Carbon in the form of diamond is the stuff of dreams, and the image of the diamond evokes deep and powerful emotions in humans. Following the successful synthesis of diamond by high-pressure methods in the 1950s, the startling development of the low-pressure synthesis of diamond films in the 1970s and 1980s almost immediately engendered great expectations of utility. The many remarkable properties of diamond due in part to its being the most atomically dense material in the universe (hardness, thermal conductivity, friction coefficient, transparency, etc.) could at last be put to use in a multitude of practical applications. “The holy grail”—it was realized early on—would be the development of large-area, doped, single-crystal diamond wafers for the fabrication of high-temperature, extremely fast integrated circuits leading to a revolution in computer technology.

Excitement in the community of chemical-vapor-deposition (CVD) diamond researchers, funding agencies, and industrial companies ran high in expectation of early realization for many of the commercial goals that had been envisioned: tool, optical, and corrosion-resistant coatings; flat-panel displays; thermomanagement for electronic components, etc. Market projection predicting diamond-film sales in the billions of dollars by the year 2000 was commonplace. Hopes were dashed when these optimistic predictions ran up against the enormous scientific and technical problems that had to be overcome in order for those involved to fully exploit the potential of diamond. This experience is not new to the scientific community. One need only remind oneself of the hopes for cheap nuclear power or for high-temperature superconducting wires available at hardware stores to realize that the lag between scientific discoveries and their large-scale applications can be very long. Diamond films are in fact being used today in commercial applications.

The last decade and a half has seen an explosive growth in the synthesis of diamond materials by a variety of chemicalvapor-deposition (CVD) processes driven by both scientific curiosity and technological exploitation for diverse applications in the fields of hard coatings, tools, optics, passive and active electronics, thermal management, corrosion protection, and radiation detection. Beginning in the 1980s, micron-sized diamond particles were reported by a few groups using hot filaments and a seemingly magical (alchemical) recipe of hydrocarbons and hydrogen. Now near the end of the 1990s, the basic science of diamond growth by CVD is well-understood. Diverse plasma- and thermal-based techniques have been developed for deposition of diamond. Polycrystalline films several mm thick and over 12 in. (30 cm) in diameter are a reality. Many companies are commercializing a wide range of products, and the cost of deposition has dropped by over three orders of magnitude. This article reviews these developments and highlights challenges for the future. It is organized along two themes: scientific advances and technological progress.

The diamond films of the early 1980s presented two quite different challenges. First how could this new form of diamond be exploited technically? Second, how could this clearly nonequilibrium generation of diamond be understood and the understanding be used to maximum effect? We shall be discussing the ideas of theory and modeling, and we will show how they have contributed to the interplay of science and technology.

The science of diamond films is the art of beating nature in the use of carbon. Theory gives the understanding to improve this art. One way in which we improve on nature is in new geometries: controlled growth over selected surfaces o surface regions. The coverage, defect density, microstructure, and rate of growth are key issues. Another way to beat nature is controlled doping. Could wmake n-type semiconductors or lasers using diamond films? A third direction might be routes to control interfaces. Grai boundaries and the regions between small, misaligned crystals affect thermal properties and electron emission. Difficulties with electrical contacts may limit the use of diamond films as semiconductors or insulators. Substrate-film adhesion can determine tribological performance.

If theory is to play a role in controlling film deposition, we need to understand the role of theory itself. Theory can add value at several distinct levels. At the highest level, modeling has the potential to provide a substitute for experiment, especially when information is needed about behavior at extreme conditions. When the phenomena are very fast or very complex, theory can be used to interpret limited experiments. At a more modest level, even simple quantitative models can illustrate the many processes occurring during film growth. Atomistic theories of this type can identify the rate-determining steps and point to ways of influencing them. Mesoscopic theories, especially combined with macroscopic approaches like elasticity theory, can identify routes to improved performance.

It has been generally believed that hydrogen plays a central role in the various processes that have been developed over the years for the chemical vapor deposition (CVD) of diamond films. In particular it has been thought that atomic hydrogen is an absolutely essential ingredient of the vapor from which the films are grown. Typically in diamond CVD, gas mixtures consisting of l-vol% CH3 in 99-vol% H2 have been used in which atomic hydrogen is generated either by thermal decomposition or by collisional processes in a plasma. With a hydrocarbon precursor such as CH3, gas-phase hydrogen-abstraction reactions lead to the generation of the methyl radical CH3, which adsorbs on a carbon radical site also created by hydrogen abstraction from the hydrogen-terminated growing diamond surface. Additional hydrogen-abstraction reactions allow the carbon in the adsorbed methyl radical to form carbon-carbon bonds and thus be incorporated into the diamond lattice. Because graphite is thermodynamically more stable than diamond, the growth of metastable diamond has been thought to require the presence of atomic hydrogen, which has been said to stabilize the diamond lattice and to remove graphitic nuclei when they do form because of the preferential etching or regasification of graphite over diamond. This description of diamond-film growth from hydrocarbon–hydrogen mixtures is of course a very highly condensed version of the detailed experimental and theoretical work that has been carried out in the field over the years. However the predominant conclusion of most of that work is that, particularly in the absence of oxygen and perhaps halogens, atomic hydrogen plays a crucial and decisive role in diamond CVD.

