Density, sp3 content and internal layering of DLC films by X-ray reflectivity and electron energy loss spectroscopy

A variety of hydrogenated and non-hydrogenated amorphous carbon thin films have been characterized by means of grazing-incidence X-ray reflectivity (XRR) to give information about their density, thickness, surface roughness and layering. We used XRR to validate the density of ta-C, ta-C:H and a-C:H films derived from the valence plasmon in electron energy loss spectroscopy measurements, up to 3.26 and 2.39 g/cm3 for ta-C and ta-C:H, respectively. By comparing XRR and electron energy loss spectroscopy (EELS) data, we have been able for the first time to fit a common electron effective mass of m*/me = 0.87 for all amorphous carbons and diamond, validating the `quasi-free' electron approach to density from valence plasmon energy. While hydrogenated films are found to be substantially uniform in density across the film, ta-C films grown by the filtered cathodic vacuum arc (FCVA) show a multilayer structure. However, ta-C films grown with an S-bend filter show a high uniformity and only a slight dependence on the substrate bias of both sp3 and layering.

Cross-sectional structure of tetrahedral amorphous carbon thin films

A cross-sectional transmission electron microscope study of the low density layers at the surface and at the substrate-film interface of tetrahedral amorphous carbon (ta-C) films grown on (001) silicon substrates is presented. Spatially resolved electron energy loss spectroscopy is used to determine the bonding and composition of a tetrahedral amorphous carbon film with nanometre spatial resolution. For a ta-C film grown with a substrate bias of -300 V, an interfacial region approximately 5 nm wide is present in which the carbon is sp2 bonded and is mixed with silicon and oxygen from the substrate. An sp2 bonded layer observed at the surface of the film is 1.3 ± 0.3 nm thick and contains no detectable impurities. It is argued that the sp2 bonded surface layer is intrinsic to the growth process, but that the sp2 bonding in the interfacial layer at the substrate may be related to the presence of oxygen from the substrate.

Scanning transmission electron microscopy investigation of the Si(111)/AlN interface grown by metalorganic vapor phase epitaxy

The structure and chemistry of the interface between a Si(111) substrate and an AlN(0001) thin film grown by metalorganic vapor phase epitaxy have been investigated at a subnanometer scale using high-angle annular dark field imaging and electron energy-loss spectroscopy. 〈1120̄〉AlN ∥ 〈110〉Si and 〈0001〉AlN ∥ 〈111〉 Si epitaxial relations were observed and an Al-face polarity of the AlN thin film was determined. Despite the use of Al deposition on the Si surface prior to the growth, an amorphous interlayer of composition SiNx was identified at the interface. Mechanisms leading to its formation are discussed. © 2010 American Institute of Physics.

Stress reduction and bond stability during thermal annealing of tetrahedral amorphous carbon

A comprehensive study of the stress release and structural changes caused by postdeposition thermal annealing of tetrahedral amorphous carbon (ta-C) on Si has been carried out. Complete stress relief occurs at 600-700°C and is accompanied by minimal structural modifications, as indicated by electron energy loss spectroscopy, Raman spectroscopy, and optical gap measurements. Further annealing in vacuum converts sp3 sites to sp2 with a drastic change occurring after 1100°C. The field emitting behavior is substantially retained up to the complete stress relief, confirming that ta-C is a robust emitting material. © 1999 American Institute of Physics.

Nitrogen incorporation into tetrahedral hydrogenated amorphous carbon

Structural changes induced by the incorporation of nitrogen into ta-C : H films have been studied by Electron Energy Loss Spectroscopy, X-Ray Photoelectron Spectroscopy, Fourier Transformed Infrared Spectroscopy and Ultraviolet-Visible Spectroscopy. ta-C:H films have been synthesised using a low pressure Electron Cyclotron Wave Resonance (ECWR) source which provides a plasma beam with a high degree of ionisation and dissociation. Nitrogen was incorporated by adding N2 to the C2H2 plasma used for the deposition of ta-C : H films. The N/C atomic ratio in the films rises rapidly until the N2/C2H2 gas ratio reaches three, and then increases more gradually, while the deposition rate decreases steeply. Chemical sputtering of the forming films and the formation of molecular nitrogen within the films limit the maximum nitrogen content to about N/C = 0.6. For low nitrogen content the films retain their diamond-like properties, however as N/C atomic ratio increases, a polymeric-like material is formed, with >C=N- structures and terminating C=N and NH groups that decrease the connectivity of the network.

