Relations between the deposition conditions,the microstructure and the defects in PECVD hydrogenated amorphous carbon films; influence on the electronic density of states

The relationships between the deposition conditions, the growth mechanisms, the microstructure and the electronic density of states of hydrogenated amorphous carbon (a-C:H) films prepared by PECVD from hydrocarbons are not yet fully understood. We therefore performed a systematic study using several complementary techniques to determine the changes in the microstructure and in the optical properties of a-C:H samples deposited from a dual micro-wave/radio-frequency discharge in methane as a function of the negative r.f. bias voltage applied to the substrate −V_b. The results reveal the existence of two successive film growth regimes when varying −V_b from −30 to −600 V, i.e. when increasing the ion bombardment during deposition, which lead to different types of sp² C atoms clustering and H incorporation, but to the same density of paramagnetic defects. Models of the a-C:H microstructure are proposed in each case, and their influence on the electronic states density is analysed in detail. It is shown that the H content and the proportion of sp² C atoms play a minor role on the electronic properties, as compared to the nature, the size, the number and probably the distortions of the π-bonded clusters.     

Effects of short-range order on the electronic structure and optical properties of amorphous carbon

We have calculated the joint density of states and absorption coefficient of amorphous carbon, a-C:H, treating it as a ternary alloy of sp² carbon, sp³ carbon and hydrogen. These optical properties have been calculated in terms of the above compositions and a short-range order parameter determining the degree of clustering of the sp² (or sp³) carbon atoms. Our calculations show that this parameter has a significant effect on the Tauc gap, which seems to be considerably smaller than the mobility gap. By comparing, however, our theoretical values of the Tauc gap with experiments we deduce that the degree of clustering found in most real samples is very small.     

Fluorination of carbon blacks: an X-ray photoelectron spectroscopy study: IV. Reactivity of different carbon blacks in CF4 radiofrequency plasma

The effect of CF₄-plasma enhanced fluorination on the surface modification of carbon blacks has been examined using XPS. Three different types of carbon blacks have been studied: a thermal black, a furnace black and a high electrical conducting black. The analysis of the XPS spectra of fluorinated carbon black samples indicates that all fluorine atoms, fixed at the surface and in a subsuperficial zone of the particles, are covalently linked to carbon atoms. The influence of the physicochemical properties and morphology of these three types of carbon blacks on the fluorination reaction has also been investigated. The proportion of different types of fluorinated carbon atoms, i.e. on one hand CF_x surface and border groups of graphitic bulk domains for which the planar configuration of the graphene layers is preserved together with the sp² character of C, i.e. structures of type I, on the other hand polyalicyclic perfluorinated structures in which sp³C form puckered layers similar to those of covalent fluorographites, i.e. structures of type II, and also the F/C ratio of the fluorinated groups are related to the surface morphology and depend on the microstructural organization of particles. When the microstructure ordering and graphitic character of the carbon increase, the size of the ordered graphitic domain also increases. At the same time the density, the size of defects and proportion of protonated sp³C entities bridging the graphene layers decrease. As a consequence, the proportion of carbon atoms, potentially able to form perfluorinated CF₂ and CF₃ groups, decreases. The relative contribution of those groups is appreciably higher in fluorinated compounds which are derived from carbon blacks with a lower structural order.     

A correlation between the microstructure and optical properties of hydrogenated amorphous carbon films prepared by RF magnetron sputtering

Comparative and systematic studies of the effect of the radiofrequency (RF) bias on the microstructure and the optical properties of hydrogenated amorphous carbon (a-C:H) have been carried out on films deposited by RF magnetron sputtering under different RF power varying from 10 to 250 W applied to the graphite target, leading to a negative bias voltage at the target in the range of −60 to −600 V.A combination of infrared (IR) absorption experiments, which give information about the local microstructure (i.e. C–C and C–H bonding), and optical transmission measurements in the UV-visible and near IR, from which we determined the optical gap E₀₄ and the refractive index n, are applied to fully characterize the samples in their as-deposited state. The results show first that the films deposited at low RF power (i.e. low negative bias) exhibit a more open microstructure (polymeric character) with a lower density than those deposited at high RF power (i.e. high negative bias). They also indicate that the total bonded H content as well as the sp³/sp² ratio of carbon atoms bonded to H decrease with increasing RF power leading to the formation of higher proportions of C-sp² sites. The same tendency is observed for the optical gap E₀₄. On the contrary, the refractive index increases with increasing RF power, suggesting the densification of the films in going to a higher RF power.     

