Doktorsavhandlingar vid Fasta tillståndets elektronik

Abstract

Radio Frequency (RF) power sources are extensively applied in various fields. Radioisotope production, i.e., the production of short-lived radioactive isotopes, for positron emission tomography (PET) is one of the most important applications in the medical and healthcare domains. Full-time operation and substantial maintenance of such systems lead to high operating expenses. Hence, the development of more efficient and reliable RF power amplifiers, which are the main contributors to the energy consumption and maintenance costs of the RF power sources, is a high priority. Solid-state technology has emerged as a viable alternative to conventional vacuum tube based high-power RF/microwave systems, offering advanced control, reliability, and ease of use. Power amplifiers based on solid-state technology enable dynamic adjustment of power to optimize the transmitted energy. Furthermore, solid-state power amplifiers (SSPA) technology shows a longer lifetime leading to increased uptime and lower maintenance costs. Concisely, with the introduction of solid-state technology in high-power RF sources, RF energy can be generated more efficiently and more controllable in a smaller form factor, allowing for more compact systems with less downtime and less maintenance. This thesis is one step further toward demonstrating the feasibility of such systems.

The thesis first introduces the RF measurement setup. It implements automation for quick measurements and supports the evaluation of the high-power RF performance of the developed SSPA modules. Moreover, a novel thru-only de-embedding approach is developed to address the calibration difficulties under multi-port excitation conditions. The second part of the thesis deals with the development and analysis of efficient kilowatt SSPA modules. A multimode SSPA with quasi-static supply control for power regulation is implemented. It achieves more than 90% efficiency over a 5 dB output power back-off range. Another compact and efficient SSPA, implemented in push-pull architecture, adopts harmonic load-pull integrated with the same quasi-static supply modulation which also achieves 90% efficiency over a 5 dB output power back-off range. The implemented SSPAs improve the state-of-the-art in these frequency bands and power ranges.

This thesis broadens RF SSPA theoretical research to the kilowatt power range and provides a new understanding of high-power SSPAs from circuits, design methodologies, and analytical approaches. And it leads to new methods and tools to improve the energy efficiency of high-power RF sources. The knowledge gained and technology developed is not limited to RF power sources in radioisotope production applications, it can also be applied in the communication industry, such as radar systems, and other RF energy systems in industrial, scientific, and medical (ISM) fields, such as particle accelerators, welding, drying, heating, and many more.

Abstract

A biosensor based on streaming current is a new and relatively unexplored subject with significant potential. This thesis attempts to gain a deeper understanding of the governing principles, and then exploit them to further improve its performance as well as develop novel applications. To this end, the underlying theoretical frameworks were examined and two critical parameters of the target: its size and electric charge, influencing the sensor’s sensitivity were identified. This was followed by experimental evaluation of the parameters, using a set of tailor-made proteins, aiming to understand the nature and extent of their influence on the sensor response in relation to simulation performed following an established model.

The dependence of the sensor response on the charge of an analyte, or specifically the charge contrast between the sensor surface and an analyte, opens a new avenue to improve the sensitivity and also to develop novel functionality. First, this aspect was exploited to improve the sensitivity by optimizing the surface functionalization strategy. Three such methods were compared in terms of the resulting zeta potential of the surface. The sensitivity was the highest when the charge contrast was maximum. The optimal functionalization strategy was then used for highly sensitive detection of extracellular vesicles (EVs), where an improvement in the limit of detection by two orders of magnitude over the previously reported results was demonstrated. Two applications of the improved method were then demonstrated: monitoring the effectiveness of targeted cancer medicines and analysis of liquid biopsy of cancer patients via sensitive profiling of EV-membrane proteins.

Improvement in the detection specificity is a critical aspect of biosensing. This was achieved by implementing a sandwich immunoassay and demonstrating the proof of concept using trastuzumab as the target and Z-domain as both the capture and detection probes. Although the improved selectivity came at the cost of a lower sensitivity, this could be mitigated via DNA-conjugation with the detection probes, a novel electrostatic labelling strategy that allows for improvement of the sensitivity by exploiting the electrostatic influence. An application of this method was then demonstrated by detecting the target from a complex medium of E. coli cell lysate. Continuing the prospect of charge engineering of antibodies, a set of positively and negatively charged antibodies were synthesized by conjugating poly-lysine and DNA oligonucleotides, respectively. This enabled stepwise, multiplexed membrane protein analysis of EVs using the alternating charge-labelled antibodies. The method was then applied to investigate EV-heterogeneity.

