Resonant Raman studies of tetrahedral amorphous carbon films

Resonant Raman scattering has been used to study the tetrahedral amorphous carbon films deposited by the filtered cathodic vacuum arc technique. The excitation wavelengths were 244, 488, 514 and 633 nm, corresponding to photon energies of 5.08, 2.54, 2.41 and 1.96 eV, respectively. In the visible Raman spectra only vibrational modes of sp²-bonded carbon (G and D peaks) are observed, while a wide peak (called the T peak) can be observed at approximately 1100 cm⁻¹ by UV-Raman spectra which is associated with the vibrational mode of sp³-bonded carbon. Both the position and the width of the G peak decrease almost linearly with increasing excitation wavelength, which is interpreted in terms of the selective ππ* resonant Raman scattering of sp²-bonded carbon clusters with various sizes. The G peak position in the UV-Raman spectra, the T peak position and the intensity ratios of I_D/I_G and I_T/I_G all exhibit maximum or minimum values at the carbon ion energy of 100 eV. The changes of these spectral parameters are discussed and correlated with the sp³ fraction of carbon atoms in the films.     

Thickness dependency of field emission in amorphous and nanostructured carbon thin films

Thickness dependency of the field emission of amorphous and nanostructured carbon thin films has been studied. It is found that in amorphous and carbon films with nanometer-sized sp² clusters, the emission does not depend on the film thickness. This further proves that the emission happens from the surface sp² sites due to large enhancement of electric field on these sites. However, in the case of carbon films with nanocrystals of preferred orientation, the emission strongly depends on the film thickness. sp²-bonded nanocrystals have higher aspect ratio in thicker films which in turn results in higher field enhancement and hence easier electron emission.     

Ultraviolet and visible Raman analysis of thin a-C films grown by filtered cathodic arc deposition

Amorphous carbon thin films with a wide range of sp² fraction from 20 to 90% grown by filtered cathodic arc deposition have been examined by ultraviolet (UV) at 325 nm and visible Raman spectroscopy at 457 nm excitation wavelength. The comprehensive study of behaviour of G, D and T band with sp²/sp³ content has been carried out. The upwards shift of the G peak with sp³ content was observed for both excitation wavelengths. It was also found that the I(D)/I(G) ratio decreases with sp³ content for UV and visible excitations, and for high sp³ content I(D)/I(G) tends to zero. The dispersion of the G peak is also investigated in this work as a function of sp² content.     

Clustering in nanostructured carbon: Evidence of electron delocalization

The electronic properties of disordered carbon-based materials can be discussed in terms of the clustering of the sp² carbon phase and delocalization of the electron wave function. In smooth amorphous carbon thin films this results in a mixed phase material of conductive sp² clusters embedded in an electrically insulating sp³ matrix. The delocalization of the electron wave function associated with the sp² clusters is shown to play an important role in understanding many of the electronic and optical properties of the films. It is demonstrated that the extent of the electron delocalization and clustering can be estimated using magnetic resonance methods. Evidence for delocalization in a range of carbon-based materials such as diamond-like carbon thin films produced by chemical vapour deposition, nanostructured carbon produced by pulsed laser ablation and ultrananocrystalline diamond is presented.     

Infrared spectra of amorphous carbon based materials

The IR absorption in amorphous carbon and carbon nitride films is discussed in terms of a phenomenological model based on similar absorptions occurring in doped polyconjugated polymers. It is proposed that the strong IR absorption band observed in the 1800–900 cm^− 1 region arises from changes in the electronic structure in the micro-domains of the sp² clusters due to long and short range charge fluxes. These charge fluxes are enhanced by the increase in the Csp² fraction and/or the increase in the nitrogen content (with the subsequent formation of sp² and sp¹ sites) since both induce clustering of the sp² phase and delocalization of π electrons.     

