Below are projects that were proposed for the 2020 intake, as examples of the types of PhD projects that might available in the Centre for Doctoral Training. These are preliminary titles and descriptions, which change as the project is co-developed by the student and supervisor(s). For the 2021 intake, the project catalogue will be available in early December of the first year of study.
1.55 μm lasers epitaxially grown on silicon for optical interconnects
Dr Qiang Li, Cardiff University
Achieving high performance, long lifetime, and electrical injection silicon-based lasers will deliver the last missing building block in silicon photonics, not only enabling optical interconnects, but also serving other growth markets such as cloud computing, light detection and ranging (LIDAR) and chip-scale wearable sensors. This project aims to develop high-performance 1.55 µm C-band quantum dot and nano lasers to benefit from the lowest fibre attenuations. Advanced heteroepitaxy techniques will be studied to yield highly strained nanostructures with superior optical qualities and low dislocation density III-V materials on silicon. The material and device characteristics and possible integration scheme with silicon photonics will be explored.
III-V quantum dot lasers grown on patterned silicon and SOI substrates by MBE
Professor Huiyun Liu, University College London
There is a strong demand for integrating of III-V materials and devices on silicon platform for high-efficient photonics light sources and high mobility electronics devices. However, there are significant challenges for direct epitaxial growth, including anti-phase boundaries (APBs), threading dislocations (TDs) and micro-cracks. In last few years, significant progress has been made in reducing these three types of defects with a few micrometre III-V buffer layers. In addition, the total thickness of III-V layers is limited to a few micrometres (< 6µm) by the formation of micro-cracks due to a mismatch in thermal expansion between the III-V epilayers and a Si substrate.
In this project, we will exploit the potential of patterned silicon to suppress the defect formation within the patterned area on Si or SOI substrates. Most importantly, we would like to grow nano-size holes within the patterned areas by grow group-IV materials, such as Si and Ge by using twin-MBE system at UCL to relax the strain between III-V layers and silicon substrates, and hence to solve the problem due to thermal expansion mismatch between III-V layers and silicon substrates.
Advanced packaging and integration for CS mm-wave integrated circuits
Dr Johnathan Lees, Cardiff University
mm-wave circuits do not only rely on the integrated circuits performing well, but require proper packaging to avoid detrimental effects on power, matching, as well as to provide good thermal management. At the same time, the trend is to integrate as many functionalities in a single package, to minimize transitions and even more issues from an EM point of view.
This project, in collaboration with the CSA Catapult, aims at studying, designing and testing advanced packaging structure for mm-wave circuits, to include in a SiP (system in package) full transceiver front ends. The project will include the study of the best materials available from both EM and thermal point of view, with the inclusion of advanced techniques to interface circuits and provide thermal dissipations. This with particular attention to the cost and manufacturability.
Advancement of back-side processes for mm-wave integrated circuits
Professor Khaled Elgaid, Cardiff University
There is an explosion of demand for high frequency circuits able to perform efficiently at mm-wave, driven by applications such as automotive radar, imaging, 5G, security.
From a manufacturing point of view, higher frequency of operation requires to decrease the dimensions of integrated circuit to keep under control the parasitic effects. For III-V technologies, where vertical integration is still limited, this means prevalently to reduce the x-y distance between components and the impact of parasitics.
This can be achieved only if the back-side processing steps follow the advancement of the top-level processing. It is necessary to guarantee that the features fabricated, such as via holes, are small enough and the fabrication tolerances are tight to avoid overlapping and damaging of the top-side features.
This project, supported by a world leader provider of backside processing machinery, focusses on the advancement of back-side processing for SiC substrates, which are the preferred solution for high performance, high efficiency integrated circuits working in the mm-wave range. The project will potentially also explore the application to Si substrates.
