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Thérèse Gorisse

Self-assembly silicon nanowires in nanoporous matrix of alumina obtained with nanoimprint process

Published on 28 March 2014
Thesis presented March 28, 2014

With the increased number of devices using functional nanostructures, e.g nanowires for photovoltaic systems, detector etc, it becomes of great importance to develop low-cost and versatile fabrication of systems with nano-objects. In this study, self-assembly properties combined with top-down methods were used to create highly dense and organized nanostructures. Indeed, flawless hexagonal porous anodic alumina arrays (PAA) were successfully used as a template for the epitaxial Silicon (Si) nanowires (NW) growth in a chemical vapor deposition reactor (CVD).PAA is naturally obtained by oxidation of aluminum in acid; however this simple process brings a poor pores organization. We present an innovative route using Thermal NanoImprint Lithography previous to aluminum anodization to prepare perfect hexagonal nanopore array on large surface (4 cm²). All the geometrical characteristics of the porous membrane can be adjusted by varying experimental parameters. Furthermore, to increase the density of the array and reduce the fabrication cost of the imprint mould, original structures with a mixed growth of NIL-guided pores and generation of naturally-guided pores (induced pores) have been developed. Shapes of the pores can be modified varying the electrolyte. To know the characteristic of these arrays and their evolution during formation, we will present the results of the hitherto unseen in situ study under Grazing Incidence Small Angle X-ray Scattering of PAA formation. The PAA is finally used as templates for the self-organized Si NW growth in a CVD reactor. Hexagonal nanowire arrays grown perpendicularly to <100> silicon substrates were successfully produced. The different process steps from the catalyst deposition to the planarization of the array are presented. The quality of the final silicon array is discussed. Densities up to 9*109 and diameter dispersion better than colloidal growth are achieved. The chemical composition and the crystalline orientation of the nanowires confirms the nanowires are in silicon and a mix between <100> and <111> orientation. We also measured the conductivity between the top of the vertical nanowire and the substrate with conductive atomic force microscopy.

Electrochemistry, Nanoporous alumina, Silicon nanowire, Nanoimprint

On-line thesis.