1,441 research outputs found

    Accurate quantum transport modelling and epitaxial structure design of high-speed and high-power In0.53Ga0.47As/AlAs double-barrier resonant tunnelling diodes for 300-GHz oscillator sources

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    Terahertz (THz) wave technology is envisioned as an appealing and conceivable solution in the context of several potential high-impact applications, including sixth generation (6G) and beyond consumer-oriented ultra-broadband multi-gigabit wireless data-links, as well as highresolution imaging, radar, and spectroscopy apparatuses employable in biomedicine, industrial processes, security/defence, and material science. Despite the technological challenges posed by the THz gap, recent scientific advancements suggest the practical viability of THz systems. However, the development of transmitters (Tx) and receivers (Rx) based on compact semiconductor devices operating at THz frequencies is urgently demanded to meet the performance requirements calling from emerging THz applications. Although several are the promising candidates, including high-speed III-V transistors and photo-diodes, resonant tunnelling diode (RTD) technology offers a compact and high performance option in many practical scenarios. However, the main weakness of the technology is currently represented by the low output power capability of RTD THz Tx, which is mainly caused by the underdeveloped and non-optimal device, as well as circuit, design implementation approaches. Indeed, indium phosphide (InP) RTD devices can nowadays deliver only up to around 1 mW of radio-frequency (RF) power at around 300 GHz. In the context of THz wireless data-links, this severely impacts the Tx performance, limiting communication distance and data transfer capabilities which, at the current time, are of the order of few tens of gigabit per second below around 1 m. However, recent research studies suggest that several milliwatt of output power are required to achieve bit-rate capabilities of several tens of gigabits per second and beyond, and to reach several metres of communication distance in common operating conditions. Currently, the shortterm target is set to 5−10 mW of output power at around 300 GHz carrier waves, which would allow bit-rates in excess of 100 Gb/s, as well as wireless communications well above 5 m distance, in first-stage short-range scenarios. In order to reach it, maximisation of the RTD highfrequency RF power capability is of utmost importance. Despite that, reliable epitaxial structure design approaches, as well as accurate physical-based numerical simulation tools, aimed at RF power maximisation in the 300 GHz-band are lacking at the current time. This work aims at proposing practical solutions to address the aforementioned issues. First, a physical-based simulation methodology was developed to accurately and reliably simulate the static current-voltage (IV ) characteristic of indium gallium arsenide/aluminium arsenide (In-GaAs/AlAs) double-barrier RTD devices. The approach relies on the non-equilibrium Green’s function (NEGF) formalism implemented in Silvaco Atlas technology computer-aided design (TCAD) simulation package, requires low computational budget, and allows to correctly model In0.53Ga0.47As/AlAs RTD devices, which are pseudomorphically-grown on lattice-matched to InP substrates, and are commonly employed in oscillators working at around 300 GHz. By selecting the appropriate physical models, and by retrieving the correct materials parameters, together with a suitable discretisation of the associated heterostructure spatial domain through finite-elements, it is shown, by comparing simulation data with experimental results, that the developed numerical approach can reliably compute several quantities of interest that characterise the DC IV curve negative differential resistance (NDR) region, including peak current, peak voltage, and voltage swing, all of which are key parameters in RTD oscillator design. The demonstrated simulation approach was then used to study the impact of epitaxial structure design parameters, including those characterising the double-barrier quantum well, as well as emitter and collector regions, on the electrical properties of the RTD device. In particular, a comprehensive simulation analysis was conducted, and the retrieved output trends discussed based on the heterostructure band diagram, transmission coefficient energy spectrum, charge distribution, and DC current-density voltage (JV) curve. General design guidelines aimed at enhancing the RTD device maximum RF power gain capability are then deduced and discussed. To validate the proposed epitaxial design approach, an In0.53Ga0.47As/AlAs double-barrier RTD epitaxial structure providing several milliwatt of RF power was designed by employing the developed simulation methodology, and experimentally-investigated through the microfabrication of RTD devices and subsequent high-frequency characterisation up to 110 GHz. The analysis, which included fabrication optimisation, reveals an expected RF power performance of up to around 5 mW and 10 mW at 300 GHz for 25 μm2 and 49 μm2-large RTD devices, respectively, which is up to five times higher compared to the current state-of-the-art. Finally, in order to prove the practical employability of the proposed RTDs in oscillator circuits realised employing low-cost photo-lithography, both coplanar waveguide and microstrip inductive stubs are designed through a full three-dimensional electromagnetic simulation analysis. In summary, this work makes and important contribution to the rapidly evolving field of THz RTD technology, and demonstrates the practical feasibility of 300-GHz high-power RTD devices realisation, which will underpin the future development of Tx systems capable of the power levels required in the forthcoming THz applications

