Stefan L. Schweizer

Upgraded Silicon Nanowires by Metal‐Assisted Etching of Metallurgical Silicon: A New Route to Nanostructured Solar‐Grade Silicon

Through metal-assisted chemical etching (MaCE), superior purification of dirty Si is observed, from 99.74 to 99.9884% for metallurgical Si and from 99.999772 to 99.999899% for upgraded metallurgical Si. In addition, large area of silicon nanowires (SiNW) are fabricated. The purification effect induces a ∼35% increase in photocurrent for SiNW based photoelectrochemical cell.

Photonic crystal gas sensors

The bandstructure of photonic crystals offers intriguing possibilities for the manipulation of electromagnetic waves. During the last years, research has mainly focussed on the application of these photonic crystal properties in the telecom area. We suggest utilization of photonic crystals for sensor applications such as qualitative and quantitative gas and liquid analysis. Taking advantage of the low group velocity and certain mode distributions for some k-points in the bandstructure of a photonic crystal should enable the realization of very compact sensor devices. We show different device configurations of a photonic crystal based on macroporous silicon that fulfill the demands to serve as a compact gas sensor.

Miniature infrared gas sensors using photonic crystals

The sensitivity of an infrared gas sensor depends on the interaction length between radiation and gas, i.e. a reduction in cell size generally results in a reduced sensitivity, too. However, low group velocity regions in the bandstructure of photonic crystals should enable the realization of very compact gas sensors. Using photonic crystals based on macroporous silicon experimental results with CO2 show an increase of the gas sensitivity in the photonic crystal compared to an empty cell of same dimensions. For practical applications the results are compared with gas measurements using conventional multireflection cells and hollow fiber setups.

Tuning 2D photonic crystals

We demonstrate three ways in which the optical band-gap of 2-D macroporous silicon photonic crystals can be tuned. In the first method the temperature dependence of the refractive index of an infiltrated nematic liquid crystal is used to tune the high frequency edge of the photonic band gap by up to 70 nm for H-polarized radiation as the temperature is increased from 35 to 59°C. In a second technique we have optically pumped the silicon backbone using 150 fs, 800 nm pulses, injecting high density electron hole pairs. Through the induced changes to the dielectric constant via the Drude contribution we have observed shifts upt to 30 nm of the high frequency edge of the E-polarized band-gap. Finally, we show that below-band-gap radiation at 2.0 and 1.7 μm can induce changes to the optical properties of silicon via the Kerr effect and tune the band edges of the 2-D macroporous silicon photonic crystal.

Morphology Dependence on Surface-Enhanced Raman Scattering Using Gold Nanorod Arrays Consisting of Agglomerated Nanoparticles

Highly ordered arrays of vertically aligned Au nanorod arrays consisting of agglomerated nanoparticles are fabricated by porous anodic aluminum oxide (AAO) template-assisted electrochemical deposition. The Au nanorod arrays with rough surfaces are then transformed to smooth surfaces by a subsequent thermal annealing step. The surface-enhanced Raman scattering (SERS) intensity of the Au nanorod arrays with rough and smooth surfaces was compared to investigate the morphology dependence of SERS. The Au nanorod arrays with agglomerated structures demonstrated axa0highly active SERS effect as abundant nanogaps are created uniformly by combination of hot spots caused by both agglomerated porous structures on each nanorod and inter-rod gaps.

Tuning of the Optical Properties in Photonic Crystals Made of Macroporous Silicon

It is well known that robust and reliable photonic crystal structures can be manufactured with very high precision by electrochemical etching of silicon wafers, which results in two- and three-dimensional photonic crystals made of macroporous silicon. However, tuning of the photonic properties is necessary in order to apply these promising structures in integrated optical devices. For this purpose, different effects have been studied, such as the infiltration with addressable dielectric liquids (liquid crystals), the utilization of Kerr-like nonlinearities of the silicon, or free-charge carrier injection by means of linear (one-photon) and nonlinear (two-photon) absorptions. The present article provides a review, critical discussion, and perspectives about state-of-the-art tuning capabilities.

Automated spray coating process for the fabrication of large-area artificial opals on textured substrates

3D photonic crystals, such as opals, have been shown to have a high potential to increase the efficiency of solar cells by enabling advanced light management concepts. However, methods which comply with the demands of the photovoltaic industry for integration of these structures, i. e. the fabrication in a low-cost, fast, and large-scale manner, are missing up to now. In this work, we present the spray coating of a colloidal suspension on textured substrates and subsequent drying. We fabricated opaline films of much larger lateral dimensions and in much shorter times than what is possible using conventional opal fabrication methods.

CMOS-compatible metal-stabilized nanostructured Si as anodes for lithium-ion microbatteries

The properties of fully complementary metal-oxide semiconductor (CMOS)-compatible metal-coated nanostructured silicon anodes for Li-ion microbatteries have been studied. The one-dimensional nanowires on black silicon (nb-Si) were prepared by inductively coupled plasma (ICP) etching and the metal (Au and Cu) coatings by successive magnetron sputtering technique. The Cu-coated nb-Si show the most promising electrochemical performance enhancements for the initial specific capacity as well as their cyclability compared to pristine nb-Si. The electrochemical and microstructural properties before and after cycling of the metal-coated nb-Si compared to their pristine counterparts are discussed in detail.

Infiltration of individual pores in macroporous silicon photonic crystals

A new and promising approach for the design and fabrication of novel optical devices is the functionalization of individual pores in 2D photonic crystals (PhC). This can be done by infiltrating the pores with polymers or dyes. We present a method to locally infiltrate individual pores. This new technique enables the fabrication of a new class of devices, such as optical switches or multiplexers. For the infiltration of individual pores 2D PhC templates made of macroporous silicon were used. Local addressing of the pores is carried out by using focused ion beam technology. For the infiltration itself the wetting assisted templating process is applied. We will present experimentally the infiltration of different polymers and different optical designs.

Purifying metallurgical silicon to solar grade silicon by metal-assisted chemical etching

Metal impurities have detrimental effects on the performance of Si solar cells. Through metal assisted chemical etching, we fabricate Si nanowires from metallurgical Si while purifying it close to solar grade Si.