Hydrogen content estimation of hydrogenated amorphous carbon by visible Raman spectroscopy

In the present study, we report the hydrogen content estimation of the hydrogenated amorphous carbon (a-C:H) films using visible Raman spectroscopy in a fast and nondestructive way. Hydrogenated diamondlike carbon films were deposited by the plasma enhanced chemical vapor deposition, plasma beam source, and integrated distributed electron cyclotron resonance techniques. Methane and acetylene were used as source gases resulting in different hydrogen content and sp2/sp3 fraction. Ultraviolet-visible (UV-Vis) spectroscopic ellipsometry (1.5-5 eV) as well as UV-Vis spectroscopy were provided with the optical band gap (Tauc gap). The sp2/sp3 fraction and the hydrogen content were independently estimated by electron energy loss spectroscopy and elastic recoil detection analysis-Rutherford back scattering, respectively. The Raman spectra that were acquired in the visible region using the 488 nm line shows the superposition of Raman features on a photoluminescence (PL) background. The direct relationship of the sp2 content and the optical band gap has been confirmed. The difference in the PL background for samples of the same optical band gap (sp2 content) and different hydrogen content was demonstrated and an empirical relationship between the visible Raman spectra PL background slope and the corresponding hydrogen content was extracted. © 2004 American Institute of Physics.

Analysis of the ellipsometric spectra of amorphous carbon thin films for evaluation of the sp3-bonded carbon content

Using spcctroscopic ellipsometry (SE), we have measured the optical properties and optical gaps of a series of amorphous carbon (a-C) films ∼ 100-300 Å thick, prepared using a filtered beam of C+ ions from a cathodic arc. Such films exhibit a wide range of sp3-bonded carbon contents from 20 to 76 at.%, as measured by electron energy loss spectroscopy (EELS). The Taue optical gaps of the a-C films increase monotonically from 0.65 eV for 20 at.% sp3 C to 2.25 eV for 76 at.% sp3 C. Spectra in the ellipsometric angles (1.5-5 eV) have been analyzed using different effective medium theories (EMTs) applying a simplified optical model for the dielectric function of a-C, assuming a composite material with sp2 C and sp3 C components. The most widely used EMT, namely that of Bruggeman (with three-dimensionally isotropic screening), yields atomic fractions of sp3 C that correlate monotonically with those obtained from EELS. The results of the SE analysis, however, range from 10 to 25 at.% higher than those from EELS. In fact, we have found that the volume percent sp3 C from SE using the Bruggeman EMT shows good numerical agreement with the atomic percent sp3 C from EELS. The SE-EELS discrepancy has been reduced by using an optical model in which the dielectric function of the a-C is determined as a volume-fraction-weighted average of the dielectric functions of the sp2 C and sp3 C components. © 1998 Elsevier Science S.A.

Analysis of amorphous carbon thin films by spectroscopic ellipsometry

Using spectroscopic ellipsometry (SE), we have measured the optical properties of amorphous carbon (a-C) films ∼ 10-30 nm thick prepared using a filtered beam of C+ ions from a cathodic arc. Such films exhibit a wide range of sp3-bonded carbon contents from 20 to 76 at.% as measured by electron energy loss spectroscopy (EELS), and a range of optical gaps from 0.65 eV (20 at.% sp3 C) to 2.25 eV (76 at.% sp3 C) as measured by SE. SE data from 1.5 to 5 eV have been analyzed by applying the most widely used effective medium theory (EMT) namely that of Bruggeman with isotropic screening, assuming a model of the material as a composite with sp2 C and sp3 C components. Although the atomic fractions of sp3 C deduced by SE with the Bruggeman EMT correlate monotonically with those obtained by EELS, the SE results range from 10 to 25 at.% higher. The possible origins of this discrepancy are discussed within the framework of an optical composite. Improved agreement between SE and EELS is obtained by employing a simple form for the EMT, in which the effective dielectric function is determined as a volume-fraction-weighted average of the dielectric functions of the two components. © 1998 Elsevier Science B.V. All rights reserved.

