Properties Of Amorphous Carbon Research Articles

Unphysical nucleation of diamond in the extended cutoff Tersoff potential

In simulations of carbon materials it is common practice to view the coefficients of the cutoff function as free parameters which can be optimised according to the system of interest. This work examines a common modification to the widely used Tersoff potential in which the coefficient of the upper cutoff is increased to improve the properties of amorphous carbon. Using molecular dynamics simulations, we show that this so-called extended cutoff Tersoff model leads to nucleation of diamond nanocrystals during annealing of amorphous carbon. By varying the density of the system, and examining the radial distribution function in conjunction with the modified cutoff function, we demonstrate that this behaviour is unphysical, and does not represent a new pathway for synthesising diamond. Viewed from a broader perspective, this observation provides a cautionary tale against altering the parameters of empirical potentials without fully considering the wider implications.

A ternary phase diagram for amorphous carbon

The sp2 phase in amorphous carbon is modeled as coexisting nc-graphite, fused aromatic ring, and olefinic chain clusters. A phenomenological linear dispersion model is derived to calculate the G peak position from weighted relative contents of the sp2 clusters and vice versa. Ternary phase diagrams based on the weighted relative contents provide quantitative predictions for G peak position, dispersion rate, sp3 content, H content, and other properties of amorphous carbon. The phase diagrams are used to classify and characterize amorphous carbon, and to track their structural changes due to modification procedures like annealing, doping, and ion irradiation. Numerous insights are drawn from patterns in the distributions or in the migration paths of the analyzed samples.

Nitrogen Effects on Optical and Electrical Properties of Amorphous Carbon

Optical and electrical properties amorphous carbon nitride (a-CN) has been investigated on films deposited by reactive R.F. sputtering source with a graphite target. The amorphous carbon nitride samples were prepared under a gas mixture of nitrogen (N2) and /or Argon (Ar).The optical transitions are governed by the  and * electronic state distributions, related to sp2- and sp1-hybridized C and N atoms. Specific lonepair electronic states arise from groups (CN) with sp1-hybridized C atoms, which may form C≡N triple bonds or —N=C=N— longer chains. Photoluminescence spectra show a maximum around 650 nm. Two conduction regimes at high and low temperature are found in a-CN samples. The corresponding activation energies decrease with the increase of target voltage.

Multi-Technology Measurements of Amorphous Carbon Films

AbstractWe demonstrate that a multi-technology approach enables the accurate characterization of the thickness and optical properties of Amorphous Carbon (α-C) films used in semiconductor manufacturing. Because the material is found to be highly birefringent, with its measured refractive index and extinction coefficient depending strongly upon the polarization and angle-of-incidence of the optical probe beam that is used, conventional single angle-of-incidence Spectroscopic Ellipsometry (SE) has insufficient information-content to detect the ordinary and extraordinary refractive indices with sufficient accuracy. On the other hand, Beam Profile Reflectometry® (BPR®) is particularly strong in this case because it measures the actual reflectances for both polarization components (rather than just the difference between them) at multiple angles-of-incidence. Furthermore, the fact that BPR is a single-wavelength technique means that no assumptions must be made about the optical dispersion of the film and an absolute measurement can be made. It is then possible to combine this with other, spectral technologies to obtain ordinary and extraordinary n and k at all wavelengths. We show that, to first order, the birefringence can be modeled by assuming a single “anisotropy parameter”, the ratio between the ordinary and extraordinary refractive indices (or, more correctly, the complex dielectric functions). Using this approach in combination with a simple Bruggeman effective-medium dispersion model enables robust characterization of these films for production.

Tribological properties of amorphous carbon thin films grown by magnetron sputtering method

Tribological properties of amorphous carbon thin films grown by magnetron sputtering method

Influence Of Deposition Parameters On The Structural, Optical And Electrical Properties Of Amorphous Carbon And Amorphous Carbon Nitride Films Grown By Pulsed Laser Deposition Using Camphoric Carbon Target

The role of laser fluence (LF) and target to substrate distance (TSD) on the surface morphology, deposition rate, composition, structural, optical and electrical properties of amorphous carbon (a-C) and amorphous carbon nitride (a-CN x ) films deposited by Xe-Cl excimer pulsed laser deposition is reported. At fixed TSD, the surface roughness, particle density and deposition rate increase, whereas the particle size decreases with higher LF. When the LF is fixed, TSD decreases resulting in an increase in the irregular small particle size, particle density, surfaces roughness and deposition rate. We found that the amorphous structure of a-C and a-CN x films is strongly dependent on the LF and TSD. The a-CN x films with high deposition rate have relatively high nitrogen content and high electrical resistivity.

