Relationship between the structural organization and the physical properties of PECVD nitrogenated carbons
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C. Guimon and G. Pfister-Guillouzo Laboratoire de Physico-Chimie Moleculaire, Universite de Pau et des Pays de VAdour, 64000 Pau, France
B. Nysten and J. P. Issi Unite de Physico-Chimie et de Physique des Materiaux, Universite Catholique de Louvain, 1, Place Croix du Sud, B-1348 Louvain-la-Neuve, Belgique (Received 11 May 1992; accepted 28 October 1992)
By a Plasma Enhanced Chemical Vapor Deposition process (PECVD), we are able to prepare nitrogenated amorphous carbon materials around room temperature from methane and nitrogen gas as precursors. We have also used chlorine gas as an additive to reduce the hydrogen content of our samples. Starting from the "as-deposited" materials, we have investigated their thermal stability by successive heat treatments up to 1400 °C. These compounds suffer a weight loss mostly due to the hydrogen departure. They become nonfusible and it turns out that nitrogen, chemically bound to sp2 hybridized carbons, induces some changes in the physical properties. In order to understand the relationship between the local structural organization and the physical characteristics, we have investigated different spectroscopic techniques such as Nuclear Magnetic Resonance, IR Absorption, and X-ray Photoelectron Spectroscopy. We have also investigated several transport properties: (i) The dc electrical conductivity shows a kind of metal/insulator transition around 700 °C. The temperature dependence for the conductive samples gives evidence for a pseudogap associated with the presence of localized states, (ii) The thermal conductivity exhibits, for the as-deposited compound, a very low value varying slowly with temperature; its magnitude as well as its temperature dependence, characteristic of noncrystalline materials, are modified by the annealing process. Finally, an electronic band model is proposed, explaining the structural evolution through a kind of Mott—Anderson pseudotransition.
I. INTRODUCTION Considerable interest exists in the structure of amorphous carbons prepared by technical processes which are working out of thermodynamical equilibrium by using generated ionic species and radicals as chemical reactive vectors.1 One major problem encompasses mixtures of different microstructures with regard to the information inferred about the fraction and the distribution of threefold and fourfold carbon sites. The polymorphic carbon phases are binary or ternary mixtures with structures ranging from sp2 hybridization (graphite) to sp3 hybridization (diamond), with also a possible amount of hydrogen atoms. In particular, hydrogenated amorphous carbons ( a - C : H ) , which can be synthesized, are approaching the hardness of diamond. Recently there have been reports on the use of nitrogen for doping these amorphous solids through the use of PECVD processes.2'3 The fabrication of a new class of nitrogenated carbons which changes the 480 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 3, Mar 1993 Downloaded: 24 Mar 2015
basic structure and improves the physical properties of a - C : H ca
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