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SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS Recent success in deposition of diamond and diamond-like coatings on a variety of substrates at practical growth rates is one of the most important technological developments in the past decade. It places within the reach of science and industry an opportunity to exploit the many desirable extreme properties of these materials (e.g., modulus, hardness, thermal conductivity, high breakdown voltage). Indeed, the ultimate economic impact of this technology may well outstrip that of high-temperature superconductors. Consequently, it is imperative that the United States take a leading role in the commercialization of diamond technology. The committee focused on materials that can be grown at practical growth rates by various vapor-phase techniques. These include principally diamond, diamond-like hydrocarbons (DLHC), and diamond-like carbons (DLC). · Diamond--This refers to crystalline diamond grown at relatively low pressure by various chemical vapor-deposition (CVD) processes in the presence of hydrogen and hydrocarbons such as methane. Substrate temperatures generally are required to be above 600°C. Surface morphology is usually faceted. · Diamond-like hydrocarbons (DLHC)--These are metastable, amorphous materials prepared by various techniques (e.g., ion beam deposition) involving thermal decomposition of hydrocarbons and having atomic number densities greater than approximately 0.19 g-atom/cm3. Hydrogen content ranges from 17 atomic percent to 60 atomic percent. Properties vary widely, depending on hydrogen content, surface finish is usually optically smooth, and substrate temperature requirement is low (<150.C). . Diamond-like carbons (DLC)--There is evidence to suggest that hydrogen-free DLC films exist. These might be considered "microcrystalline" diamond, but much work remains to further describe their structure and properties. · Other materials--In addition to the foregoing, the study includes a discussion of high- pressure diamond to provide background for the low-pressure processes and to indicate technological opportunities for the more conventional approach to diamond manufacturing Also, a brief description of related materials involving boron, carbon, nitrogen, and silicon is given, with particular emphasis on SiC and BN. The ability to deposit diamond and ~iiamond-like films on a variety of substrates (including diamond-like materials on paper and plastics) vastly expands the potential application areas for these materials over that possible with high-pressure, high-temperature technology.
2 Although current applications in actual production are limited to high-modulus tweeter speaker cones (for improved high-frequency performance) and instrument windows, other uses are being actively pursued. The committee felt that there are applications that have not been realized but which exhibit potential for high payoff, assuming adequate funding. In electronics, there is potential payoff in the following applications: . High-temperature electronics for space with improved heat dissipation capability via the T4 dependence for the radiation of energy. · Cold cathodes (current densities greater than 1000 A/cm2) that take advantage of expected low work function. . Thermal conductors and heat sinks, particularly for electronic packages. . High-power, high-efficiency radar electronics, with a potential for a hundredfold increase in power capability over silicon transistors. eV) of BN. . Solar-blind detectors, which take advantage of the large energy gap (greater than 6.4 In optics, potential payoffs could occur in the following: . Anti-abrasion, anti-erosion coatings for infrared lenses and windows. Some success has already been achieved, for example, in applying diamond-like coatings to ZnS and ZnSe windows using a Ge-C intermediate layer. . Anti-abrasion coatings on optical fibers for fiber-optic-guidec! (FOG) missiles. · Compact ultraviolet lasers (for diamond, this requires a stress-induced conversion to a direct-gap material, possibly by a strained superiattice approach). lenses). . Large single-crystal optical elements for use in infrared devices (e.g., windows and Other potential applications could include the following: . Abrasion-resistant coatings for computer disks and read-write heads. . Machine too! guides to produce precise, very reproducible parts. . X-ray lithographic masks. . Anti-friction (low-friction) coatings for prosthetics. · Corrosion protection and passivating films. As more and more potential applications of diamond and diamond-like films became apparent, it was clear that the impact of this emerging technology on the defense, space, and commercial sectors could rival that of high-temperature superconductors but with more immediate applications. For this to happen, however, certain technical and economic problem areas must be identified and decisive addressed. Also, some approaches and policies that have been carried out correctly in the past should be encouraged and promoted. Few applications have