Abstract

Graphene is widely touted as the thinnest and the most versatile material available. As an atomically thin layer of carbon atoms arranged in a hexagonal configuration, graphene has a combination of technologically important properties, such as thermal and electrical conductivity, mechanical strength, and impermeability to gases. From an industrial perspective on applications, water as a dispersing media for graphene offers safer handling and environmental benefits compared with conventional organic solvents. However, the high surface tension of water and the attractive forces between graphene surfaces drive the sheets to aggregation. Although surfactants have been an important stepping stone in the advancement of aqueous graphene dispersions, these surface-active molecules are often needed in excess and have adverse effects on coatings during film formation. These challenges limit the industrial relevance of graphene as an effective barrier in composites. In general, gas barriers against both oxygen and water vapour, made from a single coating formulation, is seemingly a holy grail for the packaging industry. In this thesis work, the aim was to gain a fundamental understanding of aqueous graphene dispersions for gas barriers used in paper packaging. Biobased materials were systematically investigated as dispersing agents for graphene based on dispersing conditions and functional barrier performance. Flavin mononucleotide (FMN), a food additive, dispersed graphene using a relatively low amount of FMN and showed intriguing spectroscopic signatures of π-π interactions with graphene. Starch nanoparticles (SNPs) realised concentrated and stable aqueous graphene dispersions for composite films. The SNP-stabilized graphene sheets in starch films lowered the gas permeability of both oxygen and water vapour simultaneously by over 70% under all the conditions tested. In general, a combined gas barrier performance is unusual for both bioplastics and common petrochemical-based plastics used in the packaging industry. Motivated by the graphene network leading to the extraordinary barrier performance, the aqueous SNP-graphene dispersion was modified for inkjet printing. The printed patterns were flexible and electrically conductive in the order of 10⁴ S m^-1 that is on par with the highest reported values in the literature. These surfactant-free aqueous SNP-graphene dispersions have the potential and versatility for paper-based gas barriers with integrated electronics. Multifunctional composite films made from these dispersions, when optimized, could become competitive with commercial plastics, and meet the current and future demands of the packaging industry.

Abstract

Silicon nanowire (SiNW) based electronic devices fabricated with a complementary metal-oxide-semiconductor (CMOS) compatible process have wide-range and promising applications in sensing area. These SiNW sensors own high sensitivity, low-cost mass production possibility, and high integration density. In this thesis, we design and fabricate SiNW electronic devices with the CMOS-compatible process on silicon-on-insulator (SOI) substrates and explore their applications for ion sensing and quantum sensing. 

The thesis starts with ion sensing using SiNW field-effect transistors (SiNWFETs). The specific interaction between a sensing layer and analyte generates a change of local charge density and electrical potential, which can effectively modulate the conductance of SiNW channel. Multiplexed detection of molecular (MB⁺) and elemental (Na⁺) ions is demonstrated using a SiNWFET array, which is functionalized with ionophore-incorporated mixed-matrix membranes (MMMs). As a follow-up, polyethylene glycol (PEG) doping strategy is explored to suppress interference from the hydrophobic molecular ion and expand the multiplexed detection range. Then, the SiNW is downscaled to sub-10 nm with a gate-oxide-free configuration for single charge detection in liquid. We directly observe the capture and emission of a single H⁺ ion with individually activated Si dangling bonds (DBs) on the SiNW surface. This work demonstrates the unprecedented ability of the sub-10 nm SiNWFET for investigating the physics of the solid/liquid interface at single charge level.

Apart from ion sensing, the SiNWFET can be suspended and act as a nanoelectromechanical resonator aiming for electrically detecting potential quantized mechanical vibration at low temperature. A suspended SiNW based single-hole transistor (SHT) is explored as a nanoelectromechanical resonator at 20 mK. Mechanical vibration is transduced to electrical readout by the SHT, and the transduction mechanism is dominated by piezoresistive effect. A giant effective piezoresistive gauge factor (~6000) with a strong correlation to the single-hole tunneling is also estimated. This hybrid device is demonstrated as a promising system to investigate macroscopic quantum behaviors of vibration phonon modes.

Noise, including intrinsic device noise and environmental interference, is a serious concern for sensing applications of SiNW electronic devices. A H₂ annealing process is explored to repair the SiNW surface defects and thus reduce the intrinsic noise by one order of magnitude. To suppress the external interference, lateral bipolar junction transistors (LBJTs) are fabricated on SOI substrate for local signal amplification of the SiNW sensors. Current gain and overall signal-to-noise ratio of the LBJTs are also optimized with an appropriate substrate voltage.

Abstract

Nanopores have emerged as a special class of single-molecule analytical tool that offers immense potential for sensing and characterizing biomolecules such as nucleic acids and proteins. As an alternative to biological nanopores, solid-state nanopores present remarkable versatility due to their wide-range tunability in pore geometry and dimension as well as their excellent mechanical robustness and stability. However, being intrinsically incompatible with biomolecules, surfaces of inorganic solids need be modified to provide desired functionalities for real-life sensing purposes. In this thesis, we presented an exploration of various surface engineering strategies and an examination of several surface associated phenomena pertaining specifically to solid-state nanopores. Based on the parallel sensing concept using arrayed pores, optical readout is mainly employed throughout the whole study.

For the surface engineering aspect, a list of approaches was explored. A versatile surface patterning strategy for immobilization of biomolecules was developed based on selective poly(vinylphosphonic acid) passivation and electron beam induced deposition technique. This scheme was then implemented on nanopore arrays for nanoparticle localization. In addition, vesicle rupture-based lipid bilayer coating was adapted to truncated-pyramidal nanopores, which was shown to be effective for the minimizing DNA-pore interaction. Further, HfO₂ coating by means of atomic layer deposition was employed to prevent the erosion of Si-based pores and to shrink the pore diameter, which enabled reliable investigations of DNA clogging and DNA polymerase docking.