Dependence of intrinsic stress and structure of ta-C film on ion energy and substrate temperature in model of the non-local thermoelastic peak

In model of the subsurface non-local thermoelastic peak (NTP) the analytical expression for the intrinsic compressive stress arising in thin coating at ion beam deposition or ion-assisted deposition is obtained. Energy dependence of compressive stress in tetrahedral amorphous carbon (ta-C) coating at C⁺ ions deposition compares with experimental data at substrate temperature 300 K and activation energy of interstitial migration. Total temperature and pressure in the NTP of C⁺ ion in ta-C matrix and its position on phase P,T-diagram of carbon depending on ion energy and substrate temperature are determined. The substrate temperature at which transition from sp³-bounded to sp²-bounded structure formation occurs is calculated depending on the ion energy. The calculation results are compared with experimental data.     

Dispersion relations and optical properties of amorphous carbons

A dispersion model based on two Tauc–Lorentz oscillators (2-TL), to describe the π–π^⁎ and σ–σ^⁎ transitions, is applied to a wide variety of amorphous carbons grown by various vapor deposition techniques. The application of identical analysis to the various samples enables the quantitative comparison between various forms of amorphous carbon. The model is applied to spectroscopic ellipsometry data and can describe accurately the optical properties of all amorphous carbons. A validation is performed based on wide-range electron energy-loss spectroscopy spectra. This approach, universal for all carbons, extends the single TL model and can determine accurately the energy position of the π–π^⁎ transition and estimate fairly the σ–σ^⁎ transitions in addition to the E_g; these parameters as well as the refractive index are correlated to the sp³ content and density of all amorphous carbons.     

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%.     

Microstructure and tribological properties of ternary BCN thin films with different carbon contents

Boron carbon nitrogen (BCN) thin films with different carbon contents are deposited on high-speed steel substrates by reactive magnetron sputtering (RMS) and their microstructure and tribological properties are studied. The BCN films with carbon contents from 26.9 wt.% to 61.3 wt.% have an amorphous structure with variable amounts of carbon bonds (sp²C–C, sp²C–N and sp³C–N bonds). A higher carbon content enhances the film hardness but reduces the friction coefficient against GCr15 steel balls in air. BCN films with higher hardness, lower friction coefficient, and better wear resistance can be obtained by increasing the carbon content.     

Structural dependence of corrosion resistance of amorphous carbon films against nitric acid

To investigate the structural dependence of the corrosion resistance of amorphous carbon (a-C:H) films, three different types of a-C:H films etched by nitric acid were evaluated using a surface plasmon resonance (SPR) device with a multilayer structure consisting of an a-C:H layer on Ag. Two non-hydrogenated amorphous carbon (a-C) films and one hydrogenated a-C:H film were synthesized to estimate the effects of the sp²/sp³ ratio and hydrogenation, respectively. A flow cell for the introduction of nitric acid solution was placed on the amorphous carbon layer of the multilayer structure. A 0.3 mM nitric acid solution was used in the etching tests. The Kretschmann configuration was used for SPR measurement, and the SPR angle was determined as the angle with minimum reflectivity. The SPR angle decreased with increasing duration of nitric acid injection into the flow cell, indicating that the film was corroded by the nitric acid. The thickness of the films was calculated from the SPR angle. The rates of decrease in the thickness were 2.2, 0.8, and 1.6 nm/h for the a-C films with lower and higher sp² contents and the 17 at.% hydrogenated a-C:H film, respectively. Although the hydrogen content had little effect on the rate of change in the film thickness, the film thickness clearly decreased with decreasing sp²/sp³ ratio. These results indicate that the sp²/sp³ ratio is an important factor determining the chemical resistance to nitric acid solution.     

The effect of CNT content on the surface and mechanical properties of CNTs doped diamond like carbon films

The carbon nanotubes (CNTs) doped diamond like carbon films were carried out by spinning coating multi-walled carbon nanotubes (CNTs) on silicon covered with diamond like carbon films via PECVD with C₂H₂ and H₂. The results show that the I_D/I_G and sp²/sp³ ratios are proportional to the CNT contents. For wettability and hydrogen content, the increase of CNT content results in more hydrophobic and less hydrogen for CNT doped DLC films. As for mechanical properties, the hardness and elastic modulus increases linearly with increasing CNT content. The residual stress is reduced for increasing CNT content. As for the surface property, the friction coefficient is reduced for higher CNT content. For CNT doped DLC films, the inclusion of horizontal CNT into DLC films increases the hardness, elastic modulus and reduces the hydrogen content, friction coefficient and residual stress. Like the light element and metal doping, the CNT doping has effects on the surface and mechanical properties on DLC which might be useful to specific application.     

