Tyndall National Institute - Doctoral Theses

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    Multiferroic investigations of Aurivillius phase thin films
    (University College Cork, 2023) Colfer, Louise; Keeney, Lynette; Long, Brenda; Royal Society; Science Foundation Ireland
    In recent years, the amount of data being created and processed is growing at a much faster rate than the rate of computational storage technology development. With CMOS technologies reaching their miniaturisation limits, new disruptive materials are needed to increase data storage capabilities. Technological road-maps have identified room temperature, non-volatile magnetoelectric multiferroic materials as promising candidates for memory scaling within future memory storage devices. Although multiferroic memory devices have the potential to revolutionise memory storage technologies, commercial devices successfully utilising multiferroics have not yet come to fruition. The focus of this thesis is to understand and optimise a rare example of a room temperature magnetoelectric multiferroic, Bi6TixFeyMnzO18 (B6TFMO; x = 2.80 to 3.04; Y = 1.32 to 1.52; Z = 0.54 to 0.64). Aurivillius phase materials, (Bi2O2)(An−1BnO3n+1), where ferroelectric perovskite units are interleaved between dielectric [Bi2O2]2+ layers, are flexible scaffolds for technological applications. While earlier studies indicated that B6TFMO is a promising material for future memory devices, my thesis presents significant advances in the characterisation, understanding and optimisation required towards implementing the material in fully realised devices. In this work, correlation between the octahedral tilting and atomic-level structural distortions with functional electronic and magnetic properties of B6TFMO were determined, revealing that crystal field splitting of the Ti4+ octahedra is influenced by its position within the Aurivillius unit cell. Theoretical calculations determined that this is predominantly driven by changes in the extent of tetragonal distortion along the c-direction. Atomic scale mapping of polar displacements reveals this has a direct impact on the ferroelectric properties. Polarisation is largest towards the outer perovskite cells, correlating with an increased extent of local tetragonal distortion of octahedral geometries. Experiments demonstrate that tilting of the BO6 octahedra competes with the extent of tetragonal distortion of the TiO6 octahedra, where the degree of octahedral tilting increases towards the central layers of this Aurivillius system, where the magnetic cations preferentially partition. This work presents the first indication that octahedral tilting might be an important enabler of long-range magnetic interactions and subsequent multiferroic behaviour in B6TFMO. Delving deeper into fundamental understandings of B6TFMO’s antipolar and magnetic behaviour, the purposeful inclusion of structural defects within the layered structure of B6TFMO and how they can impart elastic strain and electrostatic energy changes which in turn influence polar behaviour is explored. The findings show that the vicinal sapphire substrates (mis-cut angle 0.2 o to 10 o) are successful for promoting the propagation of sub-unit-cell defects and disruptions to the periodicity of the Aurivillius phases. This has a marked effect on the film morphology and ferroelectric properties. Macroscopic and local measurements show that defect, crystal grain and ferroelectric domain density increases with increasing substrate mis-cut angle. Atomic resolution polarisation mapping showed that charged domain walls alongside exotic polar vortices are facilitated by OPBs when two OPB defects are spaced 5 nm apart. This work provides insight into methods for successfully controlling defect levels and how polar vortex domain walls and charged domain walls are promoted within layered multiferroics by tailoring the underlying substrate that the film is grown on. Moving on from vicinal sapphire surfaces, patterned sapphire with 3D domes were used to encourage the growth of the Aurivillius grains towards an upright geometry. An increased number of non-(00l) reflections were present in the B6TFMO films on patterned sapphire along with evidence from STEM imaging showing that B6TFMO grains grow along the incline of the patterned sapphire domes. With the growth of the crystal grains towards an upright geometry it would be expected that access to the major a-axis polarisation via out-of-plane measurement would be improved, however with a maximum inclination angle of 60 ° achieved with the 3D dome architectures, the out-of-plane piezoresponse of the samples remained weaker than the in-plane piezoresponse. Studies of the magnetic properties of the films demonstrated that the B6TFMO samples were ferromagnetic at room temperature. These findings provide further evidence of room temperature multiferroic behaviour in B6TFMO. Lastly, the role of bismuth excess and substrate strain were investigated to optimise the epitaxial growth of B6TFMO via DLI-CVD. A single-step deposition method on epitaxial substrates was developed to allow the successful synthesis of continuous 45 nm thick B6TFMO films at thicknesses relevant to applications as piezoelectric actuators, sensors and energy harvesters. These films nucleated via a layer-by-layer growth mode and were found to have a strong in-plane ferroelectric