L.K. Cheah

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.

Properties of nitrogen doped tetrahedral amorphous carbon films prepared by filtered cathodic vacuum arc technique

The properties of nitrogen doped tetrahedral amorphous carbon films prepared by the filtered cathodic vacuum arc technique have been studied. The doping species, nitrogen ions, were produced by an ion beam source. The nitrogen flow rate was varied from 0.5 to 10 sccm while keeping other deposition conditions constant. The nitrogen content in deposited films was determined by Rutherford backscattering technique and ranged from 5 to 34 at.% depending on the nitrogen flow rate. The surface morphology, mechanical, optical, and electronic properties of the films were measured. The compressive stress, the hardness and the optical band gap all increased at low nitrogen content to a maximum at 5 at.% nitrogen and then decreased with increasing nitrogen content. The activation energy first increased and then decreased with increasing nitrogen content. We attribute these changes to the Fermi level moving up in the band gap, from below the midgap to near conduction band. We achieved continuously adjustable band gap and complex refractive index with nitrogen incorporation. Possible mechanisms of N ion in the ta-C:N films are discussed.

Transport of vacuum arc plasma through an off-plane double bend filtering duct

A new magnetic macroparticle filter design consisting of an off-plane double-bend (OPDB) filter is described. The transport of the vacuum arc plasma through this OPDB filter is investigated using Langmuir and deposition probes. Films of amorphous hard carbon have been deposited using a 90° single bend and the OPDB filter and the macroparticle contents of the films are compared. The experimental results were found to be in good agreement with the simulations results based on an improved drift approximation model. The results demonstrate that OPDB filter has a relatively better transmission efficiency than the 90° single bend filter, lower macroparticle counts and is suitable for preparation of diamond-like carbon coatings with high quality.

Field emission from undoped and nitrogen-doped tetrahedral amorphous carbon film prepared by filtered cathodic vacuum arc technique

Abstract The field emission results from undoped and nitrogen doped tetrahedral amorphous carbon (ta-C and ta-C:N) prepared by the filtered cathodic vacuum arc (FCVA) technique, deposited on both n+ and p+-type Si are reported. The effect of different types of Si substrate and the film thickness on the onset electric field has been investigated. Three sets of ta-C samples with differing doping concentrations were used in the study: undoped p-type ta-C (p-ta-C), nitrogen weakly doped intrinsic ta-C (i-ta-C) and nitrogen heavily doped n+-type ta-C (n+-ta-C). The heterojunction-based field emission model gives a reasonable explanation for the behavior of the onset electric field measured. The heavily doped hetero-junction, n+-ta-C/p+-Si, demonstrated the lowest onset field of 10 V μ−1 with current densities of 0.1 mA mm−2 at 50 V μm−1 due to the Zener tunneling arising from the severe band bending. A film thickness of 30–40 nm is more favorable for field emission due to the ease with which the film can be fully depleted. At some locations of i-ta-C films, various types of craters were formed after an electrical discharge at a high field (∼58 V μm−1) followed by a subsequent reduction in the onset field to about 15 V μm−1.

Spectroscopic ellipsometry studies of tetrahedral amorphous carbon prepared by filtered cathodic vacuum arc technique

Abstract The optical property of the tetrahedral amorphous carbon (ta-C) films deposited by the filtered cathodic vacuum are (FCVA) technique has been investigated using a spectroscopic ellipsometer. The structure of these films deposited on silicon wafer was simulated by a four-layer model consisting of a roughness layer, a ta-C layer, a graded ta-C:Si layer and the silicon substrate. The optical property of the ta-C layer was derived based on Forouhi and Bloomer amorphous semiconductor model. The graded layer consisting of the mixture of ta-C and silicon simulates the carbon ion impinging/diffusion into the surface of the silicon substrate. The dielectric constants of the random mixture microstructure were modeled by the effective medium approximation. From the fitting, the F.B. parameters for the ta-C film have been obtained following their physical significance. The complex refraction index, N = n − ik over the range of 1.5–4.2 eV, for ta-C has been determined. The Tauc (optical) gap of 2.5–2.7 eV is obtained from ta-C with 79–88% sp 3 fraction in the film. Similarity in trend is observed from the Tauc gap, F.B. optical band gap, parameter B and sp 3 fraction plotted against the carbon ion energy. The change of the slope of the optical absorption edge is observed when the ion energy is varied.