For the surface associated phenomena, several findings were made. The lipid bilayer formation on truncated pyramidal nanopores via instantaneous rupture of individual vesicles was quantified based on combined ionic current monitoring and optical observation.  The probability of pore clogging appeared to linearly increase with the length of DNA strands and applied bias voltage, which could be attributed a higher probability of knotting and/or folding of longer DNA strands and more frequent translocation events at higher voltage. A free-energy based analytical model was proposed to evaluate the DNA-pore interaction and to interpret observed clogging behavior. Finally, docking of DNA polymerase on nanopore arrays was demonstrated using label-free optical method based on Ca²⁺ indicator dyes, which may open the avenue to sequencing-by-synthesis enabled by the docked polymerase.

Abstract

The demand for wireless data communications is rapidly increasing due to several factors including increased internet access, increasingly growing number of mobile users and services, implementation of the Internet of Things (IoT), high-definition (HD) video streaming and video calling. To meet the bandwidth requirement of new and emerging applications, it is necessary to move from the existing microwave bands towards millimeter-wave bands. 

This thesis presents different antenna arrays at 60 GHz and 28 GHz that are integrated with the front-end RFIC to steer the beam in ≈ ±50° in the azimuth plane. The 5G antenna arrays at 28 GHz are designed to provide broadband high data rate services to the end users. In order to transport this high-volume data to the core network, a fixed wireless access (FWA) link demands the implementation of a broadband, high gain and steerable narrow-beam array. The 60 GHz antenna arrays, presented in this thesis, are good candidates for both FWA as well as backhaul communications. The two proposed arrays at 60 GHz (57-66 GHz) are i) a stacked patches array and ii) a connected slots array feeding a high gain lens antenna. The 2×16 stacked patches antenna array shows more than 20 dBi realized gain. The array is integrated with the front-end RFIC and the resulting module shows > 40 dBm measured effective isotropic radiated power (EIRP). The other 60 GHz antenna array is designed as linear connected slots with sixteen equidistant feeding points. The latest is then used as a feeder of a high gain dielectric lens. Peak measured gain of 25.4 dBi is achieved with this antenna.  Moreover, instead of experiencing scan loss, the lens is designed to get higher gain when the beam is steered away from the broadside direction.

Furthermore, two compact antenna arrays are designed at 28 GHz (24.25 - 29.50 GHz). A linear polarized (LP) and a circular polarized (CP) array are realized in the fan-out embedded wafer level ball-grid-array (eWLB) package. In comparison with the PCB arrays, this antenna in package (AiP) solution is not only cost-effective but it also reduces the integration losses because of shorter feed lines and no geometrical discontinuity.  The LP array is realized as a dipole antenna array feeding a novel horn-shaped heatsink.  The RF module gives 34 dBm peak EIRP with beam-steering in ±35°. Besides, the CP antenna array is realized with the help of crossed dipoles and the RF module provides 31 dBm peak EIRP with beam-steering in ±50°.

The data demands are not limited to the telecom industry as the upgradation of accelerators and experiments at the large hadron collider (LHC) at CERN will result in increased event rate thus demanding higher data rate front-end readout systems. This work thus investigates the feasibility of 60 GHz wireless links for the data readout at CERN. For this purpose, the 60 GHz wireless chips are irradiated with 17 MeV protons [dose 7.4 Mrad (RX) & 4.2 Mrad (TX)] and 200 MeV electrons [dose 270 Mrad (RX) & 314 Mrad (TX)] in different episodes. The chips have been found operational in the post-irradiation investigations with some performance degradation. The encouraging results motivate to move forward and investigate the realization of wireless links in such a complex environment.

Abstract

Nanopores are of great interest in study of DNA sequencing, protein profiling and power generation. Among them, solid-state nanopores show obvious advantages over their biological counterparts in terms of high chemical stability and reusability as well as compatibility with the existing CMOS fabrication techniques. Nanopore sensing is most frequently based on measuring ionic current through a nanopore while applying a voltage across it. When an analyte passes through the pore, the ionic current temporarily changes, providing information of the analyte such as its size, shape and surface charge. Although many magnificent reports on using solid-state nanopores have appeared in the literature, several challenges still remain for their wider applications, which include improvement of fabrication reproducibility for mass production of ultra-small nanopores and minimization of measurement instability as well as control of translocation speed and reduction of background noise. This thesis work explores different techniques to achieve robust and high throughput fabrication of sub-10 nm nanopores for different applications.

The thesis starts with presenting various fabrication techniques explored during my PhD studies. Focused ion beam method was firstly employed to drill nanopores in free-standing SiN_x membranes. Sub-10 nm nanopores could be obtained with a focused helium ion beam. But the fabrication throughput was limited with this technique. A new fabrication process combing electron beam lithography (EBL) with reactive ion etching/ion beam etching, which is compatible with the existing CMOS fabrication technology, was developed to realize a high throughput, mass production of nanopores in free-standing SiN_x membranes. However, the smallest size that could be controllably achieved with this process was around 40 nm, which is still far from sub-10 nm in size required for, e.g., DNA sequencing. Finally, by using anisotropic etching of single-crystal silicon in KOH solution, sub-5 nm truncated pyramidal nanopores were mass produced with good process controllability in a silicon-on-insulator (SOI) substrate. In addition, nanopore arrays were also successfully fabricated using a modified EBL based fabrication process.

