Effect of Substitutional or Chemisorbed Nitrogen on the Diamond (100) Growth Process
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1203-J16-01
Effect of Substitutional or Chemisorbed Nitrogen on the Diamond (100) Growth Process Karin Larsson Department of Materials Chemistry, Uppsala University, Box 538, 751 21 Uppsala, Sweden
ABSTRACT The present paper outlines the energetic and kinetic effect by substitutional N, or by coadsorbed NHx (x =1, 2), on one of the key growth steps in the CVD growth mechanism of diamond (100); H abstraction by gaseous H radical species from the (100) surface plane. Theoretical calculations were performed based on Density Functional Theory under periodic boundary conditions. Substitutionally positioned N was shown to have a large effect on the H abstraction process. The H abstraction energy from the diamond surface was greatly improved with N positioned in C layer 2. In order to outline the effect by N on the growth rate, the barriers of energies were calculated. The barrier of abstraction was shown to substantially decrease with N substitutionally positioned in the second C layer, leading to an improvement of the abstraction reaction rate by approximately a factor of 3. INTRODUCTION The growth of diamond films with desired properties and morphology requires a perfect recognition of the parameters affecting the growth process [1-3]. It is especially crucial to understand how these parameters will affect the growth on an atomic level. In the early 1990s, Locher et al. reported, for the first time, major changes in the polycrystalline diamond CVD growth by introducing a small percentage of nitrogen in the gas phase [4]. In a more recent publication, Achard et al. performed a careful study on the effects induced by the presence of N during the (100) single crystal diamond growth process in order to understand how it affects the growth rate and the surface morphology [5]. In accordance with their observations, they proposed that sub-surface nitrogen, through an electron transfer to a surface C radical, is probably responsible for the important effect observed experimentally. This assumption was initially made by Frauenheim [6] et al. who showed theoretically that this specific electron transfer largely affects the reaction energies and barriers of the different steps involved in the growth mechanism proposed by Harris and Goodwin for diamond(100) [7-8]. This mechanism is composed of 5 different steps: a) surface H abstraction by a gaseous H radical; b) CH3 adsorption on the newly formed surface radical; c) H abstraction from CH3 by a gaseous H radical; d) C-C dimer opening by a b-scission rearrangement; and e) the final formation of a six carbon ring. Independent of the surface reaction model considered, the importance of i) H abstraction from the diamond surface, ii) CH3 adsorption to the newly formed surface radical site, and iii) a following H abstraction, forming an adsorbed CH2, has earlier been stressed [9]. It is therefore of the greatest interest to analyze the effect of substitutional nitrogen on these reaction types, generally considered as initial CVD growth steps of diamond in an H /CH3 atmosphere.
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