CMPL SEMICON
National Institute of Physics
University of the Philippines - Diliman
Gallium Arsenide nanowires can be grown using Anodized Aluminum Oxide (AAO) as template. The use of AAO template is advantageous because it involves low-cost, simple and efficient fabrication method for growing nanowires with controlled size and shape. Nanomaterials can be made from different materials such as metals, ceramics, polymers, organic materials and composites. They can be in a wide variety of morphologies like spheres, rods, tubes and plates. Nanorods, nanowire and other one-dimensional nanomaterials can be fabricated chemically but the polydiversity of the final product is a problem. A solution to this limitation is by using templates with controllable dimensions that can be optimized to produce these 1D nanomaterials.
Nanostructures
AAO is one of the widely used templates in the growth of patterned nanoparticles due to its relative ease of fabrication, mechanical strength, thermal stability, structural uniformity and the tunability of its pore morphology. It has a self-ordered porous structure with geometry described as a honeycomb structure of columnar pores
Fabrication of AAO can be done by anodization of aluminum metal in an acidic medium. Shown in the image below are the steps in fabricating AAO. Oxalic acid was used as the electrolyte solution. The pore morphology of an AAO can be tuned to the desired dimensions by controlling its anodization parameters. These include the anodization potential, electrolyte concentration and anodization time. Moreover, pore widening can be done by etching the AAO with phosphoric acid.
Anodization of AAO is done in high voltage and low current conditions. The dissolution of the oxide layer is faster at higher applied anodization potential due to the high degree of hydration and ion incorporation. The figure on the right shows the variation of the AAO pore dimensions with the anodization voltage with (a) Au deposited on Si substrate with AAO and (b) Au nanoparticles after removal (etching) of AAO. As expected, it showed a strong direct relationship of the pore dimensions with the anodization voltage.
Other Applications of AAO
Other nanoparticles were also fabricated using the AAO template. Au nanoparticles were electrodeposited on the the AAO film for a patterned Au nanodot array. Au nanoparticle can be used as catalyst for the selective growth of ordered nanoparticles.
The significant decrease in the reflectivity across all wavelengths for the silicon substrate with Au nanoparticels was due to the scattering effect of gold nanoparticles. The addition gold nanoparticles varied the index of refraction of the film which affected the reflectivity of the sample. The size of the deposited Au nanoparticle ranged from 2.67-3.70nm.
Aside from its application as template for the growth of GaAs nanowires, it was also used to grow Cu nanowires as shown in the figure below where the etch time was varied from 20,30 and 40 minutes. The length of the grown nanowires ranged from 1.0 to 1.5 and the diameter ranged from 38.0 to 50.0 nm.