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.     

Enhanced cycling stability of Ni-MH battery by depositing amorphous carbon film on the Ti1.4V0.6Ni negative electrode

Amorphous carbon (a-C) films with various thicknesses depending on the reaction time are deposited on the surface of Ti_1.4V_0.6Ni alloy electrodes for Ni-MH (nickel-metal hydride) battery by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). With the increasing deposition time, the Raman spectra show a gradually disordered sp²-bonding change of the films and the changing trend of sp²/sp³ is obtained by X-ray photoelectron spectroscopy. The a-C film of depositing for 30 min with the thickness of 400 nm shows a favorable stability in alkaline electrolyte, the capacity is enhanced by 36.2% after 50 cycles than the bare electrode, and the charge voltage is 80 mV lower than the bare one. The a-C film with high sp²-bonded carbon content effectively reduces the charge transfer resistance, and as a coating layer, the dissolution of V of the alloy is also inhibited. In particular, to get a proper discharge voltage and a stable capacity simultaneously, covering completely and an appropriate thickness of the a-C film are crucial for an expected performance.     

Nanoindentation and nanoscratching studies of amorphous carbon films

Hydrogen-free amorphous carbon (a-C) films prepared by RF magnetron sputtering were deposited on Si substrates in thin films, at various negative bias voltages V_b (i.e. Ar-ion energies), and in thick layered-structure films with alternative values of V_b. The main purposes of this work are to present preliminary results concerning the effect of Ar-ion bombardment during deposition on the elastic properties of thin a-C films with Ar⁺ energies in the range 30–200 eV, and the adhesion failure which limits their thickness and usefulness for practical applications, and the enhancement of hardness and scratch resistance of sputtered a-C films developed in a layered structure. The results show a significant improvement in the elastic properties of layered structure films and their stability. The combination of high hardness and relative low elastic modulus which the layered films exhibit make them more resistant to plastic deformation during contact, as confirmed by scratch testing.     

Semiconducting amorphous carbon thin films for transparent conducting electrodes

Semiconducting amorphous carbon thin films were directly grown on SiO₂ substrate by using chemical vapor deposition. Raman spectra and transmission electron microscopy image showed that the a-C films have a short-range ordered amorphous structure. The electrical and optical properties of the a-C thin films were investigated. The films have sheet resistance of 3.7 kΩ/□ and high transmittance of 82%. They exhibit metal-oxide-semiconductor field effect transistor mobility of 10–12 cm² V⁻¹ s⁻¹ at room temperature, which is comparable to previous reported mobility of amorphous carbon. The optical band gap was calculated by Tauc’s relationship and photoluminescence spectra showed that the films are semiconductor with an optical band gap of 1.8 eV. These good physical properties make the a-C films a candidate for the application of transparent conducting electrodes.     

Formation and characteristics of undoped and doped tetrahedral amorphous carbon films

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

Thermal grafting of organic monolayers on amorphous carbon and silicon (111) surfaces: A comparative study

Thermally-assisted (160 °C) liquid phase grafting of linear alkene molecules has been performed simultaneously on amorphous carbon (a-C) and hydrogen passivated crystalline silicon Si(111):H surfaces. Atomically flat a-C films with a high sp³ average surface hybridization, sp³ / (sp² + sp³) = 0.62, were grown using pulsed laser deposition (PLD). Quantitative analysis of X-ray photoelectron spectroscopy, X-ray reflectometry and spectroscopic ellipsometry data show the immobilization of a densely packed (> 3 × 10¹⁴ cm^? 2) single layer of organic molecules. In contrast with crystalline Si(111):H and other forms of carbon films, no surface preparation is required for the thermal grafting of alkene molecules on PLD amorphous carbon. The molecular grafted a-C surface is stable against ambient oxidation, in contrast with the grafted crystalline silicon surface.     

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.     

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.     

