NASA researchers build on flat tech for better chips

Scientists at NASA’s Glenn Research Center, Cleveland, OH, and their colleagues believe they have found ways to make more reliable wide-band-gap semiconductor devices with two recent breakthroughs.

First, their web growth process, extends the size of atomically flat (or step-free) areas on semiconductor wafers. The second, step-free surface heteroepitaxy, eliminates defects in films deposited on those step-free surfaces.

Last year members of the research team announced their method for making step-free silicon carbide by first creating a pattern of square, device-size mesas on commercial wafers, then growing the steps on those mesas to the edges.

The team’s web growth process takes advantage of an unexpected

feature of the earlier work: The new silicon carbide crystals grew laterally beyond the mesa edges and formed thin cantilevers hanging over the mesa sidewalls.

“Seeing the cantilevers in the micrographs was our ‘aha,'” said Philip Neudeck, the Glenn research engineer who leads the research team.

The team created open geometry mesas, such as vee’s and crosses, and applied the flattening process to them. As expected, the cantilevers formed, and as growth continued, the cantilevers extending from adjacent legs grew together, forming a web covering the entire area between the legs and producing an atomically flat surface that is larger than the original mesa.

This web growth process allows atomically flat material to be grown over areas in wafer material that contain inherent defects like screw dislocations.

The team’s second breakthrough is an improvement in heteroepitaxy — the process of growing layers of one material on a substrate of a different material. In this case, they grew a thin film of the cubic crystal form of silicon carbide (3C-SiC) on mesas of the hexagonal crystal form of silicon carbide (4H- or 6H-SiC). The researchers first flattened the hexagonal silicon carbide mesas, then, by a careful manipulation of temperature and crystal nucleation rate, deposited a film of cubic silicon carbide on the flattened mesas.

The films formed not only were free of defects that might have propagated from defects in the substrate, but also were free of planar defects, that is, defects in the order (stacking faults) or the alignment (double positioning boundaries) of crystal planes.

“Our work shows that, for cubic silicon carbide films, too rapid crystal nucleation in the early stages is the likely cause of planar defects,” said Neudeck. The team will continue its work using step-free surface heteroepitaxy with other wide-band-gap material films on hexagonal silicon carbide. “If we can produce defect-free films with these other materials, then industrial fabrication of a wider range of much improved wide-band-gap devices is possible,” Neudeck said.

Wide-band-gap semiconductors are used widely in opto-electronic devices such as the blue and green light-emitting diodes in stadium and building displays. Improved devices may find wide use in more efficient and compact power control equipment. Short-wave-length wide band gap lasers could greatly improve the capacity of consumer products such as DVDs.

Scientists from the Ohio Aerospace Institute, Cleveland, and the State University of New York, Stony Brook, participated in the research.


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