Helmut Föll

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.

Improved effective medium approach: Application to metal nanocomposites

An improved effective medium approximation (EMA) is presented that accounts for higher order interactions between metal nanoparticles in metal-dielectric composite materials and compared to experimental results. The theoretical results of this formalism are applied to a composite material consisting of spherical gold nanoparticles randomly distributed in a dielectric matrix, which has been extensively characterized with respect to its structural and optical properties. The experimental results and theoretical predictions are compared and the results are discussed. It is shown that the modified theory expands the range to which EMA can be applied to a metal filling fraction of ∼20% at very little additional computational expenses. The improved theory also allows extracting more information from the optical characterization of the composite material such as the distribution of the interparticle distances in a composite.

Self-organized pore formation and open-loop control in semiconductor etching

Electrochemical etching of semiconductors, apart from many technical applications, provides an interesting experimental setup for self-organized structure formation capable, e.g., of regular, diameter-modulated, and branching pores. The underlying dynamical processes governing current transfer and structure formation are described by the current burst model: all dissolution processes are assumed to occur inhomogeneously in time and space as a current burst (CB); the properties and interactions between CBs are described by a number of material- and chemistry-dependent ingredients, like passivation and aging of surfaces in different crystallographic orientations, giving a qualitative understanding of resulting pore morphologies. These morphologies cannot be influenced only by the current, by chemical, material and other etching conditions, but also by an open-loop control, triggering the time scale given by the oxide dissolution time. With this method, under conditions where only branching pores occur, the ...

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.

Structural and Electrochemical Investigation during the First Charging Cycles of Silicon Microwire Array Anodes for High Capacity Lithium Ion Batteries

Silicon microwire arrays embedded in Cu present exceptional performance as anode material in Li ion batteries. The processes occurring during the first charging cycles of batteries with this anode are essential for good performance. This paper sheds light on the electrochemical and structural properties of the anodes during the first charging cycles. Scanning Electron Microscopy, X-ray diffractommetry, and fast Fourier transformation impedance spectroscopy are used for the characterization. It was found that crystalline phases with high Li content are obtained after the first lithiation cycle, while for the second lithiation just crystalline phases with less Li are observable, indicating that the lithiated wires become amorphous upon cycling. The formation of a solid electrolyte interface of around 250 nm during the first lithiation cycle is evidenced, and is considered a necessary component for the good cycling performance of the wires. Analog to voltammetric techniques, impedance spectroscopy is confirmed as a powerful tool to identify the formation of the different Si-Li phases.

Pores in n-Type InP: A Model System for Electrochemical Pore Etching

The growth mechanism of currentline-oriented pores in n-type InP has been studied by Fast-Fourier-Transform Impedance Spectroscopy (FFT IS) applied in situ during pore etching and by theoretical calculations. Several pore growth parameters could thus be extracted in situ that are otherwise not obtainable. These include the space-charge-region (SCR) width, the SCR potential, the capacitance at the pore tips, and the avalanche breakdown field strength. It could be demonstrated that the system adjusts itself in such a way that the potential across the space-charge-region at the pore tips is kept constant within a certain bandwidth of the applied potential. This provides for a constant field strength at the pore tips, ensuring that avalanche breakdown occurs, generating the necessary holes for the electrochemical dissolution of InP.

Adjustable optical anisotropy in porous GaAs

We report a theoretical investigation of the optical properties of porous (100) GaAs having crystallographic pores. An effective medium approach is used for the calculations. A biaxial anisotropy of the material is predicted for most of the material parameters. The parameter windows for different kinds of uniaxial anisotropy are predicted as well. It is shown that the type and value of the optical anisotropy, and even the direction of the optical axes, can be controlled by GaAs etching parameters, and this letter gives the theoretical blueprint for the overall pore morphology required for that.

Waveguide Structures Based on Porous Indium Phosphide

dTechnical University Hamburg-Harburg, 21073 Hamburg, Germany We researched the possibilities for engineering the morphology of porous structures in n-InP. Lithographic patterning of the sample surface before anodic etching was shown to modify considerably the electric field distribution which, in its turn, defined the direction of pore growth inside the specimen. We show that local formation of the nucleation layer combined with the possibility to introduce current-line oriented pores in a controlled manner represents a promising tool for manufacturing waveguide structures based on porous InP. Some results on simulation of the properties of these structures are presented.

Growth Mode Transition of Crysto and Curro Pores in III-V Semiconductors

The formation of crystallographically oriented and current line oriented pores in n-type InP is reviewed and compared to other semiconductors in the light of some new results. A model for the formation of crystallographically oriented pores is presented that reproduced salient features of this pore type rather well. Impedance data together with their model-based evaluation are given and discussed. Some self-organization features of current line oriented pores and their possible relation to self-induced or externally triggered growth mode transitions between the two pore types conclude the paper.

Simulating Crystallographic Pore Growth on III-V Semiconductors

In this work, the growth of crystallographically oriented pores in III-V semiconductors has been investigated. Based on new and previous results a model for pore growth has been developed, which is mainly based on a stochastic branching probability of the pores. The stochastic nature of the model allowed to implement it as the core for Monte-Carlo-Simulations of pore growth. The simulations were able to reproduce the main features of crystallographically oriented pores, like uniform pore growth in ntype InP or the formation of domains on n-type GaAs and InP. The model is also capable of reproducing the logarithmic growth law for the pore depth, as well as the pore density oscillations with depth, as recently found.