Markus Wellenzohn

Nonlinearity of optimized silicon photonic slot waveguides

In this numerical study, we show that by exploiting the advantages of the horizontal silicon slot wave-guide structure the nonlinear interaction can be significantly increased compared to vertical slot waveguides. The deposition of a 20 nm thin optically nonlinear layer with low refractive index sandwiched between two silicon wires of 220 nm width and 205 nm height could enable a nonlinearity coefficient gamma of more than 2 x 10(7) W(-1)km(-1).

Light trapping by backside diffraction gratings in silicon solar cells revisited

This numerical study investigates the influence of rectangular backside diffraction gratings on the efficiency of silicon solar cells. Backside gratings are used to diffract incident light to large propagation angles beyond the angle of total internal reflection, which can significantly increase the interaction length of long wavelength photons inside the silicon layer and thus enhance the efficiency. We investigate the influence of the silicon thickness on the optimum grating period and modulation depth by a simulation method which combines a 2D ray tracing algorithm with rigorous coupled wave analysis (RCWA) for calculating the grating diffraction efficiencies. The optimization was performed for gratings with period lengths ranging from 0.25 µm to 1.5 µm and modulation depths ranging from 25 nm to 400 nm under the assumption of normal light incidence. This study shows that the achievable efficiency improvement of silicon solar cells by means of backside diffraction gratings strongly depends on the proper choice of the grating parameters for a given silicon thickness. The relationship between the optimized grating parameters resulting in maximum photocurrent densities and the silicon thickness is determined. Moreover, the thicknesses of silicon solar cells with and without optimized backside diffraction gratings providing the same photocurrent densities are compared.

Ion multibeam nanopatterning for photonic applications: Experiments and simulations, including study of precursor gas induced etching and deposition

The capabilities of charged particle nanopatterning (CHARPAN) for photonic device fabrication are investigated. The CHARPAN tool is a proof-of-concept tool for a multi-ion beam system that the authors used to directly pattern photonic structures into both Si and Ni as well as for maskless exposure of hydrogen silsesquioxane resist. The realized structures have a regular array and show adequate roundness of the holes as well as little sidewall roughness. For the development and a better understanding of the processes they extended and used the IonShaper® simulation software. They could achieve excellent agreement between sputtering simulation and experiments. Furthermore, they developed a nonlocal recoil-based algorithm for the simulation of ion beam induced etching and deposition. Simulation results for three dimensional nanopatterning with this algorithm are presented.

Insights in the light trapping effect in silicon solar cells with backside diffraction gratings

Abstract. Light trapping by means of backside diffraction gratings can strongly increase the efficiency in silicon solar cells. However, the optimization of the grating geometry involves comprehensive multiparameter scans, which necessitates an efficient simulation method. In this study, we employ a simulation approach that combines ray tracing with rigorous coupled wave analysis. As an additional benefit, this approach provides a much better physical insight into the light trapping mechanism in contrast to fully electromagnetic simulation methods. The influence of the ray tracing simulation settings in terms of recursion depth and diffraction order on the simulation results is investigated. We show that the choice of a proper recursion depth and a sufficient number of diffraction orders is essential for obtaining fully optimized grating parameters and that the minimum recursion depth required for obtaining the correct optimized grating parameters depends on the silicon thickness. Furthermore, we investigate the influence of the angle of incidence on the optimized grating parameters. As major result, we find that the optimum grating structure does not depend on the angle of incidence on the solar cell.

A 2D numerical study on the photo current density enhancement in silicon solar cells with optimized backside gratings

In this work, we present a detailed numerical study on the optimization of the photo current density in silicon solar cells with rectangular aluminum backside gratings covered with a 100 nm thick SiO2 layer. The simulation method combines a 2D ray tracing algorithm with rigorous coupled wave analysis for calculating the grating diffraction efficiencies. The optimization was performed for gratings with period lengths ranging from 0.25 μm to 1.5 μm and modulation depths ranging from 25 nm to 400 nm under the assumption of normal light incidence. The flat front side is covered with an 80 nm thin silicon nitride anti-reflection coating. The simulations show that the optimum grating parameters resulting in a maximum photo current density strongly depend on the silicon thickness.

Optimization of Silicon Solar Cells using Backside Diffraction Gratings

This numerical study investigates the influence of backside diffraction gratings on the efficiency of silicon solar cells. In particular, the dependence of the optimum grating period and modulation depth on the silicon thickness is determined.