Applying the Resonant-State Expansion to VCSELs and non-uniform waveguides
Dr Egor Muljarov, Cardiff University
The resonant-state expansion (RSE) is a novel theoretical method in electrodynamics which was recently invented in Cardiff [DOI:10.1209/0295-5075/92/50010]. It provides a new paradigm in physics, presenting a disruptive technology outperforming known computational methods, as demonstrated in [DOI:10.1103/PhysRevA.90.013834], and offering an intuitive understanding of the properties of a physical system due to an explicit use of its resonances. The key idea of the RSE is that it uses the eigenmodes of a basis system, calculated analytically or numerically, for finding the eigenmodes of a target system, in a rigorous and efficient way.
This project on computational electromagnetics will be focused on developing an efficient computational tool based on the RSE to describe light propagation through a layered optical system. This tool will allow us to calculate the electromagnetic fields and the eigenmodes of a system consisting of a large number of layers, where each layer is described by its own dielectric constant and may have additionally an in-plane inhomogeneity. One of the main applications of the developed computational tool will be an efficient calculation of the light emission and optical modes of vertical-cavity surface-emitting lasers (VCSELs), for their study and optimisation. Other applications may include non-uniform waveguides and photonic-crystal fibres.
Assymetrical Spacer Layer Tunnel (ASPAT) Diodes Integrated Circuitsode
Professor Mohamed Missous, University of Manchester
The ASPAT is a novel high frequency which was devloped at Manchester and Cambridge. The ASPAT diode is an excellent microwave detector combining wide dynamic range with low excess noise and a high temperature stability. This project aims to make the world’s first Monolithic Microwave Integrated Circuits (MMIC) based on quantum mechanical tunneling . This project will be conducted in collaboration with linwave ltd where a range of circuits will be made and tested for a variety of high frequency aplications ( up to 100GHz).
Boosting Efficiency in Mm-Wave III-V Power Amplifiers
Dr Roberto Quaglia, Cardiff University
Several mass market applications are emerging in mm-wave bands, including 5G, satcom and mobile infrastructure (backhaul). The need for efficient power amplifiers is even more felt at these frequencies given an increased complexity of the transmitters (active arrays); this is in contrast with the intrinsic difficulty of designing at mm-wave frequencies. Moreover, efficient power amplifier design must rely on accurate waveform engineering whose feasibility at mm-wave frequencies is still unexplored.
III-V technologies have the potential of offering a boost in terms of performance compared to Si-based amplifiers. However, at mm-wave frequencies, a better understanding of non-linear operation in terms of waveforms is key in enabling more sophisticated design techniques as well as feeding-back to the transistor design o optimise operation.
The Centre for High Frequency Engineering at Cardiff University has recently acquired a £1.4m harmonic load-pull characterisation system with 110 GHz bandwidth and waveform measurement capability. The idea is to exploit the system to characterise, compare and model mm-wave transistors samples provided by Qorvo (GaAs and GaN). This will enable to explore and define the best power amplifier design technique to achieve high efficiency, as well as understanding how the devices can be improved to improve the performance. The technique developed will be tested on MMIC designs on GaAs and GaN.
Developing high performance GaN based avalanche photodiodes by using a novel growth method
Professor Tao Wang and Professor John David, University of Sheffield
There is an increasing demand for developing GaN based photodetectors, which can find a wide range of applications in harsh environment gamma ray sensing, flame sensors, atmospheric ozone detection, communications, bio-photonics. Given the wide bandgap and the high breakdown voltage of GaN in theory, GaN has been accepted to be an ideal candidate for the fabrication of high performance avalanche photodiodes (APD) for the applications in both short wavelength and visible spectral regions. It is better than SiC as the bandgap can be continuously tuned over a wide wavelength range. Although high brightness blue LEDs have been commercialised, the crystal quality of GaN is still a great challenge for some particular optoelectronics which need to work under extreme conditions, for example, GaN based APDs which need to operate under a very high bias to amplify the photocurrent and detect weak signals. So far there have been only a few papers which show any signs of reproducible impaction ionization and avalanche multiplication. A fundamental challenge is that the critical electric field reported on GaN is far from its theoretical value (3 MV/cm) due to the material issues. Very recently, the Sheffield team has developed a novel two-dimensional growth mode benefiting from our high temperature AlN buffer technology which is different from the classic two-step growth approach. With this new growth approach, our GaN demonstrates an extremely high breakdown field of 2.5 MV/cm approaching the theoretical limit of GaN.1 This project aims to achieve the 1st true GaN APD worldwide.
