The electrification of transport, the increase in renewable energies, and the power supply for large artificial intelligence data centers represent challenges that require highly efficient power conversion. To minimize power losses, optimized electronic components are required. Nitride-based compound semiconductors with a large band-gap are very promising and on the way to market entry. However, the source and drain terminals that connect the semiconductor component to integrated circuits still pose a key challenge. These ohmic contacts contribute significantly to the power loss of the component, leading to heat generation and energy losses. In addition, the manufacturing costs for nitride power components must be reduced, which makes integration into the Si-CMOS process chain essential. However, CMOS requires an Au-free metallization, which represents a major challenge for the ohmic contact development, but also promises further advantages in the long term, such as a significantly improved surface morphology. In the framework of this thesis, the state-of-the-art Au-based as well as conventional and a newly developed Au-free ohmic contact stack were fabricated on heterostructures with normal and increased Al-content and benchmarked. For the investigations, various epitaxially grown GaN and AlGaN/GaN wafers from different manufacturers were used. The origin of the power losses of Au-free contacts was investigated by electrical and microstructural characterizations and discussed. A CMOS-compatible V/Al/Ti/TiN contact stack with extremely low contact resistance is presented and demonstrated by HEMTs on GaN-on-Si substrates, as it could be used in the CMOS process chain.:1 Introduction 1.1 Power Electronics 1.2 Power Effciency 1.3 Semiconductors for Power Switching 1.4 Devices for Power Switching 1.5 Aims of this Work 2 Fundamentals 2.1 Nitride Semiconductors GaN, AlGaN, AlN 2.2 AlGaN/GaN Heterostructure 2.3 AlGaN/GaN High-Electron-Mobility Transistor 2.4 Ohmic Contacts for AlGaN/GaN High-Electron-Mobility Transistors 3 Test Device Fabrication 3.1 Employed Heterostructure Wafers 3.2 Test Die Fabrication 3.3 Mesa Module 3.4 Ohmic Module 3.5 Gate Module 4 Characterization 4.1 Electrical Characterization 4.2 Structural Characterization 5 Results 5.1 Au-based Contact Formation on High-Al AlGaN 5.2 Au-free Contact Formation on GaN Substrate 5.3 Au-free Contact Formation on AlGaN/GaN Heterostructure 5.4 Au-free V-based Ultra-Low Contact Resistance 5.5 Au-free V-based Contacts on Super-Lattice Buffered Heterostructure 5.6 Au-free V-based Contacts on High-Al AlGaN (Outlook) 6 Summary List of Publications Acknowledgements Bibliography List of Symbols List of Abbreviations List of Tables List of Figures / Die Elektrifizierung des Verkehrs, der Anstieg regenerativer Energien und die Stromversorgung großer KI-Datencenter stellen Herausforderungen dar, die eine hochgradig effiziente Stromwandlung erfordern. Um Verlustleistungen zu minimieren, sind optimierte Bauteile notwendig. Dafür vielversprechend sind nitridbasierte Verbindungshalbleiter mit großer Bandlücke, die an der Schwelle zur Marktreife stehen. Eine zentrale Herausforderung stellen noch immer die Source- und Drain-Terminals dar, die das Halbleiterbauteil in integrierte Schaltungen einbinden. Diese ohmschen Kontakte tragen signifikant zur Verlustleistung des Bauteils bei, was zu Wärmeentwicklung und Energieverlusten führt. Zudem müssen die Herstellungskosten für nitridische Leistungsbauteile gesenkt werden, was eine Integration in die Si-CMOS-Prozesskette unabdingbar macht. CMOS erfordert jedoch eine Au-freie Metallisierung, was eine große Hürde an die Kontaktwicklung stellt, langfristig aber auch weitere Vorteile wie eine deutlich verbesserte Oberflächenmorphologie verspricht. Im Rahmen dieser Arbeit wurde der state-of-the-art Au-basierte sowie herkömmliche als auch ein neu entwickelter Au-freier ohmscher Kontakstapel auf Heterostrukturen mit üblichem und erhöhtem Al-Gehalt hergestellt und hinsichtlich ihrer elektrischen Kenngrößen miteinander verglichen. Für die Untersuchungen wurden verschiedene epitaktisch gewachsene GaN- und AlGaN/GaN-Wafer von unterschiedlichen Herstellern verwendet. Die Ursachen der Verlustleistung Au-freier Kontakte wurden durch elektrische und mikrostrukturelle Charakterisierungen untersucht und diskutiert. Ein CMOS-kompatibler V/Al/Ti/TiN Kontaktstapel mit extrem niedrigem Kontaktwiderstand wird vorgestellt und an einem HEMT auf GaN-on-Si Substrat demonstriert, wie er in der CMOS-Prozesskette eingesetzt werden könnte.:1 Introduction 1.1 Power Electronics 1.2 Power Effciency 1.3 Semiconductors for Power Switching 1.4 Devices for Power Switching 1.5 Aims of this Work 2 Fundamentals 2.1 Nitride Semiconductors GaN, AlGaN, AlN 2.2 AlGaN/GaN Heterostructure 2.3 AlGaN/GaN High-Electron-Mobility Transistor 2.4 Ohmic Contacts for AlGaN/GaN High-Electron-Mobility Transistors 3 Test Device Fabrication 3.1 Employed Heterostructure Wafers 3.2 Test Die Fabrication 3.3 Mesa Module 3.4 Ohmic Module 3.5 Gate Module 4 Characterization 4.1 Electrical Characterization 4.2 Structural Characterization 5 Results 5.1 Au-based Contact Formation on High-Al AlGaN 5.2 Au-free Contact Formation on GaN Substrate 5.3 Au-free Contact Formation on AlGaN/GaN Heterostructure 5.4 Au-free V-based Ultra-Low Contact Resistance 5.5 Au-free V-based Contacts on Super-Lattice Buffered Heterostructure 5.6 Au-free V-based Contacts on High-Al AlGaN (Outlook) 6 Summary List of Publications Acknowledgements Bibliography List of Symbols List of Abbreviations List of Tables List of Figures

