Structural and Optical Anisotropy in Nanoporous Anodic Aluminum Oxide
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Porous aluminum oxide has stimulated considerable interest as a nanostructural template, primarily because of the self-organized formation of extremely well-aligned cylindrical pores. One of the fascinating aspects is the tunability of the interpore distance and pore diameter by simple variation of the anodization parameters such as voltage and electrolyte solution composition. Apart from the application of aluminum oxide films as filtration membranes, they are frequently used to fabricate nanowires with large aspect ratios. Many different materials, such as metals, both magnetic and nonmagnetic, semiconductors, nanotubes, and even heterostructures, have been grown in the porous membranes using primarily electrodeposition. Furthermore, porous aluminum oxide membranes have also been used as humidity sensors, or as cathodes for organic light-emitting diodes.
To control and optimize the formation of the aluminum oxide structure as well as the deposition of material within this porous matrix, in situ characterization techniques will certainly be required. Optical reflection techniques provide a nondestructive method to in situ monitor processes such as the anodization and the growth of nanowires. A major advantage of these techniques is their full compatibility with electrochemical or vacuum environments. Several spectroscopic ellipsometry studies of porous aluminum oxide structures, obtained under different anodization conditions, have been published. These investigations were performed on aluminum oxide films, formed by anodizing bulk aluminum samples. Different optical models have been proposed, which take into account the porosity of the film itself, but also include the barrier layer structure between the oxide matrix and the bulk substrate. In these models, the number of parameters is often large due to a proper description of the barrier layer. Determination of the properties of the anodized film in terms of actual physical quantities is not straightforward.
To avoid the aforementioned problems related to the barrier layer, and also to enable an accurate determination of the film thickness, a thin-film sample geometry is employed. The aluminum layer has a well-defined thickness, which can be determined by ex situ techniques, such as electron microscopy. Moreover, full anodization of the aluminum layer allows a smaller number of model parameters, as no bulk aluminum remains and the aforementioned barrier layer is practically absent. In this work, we show that scanning electron microscopy (SEM) enables accurate ex situ characterization of the film thickness and the pore structure after anodization. Ellipsometry spectra are analyzed using an optical model, which takes into account the anisotropy of the porous structure arising from the randomly distributed, aligned pores within the oxide. Based on the original work by Bruggeman and Wien, expressions are given for the parallel and perpendicular components of the dielectric functions, in terms of the cylinder fraction and a nanoporosity of the aluminum oxide matrix. The resulting fit parameters are quantitatively compared with the sample structure determined from electron microscopy images. Furthermore, chemical etching in phosphoric acid solutions enables controlled widening of the pores, and therewith provides a means to test the accuracy of our model. Moreover, we show that ellipsometric experiments on samples etched under different conditions allow identification of the etch process in terms of the order of the etch reaction and the corresponding rate constant.