Optical properties and new carbon forms of sputtered amorphous carbon films

Amorphous carbon films were deposited by r.f. magnetron sputtering at various bias voltages V_b applied on Si substrate. We studied the optical properties of the films using in situ spectroscopic ellipsometry (SE) measurements in the energy region 1.5–5.5 eV. From the SE data analysis the dielectric function ε(ω) of the a-C films was obtained, providing information about the electronic structure and the bonding configuration of a-C films. Based on the SE data the films are classified in three categories. In Category I and II belong the films developed with V_b≥0 V (rich in sp² bonds) and −100≤V_b<0 V (rich in sp³ bonds), respectively. The dielectric function of the films belonging in these two categories can be described with two Lorentz oscillators located in the energy range 2.5–5 eV (π–π^*) and 9–12 eV (σ–σ^*). A correlation was found between the oscillator strength and the sp² and sp³ contents. The latter were calculated by analyzing the ε(ω) with the Bruggeman effective medium theory. In films deposited with V_b<−100 V (Category III), the formation of a new and dense carbon phase was detected which exhibits a semi-metallic optical behavior and the ε(ω) can be described with two oscillators located at ∼1.2 and ∼5.5 eV.     

Structural and mechanical properties of facing-target sputtered amorphous CNx films

Structural and mechanical properties of carbon nitride films, deposited using a DC facing-target reactive sputtering system at various N₂ fractions (P_N) in the gas mixture, were studied systematically. XPS analyses indicate that N concentration is not directly proportional to P_N, and it rises quickly to a saturation value of ∼ 33 at.% at a P_N of 20%. The ratio of N–C(sp²)/N–C(sp³) increases with the rise of P_N from 0% to 20%, and then decreases with further rising P_N. However, the number and size of disordered sp²-hybridized C clusters continue to increase over the whole range of P_N, which is consistent with the Raman and high-resolution transmission electron microscopy measurements. Nanoindenter measurements show that the hardness of the films continuously decreases from ∼ 17.5 to ∼ 5.6 GPa with the increasing P_N from 0% to 100%, due to the conversion from sp³ C to sp² C and the clustering of sp² C structure.     

Structure and optical properties of Cu-DLC composite films deposited by cathode arc with double-excitation source

Copper-doped diamond-like carbon films (Cu-DLC) were fabricated on silicon and quartz substrates by cathode arc technique with direct-current and pulse double-excitation source. The microstructure, composition, morphology, hardness and optical properties of the films were studied by Raman, XPS, AFM and SEM, UV-Vis, laser ellipsometer and Vickers sclerometer. The results showed that Cu doping increases the size, ordering and amount of sp²-C clusters in the Cu-DLC composite films. The microstructure parameters enhance with increasing the Cu content to 22.4 wt%. All the films show specific nano-structural surface, however, the lower Cu content induces finer particle formation in the Cu-DLC films. When the arc current is higher than 60 A, the roughness and particle size of Cu-DLC composite films increase with increasing the Cu content. The average transmittance of the Cu-DLC films in Vis-NIR region is smaller than 40% when the Cu content exceeds 12.6 wt%. With increasing the Cu content, the optical band gap (E_g) of the films decreases from 3.54 eV to 0.25 eV. The relationships among the E_g, refractive index and extinction coefficient for the Cu-DLC films were found and indirectly revealed the correlation of microstructure and optical properties of the films with the Cu content variation.     

Optical properties of impact diamonds from the Popigai astrobleme

Impact diamonds from Popigai astrobleme were found to consist of different carbon phases: cubic and hexagonal diamond with sp³ bonding according to X-ray structural analysis as well as amorphous, crystalline and disordered graphite with sp²-bonding (Raman scattering). The sizes of graphite domains vary from 10 to 100 nm. Fundamental absorption edge for Popigai impact diamonds is shifted ~ 0.5 eV to lower energies in comparison with kimberlite diamonds (5.47 eV) as a result of the lonsdaleite input, in good agreement with ab initio calculations (E_g = 5.34 and 4.55 eV for 3C cubic and 2H hexagonal diamonds, respectively). Yellowish color of impact diamonds is due to Rayleigh light scattering on structural defects whereas graphite is responsible for gray to black coloring. In the mid-IR region there is a multi-phonon absorption of 3C diamond in the 1800 to 2800 cm^− 1 range and some new bands at 969, 1102, 1225, and 1330 cm^− 1 in the one-phonon region. Micro-Raman study shows inclusions of side noncarbon minerals (quartz, magnetite, and hematite) some of which contain Cr^3 + impurity. The vibration modes of cubic diamond and lonsdaleite exhibited in the Raman spectra were elucidated by the first-principles studies. Popigai impact diamonds demonstrate a broad-band luminescence in 2.1, 2.38, and 2.84 eV components similar to that for nanocrystal polycrystalline 3C diamond. All emissions are excited at band-to-band transitions whereas the last two are observed also at excitation into 2.4 and 3.0 bands supposedly as a result of intracenter processes within the H3(NVN) and NV⁰ centers.     