Structural investigation and gas barrier performance of diamond-like carbon based films on polymer substrates

In this report, tetrahedral amorphous carbon (ta-C), hydrogenated amorphous carbon (a-C:H), silicon doped tetrahedral amorphous carbon (ta-C:Si:H), and silicon doped hydrogenated amorphous carbon (a-C:H:Si) films with thickness in the range 50-370 nm have been produced by PECVD (Plasma Enhanced Chemical Vapour Deposition) and FCVA ( Filtered Cathodic Vacuum Arc) techniques on Polyethylene terepthalate (PET) and polycarbonate (PC) substrates. The paper is concerned with exploring the links between the atomic structure, gas barrier performance in carbon based films deposited on polymer substrates. A range of techniques including XRR, NEXAFS, Raman, surface profilometry, nano-indentation and water vapour permeation analysis were used to analyze the microstructure and properties of the films. The intensity and area of π* peak at the C K (carbon) edge of the NEXAFS spectra was lower in the FCVA films in comparison to that of PECVD ones confirming the higher sp³ content of FCVA films. The surface of ta-C films showed a network of micro-cracks, which is detrimental for gas barrier application. However, the surfaces of both ta-C:H:Si and a-C:H:Si silicon-incorporated films were almost free of cracks. We also found that the incorporation of Si into both types of DLC films lead to a significant reduction of water vapour transmission rate.     

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.     

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.     

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.     

Effect of structure of carbon films on their tribological properties

A comparative study of the tribological properties of a library of different carbon forms is presented. The library includes hydrogen free and hydrogenated carbon films with different bonding (CC, CH, different sp³ fractions) and structure configurations (amorphous, graphitic) leading to a wide range of densities and hardness. Reference samples (Si substrates, thermally evaporated amorphous carbon, graphitic foil) were studied as well. The tribological properties were measured using a reciprocal sliding tribometer under humid (50% RH) and dry (5% RH) air conditions. Friction coefficients were measured versus the number of sliding cycles and the wear was studied using optical profilometry and imaging as well as SEM.The friction and wear performance of the carbon films were found to depend on both the structure and the ambient conditions. Hydrogen free films have friction coefficients < 0.1 for 80% sp³ bonded films and > 0.1 for 100% sp² bonded films. The wear resistance of the hydrogen free films (much larger for sp³ bonded films) significantly decreases under dry conditions. In contrast, hydrogenated films show reduction in friction with decreasing humidity (from 0.2 under 50% RH to < 0.1 under 5% RH). The wear resistance of hydrogenated films is larger for dry and smaller for humid conditions.     

Effect of ion bombardment and hydrogen pressure during deposition on the optical properties of hydrogenated amorphous carbon thin films

Carbon-based thin films are ideal materials for several state-of-the-art applications, such as protective materials and as active films for organic electronics, medical, optoelectronic devices. In this work, we study in detail the effect of the ion-bombardment and the hydrogen partial pressure during deposition on the optical properties of hydrogenated amorphous carbon (a-C:H) thin films grown onto c-Si substrates by rf magnetron sputtering. The optical properties of the a-C:H films were investigated by phase modulated Spectroscopic Ellipsometry in a wide spectral region from the NIR to the Vis-far UV (0.7-6.5 eV). A dispersion model based on two Tauc-Lorentz oscillators, has been applied for the analysis of the measured < ε(ω)> of the a-C:H films to describe the π-π* and σ-σ* interband electronic transitions, that can describe accurately the optical properties of all amorphous carbons. The applied V_b influences the bombardment of the growing thin films with Ar ions affecting the content of sp² and sp³ hybridized carbon bonds in the films. As it was found, the increase of the applied negative voltage reduces the optical transparency of the a-C:H films. Also, the H incorporation has been found to change only the energy position of the σ-σ* transitions. Finally, from the study of the refractive index n(ω = 0 eV) it has been found that the increase of the ion bombardment during the films deposition is correlated to an increase in the films density.     