The low-pressure synthesis of rather pure nanocrystalline diamond films from fullerene precursors suggests that for a small enough grain size the diamond structure may be energetically preferred over graphite. Because of the small grain size of typically about 15 nm in these films, a significant fraction of the carbon atoms is situated in the grain boundaries (GBs). The surprisingly high wear resistance of these films even after the substrate is removed and their high corrosion resistance suggest that the grains are strongly bonded. Grain-boundary carbon is also believed to be responsible for the absorption and scattering of light in these films, for their electrical conductivity, and for their electron-emission properties. In spite of all these indications of a critical role played by GB carbon in achieving the remarkable properties of nanocrystalline diamond films, to date the atomic structures of the GBs are essentially not known.

It is well-known that the electronic and optical properties of polycrystalline silicon films are significantly affected by the presence of GBs. For example GBs can provide active sites for the recombination of electron-hole pairs in photovoltaic applications. Also, in electronic devices such as thin-film transistors, GBs are known to play an important role. Because of silicon's strong energetic preference for sp3 hybridization over other electronic configurations, the structural disorder in silicon GBs is accommodated by a distortion of the tetrahedral nearestneighbor bonds and in the extreme by the creation of dangling bonds—that is, of three-coordinated Si atoms each having one unsaturated, bound electron in an otherwise more or less tetrahedrally coordinated environment.

Diamond always has been considered an exciting material. In addition to its other outstanding properties, diamond's capacity for cold electron emission has become a “hot” topic of research in recent years. The electron emission from diamond films is important for both fundamental and applied purposes, which may be expressed by two questions as follows: “Why does diamond emit electrons?” and “How can an efficient cold cathode be made?”

The “diamond-emission era” started in 1991. That year three papers were published, reporting unexpectedly high electron emissivity from diamond. The first paper by Djubua and Chubun was very practical, having been written by device engineers. They tried different materials for the fabrication of pointed field emission cathodes and found that emitters made from diamondlike carbon (DLC) demonstrated a lower emission threshold compared to other materials. A second paper by Wang et al. reported a low emission threshold for chemical-vapor-deposition (CVD) diamond films, whereas a third paper by Geis et al. described the fabrication and operation of diamond cold cathodes.

Since 1991 hundreds of papers about electron emission from diamond and diamondlike materials have been published. The motivations for this increasing activity are prospective applications in vacuum microelectronics—that is, vacuum emission devices fabricated by modern microelectronic technologies. Experts expect that combining the physical advantages of emission devices and the technological progress in solid-state microelectronics will result in the development of an entirely new generation of high-performance electronic devices—among them, flat-panel displays and miniature microwave tubes.

From a commercialization standpoint, electronic applications have been particularly elusive for diamond. Market estimates of $560 million per year by the year 2000 indicate the original enthusiasm in this area. Now such projections seem unreasonably optimistic, and even a niche commercial application in the area of electronics would be considered a success. However when taken in a broader context, this extended time frame for commercialization is not at all unusual for new technologies, and many new advances have continued to bolster the enthusiasm of diamond electronics research groups. Diamond has such an extensive list of exceptional properties that it continues to be a candidate for numerous electronic applications from heat spreaders to detectors to microvacuum tubes.

A variety of theoretical calculations established diamond's potential in the early years of CVD-diamond research. As shown in Table I, figures of merit indicated that diamond's potential far exceeded the potential of more common semiconductors such as silicon and gallium arsenide for certain applications. Typically these applications revolved around high power or high temperature. However more accurate assessments of diamond's capability after further development uncovered several issues with many electronic applications of interest: (1) n-Type doping: Although scattered reports of n-type doping have been made a high-quality, low-resistance n-type material is not available, limiting the potential applications for diamond.

(2) Deep activation of p-type carriers: At reasonable dopant levels, the activation energy for p-type carriers is approximately 0.3 eV, causing diamond to be highly resistive and sensitive to temperature variations at normal operating temperatures.

Electrochemical reactions typically involve electron transfer between an electrode and a dissolved chemical species at a solid-electrode/liquid-electrolyte interface. Three broad classes of electrochemical applications may be identified: (1) synthesis (or destruction), in which an applied potential is used to bring about a desired chemical oxidation or reduction reaction; (2) analysis, in which the current/potential characteristics of an electrode are used to determine the type and concentration of a species; and (3) power generation. These broad types of applications require stable, conductive, chemically robust, and economical electrodes. Diamond electrodes, fabricated by chemical vapor deposition, provide electrochemists with an entirely new type of carbon electrode that meets these requirements for a wide range of applications.

The first reports of electrochemical studies using diamond were in the mid-1980s. During the past several years, the field has attracted increasing attention. This review summarizes the electrochemical properties of diamond that make it a unique electrode material and that distinguish it from conventional carbon electrodes.

Considering all materials, diamond has many superior properties such as elastic constant (hardness), thermal conductivity, transparency to optical visible light and x-rays, a wide-bandgap semiconductor, and negative electron affinity. These properties lead to various applications in many industries. Because of the recent successful development of vapor-phase-growth technology, large-area diamond is available at a low manufacturing cost. However the commercialization of diamond at this stage is limited to tools, speaker diaphragms, heatsinks, and optical windows. For each application, diamond utilization is limited.

In this article, three avenues on the road to commercialization of vapor-phase-grown diamond will be discussed. The categories appear in Table I with the properties of diamond and its applications. First, applications already commercialized are briefly reviewed, and the requirements for expanding their market are discussed. Second, the technologies and applications coming in the near term are introduced. Last, future candidate applications for diamond are introduced along with technical obstacles to be overcome.