response with isotropic domains. Film purity was enhanced with utilisation of epitaxial substrate with appropriate lattice match to B6TFMO and by optimising the amount of bismuth precursor used. In this work, progress was made towards the optimisation of epitaxially grown B6TFMO films, allowing greater control of film orientation and augmenting strain-induced enhancement of multiferroic properties in future data storage devices. Overall, this research has increased understanding of the fundamental mechanisms governing the ferroelectric and ferromagnetic properties of B6TFMO. The work has elucidated some of the key requirements fundamental to the manifestation of polar topologies and has created strategies for the tailoring of novel polar topologies. This combination of new material understanding and new growth optimisation of room temperature multiferroics contributes to solving the ‘big data’ problem. Application of B6TFMO in future technologies based on ultra-high density, energy efficient memory devices, spintronic devices, multilevel resistance control (memristive and synaptic devices) and energy-efficient neuromorphic “brain inspired” devices are envisioned.
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    Toward single-growth monolithically integrated electro-absorption modulated lasers
    (University College Cork, 2023) Mulcahy, Jack; Peters, Frank H.; Corbett, Brian; Science Foundation Ireland; Rockley Photonics
    Every year the demand for bandwidth is growing exponentially due to the emergence of data-intensive services such as high-definition video streaming, cloud-based computing, and machine-to-machine communication. This rapid expansion is primarily driven by the extensive deployment of fibre-based optical communication networks. Consequently, there is an increasing need for photonic components to meet the requirements of these networks, which are expanding both in geographical coverage and terminal density. To satisfy this demand, the photonics industry must enhance its production capabilities and adopt more efficient fabrication processes. A crucial aspect of streamlining fabrication involves eliminating slow and costly processes. In photonics fabrication, epitaxial regrowth and advanced lithography steps are typically time-consuming and expensive, making them prime targets for process optimisation. Moreover, the integrated electronics approach provides valuable insights by enabling the monolithic integration of multiple photonic components fabricated simultaneously. This integration technique allows for the creation of highly complex circuits while reducing overall fabrication complexity. This research focuses on a key component at the heart of photonic circuits: the tunable single-mode laser. The aim is to contribute to the development of components that can be fabricated without the need for regrowth or advanced lithography. Additionally, the study emphasises the importance of monolithic integration, specifically with electro-absorption modulators (EAMs). By integrating EAMs with tunable lasers, the resulting devices can offer enhanced functionality and performance, leading to more efficient and compact photonic systems. The issue at hand, however is the varied epitaxial requirements of lasers and EAMs, which provides a noted barrier to a monolithic, regrowth-free integration process. This thesis aims to advance the development of single-growth monolithically integrated externally modulated lasers (EMLs) based on electro-absorption modulators (EAMs). The design of quantum well structures is explored, revealing the significance of introducing an imbalance in the position of the quantum wells to optimise the transit times of carriers in EAMs, thus maximising the bandwidth. Simulation studies on epitaxial structures led to the identification of an optimal material that balances the performance of lasers and EAMs, providing an ideal platform for EML fabrication. Different laser designs are investigated, including slotted Fabry P\'erot lasers and snails, with a focus on achieving a redshifted single-mode laser. Simulation models are developed to predict laser reflectivity and spectral output, which were verified through fabrication and testing. The optimal laser design for integrated EMLs was determined through critical evaluation with a laser being produced with $>$ \SI{40}{dB} SMSR and a tuning range of \SI{60}{nm}. A high-speed process for fabricating EAMs is developed, featuring optimised lithographic mask layers for the isolation of contact pads and metal bridges to reduce parasitic capacitance. The resulting EAMs exhibited a predicted bandwidth of approximately \SI{80}{GHz}. Drawing upon the knowledge gained from laser and EAM simulation, fabrication, and characterisation, a new high-speed process for EMLs is devised. The o-band lasers and EAMs were designed based on optimal principles determined in previous chapters. The fabricated single-mode lasers were successfully matched to simulated models. Further analysis identified potential avenues for improving future EML fabrication yields. In summary, this thesis provides valuable insights and tools for the creation of single-growth monolithically integrated electro-absorption modulated lasers. The journey spans from material design to device outputs, with the aim of enabling readers to replicate and enhance the development of EMLs.