Top-surface characterization of a near frictionless carbon film

A detailed study of the top surface (∼ 2 nm) of a near frictionless carbon film has revealed new information with respect to the sp³ fraction. Previous work on near frictionless carbon films made at Argonne had shown a large fraction of sp²-hybridized carbon in the bulk of the film. However, in this study of the surface, the majority of the carbon was found to be sp³. In addition we compared and contrasted the behavior of the films after mechanical abrasion and Ar⁺ etching. The study also revealed that oxygen on untreated samples was rapidly reduced by etching or heating or mechanical abrasion; this finding was corroborated by an angle-resolved study, where different depths of the sample were probed. It was also found that the fraction of sp³ carbon decreased linearly with depth, falling in one film from ∼ 90% sp³ to ∼ 80% sp³ in the top 2 nm.     

Tribological study of amorphous BC4N coatings

Amorphous BC₄N thin films with a thickness of ∼ 2 μm have been deposited by Ion Beam Assisted Deposition (IBAD) on hard steels substrates, in order to study the wear behavior under high loads and the applicability as protective coatings. The bonding structure of the a-BC₄N film was assessed by X-ray Absorption Near Edge Spectroscopy (XANES) and Infrared Spectroscopy, indicating atomic mixing of B–C–N atoms, with a proportion of ∼ 70% sp² hybrids and ∼ 30% sp³ hybrids. Nanoindentation shows a hardness of ∼ 18 GPa and an elastic modulus of ∼ 170 GPa. A detailed tribological study is performed by pin-on-disk tests, combined with spectromicroscopy of the wear track at the coating and wear scar at pin. The tests were performed at ambient conditions, against WC/Co counterface balls under loads up to 30 N, with the sample rotating at 375 rpm. The coatings suffer a continuous wear, at a constant rate of 2 × 10^− 7 mm³/Nm, without catastrophic failure due to film spallation, and show a coefficient of friction of ∼ 0.2.     

XPS spectra of thin CNx films prepared by chemical vapor deposition

Carbon nitride (CN_x) thin films with an N/C ratio of 0.605:0.522 have been synthesized using different sources as a ksilol, CCl₄, N₂ and NH₃ by PECVD (plasma enhanced chemical vapor deposition) and hot filament-CVD reactors. X-ray photoelectron spectroscopy (XPS) analyses, which give C_1s peaks with a maximum at 285.7 eV and 287 eV, typical for C–N bonds and sp² hybridization and CN bonds and sp³ hybridization, respectively. The observed and N_1s peaks with a maximum at about 399 eV suggest the existence of different C–N bonds and polycrystallite structure in the amorphous carbide matrix. The concentration of the different CN bonds varies, depending on the deposition technique.     

Cu3N films prepared by the low-pressure r.f. supersonic plasma jet reactor: Structure and optical properties

A low-pressure r.f. supersonic plasma jet reactor (RPJ) has been used for deposition of Cu₃N thin films. From a comparison of experimental values of composition weight per cent with the theoretically predicted values, it follows that if the r.f. power absorbed in the reactor does not exceed 75 W, stoichiometric Cu₃N films can be obtained. The typical value of the deposition growth rate was found to be in the order of 16 nm min⁻¹ for r.f. power P_w≈40 W. The optical energy gap, E_g, microhardness, H, and Young's modulus, E, of the deposited Cu₃N thin films increase with decreasing r.f. power. They are E_g=1.24 eV, H=8.8 GPa and E=146 GPa for the sample deposited with a r.f. power of 40 W. Deposition of Cu₃N thin films by means of the modificated RPJ reactor (with a multi-jet system) on to internal walls of cavities, holes and on the surface of complex shapes of hollow substrates can be useful for surface-treatment technology.     

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.     

