Laserstimulierte Selbstorganisation in einfachen und komplexen Dünnschichtsystemen

Das Leitmotiv der vorliegenden Dissertation besteht in der Untersuchung laserstimulierter Selbstorganisation verschiedener Dünnschichtsysteme. Im Vergleich zu anderen Fertigungsprozessen wird die Strukturierung bei der Selbstorganisation nicht durch die direkte Kontrolle des verwendeten Werkzeuges b...

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Bibliographic Details
Main Author: Durbach, Sebastien
Contributors: Hampp, Norbert (Prof. Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2023
Online Access:PDF Full Text
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This dissertation is guided by the theme of laser-induced self-organization of various thin film systems. Contrarily to similar manufacturing techniques, during self-organization the surface is not just directly modified by the tool itself. As further explained in the introduction section, the surface is rather modified by a stimulated material-reorganization. State-of-the-art pattern formation theories are often solely based on the initial system properties. These idealized models lack the consideration of laser mediated reactions and changes of material properties, as well as substrate interactions. To tackle aforementioned shortcomings, various thin films, e.g. inert and reactive metallic thin films, as well as organo-metallic precursor films, were irradiated. The influence of the thin film’s physicochemical properties on the pattern evolution and the final nanostructure expression were evaluated. Starting with the bare substrate, the system was first extended by an inert metal and subsequently by more reactive metallic thin films. Finally, the pattern formation of metal-organic precursor thin films was studied, providing a clear contrast to the metallic thin films in terms of material properties. When generated by linearly polarized laser light, these so-called laser-induced periodic surface structures (LIPSS) consist of periodically arranged nanowires or ridges. The periodicity and orientation are functions of the structured materials itself, as well as of the irradiation parameters. Upon laser irradiation with circularly polarized light, two dimensional periodic nano-structures were obtained. As a consequence, LIPSS generated in this work are classified into 1D-LIPSS (nanowires) and 2D-LIPSS (honeycomb-like arrangements and nanoparticle-arrays). This work was done with great emphasis on the less common 2D-LIPSS. Silicon (Si) is one of the most researched, yet application-oriented and simple materials, being the ideal candidate for a model-system. Arial illumination of a silicon surface leads to the formation of 1D-LIPSS. As the directional bias of linearly polarized light was omitted upon using circularly polarized light, the LIPSS’s orientation was solely defined by the laser scan direction. The self-organization is not disturbed by the silicon’s crystallographic orientation and leads to well-defined surface topography modulations, absent from additional surface oxidation. In order to increase the system’s complexity, silicon wafers were coated with an inert metal thin film. The laser-irradiation of gold thin films leads to the formation of self-organized gold nanoparticles arrays with hexagonal and square periodic arrangements. The nanoparticles are partly sunken in the Si surface, whose crystallographic orientation determines the nanoparticles shape. On Si(100) surfaces the nanoparticles possess a lens shape, while on Si(111) the nanoparticles are formed like bulged triangles. Analogous experiments conducted by replacing gold with other metal thin films as silver, copper, iron, zinc and titanium also resulted in the formation of nanoparticle arrays. Additionally, honeycomb-like metal nanostructures, as well as 1D-LIPSS emerged. Generally, the nanostructures were found in the order of dewetting, 1D-LIPSS, honeycomb-like 2D-LIPSS and eventually nanoparticle arrays, upon gradually increasing the energy dose during irradiation. Depending on the thin film’s elemental composition and thickness, as well as the irradiation conditions, e.g. pulse fluence and pulse numbers, the pattern evolution does not necessarily follow through all aforementioned types of LIPSS. The metal’s influence on the self-organization could be attributed to the metal’s free energy of oxide formation, interaction with the substrate and alloying properties. Additionally, simultaneous patterning and alloying of two stacked thin films of different elements could be achieved. Further expanding the set of available material classes for LIPSS-generation, a metal-organic thin film of a MoS2 precursor [Mo2S4(S2CNnBu2)2] was spin coated on silicon. Upon irradiating the system with linearly polarized laser pulses, 1D-LIPSS were obtained. Stimulating the system with circularly polarized laser light resulted in a rearrangement of the thin film in honeycomb-like formations (2D-LIPSS). Unlike the structuration of metallic thin films, the laser induced self-organization of the metal-organic thin films was found to occur solely by matter reorganization, maintaining both the materials chemical integrity and quantity. Significantly increasing the pulse fluence initiates the conversion of the precursor material to dendritic MoS2 nanoparticles. On the downside, the chemical conversion process leads to a significant loss in LIPSS quality. The successful structuration of stacked metal thin films, as well as the metalorganic thin films on silicon-dioxide, confirms the laser-stimulated pattern formation to be applicable to complex layer systems. The presented methods thus allow for the fabrication of diverse semiconductor-based opto-electric nano-devices. In a last part of this work, the application of aforementioned nanostructures was exemplarily examined. The usage of laser-generated gold catalysts on silicon as catalysts for the carbothermal growth of zinc oxide (ZnO) nanostructures was investigated. The laser-generated catalysts are distinguished in two different nanoparticle types. The first type consists of nanoparticles passivated by a thin SiO2-layer. The second type consists of exposed gold surfaces, acting as main catalysts responsible for the ZnO-growth. Upon adjusting the irradiation parameters, the size and areal density of the exposed gold nanoparticles are directly controlled. As a consequence, this method allows the gradual and spatial-selective control of the subsequently grown ZnO-nanostructure’s morphology, density, height, and width.