A mechanism of CVD diamond film growth deduced from the sequential deposition from sputtered carbon and atomic hydrogen
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We describe a growth mechanism of CVD diamond films consisting of a series of surface reactions. It is derived from experimental observations of a sequential deposition process in which incident carbon flux and atomic hydrogen flux were independently varied. In this sequential process, film growth rate increased with atomic hydrogen exposure, and a saturation in the utilization of carbon was observed. These features are consistent with a surface growth process consisting of the following steps: (i) the carburization of the diamond surface, (ii) the deposition of highly disordered carbon on top of this surface, (iii) the etching of disordered carbon by atomic hydrogen, (iv) the conversion of the carburized diamond surface to diamond at growth sites by atomic hydrogen, and (v) the carburization of newly grown diamond surface. The nature of the growth sites on the diamond surface has not been determined experimentally, and the existence of the carburized surface layer has not been demonstrated experimentally. The surface growth mechanism is the only one consistent with the growth observed in conventional diamond reactors and the sequential reactor, while precluding the necessity of gas phase precursors.
I. INTRODUCTION
II. REACTOR DESCRIPTION
The wide interest in the growth of diamond films has been prompted by the material properties of diamond in combination with a broad range of potential applications. Hydrogen and hydrocarbon gas mixtures have been used extensively for synthesis,1"3 focusing attention, to date, on the gas phase precursors involved in the growth of diamond.4"6 The understanding of the nucleation and growth processes has been impeded by the complex gas phase chemistry. Additionally, the relatively high operating pressure complicates in situ diagnostics, thus impeding direct verification of proposed growth models.7'8 The goal of our work was to understand the fundamental reactions necessary to deposit diamond in a standard CVD reactor. A novel sequential reactor was constructed to develop this understanding. Our experimental results indicate that film thickness (growth rate) and film quality are functions of the exposure to carbon and atomic hydrogen bombarding the growing surface, suggesting a surface growth mechanism. We believe our results show that CVD diamond growth involves only the action of atomic hydrogen on surface-bound carbon. This carbon may originate from a gas phase precursor or from sputtered graphite, but once the diamond surface is "carburized", no other gas phase reactions are involved. This growth mechanism is presented here in as much detail as possible given the knowledge of the growing diamond surface. Nucleation issues are not addressed in this work.
In order to understand the diamond deposition process we have constructed a novel sequential reactor that has four isolated gas environments. The chemistry and excitation of each environment can be independently varied. This permits, for example, an increase in the amount of atomic hydrogen incident on the substrate independen
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