Infrared behavior of aluminum nanostructure sculptured thin films
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Infrared behavior of aluminum nanostructure sculptured thin films T. Hofmann, M. Schubert, D. Schmidt, and E. Schubert Department of Electrical Engineering and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, NE, 68588 ABSTRACT We report on fabrication, structural and infrared optical characterization of nanostructure aluminum sculptured thin films prepared by glancing angle deposition (GLAD) and controlled substrate motion on p-type silicon. We discuss two structures, one with plate-like and one with screw-like (chiral) morphology. While the plate-like sample possesses a metal Drude behavior in the infrared spectral range, the chiral nanowire sample behaves non-metallic and reveals a series of intriguing resonances, which are equally spaced in frequency by ~7.5 THz. We suggest that formation of 3D nano resonator circuits consisting of inductances and capacitances has occurred within the screw-like conductive aluminum wire sample, which might be responsible for the observed resonances. We suggest conductive GLAD nanostructures in combination with Schottky diodes to facilitate active or passive THz detector and transmitter devices. INTRODUCTION Sculptured thin films (STFs) [1] are formed from 3D building blocks (e.g., columns, spirals, screws), which are created in a self-organized growth process by using glancing angle deposition (GLAD) accompanied with a controlled substrate motion (Fig. 1). STFs with tailored designs can be grown from a broad variety of different materials (semiconductors, insulators, dielectrics, and metals) by using physical vapor deposition methods [2-4] GLAD is a method to create 3D chiral nanostructures from a concurrent growth mechanism due to geometrical shadowing in combination with kinetic limitation for surface adatoms [5]. This technique requires a particle flux reaching the substrate under an extremely oblique angle of incidence (typically > 80ยบ) [6]. These conditions support a columnar growth (Fig. 1), and the instantaneous change of the growth direction due to a simple variation of the incident vapor flux (by substrate rotation) allows for the fabrication of 3D nanostructures with manifold morphologies [7]. Such artificially grown nanostructures establish an exciting field for metamaterial preparation and characterization, and eventually for understanding of how to taylor electronic, vibronic and / or photonic resonances for suitable device architectures in nano-photonics [8], nanoelectromechanics [9], nano-magnetics [10], nano-electromagnetics [11], and nano-sensors [12]. A number of reports exist on GLAD STF preparation using semiconducting or insulating materials, such as silicon, silicon dioxide, TiO2 and others [2-5,13]. Few reports have been published on nanorods, whiskers and nanopillars GLAD grown from metals (Co, Cu, W, Al) [7, 14-16]. STFs from metal with morphologies that render chiral structures have not been attempted so far. Metal nanostructure STFs with chiral geometries should render fascinating optical effects due t
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