Then, several sensing application examples using either single nanopores or nanopore arrays were investigated. Translocation of nanoparticles, DNA and proteins were demonstrated using the fabricated single nanopores or nanopore arrays in a single freestanding membrane. Moreover, the kinetics and mechanism of the lipid bilayer formation in nanopore array, aiming to prevent non-specific adsorption, were studied using ionic current measurements. In addition, individual addressability of a solid-state nanopore array on separated freestanding membranes was realized by integrating microfluidics and a customized multiplexer.

Abstract

Gold based label-free electrochemical DNA sensors have been widely studied for biomarker diagnostics. The sensitivity and reproducibility of these sensors are determined by the sensing interface: the DNA modified gold surfaces. This thesis systematically studies the preparation processes of the DNA sensor interfaces as well as their effects on the sensor performance. First, three pretreatment methods to clean the gold electrode surface and their influence on the subsequent binding of thiolated molecules were carefully investigated. As we found that the surface pretreatment method involving cyclic voltammetry (CV) in H₂SO₄ may induce structural changes to the gold surface, thus greatly impacting the thiolated molecule binding, the factors influencing this pretreatment method were studied. Practical guidelines were summarized for preparing a clean and reproducible gold surface prior to functionalization. Afterwards, the effects of the surface coverage density of probe DNA and the salt concentration on the probe-target DNA hybridization on a gold sensing surface were systematically investigated using surface plasmon resonance (SPR) analysis. Based on the SPR results, the maximum potentiometric signal that could be generated by the DNA hybridization on the surface, and the detection limits, were estimated for different experimental conditions. These estimations were further compared with experimental results obtained using silicon nanowire field effect transistors (SiNW FET) with DNA modified gold on the gate oxide. Practical limitations for the potentiometric DNA sensor were analysed and discussed. Finally, the stability and reproducibility issues on the electrochemical impedance spectroscopy (EIS) analyses of DNA hybridization were also studied on the aptamer/mercaptohexanol (MCH)-modified gold surface. The root cause for the drift problems in this type of sensor and the temperature effects on the aptamer/MCH modified surface were identified. This thesis could serve as a practical reference for the preparation and understanding of the sensing interface of gold-based electrochemical DNA sensors.

Abstract

The rise of Big Science projects brings issues related to the energy consumption and the associated environmental impacts of such large-scale facilities. Therefore, environmentally-sustainable developments are undertaken towards the adoption of energy savings and improved energy-efficient approaches. The advent of the superconducting (SC) radio frequency (RF) accelerating cavity is bringing answers to these issues. Such superconducting RF (SRF) cavity is made of niobium that allows much higher accelerating gradients with a minimization of the energy consumption. The SC RF technology is increasingly used in many modern particle accelerators, including: the European Spallation Source (ESS), the X-ray Free Electron Laser (XFEL), the Linac Coherent Light Source (LCLS)-II and the proposed International Linear Collider (ILC).

The innovation of solid state PA technology pushes limits regarding packaging, efficiency, frequency capability, thermal stability, making them more attractive than other well-established alternative technologies, such as vacuum tube technology in mid-range power applications. Through the investigations of designs and techniques, this research goal of the thesis allows to improve solid-state based power generation systems from module to the overall system design. This thesis introduces the single-ended PA design approach in planar technology and at kilowatt level. The design solution unlocks different possibilities including: improved integration, layout flexibility for tuning, and suitably for mass productions that are demanded in future high peak power generation systems. The novel amplifier design is followed by time domain characterization to fully evaluate the pulse profiles of such amplifiers when delivering kilowatt output power level for operation in conjunction with SRF accelerating cavities. Amplitude and phase stability of those amplifiers are also investigated in time-domain. The extracted data can then be used as measurement-based model for predicting factors which could degrade the overall stability of the associated PA.

Future RF power generation systems built around solid state PAs need also efficient combining strategies. Two engineering design solutions are investigated in this thesis aiming for mid- and high- range power combination. One solution is based on a combination of the Gysel structure using suspended strip-line technology for improved power handling capability. Another solution is implementing a radial combiner, which uses re-entrant cavity resonator at 352 MHz and door-nob geometry for coupling at inputs and at the output. These solutions facilitate the scaling up 400 kW for powering ESS spoke cavities while maintaining a high degree of efficiency in RF power generation. This thesis gives insights of system integration and tuning procedures with a demonstration of combining 8 modules, delivering a total of 10 kW output power. Along with the proposed combining solutions at higher power levels, the nominal power block of 10 kW is used as an elementary block to propose scaling up in power till the 400 kW nominal power required by ESS.

Finally, this thesis focuses on implementing an optimal charging scheme for SRF cavities, which helps reducing the wasted energy and improves the overall efficiency operation at future accelerating facilities. Therefore, these results contribute further to the larger adoption of solid state technologies in the future power generation systems for particle accelerators.

