Exploring city patterns globally: The intra-urban morphology through the scope of unsupervised learning

Cities are complex systems with a unique composition of diverse elements and their relationships. Throughout history, humans form and shape cities by a range of functional, social, economical and political interactions. This diversity is reflected in the formation of individual built and non-built environments. However, similar elements and features form patterns that can be observed among multiple cities. City models try to understand the underlying processes that manifest into spatial patterns of urban form, but are often limited by a regional context and lack of comparable data. This master thesis aims to explore the urban morphology in a comparable framework on a global scale with the use of new consistent datasets such as the Local Climate Zones (LCZs) to describe the urban morphology of cities and the Morphological Urban Areas (MUAs) to delineate urban agglomerations. A search of urban morphological patterns is conducted without prior knowledge on subsets of 1523 cities. With state of the art methods of unsupervised learning 138 clusters of urban morphological patterns are found. The patterns show urban morphological configurations with similar statistical and spatial characteristics. A similarity metric is developed to compare cities based on the found patterns. Grouping similar cities leads to the formation of clusters which are partially congruent with geographic regions. The results of this work show that the formation of patterns with similar urban morphological configurations is linked to the geographic location. This master thesis is a first step towards a comprehensive knowledge on the formation of urban morphological configurations and contributes to a better understanding of cities

Analysis of onset of dislocation nucleation during nanoindentation and nanoscratching of InP

Nanoindentation and nanoscratching of an indium phosphide (InP) semiconductor surface was investigated via contact mechanics. Plastic deformation in InP is known to be caused by the nucleation, propagation, and multiplication of dislocations. Using selective electrochemical dissolution, which reveals dislocations at the semiconductor surface, the load needed to create the first dislocations in indentation and scratching can be determined. The experimental results showed that the load threshold to generate the first dislocations is twice lower in scratching compared to indentation. By modeling the elastic stress fields using contact mechanics based on Hertz's theory, the results during scratching can be related to the friction between the surface and the tip. Moreover, Hertz's model suggests that dislocations nucleate firstly at the surface and then propagate inside the bulk. The dislocation nucleation process explains the pop-in event which is characterized by a sudden extension of the indenter inside the surface during loadin

Analysis of onset of dislocation nucleation during nanoindentation and nanoscratching of InP

Nanoindentation and nanoscratching of an indium phosphide (InP) semiconductor surface was investigated via contact mechanics. Plastic deformation in InP is known to be caused by the nucleation, propagation, and multiplication of dislocations. Using selective electrochemical dissolution, which reveals dislocations at the semiconductor surface, the load needed to create the first dislocations in indentation and scratching can be determined. The experimental results showed that the load threshold to generate the first dislocations is twice lower in scratching compared to indentation. By modeling the elastic stress fields using contact mechanics based on Hertz’s theory, the results during scratching can be related to the friction between the surface and the tip. Moreover, Hertz’s model suggests that dislocations nucleate firstly at the surface and then propagate inside the bulk. The dislocation nucleation process explains the pop-in event which is characterized by a sudden extension of the indenter inside the surface during loading

Deformation mechanisms of silicon during nanoscratching

The deformation mechanisms of silicon {001} surfaces during nanoscratching were found to depend strongly on the loading conditions. Nanoscratches with increasing load were performed at 2 μm/s (low velocity) and 100 μm/s (high velocity). The load-penetration-distance curves acquired during the scratching process at low velocity suggests that two deformation regimes can be defined, an elasto-plastic regime at low loads and a fully plastic regime at high loads. High resolution scanning electron microscopy of the damaged location shows that the residual scratch morphologies are strongly influenced by the scratch velocity and the applied load. Micro-Raman spectroscopy shows that after pressure release, the deformed volume inside the nanoscratch is mainly composed of amorphous silicon and Si-XII at low scratch speeds and of amorphous silicon at high speeds. Transmission electron microscopy shows that Si nanocrystals are embedded in an amorphous matrix at low speeds, whereas at high speeds the transformed zone is completely amorphous. Furthermore, the extend of the transformed zone is almost independent of the scratching speed and is delimited by a dislocation rich area that extends about as deep as the contact radius into the surface. To explain the observed phase and defect distribution a contact mechanics based decompression model that takes into account the load, the velocity, the materials properties and the contact radius in scratching is proposed. It shows that the decompression rate is higher at low penetration depth, which is consistent with the observation of amorphous silicon in this case. The stress field under the tip is computed using an elastic contact mechanics model based on Hertz's theory. The model explains the observed shape of the transformed zone and suggests that during load increase, phase transformation takes place prior to dislocation nucleation. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cleavage Fracture of Brittle Semiconductors from the Nanometer to the Centimeter Scale

