Epitaxical nucleation of polycrystalline silicon carbide during chemical vapor deposition
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Epitaxical nucleation of polycrystalline silicon carbide during chemical vapor deposition Brian W. Sheldon Division of Engineering, Brown University, Providence, Rhode Island 02912, and Oak Ridge Associated Universities, Oak Ridge, Tennessee 37831
Theodore M. Besmann, Karren L. More, and Thomas S. Moss Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 17 June 1992; accepted 12 January 1993)
Polycrystalline silicon carbide was deposited from methyltrichlorosilane in cold-walled and hot-walled reactors, on (100) SiC surface layers that were formed on (100) Si wafers. The initial stages of the process were studied by electron microscopy after relatively short deposition times. Submicron surface features nucleated with a specific crystallographic orientation with respect to the substrate, where {111} planes in the /3-SiC substrate coincided with {0001} planes in the a - S i C features. These a - S i C features occurred only at twins on {111} planes of the /3-SiC substrate. This demonstrates that nucleation under these conditions is controlled by defects in the substrate. Surface contamination and the reactor configuration also had substantial effects on nucleation.
I. INTRODUCTION
II. DESCRIPTION OF EXPERIMENTS
Polycrystalline SiC produced by chemical vapor deposition (CVD) exhibits a wide range of different microstructures that have, in many cases, been empirically correlated to various processing parameters.1'2 Most of the research conducted on nucleation and growth mechanisms during CVD has focused on the epitaxical growth of single crystals, with particular emphasis on the formation of electronic materials such as Si. This previous work provides a starting point for understanding the formation of polycrystalline materials. Although the nucleation and growth of polycrystalline Si have been studied in detail,3"5 in general, the microstructure evolution of polycrystalline materials is not well understood.
Deposition was conducted with two different reactor configurations. Figure 1 depicts a hot-wall system, where the graphite tube surrounding the sample was heated by a radio-frequency (RF) generator operating at 455 kHz. A corresponding cold-wall configuration was also used where the graphite tube (shown in Fig. 1) was not present, so that the SiC-coated graphite sample holder acted as a susceptor in the RF field. The substrate temperature was measured with an optical pyrometer that was focused through a calibrated window and onto the graphite holder. The substrates used for this work were (100) oriented Si. Prior to SiC deposition, the substrate was heated rapidly to 1350 °C at approximately 5 0 0 700 °C/min in a flowing stream of H2 (5000 cm 3 /min)
Because of its high strength and chemical stability at elevated temperatures, SiC produced by CVD is potentially important for protective coatings. There is also considerable interest in forming ceramic matrix composites by performing CVD in a porous structure (known as chemical vapor infiltration or CVI).6"9 Researc