Abstract

In the last two decades, a significant development in the field of medical technology occurred worldwide. This development is characterized by the materialization of various body implants and worn devices, that is devices attached to the body. These devices assist doctors and paramedical staff in effectively monitoring the patient’s health and helping increase patients’ average life expectancy. Furthermore, the various implants inside the human body serve different purposes according to the humans’ needs. As this situation became more prominent, the development of protocols and of reliable transmission media is becomes essential to improve the efficiency of inter-device communications. Positive prospects of the use of human tissue for intra-body communication were proven in recent studies. Fat tissues, for example, which also work as energy banks for human beings, can be potentially used in intra-body communications as transmission media. In this thesis, the fat (adipose) tissue’s function as an intra-body communication channel was investigated. Therefore, various simulations and experimentations were performed in order to characterize the reliability of the fat tissue in terms of communication, considering, for example, the effect that the variability in the thickness of adipose and muscular tissues could have on the communication performance, and the possible effect that the variability in the transmitted signal power could have on the data packet reception. Fat tissue displays superior performance in comparison to muscle tissue in the context of a low loss communication channel. For example, at 2.45 GHz, the path losses of ~0.7 dB/cm and ~1.9 dB/cm were observed for phantom and ex-vivo measurements, respectively. At a higher frequency of 5.8 GHz, the ex-vivo path loss was around 1.4 dB/cm. It was concluded from the results that the adipose tissue could function as a reliable medium supporting intra-body communication even under low power transmitted signals. Moreover, although the presence of thick blood vessels could degrade the signal strength, the results show that communication is possible even under the presence of perturbant tissues. Overall, the results of this thesis would provide a foundation in this area and assist researchers in developing innovative and solutions for intra-body communication.

Abstract

In recent years, the use of microwave techniques for medical diagnostics has experienced impressive developments. It has demonstrated excellent competencies in various modalities such as using non-ionizing electromagnetic waves, providing non-invasive diagnoses, and having the ability to penetrate human tissues within the GHz range. However, due to anatomical, physiological, and biological variations in the human body, certain obstacles are present. Moreover, there are accuracy problems such as the absence of numerical models and experimental data, difficulty in conducting tests due to safety issues with human subjects, and also practical restrictions in clinical implementation. With the presence of these issues, a better understanding of the microwave technique is essential to further improve its medical application and to introduce alternative diagnostic methods that can detect and monitor various medical conditions in real time.

The first part of this thesis focuses on measurement systems for the microwave technique in terms of sensor design and development, numerical analysis, permittivity measurement, and phantom fabrication. The aim is to investigate the feasibility of flexible systems with different fields of application including a microwave sensor system for measuring the healing progression of bone defects present in lower extremity trauma, bone regeneration in craniotomy for craniosynostosis treatments, and dielectric variation for burn injuries. The microwave sensor which utilizes the contrast in dielectric constant between various tissues was used as the primary sensor for the proposed application. This involved detailed optimization of the sensor for greater sensitivity. The experimental work carried out in the lab environment showed that the microwave sensor was able to detect the contrast in dielectric properties so that it can give an indication of the healing status for actual clinical scenarios.

The second part of the thesis is making a significant step towards its practical implementation by establishing a system that can detect and monitor the rate of healing progression with fast data acquisition speed of microseconds, and developing an efficient user interface to convert raw microwave data into legible clinical information in terms of bone healing and burn injuries. As an extension to this thesis, clinical studies were conducted and ethical approval for conducting tests on human subjects was obtained for the development of a microwave medical system. The results showed a clear difference in healing progressions due to high detection capability in terms of dielectric properties of different human tissues. All of these contributions enable a portable system to complement existing medical applications with the aim of providing more advanced healthcare systems.

Abstract

The material supply to build renewable energy conversion systems needs to be considered from both a cost and an energy security perspective. For Cu(In,Ga)Se₂ (CIGS) thin film solar cells the use of indium in the absorber layer is most problematic. The material input per service unit can be reduced, if the absorber layers are thinned down without a loss in power conversion efficiency.

Thinning down absorber layers can increase the conversion efficiency. However, for real CIGS solar cells absorption losses and recombination rates at the rear surface between the CIGS absorber and the Mo rear contact as well as shunt-like behavior increase. Thus, both rear surface passivation and optical management are essential for maintaining high power conversion efficiencies.

In this work, thin oxide layers, so-called passivation layers, are introduced between the CIGS absorber layer and the Mo contact. They can passivate the CIGS surface, if the CIGS-oxide interface has a lower defect density than the CIGS-Mo interface and/or if they contain a negative fixed oxide charge, which increases the hole concentration and reduces the electron concentration in the CIGS in the vicinity of the oxide.