The objective of this paper is to present the fundamental phenomena occurring during the scribing and subsequent fracturing process usually performed when preparing surfaces of brittle semiconductors. In the first part, an overview of nano-scratching experiments of different semiconductor surfaces (InP, Si and GaAs) is given. It is shown how phase transformation can occur in Si under a diamond tip, how single dislocations can be induced in InP wafers and how higher scratching load of GaAs wafer leads to the apparition of a crack network below the surface. A nano-scratching device, inside a scanning electron microscope (SEM), has been used to observe how spalling (crack and detachment of chips) and/or ductile formation of chips may happen at the semiconductor surface. In the second part cleavage experiments are described. The breaking load of thin GaAs (100) wafers is directly related to the presence of initial sharp cracks induced by scratching. By performing finite element modelling (FEM) of samples under specific loading conditions, it is found that the depth of the median crack below the scratch determines quantitatively the onset of crack propagation. By carefully controlling the position and measuring the force during the cleavage, it is demonstrated that crack propagation through a wafer can be controlled. Besides, the influence of the loading configuration on crack propagation and on the cleaved surface quality is explained. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Mechanische Oberflächen-Nanostrukturierung von Indiumphosphid und Silizium für Anschliessende Elektrochemische Prozesse

We demonstrate nanoscale electrochemical porosification and deposition selectively on mechanically induced defects on semiconductors surfaces. Mechanical lithography was achieved with a diamond tip monted on an atomic force microscope (AFM) or a dedicated nanoscratch device. Two semiconductors were investigated : Indium Phosphide (InP) and Silicon (Si). The deformation processes as response to a contact loading on these semiconductors are different. Plastic deformation in InP is known to be caused by the generation, propagation and multiplication of dislocations while the plastic deformation of Si is due to dislocations formation similar to InP together with phase transformations of the single crystal. It appears that the dislocations created by nanoscratching on InP act as sites for a highly selective activation and structuring process. Nanometer size porous grooves are obtained on an n-InP(001) pre-scratched surface. The mechanical and electrochemical results obtained on indium phosphide are extended to silicon semiconductor. As already observed on InP, a nanoscratch on a n-Si(001) surface acts as an active area where selective electrochemical etching can be performed. Using this property, nano-scratches on silicon are used to trigger macro-pores formation. This is possible since the dissolution of the micro-scratches forms pre-requisite invert pyramid geometries required to trigger the macro-pore nucleation in electrochemical back-side illumination configuration. Using this Pre-Scratched Electrochemical Trench Etching (PreSETE) method well defined channels are achieved at the scratch locations.Wir zeigen selektive elektrochemische Porosifizierung und Abscheidung auf mechanisch erzeugten Defekten auf Halbleiteroberflächen im Nanometerbereich. Mechanische Lithographie wurde mit einer auf einem Rasterkraftmikroskop oder einem Nanoscratcher montierten Diamantspitze erreicht. Es wurden zwei Halbleiter untersucht: Indiumphosphid (InP) und Silizium (Si). Der durch eine Kontaktlast induzierte Deformationsprozess ist auf diesen Halbleitern unterschiedlich. In InP wird plastische Verformung durch die Erzeugung, Bewegung und Vermehrung von Versetzungen verursacht, während in Si Versetzungen ähnlich wie in InP zusammen mit Phasentransformationen dafür verantwortlich sind. Durch Nanoritzen induzierte Versetzungen auf InP dienen als hoch selektive Aktivierungszentren für Strukturierungsprozesse. Auf einer geritzten n-InP(001) Oberfläche entstehen so mittels eines elektrochemischen Ätzprozesses nanoporöse Rillen. Die mechanischen und elektrochemischen Resultate vom InP wurden auf Silizium übertragen. Wie schon auf InP beobachtet, dient ein Nanoritz auf einer n-Si(001) Oberfläche als Aktivierungszentrum für selektives elektrochemisches Ätzen. Mit dieser Eigenschaft werden Nanoritze als Auslöser für Makroporenbildung verwendet. Dies ist möglich, da durch das Auflösen der Mikroritzen die Umkehrpyramidengeometrien entstehen, die für die Makroporennukleation während des elektrochemischen Ätzens unter Rückseitenbeleuchtung verantwortlich sind. Mit dieser Methode (Pre-Scratched Electrochemical Trench Etching, PreSETE) konnten an den geritzten Stellen sehr gut definierte Kanäle hergestellt werden