Amorphous carbon (a-C) structures over a wide range of densities are generated by tight-binding molecular dynamics simulations using the recently developed environment-dependent carbon tight-binding potential. Our simulation results show that the relative concentration of the sp2 and sp3 bondings in the a-C samples changes systematically with the density of the samples. The a-C networks obtained by quenching the low density liquids consist of mostly three-fold coordinated atoms while the diamond-like tetrahedral a-C can be generated by quenching the high density (about 3.0g/cm3) liquid carbon.

We present the results of a systematic first-principles investigation of the requirements for developing realistic and reliable structural models for amorphous tetrahedral carbon (a-tC) and relate those structural models to the physical properties of this material. Within a linear combination of atomic orbitals formulation of density functional theory, we show that a large variational flexibility is required to accurately treat the highly defected and strained structures that can exist in a-tC. The average strain in the a-tC lattice is predicted to be roughly twice the strain of having all carbon atoms in three-member rings. A key figure of merit of a structural model, the proportion of three-fold bonded atoms, is shown to triple in going from a minimal basis description of a structure to a high quality basis. The basis-converged calculations agree well with experimental observables, such as the presence of four-member rings, lack of dangling bonds, and a significant gap. The simulations predict a much larger proportion of three-fold atoms than estimated in simple analyses of EELS and neutron scattering experiments. We show that the larger three-fold fraction is indeed consistent with the properties of a-tC, and imply that there are flaws in the simplifying assumptions that go into constructing experimental estimates of coordination numbers. These results highlight the perils of applying highly simplified theoretical models for a-tC before the correct physics has been identified and built into the models.

Using tight-binding molecular dynamics, we have performed two series of computer experiments assembling small fullerenes: first, we have simulated the deposition of C28 clusters on a semiconducting surface, and then we have performed calculations mimicking the gas phase growth of a small fullerene solid. The results of the two computations are discussed and compared to those obtained for ordered forms of C28 solids.

Doping in hydrogenated amorphous silicon occurs by a process of an ionised donor atom partially compensated by a charged dangling bond. The total energies of various dopant and dopant/bonding combinations are calculated for tetrahedral amorphous carbon. It is found that charged dangling bonds are less favoured because of the stronger Coulombic repulsion in ta-C. Instead the dopants can be compensated by weak bond states in the lower gap associated with odd-membered π-rings or odd-numbered π-chains. The effect is that the doping efficiency is low but there are not charged midgap recombination centres, to reduce photoconductivity or photoluminescence with doping, as occurs in a-Si:H.

Amorphous carbon is an elemental form of carbon with low hydrogen content, which may be deposited in thin films by the impact of high energy carbon atoms or ions. It is structurally distinct from the more well-known elemental forms of carbon, diamond and graphite. It is distinct in physical and chemical properties from the material known as diamond-like carbon, a form which is also amorphous but which has a higher hydrogen content, typically near 40 atomic percent. Amorphous carbon also has distinctive Raman spectra, whose patterns depend, through resonance enhancement effects, not only on deposition conditions but also on the wavelength selected for Raman excitation. This paper provides an overview of the Raman spectroscopy of amorphous carbon and describes how Raman spectral patterns correlate to film deposition conditions, physical properties and molecular level structure.

The bonding in a series of unhydrogenated amorphous carbon films has been analysed quantitatively using Raman spectroscopy with ultraviolet excitation. The Raman spectra exhibit two broad Raman peaks at 1650 cm−1 and 1100 cm−1, due to sp2 and sp3 vibrational modes respectively. The former is a resonance feature associated with a large proportion of paired sp2 sites, while the latter is a weighted vibrational density-of-states for the distorted random network of sp3 sites. The position and relative intensity of the two peaks are shown to be strongly correlated with the percentage of sp3 sites in the films, providing a reliable measure of sp3 bonding which is both quantitative and non-destructive.

We present the results of a post-deposition annealing structural study on amorphous tetrahedrally-coordinated carbon (a-tC) films on Si(100) prepared by pulsed-laser deposition. Films as-deposited and post-annealed at 200, 300, 400, 500 and 600 °C, respectively, are studied using combined x-ray reflectivity and low-angle scattering measurements. The scans are fit to the Fresnel equations to obtain values for average film density, film and interface thickness, and film and interface roughness. We observe a correlation between the evolution of film density, roughness and the spacing of quasi-periodic structures in the films as a function of annealing temperature. We relate the evolution of these structural features with previous measurements of the resistivity and the observed stress release in these films.