Optical and electronic properties of amorphous diamond

The optical and electronic properties of highly tetrahedral amorphous diamond-like carbon (amorphous diamond, a-D) films were investigated. The structure of the films grown on silicon and glass substrates, under similar deposition conditions using a compact filtered cathodic vacuum arc system, are compared using electron energy loss spectroscopy (EELS). Results from hydrogenation of the films are also reported. The hydrogenated films show two prominent IR absorption peaks centered at 2920 and 2840 cm-1, which are assigned to the stretch mode of the C-H bond in the sp3 configuration on the C-H3 and C-H sites respectively. The high loss EELS spectra show no reduction in the high sp3 content in the hydrogenated films. UV and visible transmission spectra of a-D thin films are also presented. The optical band gap of 2.0-2.2 eV for the a-D films is found to be consistent with the electronic bandgap. The relationship between the intrinsic compressive stress in the films and the refractive index is also presented. The space charge limited current flow is analyzed and coupled with the optical absorption data to give an estimate of 1018 cm-3 eV-1 for the valence band edge density of states.

Effect of deposition conditions on the chemical bonding in sputtered carbon nitride films

A balanced planar r.f. powered magnetron sputter source has been used to deposit carbon nitride films from a graphite target under various conditions. Sample temperature, bias voltage and nitrogen content in the gas mixture were varied. The effects of oxygen, methane and ammonia on the film growth were also studied. Special attention was paid to the effects of the deposition parameters on the structure of the films, in particular the hybridisation of the carbon and nitrogen bonding. The chemical bonding of the carbon and nitrogen atoms was studied by electron energy loss spectroscopy (EELS). The chemical composition was evaluated by Rutherford back-scattering. The intensity of transitions to π antibonding orbitals, as revealed by EELS, was found to increase with the nitrogen content in the films. Ion bombardment of the films during growth and the addition of oxygen or hydrogen-rich gases further increased the proportion of π bonds of both the carbon and nitrogen atoms. It is suggested that the increase in the transitions to μ antibond orbitals is to be explained by increased sp2 or possibly sp hybridisation of the carbon and nitrogen. Also, the effect of annealing on the bonding of nitrogen rich films after deposition was tested. The changes caused by nitrogen and deposition conditions are consistent with previous reports on the formation of paracyanogen structures.

Deposition of diamond-like carbon films by photon-enhanced chemical vapour deposition of methane using a windowless hydrogen lamp

Thin films of diamond-like carbon (DLC) have been deposited using a novel photon-enhanced chemical vapour deposition (photo-CVD) method. This low energy method may be a way to produce better interfaces in electronic devices by reducing damage due to ion bombardment. Methane requires high energy photons for photolysis to take place and these are not transmitted in most photo-CVD methods owing to the presence of a window between the lamp and the deposition environment. In our photo-CVD system there is no window and all the high energy photons are transmitted into the reaction gas. Initial work has proved promising and this paper presents recent results. Films have been characterized by measuring electron energy loss spectra, by ellipsometry and by fabricating and testing diode structures. Results indicate that the films are of a largely amorphous nature and are semiconducting. Diode structures have on/off current ratios of up to 106.

