Diamond Films: Recent Developments in Theory and Practice
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terpret 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. Effective medium theory, a mesoscopic approach, has been used to analyze the optical properties of films consisting of diamond crystallites and small (subwavelength) graphite crystallites.2 Other mesoscopic theories can be used to understand the effects of intercrystallite thermal barriers.3 The most important contribution of theory is as a framework for understanding information. At this level, theory is used by all experimenters. The idea that diamond is a network of sp3-bonded carbons allows one to relate defect energies to bond energies of small molecules. Theory at this level also includes much of the simple phenomenology of diamond and its relationship to graphite. The idea of a topological network allows one to understand many features of diamondlike carbons. We will consider select examples of theoretical approaches to film growth, nucleation, and film properties, and will examine their links to experiment. We will note the critical role of diamond nucleation, central to the control of film microstructure, and the role of surface processes on growth habit and surface electronic properties.
CVD-Diamond Growth Chemistries Chemical vapor deposition (CVD) of crystalline diamond films exploits the decomposition of gases, usually driven either by thermal activation or by plasma excitation, to give atomic hydrogen and reactive carbonaceous species. During the early 1990s, the focus was on hydrogen feedstock containing 1-2% of a hydrocarbonlike methane. (For earlier history, see the article by Butler and Windischmann in this issue.) Higher partial pressures of hydrocarbon tend to increase the nondiamond fraction in the film.4 For substrate temperatures from about 700°C to 1000°C, excess gas-phase carbon condenses as a film. Growth rates are usually about 0.1-1.0 /j,m/h (about one monolayer per second). Gas compositions have been varied to seek higher deposition rates, or lower growth temperatures, without compromising film quality. The main approaches involve additions of oxygen,5 and CO- and CO2based mixtures;6 halogens like fluorine and chlorine;7 and noble gases such as neon and argon.8 Much of the experimental data for crystalline diamond growth chemistries can be rationalized in the important C-H-O composition diagram of Bachmann et al.9 This diagram correlates carbonhydrogen-oxygen feedstock composition with observed deposition, or lack of it, of diamond or nondiamond carbons. One striking result is that low-pressure diamond synthesis only occurs within a specific field of gas compositions, irrespective of method of CVD growth. There are simple ways to understand the main features of this Bachmann plot. First the
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