High resolution electron energy loss spectroscopy of hydrogenated polycrystalline diamond: Assignment of peaks through modifications induced by isotopic exchange

In this work we unambiguously determine the origin of the different peaks which appear in the High Resolution Electron Energy Loss Spectrum (HREELS) of hydrogenated polycrystalline diamond films for an incident electron energy of 5 eV and loss energies extending to 700 meV. High quality diamond films deposited by hot filament chemical vapor deposition from various isotopic gas mixtures: ¹²CH₄ + H₂, ¹²CD₄ + D₂, ¹²CH₄ + D₂, ¹²CD₄ + D₂, ¹³CH₄ + H₂ were characterized. The different vibrational modes, fundamentals and overtones, were directly identified through the modifications of the HREEL spectra induced by the isotopic exchange of H by D and ¹²C by ¹³C Three types of peaks were identified: (1) pure C–C related peaks (a diamond optical phonon at ∼ 155 meV and its overtones at 300, 450 and 600 meV), (2) pure C–H related peaks (C–H bend at ∼ 150 meV and C–H stretch of sp³ carbon at 360 meV), (3) coupling of C–H and C–C peaks (510 meV peak due to coupling of the C–H stretch at 360 meV with either the C–C stretch or the C–H bend at ∼ 155 meV). The overtones at 300, 450 and 600 meV (associated with electron scattering at diamond optical phonons) indicate a well defined hydrogenated diamond surface since they are absent in the HREEL spectrum of low energy ion beam damaged diamond surface.     

A graphene flake under external electric fields reconstructed from field-perturbed kernels

The energy, dipole moment, and polarizability of a finite hydrogen terminated zigzag graphene flake (C₄₆H₂₀, in 2 × 7 rings) are calculated in the absence of and in the presence of external electric fields reaching 5 × 10⁹ Vm⁻¹ [=0.01 atomic units (a.u.)]. The field intensities studied are typically found in nanoelectronics and in the tip-sample gap of a scanning tunneling microscope (STM). The change in the total energy ΔE(eV) can be closely fitted to a quadratic function of the field {1.549 × 10⁴ [E (a.u.)]²} while the dipole moment μ(debye) to a linear function [2.7 × E (a.u.)]. The results obtained with three different chemical models [MP2, density functional theory (B3LYP), and Hartree–Fock calculations, all with a 6-311G(2d,2p) basis set] are consistent in both trends and in absolute values. These results obtained from direct calculations are reproduced with a remarkable accuracy from a linear scaling fragmentation scheme called the kernel energy method (KEM). The KEM reproduces all studied field-free and response properties of this graphene flake model, in relative and absolute terms, independently of the underlying chemical model. An observation consistent with the known stiffness of graphene is that geometry optimization under a field as strong as 0.01 a.u. insignificantly alters the total energy and the geometry of a (smaller) zigzag C₂₈H₁₄ flake, the difference in the field-induced stabilization energy (ΔΔE) being only 0.006 eV (less than 1% of ΔE) and the average field-induced displacement of nuclear positions ∼0.0046 Å [B3LYP/6-31G(d,p)].     

Low temperature reaction of point defects in ion irradiated 4H–SiC

The low temperature evolution of point defects induced in SiC by ion irradiation was investigated by deep level transient spectroscopy. The defects were introduced by irradiation with a 7.0 MeV beam of C⁺ ions at a fluence of 6 × 10⁹ cm^? 2. Annealing was then performed in the temperature range of 330–400 K in order to study the change in point defect structure with temperature. The low temperature annealing performed was observed to induce a change in the produced defects. The deep levels related to the S_x (E_C ? 0.6 eV) and S₂ defects (E_C ? 0.7 eV) recovered with annealing while, simultaneously, a new level, S₁ (E_C ? 0.4 eV), was formed. The activation energy of the S₁ defect is 0.94 eV, while the annealing of both the S_x and S₂ levels occurred with activation energy of 0.65 eV.     

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%.     

Graphene nanochains and nanoislands in the layers of room-temperature fluorinated graphite

Intercalated compound of graphite fluoride with n-heptane has been synthesized at room temperature using a multi-stage process including fluorination by a gaseous BrF₃ and a set of intercalant exchange reactions. It was found that composition of the compound is CF_0.40(C₇H₁₆)_0.04 and the guest molecules interact with the graphite fluoride layers through the van der Waals forces. Since the distance between the filled layers is 1.04 nm and the unfilled layers are separated by ∼0.60 nm, the obtained compound can be considered as a stack of the fluorinated graphenes. These fluorinated graphenes are large in area making it possible to study local destruction of the π conjugated system on the basal plane. It was shown that fluorine atoms form short chains, while non-fluorinated sp² carbon atoms are organized in very narrow ribbons and aromatic areas with a size smaller than 3 nm. These π electron nanochains and nanoislands preserved after the fluorination process are likely responsible for the value of the energy gap of the compound of ∼2.5 eV. Variation in the size and the shape of π electron regions within the fluorinated graphene layers could be a way for tuning the electronic and optical characteristics of the graphene-based materials.     