    Synthesis of Quasi-Freestanding Graphene Films Using Radical Species Formed in Cold Plasmas

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    For over a decade, the Stinespring laboratory has investigated scalable, plasma assisted synthesis (PAS) methods for the growth of graphene films on silicon carbide (SiC). These typically utilized CF4-based inductively coupled plasma (ICP) with reactive ion etching (RIE) to selectively etch silicon from the SiC lattice. This yielded a halogenated carbon-rich surface layer which was then annealed to produce the graphene layers. The thickness of the films was controlled by the plasma parameters, and overall, the process was readily scalable to the diameter of the SiC wafer. The PAS process reproducibly yielded two- to three-layer thick graphene films that were highly tethered to the underlying SiC substrate via an intermediate buffer layer. The buffer layer was compositionally similar to graphene. However, a significant number of graphene carbons were covalently bound to silicon atoms in the underlying substrate. This tethering lead to mixing of the film and substrate energy bands which degraded many of graphene’s most desirable electrical properties. The research described in this dissertation was aimed at improving graphene quality by reducing the extent of tethering using a fundamentally different plasma etching mechanism while maintaining scalability. In the ICP-RIE process, the etchant species include F and CFx (x = 1-3) radicals and their corresponding positive ions. These radicals are classified as “cold plasma species” in the sense that they are nominally in thermal equilibrium with the substrate and walls of the system. In contrast, the electrons exist at extremely high temperature (energy), and the ionic species are accelerated to energies on the order of several hundred electron volts by the plasma bias voltage that exists between the plasma and substrate. As a result, the ionic species create a directional, high rate etch that is dominated by physical etching characterized by energy and momentum transfer. In contrast, the neutral radicals chemically etch the surface at a much lower rate. In this work, the effects of physical etching due to high energy ions were eliminated by shielding the SiC substrate using a mask (e.g., quartz) supported by silicon posts. In this way, a microplasma consisting of chemically reactive cold plasma species was created in the small space between the substrate surface and the backside of the quartz mask. This process, referred to here as microplasma assisted synthesis (MPAS), was used to produce graphene films. A parametric investigation was conducted to determine the influence of MPAS operating parameters on graphene quality. The key parameters investigated included ICP power, RIE power, etch time, various mask materials, microreactor height, substrate cooling, initial surface morphology and SiC polytype. The resulting graphene films were characterized by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM). Following optimization of the MPAS process, some tethering of the graphene films remained. However, films produced by MPAS consistently exhibited significantly less tethering than those produced using the PAS process. Moreover, both XPS and Raman spectroscopy indicated that these films were quasi-free standing, and, in some cases, they approached free standing graphene. From a wide view, the results of these studies demonstrate the potential of MPAS as a technique for realizing the controlled synthesis of high-quality, lightly tethered mono-, and few-layer graphene films directly on an insulating substrate. On a more fundamental level, the results of these studies provide insight into the surface chemistry of radical species

    Scanning Probe Microscopy Studies of Petroleum Chemistry: Substrate-Dependent Catalytic Properties of MoS2 and Automating Scanning Probe Microscopy with Machine Learning

