Synthesis and characterization of sulfur-incorporated microcrystalline diamond and nanocrystalline carbon thin films by
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B.R. Weiner Department of Chemistry, University of Puerto Rico, San Juan, P.O. Box 23346, Puerto Rico 00931
G. Morell Department of Physical Sciences, University of Puerto Rico, San Juan, P.O. Box 23323, Puerto Rico 00931 (Received 23 August 2002; accepted 30 October 2002)
The synthesis of microcrystalline and nanocrystalline carbon thin films using sulfur as an impurity addition to chemical vapor deposition (CVD) was investigated. Sulfur-incorporated microcrystalline diamond (c-D:S) and nanocrystalline carbon (n-C:S) thin films were deposited on Mo substrates using methane (CH4), hydrogen (H2), and hydrogen sulfide (H2S) gas feedstocks by hot-filament CVD. These films were grown under systematically varied process parameters, while the methane concentration was fixed at 0.3% and 2% for c-D:S and n-C:S, respectively, to study the corresponding variations of the films’ microstructure. Through these studies we obtained an integral understanding of the materials grown and learned how to control key material properties. The nanocrystalline nature of the material was proposed to be due to the change in the growth mechanisms in the gas phase (continuous secondary nucleation). The growth rate (G) was found to increase with increasing TS and [H2S] in gas phase, thus following the chemisorption model that describes the surface reactions. One of the propositions for the increase was that H2S increases the production rates of methane and consequent methyl radicals without much of its own consumption, which is almost negligible and increases the carbon-containing species. This is analogous to the increase of G with increasing methane concentration, but for the relatively high S/C ratio used here, there is a possibility of its incorporation in the material, however small. This particular conjecture was verified. In this context, the results are discussed in terms of the decomposition of reactant gases (CH4/H2/H2S) that yield ionized species. The inferences drawn are compared to those grown without sulfur to study the influence of sulfur addition to the CVD.
I. INTRODUCTION
Diamond is one of the promising wide-gap semiconductor materials with a large potential for electronic as well as sensor applications.1,2 Diamond in thin-film form is attractive for several mechanical, optical, and electronic applications such as in tribological coatings and cutting tools, heat sinks,3 optical windows (wide band gap, 5.45 eV),4 high-temperature and high-power electronics (breakdown voltage of approximately 107 V/cm),
a)
Address all correspondence to this author. Present address: Department of Engineering, Cambridge University, Cambridge CB2 1PZ, United Kingdom e-mail: [email protected] J. Mater. Res., Vol. 18, No. 2, Feb 2003
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microsensors, biosensors,5,6 vacuum microelectronics in general, and field-emission arrays in particular,7 and therefore it is considered as an engineering material. The promise of diamond stems from the coexistence of several unsurpassed physical
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