Polymeric amorphous carbon films with an extended range of optical gaps

A new type of hydrogenated amorphous carbon (a-C:H) film is prepared under low bias voltage and an extended range of plasma density in a radio-frequency plasma enhanced chemical vapor deposition system (RF-PECVD). The obtained a-C:H samples are grown on electrically floating substrates instead of substrates mounted on the powered or the grounded electrode of RF-PECVD, and have structure and properties that are significantly different from regular a-C:H films. The samples have an optical gap ranging from 1 to 4 eV, while maintaining low intrinsic stress between 0.1 and 0.2 GPa. Fractions of all types of CH_x carbon–hydrogen groups of the obtained samples are measured, analyzed, and used to locate these samples in the carbon-hydrogen ternary phase diagram. The obtained samples are located in the same general area as the regular a-C:H films, indicating configurational rather than compositional structural differences. The hydrogenation ratio of sp³ carbons in the obtained samples is found to remain at a very high level, and is used to explain their unique properties.     

Surface immobilization of Mo6I8 octahedral cluster cores on functionalized amorphous carbon using a pyridine complexation strategy

Tetrahedral amorphous carbon (a-C) films have been grown by pulsed laser deposition to investigate a liquid phase process for surface immobilization of electroactive [Mo₆Iⁱ₈]^4 + transition metal cluster cores using a complexation reaction with a pyridine-terminated alkyl monolayer covalently bonded to the a-C surface (Py^S–alkyl/a-C). These films are stable against thermally-assisted grafting of alkene molecules and the covalent CC interface provides a robust monolayer/a-C assembly. Octahedral [Mo₆Iⁱ₈]^4 + cluster cores with iodine inner ligands and labile triflate apical ligands [Mo₆Iⁱ₈(CF₃SO₃)^a₆]^2 − have been immobilized through partial complexation in apical positions by surface pyridine groups (Py^S). The remaining CF₃SO₃⁻ apical ligands of [Mo₆Iⁱ₈ (Py^S)^a_y(CF₃SO₃)^a_6 − y] cluster units were further substituted with bromopyridine (Py-Br) to obtain air stable surface with expected final composition [Mo₆Iⁱ₈ (Py^S)^a_y(Py-Br)^a_6 − y]. The yield of the different reaction steps is followed by X-ray photoelectron spectroscopy, providing cluster coverage Σ_Mo6I8 = 9 × 10¹² cm^− 2. Each [Mo₆I₈]^4 + cluster is bound to the carbon surface through multiple anchoring metal sites (N_PYR = 3 or 4), indicating that pyridine-terminated alkyl chains are flexible enough to accommodate four bonds. Electrical transport through Hg//Mo₆I₈–Py^S–alkyl/a-C/p-Si(111) junctions shows rectifying current–voltage characteristics but does not reveal any signature of cluster immobilization.     

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.     

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.     

Deposition of carbon nitride films using an electron cyclotron wave resonance plasma source

Hydrogenated amorphous carbon nitride (a-C:N:H) has been synthesised using a high plasma density electron cyclotron wave resonance (ECWR) technique using N₂ and C₂H₂ as source gases, at different ratios and a fixed ion energy (80 eV). The composition, structure and bonding state of the films were investigated and related to their optical and electrical properties. The nitrogen content in the film rises rapidly until the N₂/C₂H₂ gas ratio reaches 2 and then increases more gradually, while the deposition rate decreases steeply, placing an upper limit for the nitrogen incorporation at 30 at%. For nitrogen contents above 20 at%, the band gap and sp³-bonded carbon fraction decrease from 1.7 to 1.1 eV and ∼65 to 40%, respectively. The transition is due to the formation of polymeric CN, CN and NH groups, not an increase in CH bonds. Films with higher nitrogen content are less dense than the original hydrogenated tetrahedral amorphous carbon (ta-C:H) film but, because they have a relatively high band gap (1.1 eV), high resistivity (10⁹ Ω cm) and moderate sp³-bonded carbon fraction (40%), they should be classed as polymeric in nature.     

