A.P. Burden

A study of electron field emission as a function of film thickness from amorphous carbon films

The electron field-emission properties of hydrogenated amorphous carbon and nitrogenated tetrahedral amorphous carbon thin films are examined by measuring the field-emission current as a function of the applied macroscopic electric field. The experimental results indicate the existence of an optimum film thickness for low-threshold electron field emission. The predictions of various emission models are compared to the experimental results.

Thermal stability of plasma deposited thin films of hydrogenated carbon–nitrogen alloys

The need to grow high quality semiconducting hydrogenated amorphous carbon (a-C:H) thin films to allow n-type electronic doping by nitrogenation has lead us to deposit films with low paramagnetic defect density (1017 cm−3). The films were grown on the earthed electrode of a radio frequency driven plasma enhanced chemical vapor deposition system using methane, helium and a range of nitrogen concentrations as the precursor gases. The deposited films are shown to be polymer like. Changes in the chemical structure and relative bond fractions as a function of the nitrogen flow rate into the plasma chamber and ex situ annealing are reported. Particular attention is paid to changes in the film structure after annealing at 100 °C, since an increase in the E04 optical band gap is observed as a function of nitrogen flow after the anneal. This suggests a decrease in the defect density of the film.The need to grow high quality semiconducting hydrogenated amorphous carbon (a-C:H) thin films to allow n-type electronic doping by nitrogenation has lead us to deposit films with low paramagnetic defect density (1017 cm−3). The films were grown on the earthed electrode of a radio frequency driven plasma enhanced chemical vapor deposition system using methane, helium and a range of nitrogen concentrations as the precursor gases. The deposited films are shown to be polymer like. Changes in the chemical structure and relative bond fractions as a function of the nitrogen flow rate into the plasma chamber and ex situ annealing are reported. Particular attention is paid to changes in the film structure after annealing at 100 °C, since an increase in the E04 optical band gap is observed as a function of nitrogen flow after the anneal. This suggests a decrease in the defect density of the film.

The microstructural dependence of the opto-electronic properties of nitrogenated hydrogenated amorphous carbon thin films

Abstract The microstructural properties of nitrogenated hydrogenated amorphous carbon (a-C:H:N) thin films deposited using a chemical vapour deposition system are analysed in order to evaluate their impact on the opto-electronic properties. Electron energy loss spectroscopy is used to reconstruct a joint density of states (JDOS) for the a:C:H:N films and thus used to examine the microstructure. Information obtained from optical absorption is then used to confirm the JDOS. Using this JDOS we attempt to predict the variation expected in the electronic conduction as a function of nitrogen content. The electrical data obtained for the a:C:H:N thin films appear to be bulk controlled as opposed to metal contact dominated for films that are deposited on the driven electrode and are more diamond-like in character. The bulk electronic properties at high fields are fitted to different types of conduction behaviour in order to obtain the best fits to the data. From this study it is observed that a Poole–Frenkel type fit is best for films that have a diamond-like structure. For the films that have a polymeric structure which are deposited on the earthed electrode the conductivities are very much lower, and consistent with the lower defect densities observed in the microstructural study. It is possible that the conduction in these films are Schottky barrier controlled.

In situ fullerene formation—The evidence presented

We have recently reported the real-time, in situ nucleation of fullerene molecules in a modified high resolution transmission electron microscope, providing direct visual evidence for the curvature of intact graphene sheets leading to the formation of discrete single shells. We now report a similar phenomenon on the surface of previously untreated carbon black particles, present analogous results using hexagonal boron nitride, discuss further video-captured evidence of the mechanisms involved, and draw together other related work including the formation of bucky-onions and nano-diamonds. We conclude that in situ electron microscopy experiments provide a valuable opportunity to understand the allotropic transformations of carbon, leading to a better appreciation of the parameters necessary for fullerene nucleation. However, we also highlight the importance of taking into account the specifications of the particular electron microscopes used for such investigations, and in particular the vacuum systems employed.

Enhancing the field emission properties of amorphous carbon films by thermal annealing

Abstract Polymer-like hydrogenated and nitrogenated amorphous carbon films (a-C:H:N) have been deposited on silicon and glass substrates using a Plasma Technology DP800 radio frequency plasma enhanced chemical vapour deposition system. This equipment was configured with an earthed water-cooled substrate table, allowing the carbon films to grow under low bias conditions yielding films with refractive indices of ∼1.5−1.7, and E 04 optical band-gaps of ∼3−4 eV. The field emission properties of these films have been studied using a sphere-to-plane anode–cathode configuration, and the dependence on annealing treatment investigated. A significant lowering of the emission threshold field has been measured after treating the films for 1800 s at 400°C. This has been correlated with a simultaneous study of the change in the films’ microstructural properties. We conclude that the changes commensurate with the onset of graphitization are beneficial for field emission, and that the associated improvements in the films’ mechanical stability will aid incorporation into large-area displays.

