J. Carstensen

Crystal orientation and electrolyte dependence for macropore nucleation and stable growth on p-type Si

Abstract Macropore formation in moderately doped p-type Si was studied in mostly galvanostatic experiments (2–10 mA cm −2 ) with various fluoride containing electrolytes and substrate orientations [(100), (511), (5 5 12), (111)] from the nucleation phase to the phase of stable pore growth. Macropores on p-type Si always grow anisotropically in 〈100〉- and 〈113〉-directions. The most important parameter of the electrolyte is its ability to supply oxygen and hydrogen. Whereas oxygen is necessary for smoothing the pore tips, hydrogen is the decisive factor for the anisotropic growth and the passivation of macropore side walls. Based on a better theoretical understanding of the electrode processes in general pore formation in particular, etching conditions could be optimized for the generation of macropores in p-type Si with better aspect ratios, better stability, and smaller diameters than those in n-type Si.

Uniform and Nonuniform Nucleation of Pores during the Anodization of Si, Ge, and III-V Semiconductors

Morphology is one of the basic characteristics of porous layers. For electrochemically grown pores, morphology is strongly dependent on the starting phase of pore growth, the so-called nucleation phase. This paper addresses uniform and nonuniform nucleation of pores on the surface and consequently the development of pores into the bulk of the following semiconductor substrates: Si, Ge, and III-V compounds (GaAs, InP, and GaP). It was found that nonuniform nucleation can cause formation of domainlike porous structures in all investigated semiconductors. However, depending on the anisotropy of the substrate, these domains show significant differences between them. The particularities of each type of domains are discussed.

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

Crystal Orientation Dependence of Macropore Formation in n‐Type Silicon Using Organic Electrolytes

The crystal orientation dependence of macropore formation in n-type Si using aqueous electrolytes provided important information on pore formation mechanisms. Since macropores can also be obtained in organic electrolytes (especially for p-type Si), the orientation dependence for these electrolytes is of interest and therefore was studied. The observed pore morphologies are often remarkably different from the ones observed in aqueous electrolytes under comparable conditions. The results can be understood in terms of the current burst model, Carstensen et al., Mater. Sci. Eng. B 69/70, 23 (2000).

Strongly frequency dependent focusing efficiency of a concave lens based on two-dimensional photonic crystals

Results of an experimental study of a concave lens based on a two-dimensional microwave photonic crystal with neff<1 are shown. We demonstrate that the lens focuses electromagnetic radiation for transverse electric (TE) and transverse magnetic (TM) polarizations. Intensity gains as high as 5.4 for TE polarization and 6.3 for TM polarization were achieved for definite frequencies lying in the explored interval from 6to15GHz, the smallest area of the focal spot being equal to 0.24λ2 and 1.02λ2 for TE and TM polarizations respectively. The proposed lens serves as a model system that can be scaled to THz and optical frequencies.

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.

A New Way to Silicon Microstructuring with Electrochemical Etching

Porous silicon layers etched through openings in an inert masking layer have been employed to produce microstructures in silicon. In order to overcome the isotropic nature of the porous silicon formation process around the mask edges, optimized organic electrolytes for etching macropores on p-type silicon were employed. New kinds of pores (dubbed “trenches”) were found, which are sensitive to mechanical stress and may prove useful for microstructuring silicon. Systematic experiments are presented which investigate the stability, the crystal orientation dependence, and limitations of trench formation. A possible mechanism for the trench formation is discussed in terms of the “current burst” model.

Smoothening the Pores Walls in Macroporous n-Si

Our work reports on the etching of very smooth pores in n-Si with backside illumination. From the point of view of the so- called current burst model we explain the origin of the roughness on macropore walls. By trying different types of electrolytes at very low or moderate pH, electrolytes with very small dissolution rates of SiO2, or etching at very low temperatures, we show that the roughness of the walls can be controlled. Finally we propose to use a viscous electrolyte which lead to a roughness of only 9 nm on a 800 x 800 nm2 area which is a factor 5 smaller than the roughness observed for aqueous electrolytes.

Ohmic loss analysis for lateral balancing currents by CELLO and photoluminescence measurements

Inhomogeneous local photocurrent generation, as typical for multicrystalline silicon (mc-Si) solar cells, leads to lateral balancing currents, occurring also under open-circuit conditions. In general, all currents passing emitter and grid of a solar cell lead to ohmic losses which increase with the distance the currents have to flow through grid and emitter. Therefore, for the ohmic losses related to lateral balancing currents, the distribution of sites with low photocurrent production plays a crucial role: a 2-D clustering leads to significantly larger losses than a 1-D arrangement (or even an isolated occurrence) of such sites. These ohmic losses can be made visible both in CELLO and luminescence series resistance measurements, which also show that the strength of the losses varies with the coupling of such clusters to the grid.

Making metal surfaces strong, resistant, and multifunctional by nanoscale-sculpturing

Surfaces are the crucial and limiting factor in nearly all metal applications, especially when technologically relevant alloys are employed. Insufficient surface properties on the nano- and microscale of metals determine, e.g. metal-polymer composite stability, implant biocompatibility, or corrosion resistance. Conventional surface preparation is just like an arbitrary cut through the metal body optimized for bulk behavior so that such surfaces contain various element mixtures and complex microstructures in which grains and lattice planes vary in their chemical stability from weak to strong. In contrast, the here described novel nanoscale-surface sculpturing based on semiconductor etching knowledge turns surfaces of everyday metals into their most stable configuration, but leaves the bulk properties unaffected. Thus, nanoscale-sculpturing ensures stronger, reliable joints to nearly all materials, reduces corrosion vastly, and generates a multitude of multifunctional surface properties not limited to those shown below.