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    Macroporous metal oxide battery electrode performance analysis and operando spectroscopy
    (University College Cork, 2023) Grant, Alex; O'Dwyer, Colm; Irish Research Council for Science, Engineering and Technology
    Vast consumption of fossil fuels has catapulted greenhouse gas emissions which have contributed to increasing the global average temperature at a frightening rate. Thankfully, sun, wind and tides have massive potential to provide energy in an environmentally friendly way. However, maintaining these energy sources at the global scale required to realise the transition from fossil fuels to renewable energy is difficult, given that their supply is often location and weather dependent. Fortunately, there is a solution, and energy storage is the key to a green energy future. Currently, battery systems are the most suitable option. These systems can be implemented in large-scale grid storage facilities, electric vehicles (EVs), portable electronics like laptops and mobile phones, right down to fingernail-sized microbatteries for medical devices and hearing aids. Despite decades of research, the demand for increased functionality has grown at such a rate that the development of appropriate technology has proven difficult. Additionally, the materials typically used in production of commercial batteries such as cobalt, graphite and Li are coupled with concerns related to ethics, CO2 emissions, and availability. This thesis delves into three realms, all related to the development and optimisation of battery electrode materials, and efficient approaches to monitor their deterioration. The first realm is optical analysis of opal photonic crystals (PhCs) which are used as templates for battery electrodes. PhCs are a particular type of nanostructure which demonstrate iridescent colours when exposed to light. The observed colours originate from a phenomenon known as the photonic band gap (PBG), the band of frequencies which are reflected from the crystal. The PBG is dependent on the structure and refractive index contrast of the material. Consequently, the PBG can be controlled by changing the structural parameters of the crystal. Opals are a type of PhC which are used as templates for inverse opals (IOs), which have shown promise as battery electrodes due to their high surface area to volume ratio, thin pore walls and interconnectivity, which removes the need for conductive additives and binders. The most fascinating aspect of their potential is the correlation between their structural properties and their optical response since the primary route of battery performance deterioration is through the structural degradation of the battery electrode materials. Opals of different thicknesses can be constructed while the reflected colours remain unchanged once the periodicity of the crystal is maintained. However, the intensities of the reflected frequencies are thickness dependent. The optical response of these materials can therefore be used to determine crystal thickness, and vice versa. Optimising the electrochemical performance of IO (IO) battery electrodes requires formation of an excellent opal template which can be directed and evaluated by this non-destructive optical spectroscopy approach. With a method established for perfecting opal quality established, the second realm explores the electrochemical performance of sustainable battery systems, in the form of SnO2-based electrode materials in lithium-ion (Li-ion)and sodium-ion (Na-ion) chemistries. Li-ion is the most widely used commercial battery chemistry. As Li reserves are depleted, alternatives must be considered. Na stands as a practically infinite resource, extracted easily from sea salt. Na-ion batteries operate under similar principles to Li-ion, but optimisation of appropriate electrode materials has proved challenging. Graphite stands as the gold standard anode material for Li-ion, but its capacity is dramatically reduced in Na-ion systems. Furthermore, the carbon emissions from the processing of natural graphite and production of synthetic graphite are worrying. SnO2 is one of the thirty most abundant elements on earth and stands as a promising replacement for graphite in Na-ion batteries. However, massive volume expansion prevents its widespread adoption. Routes to overcome this volume expansion involve the design of state-of-the-art nanostructures, the route taken in this thesis using the IO architecture. While this electrochemical exploration provides an explanation of the underlying chemistry governing the rate behaviour and stability of the IO electrodes, the structural changes which arise in the electrode during