A study of nanoporous carbon obtained from ZC powders (Z=Si, Ti, and B)

Nanoporous carbon (NPC) material prepared from polycrystalline powders of SiC, TiC, or B₄C carbides, using different techniques, was studied by means of ESR, Hall and conductance measurements for three groups of each type of sample—control group, and those annealed at T=960 and 1180 °C. It is found, according to Hall measurements, that holes are the majority charge carriers in NPC. The concentration of free carriers is found to be rather high (about 10¹⁹–10²¹ cm⁻³). The ESR spectrum has the so-called ‘Dyson line shape’ with a large signal asymmetry. Two groups of carriers with strongly different electronic and magnetic properties are found. The values of the g-factor and their variability are related to the existence of two different kinds of particles: small carbon nanoclusters (1–3 nm) and large-scale carbon skeleton structural elements (up to 1000 nm) of complex shape, where the holes move along graphite-like sp² bonds. The temperature dependence of the integral intensities shows the existence of two non-separable spin subsystems—charge carriers and localised spins. A role of oxygen atoms in the electronic system of NPC is studied.     

Long-term H-release of hard and intermediate between hard and soft amorphous carbon evidenced by in situ Raman microscopy under isothermal heating

We study the kinetics of the H release from plasma-deposited hydrogenated amorphous carbon films under isothermal heating at 450, 500 and 600 °C for long times up to several days using in situ Raman microscopy. Four Raman parameters are analyzed. They allow the identification of different processes such as the carbon network reorganization and the H release from sp³ or sp² carbon atoms and the corresponding timescales. Carbon reorganization with aromatization and loss of sp³ hybridization occurs first in 100 min at 500 °C. The final organization is similar at all investigated temperatures. Full H release from sp³ carbon occurs on a longer timescale of about 10 h while H release from sp² carbon atoms is only partial, even after several days. All these processes occur more rapidly with higher initial H content, in agreement with what is known about the stability of these types of films. A quantitative analysis of these kinetics studies gives valuable information about the microscopic processes at the origin of the H release through the determination of activation energies.     

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.     

Semi-quantitative description of C hybridization via s- and p-partial density of states probing: an electron spectroscopy study

The paper is a contribution to the characterization of bonding in carbon films. C is unique in establishing different kinds of CC bonds that are described in terms of variously hybridized s and p electron states of its VB (Valence Bond). On this basis, the idea is put forward that the sp hybridization of C in different environments is best characterized if the s- and p-partial density of states (DOS) are independently mapped. This is made possible by sampling the occupied DOS via probes that are selective with respect to the angular momentum symmetry of electron states, such as X-ray excited VB emission (s-selective) and C KVV (K Valence Valence) Auger emission (p-selective). The VB electronic structure appears to be well-characterized in this way. Systems made of purely sp² or purely sp³-hybridized carbons are unambiguously singled out while for systems where the two hybridizations coexist, the ability to reveal changes in the relative amount of sp² and sp³ hybridized carbons is demonstrated.     

Properties of a-C:H films grown in inert gas ambient with camphoric carbon precursor of pulsed laser deposition

The influence of the ambient argon gas (Ar) pressure on the properties of the hydrogenated amorphous carbon (a-C:H) films deposited by pulsed laser deposition (PLD) using camphoric carbon (CC) target have been studied. The a-C:H films are deposited with varying Ar pressure range from 0.01 to 0.23 Torr. SEM and AFM show that the particle size of films is decreases, while the roughness increases with higher Ar pressure. The FTIR measurement revealed the presence of hydrogen in the a-C:H films. We found the surface morphology, structural and physical properties structure of a-C:H films are influenced by the presence of inert gas and the ratio of sp² trigonal component to sp³ tetrahedral component is strongly dependent on the inert gas pressure. We suggest that these phenomena are due to the effect of the optimum concentration of the Ar atoms in the C lattice. Improvement of the structural properties of the a-C:H films deposited in inert gas environment using CC target reveals different behaviour than reported earlier.     

