Researchers Unlock the Secrets of Diamonds
Daniel S. Burgess
HONG KONG -- The captivating beauty of diamonds can overwhelm our sense of their utility. Still, their position as the hardest, most thermally conductive and most chemically resistant material known has a greater impact on us.
Diamond coating technology has been applied to produce superior abrasives and cutting tools, such as diamond-turning machinery for optics fabrication. But the technique also promises additional applications in electronics and optics if methods for epitaxial growth can be perfected.
In the Jan. 7 issue of Science, researchers from the City University of Hong Kong reported new insights into how diamond films grow -- insights that they say may enable the production of low-cost, large-area epitaxial films for these applications.
"The nucleation process of diamond is one of the most studied and important issues but one of the least understood in [chemical vapor deposition] diamond growth," said Shuit-Tong Lee of the university's department of physics and materials science. "Our work reported a major advance in the observation and understanding of this nucleation step for epitaxial growth."
Controlling nucleation, the formation of seed crystals on the target substrate, could have a significant impact on the quality of the films that vapor deposition processes yield. When carbon gas is induced to form these crystals, it produces a mix of epitaxial, partially epitaxial and random nuclei.
A better understanding of diamond nuclei formation may lead to the production of low-cost, large-area diamond films by selective chemical vapor deposition. Researchers at the City University of Hong Kong monitored the deposition of diamond on a silicon substrate.
The researchers made the diamond seeds with double-bias-assisted hot filament chemical vapor deposition, which dissociated a CH4/H2 mixture with 2000 °C tungsten filaments and applied voltage to the silicon target substrate and the filaments. The team analyzed the 2-nm particles with high-resolution scanning electron microscopy and Raman microspectroscopy, and found that they grow preferentially on steps or kinks in the substrate and without an interlayer.
Lee said the observations suggest that epitaxial diamond is achievable. Manufacturers could monitor the growth of seed crystals and selectively remove those that display imperfect alignment and the graphite and amorphous carbon that forms in the process.
"We are now in a good position to design a [chemical vapor deposition] growth of single crystalline diamond on crystalline silicon," said Lee, noting that the group needs to study more examples of diamond nuclei. "If this is realized, it will make possible many more important applications of diamond, particularly diamond electronics." Other applications include single-crystal diamond broadband optics for corrosive or hostile environments.
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