Enhancing the electrical conduction in amorphous carbon and prospects for device applications

The problems and possible solutions associated with producing practical electronic devices based upon amorphous carbon (a-C) thin films are discussed. The clustering of the carbon sp² phase is shown to be a critical aspect in understanding the current device limitations. In order to exploit the sp² clustering we show that the use of ion beams to deposit energy into the microstructure in a controlled manner, as opposed to conventional thermal anneal treatments, results in a delocalised electron wavefunction and an enhancement of conductivity. A barrier controlled device is demonstrated by carefully choosing the ion energy and dose. One of the consequences of ion implantation is that film can now be considered as consisting of conductive sp² C clusters within an insulating sp³ C matrix. We show that the presence of this dielectric inhomogeneity between the conductive sp² regions and the sp³ matrix plays an important role in understanding the field emission behaviour from a-C based materials.     

Infrared spectra of amorphous carbon based materials

The IR absorption in amorphous carbon and carbon nitride films is discussed in terms of a phenomenological model based on similar absorptions occurring in doped polyconjugated polymers. It is proposed that the strong IR absorption band observed in the 1800–900 cm^− 1 region arises from changes in the electronic structure in the micro-domains of the sp² clusters due to long and short range charge fluxes. These charge fluxes are enhanced by the increase in the Csp² fraction and/or the increase in the nitrogen content (with the subsequent formation of sp² and sp¹ sites) since both induce clustering of the sp² phase and delocalization of π electrons.     

Preparation and blood compatibility of carbon/TiO2 nanocomposite

A carbon/TiO₂ nanocomposite, which consists of carbon film with various sp³C content and TiO₂ nanowire arrays, has been synthesized, in which the top surface of TiO₂ nanowire arrays prepared using hydrothermal method on fluorine-doped tin oxide glass were coated with carbon thin films. The carbon thin films with a higher, medium and lower sp³C content were deposited by pulsed magnetic filtered cathodic vacuum arc deposition, plasma-enhanced chemical vapor deposition and magnetron sputtering deposition, respectively. The surface morphology and structure of TiO₂ nanowire arrays were investigated by scanning electron microscopy, transmission electron microscope and X-ray diffraction. The sp³C content in carbon films was characterized using Raman spectroscopy. The blood compatibility of the samples including the TiO₂ nanowire arrays, carbon films and carbon/TiO₂ nanocomposite was assessed by tests of platelet adhesion in vitro. Results showed that the carbon/TiO₂ composite can effectively improve the anticoagulant function compared to the single materials. It is believed that the excellent blood compatibility of the carbon/TiO₂ nanocomposite is attributed to a joint function of surface properties adjusted by nanowire arrays and electronic structure of carbon thin films.     

Spatial organization of the sp-hybridized carbon atoms and electronic density of states of hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films prepared by plasma decomposition of hydrocarbons exhibit a wide variety of electronic and mechanical properties depending on their deposition conditions, which makes them very interesting for applications in several domains. This versatility is essentially due to the presence of both sp²- and sp³-hybridized carbon atoms in variable proportions, and to the tendency of the sp² C atoms to gather into π-bonded clusters with different bonding configurations. The relationships between the film microstructure and their electronic density of states, as deduced from a detailed analysis of their optical properties over a large spectral range, are described and discussed, taking as reference materials the purely sp² (graphite) and purely sp³ (diamond) carbon crystalline phases, as well as the prototype hydrogenated amorphous tetra-coordinated semiconductor, hydrogenated amorphous silicon. It is shown that the type of clustering of the sp² C atoms is certainly more determinant for the electronic density of states, and especially for the optical gap value, than the proportion of these atoms in the material.     

Contribution of sp3 clusters in films and fibers obtained from camphor

Considerable attention has been focused on the growth of carbon-based films or fibers by various methods. Diamond-like carbon (DLC) films may be of greater importance in some specific electronic applications such as flat panel displays, which represent a very large market. In this study, carbon-based thin films and fibers obtained from doped camphor soot were studied by confocal microRaman spectroscopy at 632.8 nm.Different contributions were identified between 1000 and 1650 cm⁻¹ in the Raman spectra of the as-grown and laser-annealed films and fibers. The contributions were the D-like and G-like peaks of polycrystalline graphite at about 1345 and 1530 cm⁻¹, respectively, with a FWHM value about 5 times larger than in a:C. It is now well established from correlation between Raman signature and grain size measurements that the width of the Raman line is a decreasing function of the graphite grain size. From these results, one can estimate that the grain size of this polycrystalline graphitic phase was small. An additional feature is observed at about 1240 cm⁻¹ which could be due to sp³-bonded carbon clusters.     