1. S. Jiang, T. Wang et al, ACS Appl. Mater. Interfaces, 12, 12949−12954(2020);
Development III-nitrides HEMTs for microLED display applications
Professor Tao Wang, University of Sheffield
There is a significantly increasing demand on III-nitride based power electronics due to their intrinsic properties of high breakdown voltage and high saturation electron velocity. So far, AlGaN/GaN-based high electron mobility transistors (HEMTs) are developed mainly based on c-plane GaN where a large sheet carrier density is produced as a result of spontaneous and piezoelectric polarisations. As a consequence, such polarisations generally lead to a depletion-mode transistor, while it is ideal to have enhancement-mode GaN devices in practical applications due to safety requirements. Generally speaking, like GaAs based HEMTs the growth of an AlGaN/GaN heterostructure with a modulation doping along a non-polar direction provides a simple but promising solution, where the polarisations are eliminated and thus the sheet carrier density can be tuned simply through optimising the doping level. This HEMTs will be epitaxially integrated with LEDs or microdisk laser, aiming at ultra-fast response Li-Fi application.
Development of artificial intelligence techniques for analysis of Low Energy Electron Microscopy videos of Compound Semiconductor thin film and nanostructure growth dynamics
Dr Juan Pereiro Viterbo, Cardiff University
The project aims to facilitate the integration of new compound semiconductor materials and new nanostructures to develop new technologies (e.g. photonics, spintronics). The group is developing a new methodology to study key problems of epitaxial growth in a more efficient way.
The project has two main priorities:
- Development of computational methods based on machine learning techniques: Low Energy Electron Microscopy (LEEM) produces vast amounts of information, in order to process it, we are developing Artificial Intelligence (AI) techniques. The student will use reinforcement learning to explore experimental phase space and simplify the use of complex instruments to their full potential. The development of unsupervised learning techniques to reduce dimensionality of data, cluster the data and carry out comparisons that assist analysis of large amounts of data.
- Development of new methods for integration of dissimilar materials: The student will focus on imaging of growth dynamics of GaAs on Silicon. The student will test novel methodologies (e.g., use of liquid or graphene buffer layers). The student will monitor the dynamic behaviour of surface phases, adatoms and atomic steps to find changes in nucleation mechanisms of III-As on Silicon caused by the use of surfactants or dislocation filters.
Dilute nitride and other approaches to 1300nm VCSELs
Professor Peter Smowton, Cardiff University
Vertical Cavity Surface Emitting Lasers (VCSELs) are commercially important with an increasing range of applications and are required over an increasing range of wavelengths. While shorter wavelengths are in commercial manufacture VCSELs emitting at 1300nm and longer wavelengths have been claimed but are not available on a commercial basis. This stems from a failure to understand the active region materials necessary for these wavelengths and associated degradation processes. The project will focus on developing efficient processes for new materials such as dilute nitrides and bismides for long wavelength operation. We envisage new device designs resulting in excellent laser performance and also uniformity and yield over large area substrates. Students will utilise modelling packages such as nextnano and photon design for device design and a series of advanced characterisation techniques for materials and devices for understanding results before developing new device concepts.
Earth-abundant catalysts immobilised on light-absorbing III-V semiconductor electrodes for sustainable solar-driven CO2 reduction
Professor Julia Weinstein, University of Sheffield
Solar-driven production of chemicals is the most promising way to sustainable future. A useful process is the light-driven reduction of CO2 into small molecules, methanol or formate. However, there is no efficient, robust system for CO2-reduction; as all existing approaches use expensive catalysts.
