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I. INTRODUCTION Low-pressure gas-phase synthesis of crystalline diamond and diamond-like carbon (DLC) has become a subject of intensive research.1"* The main focus of these studies has been placed on the properties of the material obtained and the rate of growth. It was noted that diamond and DLC can be formed from various hydrocarbons under different experimental conditions and that the rate of growth is significantly increased with the addition of hydrogen. However, very little is known concerning the mechanisms of growth of either DLC or crystalline diamond. The low-pressure synthesis takes place under temperature-pressure conditions in which diamond is metastable with respect to graphite.' When the growing layer is bombarded by high-energy particles, DLC can be produced.2 One school of thought is that bombardment supplies energy to the surface for the creation of local conditions under which diamond is the stable phase.1'4 These DLC films typically appear amorphous when characterized by diffraction methods since longrange order does not exist.2 Crystalline diamond can be grown at low pressures if the precursor gas is "activated" prior to deposition. l~* The Russian school of Derjaguin, Fedoseev, and coworkers4"10 provides a persuasive argument, based on an extensive experimental base, that the growth of diamond is controlled by kinetics. Thus, for instance, in a recent article Varnin etal.9 wrote that "if the condensate as a whole under the regular conditions of crystallization mostly reveals properties close to those of graphite, then this is a consequence of not the energetic advantage of the graphite (which, by the way, is quite insignificant), but the kinetic preferences in the growth of the graphite structure." The ideas on the mechanism of growth developed by the Russian school can be briefly summarized as follows. J. Mater. Res. 3 (1), Jan/Feb 1988

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The formation of diamond kinetically competes with the formation of graphite. Atomic hydrogen at superequilibrium concentration suppresses the formation of graphite but not that of diamond. Then, there are two somewhat conflicting proposals. One is that the surface of the growing crystal is covered with "hydrocarbon complexes having different numbers of C and H atoms. The processes occurring on the surface are: adsorption of hydrogen atoms and hydrocarbons Cn H m ; recombination of H atoms; hydrogenation and dehydrogenation of adsorbed complexes; formation and decomposition of diamond and non-diamond nuclei; desorption of H 2 and 0,-H,- molecules."8 The other suggestion is that "in the vicinity of the growing crystal surface there is a hyper-concentrated layer, where homogeneous formation of nuclei and clusters of the new phase is possibly taking place, which then incorporate themselves into the lattice of the crystal." 10 A global kinetic theory was suggested that could account for various experimental observations.5>6a The theory was developed based on macroscopic concepts— classical nucleation theory11—and the Langmuir model