Density Functional Based Tight Binding Study of C 2 and CN Deposition On (100) Diamond Surface
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DENSITY FUNCTIONAL BASED TIGHT BINDING STUDY OF C2 and CN DEPOSITION ON (100) DIAMOND SURFACE Michael Sternberg§, Peter Zapol‡, Thomas Frauenheim§, John Carlisle‡, Dieter M. Gruen‡, and Larry A. Curtiss‡ ‡
Materials Science and Chemistry Divisions, Argonne National Laboratory, Argonne, IL 60439 Universität Paderborn, Fachbereich Physik, Theoretische Physik, D-33098 Paderborn, Germany
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ABSTRACT A density-functional based tight binding method was used to study elementary steps in the growth of ultrananocrystalline (UNCD) diamond. It was shown previously that C2 dimers are the dominant growth species in hydrogen-poor argon plasmas. Recent experimental evidence shows that nitrogen addition to the plasma profoundly changes the morphology of the UNCD film. CN species are believed to play a major role. Reactions of C2 and CN molecules with reconstructed diamond (100) surfaces were studied. A single CN prefers an end-on attachment to a surface atom on the unhydrided (100) surface with its C end down. It is shown how further C2 addition to the surface leads to CN-mediated diamond growth and how the CN species remain on top of the growing diamond layer. INTRODUCTION Diamond film growth from argon/methane plasmas produces ultrananocrystalline diamond (UNCD) with an average size of 3-10 nm.1 Nitrogen addition to the plasma in the amount of 1% to 20% strongly influences growth and renucleation rates as well as the composition of the resulting films. These films have a range of unique mechanical and electronic properties, which are associated with small crystallite size and large number of grain boundaries.2 The renucleation rate in UNCD growth is several orders of magnitude higher than that in the conventional diamond synthesis from hydrogen/methane plasmas. Examination of plasma emission spectra detected a Swan band produced by carbon dimers.3 This led to studies of diamond growth mechanisms on (100) and (110) surfaces using carbon dimers as precursors.4,5,6,7 In contrast, hydrogen abstraction and surface reactions with hydrocarbons such as methyl radicals play a central role in the established mechanisms of conventional microcrystalline diamond growth.8 On the basis of quantum chemical modeling of C2 reactions with the unhydrided and hydrogen-covered diamond surfaces, it was proposed that reconstructed hydrogen-free surface sites can act as chemically active nucleation sites after dicarbon insertion.5,6 Strongly exothermic reactions leading to diamond growth on unhydrided and monohydrided diamond surface were calculated as well. Subsequently, a growth mechanism on the (110) surface based on C2n chain formation and coalescence was investigated using a densityfunctional-based tight-binding method (DFTB).7 In view of the strong modification of the growth process by nitrogen addition to the plasma, it is interesting to investigate the nitrogen role in the growth process. One of the common plasma species in this case is the CN molecule.3 In the present paper we examine the interaction of the CN radical with the unhydrided diamo
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