Aim: to develop a photoelectrochemical cell for light-driven CO2-reduction using III-V light-harvesting semiconductor electrodes and immobilised Earth-abundant catalysts.
We have recently discovered electro-catalysts for CO2-reduction based on Earth-abundant Mn instead of Nobel-metals (Re, Ru). But Mn-catalysts decompose under light! A light-absorbing semiconductor electrodes, such as III-nitride semiconductors whose bandgap covers from ultraviolet through visible to infrared, can enable light-activation of these highly promising catalysts, bringing a step-change in the area.
Effects of plasma etching and plasma deposition on the performance of VCSELs
Professor Peter Snowton, Cardiff University
Vertical Cavity Surface Emitting Lasers (VCSELs) are commercially important with an increasing range of applications and are required over an increasing range of wavelengths. The plasma etching of the cylindrical VCSEL geometry and, following steam oxidation of the current and optical apertures, passivation of the exposed sidewalls using plasma deposition are key steps in producing high efficiency devices. The project will focus on developing efficient processes for new materials such as dilute nitrides and bismides for long wavelength operation. The student will also research how the etch process effects the subsequent steam oxidation process and how leakage current and subsequent degradation are affected by etch and passivation processes. We envisage new devices designs where the surface passivation layer also creates localised strain in the surface region affecting near surface recombination and refractive index. We also envisage developing processes to provide the greatest uniformity and yield over large area substrates. Students will utilise modelling packages such as nextnano and photon design for device design and understanding device results.
Energy Harvesting for Autonomous Systems
Professor Peter Smowton & Professor Gao Min, Cardiff University
A key part of the vision of the Internet of Things is the large number of autonomous sensors relaying information back through the web. To be truly autonomous a sensor must be able to derive energy from the environment to carry out the sensing action and to relay the information back to the connected system. Here we focus on the mechanisms by which communications based on light emission by LEDs or lasers can be powered by energy harvested from the environment. The PhD project will involve understanding compound semiconductor heterostructure devices and the absorption and emission of radiation, the change and exchange of energy and entropy in such a system, designing a laser or LED powered by the heat and light available in the environment, fabricating such device and demonstrating communication of information at some level.
Engineering interaction effects in semiconductor nanostructures
Dr Sanjeev Kumar, University College London
Recently discovered fractional quantisation of conductance in the absence of quantising magnetic field in one-dimensional semiconductors using GaAs/AlGaAs heterostructure has allowed many new quantum phenomena to be envisaged which were inaccessible before. This PhD project will involve investigating interaction effects in coupled and uncoupled bilayer electron gases. A bilayer system provides an outstanding platform to investigate quantum transport as the separation between electrons can be as close as the Bohr radius therefore the quasi-particles which give rise to a fractional quantisation in conductance could be investigated in a variety of interacting regimes. The project will involve answering a number of original questions as how nature allows electrons to self-organise and give rise to fractional quantisation and the spin and charge phases of quasi-particles including entanglement. The project is experimental as well as theoretical and will involve training in the cleanroom for high-quality device fabrication and low-noise measurements at extremely low temperatures and high magnetic field.
Enhanced mm-wave load-pull for characterisation of CS technology
Dr Roberto Quaglia, Cardiff University
CS technology will be winning its race with Si-based solutions if it will keep proving itself as the best choice in terms of performance for transmitters. At mm-wave, the intrinsic better properties of CS technology can translate in better output power and efficiency only if the design of power amplifiers exploits at maximum these properties. In order to a) guarantee that the devices have the potential to be outperforming competition, and b) provide designers with the needed information to exploit the outstanding characteristics of CS transistors, it is necessary to support technology with high quality transistor characterisation capabilities. EPSRC has recently funded £1.4m for a mm-wave large signal characterisation setup which provides a world-unique platform to perform these characterisation campaigns. This project aims at boosting the capabilities of the system by introducing i) hybrid active/passive capabilities, ii) modulated signal capabilities, and iii) further frequency extension. In collaboration with the company which built the original measurement system and an end-user of the technology, which provides clear steer on what is required from the system.