As these oxides are insulators, electrical conduction through the passivation layer has to be ensured. In this work, nanopoint contacts were etched into ALD-Al₂O₃ passivation layers in CIGS solar cells. These solar cells had 0.5 -1.5 µm thin absorber layers with a low In content and a high band gap. Ga grading was not used. Although absorber layers with a high Ga content have a short minority carrier diffusion length, a passivation effect could be discerned with the help of external quantum efficiency measurements and current-voltage measurements under varying temperatures in combination with optical and electrical modeling with a two-diode model. Moreover, the possibility of leaving out the additional fabrication step has been explored for ALD-Al₂O₃ and HfO₂ as passivation layers. The results suggest that the passivation layer does not necessarily need to be opened for electrical conduction in an additional fabrication step, if sodium fluoride (NaF) is deposited onto Al₂O₃ layers prior to CIGS evaporation. In this case solar cells with 215 nm absorber layers and 6 nm thin passivation layers have a power conversion efficiency of 8.6 %, which is 3 % (absolute) higher than the conversion efficiency on a reference. Shunt-like behavior is additionally reduced. For the HfO₂ layers photoluminescence data indicate a good passivation effect, but the layers need to be opened up to ensure conduction.

Abstract

Three-dimensional integrated circuits have great potential for further increasing the number of transistors per area by stacking several device tiers on top of each other and without the need to continue the evermore complicated and expensive down-scaling of transistor dimensions. Among the different approaches towards the realization of such circuits, the monolithic approach, i.e. the tier-by-tier fabrication on a single substrate, is the most promising one in terms of integration density. Germanium is chosen as a substrate material instead of silicon in order to take advantage of its low fabrication temperatures as well as its high carrier mobilities. In this thesis, the work on two key components for the realization of such germanium-based three-dimensional integrated circuits is presented:the source/drain contacts to germanium the interconnects.

As a potential source/drain contact material, nickel germanide is investigated.In particular, the process temperature windows for the fabrication of morphologically stable nickel germanide layers formed from initial nickel layers below 10 nm are identified and the reaction between nickel and germanium is further studied by means of in-situ x-ray diffraction. The agglomeration temperature of nickel germanide is increased by 100 °C by the addition of tantalum and tungsten interlayers and capping layers. In an effort to more thoroughly characterize the contacts, a method to reliably extract the specific contact resistivity is implemented on germanium.

As a potential interconnect material cobalt is investigated. In a first step, highly conductive cobalt thin films are demonstrated by means of high-power impulse magnetron sputtering. The high conductivity of the cobalt films is owing to big grains, high density, high purity, and smooth interfaces. In a second step, the potential of high-power impulse magnetron sputtering for the metallization of nanostructures is further explored.

Abstract

Nanopore based sensing technology has been widely studied for a broad range of applications including DNA sequencing, protein profiling, metabolite molecules, and ions detection. The nanopore technology offers an unprecedented technological solution to meeting the demands of precision medicine on rapid, in-field, and low-cost biomolecule analysis. In general, nanopores are categorized in two families: solid-state nanopore (SSNP) and biological nanopore. The former is formed in a solid-state membrane made of SiN_x, SiO₂, silicon, graphene, MoS₂, etc., while the latter represents natural protein ion-channels in cell membranes. Compared to biological pores, SSNPs are mechanically robust and their fabrication is compatible with traditional semiconductor processes, which may pave the way to their large-scale fabrication and high-density integration with standard control electronics. However, challenges remain for SSNPs, including poor stability, low repeatability, and relatively high background noise level. This thesis explores SSNPs from basic physical mechanisms to versatile applications, by entailing a balance between theory and experiment.

The thesis starts with theoretical models of nanopores. First, resistance of the open pore state is studied based on the distribution of electric field. An important concept, effective transport length, is introduced to quantify the extent of the high field region. Based on this conductance model, the nanopores size of various geometrical shapes can be extracted from a simple resistance measurement. Second, the physical causality of ionic current rectification of geometrically asymmetrical nanopores is unveiled. Third, the origin of low-frequency noise is identified. The contribution of each noise component at different conditions is compared. Forth, a simple nano-disk model is used to describe the blockage of ionic current caused by DNA translocation. The signal and noise properties are analyzed at system level.

Then, nanopore sensing experiments are implemented on cylinder SiN_x nanopores and truncated-pyramid silicon nanopores (TPP). Prior to a systematic study, a low noise electrical characterization platform for nanopore devices is established. Signal acquisition guidelines and data processing flow are standardized. The effects of electroosmotic vortex in TPP on protein translocation dynamics are excavated. The autogenic translocation of DNA and proteins driven by the pW-level power generated by an electrolyte concentration gradient is demonstrated. Furthermore, by extending to a multiple pore system, the group translocation behavior of nanoparticles is studied. Various application scenarios, different analyte categories and divergent device structures accompanying with flexible configurations clearly point to the tremendous potential of SSNPs as a versatile sensor.

Abstract

This thesis work explores electrical conductors based on few-layer graphene flakes as an enabler for low-cost, mechanically flexible, and high-conductivity conductors in large area flexible and printed electronic devices. The flakes are deposited from aqueous solutions and processed at low temperature.

