505 research outputs found
Electronic and optical properties of boron containing GaN alloys: The role boron atom clustering
Boron (B) containing III-nitride materials, such as wurtzite (B,Ga)N alloys,
have recently attracted significant interest to tailor the electronic and
optical properties of optoelectronic devices operating in the visible and
ultraviolet spectral range. However, the growth of high quality samples is
challenging and B atom clustering is often observed in (B,Ga)N alloys. To date,
fundamental understanding of the impact of such clustering on electronic and
optical properties of these alloys is sparse. In this work we employ density
functional theory (DFT) in the framework of the meta generalized gradient
approximation (modified Becke Johnson (mBJ) functional) to provide insight into
this question. We use mBJ DFT calculations, benchmarked against
state-of-the-art hybrid functional DFT, on (B,Ga)N alloys in the experimentally
relevant B content range of up to 7.4%. Our results reveal that B atom
clustering can lead to a strong reduction in the bandgap of such an alloy, in
contrast to alloy configurations where B atoms are not forming clusters, thus
not sharing nitrogen (N) atoms. We find that the reduction in bandgap is linked
mainly to carrier localization effects in the valence band, which stem from
local strain and polarization field effects. However, our study also reveals
that the alloy microstructure of a B atom cluster plays an important role: B
atom chains along the wurtzite c-axis impact the electronic structure far less
strongly when compared to a chain formed within the c-plane. This effect is
again linked to local polarization field effects and the orbital character of
the involved valence states in wurtzite BN and GaN. Overall, our calculations
show that controlling the alloy microstructure of (B,Ga)N alloys is of central
importance when it comes to utilizing these systems in future optoelectronic
devices with improved efficiencies
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
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
Investigation Of Metal Modulated Epitaxy Grown III-Nitride High-Power Electronic And Optoelectronic Devices
The wide-bandgap material GaN (Eg = 3.4 eV) continues to mature due to its achievements in high-power electronic and optoelectronic devices. The fully vertical GaN high-power devices show high performance but are very expensive. Quasi-vertical GaN devices are cost-effective but lack high performance due to low quality films. Improvement in the performance of quasi-vertical devices would make this technology suitable for high volume production. Although the market of GaN based devices is still growing, significantly higher performance parameters can potentially be achieved with the ultrawide-bandgap semiconductor material AlN with the bandgap as high as 6.1 eV. The only limitation to AlN-based devices so far was doping.
An extensive study is performed to explore the 3D phase diagram of MME to find the optimized morphological, electrical, structural, and optical growth conditions to achieve high performance high-power quasi-vertical GaN pin diodes. Thick abrupt Beryllium step-doped GaN i-layers were used to demonstrate these high-power devices. Novel fabrication methods and current spreading layers are introduced in these devices.
Furthermore, for the first time in more than 8 decades of AlN research, substantial bulk conduction, both p-type and n-type, was achieved in MME AlN and the first known p-n junction AlN diodes are demonstrated to extend the high-power performance of nitride technology. Also, MME mixed AlN/GaN Pin and junction barrier Schottky diodes are demonstrated to achieve low turn-on voltage and higher breakdown performance simultaneously. The conductive AlN films show great promise for AlN-based device applications that could potentially revolutionize deep ultraviolet light based viral and bacterial sterilization, polymer curing, high-temperature, high-voltage and high-power electronics among many societal impacts.Ph.D
Band structure renormalization at finite temperatures from first principles
In dieser Doktorarbeit untersuchen wir den Einfluss von Elektron-Phonon-Wechselwirkungen (EPW) auf die Bandlueckenrenormierung in kristallinen Festkoerpern bei endlichen Temperaturen. Das Hauptziel besteht darin, den Einfluss der Kernbewegung und der thermischen Ausdehnung des Gitters auf die Bandstruktur in einer Vielzahl von Materialien zu quantifizieren. Zu diesem Zweck wird der Temperatureinfluss auf das EPW in harmonischen Naeherungen unter Verwendung der stochastischen Abtastmethode und vollstaendig anharmonisch durch Durchführung von ab initio Molekulardynamiksimulationen (aiMD). Die Bandluecke bei endlichen Temperaturen wird aus der thermodynamisch gemittelten Spektralfunktion extrahiert, die unter Verwendung der Bandentfaltungstechnik berechnet wird. Waehrend die Verwendung von aiMD bereits fuer Berechnungen von EPW verwendet wurde, wurde die Kombination von aiMD und Bandentfaltung zur Behandlung der Bandluecken renormalisierung erst