Experimental Investigations into Localization and New Concepts for Qubits
Professor Sir Michael Pepper, University College London
The key element here is the role of the interaction between electrons allied to disorder which results in a new form of electron localization in which their distribution becomes frozen. The unusual consequences of this state include a finite temperature zero conductance state never previously observed, one of the consequences is the localization of energy such that heat does not diffuse. The stability of the electron states in this regime may result in the slow and measurable spread of entanglement and a decoherence free qubit structure. In a similar manner to the localization of light in certain Cold Atom structures we will be able to modify the localization of energy with the speed of its transmission leading to a study of this process in a way not previously possible. Although there is a large amount of new physics to be investigated there are also a number of aspects which point to new methods of storing and manipulating quantum information. The different configurations of electrons allow themselves to be used as elements for storage of information in a way not previously considered which can be investigated. In this project, experimental samples based on III-V semiconductors (GaAs, InGaAs, InAs, etc) will be processed in the cleanroom and measurements will be performed at UCL. Advanced 111-V device structures will form an important part of this project which will seek to develop new device concepts for quantum information.
Exploring the nanoscale optoelectronic properties of low-dimensional materials via terahertz spectroscopy and near-field terahertz microscopy
Dr Jessica Boland, University of Manchester
Low-dimensional semiconductor materials are extremely attractive in the field of nanotechnology owing to their potential as building blocks for ultrafast optoelectronic devices, including solar cells and photodetectors. Dirac materials, in particular, have emerged as promising candidates for more energy-efficient devices, owing to their perfectly-conducting surface states and doping tuneability. However, to develop functional optoelectronic devices, an in-depth understanding of carrier transport in these materials is essential. Terahertz spectroscopy provides a perfect, non-contact, non-destructive tool for examining the electrical conductivity of a material and extracting key transport parameters, such as mobility, carrier lifetime and extrinsic carrier concentration. Recent advances have also pushed the spatial resolution of this technique down to the nanoscale. By combining terahertz spectroscopy with scattering-type near-field optical microscopy (SNOM), electrical conductivity and ultrafast carrier transport in these materials can be mapped in 3D with <1ps temporal resolution and <30nm spatial resolution. This project will exploit this technique to conduct the first investigation of nanoscale carrier transport in III-V nanowires and topological insulator nanowires. It will employ surface-sensitive measurements to examine the exotic surface conductivity response in Dirac materials independently from the bulk for the first time. This will provide a unique insight into the underlying physical mechanisms governing transport in these materials that will directly feed into development of next-generation devices (namely terahertz photodetectors).
GaN based mm-wave Integrated Receiver for Communications and Radar applications
Professor Khaled Elgaid & Dr Roberto Quaglia, Cardiff University
This project concerns the development of GaN based millimetre-wave arrays receiver technology that utilise pixels composed of antenna-coupled Schottky detectors for security, radar and communications applications. The aims and objectives of this research are to design, fabricate, experimentally validate, and integrate GaN based Schottky barrier diodes, with matching circuits and antennas into an array configuration. The project will investigate key active/passive devices and integrated circuit design and technology optimisations for performance. This includes: Schottky barrier diodes intrinsic and extrinsic parameters, transmission media, integrated power dividers combiners and integrated antenna. The project will be linked with industry needs and include student interacting with an industry partner in the form of devices/circuits design, characterisation and verifications.