cycling prompt deeper investigations. Furthermore, the rapid nature of the associated electrochemical processes prompt real-time measurements, which are addressed next in the thesis in the form of operando Raman spectroscopy. The third realm is the application of operando methods to battery performance analysis, an approach which litters the literature in recent years. Traditionally, analysis of battery electrodes and the electrolyte were performed ex-situ. The battery was disassembled, the materials were removed and analysed. Ex-situ analysis is useful prior to cycling and for postmortem analysis. A more effective analytical approach is the use of in-situ techniques. These involve stopping the battery at a certain voltage, disassembly, and analysing the electrodes and electrolyte in that state. This approach is useful for determining what changes occur to the battery at different states of charge. However, these materials are often sensitive to the environment, and the in-situ results are not representative of the state of the material prior to disassembly. Furthermore, electrochemical reactions can occur over nanoseconds or even less, while the structural changes can be short-lived. As a result, in-situ analysis can fail to capture many of the changes which occur during cycling. Operando techniques involve analysis of battery components during electrochemical cycling. Disassembly is not required, and structural changes can be tracked continuously, capturing any metastable and ultrashort phases. Operando Raman spectroscopy can be used to monitor structural changes to the electrode at the atomic scale, along with evolution of the electrolyte non-destructively. This thesis has been written to showcase the benefits of combining optical, structural, and electrochemical analysis, moving from ex-situ to operando techniques, and optimising performance of sustainable battery technologies with a view to transitioning from Li-ion to Na-ion chemistry with environmentally friendly electrode materials to develop the first PhC, lab-based operando battery diagnostics system.
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    Theory of radiative and nonradiative recombination processes in nitride-based heterostructures
    (University College Cork, 2023) McMahon, Joshua M.; Schulz, Stefan; Science Foundation Ireland; Sustainable Energy Authority of Ireland
    Most modern blue-violet short wavelength visible light-emitting diodes (LEDs) incorporate group III-nitride (III-N) semiconductor quantum wells (QWs), and ultraviolet (UV) LEDs could be developed using similar materials and heterostructures. Relative to filament and fluorescent bulb technologies, modern blue-violet short wavelength emitting III-N QW-based devices are significantly more efficient. However, they suffer from efficiency ``droop'' effects, the fundamental causes of which are under debate. For example, the efficiency of III-N LEDs drops with increasing drive current, and thus increased carrier density in the well, an effect known as ``current droop''. Furthermore, even at fixed drive current, when the temperature of the device rises, there is a corresponding drop in device efficiency, the so-called ``thermal droop''. On top of current and temperature dependent droop, III-N QW-based light emitting devices also suffer significant reductions in efficiency at longer (green to red) and shorter (deep UV) wavelengths. The reduction in efficiency as III-N devices are engineered to emit at longer wavelengths is a major contributor to the so-called ``green gap'' phenomenon, wherein neither III-N nor other III-V heterostructure-based devices (such as those based on phosphides or arsenides emitting in the red to infrared spectral range) can be designed to emit efficiently in the green to yellow spectral range. Input from theory is important for understanding the fundamental origins of these droop phenomena, as well as to guide device design to improve the efficiency of not only blue-violet visible wavelength emitters that suffer from droop, but also devices emitting at the shorter and longer wavelengths mentioned above. However, theoretical results obtained for other semiconductor structures (such as those based on II-VI and other III-V heterostructures) cannot be carried over to III-N systems due to significant differences in the fundamental material properties. For instance, alloy disorder causes strong carrier localisation in III-N semiconductor alloys, which can significantly alter the electronic and optical properties of III-N heterostructures. Although experimental data indicates the importance of such effects, only very recently have they been accounted for in theoretical studies, as they present