Microstructure effect on field emission from tetrahedral amorphous carbon films annealed in nitrogen and acetylene ambient

Tetrahedral amorphous carbon (ta-C) films have been deposited by filtered cathodic vacuum arc technique. The samples were then annealed at various temperatures in nitrogen and acetylene ambient. The surface morphologies and microstructure of the films were characterized using atomic force microscopy, scanning electron microscopy, visible and ultraviolet Raman spectroscopy. A thin layer of amorphous carbon was deposited on the surface of the ta-C films after annealed at 700 and 800 °C while submicro crystalline pyrolytic graphite was formed on the surface of the ta-C film annealed at 900 °C. The surface layer was found to enhance the sp² clustering of the underlying ta-C layer. Field emission results reveal that the sp² cluster size plays an important role in electron field emission properties. The threshold field decreases as the sp² cluster size increases. For the film annealed at 800 °C, the lowest threshold field and the largest cluster size concurred.     

Thickness dependency of field emission in amorphous and nanostructured carbon thin films

Thickness dependency of the field emission of amorphous and nanostructured carbon thin films has been studied. It is found that in amorphous and carbon films with nanometer-sized sp² clusters, the emission does not depend on the film thickness. This further proves that the emission happens from the surface sp² sites due to large enhancement of electric field on these sites. However, in the case of carbon films with nanocrystals of preferred orientation, the emission strongly depends on the film thickness. sp²-bonded nanocrystals have higher aspect ratio in thicker films which in turn results in higher field enhancement and hence easier electron emission.     

Field emission enhancement and microstructural changes of carbon films by single pulse laser irradiation

The effect of single nanosecond laser pulse irradiation on the microstructure and field emission (FE) properties of carbon films is studied. Amorphous carbon films were exposed to a single pulse of a 248 nm Excimer laser with pulse width of 23 ns. Microstructural changes of the films were investigated by Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. FE study was conducted in a parallel plate configuration. It was found that the landscape of the FE properties is not directly correlated to the laser energy in a simple way, whereas low energy laser irradiation (<117 mJ/cm²) leads to a lower emission threshold field due to the formation of sub-nanometer conductive sp² clusters within the insulating sp³ matrix. A medium energy range (117–362.5 mJ/cm²) would actually reduce field enhancement and increase the threshold field because of the increased size of the same sp² clusters. Interestingly, a much higher laser energy (>362.5 mJ/cm²) would reverse this effect by forming multiple continuous conductive sp² channels and thereby reduce the threshold field sharply again.     

Electrochemical characterization of diamond like carbon thin films

Diamond-like carbon (DLC) thin films were deposited from pure graphite target by DC magnetron sputtering method. Experimental parameters, i.e., substrate temperature and negative bias voltage, have been changed to finely tune the chemical bonding property (sp²/sp³) of the as-deposited DLC films. The as-deposited DLC films were characterized as anode materials for Li–ion batteries and special attentions were paid to the effects of sp²/sp³ ratio on the electrochemical properties of the DLC films. The results indicated that a high fraction of sp² bonding in the DLC films is preferred for high lithium storage capacity, flat and low charge voltage plateau, and long cycling retention.     

Real-time monitoring,growth kinetics and properties of carbon based materials deposited by sputtering

The nucleation stages of growth of carbon-based materials such as amorphous carbon (a-C) and carbon nitride (a-CN_x) films control the bonding configuration, sp² and sp³ formation, N concentration in a-CN_x films and consequently their final properties. Thus, in order to obtain insights into the deposition mechanisms, the study of growth kinetics by applying real-time monitoring is required. We present here, the development of a-C and a-CN_x thin-films by conventional and unbalanced magnetron sputtering, studied by in situ spectroscopic ellipsometry (SE) and multi-wavelength ellipsometry (MWE) in the energy region 1.5–5.5 eV. SE and MWE were used to monitor the deposition processes and to study the films properties. The MWE provides the ability for simultaneous acquisition of the dielectric function, in 16 different wavelengths distributed in the VIS-UV energy region and to investigate the nucleation and growth stages, the deposition rate and films composition, as well as their optical properties during deposition. The results deduced either by real-time MWE monitoring, or by the analysis of in situ SE and MWE data, for the carbon based materials prepared by the above techniques, were certified by X-ray reflectivity studies and were correlated to various mechanisms taking place during film growth (e.g. diffusion, surface adsorption, chemical reactions, etc.), which were investigated by applying a phenomenological model of growth kinetics.     