kuerzlich verwendet. In dieser Doktorarbeit haben wir eine verbesserte Bandentfaltungstechnik verwendet, um die Berechnung effektiv zu verwalten. Diese verbesserte Methode enthaelt mehrere methodische Neuerungen, die dazu dienen, den Rechenaufwand zu verringern und das statistische Rauschen in den Endergebnissen zu minimieren. Die aktualisierte Methode wurde gruendlich bewertet, dokumentiert und mit einer benutzerfreundlichen Oberflaeche gestaltet. Wir praesentieren eine umfassende Untersuchung der numerischen Aspekte der thermodynamischen Mittelung, der Schaetzung von Fehlerbalken und der Bewertung der Konvergenz in Bezug auf die Groesse der Simulationssuperzelle. Unser etabliertes Protokoll ermoeglicht die Berechnung der Bandlückenrenormierung bei endlichen Temperaturen, was in guter Uebereinstimmung mit frueheren theoretischen Studien und experimentellen Daten steht.In this thesis, we investigate the influence of electron-phonon interactions (EPI) on the band gap renormalization in crystalline solids at finite temperatures. The main goal is to identify the impact of the nuclear motion and the lattice thermal expansion on the band structure in a wide range of materials. For this purpose, the temperature influence on the EPI is calculated in the harmonic approximations by utilizing the stochastic sampling methodology and fully anharmonically, by performing ab initio molecular dynamics simulations (aiMD). The band gap at finite temperatures is extracted from the thermodynamically averaged spectral function, which is calculated using band-unfolding technique. While utilization of aiMD was already used for calculations of EPI the combination of aiMD and band-unfolding to treat the band gap renormalization was used only recently. In this thesis, we employed an improved band unfolding technique in order to effectively manage the calculations. This improved method incorporates several methodological innovations that serve to mitigate computational cost and minimize statistical noise in the final results. The updated method was thoroughly benchmarked, documented, and designed with a user-friendly interface. We present a comprehensive examination of the numerical aspects of thermodynamic averaging, the estimation of error bars, and the evaluation of convergence with respect to the size of the simulation supercell. Our established protocol enables the calculation of band gap renormalization at finite temperatures, which is in good agreement with prior theoretical studies and experimental data
Transition metal doped phosphors for LEDs: A study of temperature dependent luminescence of manganese and chromium
The luminescence properties of various transition metal ions in inorganic host lattices have been studied in this thesis, with a strong emphasis on their temperature-dependent behavior also in relation with application as phosphors in white light LEDs for lighting and broad band NIR LEDs for sensing. The following paragraphs present an overview of the key results from each chapter. This is followed by an outlook on future research topics and challenges
Lattice deformation at the sub-micron scale: X-ray nanobeam measurements of elastic strain in electron shuttling devices
The lattice strain induced by metallic electrodes can impair the
functionality of advanced quantum devices operating with electron or hole
spins. Here we investigate the deformation induced by CMOS-manufactured
titanium nitride electrodes on the lattice of a buried, 10 nm-thick Si/SiGe
Quantum Well by means of nanobeam Scanning X-ray Diffraction Microscopy. We
were able to measure TiN electrode-induced local modulations of the strain
tensor components in the range of with ~60 nm lateral
resolution. We have evaluated that these strain fluctuations are reflected into
local modulations of the potential of the conduction band minimum larger than 2
meV, which is close to the orbital energy of an electrostatic quantum dot. We
observe that the sign of the strain modulations at a given depth of the quantum
well layer depends on the lateral dimensions of the electrodes. Since our work
explores the impact of device geometry on the strain-induced energy landscape,
it enables further optimization of the design of scaled CMOS-processed quantum
devices.Comment: 16 pages, 6 figure
First Principles Investigation of Covalently Immobilised Metalloporphyrins on Carbon Supports for the Electrochemical Reduction of CO2
Metal-organic molecular catalysts used for CO2 electroreduction (CO2ERR) have recently been shown to exhibit remarkable CO2ERR performance (particularly when immobilised). One of the main challenges, however, is determining accurate entropy values of molecules in the aqueous-phase. We perform a bench marking study of different solvation entropy calculation methods for a set of 56 molecules ranging in size from H2 to tetrabutylammonium (C16H36N). Attention is paid to the cavity packing parameters which are important for calculating accurate cavity formation entropies. It is found that our developed approach provides the more accurate solvation entropy values.
