Thermal Stability of Thin Films of Ion Beam Deposited CN x
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Thermal Stability of Thin Films of Ion Beam Deposited CNx David C. Ingram, Asghar Kayani Condensed Matter and Surface Science Program, Department of Physics and Astronomy Edwards Accelerator Laboratory, Ohio University, Athens OH 45701 William C. Lanter Innovative Scientific Solutions, Corp., 2766 Indian Ripple Rd., Beavercreek, OH 45440 Charles A. DeJoseph Air Force Research Laboratory (AFRL/PRPE), Wright-Patterson AFB, OH 45433
ABSTRACT A dual ion beam deposition system was used to deposit thin films of CNx from a carbon target. A 1 keV nitrogen ion beam from a 3 cm Kaufman source was used to sputter carbon from a graphite target, and a second nitrogen ion beam of 50 eV, from an RF ion source, was used to bombard the growing film with nitrogen ions. Using this technique, rather than direct ion beam deposition from methane, it is possible to reduce the amount of hydrogen in these films to less than 5% (atomic), and to boost the nitrogen content to over 30%. These films were then subjected to isochronal heating up to 900oC to determine the stability of the films as compared to those with much higher concentrations of hydrogen. CNx is a material that is difficult to fabricate without the inclusion of large amounts of hydrogen. A high hydrogen content has the tendency to make the material sensitive to property changes as it is heated over 200oC. Concomitant with a loss of hydrogen is the loss of nitrogen. In the films that had lower amounts of hydrogen it was found that the loss of nitrogen during heating was delayed until higher temperatures were reached. However, instead of hydrogen being evolved during heating, the amount of hydrogen in the film increases, reaching a maximum concentration of ~45% at 800oC.
INTRODUCTION Ever since the work of Liu and Cohen [1,2] on the properties of C3N4 was published, attempts have been made to fabricate it. Their predictions were that this material would be a wide band gap semiconductor and have mechanical properties similar to diamond. Many attempts have been made to make the material but to date only nanometer sized particles have been fabricated in a matrix of substoichiometric amorphous material. An objective of this study is to use ion beams to deposit carbon nitride films, CNx, with minimal hydrogen contamination, while maximizing the nitrogen content. In previous work on ion beam deposited CNx it was found that the amount of hydrogen trapped in the films was about 35% and the maximum nitrogen content was about 15%,
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[3]. For that work the ion source was fed with a mixture of methane and nitrogen and the films were deposited by direct ion beam deposition. For this work sputter deposition of graphite is the source of carbon. Both the sputtering ion beam source and the direct ion beam source, used to irradiate the growing films, are fed with nitrogen, in order to reduce the partial pressure of hydrogen bearing gases to background levels, and also to enhance the nitrogen content of the environment, in particular the atomic nitrogen content.
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