We have investigated the effect of dopants on the reduction of internal compressive stress in diamond-like carbon (DLC) films prepared by pulsed laser deposition on Si(100) substrates. A novel target configuration was used to incorporate dopants into DLC films by sequential pulsed laser ablation of two targets. These dopants include copper, titanium and silicon. The thickness of the DLC films deposited was measured in the range 400nm - 600nm using a profilometer. Raman spectroscopy was employed to analyze the chemistry of the films. The shifts of the G-peak position in the Raman spectrum, due to different concentrations of dopant, were used to estimate the internal stress changes. All of the films showed a Raman spectrum typical of DLC films containing a high fraction of sp3 species, with the G-peak centered at around 1510–1560cm−1. The shift of the G-peak due to the presence of dopants was observed for all the DLC films as compared to the undoped one. It was found that Ti has the strongest tendency to reduce the compressive stress of DLC films. This effect increases with increasing concentration of dopants. Silicon was also observed to have this effect, but the G-peak position did not appear to shift with different Si concentrations. Buckling occurred in the as-deposited, undoped DLC film because of the relief of the large compressive stress accumulated in the film, while all the doped DLC films showed good adhesion to the substrate. The results are discussed combining the atomic structure of DLC and the structure and properties of the dopants.

Thin films of light atomic weight elements in amorphous, partially-crystalline, or crystalline forms have applications in a broad range of technologies. For example, amorphous tetrahedral carbon (a-tC) and polymeric thin films impact electronic materials technology as electron- and light-emitting device elements, respectively. A lack of crystallinity introduces complexity in the experimental and theoretical characterization of these materials but is not necessarily a limiting factor in their performance. While the growth process is clearly a major factor governing the physical properties of a film, interactions with the substrate are also important, so surface and interface analysis provides an important complement to bulk measurements. Currently, the fundamental and applied aspects of the atomic, electronic and vibrational structure of these complex materials are being elucidated by novel approaches combining several experimental techniques with theoretical calculations. This paper focuses on several approaches in the characterization and modification of thin films made possible by recent experimental advances. The structural and electronic properties of two model systems are considered as examples: a-tC thin films grown by pulsed laser deposition (PLD) and poly aniline thin films grown by vapor deposition. First, scanning probe microscopies and x-ray scattering are used to investigate the structural aspects of a-tC films as a function of PLD growth conditions. The possible connection of nanoscale surface modification and characterization with electron emission properties will be discussed. Second, the results of inelastic scattering spectroscopy and other surface techniques will be discussed to obtain information on both interfacial aspects of the growth of polyaniline thin films and microscopic and macroscopic aspects of electrical conductivity upon doping. Comparisons will be made with other studies that address properties of analogous crystalline systems as appropriate. A brief assessment of the broader problem of analyzing these systems will be given.

The modification of the diamond surface through adsorbants offers the opportunity to adjust the electronic and electron emission properties of the surface. In the study presented here, we deposited between 0.1 and 100 monolayers of carbon from an electron beam evaporation source on polycrystalline diamond films. Photoelectron spectroscopy in the ultraviolet and X-ray regime was employed to characterize the surface. Observations on a (100) polycrystalline diamond film show, that the surface is first depleted of hydrogen and subsequent growth of an amorphous carbon film (a-C) occurs on the reconstructed surface. The deposition of these ultrathin carbon films allows the controlled introduction of sp2carbon and p-π states onto the diamond surface. The field emission current increases considerably with the amount of sp2-carbon accumulated at the diamond surface. The current-voltage characteristics only partially follow the Fowler-Nordheim equation, and the results obtained for different films are described and possible emission mechanism discussed.

The microstructure of highly sp2 bonded amorphous carbon and partially tetrahedrally bonded amorphous carbon deposited on needle-shaped molybdenum field emitters by pulsed laser ablation and cathodic arc deposition techniques was studied using transmission electron microscopy and electron energy loss spectroscopy. Undoped and nitrogen-doped films were included in this study. The undoped laser ablation films were approximately 50% sp2 at the emitter tip and 65% sp2 along the shank, while the N-doped laser ablation films were highly sp2 bonded both at the tip and along the shank. These laser ablation films were continuous and relatively uniform, exhibiting an isotropie microstructure at the emitter tip and a columnar microstructure along the shank. The cathodic arc deposited films were predominantly sp2 bonded both at the tip and along the shank; these films were non-uniform, with an isotropie microstructure at the tip and regions of isotropie, columnar, and mixed microstructure appearing along the shank.

In this work we report some results concerning Raman characterisation of covalently bonded amorphous carbon based thin films (∼ 100 nm) of binary alloys, obtained by ion implantation of carbon ions into silicon and germanium matrices. It will be shown that compositional disorder is present in our samples even below the stoichiometric concentration (∼50 carbon at.%) as deduced by the presence of three vibrational features in the Raman spectra. A quantitative measure of the compositional disorder will be also given by using either Raman or infrared spectroscopies in samples with different carbon content.