The stability of nitrogen-containing amorphous carbon films after annealing at moderate temperatures

Abstract The future of electronic devices fabricated from amorphous carbon films will only be ensured if the optical, electrical, and structural properties of the material remain stable in a variety of operating conditions, including moderate short-term elevations in temperature. In addition, the sensitivity of these metastable films to structural changes as a consequence of their thermal history provides a good opportunity to further modify and tune the material by using carefully applied heat treatments. This paper begins to address both these concerns by studying the variation of the optical band-gap and refractive index of a variety of amorphous carbon films deposited using plasma-enhanced chemical vapour deposition techniques. It shows that the expected graphitisation of the carbon films occurs after relatively short annealing times even at moderate temperatures, but that there is scope to improve the quality of the films prior to the onset of degradation.

A study of the effects of nitrogen incorporation and annealing on the properties of hydrogenated amorphous carbon films

Abstract The electronic properties of hydrogenated amorphous carbon films deposited using a Plasma Technology DP800 radio frequency plasma-enhanced chemical vapour deposition system are investigated. Films deposited on the driven electrode have a Tauc optical band-gap of 0.9–1.2 eV, a refractive index of 1.8–2.3, and are hard and diamond-like. However, films deposited on the earthed electrode are softer and polymer-like with a Tauc optical band-gap of 2–3 eV and a refractive index of 1.5–1.7. Both types of film have been grown with varying amounts of nitrogen in an attempt to dope them and measure their characteristics. Films grown on the driven electrode showed current versus voltage (I/V) characteristics indicative of Poole-Frenkel type conduction. However, the I/V characteristics of the films grown on the earthed electrode exhibited high resistivity (typically 10 14 –10 15 Ωcm). Thermal annealing of the films grown on the earthed electrode has also been investigated. The films containing nitrogen were found to be more sensitive to annealing.

Fullerene and nanotube formation in cool terrestrial “dusty plasmas”

The simultaneous generation of dust during the deposition of semiconducting thin films by radio frequency plasma enhanced chemical vapor deposition has so far been regarded as a troublesome by-product. However, we present results from recent microstructural investigations of carbonaceous dust particles from a methane precursor that demonstrate that the technique may be suited to generating fullerene molecules, nanotubes, and nanoparticles. Chemical analysis reveals that these particles contain few contaminant species, and we deduce that they nucleated in the plasma, with the carbon ions possibly self-arranging through the action of coulombic forces.

Modification of electron field emission properties from surface treated amorphous carbon thin films

The field emission properties of amorphous hydrogenated carbon (a-C:H) films and nitrogenated a-C:H (a-C:H:N) films subjected to 10 keV Co60 ion implantation are investigated as a function of ion dose. The average threshold electric field for conditioned nitrogenated and non-nitrogenated unimplanted films is found to be 27 and 29 V/μm. Implantation of C60 ions to a dose of 7.5×1013 cm−2 results in an increase in the threshold field of both the nitrogenated and non-nitrogenated films. However, implantation to a dose of 1.25×1014 cm−2 results in a reduction in the threshold fields to values close to those found in the unimplanted samples. At larger doses of 2.65×1014 cm−2, the threshold field for the non-nitrogenated samples remain largely unchanged though there is an increase in the refractive index consistent with an increase in the optical density of the film. In the nitrogenated film, the average threshold field at the highest dose again increases to values comparable to those found for C60 implantation...

Microstructural characterisation of carbonaceous dust generated during the deposition of diamond-like carbon coatings

During plasma enhanced chemical vapour deposition (PECVD) of diamond-like carbon coatings, material is routinely deposited on the electrodes and chamber walls of the system. After prolonged deposition times, this material exfoliates, contributing to dusty plasmas and affecting the quality and microstructural properties of the films. We have generated such dust in a Plasma Technology DP800 radio frequency PECVD system, using a methane precursor at a variety of powers, pressures and electrode temperatures. A JEOL 2010 analytical electron microscope equipped with X-ray analysis (EDX) has confirmed that this dust is predominantly carbonaceous with negligible metal contamination from the chamber. JEOL 2000 FX and JEOL 4000 EX high resolution transmission electron microscopes (HRTEMs) have been used to investigate the morphology and microstructure of these dust particles for comparison with the amorphous diamond-like carbon films that are more typically produced using the apparatus. A significant number of the dust particles analysed revealed highly curved graphene layers and microcrystalline nano-particles more typically observed in the soot following the arc-discharge generation of fullerenes. This is in contrast to the amorphous nature of the carbon films deposited over shorter times on silicon and glass substrates, which rarely show any microcrystalline inclusions.