roughness and hardness, growth rate, as well as the optical properties of NCD films were investigated. The results indicated that with a low total gas pressure and high CH₄ concentration, high frequency of secondary nucleation can be obtained. In addition, low substrate temperature can increase the rate of the hydrogen atom etched sp² graphite carbon in the film, yielding a smooth surface of NCD films and very high sp³ content. Under optimized conditions, the hardness can be enhanced up to 65 Gpa, with 80% maximum transmittance in the visible light region. The aforementioned reaction platform outcomes a 1.2 μm thickness of NCD coating with a low root-mean-square (r.m.s.) surface roughness around 12–13 nm and a high growth rate around 1 μm/h. The influences of the total gas pressure, substrate's temperature, and CH₄ concentration for growing NCD films were also discussed in this paper.     

Substrate temperature effects on the microhardness and adhesion of diamond-like thin films

Diamond-like films were deposited on silicon substrates by r.f. plasma-enhanced chemical vapor deposition from gas methane. In this study, the substrate temperature, T_S, was varied in a wide range from 20 to 370°C while maintaining fixed other important process parameters such as r.f. power (70 W) or pressure (2.5 Pa). The increase of T_S causes an increase of the sp²/(sp²+sp³) bonded carbon ratio and a decrease of the hydrogen content. These changes produce a great modification of the mechanical properties: microhardness, friction coefficient and adhesion. The variations of mechanical properties with T_S correlate well with the sp²/(sp²+sp³) bonded carbon ratio and the hydrogen content in the films showing a gradual transformation of the diamond-like structure into a more sp²-rich one.     

Plasma-assisted chemical vapor deposition process for depositing smooth diamond coatings on titanium alloys at moderate temperature

A simple process has been perfected to deposit smooth fine-grained diamond coatings at 600°C on titanium alloys or titanium-coated surfaces. It consists of a two-step microwave plasma-assisted chemical vapor deposition (PACVD) procedure including first the deposition of a sacrificial sp²-carbon containing layer from a methane-rich CH₄–H₂ mixture and then the diamond growth from a CO₂–CH₄ inlet mixture. Scanning electron microscopy, X-ray diffraction, visible and UV Raman spectroscopy show that the coatings are smooth and mainly composed of crystalline diamond with a fine-grained morphology. The results are compared with the results obtained with classical rough polycrystalline coatings deposited from 8% CO–H₂. Optical emission spectroscopy reveals important differences between the plasma species produced for the deposition of these smooth coatings and the plasma species produced for the deposition of both polycrystalline coatings from 1% CH₄–H₂ or 8% CO–H₂ mixture and nanocrystalline films from Ar–CH₄(–H₂).     

Influence of hydrogen on the residual stress in nanocrystalline diamond films

Nanocrystalline films were deposited by microwave-plasma CVD at a pressure of 200 mbar from an Ar/H₂/CH₄ plasma where the hydrogen fraction in the process gas was varied between 2 and 7%.Residual stress is a critical parameter in thin film deposition and especially important for technical applications of nanocrystalline diamond because high residual stress can lead to cracking or even to delamination of the film from the substrate. An ex-situ optical device was used to measure the residual stress of the substrate.It is shown that by controlling the process parameters the residual stress in the NCD films can be adjusted in a wide range even from compressive to tensile.The films were characterized by two wavelength scanning micro Raman spectroscopy and SEM.In this work a correlation is made between the intrinsic stress measurements and the Transpolyacetylene peaks (around 1120 cm^− 1 and 1450 cm^− 1) in the Raman spectra of NCD films. It is shown that the intensity and the FWHM of the peaks correlate with the tensile stress in the films. A model correlating the Raman spectra to the grain size and thus to the intrinsic stress measurements is given in this paper.     

Diamond-like carbon films: electron spin resonance (ESR) and Raman spectroscopy

Tetrahedral diamond-like carbon (ta-C) films and hydrogenated a-C:H films were deposited onto Si substrates using filtered cathodic vacuum arc (FCVA) process and direct ion beam deposition from CH₄/C₂H₄ plasma, respectively. Stress of deposited films was varied in the range 2.8–8.5 GPa depending on deposition conditions. Stationary and pulse electron spin resonance (ESR), and Raman spectroscopy techniques were used to analyze sp² related defects in pseudo-gap of undoped as deposited and annealed 20–100 nm thick films.¹ High density of ESR active paramagnetic centers (PC) N_s=(1.0–4.5)×10²¹ cm⁻³ at g=2.0025 was observed in the films. The dependence of ESR line width and line shape vs. deposition conditions and internal film stress were investigated. The several actual mechanisms for ESR line width broadening were considered: spin–spin dipole–dipole and exchange interactions, super-hyperfine interaction (SHFI) with ¹H (for a-C:H), averaging of SHFI due to electron jumps between PC positions with different SHFI values, and broadening due to Mott's electron hopping process. Three types of samples were revealed depending on relative contribution of these mechanisms. Effects of annealing on mechanical and paramagnetic properties of films were studied. An electrical resistance anisotropy at room temperature for ta-C films and g-value anisotropy at low temperature (T<77 K) for both ta-C and a-C:H films were found for the first time. Nature and distribution details of paramagnetic defects in DLC films, anisotropy effects and Raman spectroscopy data are discussed.     