The electronic transport mechanism in tetrahedrally-coordinated amorphous carbon was investigated using measurements of stress relaxation, thermal evolution of electrical conductivity, and temperature-dependent conductivity measurements. Stress relaxation measurements were used to determine the change in 3-fold coordinated carbon concentration, and the electrical conductivity was correlated to this change. It was found that the conductivity was exponentially proportional to the change in 3-fold concentration, indicating a tunneling or hopping transport mechanism. It was also found that the activation energy for transport decreased with increasing anneal temperature. The decrease in activation energy was responsible for the observed increase in electrical conductivity. A model is described wherein the transport in this material is described by thermally activated conduction along 3-fold linkages or chains with variable range and variable orientation hopping. Thermal annealing leads to chain ripening and a reduction in the activation energy for transport.

We have studied the photoconductivity in pure tetrahedral amorphous carbon (ta-C) and hydrogenated tetrahedral amorphous carbon (ta-C:H). Good photoconductive properties are demonstrated for ta-C:H, showing that the hydrogenated form of ta-C is of a higher electronic quality. Transport and recombination parameters are derived. Ta-C:H are low mobility solids with a μτ product of the order of 10−12 cm2 V−1 and a recombination time τr of about 10−7 s. At low energy excitation, the photoconductivity shows a sublinear dependence on the light intensity over a wide temperature range. The relationship between the photoconductivity and the density of spin defect centers is discussed. UV light is used to excite carriers into the extended states. Competitive recombination centers may be involved at high excitation energy.

Hydrogenated-amorphous carbon films are deposited in a microwave ECR plasma reactor with an rf biased substrate holder using methane-argon and acetylene-argon gas mixtures. The film's optical properties are characterized versus deposition conditions including pressure (1–5 mTorr), rf induced bias (-100 to 20 volts), and argon/hydrocarbon flow ratio. The acetylene-argon discharges show that ion species formed from acetylene are more important than the argon ions, conversely, the methane-argon discharges showed that argon ions are a major ionic species in the deposition. The influence of the thickness of insulating substrates is studied and the feasibility of depositing multilayer a-C:H films with different film properties in each layer is demonstrated.

The 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.

Much interest has been shown in the use of tetrahedral amorphous carbon (ta-C) deposited by filtered cathodic arc as an inexpensive, easily produced, wide band-gap semiconductor in the fabrication of electronic devices. There has, however, been limited success in producing devices with properties that might make its use in electronic applications commercially viable, which in part may be due to the high density of electronic trap states as reflected in ta-C's rather high ESR signal of ∼ 1020 spin/g. Recent results at the University of Sydney suggest, however, that a new range of possibilities exist in the utilisation of these traps as a means of producing nonvolatile digital information storage. Devices with write times of 100 μs, read times of 100 ns, and effective memory retention times approaching 1 year, have been fabricated.

A characteristic ion energy and substrate temperature dependence for the formation of tetra-hedral amorphous carbon (ta-C), cubic boron nitride (c-BN) and related diamondlike materials by ion deposition has been experimentally observed. We present an analytical model for diamondlike film growth that describes the transient modification of the film structure by successive individual ion impact events. Each ion impact is treated as a cylindrical thermal spike with a finite initial width, taking into account energy loss and energy dissipation processes. We show that rearrangement of atoms during a cylindrical spike is the dominant mechanism leading to the formation of the diamondlike phase. The model explains quantitatively the observed ion energy dependence of the sp3 bond fraction in ta-C, as well as the broad range of ion energies for which c-BN growth is observed.

The cathodic arc provides a method for depositing extremely hard thin carbon films. These films are potentially useful in a variety of tribological applications including protection of magnetic media and recording heads. Effective application of this technology requires an improved understanding of the influence of process parameters on the microstructure and hardness of the deposited films. In this work, we use a commercially available filtered cathodic arc to deposit tetrahedral amorphous carbon at several deposition angles.

Hard carbon films can be prepared by the condensation of energetic carbon species at or below room temperature. These amorphous films are primarily tetrahedrally coordinated and contain high fractions of sp3 bonds leading to the terminology amorphous diamond. These films have been successfully doped with phosphorus up to 1 at.%, by other researchers by using a phosphorus doped graphite target. We have also investigated evaporated phosphorus in conjuction with a filtered cathodic arc to incorporate phosphorus into the films and have successfully incorporated phosphorus up to 40 at.% into our films using this technique. XPS showed that some of the phosphorus was clustered. PEELS revealed that with an incorporation of 40 at.% of phosphorus, the sp3 content was approximately 20%.