ICMCTF1999 Session DP: DP Poster

Diamond and diamond-like carbon (DLC) are considered as promising materials for electron emitter applications such as flat-panel displays. These materials have usually been prepared by vapor-phase deposition techniques. We have carried out liquid-phase deposition of DLC films by an electrolysis of methanol and investigated the field emission properties from the electrodeposited DLC films.

The DLC films were deposited on Si substrate mounted on a negative electrode. A graphite rod was used as a positive electrode. The distance between the two electrodes was 10 mm. The current density was in the range of 10 to 20 A/cm² when the high DC voltage was applied to the substrate. The field emission current from the deposited DLC film was measured by using a parallel plate configuration. Indium-Tin-Oxide coated glass plate was used as an anode whereas the deposited DLC film was a cathode.

The current - electric field characteristics shows a threshold field for electron emission as low as 1 V/µm. This threshold field is the lowest one for undoped DLC films ever reported, indicating that the electrodeposited DLC film has a potential advantage for the field emitter applications. We also find that that the emission properties can be explained by Fowler-Nordheim relationship based on the tunneling theory. To visualize the spatial distribution of the emission site, we observed a light emission from a fluorescent screen mounted at 200 µm above the DLC films. We found that the electrons are emitted from the isolated emission site within the DLC film and that a small portion of the surface area contributes to the field emission.

There are increasing researches into use of carbon-based films as a field emitter material because it has many attractive properties such as low or negative electron affinity, high chemical and mechanical stability, and low field emission characteristics. There have been many reports to identify the origin of low field emission of carbon-based films. However, the low field emission behavior has not been clearly understood.

On the other hand, hydrogen in atomic and molecular states is a main constituent of the vapor phase of chemical vapor deposition (CVD) diamond growth process and the diamond films synthesized by CVD process contain typically up to a few at. %. Although hydrogen exists in diamond films with quite a small amount it can critically affect electrical conductivity of diamond films.

It is expected that the change of hydrogen content or hydrogen-related bond in diamond films can affect field emission property of diamond. It should be also considered that the grain size and surface morphology could change field emission property. Until now there has been no report on the effect of hydrogen in diamond on the field emission property.

In this study, in order to change the hydrogen content and structural property of diamond films, diamond films are grown at various bias conditions using an electron assisted CVD system. A comparison has been performed on the qualitative amount of hydrogen in the films grown at various bias conditions and diode current-voltage characteristics are investigated. Difference of the field emission property is discussed by comparing both hydrogen content and structural properties with electrical resistivity.

Diamond-like carbon (DLC) films were deposited on various substrates using highly reproducible direct ion beam deposition from an RF IC CH₄ plasma source ¹. Combinations of gases such as CH₄, CH₄ -N₂ were used to form plasma. The mechanical, electrical and optical properties of the films were examined as a function of deposition conditions and N₂ content in gas composition. A small amount of N₂ (6-8 sccm) did not markedly change hardness and stress, while electrical conductivity was significantly increased. In addition, a small amount of N₂ improved tribological performance of the films reducing amount of debris and wear track size. Introduction of high N₂ flow into the system significantly deteriorates value of these parameters. It was found that N₂ essentially increases absorption coefficient, and reduces optical band gap. Analysis of the experimental results shows that observed effects can be explained by incorporation of N₂ into carbon-strained network that induces structural changes and, in turn, leads to an increase of sp² fraction in the DLC films. For investigation of the bulk and surface electrically active defects, the deep level transient spectroscopy (DLTS) was used. The density, activation energies and capture cross-sections of the bulk and surface defects in the DLC films with different N₂ content were found. The role of the point defects in the DLC films is discussed.

¹ B. Druz et al., Diamond and Related Materials 7 (1998) 965 - 972.

Amorphous hard CN_x coatings were deposited by DC magnetron sputtering in pure N₂ atmosphere. Increase of the samples nitrogenation x = N/C was controled by: a) increase of working gas N₂ pressure, b) decrease of DC magnetron current. These technologigal condition changes led to monotonous ( with systematic trend ) decrease of plastical microhardness of the investigated CN_x coatings sequence.

Evaluation of optical properties combining the spectroscopic ellipsometry (1.5-4 eV) and the VUV reflection spectroscopy (4-14 eV) by means of Kramers-Kronig analysis showed increase of the π-plasmon resonance peak, which indicates enhancement of amount of π-bonded electrons. It is linked with increase of sp² hybridization. The optical energy gap extrapoled by Tauc's plot indicates more semimetallic properties of the CN_x coatings due to nitrogenation. A shift in the ε₂(ω) maximum corresponding to σ-σ^* electron transitions in carbon demonstrates deeper band structure changes in higly nitrogenated CN_x films.

Hard coatings (as diamond or diamond-like carbon layers) are widely used as protective coatings on metal substrates. The performance of the composite depends on material parameters like the layer's Hardness, film fracture toughness, interface toughness between layer and substrate or the yield strength of the substrate. Residual stress in the layers and geometrical design parameters like the layer thickness, the tribology of the system and the loading conditions are of similar importance. Ideally, one would like to know all these parameters and their influence on coating failure. This implies an absolute measure of the materials parameters involved and a knowledge of the failure mechanisms.

Failure mechanisms of the substrate/coating composite are studied in this paper by a rigorous elastic-plastic finite element analysis. The common load case considered here is the indentation of spherical bodies into a layered surface, which typically would represent the mechanics in a ball bearing or penetration of wear debris in a layered surface.

For diamond-like carbon on steel and diamond on steel the onset of plastic deformation in the substrate and the coating fracture, dependent on the Young's modulus of the layer, the yield stress of the steel, the fracture stress of the layers and the indentor radius and coating thickness is calculated. Indentation experiments are carried out parallely to identify the failure mechanisms experimentally. Asymptotic analytical solutions are also discussed which allow extrapolating the results to other substrate coating systems. The results are summarized in form of a failure mechanism map, that allow to determine the optimal coating and coating thickness for a special load case, described by indentor radius and indentor force.

This paper reports on the evolution of stress with the layer thickness in the case of heteroepitaxial diamond layers on silicon [001] substrates. It is shown by finite element calculations that the thermal stresses developing in the films can be treated analytically by plate theory. The calculated thermal stress, however, differs from the by Raman spectroscopy measured stress, because higher compressive stresses at small layer thickness, lateral stress gradients in the layers, stress inhomogeneities within single grains and plastic deformation of the substrate were found experimentally.

By the aid of finite element calculations and transmission electron microscopy (TEM), these differences are attributed to clear physical origins, i.e. temperature inhomogeneities during growth, the layer morphology (island growth, surface roughness) and microstructural sources as coherency strains and disclination formation. Temperature gradients during deposition induce radial stress gradients in the layer, which leads to more compressive stress at the edge of the specimen and stresses in free standing membranes. Creep of the substrate during deposition can be induced by temperature gradients and stresses due to the microstructure. In case of a noncontinuous layer or an important surface roughness finite element calculations show that an important part of the stress can be relaxed elastically. Finally, disclinations found by TEM investigations explain stress inhomogeneities within single grains.