Argon ion multibeam nanopatterning of Ni–Cu inserts for injection molding

The authors have successfully employed the charged particle nanopatterning (CHARPAN) technology for nanostructuring of a metal mold insert for a conventional injection molding machine. High-precision diamond-milled Ni–Cu mold inserts have been nanopatterned with 10 keV argon ion multibeam milling with feature sizes as small as 50 nm. A variety of structures such as circles, hexagons, and lines in different dimensions, with positive and negative shapes, have been fabricated in the metal mold. These structures have been successfully replicated in polymethylpentene samples by injection molding. To the authors’ best knowledge, the CHARPAN technology is one of the very few technologies that allow for resistless nanostructuring a field size of 25×25 μm2 into a metal mold in a single shot. This is of high importance for the practical injection molding fabrication of nanostructured polymer devices such as optical biosensors.The authors have successfully employed the charged particle nanopatterning (CHARPAN) technology for nanostructuring of a metal mold insert for a conventional injection molding machine. High-precision diamond-milled Ni–Cu mold inserts have been nanopatterned with 10 keV argon ion multibeam milling with feature sizes as small as 50 nm. A variety of structures such as circles, hexagons, and lines in different dimensions, with positive and negative shapes, have been fabricated in the metal mold. These structures have been successfully replicated in polymethylpentene samples by injection molding. To the authors’ best knowledge, the CHARPAN technology is one of the very few technologies that allow for resistless nanostructuring a field size of 25×25 μm2 into a metal mold in a single shot. This is of high importance for the practical injection molding fabrication of nanostructured polymer devices such as optical biosensors.

Integrated optical waveguide and nanoparticle based label-free molecular biosensing concepts

We present our developments on integrated optical waveguide based as well as on magnetic nanoparticle based label-free biosensor concepts. With respect to integrated optical waveguide devices, evanescent wave sensing by means of Mach- Zehnder interferometers are used as biosensing components. We describe three different approaches: a) silicon photonic wire waveguides enabling on-chip wavelength division multiplexing, b) utilization of slow light in silicon photonic crystal defect waveguides operated in the 1.3 μm wavelength regime, and c) silicon nitride photonics wire waveguide devices compatible with on-chip photodiode integration operated in the 0.85 μm wavelength regime. The nanoparticle based approach relies on a plasmon-optical detection of the hydrodynamic properties of magnetic-core/gold-shell nanorods immersed in the sample solution. The hybrid nanorods are rotated within an externally applied magnetic field and their rotation optically monitored. When target molecules bind to the surfaces of the nanorods their hydrodynamic volumes increase, which directly translates into a change of the optical signal. This approach possesses the potential to enable real-time measurements with only minimal sample preparation requirements, thus presenting a promising point-of- care diagnostic system.

On the light trapping mechanism in silicon solar cells with backside diffraction gratings

In this numerical study, we investigate the light trapping mechanism in silicon solar cells with backside diffraction gratings. In order to obtain a clearer view on the physical mechanisms underlying the light trapping we employ a simulation scheme that combines ray tracing with rigorous coupled wave analysis (RCWA). This combined simulation approach treats the light propagation inside the silicon absorber layer incoherently and averages out Fabry-Perot resonances, which otherwise would obscure characteristic humps in the absorption spectra that are directly related to light trapping effect of the diffraction gratings. We provide an in-depth explanation for the origin of these characteristic humps and their interrelationship with the silicon absorber thickness. A major benefit of this combined RCWA/ray tracing approach compared to the fully electromagnetic simulation methods RCWA and finite difference time domain (FDTD) is the more efficient use of computational power accompanied by a gain in simulation precision, in particular for cells with an absorber thicker than 10 μm.

Ion multi-beam direct sputtering of Si imprint stamps and simulation of resulting structures

The multibeam charged particle nanopatterning tool has been used to fabricate template structures with 10 keV Ar+ ion beam sputtering in Si. Trench array structures of 130 nm width, 280 nm depth and 80° sidewall angle have been cross sectioned and examined by the scanning electron microscopy. The experiments have been compared with the sputter simulation code IonShaper®. A double Lorentz tool point spread function with 21.5 nm full width at half-maximum has been used for the simulations. For different layout geometries but identical simulation parameters, the simulation results are in good agreement with the experiments.