Field emission from nano-cluster carbon films

Field emission has been reported to occur at much lower fields in carbon based thin film systems than from any other material systems. The emission has been shown to depend on the various material parameters, but whichever carbon based system is used, it is found that emission occurs at localised sites rather than uniformly over the entire surface. Carbon films with mixed sp³/sp² bonding, like nanocrystalline diamond and nanocluster graphitic films emit at lower fields with a higher emission site density than single-phase films. The sp² cluster size in any carbon film can be altered during deposition, but it is easier to control nanocluster size by post-deposition annealing. Annealing increases the sp² cluster size embedded in a sp³ matrix until the sp³ matrix disappears completely and the film transforms into nanocrystalline graphite. To distinguish the effects of the sp² cluster size from other material parameters, a series of different carbon films were annealed post-deposition and the sp² cluster size was measured using visible Raman. Field emission was then measured at a vacuum of 10⁻⁸ mbar on all films using a parallel plate configuration. It was found that the field emission for all films tested depended upon the clustering of the sp² phase and this effect dominates the effects of the other parameters, such as chemical composition, surface termination, sp³ content or conductivity. The optimum size of the sp² was of the order of 1 nm for all systems tested. We believe that field emission occurs form the localised conducting, predominantly sp² bonded regions, which provideds the large field enhancement required for effective emission.     

Surface-enhanced Raman Scattering Study on 1D-2D Graphene-based Structures

In order to explore novel functional nanomaterials, we have produced sp²-sp hybrid carbon structures composed of graphene layers (2D) and linear carbon chains (1D). A remarkable change of the graphene electronic and phononic behaviour is observed after the interaction with 1D carbon nanostructures. Raman and surface enhanced Raman spectroscopies together with a density functional theory approach are used to explain charge transfer phenomena as a function of linear molecule orientation in the produced 1D-2D carbon-based structures, inducing hole-doping in graphene layers.     

Effect of surface treatments on the electronic properties of ultra-nanocrystalline diamond films

We present the soft x-ray spectroscopic study of the ultra-nanocrystalline diamond (UNCD) films with different surface treatments. The samples were prepared by means of microwave plasma enhanced chemical vapor deposition (MPECVD) and the different surface treatments are applied to alter their field emission properties. The electronic properties were subsequently examined by the soft x-ray absorption and x-ray emission spectroscopy at carbon 1s threshold. From the experimental results, there is no significant variation in electronic structure of oxygen- and hydrogen-plasma treated UNCD films. On the other hand, the biased treated UNCD film shows more remarkable change on the sp² and sp³ states. The formation of sp² bonding and the reduction of sp³ bonding are the consequence of the improved electron field emission properties.     

Relations between the deposition conditions,the microstructure and the defects in PECVD hydrogenated amorphous carbon films; influence on the electronic density of states

The relationships between the deposition conditions, the growth mechanisms, the microstructure and the electronic density of states of hydrogenated amorphous carbon (a-C:H) films prepared by PECVD from hydrocarbons are not yet fully understood. We therefore performed a systematic study using several complementary techniques to determine the changes in the microstructure and in the optical properties of a-C:H samples deposited from a dual micro-wave/radio-frequency discharge in methane as a function of the negative r.f. bias voltage applied to the substrate −V_b. The results reveal the existence of two successive film growth regimes when varying −V_b from −30 to −600 V, i.e. when increasing the ion bombardment during deposition, which lead to different types of sp² C atoms clustering and H incorporation, but to the same density of paramagnetic defects. Models of the a-C:H microstructure are proposed in each case, and their influence on the electronic states density is analysed in detail. It is shown that the H content and the proportion of sp² C atoms play a minor role on the electronic properties, as compared to the nature, the size, the number and probably the distortions of the π-bonded clusters.     

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.     