X-ray photoemission spectroscopy of nitrogen-doped UNCD/a-C:H films prepared by pulsed laser deposition

Nitrogen-doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) films were deposited by pulsed laser deposition (PLD). Nitrogen contents in the films were controlled by varying a ratio in the inflow amount between nitrogen and hydrogen gases. The film doped with a nitrogen content of 7.9 at.% possessed n-type conduction with an electrical conductivity of 18 Ω^? 1 cm^? 1 at 300 K. X-ray photoemission spectra, which were measured using synchrotron radiation, were decomposed into four component spectra due to sp², sp³ hybridized carbons, C=N and C–N. A full-width at half-maximum of the sp³ peak was 0.91 eV. This small value is specific to UNCD/a-C:H films. The sp²/(sp³ + sp²) value was enhanced from 32 to 40% with an increase in the nitrogen content from 0 to 7.9 at.%. This increment probably originates from the nitrogen incorporation into an a-C:H matrix and grain boundaries of UNCD crystallites. Since an electrical conductivity of a-C:H does not dramatically enhance for this doping amount according to previous reports, we believe that the electrical conductivity enhancement is predominantly due to the nitrogen incorporation into grain boundaries.     

Synthesis of diamond using ultra-nanocrystalline diamonds as seeding layer and their electron field emission properties

A modified nucleation and growth process was adopted so as to improve the electron field emission (EFE) properties of diamonds films. In this process, a thin layer of ultra-nanocrystalline diamonds (UNCD), instead of bias-enhanced-nuclei, were used as nucleation layer for growing diamond films in H₂-plasma. The morphology of the grains changes profoundly due to such a modified CVD process. The geometry of the grains transform from faceted to roundish and the surface of grains changes from clear to spotty. The Raman spectroscopies and SEM micrographs imply that such a modified diamond films consist of UNCD clusters (~ 10–20 nm in size) on top of sp³-bonded diamond grains (~ 100 nm in size). Increasing the total pressure in CVD chamber deteriorated the Raman structure and hence degraded the EFE properties of the films, whereas either increasing the methane content in the H₂-based plasma or prolonged the growth time improved markedly the Raman structure and thereafter enhanced the EFE properties of diamond films. The EFE properties for the modified diamond films can be turned on at E₀ = 11.1 V/μm, achieving EFE current density as large as (J_e) = 0.7 mA/cm² at 25 V/μm applied field.     

Annealing-induced structural changes of carbon onions: High-resolution transmission electron microscopy and Raman studies

The first- and second-order Raman spectra of carbon nano-onions (CNOs), produced via annealing of detonation nanodiamonds with a mean grain size of ∼5 nm in the argon ambience at the maximal temperature of annealing process (T_MAX) varying from 1500 to 2150 °C, are analyzed together with the high-resolution transmission electron microscopy (HRTEM) images. The combined analysis provides a deep insight into the annealing-induced atomic-scale structural modifications of the CNO nanoparticles. The Raman and HRTEM data unambiguously demonstrate the reduction in the number of defects in the CNO structure, as well as indicate the conversion from the diamond sp³-bonded carbon phase to the sp²-bonded carbon phase with increasing T_MAX and its almost full completion for T_MAX = 1600 °C.     

Spectroscopic evaluation of out-of-plane surface vibration bands from surface functionalization of graphite oxide by fluorination

The fluorination of graphite/graphite oxide (GO) and their derivatives has been widely investigated for how fluorine interacts with sp²/sp³ carbon; however, the mechanism of this interaction has not yet been elucidated. Fluorination of GO (FGO) at either 10 or 15 psi for 24 h, produced two new absorption bands at ∼743 cm⁻¹ and 482 cm⁻¹, and are attributed to the presence of out-of-plane surface fluorine bonds in FGO (absent in fluorographite – FG). IR studies confirmed the stability of the formed C–F bonds and defect formation due to the introduction of oxyfluorinated species into the graphitic carbon through fluorination of epoxides. Fluorination of GO resulted in ∼4–5 times more fluorine incorporation in bulk as compared to FG. (4.57 vs. 0.8 at.% and 6.64 vs. 1.4 at.% at 10 and 15 psi, respectively). PXRD analyses also showed that the interlayer spacing of FGO expanded in the presence of intercalated C–F species and a defect formation was observed with the evidence of increase of the I_D/I_G ratio from Raman spectra. To this end, understanding the origin of surface C–F bonds and structural changes in FGO therefore leads to new applications such as implementation of FGO for sensing, nano-electronics and energy storage.     

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%.     

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.