Thomas G. McCauley

Control of diamond film microstructure by Ar additions to CH4/H2 microwave plasmas

The transition from microcrystalline to nanocrystalline diamond films grown from Ar/H2/CH4 microwave plasmas has been investigated. Both the cross-section and plan-view micrographs of scanning electron microscopy reveal that the surface morphology, the grain size, and the growth mechanism of the diamond films depend strongly on the ratio of Ar to H2 in the reactant gases. Microcrystalline grain size and columnar growth have been observed from films produced from Ar/H2/CH4 microwave discharges with low concentrations of Ar in the reactant gases. By contrast, the films grown from Ar/H2/CH4 microwave plasmas with a high concentration of Ar in the reactant gases consist of phase pure nanocrystalline diamond, which has been characterized by transmission electron microscopy, selected area electron diffraction, and electron energy loss spectroscopy. X-ray diffraction and Raman spectroscopy reveal that the width of the diffraction peaks and the Raman bands of the as-grown films depends on the ratio of Ar to H2 in...

SYNTHESIS AND ELECTRON FIELD EMISSION OF NANOCRYSTALLINE DIAMOND THIN FILMS GROWN FROM N2/CH4 MICROWAVE PLASMAS

Nanocrystalline diamond films have been synthesized by microwave plasma enhanced chemical vapor deposition using N2/CH4 as the reactant gas without additional H2. The nanocrystalline diamond phase has been identified by x-ray diffraction and transmission electron microscopy analyses. High resolution secondary ion mass spectroscopy has been employed to measure incorporated nitrogen concentrations up to 8×1020 atoms/cm3. Electron field emission measurements give an onset field as low as 3.2 V/μm. The effect of the incorporated nitrogen on the field emission characteristics of the nanocrystalline films is discussed.

Synthesis of nanocrystalline diamond thin films from an Ar–CH4 microwave plasma

Nanocrystalline diamond thin films have been synthesized in an Ar{endash}CH{sub 4} microwave discharge, without the addition of molecular hydrogen. X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy characterizations show that the films consist of a pure crystalline diamond phase with very small grain sizes ranging from 3 to 20 nm. Atomic force microscopy analysis demonstrates that the surfaces of the nanocrystalline diamond films remain smooth independent of the film thicknesses. Furthermore, the reactant gas pressure, which strongly affects the concentration of C{sub 2} dimer in the Ar{endash}CH{sub 4} plasma as well as the growth rate of the films, has been found to be a key parameter for the nanocrystalline diamond thin film depositions. {copyright} {ital 1998 American Institute of Physics.}

Temperature dependence of the growth rate for nanocrystalline diamond films deposited from an Ar/CH4 microwave plasma

We have investigated the effect of substrate temperature on the growth rate and properties of nanocrystalline diamond thin films prepared by microwave plasma-assisted chemical vapor deposition on (100) Si from a 1% methane (CH4) precursor in argon (Ar). In previous work we have shown that the carbon dimer C2 is the dominant growth species for this CH4/Ar system without the addition of molecular hydrogen. In the present work, the apparent activation energy for this growth process from C2 was determined from a standard Arrhenius-type analysis of the growth rate data for substrate temperatures between 500 and 900 °C. The measured value of 5.85±0.438 kcal/mol (0.254±0.019 eV/atom) is shown to be in close agreement with the results of recent modeling studies of the energetics of C2 addition to the diamond (110)–(1×1):H surface. These results have important implications for low-temperature diamond coating of nonrefractory materials such as glasses.

Swan band emission intensity as a function of density

We report the systematic comparison of the optical emission intensity of the vibrational band of the Swan system with the absolute concentration in and microwave plasmas used in the deposition of nanocrystalline diamond. The absolute concentration is obtained using white-light absorption spectroscopy. Emission intensity correlates linearly with density for variations of several plasma parameters and across two decades of species concentration. Although optical emission intensity generally is not an accurate quantitative diagnostic for gas phase species concentrations, these results confirm the reliability of the (0,0) Swan band for relative determination of density with high sensitivity under conditions used for hydrogen-deficient plasma-enhanced chemical vapour deposition of diamond.

