Anodic porous alumina films are unique due to their characteristic honey-comb like structure of vertical cylindrical pores homogeneously distributed in hexagonal close-packed arrays. Pore diameter and density are controlled accurately by the anodization procedure, while a very homogeneous pore distribution can be succeeded on very large areas. For these reasons, porous alumina films grown by anodization on various substrates are commonly used as a template material for the growth of nanowires of metals and semiconductors by electrodeposition inside the pores. Electrodeposition of Aluminium though suggests a very difficult task and for this reason template techniques have not been used for the fabrication of Al nanowires.
In this study, porous anodized alumina templates were used to grow Al nanowires inside the alumina pores by electron gun evaporation. Filling of the pores by evaporation is not possible if they have a high diameter to depth aspect-ratio. To overcome this problem, we fabricated ultra-thin porous alumina templates on Si substrates with low aspect ratios (< 1:4). For the sample fabrication, very thin evaporated Al films, with thicknesses of 30 and 50 nm, were deposited onto Si substrates and anodized in sulphuric acid aqueous solution under constant voltage, followed by pore widening in a phosphoric acid aqueous solution. The obtained porous alumina films had thicknesses and pore diameters of approximately 45 and 70 nm and 25 to 35 nm, respectively, depending on the duration of the pore widening step, resulting in aspect ratios of 1:3, 1:2 and 1:1.5. Al was then deposited onto the porous alumina templates by electron gun evaporation and the samples were analysed using cross-sectional bright and dark field transmission electron microscopy (TEM). Images revealed that the Al completely filled the pores down to the silicon substrate. Initial experiments showed that Al nanowires can be obtained by dissolving the porous alumina template in a mixture of phosphoric and chromic acid, resulting in free-standing Al nanowires.
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