Growth of Single Crystal Tungsten Nanorods by Oblique Angle Sputter Deposition
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Growth of Single Crystal Tungsten Nanorods by Oblique Angle Sputter Deposition Tansel Karabacak, Pei-I Wang, Gwo-Ching Wang, and Toh-Ming Lu Department of Physics, Applied Physics, and Astronomy Rensselaer Polytechnic Institute Troy, NY 12180-3590 ABSTRACT We report the creation of single crystal tungsten nanorods with unusual simple cubic βphase. These novel nano-structures were grown by oblique angle sputter deposition with substrate rotation through a shadowing effect. Transmission electron microscopy (TEM) diffraction patterns from individual nanorods clearly show the single crystal structure. It is evident from TEM diffraction measurements, during the oblique angle deposition, both β-phase W(100) and α-phase W(110) islands exist at the initial stages of growth. However, at later stages of the growth the β-phase structure dominates. This is in contrast to the sputter deposition at normal incidence where only the thermodynamically stable bcc α-phase W(110) polycrystalline films were formed when the film grew to a certain thickness. We explain our results by using the shadowing and adatom mobility mechanisms: At the initial stages of growth, the β-phase W(100) islands grow taller due to the lower adatom mobility on these islands. The taller β-phase W(100) islands survive in the competition during oblique angle growth and form isolated nanorods in the later stages, while the shorter α-phase W(110) islands stop growing due to the shadowing effect. In addition, our Monte Carlo simulation results agree well with the experimental measurements. INTRODUCTION Tungsten films have been the subject of intense research due to their importance in various technological applications, for example, field emitters [1], photonic crystals [2], diffusion barriers in semiconductor interconnect structures [3], absorbing layers in X-ray masks [4], and X-ray mirrors [5]. Depending on the growth conditions and thickness of the films, normal incidence sputter deposition (NISD) of tungsten films can give rise to either the α-phase W, which has the equilibrium bcc structure, or the metastable β-phase W, which has an A15 (cubic) o
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structure, or a mixture of both phases [6,7]. The lattice constants are 3.16 A and 5.04 A for αW and β-W, respectively. These two phases may have very different properties, for example, the measured resistivity of β-W film is an order of magnitude higher than that of the α-W film [3]. It was suggested that oxygen incorporation might play a role in the formation of the metastable βW. In a previous study [8] we reported a sequence of X-ray θ-2θ measurements for the NISD and oblique angle sputter deposited (OASD) tungsten films at different deposition times. It was realized that α- and β-phases are present at the initial stages of growth for both normal incidence and oblique angle deposition. As the growth progressed to a larger thickness the α-phase for the NISD grew faster and the β-phase was suppressed. This behavior is consistent with that reported in the literature [7,9]. On the other hand, for the OAS
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