Preparation of tetrahedral amorphous carbon films by filtered cathodic vacuum arc deposition

Tetrahedrally bonded amorphous carbon (ta-C) and nitrogen doped (ta-C:N) films were obtained at room temperature in a filtered cathodic vacuum arc (FCVA) system incorporating an off-plane double bend (S-bend) magnetic filter. The influence of the negative bias voltage applied to substrates (from -20 to -350 V) and the nitrogen background pressure (up to 10-3 Torr) on film properties was studied by scanning electron microscopy (SEM), electron energy loss spectroscopy (EELS), Raman spectroscopy, X-ray photoemission spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and X-ray reflectivity (XRR). The ta-C films showed sp3 fractions between 84% and 88%, and mass densities around 3.2 g/cm3 in the wide range of bias voltage studied. In contrast, the compressive stress showed a maximum value of 11 GPa for bias voltages around -90 V, whereas for lower and higher bias voltages the stress decreased to 6 GPa. As for the ta-C:N films grown at bias voltages below -200 V and with N contents up to 7%, it has been found that the N atoms were preferentially sp3 bonded to the carbon network with a reduction in stress below 8 GPa. Further increase in bias voltage or N content increased the sp2 fraction, leading to a reduction in film density to 2.7 g/cm3.

X-ray propagation in carbon nanotubes

The maintenance of the growth of the multibillion-dollar semiconductor industry requires the development of techniques for the fabrication and characterisation of nanoscale devices. Consequently, there is great interest in photolithography techniques such as extreme UV and x-ray. Both of these techniques are extremely expensive and technologically very demanding. In this paper we describe research on the feasibility of exploiting x-ray propagation within carbon nanotubes (CNT's) for the fabrication and characterisation of nanoscale devices. This work discusses the parameters determining the design space available. To demonstrate experimentally the feasibility of x-ray propagation, arrays of carbon nanotubes have been grown on silicon membranes. The latter are required to provide structural support for the CNT's while minimising energy loss. To form a waveguide metal is deposited between the nanotubes to block x-ray transmission in this region at the same time as cladding the CNT's. The major challenge has been to fill the spaces between the CNT's with material of sufficient thickness to block x-ray transmission while maintaining the structural integrity of the CNT's. Various techniques have been employed to fill the gaps between the nanotubes including electroplating, sputtering and evaporation. This work highlights challenges encountered in optimising the process.

Properties of tetrahedral amorphous carbon prepared by vacuum arc deposition

The structural, optical, electrical and physical properties of amorphous carbon deposited from the filtered plasma stream of a vacuum arc were investigated. The structure was determined by electron diffraction, neutron diffraction and energy loss spectroscopy and the tetrahedral coordination of the material was confirmed. The measurements gave a nearest neighbour distance of 1.53 Å, a bond angle of 110 and a coordination number of four. A model is proposed in which the compressive stress generated in the film by energetic ion impact produces pressure and temperature conditions lying well inside the region of the carbon phase diagram within which diamond is stable. The model is confirmed by measurements of stress and plasmon energy as a function of ion energy. The model also predicts the formation of sp2-rich materials on the surface owing to stress relaxation and this is confirmed by a study of the surface plasmon energy. Some nuclear magnetic resonance, infrared and optical properties are reported and the behaviour of diodes using tetrahedral amorphous carbon is discussed. © 1991.

Microstructural evolution of SiOx films and its effect on the luminescence of Si nanoclusters

In this paper we demonstrate that the structural and optical properties of Si nanoclusters (Si ncs) formed by thermal annealing of SiOx films prepared by plasma enhanced chemical vapor deposition (PECVD) and magnetron sputtering are very different. In fact, at a fixed Si excess and annealing temperature, photoluminescence (PL) spectra of sputtered samples are redshifted with respect to PECVD samples, denoting a larger Si ncs size. In addition, PL intensity reaches a maximum in sputtered films at annealing temperatures much lower than those needed in PECVD films. These data are correlated with structural properties obtained by energy filtered transmission electron microscopy and electron energy loss spectroscopy. It is shown that in PECVD films only around 30% of the Si excess agglomerates in clusters while an almost complete agglomeration occurs in sputtered films. These data are explained on the basis of the different initial structural properties of the as-deposited films that become crucial for the subsequent evolution. © 2008 American Institute of Physics.