This thesis investigates the electrical and electrochemical properties and the long-term stability of anodes of the solid oxide fuel cell (SOFC). A model is suggested, which describes the impedance spectra of symmetrical anode cells. According to this, the series resistance in the spectra is caused by the resistance of the electrolyte (YSZ), ohmic parts of the anodes, which are described as porous electrodes, and by the partial contacting of the anodes. A major contribution to it is provided by the nickel matrix in the anodes. The high frequency relaxation in the spectra is assigned to the transfer reaction, the low frequency to a gas diffusion inhibition along the gas supply channels. The degradation of the symmetrical anode cells, which has been observed in long-term experiments, is ascribed to a degradation of the electrolyte material, of the transfer reaction, of the nickel matrix in the anodes and of the contact resistance between the anodes and the current collecting nickel grids. The degradation rate of the last two depends on the gas composition. A model for the observed behaviour in time is presented. / Diese Arbeit untersucht die elektrischen und elektrochemischen Eigenschaften und die Langzeitbeständigkeit der Anoden von Festoxid-Brennstoffzellen (SOFC). Ein Modell wird vorgestellt, mit dem die Impedanzspektren symmetrischer Anodenzellen beschrieben werden können. Demnach ist der Serienwiderstand in den Spektren verursacht durch den Widerstand des Elektrolyten (YSZ), ohmsche Anteile in den Anoden, die als poröse Elektroden beschrieben werden, und durch die partielle Kontaktierung der Anoden. Maßgebliche Beiträge liefert hier die Nickelmatrix in den Anoden. Die hochfrequente Relaxation in den Spektren wird der Durchtrittsreaktion, die niederfrequente einer Gasdiffusionshemmung entlang der Gasversorgungskanäle zugeordnet. Die in Langzeitversuchen beobachtete Degradation der symmetrischen Anondenzellen wird zurückgeführt auf eine Degradation des Elektrolytmaterials, der Durchtrittsreaktion, der Nickelmatrix in den Anoden und des Kontaktwiderstandes zwischen den Anoden und den stromabnehmenden Nickelnetzen. Die Degradation der beiden letzteren ist in ihrer Rate abhängig von der Gaszusammensetzung. Ein Modell für das beobachtete zeitliche Verhalten wird vorgestellt.

Stainless steel bipolar plates are a possible replacement for graphite and composite bipolar plates in fuel cells. However, due to a native oxide layer they exhibit a high interfacial contact resistance (ICR) which lowers the performance. Conductive coatings like gold are a possible solution because they can reduce the contact resistance of metallic bipolar plates. We investigate the pulsed cathodic arc technique for deposition of carbon-based thin films on austenitic stainless steel 316L as cost-efficient alternative. Different types of coatings were prepared by varying the layer structure and processing parameters. Potentiodynamic polarization tests and ICR measurements were conducted to evaluate the performance of the films as conductive and corrosion resistant coatings. It was found that the corrosion resistance of coated austenitic steel samples is improved by both coatings and that measured ICR-values are well below the DOE 2020 target of 10 mΩ/cm2.