- as yet reached the marketplace; the enthusiasm of workers in the field is based substantially on the feelings that applications~ften completely unforeseen follow when a product with unique properties is developed. CONCLUSIONS AND RECOMMENDATIONS This emerging technology must be exploited. The following conclusions and recommendations address those areas that the committee feel will expedite this exploitation. Conclusion 1 While General Electric and DeBeer have the bulk of the high-pressure diamond market, Japan apparently leads in vapor-deposited diamond research and development. However, the ability to transfer a technological development from the laboratory stage into useful applications can be the most difficult part in the growth of a new technology. Fortunately, in the case of diamond and d~amond-like films, it appears that transfer has been facilitated by the excellent university, industry, and government interactions that have developed in this country. As a result, many companies, large and small, are now growing films in the United States after receiving training from government- and industry-sponsored programs. Recommendation Funding for university, industry, and government programs that will turn this emerging technology into a national pivotal technology with important useful applications must be continued and expanded in the future. The programs should be interdisciplinary, with theoreticians working with chemists, optical scientists, electrical engineers, and materials scientists to establish relationships among growth processes, structure, and properties. They should be carefully coordinated with and should draw upon the results of the individual basic research programs discussed in Conclusion and Recommendation 2. In order to ensure the greatest impact, emphasis should be placed on the optimization and use of combinations of properties (e.g., hardness and infrared transparency for window coatings) for well-defined applications. Conclusion 2 The new films can exhibit many of the extreme properties that make diamond attractive but the effect of variables such as hydrogen content, other impurities (dopants), crystallinity, and sp3/sp2 ratio on mechanical, electrical, and optical properties is not well understood. An appreciation of the influence of additional elements in the diamond lattice on, for example, tribology could be of considerable industrial significance. Thus, the ability to produce films with predictable material properties to satisfy specific application requirements is currently lacking. Recommendation A coordinated interagency effort is recommended to address fundamental problems that have arisen in attempts to exploit diamond and diamond-like technology applications. Among the most pressing problems that should be addressed are: . Nucleation and growth processes, with specific emphasis on increasing growth rates of both diamond and diamond-like films and on growing larger high-quality single crystals of diamond for electronic and optical applications. An integral part of this study should be substrate development and adhesion to substrates. Current work involves deposition from vapor;
4 other possibilities should be explored. Much needs to be done to enable deposition over large areas and at lower temperatures. · Growth of cubic BN films and crystals, with specific emphasis on their use for ultraviolet detectors (BN can be doped both p and n type) and solar-blind detectors. · Modulated structures incorporating layers of BN and diamond, for example, to develop materials that are harder and/or tougher than diamond. · Colt! cathode development for high-power electron sources (greater than 1000 A/cm2~. The basic question to be resolved relates to determining the degree of (negative) electron affinity of the film materials. . Development of a predictive capability for the stability of various boron, nitrogen, and carbon alloys and "superdense" carbons through the use of fundamental structure calculations. One question that should be answered: "Is there anything beyond diamond with useful extreme properties?" Conclusion 3 . Attaining successful electronic devices requires successful controlled doping of diamond both by CVD and by ion implantation. Recommendation Basic understanding should be developed, through theoretical studies, of doping and defect formation and their influence on the band structure. Development and understanding of processes for both p-type and e-type doping should also be pursued. Conclusion 4 The rapid growth of this emerging technology coupled with the wide variety of materials it can produce (e.g., crystalline and amorphous materials containing large or small amounts of hydrogen) has led to several different nomenclature schemes, with some resulting confusion. Recommendation The committee recommends that a uniform system of nomenclature be used based on the following principles: 1. Diamond films - true crystalline diamond films as produced by a variety of CVD techniques. 2. Diamond-like films {DL) are of two types: . Diamond-like hydrocarbons (DLHC) - amorphous films grown by a variety of ion- assisted deposition processes and the decomposition of hydrocarbon gases. These films generally contain between 15 percent and 60 percent hydrogen and have atomic number densities greater than 0.19 g-atom/cm3. . Diamond-like carbons (DLC) - amorphous hydrogen-free carbon films having atomic number densities greater than 0.19 g-atom/cm3. .