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    With the growth of the population, society’s energy demands are mostly reliant on petroleum products that come from the refining of crude oil. Most of these refining reactions have been developed through averaging spectroscopic techniques, but scientists do not know exactly what is happening in these processes at the nano and atomic levels. This information is crucial when designing an efficient refining process that produces petroleum products that emit fewer harmful gases when combusting. Scanning probe microscopy techniques have become a powerful tool to look into the chemical structures found in petroleum products, to understand catalytic reactions in refining processes, and to find new non-combustible uses for these products. In this dissertation, I show how scanning probe microscopy (SPM) techniques, especially non-contact atomic force microscopy (NC-AFM) can provide an atomic-level understanding of the chemical structures and active catalytic sites that play a role in these refining processes. First, I studied hydrodesulfurization reactions that use molybdenum disulfide as a main catalyst to explore the effect of layer thickness, strain, and underlying substrates on its electronic and catalytic properties. Here, I present the first NC-AFM experiments investigating the active catalytic sites of molybdenum disulfide on industrially relevant substrates. Through these experiments, I found how NC-AFM techniques on insulators need to be improved to achieve high-resolution images that are comparable to those collected on metal substrates. Second, I created Auto-HR-AFM, a machine-learning script that collects optimal high-resolution NC-AFM images. Auto-HR-AFM is a modular and open-source script that provides an initial framework for a fully automated SPM. Expanding on this framework will widen the use of scanning probe microscopy techniques to non-experts and the automation will increase the time the system is kept running to collect large optimal datasets. Ultimately, these studies will broaden the use of high-resolution SPM techniques and help create more efficient catalysts and refining processes to produce cleaner and more efficient petroleum products

    A Contribution Towards Intelligent Autonomous Sensors Based on Perovskite Solar Cells and Ta2O5/ZnO Thin Film Transistors

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    Many broad applications in the field of robotics, brain-machine interfaces, cognitive computing, image and speech processing and wearables require edge devices with very constrained power and hardware requirements that are challenging to realize. This is because these applications require sub-conscious awareness and require to be always “on”, especially when integrated with a sensor node that detects an event in the environment. Present day edge intelligent devices are typically based on hybrid CMOS-memristor arrays that have been so far designed for fast switching, typically in the range of nanoseconds, low energy consumption (typically in nano-Joules), high density and endurance (exceeding 1015 cycles). On the other hand, sensory-processing systems that have the same time constants and dynamics as their input signals, are best placed to learn or extract information from them. To meet this requirement, many applications are implemented using external “delay” in the memristor, in a process which enables each synapse to be modeled as a combination of a temporal delay and a spatial weight parameter. This thesis demonstrates a synaptic thin film transistor capable of inherent logic functions as well as compute-in-memory on similar time scales as biological events. Even beyond a conventional crossbar array architecture, we have relied on new concepts in reservoir computing to demonstrate a delay system reservoir with the highest learning efficiency of 95% reported to date, in comparison to equivalent two terminal memristors, using a single device for the task of image processing. The crux of our findings relied on enhancing our capability to model the unique physics of the device, in the scope of the current thesis, that is not amenable to conventional TCAD simulations. The model provides new insight into the redox characteristics of the gate current and paves way for assessment of device performance in compute-in-memory applications. The diffusion-based mechanism of the device, effectively enables time constants that have potential in applications such as gesture recognition and detection of cardiac arrythmia. The thesis also reports a new orientation of a solution processed perovskite solar cell with an efficiency of 14.9% that is easily integrable into an intelligent sensor node. We examine the influence of the growth orientation on film morphology and solar cell efficiency. Collectively, our work aids the development of more energy-efficient, powerful edge-computing sensor systems for upcoming applications of the IOT

    InP membrane photonics for large-scale integration

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    Exploring Nanoscale Optoelectronic Properties of Kesterite- and Perovskite-based Devices

