Conformality of PVD shell layers on vertical arrays of rods with different aspect ratios investigated by Monte Carlo sim
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Conformality of PVD shell layers on vertical arrays of rods with different aspect ratios investigated by Monte Carlo simulations M. Yurukcu1, H. Cansizoglu2, M. F. Cansizoglu3 and T. Karabacak1 1
Department of Physics and Astronomy, University of Arkansas at Little Rock, Little Rock, AR, 72211, United States 2 Department of Electrical and Computer Engineering, University of California, Davis, CA, 95616, United States 3 Green Center for Systems Biology, University of Texas Southwestern Medical Center, Forest Park, Dallas, TX, 75390, United States Email: [email protected] Abstract Applications such as batteries, fuel cells, solar cells, and sensors, can benefit from high surface-to-volume ratio core/shell arrays of nanorods. The fabrication of the conformal shell layers on nanorod arrays has been a formidable task. In order to assess the deposition conditions for the production of conformal shell coatings by physical vapor deposition (PVD) techniques, we employed Monte Carlo (MC) simulations that involved shell depositions under different flux distributions and angles on arrays of rods. We investigated the conformality of PVD shell layers on nanorod arrays of different aspect ratios, which is defined to be the ratio of rod height to the gaps between nearest-neighbor rods. MC simulated core/shell structures were analyzed for the thickness uniformity of the shell layer across the sidewalls of rods. Our results show that a small angle deposition approach involving a uniform oblique flux (U-SAD) with a small incidence angle ≤ 30o can generate a fairly conformal shell coating around small aspect-ratio rods. However, normal angle deposition with an angular flux distribution (A-NAD) achieves superior conformality both on small and high-aspect-ratio structures compared to U-SAD, conventional uniform normal angle deposition (U-NAD), and SAD with an angular flux distribution (A-SAD). A-NAD can be realized in a PVD system such as by high pressure sputter deposition; while USAD can be achieved in thermal evaporation system with a small angle incident flux. In addition, U-NAD and A-SAD can correspond to film growth by normal incidence thermal evaporation and SAD-high pressure sputter deposition, respectively. 1. Introduction Core/shell nanostructures are promising candidates of being the building blocks for future nanostructured electrodes in fuel cells, and active components in solar cells and photodetectors [1-4]. The nanostructured proton exchange membrane (PEM) fuel cell electrodes with platinum proved to be controlling the porosity [5]. Three-dimensional electrodes in lithium-ion batteries and cells can also make use of the core/shell nanorod arrays [1,2]. However, fabrication of the conformal shell layers on nanorod arrays with minimized defect at the interface has been a great challenge. Chemical vapor deposition (CVD) [6], and atomic layer deposition (ALD) [6] have been the most common techniques in producing highly conformal and uniform films on nanostructures. Nevertheless, having required higher cost and limited availa
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