Ordered arrays of Si nanopillars with alternating diameters fabricated by nanosphere lithography and metal-assisted chemical etching

Abstract

Ordered Si nanopillar arrays have a great potential for e.g. photonic, sensing and electronic devices. In the present article, we investigate the tailoring of Si nanostructure geometries obtained on large areas by nanosphere lithography combined with metal-assisted wet-chemical etching. In particular, the formation of Si nanopencils and, as a new feature of metal-assisted chemical etching, of nanopillars with single and multiple constrictions is demonstrated, which is accomplished by varying the HF/H2O2 ratio of the etching solution without modifying the noble metal film. As a result, it is possible to obtain defined constrictions in high aspect ratio Si nanopillars (length-to-diameter up to 27) with relative diameter reductions of up to around 0.6. Moreover, by controlled oxidation in water vapor nanoscale Si inclusions surrounded by an amorphous SiO2 shell are obtained. The morphology and structure of these pillar arrays are analyzed by scanning and transmission electron microscopy, complemented by optical reflection measurements. For the pencil-shaped Si nanostructures a reflectivity lower than 3% is found in the visible and infrared wavelength ranges. Nanopillars with multiple constrictions exhibit a significantly reduced thermal conductivity due to phonon backscattering. Therefore, the presented method could be implemented in the large-scale fabrication of advanced thermoelectric devices.