The Mechanism for Ultrananocrystalline Diamond Growth: Experimental and Theoretical Studies
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0956-J07-04
The Mechanism for Ultrananocrystalline Diamond Growth: Experimental and Theoretical Studies Paul William May1 and Yuri A. Mankelevich2 1 School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, United Kingdom 2 Nuclear Physics Institute, Moscow State University, Moscow, 119992, Russian Federation
ABSTRACT Ar/CH4/H2 gas mixtures have been used to deposit microcrystalline diamond, nanocrystalline diamond and ultrananocrystalline diamond films using hot filament chemical vapor deposition. A 3-dimensional computer model was used to calculate the gas phase composition for the experimental conditions at all positions within the reactor. Using the experimental and calculated data, we show that the observed film morphology, growth rate, and across-sample uniformity can be rationalized using a model based on competition between H atoms, CH3 radicals and other C1 radical species reacting with dangling bonds on the surface. Proposed formulae for growth rate and average crystal size are tested on both our own and published experimental data for Ar/CH4/H2 and conventional 1%CH4/H2 mixtures, respectively.
INTRODUCTION Recently, so-called ultrananocrystalline diamond (UNCD) films have become a topic of great interest, since they offer the possibility of making smooth, hard coatings at relatively low deposition temperatures, which can be patterned to nm resolution.1,2 These differ from nanocrystalline diamond (NCD) films,3 since they have much smaller grain sizes (~2-5 nm), and have little or no graphitic impurities at the grain boundaries. Most reports of the deposition of these films describe using a microwave (MW) plasma CVD reactor and gas mixture of 1%CH4 in Ar, usually with addition of 1-5% H2.1 We have previously reported the use of similar Ar/CH4/H2 gas mixtures to deposit microcrystalline diamond (MCD), NCD and UNCD in a hot filament (HF) reactor,4 with the compositional diagram for mixtures of Ar, CH4 and H2 being mapped out corresponding to the type of film grown. For the majority of the composition diagram, diamond films are deposited only in a very narrow region around [CH4]/([CH4]+[H2]) ~ 0.5-6%, with UNCD films being deposited only in the region of the MCD/‘no-growth’ boundary.
Originally it was suggested 5 that the C2 radical played an important role in the growth mechanism for UNCD. However, recent work by ourselves 6,7 and others 8 has shown that C2 is only a minority species close to the substrate surface and plays no significant role in growth. In our previous paper,7 we used a 2-dimensional model of the gas chemistry, including heat and mass transfer, in our HF reactors to understand the experimental observations. The conclusions led to a generalized mechanism7 for the growth of diamond by CVD which was consistent with all experimental observations, both from our group and from others in the literature. The proposed mechanism involves competitive growth by all the C1 radical species that are present in the gas mixture close to the growing diamond surface. Previous models o
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