Graphene is selected for its excellent properties in mechanical, optical, electronic, and electrical aspects. However, thin films of pristine few-layer graphene flakes deposited from dispersions normally exhibit inferior electrical conductivity. One cause responsible for this problem is the loose stacking and random orientation of graphene flakes in a graphene deposition. We have solved this problem by implementing a simple post-deposition treatment leading to dramatically densified and planarized thin films. Significantly increased electrical conductivity by ~20 times is obtained. The 1-pyrenebutyric acid tetrabutylammonium salt as an exfoliation enhancer and dispersant in water yields ~110 S/m in conductivity when the graphene based thin films are processed at 90 °C. In order to achieve higher conductivity, a room-temperature method for site-selective copper electroless deposition has been developed. This method is of particular interest for the self-aligned copper deposition to the predefined graphene films. The resultant two-layer graphene/copper structure is characterized by an overall conductivity of ~7.9 × 10⁵ S/m, an increase by ~7000 times from the template graphene films. Several electronic circuits based on the graphene/copper bilayer interconnect have been subsequently fabricated on plastic foils as proof-of-concept demonstrators. Alternatively, highly conductive composites featuring graphene flakes coated with silver nanoparticles with electrical conductivity beyond 10⁶ S/m can be readily obtained at 100 ^oC. Moreover, a highly conductive reduced-graphene-oxide/copper hybrid hydrogel has been achieved by mixing aqueous graphene oxide solution and copper-containing Fehling's solution. The corresponding aerogel of high porosity exhibits an apparent electrical conductivity of ~430 S/m and delivers a specific capacity of ~453 mAh g⁻¹ at current density of 1 A/g. The experimental results presented in this thesis show that the solution-phase, low-temperature fabrication of highly conductive graphene-based materials holds promises for flexible electronics and energy storage applications. 

Abstract

In the past decades, silicon nanowire field-effect transistors (SiNWFETs) have been explored for label-free, highly sensitive, and real-time detections of chemical and biological species. The SiNWFETs are anticipated for sensing analyte at ultralow concentrations, even at single-molecule level, owing to their significantly improved charge sensitivity over large-area FETs. In a SiNWFET sensor, a change in electrical potential associated with biomolecular interactions in close proximity to the SiNW gate terminal can effectively control the underlying channel and modulate the drain-to-source current (I_DS) of the SiNWFET. A readout signal is therefore generated. This signal is primarily determined by the surface properties of the sensing layer on the gate terminal, with sensitivity close up to the Nernstian limit widely demonstrated. To achieve a high signal-to-noise ratio (SNR), it is essential for the SiNWFETs to possess low noise of which intrinsic device noise is one of the major components. In metal-oxide-semiconductor (MOS)-type FETs, the intrinsic noise mainly results from carrier trapping/detrapping at the gate oxide/semiconductor interface and it is inversely proportional to the device area.

This thesis presents a comprehensive study on design, fabrication, and noise reduction of SiNWFET-based sensors on silicon-on-oxide (SOI) substrate. A novel Schottky junction gated SiNWFET (SJGFET) is designed and experimentally demonstrated for low noise applications. Firstly, a robust process employing photo- and electron-beam mixed-lithography was developed to reliably produce sub-10 nm SiNW structures for SiNWFET fabrication. For a proof-of-concept demonstration, MOS-type SiNWFET sensors were fabricated and applied for multiplexed ion detection using ionophore-doped mixed-matrix membranes as sensing layers. To address the fundamental noise issue of the MOS-type SiNWFETs, SJGFETs were fabricated with a Schottky (PtSi/silicon) junction gate on the top surface of the SiNW channel, replacing the noisy gate oxide/silicon interface in the MOS-type SiNWFETs. The resultant SJGFETs exhibited a close-to-ideal gate coupling efficiency (60 mV/dec) and significantly reduced device noise compared to reference MOS-type SiNWFETs. Further optimization was performed by implementing a three-dimensional Schottky junction gate wrapping both top surface and two sidewalls of the SiNW channel. The tri-gate SJGFETs with optimized geometry exhibited significantly enhanced electrostatic control over the channel, thereby confined I_DS in the SiNW bulk, which greatly improved the device noise immunity to the traps at bottom buried oxide/silicon interface. Finally, a lateral bipolar junction transistor (LBJT) was also designed and fabricated on a SOI substrate aiming for immediate sensor current amplification. Integrating SJGFETs with LBJTs is expected to significantly suppress environmental interference and improve the overall SNR especially under low sensor current situations.

Abstract

The outstanding properties of graphene make it attractive ink filler for conductive inks which plays an important role in printed electronics. This thesis focuses on the ink formulation based on graphene and graphene oxide (GO).

Liquid phase exfoliation of graphite is employed to prepare graphene dispersions, i.e., shear- and electrochemical exfoliation. High concentration graphene dispersions with small size, few-layer graphene platelets are obtained by both methods. With the addition of ethyl cellulose stabilizer, shear-exfoliated graphene platelets in NMP were successfully inkjet printed on different substrates. The printed graphene film with electrical conductivity of ~3^10⁴ S/m was obtained after annealing at 350 °C for one hour. Alternatively, the electrochemically exfoliated graphene nano-platelets were collected and redispersed in DMF to form inks. The printed film of conductivity ~2.5^10³ S/m was obtained after annealing at 300 °C for one hour.

