A5045827051- GaN-based semiconductor devices and materials
- Metal and Thin Film Mechanics
- Semiconductor materials and devices
- Semiconductor Quantum Structures and Devices
- Ga2O3 and related materials
- Perovskite Materials and Applications
- Radio Frequency Integrated Circuit Design
- Acoustic Wave Resonator Technologies
- Advanced DC-DC Converters
- Quantum Dots Synthesis And Properties
- ZnO doping and properties
- Optical properties and cooling technologies in crystalline materials
- Advanced Power Amplifier Design
- X-ray Diffraction in Crystallography
- Silicon Carbide Semiconductor Technologies
- Advanced Materials Characterization Techniques
- Crystallization and Solubility Studies
- Low-power high-performance VLSI design
- Advanced Photocatalysis Techniques
- Catalytic Processes in Materials Science
- Nanowire Synthesis and Applications
- Solid-state spectroscopy and crystallography
- Metal-Organic Frameworks: Synthesis and Applications
- 2D Materials and Applications
- Electric Motor Design and Analysis
University of Cambridge
2014-2023
Cardiff University
2018
University of Sheffield
2018
University College Cork
2007-2014
National University of Ireland
2014
ITT Technical Institute
2002-2005
A significant fraction of global electricity demand is for lighting. Enabled by the realization and development efficient GaN blue light-emitting diodes (LEDs), phosphor-based solid-state white LEDs provide a much higher efficiency alternative to incandescent fluorescent lighting, which are being broadly implemented. However, key challenge this industry achieve right photometric ranges application-specific emission spectra via cost-effective means. Here, we synthesize organic–inorganic lead...
Abstract Geometric or electronic confinement of guests inside nanoporous hosts promises to deliver unusual catalytic opto-electronic functionality from existing materials but is challenging obtain particularly using metastable hosts, such as metal–organic frameworks (MOFs). Reagents (e.g. precursor) may be too large for impregnation and synthesis conditions also destroy the hosts. Here we use thermodynamic Pourbaix diagrams (favorable redox pH conditions) describe a general method...
Three-dimensional carbon-based structures have proven useful for tailoring material properties in structural mechanical and energy storage applications. One approach to obtain them has been by carbonization of selected metal-organic frameworks (MOFs) with catalytic metals, but this is not applicable most common MOF structures. Here, we present a strategy transform MOFs, guest inclusions high-temperature MOF-guest interactions, into complex carbon-based, diatom-like, hierarchical (named the...
Cesium lead halide nanocrystals, CsPbX3 (X = Cl, Br, I), exhibit photoluminescence quantum efficiencies approaching 100% without the core–shell structures usually used in conventional semiconductor nanocrystals. These high make these crystals ideal candidates for light-emitting diodes (LEDs). However, because of large surface area to volume ratio, halogen exchange between perovskite nanocrystals different compositions occurs rapidly, which is one limiting factors white-light applications...
In this paper we present a detailed analysis of the structural, electronic, and optical properties an $m$-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized x-ray diffraction, scanning transmission electron microscopy, 3D atom probe tomography. have studied photoluminescence (PL), time-resolved PL spectroscopy, polarized excitation spectroscopy. spectrum consisted very broad line with high degree linear...
Atom probe tomography (APT) has been used to characterize the distribution of In atoms within non-polar a-plane InGaN quantum wells (QWs) grown on a GaN pseudo-substrate produced using epitaxial lateral overgrowth. Application focused ion beam microscope enabled APT needles be prepared from low defect density regions sample. A complementary analysis was also undertaken QWs having comparable contents polar c-plane sample pseudo-substrates. Both frequency and modified nearest neighbor analyses...
Recently, there has been an increase in research relating to the search for new optoelectronic materials photovoltaic devices and light-emitting diodes. This focuses on developing that not only possess required properties (high absorption, efficient photon emission, high charge-carrier mobilities) but are also solution-processable, which can reduce fabrication costs. Ternary ionic crystal structures, such as lead-halide perovskites, have emerged a class of suitable materials. However, most...
This paper presents an electromagnetic system for five degrees of freedom position and orientation sensing with application in a virtual bronchoscopy system. Planar magnetic source coils were manufactured on printed circuit boards reproducibility low-cost manufacturing. The exact field each coil is calculated using filament-based approach. A constant ac current amplifier used to drive at independent frequency synchronous demodulator calculates associated component the waveform as reference...
Nanocathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation emission wavelength each well, correlation with Si dopant concentration barriers. This is reproduced by band profile simulations, which reveal reduction Stark shift doping. demonstrate nanocathodoluminescence powerful technique to optimize doping optoelectronic devices.
Control of leakage currents in the buffer layers GaN based transistors on Si substrates is vital for demonstration high performance devices. Here, we show that growth conditions during metal organic chemical vapour deposition graded AlGaN strain relief (SRLs) can significantly influence vertical leakage. Using scanning capacitance microscopy, secondary ion mass spectrometry, and transmission electron investigate origins paths they result from preferential incorporation oxygen impurities side...