Hard and sp2-rich amorphous carbon structure formed by ion beam irradiation of fullerene,a-C and polymeric a-C:H films

In this work, we present a comparative study of the ion irradiation effect on the mechanical and optical properties of fullerene, amorphous carbon (a-C) and polymeric hydrogenated amorphous carbon (a-C:H) films, irradiated with N ions at 400 keV in the fluence range from 10¹³ to 3×10¹⁶ N cm⁻². Modifications in the carbon structure, as function of the irradiation fluence, were investigated using the Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), Fourier transform infrared (FTIR), Raman spectroscopy, UV–Vis–near infrared (NIR) spectrophotometry and nanoindentation techniques. After high fluence, the three carbon samples are transformed into very similar hard (≈14 GPa) and non-hydrogenated amorphous carbon layers with very low optical gap (≈0.2 eV) and an unusual sp²-rich bonded atomic network. The mechanical properties of the irradiated films correlated with the bonding topologies of this new sp² carbon phase are investigated through the constraint-counting model. The results show that the structural modifications and the unusual rigidity were achieved by the distortion of the sp² carbon bond angles, giving origin to a constrained three-dimensional sp² carbon bonded network.     

Enhancing the electrical conduction in amorphous carbon and prospects for device applications

The problems and possible solutions associated with producing practical electronic devices based upon amorphous carbon (a-C) thin films are discussed. The clustering of the carbon sp² phase is shown to be a critical aspect in understanding the current device limitations. In order to exploit the sp² clustering we show that the use of ion beams to deposit energy into the microstructure in a controlled manner, as opposed to conventional thermal anneal treatments, results in a delocalised electron wavefunction and an enhancement of conductivity. A barrier controlled device is demonstrated by carefully choosing the ion energy and dose. One of the consequences of ion implantation is that film can now be considered as consisting of conductive sp² C clusters within an insulating sp³ C matrix. We show that the presence of this dielectric inhomogeneity between the conductive sp² regions and the sp³ matrix plays an important role in understanding the field emission behaviour from a-C based materials.     

Sp content in ta-C films vs pulse bias width to the substrate: A correlative structural analysis

Tetrahedral amorphous carbon (ta-C) films were studied with and without applying fixed pulse bias and frequency at variable pulse widths in double bent Filtered Cathodic Vacuum Arc (FCVA) system. Both from Raman and X-ray photoelectron spectroscopy (XPS) analyses it has been observed that the ratio of sp³/sp² is maximal at pulse width of 15 µs with fixed pulse bias 3 kV and frequency 200 Hz. Increasing or decreasing pulse width from this threshold value accompanies the decreasing sp³ content in the film. It is also observed that with applying pulse bias width at said frequency and bias voltage G peak position was shifted to lower values and after reaching a minimum at 15 µs G peak position shifted to higher wave numbers. At the 15 µs pulse width, 3 kV bias voltage and 200 Hz frequency we have formed ta-C films with maximum sp³ content. This study clearly suggests that it is possible to tune the ta-C film's most important properties such as percentage of sp³ content, internal stress, and hardness by applying pulse width at particular frequency and bias voltage.     

Spatial organization of the sp-hybridized carbon atoms and electronic density of states of hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films prepared by plasma decomposition of hydrocarbons exhibit a wide variety of electronic and mechanical properties depending on their deposition conditions, which makes them very interesting for applications in several domains. This versatility is essentially due to the presence of both sp²- and sp³-hybridized carbon atoms in variable proportions, and to the tendency of the sp² C atoms to gather into π-bonded clusters with different bonding configurations. The relationships between the film microstructure and their electronic density of states, as deduced from a detailed analysis of their optical properties over a large spectral range, are described and discussed, taking as reference materials the purely sp² (graphite) and purely sp³ (diamond) carbon crystalline phases, as well as the prototype hydrogenated amorphous tetra-coordinated semiconductor, hydrogenated amorphous silicon. It is shown that the type of clustering of the sp² C atoms is certainly more determinant for the electronic density of states, and especially for the optical gap value, than the proportion of these atoms in the material.     