We also study the impact that intrinsic defects in graphene and carbon nanotube supports have on the CO2ERR reaction performance of cobalt-centred phthalocyanine (CoPc) and tetraphenylporphyrin (CoTPP) catalysts. We find that up-right immobilised CoPc on a Stone-Wales defect, and CoTPP on an octagon-pentagon line defect, respectively have the most favourable reaction pathways. The pyridine linker immobilising CoPc (via its Co active site) and the oxygen linker immobilising CoTPP are also studied with the CoPc-pyridine system demonstrating superior
CO2ERR reaction pathway performance compared to all other systems considered.
Finally, we perform a systematic study of the immobilisation of CoPc via its Co active site for eight different atomic-style linkers on a carbon nanotube support. The bonding between the linker and the Co active site can change the d-state ordering, shifting components of the dz2 states, which facilitate CO2 adsorption, to higher energies making them unoccupied and close to the Fermi level. This was found to be the case for the NH, S and PH linkers which exhibited the most favourable reaction free energy pathways compared to all other linker systems considered. We therefore propose that the NH, S and PH linkers would be promising candidates for future experimental studies
Computational study of native defects and defect migration in wurtzite AlN: an atomistic approach †
We derive an empirical, lattice energy consistent interatomic force field model for wurtzite AlN to predict consistently a wide range of physical and defect properties. Using Mott–Littleton techniques, we calculate formation energies of vacancies and interstitials, which show good agreement with previous ab initio calculations at the edge of the band gap. A novel N3− interstitialcy configuration is proposed to be of lower energy than the octahedral-channel-centred counterpart. With the assistance of the QM/MM method, our potential can predict a VBM level (−7.35 eV) comparable to previous experimental measurements. We further investigate the migration mechanisms and energy barriers of the main intrinsic defects. For the vacancy migration, the axial migration barrier is found to be lower than the basal migration barrier, in contrast to previous calculations. Two interstitialcy migration mechanisms for the interstitial defects are proposed, the “knock-out” mechanism for Al interstitial and the “hand-over” mechanism for N interstitialcy defects. The new force field model proposed here demonstrates that the empirical two-body interatomic potential is still effective for the study of defect properties, electronic states, and other extended systems of III/V semiconductors and further can be employed with QM/MM embedded techniques
Operational Research: methods and applications
This is the final version. Available on open access from Taylor & Francis via the DOI in this recordThroughout its history, Operational Research has evolved to include methods, models and algorithms that have been applied to a wide range of contexts. This encyclopedic article consists of two main sections: methods and applications. The first summarises the up-to-date knowledge and provides an overview of the state-of-the-art methods and key developments in the various subdomains of the field. The second offers a wide-ranging list of areas where Operational Research has been applied. The article is meant to be read in a nonlinear fashion and used as a point of reference by a diverse pool of readers: academics, researchers, students, and practitioners. The entries within the methods and applications sections are presented in alphabetical order. The authors dedicate this paper to the 2023 Turkey/Syria earthquake victims. We sincerely hope that advances in OR will play a role towards minimising the pain and suffering caused by this and future catastrophes
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