a significant modelling challenge. The aim of this thesis is to address this challenge using atomistic modelling. Auger recombination has been discussed in the literature as an important nonradiative process in c-plane InGaN/GaN QWs in which an electron recombines with a hole, but instead of a photon being emitted (as in radiative recombination) another carrier is excited. If the rate of Auger recombination grows faster than the rate of radiative recombination as temperature or carrier density increases, overall, there will be a negative impact on device performance. For instance, there is much evidence that Auger recombination plays a significant role in carrier density dependent droop, but the exact nature of the Auger process underlying this droop phenomenon is still under debate, and several Auger recombination mechanisms have been suggested, including a defect-assisted process as well as alloy disorder enhanced Auger recombination. Auger recombination has also been explored as a possible cause of thermal droop but there is still much debate over its relevance to this droop phenomenon. Despite experimentally established links between Auger recombination and these droop phenomena, on the theoretical side the impact of alloy disorder on the Auger recombination process in III-N QW systems is widely unexplored. Our theoretical framework is based on a nearest neighbour sp3 tight-binding model, which takes input from a valence force field model to determine the equilibrium lattice positions of the alloy disordered QW structures studied. On top of this, local variations in strain and polarisation fields are accounted for in the framework, along with polarisation field screening effects at high carrier densities in the well. The tight-binding energies and wave functions are then used to calculate the radiative and Auger recombination rates. For InGaN/GaN QW systems, our predicted values for radiative and Auger recombination rates lie within the wide range of experimentally reported values, and confirm that the coefficient of Auger recombination is not as small as one may expect from the expected dependence of Auger coefficient on band gap. To study the thermal droop, we have evaluated the radiative and Auger recombination rates at a fixed carrier density but as a function of temperature. Our results reflect the unconventional but experimentally observed increase of the radiative recombination rate with increasing temperature. When focusing on the competition between radiative and alloy-enhanced Auger effects, neglecting, e.g., defect-related processes such as Shockley-Read-Hall recombination, our results indicate an improvement of device performance with increasing temperature, in contrast to experimentally determined efficiency data. Thus, we expect that alloy-enhanced Auger recombination, intrinsic to InGaN-based QWs, is not responsible for thermal droop. As a result, efficiency improvement strategies that target, for instance, factors extrinsic to the well, such as reducing defect densities, should be considered. Turning to the carrier density dependent droop (i.e. the current droop), the competition between radiative and Auger rates as a function of carrier density (but at fixed temperatures) was determined. In InGaN/GaN QW systems, already at low carrier densities our model predicts Auger recombination rates large enough to be considered significant contributors to this droop phenomenon (based on expectations from the literature). When investigating the carrier density dependence of the recombination rates, we find that the Auger rate grows faster than the radiative rate as carrier density increases in the c-plane InGaN/GaN QWs studied here. Working within a commonly used model of efficiency in LEDs, we compared the theoretically determined carrier density dependent efficiency data to experimental results from collaborators at the Universities of Manchester and Cambridge for samples with varying defect densities. We found a good agreement between theory and experiment at carrier densities where this droop phenomenon is observed; the temperature is kept constant in the experiments and calculations. Furthermore, the drop in efficiency with increasing carrier density was essentially independent of sample, and thus independent of defect density. Overall, these results indicate that defect-assisted Auger processes may be of secondary importance, and alloy-enhanced Auger recombination can be sufficient to explain the current droop. Moreover, despite the large Auger coefficients, the green emitting samples studied in our theory experiment comparison were found to have relatively high internal quantum efficiencies, suggesting that Auger recombination