Fluence-dependent photoluminescence and ultrafast transient absorption spectroscopy are used to study the dynamic behavior of carriers in CsPbCl3 perovskite nanocrystals. At low excitation fluences, radiative recombination rate is outcompeted by significant trapping charge which then recombine nonradiatively, resulting weak photoluminescence. As fluence increased, saturation trap states deactivates these nonradiative relaxation paths giving rise an increase at first. However, with further...
We have used high resolution transmission electron microscopy (HRTEM), aberration-corrected quantitative scanning (Q-STEM), atom probe tomography (APT) and X-ray diffraction (XRD) to study the atomic structure of (0001) polar (11-20) non-polar InGaN quantum wells (QWs). This paper provides an overview results. Polar in QWs is a random alloy, with In replacing Ga randomly. The height interface steps, resulting QW width fluctuations. electrons are localised at top by built-in electric field...
III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal nanoscale structural properties InGaN formed by combined top-down etching regrowth to achieve non-polar sidewalls with a low density extended defects. While luminescence is uniform along {1–100} sidewalls, nano-cathodoluminescence shows sharp reduction in luminescent intensity at intersection facets. The accompanied emission energy localised apex...
The measurement of ζ potential Ga-face and N-face gallium nitride has been carried out as a function pH. Both the faces show negative in pH range 5.5-9. an isoelectric point at 5.5. shows more due to larger concentration adsorbed oxygen. data clearly showed that H-terminated diamond seed solution 8 will be optimal for self-assembly monolayer nanoparticles on GaN surface. subsequent growth thin films seeded with seeds produced fully coalesced films, confirming seeding density excess 1011...
Atom probe tomography and quantitative scanning transmission electron microscopy are used to assess the composition of non-polar a-plane (11-20) InGaN quantum wells for applications in optoelectronics. The average well measured by atom quantitatively agrees with measurements X-ray diffraction. is further applied study distribution indium atoms wells. An inhomogeneous observed frequency analysis measurements. optical properties compositions varying from 7.9% 20.6% studied. In contrast m-plane...
The use of magnets for anchoring instrumentation in minimally invasive surgery and endoscopy has become increased interest recent years. Permanent have significant advantages over electromagnets these applications; larger retraction force comparable size volume without the need any external power supply. However, permanent represent a potential hazard operating field where inadvertent attraction to surgical is often undesirable. current work proposes an interesting hybrid approach which...
Non-polar (11-20) InGaN quantum dots (QDs) were grown by metal organic vapour phase epitaxy. An epilayer was and subjected to a temperature ramp in nitrogen ammonia environment before the growth of GaN capping layer. Uncapped structures with without for reference imaged atomic force microscopy. Micro-photoluminescence studies reveal presence resolution limited peaks linewidth less than ∼500 μeV at 4.2 K. This is significantly narrower that non-polar alternate methods may be indicative...
We report on the optical properties of non‐polar m‐plane InGaN/GaN multi‐quantum wells (MQWs) grown ammonothermal bulk GaN substrates. The low temperature continuous wave (CW) photoluminescence spectra are broad with a characteristic energy tail. majority emission bands decay time constant ~300 ps, but detailed and resolved spectroscopy measurements revealed existence distinct slowly decaying band. This component is responsible for tails observed in CW spectra. Scanning electron...
Abstract In this paper we describe the implementation and characterisation of five different in‐situ defect reduction techniques for non‐polar ( a ‐plane) GaN growth on r ‐plane sapphire. Sample 1 (3D/2D) employs methodology frequently applied c ‐plane, involving low temperature nucleation layer (LTNL) followed by 3D island formation lateral coalescence. For 2 (d3D) islands are grown directly onto sapphire with no LTNL, growth. 3 (d3D Si) follows similar procedure, but high silicon doping in...
Nano-cathodoluminescence (Nano-CL) reveals optical emission from individual InGaN quantum wells for applications in optoelectronic devices. We show the luminescent intensity decays over time with exposure to electron beam energies between 80 and 200 keV. Measurements of CL an exponential decline intensity, which we propose is due formation nitrogen Frenkel defects. The measured damage decreases reductions accelerating voltage suggest that induced structural may be suppressed below proposed...
In an RF system power amplifiers (PAs) typically consume the most power. This paper presents a buck converter design optimised for wideband code division multiple access (WCDMA) PA. The approach taken focuses on optimization of switch sizing based overall losses including output inductor losses. is 20 MHz switching and currents in range 200-500 mA. Experimented results are presented fabricated converter, with maximum measured efficiency 82%.
Perovskite colloidal nanocrystals have emerged as important new optical materials, with tunable light emission across the visible spectrum, narrow line widths for high color purity, and quantum efficiencies approaching unity. These materials can be solution processed in large volumes at low cost making them promising optoelectronic devices. The structure of influences radiative nonradiative recombination carriers within through trap states Auger recombination. To optimize properties it is...
We present a combined theoretical and experimental analysis of the optical properties m-plane InGaN/GaN quantum wells. The sample was studied by photoluminescence excitation spectroscopy at low temperature. spectra show large Stokes shift between lowest exciton peak in spectrum. This behavior is indicative strong carrier localization effects. These results are complemented tight-binding calculations, accounting for random alloy fluctuations Coulomb data explain main features spectra....