DFG-Project

Lateral and vertical nanostructuring of silicon electrodes by controlled self-organized processes

Funded by the German Research Foundation (DFG)
Project No, LE 1192/4-1/2

Surface structuring and patterning of silicon substrates can be achieved by a great variety of modern techniques. Photolithographic masking employing extreme ultraviolet or X-ray radiation, the application of electron beams, ion beams, or direct laser writing are some advanced examples in a field likewise important to research and industry. These techniques allow selective and regular manipulation of surface areas and the resolution limit reaches, with increasing costs of operation and implementation, values below 10 nm.

Purpose of the project introduced here and funded by the DFG, is to determine experimental conditions for self-organized electrochemical fabrication of silicon and silicon dioxide structures on the nanoscale such as single pores, pore networks, nanocrystals and nanostructure arrays. Initial surface modifications after electrochemical preparation are analyzed by a combinatorial approach of surface-sensitive measurement techniques. Model experiments by, for instance, AFM-nanoindentation are furthermore employed in order to analyze the interplay of chemical, electrochemical and elastic properties at local sites. Empirical efforts are accompanied by model considerations whose predictions are tested against experimental findings. It is thereby attempted to predict structure formation and propagation and to optimize the desired forms, densities and sizes. For this purpose, also self-organized fractal microstructures are prepared and analyzed as large-scale counterparts of the nanoscopic topographies. These larger patterns can be easier investigated by spatially resolved measurement techniques and provide valuable information about interdependencies that also influence the electrochemical processes at the nano-scale.

Examples of electrochemically prepared surface topographies are shown on the pages in the research section. The morphological and chemical analyses, presented there, were achieved by synchrotron radiation photoelectron spectroscopy (SRPES), atomic force microscopy (AFM), scanning electron microscopy (SEM) and in-situ real time Brewster-angle reflectometry (BAR). Results of model considerations are illustrated by two movies simulating the self-organized propagation of fractal structures on Si(100) surfaces. Other models developed so far refer to nanostructure formation principles in diluted NH4F and the mathematical relation between in situ reflectance data and charge flow during dissolution of the silicon electrode.

Please follow the links on the publications section for further details.

Current research topics

Nano– and microstructured silicon

Funded by the DFG, methods for nano– and micro-manipulation of silicon surfaces are investigated. Emphasis is laid on self-organization principles in (photo-)electrochemical conditioning. Model experiments also comprise induced local manipulation by, e.g., AFM indentation techniques.

Semiconductor surface analysis

Thorough surface analysis is inevitable for the understanding of dissolution processes at the semiconductor interface. We follow a combinatorial approach comprising photoelectron spectroscopy, atomic force, scanning electron and scanning tunneling microscopy. These methods are accompanied by Brewster-angle analysis and reflectometry.

Application to photoelectrochemical solar cells and photoelectrocatalysis

The formation of Schottky-nanocontacts on pre-structured silicon surfaces is investigated. Effects of the structuring process on the efficiency of silicon based photoelectrochemical solar cells (n-type Si) and hydrogen producing devices (p-type Si) are studied.

Self-organized electrochemical reactions

Dynamical electrochemical systems, showing recurrent patterns in time or space, are investigated. Based on identified self-organization principles, the systems are exploited for optimized chemical or topographical surface conditions.

Fractal semiconductor electrodes

Fractal microtopographies on varying silicon surface orientations are a fascinating example of self-organization in electrochemical systems. According to our current understanding, the formation principle can be described by time-dependent elasto-mechanical properties in the near-surface region.

In situ optical spectroscopy

Valuable information about the chemical and topographical interface state can be obtained using Brewster-angle reflectometry. The method benefits, in ex situ or in situ/real-time investigations, from the enhanced surface sensitivity at the Brewster-angle of the specimen.

Surface plasmonics

Nobel metal particles on silicon surfaces are investigated in the range of their plasmonic resonance energies. Size and density variations are correlated with the resulting optical behavior. Data will be presented soon.

Mathematical modeling

Numerical and analytical models are developed to understand and predict the spatiotemporal behavior of self-organized electrochemical systems. Two movies are available illustrating the formation of fractal silicon microstructures.