Deposition of iron containing amorphous carbon films by filtered cathodic vacuum arc technique

Iron containing amorphous carbon (a-C:Fe) films have been deposited with an Fe/graphite composite target with different Fe content by filtered cathodic vacuum arc (FCVA) technique. X-Ray induced photoelectron spectroscopy (XPS) was used to analyze the Fe content in the films. Micro-Raman spectroscopy was employed to characterize the structural changes of a-C:Fe films. The properties of the a-C:Fe films such as the intrinsic stress, morphology and roughness investigated by the profiler, atomic force microscope (AFM). The XPS results show that there exists small amount of oxygen in the form of FeO in the films and the Fe fraction in the films is always larger than that in the target. Compared with pure amorphous carbon films the intrinsic stress was effectively reduced by incorporating Fe into the films, and decreases with increasing Fe content. As increasing the Fe content, the clusters in the films become finer and the roughness increases The studies of Raman spectra show that the positions of G peak and D peak shift to low and high wavenumbers, respectively, and the ratio of the intensity of D and G peaks increases with an increase in Fe content, that suggests that the sp²-bonded carbon and the size of the sp²-bonded cluster increases with an increase in the Fe content.     

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.     

Electronic and bonding properties of Fe- and Ni based hydrogenated amorphous carbon thin films by X-ray absorption, valence-band photoemission and Raman spectroscopy

Electronic and bonding properties of Me-based hydrogenated amorphous carbon (a-CH:Me, Me = Fe, Ni) thin films have been studied by X-ray absorption near-edge structure (XANES), valence-band photoemission (VB-PES) and Raman spectroscopy. Raman and XANES results show enhancement of the content of sp³-rich diamond-like carbon (DLC) by doping with Fe and Ni. The VB-PES spectrum of a-CH:Fe shows emergence of a prominent feature due to states of sp³-bonded clusters, indicating that a-CH:Fe induced enhancement of DLC structure. The nano-indentation measurement reveals that a-CH:Fe has a greatly enhanced hardness, while electrical resistance measurement shows that a-CH:Me reduces resistivity.     

Effect of acrylic acid additive on electric conductivity of polymer-derived amorphous silicon carbonitride

The effect of acrylic acid additive on the electric conductivity of amorphous SiCN derived from polymeric precursor was studied. The conductivity showed to follow the Arrhenius dependence on pyrolysis temperature, but with much smaller activation energy, as compared to the unmodified SiCN. Structural analysis using Raman and XPS revealed that the size of the free-carbon clusters within the AC-modified SiCN changed with pyrolysis temperature, but the sp²-to-sp³ ratio remained almost the same. The reason for the effect of AC on the carbon cluster was speculated. The mechanisms governing the conductivity behavior of the AC-modified SiCN were discussed.     

Rearrangements of sp/sp hybridized bonding with phosphorus incorporation in pulsed laser deposited semiconducting carbon films by X-ray photoelectron spectroscopic analysis

The carbon films were grown on p-type silicon substrate at room temperature by pulsed (XeCl) laser deposition technique using camphoric carbon target containing 1%, 3%, 5% and 7% of phosphorus (P) by mass. The analysis of X-ray photoelectron spectroscopy spectra of the C1s region in these films shows the presence of sp² and sp³ hybridized carbon and a sp² satellite peak due to π–π shake up. The sp² content is seen to remain almost constant with P content. The FWHM of the sp² peak increases up to 5% P but decreases for 7% P probably due to clustering of sp² chains and this clustering in the sp² phase probably decreases the band gap for the 7% P film. With P incorporation, the tetrahedral bonding configurations of the carbon network do not change appreciably, therefore, suggesting the scope of phosphorus as a potential dopant in carbon films.