Nanostructuring of Si substrates by a metal-assisted chemical etching and dewetting process.

In this work, we reported on the development of lithography-free technology for the fabrication of nanopatterned Si substrates. The combination of two phenomena, the solid-state dewetting process and metal-assisted wet chemical etching, allowed for fabrication of Si nanocolumns on large areas in a relatively simple way. The process of dewetting the thin metal layer enabled formation of nickel nanoislands, which were used as a shadow mask in the deposition of a catalytic metal pattern. Application of the two-stage dewetting process with the repetition of the metal deposition and annealing step enabled us to obtain a significant increase in the surface coverage ratio and the surface density of the nanoislands. As a catalytic metal, a gold layer was applied in the metal-assisted wet chemical etching process. The obtained columnar nanostructures showed a great verticality and had a high aspect ratio. In the conducted studies, the maximum etching rate (at RT) was higher than 1.2 µm min-1. The etching rate increased with increasing concentration of oxidizing (H2O2) and etching (HF) agent, with a tendency to saturate for more concentrated solutions. The etching rate was significantly higher for Si substrates with a crystallographic orientation (115) than for (111), but there was no privileged direction of etching except for the direction vertical to the substrate. With increasing layer thickness of the catalytic metal a decrease in the metal-assisted wet chemical etching process efficiency was observed. The developed technology allows for fabrication of patterned substrates with a wide range of lateral dimension of nanocolumns and their density.

SbSI Nanosensors: from Gel to Single Nanowire Devices.

The gas-sensing properties of antimony sulfoiodide (SbSI) nanosensors have been tested for humidity and carbon dioxide in nitrogen. The presented low-power SbSI nanosensors have operated at relatively low temperature and have not required heating system for recovery. Functionality of sonochemically prepared SbSI nanosensors made of xerogel as well as single nanowires has been compared. In the latter case, small amount of SbSI nanowires has been aligned in electric field and bonded ultrasonically to Au microelectrodes. The current and photocurrent responses of SbSI nanosensors have been investigated as function of relative humidity. Mechanism of light-induced desorption of H2O from SbSI nanowires' surface has been discussed. SbSI nanosensors have been tested for concentrations from 51 to 106 ppm of CO2 in N2, exhibiting a low detection limit of 40(31) ppm. The current response sensitivity has shown a tendency to decrease with increasing CO2 concentration. The experimental results have been explained taking into account proton-transfer process and Grotthuss' chain reaction, as well as electronic theory of adsorption and catalysis on semiconductors.

Surface topography analysis with application of roughness area dependence method.

At the moment the root mean square roughness (Rq) is the most commonly used parameter for quantitative description of surface properties. However, this parameter has one main disadvantage: for its calculation only height variations of surface profile are used which are then represented by a single number. To eliminate this restriction authors of the paper have developed a surface analysis method which is based on roughness calculation in the function of gradually increasing dimensions of a sampling area. By setting proper measurement parameters and further data processing, from Rq dependence on sampling area plot size there is a possibility to obtain more useful, additional information about specific surface properties than using the single roughness value. Roughness area dependence plots, obtained from AFM images, were analyzed to study the influence of different growth parameters on surface properties of GaN layers and AlGaN/GaN heterostructures grown on sapphire and silicon substrates by Metal Organic Chemical Vapor Deposition (MOVPE) epitaxy. Although the method was used to characterize the semiconductor material in micrometer range, it can be applied also for any topography imaging technique in wide scale ranges.