This thesis investigates the electrical and electrochemical properties and the long-term stability of anodes of the solid oxide fuel cell (SOFC). A model is suggested, which describes the impedance spectra of symmetrical anode cells. According to this, the series resistance in the spectra is caused by the resistance of the electrolyte (YSZ), ohmic parts of the anodes, which are described as porous electrodes, and by the partial contacting of the anodes. A major contribution to it is provided by the nickel matrix in the anodes. The high frequency relaxation in the spectra is assigned to the transfer reaction, the low frequency to a gas diffusion inhibition along the gas supply channels. The degradation of the symmetrical anode cells, which has been observed in long-term experiments, is ascribed to a degradation of the electrolyte material, of the transfer reaction, of the nickel matrix in the anodes and of the contact resistance between the anodes and the current collecting nickel grids. The degradation rate of the last two depends on the gas composition. A model for the observed behaviour in time is presented. / Diese Arbeit untersucht die elektrischen und elektrochemischen Eigenschaften und die Langzeitbeständigkeit der Anoden von Festoxid-Brennstoffzellen (SOFC). Ein Modell wird vorgestellt, mit dem die Impedanzspektren symmetrischer Anodenzellen beschrieben werden können. Demnach ist der Serienwiderstand in den Spektren verursacht durch den Widerstand des Elektrolyten (YSZ), ohmsche Anteile in den Anoden, die als poröse Elektroden beschrieben werden, und durch die partielle Kontaktierung der Anoden. Maßgebliche Beiträge liefert hier die Nickelmatrix in den Anoden. Die hochfrequente Relaxation in den Spektren wird der Durchtrittsreaktion, die niederfrequente einer Gasdiffusionshemmung entlang der Gasversorgungskanäle zugeordnet. Die in Langzeitversuchen beobachtete Degradation der symmetrischen Anondenzellen wird zurückgeführt auf eine Degradation des Elektrolytmaterials, der Durchtrittsreaktion, der Nickelmatrix in den Anoden und des Kontaktwiderstandes zwischen den Anoden und den stromabnehmenden Nickelnetzen. Die Degradation der beiden letzteren ist in ihrer Rate abhängig von der Gaszusammensetzung. Ein Modell für das beobachtete zeitliche Verhalten wird vorgestellt.

The Antimatter Experiment: Gravity, Interferometry, Spectroscopy (AEGIS) at the European Organization for Nuclear Research (CERN) is an experiment investigating the influence of earth’s gravitational force upon antimatter. To perform precise measurements the antimatter needs to be cooled to a temperature of 100 mK. This will be done in a Penning trap, formed by several electrodes, which are charged with several kV and have to be individually electrically insulated. The trap is thermally linked to a mixing chamber of a 3He-4He dilution refrigerator. Two link designs are examined, the Rod design and the Sandwich design. The Rod design electrically connects a single electrode with a heat exchanger, immersed in the helium of the mixing chamber, by a copper pin. An alumina ring and the helium electrically insulate the Rod design. The Sandwich uses an electrically insulating sapphire plate sandwiched between the electrode and the mixing chamber. Indium layers on the sapphire plate are applied to improve the thermal contact. Four differently prepared test Sandwiches are investigated. They differ in the sapphire surface roughness and in the application method of the indium layers. Measurements with static and sinusoidal heat loads are performed to uncover the behavior of the thermal boundary resistances. The thermal total resistance of the best Sandwich shows a temperature dependency of T-2,64 and is significantly lower, with roughly 30 cm2K4/W at 50 mK, than experimental data found in the literature. The estimated thermal boundary resistance between indium and sapphire agrees very well with the value of the acoustic mismatch theory at low temperatures. In both designs, homemade heat exchangers are integrated to transfer the heat to the cold helium. These heat exchangers are based on sintered structures to increase the heat transferring surface and to overcome the significant influence of the thermal resistance (Kapitza resistance). The heat exchangers are optimized concerning the adherence of the sinter to the substrate and its sinter height, e.g. its thermal penetration length. Ruthenium oxide metallic resistors (RuO2) are used as temperature sensors for the investigations. They consist of various materials, which affect the reproducibility. The sensor conditioning and the resulting good reproducibility is discussed as well.

The Antimatter Experiment: Gravity, Interferometry, Spectroscopy (AEGIS) at the European Organization for Nuclear Research (CERN) is an experiment investigating the influence of earth’s gravitational force upon antimatter. To perform precise measurements the antimatter needs to be cooled to a temperature of 100 mK. This will be done in a Penning trap, formed by several electrodes, which are charged with several kV and have to be individually electrically insulated. The trap is thermally linked to a mixing chamber of a 3He-4He dilution refrigerator. Two link designs are examined, the Rod design and the Sandwich design. The Rod design electrically connects a single electrode with a heat exchanger, immersed in the helium of the mixing chamber, by a copper pin. An alumina ring and the helium electrically insulate the Rod design. The Sandwich uses an electrically insulating sapphire plate sandwiched between the electrode and the mixing chamber. Indium layers on the sapphire plate are applied to improve the thermal contact. Four differently prepared test Sandwiches are investigated. They differ in the sapphire surface roughness and in the application method of the indium layers. Measurements with static and sinusoidal heat loads are performed to uncover the behavior of the thermal boundary resistances. The thermal total resistance of the best Sandwich shows a temperature dependency of T-2,64 and is significantly lower, with roughly 30 cm2K4/W at 50 mK, than experimental data found in the literature. The estimated thermal boundary resistance between indium and sapphire agrees very well with the value of the acoustic mismatch theory at low temperatures. In both designs, homemade heat exchangers are integrated to transfer the heat to the cold helium. These heat exchangers are based on sintered structures to increase the heat transferring surface and to overcome the significant influence of the thermal resistance (Kapitza resistance). The heat exchangers are optimized concerning the adherence of the sinter to the substrate and its sinter height, e.g. its thermal penetration length. Ruthenium oxide metallic resistors (RuO2) are used as temperature sensors for the investigations. They consist of various materials, which affect the reproducibility. The sensor conditioning and the resulting good reproducibility is discussed as well.