Verbesserung der Anwendbarkeit von organischen Leuchtdioden durch integrierte Nanostrukturen

The organic light-emitting diode (OLED) is a promising technology for a variety of applications, such as displays, large-area lighting, integrated sensing, smart packaging, and signage. OLEDs are thin-film devices comprising organic semiconductors, which allow for cost-efficient high-volume manufacturing using solution-based fabrications methods and therefore hold great potential towards disposable and recyclable electronic products. In this thesis, three different approaches to improve the applicability of OLEDs through integrated nanostructures are explored. Nanostructuring the carrier substrate's outside surface provides a way to enhance light extraction as well as customize tactile and visual device perception. Here, a polymer coating containing tetrapodal zinc oxide nanoparticles and color pigments is investigated with respect to surface roughness characteristics and optical properties. Electrical device properties can be altered by integrating nanostructures directly into the OLED semiconductor stack. In this work, periodic nanopatterning of a metal electrode is shown to improve charge injection into the organic semiconductor layer of a single-carrier device through local electric field enhancements. A current increase of up to 300 % is observed, exceeding the planar current injection limit and indicating a local transition to space charge limited operation. Integration of a photonic crystal slab into the waveguide formed by the OLED can also lead to resonant light outcoupling. Here, a fabrication method is presented to create two-dimensional nanogratings with variable grating designs in the commonly used electrode material indium tin oxide. Furthermore, a novel device structure is investigated in which a fluorescent nanopatterned waveguide is placed outside the OLED for directional light emission leading to sharp angle-dependent outcoupling peaks in the emission spectra

Nanoimprint Lithography - Next Generation Nanopatterning Methods for Nanophotonics Fabrication

The application of different electrochemical techniques to surfactant systems, namely polarography and cyclic voltammetry, differential capacitance, chronocoulometry and electrochemical impedance spectroscopy, is reviewed

Development of nano-patterned sapphire substrates for deposition of AlGaInN semiconductors by molecular beam epitaxy

Thesis (M.Sc.Eng.)This research addressed the design and fabrication of nano-patterned sapphire substrates (NPSS) as well as the growth by molecular-beam epitaxy (MBE) on such substrates of AlGaN and InGaN multiple quantum wells (MQWs). In recent years a number of LED manufacturers are developing nitride LED devices emitting in the visible part of the electromagnetic spectrum on micron-patterned sapphire substrate (MPSS). These devices are reported to have lower threading dislocation densities, resulting in improvement of the LED internal quantum efficiency (IQE). Furthermore, the LED devices fabricated on MPSS were also found to have improved light extraction efficiency (LEE), due to light scattering by the patterned substrate. My research focuses on the development of nano-patterned sapphire substrate aiming to improve the performance of LEDs grown by MBE and emitting at the deep ultraviolet region of the electromagnetic spectrum. In order to optimize the nano-patterning of the sapphire substrates for maximum light-extraction, the Finite-Difference Time-Domain (FDTD) simulation method was employed. The LEE enhancement was calculated as a function of the diameter, height and perion of the pattern. The calculations were performed only at a single wavelength, corresponding to the maximum of the emitted LED spectrum, which was taken to be 280 nm. These calculations have shown that the best sapphire substrate patterning strategy for this wavelength is the cone shape pattern in hexagonal array structure. Based on limited number of calculations I found that the optimum period, diameter and height of this cone shaped pattern are 400nm 375nm and 375nm respectively. Experimentally, nano patterned substrates were fabricated through natural and nano-imprint lithography. In natural lithography the first step for the definition of the nano-pattern consists of coating the sapphire substrate with photoresist (PMMA) followed by depositing a monolayer of polystyrene nanospheres, 400nm in diameter, using the Langmuir–Blodgett method. These spheres assemble on the substrate and form a closed packed hexagonal array pattern. Following this step the size of the spheres was slightly reduced using reactive-ion etching (RIE) in oxygen plasma. This was followed by the deposition a chromium film, lift-off to remove the polystyrene spheres and RIE to remove the PMMA from the footprints of the spheres. The substrate was then coated with a nickel or chromium films followed by another lift-off which defines the final mask for the formation of cone shaped features by RIE in a CHF3 plasma. An alternative method for pattern definition was the nanoimprint lithography; the stamp for this method (2 mm2 in size) was formed on Silicon substrate using e-beam lithography. NPSS with high quality pillar shape was also fabricated by this method, however, this method can produce only small size patterns. AlGaN films and GaN/InGaN MQWs were deposited on the NPSS by MBE, and characterized by Scanning electron microscopy and photoluminescence and cathodoluminescence measurements. The cathodoluminescence and photoluminescence spectra show that films grown on NPSS has much stronger luminescence than the films grown on flat sapphire substrate, consistent with enhanced light extraction efficiency