Study of the influence of Ar on the formation of diamond-like bonds in films obtained from CH 4+Ar, RF plasma deposition

The concentration of the sp 3 bonds in the volume (ratio Φ=N 3 sp 3/(N 3 sp 3+N 2 sp 2+N 1 sp 1) , where N 1, N 2 and N 3 are the concentrations of sp 1, sp 2 and sp 3 bonds, respectively), is the basic parameter which determines the `diamond-like' properties of amorphous carbon (a:DLC) films. Electrical, morphological and mechanical properties, as a function of the partial pressure of Ar, of a:DLC films, were measured and their relation to Φ was shown. From Auger electron spectroscopy (AES) data it was shown that the presence of Ar leads to sputtering of the low adhesive carbon atoms from the deposited film, consequently, the a-a:DLC is much more adhesive and harder which make thickness ( t) a function of Ar concentration. Ar atoms in the film were found to be less than 2%. Visible and UV Raman spectroscopy, FTIR and XPS, were used to determine the concentration of sp 3 bonds, ( Φ), which was maximum for Ar=30%. Studies of morphology of the a:DLC films have shown that the average height profile (AHP) obtained from AFM data show an abrupt change for Ar=30%. Results of electrical and mechanical (microhardness) measurements have shown a maximum value of resistivity ( ρ=1.4×10 9 Ω cm) and microhardness ( H=2700 kg/cm 2 ) is obtained also for Ar=30%. Since the measurements of Φ is maximum at concentration Ar=30% and for AHP appears abrupt changes, for ρ and H maximum, we may assume that this behaviour of the physical parameter, is due to the diamond-like structure with maximum Φ. The results obtained in this paper also have practical importance since the sample deposited at 30% Ar exhibits 50% sp 3 bonding or more, which is high enough to use this material where diamond-like properties are needed.

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

Abstract 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.

Microstructure and Nanomechanical Properties of Amorphous Carbon Thin Films Prepared by Pulsed Laser Deposition in Various Atmospheres

AbstractWe have investigated the effect of chamber pressure and atmosphere on the microstructure and nanomechanical properties of amorphous diamondlike carbon (DLC) thin films prepared by pulsed laser deposition. The amorphous carbon films were deposited in various atmospheres such as nitrogen and argon at different pressures. We used Raman spectroscopy and optical microscopy to study the bonding characteristics and microstructures of the DLC films. Nanoindentation measurements were carried out on various samples prepared under different conditions to study the effect of chamber pressure and atmosphere on the elastic modulus and nano-hardness of the films. It was found that reduced vacuum leads to softer amorphous carbon films. Amorphous carbon films prepared in higher pressures exhibit increased density of particulates, and significantly rough surface. The results were understood in terms of thermalization of the laser plasma due to increased possibility of collision.

Effects of nitrogen addition on the structure and field emission properties of amorphous carbon

The films of amorphous carbon with different amounts of incorporated nitrogen are deposited by helical resonator plasma enhanced chemical vapor deposition using CH4, Ar, and N2 gas mixtures. As the nitrogen content in the films increases, the optical band gap and sp3 bond fraction decreased. The higher field emission current and the lower turn-on voltage are obtained from the sample of the higher nitrogen content. The calculated emission barrier heights for a-C:N (5/5) and (10/0) are 0.76 and 0.70 times that for a-C, respectively. The increase of the conducting part of the films and the role of nitrogen as an electrical donor are responsible for the enhanced field emission.

Nanomechanical Properties of Amorphous Carbon and Carbon Nitride Thin Films Prepared by Shielded Arc Ion Plating

AbstractHydrogen free amorphous carbon (a-C) and carbon nitride (a-C:N) were synthesized by means of shielded arc ion plating in which a shielding plate was inserted between a target and a substrate in order to reduce macroparticle deposition onto the substrate. Using a graphite target as a cathode, thin films of a-C and a-C:N were prepared in an arc discharge plasma of argon or nitrogen gas, respectively, at a pressure of 1 Pa. Based on nanoindentation, mechanical properties of these films were studied in relation to substrate bias voltage (VS). The a-C films prepared at VS ranging from −50 to −200 V consisted of diamond-like phase and showed hardness higher than 20 GPa with its maximum of 35 GPa at Vs = −100 V. However, hardness of the films deposited at VS < -300 V was less than 7 GPa indicating that the films were converted to graphite-like phase due to excessive ion impact in Ar plasma. Wear resistance of the a-C films was closely related to their hardness. Namely, a harder a-C film was more wear resistant. On the contrary, hardness of the a-C:N films was less dependent on VS. It remained in the range of 10 to 15 GPa and was much lower than the maximum hardness of the a-C films. Nevertheless, the wear resistance of the a-C:N films was comparable to or much better than the a-C films. In particular, the a-C:N film prepared at VS = -300 V was so wear resistant that the film showed no apparent wear when rubbed with a diamond tip less than 100 nm in tip-diameter at a contact force of 20 μN. The presence of β-C3N4 like phase characterized by a Nls XPS peak at 400.5 eV has found to be crucial for high wear resistance of the a-C:N films.