Temperature effect on bonding structures of amorphous carbon containing more than 30at.% silicon

Bonding evolution of amorphous carbon incorporated with Si or a-C(Si) in a thermal process has not been studied. Unhydrogenated a-C(Si) films were deposited by magnetron sputtering to undergo two different thermal processes: i) sputter deposition at substrate temperatures from 100 to 500 °C; ii) room temperature deposition followed by annealing at 200 to 1000 °C. The hardness of the films deposited at high temperature exhibits a monotonic decrease whereas the films deposited at room temperature maintained their hardness until 600 °C. X-ray photoelectron spectroscopy and Raman spectroscopy were used to analyze the composition and bonding structures. It was established that the change in the mechanical property is closely related to the atomic bonding structures, their relative fractions and the evolution (conversion from C–C sp³ → CC sp² or CC sp² → C–Si sp³) as well as clustering of sp² structures.     

Ion beam deposition of amorphous hydrogenated carbon films on amorphous silicon interlayer: Experiment and simulation

A thick layer of amorphous silicon (a-Si) was deposited on industrial grade crystalline n-Si < 111 > substrate by means of electron beam evaporation. On top of a-Si layer, amorphous hydrogenated carbon (a-C:H) film was grown by direct ion beam deposition from acetylene precursor gas. In order to study on atomic level the a-C:H film growth on amorphous silicon, a theoretical model was developed in a form of reaction rate (kinetic) equations. Numerical simulation using this model has revealed that the ratio of sp³/sp² content in the film is heavily influenced by relaxation rate of the carbon atoms in a sub-surface region of the film that were activated by ion irradiation. The final structure of a-C:H film does not depend much on elemental composition and structure of amorphous Si coating, provided that deposition procedure is not terminated at its initial stage but continues for more than 60 s. It became evident, therefore, that the use of a-Si interlayer with a-C:H films could be particularly beneficial when a need arises to minimize or eliminate the effect of the substrate. As one of such cases, a poor adhesion of amorphous carbon on steel and other ferrous alloys could be mentioned.     

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.     

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.     

Relationship between mechanical properties and coefficient of friction of sputtered a-C/Cu composite thin films

The article reports on properties of a-C films containing different amount of Cu. Films were sputtered by unbalanced magnetron from a graphite target with Cu fixing ring in argon under different deposition conditions. Relationships between the structure, mechanical properties, macrostress σ and coefficient of friction (CoF) μ of a-C/Cu films sputtered on Si substrates were investigated in detail. Besides, a special attention was concentrated on investigation of the effect of a deposition rate a_D of the a-C/Cu film on its hardness H and macrostress σ. Four main issues were found: (1) the addition of Cu into a-C film strongly influences its structure and mechanical properties, i.e. the hardness H, effective Young's modulus E⁎ macrostress σ and CoF, and makes it possible to form electrically conductive films; here E⁎ =  E / (1 − ν²), E is the Young's modulus, and ν is the Poisson's ratio, (2) the hardness H and compressive macrostress σ of the a-C/Cu film decrease with increasing a_D due to decreasing of total energy E_T delivered to the film during its growth, (3) hard a-C/Cu films with low value of CoF (μ ≈ 0.1) can be sputtered at high deposition rates a_D ranging from ~ 10 to ~ 80 nm/min, and (4) CoF decreases with increasing (i) hardness H and (ii) resistance of film to plastic deformation characterized by the ratio H³/E⁎² but only in the case when compressive macrostress σ is low.     

CNx coatings sputtered by DC magnetron: hardness,nitrogenation and optical properties

CN_x coatings were deposited by DC magnetron sputtering at lower constant N₂ pressure 0.1 Pa. Nitrogenation of samples was controlled by increase in the DC current. It led a decrease in the nitrogen content N/C in the films, but increased their microhardness. Evaluation of optical properties combining spectroscopic ellipsometry (1.5–4 eV) and the VUV reflection spectroscopy (4–14 eV) by means of Kramers–Kronig analysis showed increase of the π-plasmon resonance peak, which indicates enhancement of amount of π-bonded electrons. It is linked with increase of sp² hybridization. The optical energy gap values indicate semimetallic properties of the CN_x films. A shift in the ε₂(ω) maximum corresponding to σ–σ1 electron transitions in carbon demonstrates deeper band structure changes in highly nitrogenated samples.     

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%.