Nanoimprint Lithography Technology and Applications

Nanoimprint Lithography (NIL) has been an interesting and growing field in recent years since its beginnings in the mid-1990s. During that time, nanoimprinting has undergone significant changes and developments and nowadays is a technology used in R&D labs and industrial production processes around the world. One of the exciting things about nanoimprinting process is its remarkable versatility and the broad range of applications. This reprint includes ten articles, which represent a small glimpse of the challenges and possibilities of this technology. Six contributions deal with nanoimprint processes aiming at specific applications, while the other four papers focus on more general aspects of nanoimprint processes or present novel materials. Several different types of nanoimprint processes are used: plate-to-plate, roll-to-plate, and roll-to-roll. Plate-to-plate NIL here also includes the use of soft and flexible stamps. The application fields in this reprint are broad and can be identified as plasmonics, superhydrophibicity, biomimetics, optics/datacom, and life sciences, showing the broad applicability of nanoimprinting. The sections on the nanoimprint process discuss filling and wetting aspects during nanoimprinting as well as materials for stamps and imprinting

Digital and Gradient Refractive Index Planar Optics by Nanoimprinting Porous Silicon

Due to the drawbacks of traditional refractive optics, the implementation of planar or nearly planar optical devices has been of research interest for over a century. Subwavelength gratings are a particularly promising option for creating flat optical devices; however, the implementation of subwavelength grating-based optics is limited by fabrication constraints. Recently, we implemented flat optical devices using the nanoimprinting of refractive index (NIRI) process, a process which was pioneered in a previous study but remained largely unproven in terms of device fabrication. The planar, gradient index microlenses we fabricated were found to possess an effective medium similar to a subwavelength grating. We determined that the gradient index planar microlenses successfully focused collimated incident light with focal full-width-half-maximums of less than 14 μm at wavelengths as low as 406 nm. We also fabricated digitally patterned waveguides between 0.35 and 2 μm in width using the NIRI process. We found a propagation loss in the non-oxidized waveguides of 8.1 ± 0.245 dBm/mm, which we were able to reduce by roughly 8 times following a full oxidation of the waveguides

Towards High Throughput Large Area Metalens Fabrication using UV-Nanoimprint lithography and Bosch Deep Reactive Ion Etching

We demonstrate the fabrication of diffraction-limited dielectric metasurface lenses for NIR by use of standard industrial high throughput silicon processing techniques: UV Nano Imprint Lithography (UV-NIL) combined with continuous Reactive Ion Etching (RIE) and pulsed Bosch Deep Reactive Ion Etching (DRIE). As the research field of metasurfaces moves towards applications these techniques are relevant as potential replacements of commonly used cost-intensive fabrication methods utilizing Electron Beam Lithography. We show that washboard-type sidewall surface roughness arising from the Bosch DRIE process can be compensated for in the design of the metasurface, without deteriorating lens quality. Particular attention is given to fabrication challenges that must be overcome towards high throughput production of relevance to commercial applications. Lens efficiencies are measured to be 30% and 17% at wavelengths {\lambda} = 1.55$\mu$m and {\lambda} = 1.31$\mu$m, respectively. A number of routes towards process optimization are proposed in relation to encountered challenges