Nanomechanical Properties of Amorphous Carbon and Carbon Nitride Thin Films Prepared by Shielded Arc Ion Plating

ABSTRACTHydrogen free amorphous carbon (a-C) and carbon nitride (a-C:N) were synthesized by means of shielded arc ion plating in which a shielding plate was inserted between a target and a substrate in order to reduce macroparticle deposition onto the substrate. Using a graphite target as a cathode, thin films of a-C and a-C:N were prepared in an arc discharge plasma of argon or nitrogen gas, respectively, at a pressure of 1 Pa. Based on nanoindentation, mechanical properties of these films were studied in relation to substrate bias voltage (Vs). The a-C films prepared at Vs ranging from -50 to -200 V consisted of diamond-like phase and showed hardness higher than 20 GPa with its maximum of 35 GPa at Vs = -100 V. However, hardness of the films deposited at Vs < 300 V was less than 7 GPa indicating that the films were converted to graphite-like phase due to excessive ion impact in Ar plasma. Wear resistance of the a-C films was closely related to their hardness. Namely, a harder a-C film was more wear resistant. On the contrary, hardness of the a-C:N films was less dependent on Vs. It remained in the range of 10 to 15 GPa and was much lower than the maximum hardness of the a-C films. Nevertheless, the wear resistance of the a-C:N films was comparable to or much better than the a-C films. In particular, the a-C:N film prepared at Vs = -300 V was so wear resistant that the film showed no apparent wear when rubbed with a diamond tip less than 100 nm in tip-diameter at a contact force of 20μN. The presence of β-C3N4like phase characterized by a N1 s XPS peak at 400.5 eV has found to be crucial for high wear resistance of the a-C:N films

Structural, morphological, and mechanical properties of amorphous carbon and carbon nitride thin films deposited by reactive and ion beam assisted laser ablation

Amorphous carbon nitride thin films have been prepared on Si (100) wafers by nitrogen ion beam assisted Nd:YAG laser ablation techniques. Amorphous carbon and carbon nitride films have also been prepared by the conventional laser ablation techniques for comparison. Raman spectroscopy and spectroscopic ellipsometry have been performed for the films to analyze structural properties, atomic force microscopy to observe surface morphologies, and scratch, acoustic emission, and Vicker hardness test to examine mechanical properties. The amorphous carbon nitride films deposited by the ion beam assisted laser ablation techniques had generally better mechanical properties compared to the amorphous carbon films and amorphous carbon nitride films deposited in N2 atmosphere. The amorphous carbon nitride films deposited at optimum ion beam current of 10 mA and laser power density of 1.7 × 109 W/cm2 showed excellent mechanical properties: root mean square surface roughness of 0.33 nm, friction coefficient of 0.02–0.08, the first crack and critical load of 11.5 and 19.3 N respectively, and Vicker hardness of 2300 [Hv]. It is considered that the films have high potential for protective coatings for microelectronic devices such as magnetic data storage media and heads.

Modification of the electron spectrum and vibrational properties of amorphous carbon by copper doping

The near-gap electron spectrum and the effective charge distribution in graphite-like carbon nanoclusters of simple geometry in a-C: H containing a single Cu atom are calculated in the tight-binding approximation. Only the coupling between π electrons of the constituent C atoms and one valence s electron of the Cu atom is taken into account. The binding energy of the Cu atom in the clusters and the static dipole moment of the clusters are calculated. The results are invoked to interpret the experimentally observed activation of the Raman G band in the IR spectrum of a-C: H: Cu as a consequence of a lowering of the symmetry of the graphite-like clusters due to copper intercalation. Experimental data on the time dependence of the G band intensity during isothermal annealing of a-C: H: Cu are presented. The data suggest the possibility of reversible transfer of Cu atoms between the impurity states in the copper-carbon clusters and the impurity states in the purely copper clusters. The average activation energies of direct and reverse transfer are estimated from the experiment.

Comparison of bonding in amorphous silicon and carbon

We present detailed calculations using the total energy pseudopotential method of relaxed amorphous structures at various densities formed metastably in silicon and carbon. These calculations show a number of interesting features: fivefold coordination of atoms in silicon but not in carbon, bond lengths in carbon increased at reduced volume, with the opposite trend in silicon, and three-center orbitals in carbon. Also, the energy is nearly independent of volume in carbon, suggesting that properties of amorphous carbon will depend strongly on the growth process.