Water based GO/Ag hybrid inks were developed for screen printing. When high concentration GO aqueous dispersion was mixed with reactive silver ink, the viscosity of the mixture increased instantly to above 1000 cP as a result of reactions between oxygen functional groups (OFGs) on GO sheets and ingredients in the reactive silver ink. When the screen printed lines with different GO:Ag ratios were annealed in air, the conductivity of the resultant reduced graphene oxide/silver nanoparticles (RGO/AgNP_X) composites decreased as silver content increased. As oxygen enriched compounds in RGO/AgNP_X composites were detected, we proposed that AgOx compounds were generated on the AgNPs surface, which raised the contact resistance between AgNPs and RGO flakes. To solve this problem, the printed patterns were instead annealed in reducing gas (Ar/H₂ 5%). The electrical conductivity ~2.0^10⁴ S/m was then achieved.

Furthermore, the reduction of GO using ammonium formate as reducing reagent was investigated. When applying a hydrothermal method, ammonium formate shows excellent reduction ability, surpassing the widely used reducing agent, L-ascorbic acid, under same condition. Elemental analysis shows the C/O ratio of RGO as high as ~11 and most OFGs were removed in the reduction process. Meanwhile, incorporated nitrogen atoms introduced active sites in resultant RGO, making it a promising electrocatalyst for oxygen reduction reaction.

Abstract

The aim of this thesis is to understand the influence of the deposition process and resulting film properties on Cu₂ZnSnS₄ (CZTS) thin film solar cells. Two main aspects are studied, namely formation of absorber precursors by sputtering, and alternative Cd-free buffer materials with improved band alignment.

Reactive sputtering is used to grow dense and homogeneous precursor films containing all elements needed for CZTS absorbers. The addition of H₂S gas to the inert Ar sputter atmosphere leads to a drastic decrease of Zn-deposition rate due to the sulfurization of the target. Sulfurization also leads to instabilities for targets made of CuSn, Cu and Cu₂S, while sputtering from CuS gave acceptable process stability.

The H₂S/Ar-ratio also affects film morphology and composition. Precursors with sulfur content close to stoichiometric CZTS have a columnar, crystalline structure. Materials analysis suggests a non-equilibrium phase with a cubic structure, where each S atom is randomly surrounded by 2:1:1 Cu:Zn:Sn-atoms, respectively. Substrate heating during sputtering is shown to be important to avoid cracks in the annealed films while stress in the precursor films is not observed to affect the absorber or solar cell quality.

Sputtering from compound targets in Ar-atmosphere yields precursor properties similar to those from reactive sputtering at high H₂S/Ar-ratios and both types can be processed into well-performing solar cells.

Additionally, a low temperature treatment of CZTS absorbers in inert atmosphere prior to buffer layer growth is shown to affect the device properties, which indicates that the thermal history of the CZTS absorber is important.

The alternative buffer system ZnO_1-xS_x is found to yield lower efficiencies than expected, possibly due to inferior interface or buffer quality. The Zn_1-xSn_xO_y (ZTO) buffers instead give better performance than their CdS references. For optimized parameters, the activation energy for recombination coincides with the energy of the photoluminescence peak of the absorber. This can be interpreted as a shift of dominant recombination path from the interface to the CZTS bulk. A well-performing CZTS-ZTO device with antireflective coating yielded an efficiency of 9.0 %, which at the time of publication was the highest value published for a Cd-free pure-sulfide CZTS solar cell.

Abstract

The CIS solar cell family features both a high stability and world-class performances. They can be deposited on a wide variety of substrates and absorb the entire solar spectrum only using a thickness of a few micrometers. These particularities allow them to feature the most positive Energy returned on energy invested (EROI) values and the shortest Energy payback times (EPBT) of all the main photovoltaic solar cells. Using mainly electron microscopy characterization techniques, this thesis has explored the questions related to the interface control in thin-film photovoltaic solar cells based on CuInSe₂ (CIS) absorber materials. Indeed, a better understanding of the interfaces is essential to further improve the solar cell conversion efficiency (currently around 23%), but also to introduce alternative substrates, to implement various alloying (Ga-CIS (CIGS), Ag-CIGS (ACIGS)…) or even to assess alternative buffer layers.

The thread of this work is the understanding and the improvement of the interface control. To do so, the passivation potential of Al₂O₃ interlayers has been studied in one part of the thesis. While positive changes were generally measured, a subsequent analysis has revealed that a detrimental interaction could occur between the NaF precursor layer and the rear Al₂O₃ passivation layer. Still within the passivation research field, incorporation of various alkali-metals to the CIS absorber layer has been developed and analyzed. Large beneficial effects were ordinarily reported. However, similar KF-post deposition treatments were shown to be potentially detrimental for the silver-alloyed CIGS absorber layer. Finally, part of this work dealt with the limitations of the thin-barrier layers usually employed when using steel substrates instead of soda-lime glass ones. The defects and their origin could have been related to the steel manufacturing process, which offered solutions to erase them.

Electron microscopy, especially Transmission electron microscopy (TEM), was essential to scrutinize the local changes occurring at the different interfaces within a few nanometers. The composition variation was measured with both Electron energy loss spectroscopy (EELS) and Energy dispersive X-ray spectroscopy (EDS) techniques. Finally, efforts have been invested in controlling and improving the FIB sample preparation, which was required for the TEM observations in our case.