Ala Cojocaru

Temperature and structure size dependence of the thermal conductivity of porous silicon

In order to assess the potential of porous Si as thermoelectric material we have performed thermal conductivity measurements in dependence of both temperature and average structure size. Investigations on samples with average structure sizes between 7 nm and 100 nm reveal a pronounced reduction in thermal conductivity which is encouraging for the future use of porous Si as thermoelectric material. The observed dependence on structure size and temperature can be explained in terms of a simple kinetic model based on the reduction of the effective phonon mean free path.

Optimized Cu-Contacted Si Nanowire Anodes for Li Ion Batteries Made in a Production Near Process

Anodically etching macropores in Si substrates followed by chemical over-etching and Cu galvanics allows producing Si nanowire anodes for Li ion batteries with optimized geometry. This paper focuses on the optimizations of the process chain. The times for key processes could be substantially reduced while concomitantly improving the quality and the process window. The process chain now is close to enabling mass production on 200 mm wafers

New Applications of Electrochemically Produced Porous Semiconductors and Nanowire Arrays

The growing demand for electro mobility together with advancing concepts for renewable energy as primary power sources requires sophisticated methods of energy storage. In this work, we present a Li ion battery based on Si nanowires, which can be produced reliable and cheaply and which shows superior properties, such as a largely increased capacity and cycle stability. Sophisticated methods based on electrochemical pore etching allow to produce optimized regular arrays of nanowires, which can be stabilized by intrinsic cross-links, which serve to avoid unwanted stiction effects and allow easy processing.

Production of High Aspect Ratio Single Holes in Semiconductors

M.-D. Gerngros, H. Foll, A. Cojocaru, and J. Carstensen Institute for Materials Science, Christian-Albrechts-University of Kiel, Kaiserstr. 2, D-24143 Kiel, Germany In many areas of research exists a need for single small holes with diameters from a few nm to several µm and large aspect ratios. Ex-isting technologies for that are complex and rather limited. It is shown by a simple proof of principle that suitable single holes or specific arrays of some single holes can be made by first etching a very large number of small and deep holes or pores into semicon-ductors like Si or InP by established electrochemical means, fol-lowed by masking the desired holes and filling all others with, e.g., a metal in a galvanic process. The potential and limitations of this technique are discussed in some detail.

Effect of Al Sn — Doping on properties of zinc oxide nanostructured films grown by magnetron sputtering

Metal doping in nanostructured zinc oxide is important for device applications. To obtain improved performances for practical applications, Aluminum (Al) and Tin (Sn)-doping in zinc oxide nanostructured layers were investigated. Samples were grown by magnetron sputtering and studied by X-ray diffraction (XRD), micro-Raman, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) techniques. It was observed that nanoparticles are interconnected and form porous network of individual nanoparticles. It is found clear evidence of changes of different properties after doping with aluminum or tin in zinc oxide nanostructured films grown by magnetron sputtering.

Dynamics of Macropore Growth in n-type Silicon Investigated by FFT In-Situ Analysis

The dynamics of macropore growth in n-type silicon were investigated by in-situ FFT impedance spectroscopy and transient analysis. In particular the response to fast growing pores to current density steps in the context of so-called anti-phase diameter oscillations was investigated. These pore growth mode allows for a very fast growth of deep macropores and could for the first time be stimulated by external current steps.

Photocatalytic Applications of Doped Zinc Oxide Porous Films Grown by Magnetron Sputtering

Undoped, Al-doped and Sn-doped ZnO was sputter-deposited on glass substrates and on trenched Si substrates and some properties of these samples were evaluated. It was shown that the photocatalytic performance of the thin layers is improved by doping with Sn and Al. The samples doped with Al-doping provided a more pronounced effect compared to the control sample, as well as compared to the greater volume of undoped ZnO deposited on Si substrates. Morphology of produced particles was studied using transmission electron microscopy. Electron diffraction patterns were taken to confirm lattice parameters of the materials. Size and shape variations were monitored for un-doped, Al-doped and Sn-doped ZnO. EDX spectroscopy was utilized to assay chemical composition for doped and un-doped samples.

Tetrapodal ZnO Particles for Substrate Mode Scattering in Flexible Organic Light-Emitting Diodes

A thin polymer foil containing tetrapodal zinc oxide microparticles for volume scattering is produced. This foil is used as bendable substrate for an organic light-emitting diode.

Production and Investigation of Porous Si-Ge Structures for Thermoelectric Application

Mesoporous Si and Si-Ge are promising candidates for efficient thermoelectric converters needed for energy harvesting, e.g. from hot exhaust pipes of cars. Suitable mesoporous structures must meet a number of requirements concerning their geometry and processing costs, and suitable pore etching techniques have been developed. The paper addresses these issues, including the use of Si-Ge substrates from specially grown Si-Ge crystals. It is shown that most geometric and process requirements can be met by etching so-called current line pores under special conditions. First results concerning thermal and electrical properties are also presented.

Impedance Spectroscopy as a Powerful Tool for Better Understanding and Controlling the Pore Growth Mechanism in Semiconductors

This work shows new results toward a better understanding of macropore growth in semiconductors by using in situ FFT impedance spectroscopy. A new interpretation of the voltage impedance is proposed. In particular, the pore quality could be quantified for the first time in situ, especially by extracting the valence of the electrochemical process. The study paves the way toward an automatized etching system where the pore etching parameters are adjusted in situ during the pore etching process.