R.U.A. Khan

Influence of sp2 clusters on the field emission properties of amorphous carbon thin films

The influence of the concentration and size of sp2 carbon clusters on the field emission properties of hydrogenated amorphous carbon thin films is investigated. In combination with electron paramagnetic resonance and optical measurements, it is shown that the trend in the threshold field for emission for films deposited under certain conditions can be explained in terms of improvements in the connectivity between sp2 clusters. These clusters are believed to be located near the Fermi level, and the connectivity is primarily determined by the cluster size and concentration, which in turn is determined by the choice of deposition conditions. Details of the appropriate emission mechanisms for different types of deposited carbon films are discussed.

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.

Amorphous carbon thin films

Publisher Summary This chapter focuses on the evolution of a carbon (C) thin films and maps out the significant contributions to the field by numerous research laboratories. The impact of the microstructure and growth on the optical and electrical properties is also examined in the chapter. New results show how ion implantation allows a methodology to delocalize gap states within these films. Carbon is unique in its structure by being able to form one of the strongest materials known to man—diamond—or one that is soft—graphite—by virtue of the way in which each atom bonds to another. All of these variations are made possible by the three different bond hybridizations that are available to carbon. Diamond-like carbon (DLC) should generally be reserved for polycrystalline or nanocrystalline carbon films, whereas amorphous carbon films should generally fall into the categories of polymer such as amorphous carbon (PAC), graphite-like amorphous carbon (GAC), diamond-like amorphous carbon (DAC), tetrahedral amorphous carbon (TAC), and nano-composite amorphous carbon (NAC).

A review of the effects of carbon self-implantation into amorphous carbon

Abstract Studies carried out on the ion implantation of diamond and amorphous carbon (a-C) in its polymer-like (PAC), tetrahedral (TAC) and diamond-like (DAC) forms have been reviewed. It has been reported that implantation into amorphous carbon results in a re-ordering into a sp 2 -rich state. However, this does not explain the range of optical and electrical results which have been observed. It was shown that the resistivity variation of TAC and DAC films are similar at high doses, although the corresponding band gap data show a significant difference. Based on these data, a model for the increase of electron delocalisation with ion dose within these films is postulated. Also, the role of nascent hydrogen within the films as the ion dose is increased is discussed.

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.

Ion-implantation into amorphous hydrogenated carbon films

Abstract Amorphous hydrogenated carbon is being investigated as a possible semiconducting material. A required property of a semiconductor is electronic doping, and this may be achieved either by in situ addition of gaseous precursors during deposition or ex situ ion-implantation. This study shows that resulting ion beam damage produced by ion-implantation may be kept to a minimum as long as the ion dose is less than approximately 10 14 cm −2 . Polymer-like carbon films grown using radio frequency plasma enhanced chemical vapour deposition have been implanted with carbon, nitrogen and boron ions, and the electronic and optical properties of the material analysed. An analysis of the electrical and optical data is carried out to distinguish between the physical and chemical changes that occur within the microstructure of the films.

Electron paramagnetic resonance study of ion implantation induced defects in amorphous hydrogenated carbon

Electron paramagnetic resonance (EPR) measurements have been made of defects in amorphous hydrogenated carbon (a-C:H) thin films. The films were grown on silicon substrates on the earthed electrode of an rf-powered plasma enhanced chemical vapour deposition reactor and were subsequently implanted with a range of doses of boron, carbon or nitrogen ions with energies from 20 keV to 32.5 keV. Two paramagnetic centres are observed, the carbon defect in the film with g = 2.0028(1) and a silicon defect in the substrate with g = 2.0058(6). The volume concentration of the carbon defect increases approximately linearly with dose, from approximately 3 x 10 17 cm -3 for unimplanted samples to 2.7 × 10 20 cm -3 at the highest implantation of 2 × 10 16 B + ions cm -2 . The increase in dose over this range also causes a narrowing of the EPR line (from 0.83 mT to 0.13 mT) and a significant decrease in the spin-lattice relaxation time (from 3 x 10 -5 s to 6 x 10 -8 s) which approaches the spin-spin relaxation time at the highest dose. The narrowing is attributed to motional averaging produced by either exchange or hopping. We also report the effects of annealing samples implanted with a range of boron doses. The prime novelty of this paper is that it is the first EPR study of defects produced by the implantation of a range of ions into polymer-like amorphous hydrogenated carbon.

Growth and characterisation of amorphous carbon films doped with nitrogen

Abstract Hydrogenated amorphous carbon (a-C:H) thin films are of interest to the electronics industry as possible inexpensive semiconductors, especially for device passivation. Nitrogenation is a promissing method for the doping of the films. We have grown a-C:H films on the earthed electrode of a radio frequency driven plasma enhanced chemical vapour deposition system using methane, helium and a range of nitrogen concentrations as precursor gases. We subsequently studied the composition of the films with heavy ion Elastic Recoil Detection analysis. Nitrogen concentrations increasing from 0 to 8 at.% with N flow rate are observed, with a corresponding decrease of the carbon concentration. The hydrogen concentration remains approximately constant as a function of nitrogen flow into the chamber. Fourier Transform Infrared analysis confirmed that with increasing nitrogen content there is a C–N and N–H bond concentration increase. This is accompanied by a reduction in the C–H bond concentration.

Tailoring of the field emission properties of hydrogenated amorphous carbon thin films by nitrogen incorporation and thermal annealing

Abstract Amorphous carbon (a-C) films have been shown to be a potential cold cathode for future flat panel displays. In this paper we report the changes in the threshold electric field of hydrogenated a-C (a-C:H) thin films as a function of nitrogen content and thermal annealing. Films deposited with a plasma enhanced chemical vapour deposition system were characterised using optical techniques, Rutherford backscattering spectroscopy (RBS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and ellipsometry. Field emission testing was performed using a plane-to-sphere electrode configuration. Our results show that the threshold field for electron emission can be reduced with the incorporation of nitrogen. Annealing was also shown to alter the threshold field along with film thickness, bulk material properties, optical band-gap and refractive index. These results are discussed in terms of a space charge controlled emission model.

Switching phenomena in boron-implanted amorphous carbon films

Abstract Amorphous hydrogenated carbon (a-C:H) has been extensively researched as an electronic material. In this study, a-C:H is implanted with boron ions at a dose of 2×1015 cm−2. It is observed that the current versus voltage shows a number of distinct conduction regimes. The likely mechanism is a combination of space charge and barrier effects. A uniform implant throughout the film results in symmetrical characteristics, whilst an implant through only the first half of the film results in switching only on one side, reinforcing the suggestion that the conduction mechanism is barrier controlled.