Disorder and optical properties of amorphous carbon

Tight-binding calculations shed light onto the link between disorder and optical properties in pure amorphous carbon films. It is shown that the Urbach energy E_U is an excellent probe of disorder in such films. Our striking finding is that E_U varies non-monotonically with sp³ fraction and optical gap, contrary to what is seen experimentally in hydrogenated samples. Firm evidence is provided, supported by experimental measurements, that in dense films the higher the sp³ fraction the lower the disorder. The Urbach energy has a value of ∼ 0.09 eV for the 100% sp³-bonded network, it reaches a maximum of ∼ 0.3 eV for 65% sp³ content, and it then declines to low values for low-sp³ content films. Analysis of cluster distributions and bond-angle and -length distortions reveals that the Urbach edge is associated to both topological and structural disorder. Another notable finding is that the edge is rather insensitive to the spin density due to unpaired sp² sites.     

AES and core level photoemission in the study of a-C and a-C:H

The state of the art in the use of two probes of the occupied electron states, namely C KVV Auger emission and C 1s photoemission, in the study of a-C and a-C:H is reviewed, with particular attention to the issue of deriving the sp² fraction. The local character of the two probes justifies decomposition of the relative spectra into an sp² and an sp³ component. While however decomposition of the C 1s spectrum relies upon a theoretical basis and allows accounting for disorder effects in the amorphous state, no theory is available to support C KVV spectrum decomposition which has therefore to rely upon a purely empirical basis. In addition, the introduction of disorder related effects is not straightforward for this spectrum. A real validation of the sp² fraction measurement is lacking for both techniques, though there are indications that both allow qualitative or even semi-quantitative (C 1s spectrum) understanding of the electronic structure of amorphous carbon systems. Beside the sp² fraction evaluation, other pieces of information, concerning the spatial organization of the sp² sites, are possibly extracted from these spectra.     

First-principle study of nitrogen incorporation in amorphous carbon

First-principle calculations using quantum-mechanical density functional theory are carried out to study nitrogen incorporation in amorphous carbon, in which the structural models from liquid quench containing 64 atoms are introduced. The properties simulated for N incorporated amorphous carbon are in agreement with the available experimental results. The topological and electronic properties for nitrogen incorporation structures with various densities are investigated, and it is found that the bonding configuration of nitrogen atoms strongly depends on the density and the nitrogen concentration in the network. The simulations provide a qualitative support for a low nitrogen doping efficiency observed in the experiment since no true doping (N atoms substitutionally occupy C sites, leading to a donor level below conduction band mobility edge E_c) exists in any cases studied. For the tetrahedral amorphous carbon (at density of 2.9 g/cm³) with a low concentration of nitrogen (1.6 at% and 3.2 at%), the incorporated N atoms are found to adopt the auto-compensated sites or to be in threefold coordination. In particular, a new threefold C defect is found, which is introduced by nitrogen auto-compensation. Nitrogen incorporated amorphous carbon (2.0 g/cm³) leads to the formation of a graphite-like structure with twofold nitrogen coordination.     

Studies of carbon ion self-implantation into hydrogenated amorphous carbon films

The properties of polymer-like amorphous hydrogenated carbon thin films with low defect density have been studied. These films were implanted with carbon ions with a dose range of 10¹²–10¹⁶ cm⁻². The purpose of the study is to investigate the effects of ion beam damage on this type of film. Optical absorption measurements observe a narrowing of the optical band gap, suggesting the introduction of a large number of defect states subsequent to the implantation resulting in the broadening of the band tails, only after a threshold ion dose of 10¹⁵ cm⁻². Nuclear reaction analysis suggests also a reduction in the hydrogen content of the film which coincides with film thinning.     

Highly spin-polarized carbon-based spinterfaces

We deploy topographical and spectroscopic techniques to show that a strongly spin-polarized interface arises between ferromagnetic cobalt and an amorphous carbon layer. Scanning tunneling microscopy and spectroscopy show how a semiconducting carbon film with a low band gap of about 0.4 eV is formed atop the metallic interface. To understand how the cobalt/carbon interface is formed, we used X-ray photoemission spectroscopy to study the hybridization state of carbon. We find that the semiconducting layer consists mainly of sp²-bonded carbon atoms with a sp²-to-sp³ ratio between 1.4 and 1.8. The spin-polarized properties of the cobalt/carbon interface are studied by spin-resolved photoemission spectroscopy. We observe interface states close to the Fermi energy that are not exclusive to cobalt. These electronic states reveal a high degree of spin polarization at room temperature.