Low Density Supersonic Plasma Jet Synthesis of Thin Films of Refractory and Wide Band-Gap Nitrides

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Abstract - Cappelli CTS-9707923 This is a combined experimental and modelling study oriented toward an efficient process for the synthesis of advanced materials in the nitride family. A supersonic plasma jet generated by an arc-heated flow and expanded to low densities provides a large flux of energetic dissociated (atomic) nitrogen into which a wide variety of reactants (such as gallium trichloride, aluminum trichloride, methane, or boron trichloride) are injected to generate a reacting flow from which products condence onto a substrate to form films, coatings, or epitaxial layers. The ability to apply a controlled electrical bias to the substrate provides an added variable to influence film structure and properties. The project includes modelling of reactant injection and subsequent decomposition in the plume; modelling of ion transport through the plasma sheath surrounding the substrate; development of empirical heterogeneous models of film growth; measurement of concentratures of critical gas-phase species during film deposition by optical absorption spectroscopy; measurement of concentrations and energies of a broader range of neutral and ionic radicals by molecular-beam mass spectrometry; and correlation of vapor-phase properties to film structure as ascertained by infrared absorption (IR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Optical and mechanical properties of the deposited films are also measured. This study addresses the synthesis of a number of nitrides that form the basis of "space-age" ceramics. These include cubic boron nitride (c-BN), carbon nitride (C3 N4), gallium nitride (GaN), aluminum nitride (AlN), and the mixed aluminum/gallium nitrides. The unusual mechanical, electrical, and optical properties of these materials make them attractive for many applications, but their wide use is inhibited by the lack of an economic method of s ynthesis.