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Members of TU Berlin can publish different kinds of (text) publications on DepositOnce:
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- 2023Doctoral Thesis
28Si, Ge epitaxy for qubitsSilicon (Si), Germanium (Ge) related materials such as SiGe heterostructures or silicon-on-insulator structures (SOI) stand out as excellent material platforms for spin qubits or optical quantum emitters. To achieve the high-performance devices built on these materials, there are two main paths to go for material optimisation. One is using isotopically enriched materials, such as enriched with 28Si, to achieve a nuclear spin depleted environment. Another one is improving the crystal quality of the materials by reducing defects, such as pits on the layers or dislocations at the interfaces. Both paths have been explored within the scope of this work: First, an isotope engineered molecular beam epitaxy (MBE) was established with 28Si monocrystal as source material. A hybrid method combining this isotope engineered MBE / chemical vapour deposition (CVD) was applied to grow 28SiGe/28Si(10 nm)/28SiGe heterostructures for electron spin qubits and 28SiGe/Ge(20 nm)/28SiGe heterostructures for hole spin qubits. Here, the thick relaxed SiGe substrates are realised by CVD and the 28SiGe/28Si/28SiGe stacks or the 28SiGe/Ge/28SiGe stacks are grown by the isotope engineered MBE. The CVD growth in this work is undertaken by Dr. Yamamoto in IHP and a research group from Siltronic. A 28Si quantum well layer with 29Si concentration as low as 200 ppm is achieved within a 28SiGe/28Si/28SiGe heterostructure. This 28Si quantum well layer has 1.2 % tensile strain, that is fully strained respective to the relaxed 28Si0.7Ge0.3 substrate, which was shown in the reciprocal space maps by X-ray diﬀraction (XRD). In the case of the grown 28SiGe/Ge/28SiGe heterostructure for hole spin qubits, the Ge quantum well layer has 1.3 % compressive strain, that is also fully strained according to the reciprocal space maps. The isotope engineered MBE was also applied in the growth of a high-quality 28SOI for optical quantum emitters. A 400 nm 28Si layer was grown on a 70 nm thin SOI seed. The 28SOI shows low surface roughness of 3.4 Å. There rippels on the surface due to unintentional miscut of the substrate. SIMS reveals 29Si concentration in the 28Si homoepitaxial layer is below 60 ppm. Second, the dislocations in the SiGe heterostructures have been investigated regarding the misﬁt dislocation formation, kinetics, and interactions. The critical thickness for the plastic relaxation of the Si quantum well layer embedded in a SiGe/Si/SiGe heterostructure for qubits is studied by plan-view transmission electron microscopy (TEM) and electron channelling contrast imaging (ECCI). Misﬁt dislocation segments have been observed in the CVD grown SiGe/Si(10 nm)/SiGe heterostructures with both high (1.4 × 10^7 cm^−2) and low (3 × 10^5 cm^-2) threading dislocation densities. This misﬁt dislocations form the glide of pre-existing threading dislocations at the interface of the Si quantum well layer, when the Si quantum well layer thickness beyond a critical value hc (hc = 8.5 nm in this material system) given by the Matthews-Blakeslee criterion. A Burgers vector analysis was conducted based on the TEM images. The analysis reveals the misﬁt dislocations are mostly 60° dislocations with Burgers vectors a/2<1 0 1> that are split into partials a/6<1 1 2> due to the tensile strain ﬁeld of the Si quantum well layer. By reducing the quantum well thickness from 10 nm to 5 nm below critical thickness, misﬁt dislocations can be avoided. We discuss the consequences of our ﬁndings for the layer stack design of SiGe/Si/SiGe heterostructures for usage in qubits. Furthermore, the misﬁt dislocation propagation kinetics and interactions have been studied by annealing the strained Si or Ge layers grown by MBE and investigating these layers in ECCI. The strained Si layers have been annealed at temperatures from 500°C to 600°C in an ultra high vacuum chamber. The strained Ge layers have been annealed temperatures from 300°C to 400°C in the same chamber. The misﬁt dislocations in these annealed layers were imaged by ECCI. The results conﬁrm that the misﬁt dislocation propagation is a thermally activated process following an Arrhenius-type law for the propagation velocity v: v = v (σ) exp(−Ea(σ)/KT). The activation energies Ea(σ) is obtained as 0.49±0.01 eV for the tensile strained Si and 0.39±0.10 eV for the compressively strained Ge in this work. These observations imply that it is possible to suppress the misﬁt dislocation formation kinetically by reducing the temperatures during the SiGe heterostructure epitaxy and post-epitaxy processes in order to develop the low-defect materials for the well-functional SiGe-based spin qubits.
