Jürgen Probst

5 × 5 cm² silicon photonic crystal slabs on glass and plastic foil exhibiting broadband absorption and high-intensity near-fields.

Crystalline silicon photonic crystal slabs are widely used in various photonics applications. So far, the commercial success of such structures is still limited owing to the lack of cost-effective fabrication processes enabling large nanopatterned areas (≫ 1 cm2). We present a simple method for producing crystalline silicon nanohole arrays of up to 5 × 5 cm2 size with lattice pitches between 600 and 1000 nm on glass and flexible plastic substrates. Exclusively up-scalable, fast fabrication processes are applied such as nanoimprint-lithography and silicon evaporation. The broadband light trapping efficiency of the arrays is among the best values reported for large-area experimental crystalline silicon nanostructures. Further, measured photonic crystal resonance modes are in good accordance with light scattering simulations predicting strong near-field intensity enhancements greater than 500. Hence, the large-area silicon nanohole arrays might become a promising platform for ultrathin solar cells on lightweight substrates, high-sensitive optical biosensors, and nonlinear optics.

Quasicrystalline-structured light harvesting nanophotonic silicon films on nanoimprinted glass for ultra-thin photovoltaics

We present nanophotonic light harvesting crystalline silicon (c-Si) thin films on glass exhibiting ten-fold transversely quasicrystalline lattice geometry on 6 x 8 mm2 area. The c-Si architectures with a nearest neighbor distance of 650 nm are fabricated by nanoimprinting the desired quasicrystalline geometry into sol-gel coated glass substrates followed by Si deposition of 240 nm to 270 nm thickness, self-organized solid phase crystallization and selective chemical etching. Broadband absorption measurements on these quasicrystalline-structured c-Si architectures yield a very significant improvement in light trapping in the near infrared regime and an enhanced light coupling due to a graded index effect in comparison to the unstructured sample. The average value of maximum achievable short circuit current density jsc, max of solar cells with such quasicrystalline-structured c-Si absorber geometry (19.3 mA/cm2) is more than double in comparison to the jsc, max of unstructured planar films of the same thickness (9.3 mA/cm2) and remains stable for light incident angles up to 60°. In comparison to a 320 nm thick c-Si film on textured ZnO:Al substrate as widely used for light trapping in amorphous-microcrystalline Si thin-film photovoltaics, still a 65% increased jsc, max is observable for the presented quasicrystalline c-Si structures. The nanophotonic light trapping efficiency of these transversely quasicrystalline c-Si nanoarchitectures is among the highest values for experimentally realized structures, revealing their promising influence for broadband and isotropic light trapping for economically viable and efficient ultra-thin solar cells.

A realistic fabrication and design concept for quantum gates based on single emitters integrated in plasmonic-dielectric waveguide structures.

Tremendous enhancement of light-matter interaction in plasmonic-dielectric hybrid devices allows for non-linearities at the level of single emitters and few photons, such as single photon transistors. However, constructing integrated components for such devices is technologically extremely challenging. We tackle this task by lithographically fabricating an on-chip plasmonic waveguide-structure connected to far-field in- and out-coupling ports via low-loss dielectric waveguides. We precisely describe our lithographic approach and characterize the fabricated integrated chip. We find excellent agreement with rigorous numerical simulations. Based on these findings we perform a numerical optimization and calculate concrete numbers for a plasmonic single-photon transistor.

Metrology of nanoscale grating structures by UV scatterometry

In this contribution we demonstrate goniometric scatterometry measurements of gratings with linewidths down to 25 nm on silicon wafers with an inspection wavelength of 266 nm. For each sample, measurements have been performed in four different configurations and the obtained data have been evaluated in parallel. As results we present the reconstruction of the complete cross-section profile. We introduce a novel geometry parameterization which overcomes some limitations of the default parameterization. A co-variance analysis of the parameters is offered to indicate the soundness of the results. A qualitative comparison with cross-section scanning electron microscope (SEM) images shows excellent agreement.

Nanometrology on gratings with GISAXS: FEM reconstruction and fourier analysis

The aim of the semiconductor industry to decrease the feature size of integrated circuits poses a huge technological endeavor. Consequently, new challenges are arising for metrology on structures in the nanometer regime. Scatterometry is a fast method which provides non-contact non-destructive characterization of structures on photomasks or exposed wafers. However, the determination of important line structure parameters with subnanometer accuracy still needs further investigation. Grazing incidence small-angle X-ray scattering (GISAXS) is a scatterometry technique to measure both vertical and lateral structural features in the nanometer range with high sensitivity. We apply GISAXS to the investigation of structural parameters such as period length, sidewall angle, linewidth and height on silicon gratings. Our test structures with nominal widths of 35 nm to 100 nm and a pitch from 100 nm to 250 nm were fabricated by electron beam lithography. The diffraction patterns have been analyzed by power spectral density analysis which directly yields periodical modulations of the structured surface such as line width or groove width. We also apply a finite element method (FEM) to the diffraction peak intensity of the grating structure obtained with GISAXS for the geometric reconstruction of the line shape.