Graphene in III-V Semiconductor Photonics
Professor Michael J. Wale, UCL
III-V semiconductors provide the basis for highly capable integrated photonics platforms, embodying laser sources, amplifiers, modulators, detectors and other functions. Work by UCL and Cardiff has led to world-leading demonstration of integration of III-V active components on silicon, promising low-cost manufacture on large (200-300mm) substrates. 2D materials such as graphene are exciting great interest on account of their unique electronic and photonic properties, as transparent conductors, saturable absorbers, detectors, modulators and non-reciprocal elements operating over a wide spectral range. The project will explore how the unique properties of these two material systems can be combined in a single technology, using the newly developed techniques of remote epitaxy (where a III-V material can be grown on another substrate carrying a graphene film on its surface) and van der Waals bonding. This inter-disciplinary project will build on collaboration with a leading company in the field of graphene, as well as the world-leading epitaxy and photonics expertise of the academic partners. Following on from demonstration of III-V and III-V/Si structures incorporating graphene, the project will investigate the integration of complex functionality into the material platform, e.g. lasers, modulators, detectors and other elements, for use in high speed communications and sensor applications.
Hardware-in-the-Loop GaN Power Amplifier Design, Realisation and Optimisation for 5G Systems
Dr James Bell & Professor Johannes Benedikt, Cardiff University
High-data-rate, high-efficiency front-end systems are becoming key components in realising highly efficiency, long-range telecommunications systems for 5G and beyond. This project will implement a novel hardware-in-the-loop Gallium Nitride power amplifier development procedure, that will automate designs for performance and predict output metrics, allowing for optimisation prior to amplifier manufacturing and testing. The project will utilise Cardiff University’s broadband characterisation capabilities, with advanced design software and LabVIEW used to control of the characterisation steps. Amplifier designs will be fabricated using compound semiconductor technologies and tested to verify the novel design techniques and design algorithms work. Optimisation of approaches will allow for rapid load-pull characterisation – a key element volume production using emerging device technologies for future applications.
High-performance optical sources for photonic integrated platforms
Dr Samuel Shutts, Cardiff University
Compound Semiconductor (CS) lasers are the key enabling technology for optical communication and sensing. The doubling of inter-chip transmission bandwidth in servers every two years has put an increasing need on photonic integrated systems (e.g. silicon-photonic based optical-interconnects) to solve the bandwidth bottleneck problem. In addition, the emergence of applications such as light-detection and ranging (LiDAR) used, for example, in autonomous transportation, requires integration of optoelectronic components with advanced functionality. Such technologies rely on lasers that are capable of producing optical powers beyond 100mW and spectral line-widths approaching 100kHz. Large-scale uptake of integrated photonic systems will require lasers featuring low-power consumption and the ability to withstand harsh environments, e.g. temperatures beyond 100˚C.
Quantum dot lasers offer several key advantages when it comes to integration with other optical components; they are relatively insensitive to elevated temperatures and optical feedback, and are tolerant to material defects. This project will aim to develop and test quantum dot integrated optoelectronic devices, targeting applications in telecoms and LiDAR. There are a range of approaches and directions that can be taken by the student, which will be discussed with the academic and industry supervisors.
Highly-efficient SiC-based Power Converter for Electric Vehicles
Dr Wenlong Ming, Cardiff University
In this PhD project, a comprehensive comparison will be made between Silicon Carbide (SiC)-based voltage-link inverters and current-link inverters for the application of Electric Vehicles. Two 50kW 600V SiC MOSFET motor drives will be built to demonstrate the comparison, i.e., one with a traditional dc link and output EMI filter and the other with a current dc link and combined EMC and voltage harmonic filter. The study will include analysis of the cost, size and EMC performance of both methods. Key innovations of this project come from the design of the filters and control scheme for voltage/current-link inverters and measurement of the whole system efficiency.