Rearrangements of sp/sp hybridized bonding with phosphorus incorporation in pulsed laser deposited semiconducting carbon films by X-ray photoelectron spectroscopic analysis

The carbon films were grown on p-type silicon substrate at room temperature by pulsed (XeCl) laser deposition technique using camphoric carbon target containing 1%, 3%, 5% and 7% of phosphorus (P) by mass. The analysis of X-ray photoelectron spectroscopy spectra of the C1s region in these films shows the presence of sp² and sp³ hybridized carbon and a sp² satellite peak due to π–π shake up. The sp² content is seen to remain almost constant with P content. The FWHM of the sp² peak increases up to 5% P but decreases for 7% P probably due to clustering of sp² chains and this clustering in the sp² phase probably decreases the band gap for the 7% P film. With P incorporation, the tetrahedral bonding configurations of the carbon network do not change appreciably, therefore, suggesting the scope of phosphorus as a potential dopant in carbon films.     

Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film

Amorphous carbon (a-C) films with high contents of tetrahedral carbon bonding (sp³) were synthesized on smooth Si(100) surfaces by cathodic arc deposition. Before diamond growth, the a-C films were pretreated with a low-temperature methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. The evolution of the morphology and microstructure of the a-C films during the pretreatment and subsequent diamond nucleation and initial growth stages was investigated by high-resolution transmission electron microscopy (TEM). Carbon-rich clusters with a density of ∼10¹⁰ cm⁻² were found on pretreated a-C film surfaces. The clusters comprised an a-C phase rich in sp³ carbon bonds with a high density of randomly oriented nanocrystallites and exhibited a high etching resistance to hydrogen plasma. Selected area diffraction patterns and associated dark-field TEM images of the residual clusters revealed diamond fingerprints in the nanocrystallites, which played the role of diamond nucleation sites. The presence of non-diamond fingerprints indicated the formation of Si–C-rich species at C/Si interfaces. The predominantly spherulitic growth of the clusters without apparent changes in density yielded numerous high surface free energy diamond nucleation sites. The rapid evolution of crystallographic facets in the clusters observed under diamond growth conditions suggested that the enhancement of diamond nucleation and growth resulted from the existing nanocrystallites and the crystallization of the a-C phase caused by the stabilization of sp³ carbon bonds by atomic hydrogen. The significant increase of the diamond nucleation density and growth is interpreted in terms of a simple three-step process which is in accord with the experimental observations.     

The carbon nitride films prepared at various substrate temperatures by vacuum cathodic arc method

Carbon nitride films have been grown by vacuum cathodic arc method in the substrate temperature range of 100–500 °C. The bonding structure of the films was investigated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and infrared (IR) spectroscopy. With increasing substrate temperature, the films indicate various characteristics. At 100 °C, it can be described as a network similar to DLC in which aromatic sp²C phase is cross-linked by sp³C phase. Between 200 and 400 °C, with increasing substrate temperature the films become graphitized and the sp²CN phase increases, meanwhile the non-aromatic sp²CN phase appears at the edges of aromatic clusters in planar position as well as in out-of-planar regions. While at 500 °C the non-aromatic sp²CN phase almost comes to the same level as the aromatic sp²CN phase. So in the network of the film the aromatic sp²C phase is cross-linked by the non-aromatic sp²C phase. Based on the variation of the microstructure of the films, a comprehensive assignment pattern for the XPS C1s and N1s at different substrate temperature is proposed. In addition, the interpretation of p electron band in valence band spectra at various substrate temperatures is also discussed.     

Quantification of sliding-induced phase transformation in N3FC diamond-like carbon films

We investigate magnetron-sputtered “N3FC” diamond-like carbon films at the nanoscale using an in situ transmission electron microscopy sliding experiment. We measure the sliding-induced changes in sp³- and sp²-hybridized carbon using electron energy loss spectroscopy, taking into account the effect of the electron beam. The rate of the phase transformation from sp³ to sp² bonding is quantified as being between 0.009% and 0.018% volume transformed per sliding pass.     

Detecting sp2 phase on diamond surfaces by atomic force microscopy phase imaging and its effects on surface conductivity

We show correlation of microscopic surface quality and morphology of nanocrystalline diamond films as a function of deposition temperature and post-growth acid treatment detected by atomic force microscopy in phase detection regime, X-ray photoelectron spectroscopy, X-ray induced Auger electron spectroscopy, Scanning Electron Microscopy, Raman spectroscopy, and the electrical conductivity of H-terminated diamond surfaces. The correlation reflects the decrease in sp² amount and enhanced surface conductivity of the diamond surface after the chemical treatment. These results indicate that the AFM phase can detect clearly and microscopically carbon sp² phase on facets and grain boundaries of nanocrystalline diamond films.