Grazing-incidence small-angle X-ray scattering (GISAXS) on small periodic targets using large beams

The shallow incidence angles used in GISAXS lead to a very large footprint of the X-ray beam on the sample. Here it is shown that, despite these large footprints, GISAXS measurements of targets with sizes down to 4 µm × 4 µm are possible.

Reconstructing detailed line profiles of lamellar gratings from GISAXS patterns with a Maxwell solver

Laterally periodic nanostructures were investigated with grazing incidence small angle X-ray scattering (GISAXS) by using the diffraction patterns to reconstruct the surface shape. To model visible light scattering, rigorous calculations of the near and far field by numerically solving Maxwells equations with a finite-element method are well established. The application of this technique to X-rays is still challenging, due to the discrepancy between incident wavelength and finite-element size. This drawback vanishes for GISAXS due to the small angles of incidence, the conical scattering geometry and the periodicity of the surface structures, which allows a rigorous computation of the diffraction efficiencies with sufficient numerical precision. To develop dimensional metrology tools based on GISAXS, lamellar gratings with line widths down to 55 nm were produced by state-of-the-art e-beam lithography and then etched into silicon. The high surface sensitivity of GISAXS in conjunction with a Maxwell solver allows a detailed reconstruction of the grating line shape also for thick, non-homogeneous substrates. The reconstructed geometrical line shape models are statistically validated by applying a Markov chain Monte Carlo (MCMC) sampling technique which reveals that GISAXS is able to reconstruct critical parameters like the widths of the lines with sub-nm uncertainty.

Light harvesting quasicrystalline nanophotonic structures for crystalline silicon thin-film solar cells

We present our results on optical absorption enhancement in crystalline silicon (c-Si) absorber structured with transversely quasicrystalline lattice geometry for thin-film photovoltaics. c-Si nanoarchitectures are prepared on the nanoimprinted ten-fold symmetry quasicrystalline textured substrate. The structural features of the fabricated Si nanostructures are analyzed to confirm the defining characteristics of the quasicrystalline texturing of the absorber film. We present the optical absorption plots for a spectrum of incident light for varying angle of light incidence in these fabricated higher symmetry crystalline Si architectures. Neither any back reflector nor antireflection coating is considered in the present study, where use of such layers could further improve the light absorption. The realized quasicrystalline textured silicon nanoarchitectures with higher rotational symmetry lattice geometry are observed to improve the isotropic and broad band absorption properties of the thin film c-Si absorber and envisaged to have efficiency enhanced thin film photovoltaics effective in terms of cost and performance.

Nanoimprinted deterministic aperiodic nanostructures for tailored light matter interactions

Engineered manipulation of photon flux in structured semiconductor materials with an aperiodic distribution of nanostructures plays a key role in efficiency-enhanced and industrially viable broadband photonic and plasmonic integrated technologies [1, 2]. The dense Fourier spectra of such aperiodic lattice-embedded nanostructured materials could strongly modulate and enable flexible tailoring of the light-matter interaction for broad band nanophotonic applications in comparison to unstructured bulk materials. Through a deterministic bottom-up technological route, we show a versatile generic strategy for very large area nanoengineered c-Si thin films embedded with desired aperiodic nanostructures precisely tailorable for varying integrated applications such as spectrally tunable biosensors, exotic white LEDs, slow light integrated chips, structured thin film photovoltaics, novel surface-enhanced Raman scattering nanoplasmonic building blocks, tailorable photonic bandgap materials and efficient nanophotonic test beds for nonlinear light-matter interactions like Anderson localization[3].

Characteristic diffuse scattering from distinct line roughnesses

Lamellar gratings are widely used diffractive optical elements; gratings etched into Si can be used as structural elements or prototypes of structural elements in integrated electronic circuits. For the control of the lithographic manufacturing process, a rapid in-line characterization of nanostructures is indispensable. Numerous studies on the determination of regular geometry parameters of lamellar gratings from optical and extreme ultraviolet (EUV) scattering highlight the impact of roughness on the optical performance as well as on the reconstruction of these structures. Thus, a set of nine lamellar Si gratings with a well defined line edge roughness or line width roughness were designed. The investigation of these structures using EUV small-angle scattering reveals a strong correlation between the type of line roughness and the angular scattering distribution. These distinct scattering patterns open new paths for the unequivocal characterization of such structures by EUV scatterometry.