Influence of ion energy and substrate temperature on the optical and electronic properties of tetrahedral amorphous carbon (ta-C) films

The properties of amorphous carbon (a-C) deposited using a filtered cathodic vacuum arc as a function of the ion energy and substrate temperature are reported. The sp3 fraction was found to strongly depend on the ion energy, giving a highly sp3 bonded a-C denoted as tetrahedral amorphous carbon (ta-C) at ion energies around 100 eV. The optical band gap was found to follow similar trends to other diamondlike carbon films, varying almost linearly with sp2 fraction. The dependence of the electronic properties are discussed in terms of models of the electronic structure of a-C. The structure of ta-C was also strongly dependent on the deposition temperature, changing sharply to sp2 above a transition temperature, T1, of ≈200 °C. Furthermore, T1 was found to decrease with increasing ion energy. Most film properties, such as compressive stress and plasmon energy, were correlated to the sp3 fraction. However, the optical and electrical properties were found to undergo a more gradual transition with the deposition temperature which we attribute to the medium range order of sp2 sites. We attribute the variation in film properties with the deposition temperature to diffusion of interstitials to the surface above T1 due to thermal activation, leading to the relaxation of density in context of a growth model.

O( N) tight-binding molecular dynamics study of amorphous carbon

We have carried out molecular dynamics simulations of amorphous carbon using the recently developed density-matrix order N tight-binding molecular dynamics scheme. Structures and properties of amorphous carbon at three different regimes (i.e., porous, graphite-like, and diamond-like) have been studied. The simulation results demonstrate that the order N tight-binding molecular dynamics scheme is accurate and efficient for atomistic simulation of large and complex systems.

Optical Properties of Carbon thin films Containing Nanoparticles

ABSTRACTThe incorporation of carbon nanoparticles in the forms of nanotubes and bucky onions have shown to improve the mechanical properties of amorphous carbon (a-C) thin films. In this paper we report on the change in the optical properties of a-C films containg nanoparticles. The optical band gap, index of refraction and extinction coefficient are studied. The optical band gap in highly tetrahedral amorphous carbon (ta-C) is found to be around 2 eV. It decreases almost linearly with the sp2 fraction. It is theorized that the clustering of the sp2 sites leads to a reduction in the band gap. In this paper, we study the influence of large sp2 clusters in forms of graphitic nanoparticles on the optical of ta-C. We find that the optical gap remains around 1.8 eV even with the large inclusions of clustered nanoparticles. Furthermore, the gap remains close to 1.8 eV even when the sp2 fraction in the amorphous matrix is increased. The index of refraction however is found to decrease with the sp2 fraction indicating a reduction in density.

Model studies of the structure and chemical bonding in amorphous carbon films prepared by magnetron sputtering

Abstract The microstructure and chemical bonding properties of amorphous carbon with 2.0 g/cm 3 mass density prepared by magnetron sputtering have been studied by electron diffraction measurements, molecular dynamics and Monte Carlo simulations. The model structures were generated by simulated annealing techniques applying both classical Tersoff and Keating potentials and a quantum mechanical density functional method. By comparison of the experimental results with theoretically simulated diffraction data, the atomic scale structure is analyzed. The chemical bonding properties of the models are evaluated within a local valence orbital description. In this description, the electronic band gap properties are related to the formation of π-bonded sp 2 clusters. Good agreement is found between experimental and simulated diffraction data for models showing a minimum in their electronic density of states at the Fermi energy. In these models, 60% of the atoms belong to one large π-bonded cluster and, hence, from the matrix of the structures. Isolated aromatic clusters of any size are not found. In the best fitting model, only 14% of all five-, six- and seven-membered rings are completely π-bonded.

Stability, Chemical Bonding and Vibrational Properties of Amorphous Carbon at Different Mass Density

ABSTRACTWe describe correlations between the atomic-scale structure and the global electronic and vibrational properties in amorphous carbon versus mass density. The model structures have been generated by applying different annealing regimes using a density-functional based molecular-dynamics (DF-MD) method. The stability of the amorphous modifications and the calculated vibrational density of states (VDOS) are strongly affected by the density and the annealing sequences, altering the chemical composition, the sp/sp2/sp3-clustering, the structure and related physical properties. A mass density of 3.0 g/cm3 is confirmed as a magic density favoring the formation of most stable a-C modifications having lowest defect densities and maximum band gap. In analyzing the vibrational spectra and the localization of modes in comparison with crystalline diamond and graphite, we identify the spectral signatures for chemically different bonded species and defects, that may be used for comparison with related Neutron-, Raman- and IR- work.