- 2023Doctoral Thesis
Further development and application of advanced exergy-based methodsTechnical systems for energy conversion and chemical production processes undergo continuous technological changes triggered by evolutionary and revolutionary developments which are related to changing requirements regarding their thermodynamic efficiency, economic performance, and environmental impact. In order to make robust decisions for developing and improving such process systems, a systematic, hierarchically structured conceptual process design approach is used. For this purpose, different iterative and automatic as well as qualitative and quantitative methods are applied. The concept of exergy analysis is attributed to the group of thermodynamic methods, being used iteratively and providing quantitative results for the analysis and evaluation of process systems. It provides tangible information on the location, the magnitude, and the sources of thermodynamic inefficiencies, and can be used to determine meaningful measures for thermodynamic efficiencies. However, conventional exergy-based analyses cannot provide any information on the interaction and possible improvement potential of process system components. For this reason, the framework and methods of advanced exergy-based analysis was developed. In this work, methodological developments and improvements within the advanced exergy-based framework are carried out and subsequently tested on various examples of energy conversion and chemical process systems. The methodological developments for determining the endogenous and exogenous as well as avoidable and unavoidable portions of the exergy destruction of a process system component follow a top-down oriented approach, which itself builds on and advantageously complements well-established systematic, hierarchically structured conceptual process design procedures. The new, thermodynamically sound methodology overcomes the problems and shortcomings of previous approaches thereby simplifying and facilitating the analyses. By having the opportunity to identify the root causes of thermodynamic inefficiencies and their effect on a process system component's efficiency provides the possibility for individual design decisions to be analyzed, evaluated, and discussed holistically. This opens the design space for discussions of changes regarding parameters and structure of the process system. It can further be used on different system aggregation levels thereby allowing the user to set the scope and context of the analysis. Supporting, both, the qualitative understanding and the quantitative analysis, an advanced exergy-based analysis is a promising tool for designing, analyzing, and improving energy conversion and chemical process systems towards higher thermodynamic efficiencies, lower costs, and reduced environmental impact.
- 2023Doctoral Thesis
Service-oriented design and verification of hybrid systemsNowadays, cyber-physical systems find application in many areas. These systems consist of multiple control components that interact with each other and with the physical environment. Model-driven development methods are used to handle their complexity and to ease their development. Furthermore, variability in the model design enables to customize a system for different environments. In safety-critical areas, where faulty behavior can injure or even kill people, it is desirable that the development and quality assurance of these systems is performed rigorously. Cyber-physical systems have properties that make their development and quality assurance challenging. They contain hybrid behavior, which is the interaction of discrete changes and continuous behavior that is described by differential equations. Data flow-oriented modeling languages that are used in industry aim at enabling the comprehensible and fast development of hybrid control systems. However, they lack formal semantics, which precludes the application of rigorous verification methods and thus makes it impossible to provide guarantees about the behavior of the modeled systems. Formal modeling languages enable us to precisely model the mathematical processes and to formally verify that the system fulfills its requirements. However, correct behavior for each system variant must be ensured, which requires high effort, and formal approach often have scalability issues when handling industrially used models. Two major barriers prevent formal methods from being applied in practice: The lack of formal semantics of industrially used modeling languages, but also the high cost of rigorous formal verification. In this thesis, we present an approach for the service-oriented design and verification of hybrid control systems. We define the concept of a Service for hybrid systems, with which we can formally define the functionality of the system and customize it for different contexts. As representative for data flow-oriented modeling languages, we present a formalization for Simulink into differential dynamic logic (dL), which is a formal language to design and verify hybrid systems. Our formalization enables the formal verification of hybrid Simulink models. With the verification results, we create hybrid contracts that describe the interface behavior of these services containing hybrid behavior. Additionally, we present an abstraction mechanism for services with hybrid contracts that enables the scalable verification of systems consisting of interacting services. With the addition of a feature model, we enable the customization for services for the reuse in different contexts. We have created a framework that implements our service-oriented design in Simulink and provides an automatic transformation of Simulink models into dL to enable the service-oriented design and verification of Simulink models. The hierarchical nature of services enables the development of larger models that are verifiable. Our approach combines the strengths of model-driven development of hybrid systems with the power of formally ensuring the correct behavior of modeled systems under all circumstances. We provide a formal foundation for hybrid Simulink models, for which we have developed an automatic transformation of Simulink models into dL. With our abstraction with hybrid contracts, we can provide safety guarantees for larger systems that consist of multiple interacting services. With a feature model and automatic service generation, we enable easy customization of services and their reuse. We demonstrate the applicability of our approach with different experimental results.
