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regions, W was deposited on the exposed gate, source, and drain regions. After the formation of an initial thin W film, a SiH4 reduction reaction took place with a deposition rate of ~240 nm/min. The WF6 / SiH4 gas flow rate was kept at 40/12 and the process temperature was 300°C. Conventional devices without W film deposition were also fabricated to serve as controls. A 500 nm Al film was deposited, patterned, and sintered at 400°C for 30 min to form metal pads. To reduce trap density and improve interface quality, wafers were also immersed in a NH3 plasma. The poly-Si gate was 10 µm in length and 3 µm in width. The oxide gate thickness was ~30 nm. The researchers observed that the WTFTs have a larger driving current then conventional TFTs, especially under high gate bias. The parasitic resistance of the W-TFTs is ~4 kΩ, which is three times smaller than that of conventional TFTs. In addition, the researchers found that a NH3 plasma treatment before the deposition of W contacts achieves a better inter-

face quality and junction integrity of WTFTs than for NH 3 plasma treatment after deposition of the W contacts. The researchers said this happens because the W blocks the plasma passivation from reaching the active regions under the contacts in the latter case. MAXIM NIKIFOROV

In Situ TEM Observations Reveal Growth Mechanisms for Carbon Nanofibers The synthesis of carbon nanotubes and nanofibers with specific structures and functionality is very important for nanotechnology applications. Carbon nanofibers are grown from the vapor phase in the presence of a catalyst. The atomicscale growth mechanisms for forming nanofibers have not been understood thus far. This understanding is crucial for manufacturing carbon nanofibers with tailored properties and characteristics. Now, S. Helveg at the Danish company Haldor Topsøe, F. Abild-Pedersen at the Technical University of Denmark, and

their colleagues have used time-resolved, high-resolution in situ transmission electron microscopy (TEM) to elucidate the growth mechanisms of carbon nanofibers. As the researchers reported in the January 29 issue of Nature, carbon nanofibers are formed by the decomposition of methane vapor at 500°C over a catalyst consisting of Ni nanoclusters with diameters of 5–20 nm supported on MgAl2O4. The growth experiments were performed in an in situ TEM, and a large number of consecutive real-time TEM images were obtained and stitched into a TEM movie. The nanofibers formed through a reactioninduced reshaping of the Ni nanocrystals into highly elongated shapes. The reshaping of the nanoclusters helped align the graphene sheets into tubular structures forming the nanofibers. The researchers said that this reshaping is abrupt, oscillating between spherical and elongated, but is critical for the formation of the fibers. The Ni nanoparticles were found to remain crystalline throughout the growth process. The nucleation and growth of the

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