may not be limiting the performance of longer wavelength visible light-emitting devices. Lastly, UV LEDs based on AlGaN QW systems have attracted significant attention in recent years for the potential development of more efficient and environmentally friendly UV emitters. Despite this, and despite the experimental observation of both thermal and current droop in AlGaN-based QWs, Auger recombination and carrier localisation have also been widely unexplored in these systems, particularly from a theoretical perspective. Thus, we applied our theoretical framework to determine the radiative and Auger recombination rates at a fixed carrier density but as a function of temperature in c-plane AlGaN/AlN QWs, thus providing insight into the thermal droop of such emitters. We found Auger recombination rates on the same order of magnitude as those determined for the InGaN/GaN QWs studied above. Based on our results, we expect that Auger recombination is not the driving cause of thermal droop in AlGaN/AlN QWs. However, based on expectations from the literature, the values calculated here suggest that Auger recombination could be a significant contributor to current droop in AlGaN QWs. Overall, our calculations show that alloy-enhanced Auger recombination is indeed strongly contributing to nonradiative recombination processes in III-N heterostructures. However, the thermal droop does not seem to be driven by this intrinsic nonradiative process. Instead, other factors, such as defect-assisted Auger recombination or carrier injection deficiencies, may be responsible. Thus, device optimisation strategies that target these extrinsic effects may still improve efficiency. With regard to the current density dependent droop, our calculations indicate that alloy-enhanced Auger recombination plays a significant role. Efforts to mitigate the impact of alloy-enhanced Auger recombination on the internal quantum efficiency could consider targeting larger active regions to reduce current density, or improved current spreading through the multiple QWs employed in LEDs. Although alloy-enhanced Auger recombination may present an intrinsic roadblock to reducing current droop, our results suggest that the reduction in efficiency of longer wavelength III-N devices emitting in the visible range (i.e. green) is not driven by this Auger process. Thus, device performance optimisation may still be achievable by targeting extrinsic factors such as carrier injection efficiency and homogeneity.
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    Monolithically integrated, high coherence frequency comb generation through on-chip gain switching
    (University College Cork, 2023) McCarthy, John T.; Peters, Frank H.; Corbett, Brian; Kelleher, Bryan; Science Foundation Ireland
    As the number of internet users continuous to increase, methods of developing new forms of communication networks are being widely considered. Optical frequency comb sources have the potential to reduce or eliminate the spectrally inefficient guard bands that are currently used to prevent cross-talk between adjacent channels. With their common laser source and fixed phase relation, frequency combs can offer to not only to replace the hundreds of lasers being used to generate hundreds of channels, but also reduce the present day separation between channels. This thesis demonstrates on-chip frequency combs that are generated through gain switching. Simulation analysis is carried out to investigate the effects of experimental parameters on the quality of gain switched combs and extensive experimental analysis is carried out to examine the experimental conditions required to enhance the quality of these combs. Typically a two laser design is used where a gain switched Fabry–Pérot laser is phase locked to a single mode laser creating a primary-secondary configuration. Different coupling techniques were investigated and developed, with stable combs being generated as a result of bidirectional coupling, and greater comb enhancement being demonstrated using mutually coupled techniques. Utilising both the combs generated through mutual coupling and the knowledge of on-chip, stable, bidirectionally coupled combs, the conditions required to generate an enhanced comb with additional comb lines are developed. Methods of on-chip comb line filtering are demonstrated for the purpose of future de-multiplexing systems. Finally, a new method of frequency comb generation through on-chip gain switching, without the previously required additional optical injection, is analysed and developed. The versatile design and ease of integration of these comb sources shows great promise for future generation designs and optical networks.