Gecko Foot Hair Research Feeds Adhesive Development
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occur. I–V measurements show a linear relationship across the grain boundary in the nonevaporated bicrystal, while the cobalt-evaporated sample displays distinctly nonlinear I–V characteristics. Since the grain boundaries were shown to be highly coherent, only the presence of cobalt can account for the potential barrier that developed. The researchers attributed this phenomena to the formation of additional interface states at a lower level in a bandgap generated by the solution of cobalt ions at the grain boundary. TIM PALUCKA
Adding HCl during Chemical Vapor Deposition Produces Controlled Growth of 6H-SiC on On-Axis 6H-SiC(0001) Substrates Researchers from the Department of Chemical Engineering at Kansas State University and the Department of Mechanical Engineering at Wichita State University have found a reliable epitaxialgrowth method for 6H-SiC on 6H-SiC substrates. As reported in the August issue of Electrochemical and Solid-State Letters, they achieved this process by adding HCl as a reaction gas during the growth procedure. Traditionally, chemical vapor deposition (CVD) processes are used to grow various polytypes of SiC. Two methods for the growth of 6H-SiC(0001) polytype are available. The first is on-axis homoepitaxial growth of 6H-Si between 1700°C and 1800°C, and the second is off-axis homoepitaxial, step-controlled growth of 6H-SiC between 1400°C and 1500°C. Both methods are sound, except that slight variations may result in the formation of 3C-SiC. These defects are caused by substrate imperfections, which in turn induce triangular stacking faults. The researchers suggest a method that can grow reliable 6H-Si films without the 3C-SiC defect. The substrates used were on-axis Si-face n-type 6H-SiC(0001), ultrasonically degreased, etched, and rinsed. The gases used for deposition were SiH4, C2H4 (as source), and HCl. The ratio of HCl/Si was kept at 50. The 6H-SiC film was grown on-axis at a reaction temperature of 1475°C. The researchers said that the benefits of HCl addition are twofold: Not only does HCl act as a pregrowth etch that provides stepped surfaces, it also continuously etches away 3C-SiC nucleation sites so that defects cannot develop. The conclusion from this work is that 6H-SiC films may be deposited reliably (i.e., defect-free) on on-axis substrates at a lower temperature—1475°C, as compared with 1700– 1800°C for conventional deposition. JUNE LAU MRS BULLETIN/AUGUST 2000
Gecko Foot Hair Research Feeds Adhesive Development In a study on the adhesive nature of the microscopic hairs between the toes of Tokay geckos (native to Southeast Asia) that enable the reptiles to cling to surfaces, biologists at the University of California— Berkeley and Lewis and Clark College have determined that van der Waals forces may account for the adhesion, though they have not ruled out the possibility of water adsorption or other types of water interaction. Robert J. Full, head of the PolyPEDAL (Performance, Energetics, Dynamics, Animal Locomotion) Laboratory at UC—Berkeley; Kellar Autumn, assistant profes
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