- 2023Doctoral Thesis
Productive green infrastructure for optimising the urban metabolism the case of Da Nang, VietnamThe thesis reflects current discourses surrounding human-induced planetary alterations in the Anthropocene that have led to problematic changes in the biosphere. Resource utilisation and food systems are particular drivers of these alterations. Over the past 150 years, the metabolism of cities has continuously expanded into the so-called ‘global hinterland’. As the biosphere’s regenerative capacities have gradually become exhausted, the interdependency of urban and natural systems becomes increasingly evident again. Emerging urban metabolism approaches aim at deciphering urban-natural interrelations by viewing cities as ecosystems of the ‘technosphere’ that can be optimised by mimicking circular cycling processes in natural ecosystems. This work focusses on the urban food system and green infrastructure planning as well as further develops the conceptual approach of productive green infrastructure (PGI). PGI aims to address the challenge of assessing the spatial extent of dispersed and informal practices of urban agriculture and its integration into a strategically planned green infrastructure network. The PGI approach is exemplified through a case study of Da Nang, a rapidly growing city in Vietnam. Da Nang has a rich culture of urban agriculture that has not been subject to urban planning. Through an interlinked process of mapping, aerial picture interpretation, and processing of existing land use data, characteristic typologies of urban agriculture are identified and quantified. The relation between the typologies and the urban metabolism is conceptualised and substantiated with data. The research project ‘Rapid Planning’ made it possible to integrate the typologies into a simulation model of Da Nang’s urban metabolism and to simulate different development scenarios in a collaborative process. One scenario showed, that despite a growing population, the supply of horticultural produce from the region could be stabilised by preserving some of the grown structures of urban agriculture, facilitating their integration into urban extension areas, and gradually shifting from staple crops to vegetables and fruits on remaining agricultural land. Based on the scenario, the approach of green infrastructure is used to draft a spatial framework for a city-region-wide network of green spaces, which marks the turning point of when single elements of urban agriculture become a PGI. Potential links to the existing planning goals, framework and procedures are also reflected. PGI combines the agenda of green infrastructure and food system planning. It can potentially contribute to make use of urban resources, rendering material flows more sustainable and increasing food security by expanding the choices for healthy and sustainable diets. PGI broadens the search space for developing multifunctional and connected green infrastructure, which is urgently needed for the well-being of urban residents and to buffer the effects of climate change.
Evaluation of a Modular Filter Concept to Reduce Microplastics and Other Solids from Urban Stormwater RunoffThis paper describes an innovative Decentralized Technical Sustainable Drainage System (SuDS) concept, which is based on technical devices, such as sieves, sedimentation barriers, floating barriers and a magnetic module, which addresses, mainly, the fine matter. The SuDS is designed as a retrofit system so that no costly and time-consuming conversion measures are necessary. Due to the possibility of free configurability of individual modules in the three levels, road, gully and drain, a novel solution approach is presented, which is not available on the market, for a reduction in solids in general and microplastics in particular. The retention performance of selected modules and their combinations is demonstrated by means of bench tests according to the test procedure of the German Institute for Construction Engineering (DIBt) for the evaluation of decentralized treatment systems. Four different rain intensities, from light to medium up to heavy rain, are charged to the filter modules. Collected and fractionated road-deposited sediment (RDS) was selected as the test substance (10 kg). Additional tests with tyre powder, PE pellets, cigarette butts and candy wrappers helped to make clear the filter process of the particulate matter. The retention performance was determined by the mass balance between the defined dosage and at the outlet. For this purpose, the total volume flow of the effluent was passed over a stainless-steel sieve with a diameter of 600 mm and a mesh size of 20 µm. For the test substance, RDS retention rates up to 97% were measured. Very fine matter, particularly, was technically challenging to obtain; <63 µm up to 66% could be retained by the filter modules. Modules in the road space, such as porous asphalt or additional retention spaces, in the area of the curb as well as direct infiltration in the road drainage shaft are theoretically described and discussed. The outlook also addresses the potential of an intelligent network to reduce the input of pollution from urban stormwater runoff.