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    Emerging photovoltaic materials such as kesterite and perovskite for optoelectronic devices have inevitable defects that affect the device performance. Therefore, examining the optoelectronic properties of defects is crucial to improve the device performance. Scanning probe microscopy (SPM) is one of the essential techniques to investigate various properties of materials at nanoscale with high resolution. This thesis focuses on characterising kesterite and perovskite using SPM technique to elucidate nanoscale charge transport properties. This primary objective of the first part of this thesis is to understand the effect of compositional variation of kesterite on optoelectronic properties at nanoscale by controlling tin content during the precursor process. This work demonstrated the highest A-type [ZnCu + VCu] defect and altered distribution of sulfur with high near-surface accumulation at an optimised compositional ratio. This synergetic outcome facilitated carrier separation across grain boundaries (GBs). Moreover, the open-circuit voltage deficit was significantly reduced at an optimum compositional ratio owing to improving charge transport through GBs, thereby exhibiting an improved power conversion efficiency. Furthermore, this thesis examines the effect of passivation strategies in halide perovskite for indoor applications using phenethylammonium iodide (PEAI) and lead(II) chloride (PbCl2). A homogenous charge separation across a surface of perovskite was observed when an adequate amount of PEAI was deposited on the perovskite surface. In addition, incorporation of PbCl2 facilitated effective charge transport through GBs. Both strategies facilitated carrier transport towards the surface of perovskite with less ion migration and reduced non-radiative recombination, thereby improving the performance of indoor perovskite solar cells. Finally, another aspect of this thesis is to investigate the optoelectronic properties of two-dimensional perovskite single crystal (butylammonium lead bromide, BA2PbBr4) and elucidate their effect of these optoelectronic properties on the performance of photodetectors (PDs). Accumulation of charge carriers increased at edges with increasing the edge height. Furthermore, the existence of multiple sub-bandgap states in BA2PbBr4, and increasing electron transitions from sub-bandgap states with higher edge height were observed with increasing edge height. This work suggests that edge-height dependence of charge-carrier behaviour in BA2PbBr4 can be utilised in broadband PDs

    InP membrane photonics for large-scale integration

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    Engineering Photon Sources for Practical Quantum Information Processing:If you liked it then you should have put a ring on it

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    Integrated quantum photonics offers a promising route to the realisation of universal fault-tolerant quantum computers. Much progress has been made on the theoretical aspects of a future quantum information processor, reducing both error thresholds and circuit complexity. Currently, engineering efforts are focused on integrating the most valuable technologies for a photonic quantum computer; pure single-photon sources, low-loss phase shifters and passivecircuit components, as well as efficient single-photon detectors and corresponding electronics.Here, we present efforts to target the former under the constraints imposed by the latter. We engineer the spectral correlations of photons produced by a heralded single-photon source, such that they produce photons in pure quantum states (99.1±0.1 % purity), and enable additional optimisation using temporal shaping of the pump field. Our source also has a high intrinsicheralding efficiency (94.0 ± 2.9 %) and produces photon pairs at a rate (4.4 ± 0.1 MHz mW−2) which is an order of magnitude better than previously predicted by the literature for a resonant source of this purity. Additionally, we present tomographic methodologies that fully describe the photonic quantum states that we produce, without the use of analytical models, and as a means of verifying the quantum states we create, entitled – "Quantum-referenced SpontaneousEmission Tomography" (Q-SpET). We also design reconfigurable photonic circuits that can be operated at cryogenic temperatures, with zero static power consumption, entitled – "Cladding Layer Manipulation" (CLM). These devices function as on-chip phase shifters, enabling the local reconfiguration of circuit elements using established technologies but removing the need for active power consumption to maintain the reconfigured circuit. These devices are capable ofan Lπ = 12.3 ± 0.3 µm, a ∼7x reduction in length when compared to the thermo-optic phaseshifters used throughout this thesis. Finally, we investigate how pure photon sources operate as part of larger circuits within the typical design rules of photonic quantum circuits. Using this information to accurately model all of the spurious contributions to the final photonic quantumstate, which we call a form of nonlinear noise. This noise can decrease source purity to below 40 %, significantly affecting the fidelity of Hong-Ou-Mandel interference, and subsequently, our ability to reliably create fundamental resources for photonic quantum computers. All of this contributes to our design of a fundamental building block for integrated quantum photonic processors, the functionality of which can be predicted at scale, under the conditions imposed by the rest of the processor
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