Tribological behaviour of different diamond-like carbon materials

Abstract This paper comparatively studies the tribological behaviour of different types of diamond-like carbon (DLC) coatings in a reciprocating sliding wear test. The results are interpreted in terms of structure and surface morphological characteristics. At the beginning of each reciprocating sliding wear test, the higher coefficient of friction of the DLC coatings reflects the original surface conditions of contacting counterfaces, whereas the low coefficient of friction achieved under the steady regime is linked to the presence of wear debris and an enlarged real contact area. sp2 carbon bonding has been related to the occurrence of a higher coefficient of friction and surface damage of DLC materials compared to bulk diamond. The a-C coatings and ta-C films have a lower coefficient of friction at the steady regime than the a-C:H coatings, possibly due to a significant amount of hydrogen in the a-C:H coatings.

Electronic transport properties of nitrogen doped amorphous carbon films deposited by the filtered cathodic vacuum arc technique

Highly tetrahedral amorphous carbon thin films were deposited by the filtered cathodic vacuum arc technique at room temperature. Nitrogen was found to be a good n-type dopant of the tetrahedral amorphous carbon thin films. The Fermi level shifts from 0.91 eV above the valence band to 0.65 eV below the conduction band with increasing nitrogen flow rate from null to 16 sccm (nitrogen partial pressure from 0 to ). At the same time, the optical band gap drops from 2.7 to 1.8 eV. Three electronic transport mechanisms, namely, transport in extended states, in band tails by hopping and variable range hopping (VRH) near the Fermi level, were observed from the thermal activation measurements in the temperature range from 100 to 450 K. The VRH transport parameters for ta-C films are studied, and the density of states near the Fermi level extracted from the hopping transport parameters was found in the range of . The dominant doping configuration is the substitution in the coordination at low N concentration and adoption of bonding at high N concentration.

Micro-Raman spectroscopic analysis of tetrahedral amorphous carbon films deposited under varying conditions

The structure of tetrahedral amorphous carbon (ta-C) films deposited by a filtered cathodic vacuum arc has been studied using micro-Raman spectroscopy in terms of substrate bias, nitrogen gas partial pressure (ta-C:N films) or aluminum content in a mixed aluminum/carbon target (ta-C:Al films) during deposition. The first-order Raman spectra generally show a broad feature overlaid by a disordered (D) peak and a graphitic (G) peak. The contribution of sp3 bonding to the Raman spectrum is not explicit, since the Raman phonon line is more sensitive to the sp2 carbon bonding due to its larger Raman scattering cross section. However by comparing the ratios of the intensities, the full widths at half maximum (FWHM), and the peak areas between the D and G peaks, the sp3 contribution may indirectly be reflected by the complex Raman features. The G peak position for the ta-C and ta-C:N films appears to not change significantly with the change of substrate bias voltage or N2 partial pressure, whereas the shift of th...

Electron field emission properties of tetrahedral amorphous carbon films

The electron field emission of tetrahedral amorphous carbon (ta-C) films deposited by filtered cathodic vacuum arc is reported. The ta-C films were found to have a threshold field ranging from 18 to 28 V μm−1, depending on the sp3 content. The nitrogenated ta-C (ta-C:N) films show a lower threshold field of 12 V μm−1 as compared to the ta-C films. The threshold field appears to be dependent on the film thickness. There is a minimum threshold field with the film thickness of around 30 nm for the ta-C:N film. Although the ta-C and ta-C:N films have relatively low threshold fields, the density of emission sites is not high for these films. The density of emission sites can be increased when the film surface is treated with H+, O+, or Ar+ ions after deposition. Moreover, the posttreated films show even lower threshold fields compared to the untreated films. The improvement in the emission after the ion beam treatment appears to be independent of the ions used. The surface before and after ion bombardment was ...

Electron field emission from surface treated tetrahedral amorphous carbon films

The electron field emission properties of tetrahedral amorphous carbon thin films deposited using a filtered cathodic vacuum arc system have improved as a result of surface treatment with H, O, and Ar ions. The limiting factor of the emission process does not appear to be only the front surface of the films. The improvement in the emission after ion beam treatment appears to be independent of the ions used. The surface which has been analyzed using ultraviolet photospectroscopy, reflected electron energy loss spectroscopy, and scanning tunneling microscopy shows evidence of the creation of segregated sp2 and sp3 rich regions of less than 20 nm in dimension. An extension to the space charge-induced band bending model including a multistep emission process that occurs in this mixed phase material is proposed.