Walter A. Yarbrough

Current Issues and Problems in the Chemical Vapor Deposition of Diamond

Current issues and problems in the chemical vapor deposition (CVD) of diamond are those which relate to its characterization, its nucleation on foreign surfaces, the question of its formation in preference to the other phases of solid carbon (for example, graphite, chaoite, or lonsdaleite), why different morphologies and crystallographic orientations (textures) are seen in different experiments or with different parameters in the same experiment, and finally whether well-crystallized metastable phases can be obtained by CVD in other material systems or are only a peculiarity of carbon chemistry. Whether a given carbon coating is justly described as diamond has been such an issue, and coatings should clearly show evidence for diamond by x-ray diffraction and Raman spectroscopy before the claim of diamond is made. Experimental results have not been consistent in many cases, and much work remains to be done before an accurate assessment can be made of the technological impact of the development.

Role of heat transfer and fluid flow in the chemical vapor deposition of diamond

The role of fluid flow and heat transfer in determining the quality of the diamond films and the rate of their deposition in a hot‐filament chemical vapor deposition (HFCVD) reactor was investigated both experimentally and theoretically. The equations of conservation of mass, momentum, and enthalpy were solved numerically to calculate the temperature and fluid flow fields. Experiments were conducted with various flow configurations, and the deposition rates and the spatial variations of film thickness were examined in each case. The films were characterized by Raman spectroscopy, x‐ray, and scanning electron microscopy. The influences of free and forced convection, and diffusion due to concentration and temperature gradients (Soret effect) were examined. Comparison of the computed results with the experimental data revealed the importance of thermal diffusion in the HFCVD of diamond.

Preparation and characterization of nanocrystalline cubic boron nitride by microwave plasma‐enhanced chemical vapor deposition

Polycrystalline boron nitride films have been deposited using microwave plasma‐enhanced chemical vapor deposition. IR absorption spectra of films deposited using NaBH4 as the boron source in NH3 and H2 gases showed absorptions which are nearly the same as the characteristic vibrational modes seen in cubic and pyrolytic boron nitrides. Films deposited at 5 Torr also showed electron diffraction patterns for pyrolytic boron nitride, turbostratic boron nitride and cubic boron nitride. At higher gas pressures, only rings consistent with the formation of amorphous and cubic boron nitride were observed. Although the Raman spectra from a film deposited at 60 Torr showed broad peaks at ∼1080 and ∼1310 cm−1, the positions of the Raman lines for cubic boron nitride, no x‐ray diffraction lines could be observed except that of the silicon substrate.

Microstructural evolution in sintering of ALOOH gels

Materials derived by precipitation or polymerization chemistry (e.g., “sol-gel” methods) are usually obtained in noncrystalline or otherwise metastable phases, and transformation to more thermodynamically stable phases generally occurs by a nucleation and growth process. In a fully reconstructive transformation, such as occurs in the alumina system, the activation energy for nucleation may be higher than that for simple short-range diffusion. Hence nucleation frequency can be a controlling factor in the development of microstructurc. The efficacy of seeding as a method of microstructural and phase control in solution-derived or so-called sol-gel materials has been clearly demonstrated for the alumina system. The epitaxial nature of this phenomenon is explored, using the polarizing microscope to follow the crystallographic orientation of the transformed material as the transformation proceeds, showing that this is epitaxial in nature, and that the nucleation frequency in unseeded material is relatively low (∼ 10 10 cm −3 ). The microscope was then used to demonstrate the effect on nucleation frequency of seeding with materials selected to be isostructural, isotypic, and having little or no similarity to the corundum structure. Using these and other methods, the seeding phenomenon in alumina gels is shown to result from epitaxial growth of the stable corundum phase on isostructural or isotypic nuclei in the solid state. This approach is applied to formulate hypotheses for the mechanisms by which some of the previously reported effects of seeding, e.g., enhanced densification and microstructural refinement, can be understood and to formulate a set of generalizations for its potential application to other systems.

Graphite as a substrate for diamond growth

The nucleation of diamond on highly oriented pyrolytic graphite (HOPG) substrates using hot‐filament‐assisted chemical‐vapor deposition has been studied. Significant differences were observed between the basal {0001} and prism plane {hk*0} surfaces, and for various pretreatments of these surfaces. Observed nucleation densities were found correlate with the C KVV Auger transition signatures observed ex situ on these surfaces prior to diamond growth. An enhanced diamond nucleation density is observed on those carbon surfaces which give an Auger signature close to that observed for the prism or edge plane surface(s) of HOPG. The cleaved basal plane surface of HOPG appears to give by far the lowest nucleation density for diamond.

