Fabrication of hollow coaxial Al2O3/ZnAl2O4 high aspect ratio freestanding nanotubes based on the Kirkendall effect

In this communication, fabrication of high aspect ratio Al2O3/ZnO/Al2O3 nanotubes is reported and morphological changes at elevated temperatures are investigated. The structures were made by implementing several fabrication methods, such as deep-UV lithography, atomic layer deposition (ALD), and plasma etch methods. During the fabrication, the ALD deposited Al2O3 and ZnO conformally passivated the prepared Si-holes template, resulting in the complex coaxial Al2O3/ZnO/Al2O3 pillars. By utilizing several scanning and transmission electron microscopy techniques, it is experimentally shown that at elevated temperatures, internal voids form in the nanotube due to diffusion of ZnO into surrounding Al2O3 and also ZnAl2O4 spinel structure forms. Finally, the porous tubes have been isolated from the surrounding silicon core using a conventional isotropic selective Si plasma etch process. The presented approach opens the opportunity to build complex optical metamaterial compositions, for example, for a new generation of sensors for gas and biomarker detection.