The role of inert gas in MW-enhanced plasmas for the deposition of nanocrystalline diamond thin films

Nanocrystalline diamond thin films have been deposited using microwave plasma enhanced deposition with gas mixtures of composition H₂/CH₄/X, where X was one of the inert gases He, Ne, Ar and Kr and typically constituted > 90% of the total gas flow. The diamond films obtained with each gas mixture deposited at approximately the same rate (0.15–0.5 µm h^? 1), and all showed similar morphologies and average grain sizes, despite very obvious differences in the appearance and gas temperatures of the respective plasmas. These plasmas were probed by optical emission and cavity ring-down spectroscopy, and results from companion 2D chemical kinetic modelling of the Ar/H₂/CH₄ and He/H₂/CH₄ plasma were used to guide interpretation of the experimental observations. We conclude that the inert gas, though acting primarily as a buffer, also has significant effects on the thermal conduction of the gas mixtures, the electron temperature and electron energy distribution, and thereby changes the main channels of ionization and input power absorption. As a result, inert gas dilution elevates the electron and gas temperatures, enhances the hydrogen dissociation degree and affects the H/C mixture composition and deposition mechanisms.     

Bonding regimes of nitrogen in amorphous carbon

Nitrogen can have numerous effects on diamond-like carbon: it can dope, it can form the hypothetical superhard compound C₃N₄, or it can create fullerene-like bonding structures. We studied amorphous carbon nitrogen films deposited by a filtered cathodic vacuum arc as a function of nitrogen content, ion energy and deposition temperature. The incorporation of nitrogen from 10⁻² to 10 at% was measured by secondary ion mass spectrometry and elastic recoil detection analysis and was found to vary slightly sublinearly with N₂ partial pressure during deposition. In the doping regime from 0 to about 0.4% N, the conductivity changes while the sp³ content and optical gap remain constant. From 0.4 to ∼10% N, existing sp² sites condense into clusters and reduce the band gap. Nitrogen contents over 10% change the bonding from mainly sp³ to mainly sp². Ion energies between 20 and 250 eV do not greatly modify this behaviour. Deposition at higher temperatures causes a sudden loss of sp³ bonding above about 150°C. Raman spectroscopy and optical gap data show that existing sp² sites begin to cluster below this temperature, and the clustering continues above this temperature. This transition is found to vary only weakly with nitrogen addition, for N contents below 10%.     

Optical and electrical properties of a-C:H deposited by magnetic confinement of r.f. PECVD plasma

Amorphous hydrogenated carbon (a-C:H) thin films have been deposited in an r.f. PECVD chamber using a magnetic multipole system to confine the plasma. The influence of magnetic field on both the plasma parameters and the film properties has been studied. The results are compared with those obtained under similar conditions using a standard PECVD system. Optical emission spectroscopy (OES) shows an increase in the intensity of the hydrogen and C–H lines in the plasma. EELS, optical, electrical and electron field emission measurements have been used to characterize the deposited films. The sp³/sp² ratio was increased using the magnetic field and the optical gap was also increased as compared to films grown using a standard process. The electron field emission was found to be improved (higher current density and smaller barrier height) for samples deposited in the presence of the magnetic field.     

Boron-doped ultrananocrystalline diamond synthesized with an H-rich/Ar-lean gas system

This paper reports the recent development and applications of conductive boron-doped ultrananocrystalline diamond (BD-UNCD). The authors have determined that BD-UNCD can be synthesized with an H-rich gaseous chemistry and a high CH₄/H₂ ratio, which is opposite to previously reported methods with Ar-rich or H-rich gas compositions but utilizing very low CH₄/H₂ ratios. The BD-UNCD reported here has a resistivity as low as 0.01 ohm cm, with low roughness (<10 nm) and a wide deposition temperature range (450–850 °C). The properties of this BD-UNCD were studied systematically using resistivity characterization, scanning and transmission electron microscopy, Raman spectroscopy, and roughness measurements. Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy confirms that up to 97% of the UNCD is deposited as sp³ carbon. These various measurements also reveal additional special properties for this material, such as an “M” shape Raman signature, line-granular nano-cluster texture and high CH bond surface content. A hypothesis is provided to explain why this new deposition strategy, with H-rich/Ar-lean gas chemistry and a high CH₄/H₂ ratio, is able to produce high sp³-content and/or heavily doped UNCD. In addition, a few emerging applications of BD-UNCD in the field of atomic force microscopy, electrochemistry and biosensing are reviewed here.     

Optical and microstructural properties of diamond-like carbon films grown by pulsed laser deposition

Diamond-like carbon films have been fabricated using 308 nm excimer laser ablation in vacuum followed by deposition at temperatures between 77 K and 573 K. Optical band gap energies are obtained from UV/optical spectroscopy. Raman spectra and X-ray photoelectron spectra (XPS) show that the sp³/(sp² + sp³) ratio in these films is in excess of 0.7 in films deposited at 77 K and 300 K. This ratio decreases to 0.2 in films deposited at 573 K. It is found that films deposited at cryogenic temperatures consist of a matrix structure assembled from embedded nanometer clusters, while films deposited at 300 K or higher temperature are amorphous and atomically flat. Microstructural features in cryogenic films are discussed in relation to the mechanism of deposition and possible phase transitions during assembly of these films.     

Diamond deposition in a DC-arc Jet CVD system: investigations of the effects of nitrogen addition

Studies of the chemical vapour deposition of diamond films at growth rates >100 μm h⁻¹ with a 10-kW DC-arc jet system are described. Additions of small amounts of N₂ to the standard CH₄/H₂/Ar feedstock gas results in strong CN(B→X) emission, and quenches C₂(d→a) and H_α emissions from the plasma. Species selective, spatially resolved optical emission measurements have enabled derivation of the longitudinal and lateral variation of emitting C₂, CN radicals and H (n=3) atoms within the plasma jet. Scanning electron microscopy and laser Raman analyses indicate that N₂ additions also degrade both the growth rate and quality of the deposited diamond film; the latter technique also provides some evidence for nitrogen inclusion within the 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.