Growth of cubic boron nitride from vapor phase

Abstract Both physical and chemical vapor deposition (PVD and CVD) methods for the growth of cubic boron nitride ( c -BN) have been reported. These experiments, reviewed in this article, produced a mixture of phases with some evidence of cubic materials. These materials were of limited crystallite size and perfection, much too poor to be useful for electronic devices or for hard coating applications. Using a solid boron source, NaBH 4 , the preparation of c -BN by microwave plasma enhanced CVD has been investigated. When the deposition rate is low, the BN film deposited on an untreated single crystal silicon wafer, displayed a weak infrared absorption spectra for the turbostratic structure of BN. With submicron diamond powder scattered on the substrate, the BN films displayed strong infrared absorption attributable to c -BN. Transmission electron microscopy revealed that c -BN crystals, 50–100 nm in size, grew from the diamond submicron crystals. This suggests that c -BN preferentially nucleates on diamond.

Growth of cubic boron nitride on diamond particles by microwave plasma enhanced chemical vapor deposition

The nucleation and growth of cubic boron nitride (c‐BN) onto diamond powder using solid NaBH4 in low pressure gas mixtures of NH3 and H2 by microwave plasma enhanced chemical vapor deposition has been studied. Boron nitride was deposited on submicron diamond seed crystals scattered on (100) silicon single crystal wafers and evidence was found for the formation of the cubic phase. Diamond powder surfaces appear to preferentially nucleate c‐BN. In addition it was found that the ratio of c‐BN to turbostratic structure boron nitride (t‐BN) deposited increases with decreasing NH3 concentration in H2. It is suggested that this may be due to an increased etching rate for t‐BN by atomic hydrogen whose partial pressure may vary with NH3 concentration.

Hydrogen assisted heat transfer during diamond growth using carbon and tantalum filaments

Much of the previous work on the role of atomic hydrogen in diamond growth has been focused on its formation on various refractory metal filaments, its reaction in the gas phase and its role in the growth mechanism. In contrast, the effect of atomic hydrogen recombination on substrate heating is addressed in this letter. Experiments were conducted in vacuum, helium, and hydrogen environments. Tantalum and carbon filaments were used to vary atomic hydrogen generation rates. Furthermore, methane was added in some experiments to determine its effect on hydrogen assisted ‘‘chemical’’ heating of the substrate. The results indicate that when substantial amounts of atomic hydrogen are generated at the filament, reactions of atomic hydrogen at the diamond growth surface have a pronounced effect on the substrate temperature. Use of carbon filaments lead to significantly diminished atomic hydrogen generation rates and much lower substrate temperatures. Additions of small amounts of methane to hydrogen also resulted...

STRUCTURAL STABILITY OF HYDROGENATED (100) SURFACE OF CUBIC BORON NITRIDE IN COMPARISON WITH DIAMOND

In view of (1×1):2H dihydride/(2×1):H monohydride reconstruction, structural stability of (100) surfaces of both cBN and diamond was comparatively investigated by semiempirical molecular orbital methods using isoelectronic clusters of B52N42H80−2n(10−), N52B42H80−2n(10+), and C94H80−2n, to model (100)B and (100)N of cBN, and diamond surface, respectively, where n=0, 1, 2, or 3. The n denotes the number of monohydride dimers formed. These clusters were nanometer-sized pyramidal crystallites bound by four of {111} faces and one (100). The (100)N of cBN was found unique because of the great stability as (1×1):2H dihydride phase, which retains the bulk structure truncated at the surface without reconstruction and is expected to be chemically inert. This passivation seems to be related to the difficulty in chemical vapor deposition of high quality cBN. The (100)B of cBN was predicted to stabilize as (2×1):H monohydride phase as much as hydrogenated (100) of diamond does.

Non-equilibrium thermodynamics and the vapor phase preparation of diamond for electronic applications

Bulk diamond is unstable relative to bulk graphite except at high pressure and temperature. In spite of this, well crystallized diamond has been grown using numerous CVD methods, many of which have in common the production of atomic hydrogen and hydrocarbon radicals in regimes where solid carbon is expected to be a stable product. Several fundamentally different points of view have emerged in the effort to explain why well crystallized diamond, and not graphite or vitreous carbon, is observed in these experiments. One of the earliest argues that graphite is “etched” by atomic hydrogen at a rate higher than diamond and hence diamond is kinetically stable with respect to graphite. If diamond formation is kinetically controlled the deposition mechanism is critical and much debate has centered on the mechanism and species involved. Alternatively it can be argued that at the growth interface, diamond surfaces are stabilized by termination with hydrogen. If this is correct, and bulk reorganization ignored, then it is shown that a global understanding of the parameters important to the growth of diamond can be obtained without detailed kinetic analyses. Thus it is argued that single crystal diamond films of arbitrarily high purity and perfection are theoretically possible by CVD in spite of the bulk instability of diamond. It is also suggested that general principles exist which might be applied to the growth of other well crystallized metastable phases-notably cubic boron nitride.