Fabrication of Highly Porous Zinc and Zinc Oxide Nanostructures
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Fabrication of Highly Porous Zinc and Zinc Oxide Nanostructures Joshua M. LaForge1 and Michael J. Brett1,2 Electrical and Computer Engineering, University of Alberta, Edmonton, AB, Canada 2 National Institute for Nanotechnology, Edmonton, AB, Canada 1
ABSTRACT Glancing angle deposition (GLAD) is a physical vapor deposition technique that depends on a highly oblique flux angle to create porous, large surface area thin films via self-shadowing. Control of the deposition parameters may provide a means to tune film porosity for zinc oxide sensors and photovoltaic devices. However, the self-shadowing mechanism requires a collimated particle flux, and therefore GLAD performs best under high vacuum. Creating structured films with sputtered GLAD is difficult since the high chamber pressure (>1 mTorr) necessary to maintain the sputter plasma reduces the mean-free-path of flux particles to less than 100 mm. By using an aperture to reduce the angle subtended by the target from the perspective of the substrate, maintaining an argon plasma pressure of 1.4 mTorr, and reducing the throw distance to less than 50 mm we were able to produce structured, polycrystalline, zinc thin films via GLAD. At oblique flux angles, highly porous films consisting of randomly oriented nanorods are grown. The nanorods have diameters between 10-100 nanometers with lengths up to several micrometers. Annealing at temperatures up to 250 °C in air produces polycrystalline zinc oxide with minimal changes to the film structure. We present details of the thin film fabrication process for the convoluted nanorod film morphology. We report characterization results for films produced at several deposition angles before and after annealing using scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). INTRODUCTION Zinc oxide (ZnO) is a suitable semiconductor material for device applications including sensing, catalysis, and photovoltaics due to its unique combination of properties: piezoelectricity, a direct band-gap of 3.37 eV, large exciton and bi-exciton binding energies of 60 meV and 15 meV respectively, optical transparency, and biocompatibility [1-3]. Fabrication of ZnO thin films with a nano/micro-structured morphology is desirable for several applications due to the increase in effective surface area. Several methods are used to successfully grow ZnO nanostructures [2, 3]: vapor phase growth, vapor-liquid-solid growth, metal organic chemical vapor deposition, hydrothermal synthesis, and RF magnetron sputtering [4]. GLAD is a physical vapor deposition technique used to control film morphology (e.g. columns, square spirals, helices, etc) and porosity by careful manipulation of the substrate orientation during deposition [5, 6]. In this study we apply GLAD to the fabrication of highly porous ZnO thin films. Magnetron sputtering of metal-coated Si substrates at normal incidence has been used previously to fabricate ZnO nanorods [4]. Our technique forms porous Zn films with a different morphology o