Spectroscopic determination of carbon dimer densities in and plasmas

In contrast to conventional methods of diamond chemical vapour deposition (CVD), nanocrystalline diamond CVD takes place with only a small fraction of feed gas hydrogen. Minimal amounts of , believed critical in hydrogen-rich CVD, are expected to be produced in hydrogen-deficient systems and alternative mechanisms for diamond growth must be considered. The carbon dimer, , is believed to be an important species in these growth environments. We have experimentally determined the density of gas phase in and microwave plasmas used to deposit nanocrystalline diamond. The density is monitored using high-sensitivity absorption spectroscopy of the (0, 0) band as chamber pressure, microwave power, substrate temperature and feed gas mixtures are varied for these two chemical systems. The absolute density of is most sensitive to the total chamber pressure and fraction of carbon in all molecular species in the feed gas in discharges and to the total chamber pressure and substrate temperature in plasmas. We discuss possible production channels in both chemical systems. The efficiency of production from fullerene precursors is over an order of magnitude greater than that from hydrocarbon precursors.

Microstructure and field emission of nanocrystalline diamond prepared from C{sub 60} precursors

Cold cathode electron field emission from nanocrystalline diamond thin films produced by microwave plasma-enhanced CVD with C{sub 60} as the growth precursor has been observed. The lowest onset field obtained is about 2 V/{micro}m, and a 4 V/{micro}m field is required to emit a current density of 0.4 mA/cm{sup 2}. It has been found that hydrogen content of the plasma, which can be varied over a wide range, strongly affects the microstructure and electron field emission properties of the nanocrystalline diamond thin films. This is an important new development which allows one to exercise powerful control over many properties of diamond films. Based on TEM characterization and field emission measurements, the effects of the structural defects and the orientations of the diamond grains on the field emission properties are discussed.

Morphology and electron emission properties of nanocrystalline CVD diamond thin films

Nanocrystalline diamond thin films have been produced by microwave plasma-enhanced chemical vapor deposition (MPECVD) using C{sub 60}/Ar/H{sub 2} or CH{sub 4}/Ar/H{sub 2} plasmas. Films grown with H{sub 2} concentration {le} 20% are nanocrystalline, with atomically abrupt grain boundaries and without observable graphitic or amorphous carbon phases. The growth and morphology of these films are controlled via a high nucleation rate resulting from low hydrogen concentration in the plasma. Initial growth is in the form of diamond, which is the thermodynamic equilibrium phase for grains {le}5 nm in diameter. Once formed, the diamond phase persists for grains up to at least 15-20 nm in diameter. The renucleation rate in the near-absence of atomic hydrogen is very high ({approximately} 10{sup 10} cm{sup {minus}2} sec{sup {minus}1}), limiting the average grain size to a nearly constant value as the film thickness increases, although the average grain size increases as hydrogen is added to the plasma. For hydrogen concentrations less than {approximately}20%, the growth species is believed to be the carbon dimer, C{sub 2}, rather than the CH{sub 3}* growth species associated with diamond film growth at higher hydrogen concentrations. For very thin films grown from the C{sub 60} precursor, the threshold field (2 to {approximately}60 volts/micron) for cold cathode electron emission depends on the electrical conductivity and on the surface topography, which in turn depends on the hydrogen concentration in the plasma. A model of electron emission, based on quantum well effects at the grain boundaries is presented. This model predicts promotion of the electrons at the grain boundary to the conduction band of diamond for a grain boundary width {approximately} 3--4 {angstrom}, a value within the range observed by TEM.

Electron emission properties of Si field emitter arrays coated with nanocrystalline diamond from fullerene precursors

In this paper, we report on a substantial lowering of the threshold field for electron field emission from Si field emitter arrays (FEA), which have been coated with a thin layer of nanocrystalline diamond by microwave plasma-assisted chemical vapor deposition (MPCVD) from fullerene (C 60 ) and methane (CH 4 ) precursors. The field emission characteristics were investigated and the emission sites imaged using photoelectron emission microscopy (PEEM). Electron emission from these Si FEAs coated with nanocrystalline diamond was observed at threshold fields as low as 3 V/μm, with effective work functions as low as 0.59 eV.

Synthesis of nanocrystalline diamond thin films from an Ar{endash}CH{sub 4} microwave plasma

Nanocrystalline diamond thin films have been synthesized in an Ar{endash}CH{sub 4} microwave discharge, without the addition of molecular hydrogen. X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy characterizations show that the films consist of a pure crystalline diamond phase with very small grain sizes ranging from 3 to 20 nm. Atomic force microscopy analysis demonstrates that the surfaces of the nanocrystalline diamond films remain smooth independent of the film thicknesses. Furthermore, the reactant gas pressure, which strongly affects the concentration of C{sub 2} dimer in the Ar{endash}CH{sub 4} plasma as well as the growth rate of the films, has been found to be a key parameter for the nanocrystalline diamond thin film depositions. {copyright} {ital 1998 American Institute of Physics.}