Hybrid inorganic/organic semiconductor hetero structures for high-performance LEDs
Dr Leszek Majewski, University of Manchester
An excellent progress has recently been made in the development and commercialization of semiconductor light-emitting devices that operate in the visible part of the electromagnetic spectrum, mainly due to the rapidly growing demand for displays, solid-state lighting, and related applications. Of particular significance are the developments in gallium nitride and organic semiconductor technologies. GaN-based inorganic semiconductor structures have found application as efficient, robust, short-wavelength-visible- and UV-light sources. Organic semiconductor light-emitting diodes (LEDs) in turn can span the entire visible spectrum and have low-cost potential due to high-throughput manufacturing, opening attractive prospects for displays and lighting. These two semiconductor families have their own especial advantages and disadvantages. Most notably, GaN structures offer excellent electrical properties but are inherently prone to the complex and expensive fabrication techniques that characterize inorganic semiconductors. Their spectral coverage is also limited, although down-conversion with phosphors allows access to other colours and white-light emission. On the other hand, organic semiconductors offer excellent luminescence properties, with a greater variety of emission wavelengths and significantly higher photoluminescence efficiencies, but exhibit less good electrical behaviour. Combining the best attributes of each family would clearly be an attractive proposition. The aim of this project is to explore novel combinations of inorganic/organic semiconductor hetero structures. This will involve characterisation of the employed semiconductor materials but also evaluation of the fabricated nanostructures and light-emitting devices.
Integrated Colloidal Quantum Dot (QD) smart optoelectronic devices for the Internet of Things (IoT)
Dr Bo Hou, Cardiff University
QD and related optoelectronic devices are promising candidates for IoT applications. Benefit from their customisable bandgap, solution processability, strong light-matter interaction and robust photo/electrical stability; QDs have shown their great promise in various optoelectronic devices. According to Touch Display, Research and IDTex recently estimated that the overall QD market would reach $10.8 billion by 2026. However, it is highly challenging to obtain a comprehensive and fundamental understanding of underlying physics before they can be well integrated into IoT technology. To develop QD IoTs with minimum power consumption and sustainable energy supply, three key challenges will explore: i) Low power consumption Cd-free QD LEDs; ii)High-detectivity QD Phototransistors; iii) the Cardiff University, Physics Department and Institute for Compound Semiconductors, have extensive experience in PV and integrated electronics/photonics which can be complementary by introducing high-efficiency QD indoor solar cells technologies to demonstrate an entire solution-processed integrated IoT module.
Integration of III-nitride electrically driven single photon source and waveguide for optical communication
Professor Tao Wang, University of Sheffield
The only room temperature electrically driven single-photon source is based on a single neutral nitrogen-vacancy centre in diamond. However, it is well-known that it is a great challenge to achieve large size diamond heterostructures with high crystal quality. Therefore, it is impossible to achieve a high performance single-photon source using diamond.
Similar to diamond, III-nitride based semiconductors are also wide bandgap semiconductors, where it is easy to form a hetero-structure with a tunable band diagram, using InAlGaN alloys and well-controlled doping. Generally speaking, III-nitride semiconductors contain a large number of point defects such as Ga or nitrogen-vacancy. Such a defect in the wide bandgap semiconductors can also be used as a single-photon source for quantum cryptography due to their excellent photostability at room temperature. The last two decades have seen major progress on developing III-nitride based optoelectronics, in particular, blue LEDs/LDs, which have been commercialised. It means that both epitaxial growth technologies and device fabrication have been matured and possible growth on patterned Silicon or GaN templates allows production of emitters in novel 3D structures on semi-and non-polar GaN crystal planes.
Therefore, it is good timing to explore the possibly of developing an electrically driven single-photon source based on a III-nitride hetero-structure, which will be further integrated with III-nitride waveguides to improve light extraction. The project aims at optical communication at 1.3-1.55 µm.
Low Energy Electron Microscopy studies of GaAs growth on Silicon. Search for alternative integration strategies based on direct observation of GaAs growth dynamics
Dr Juan Pereiro Viterbo, Cardiff University
III-As semiconductors’ properties make them ideal candidates for the development of photonic circuits. The integration of photonics and current electronics requires epitaxial growth of III-As on Silicon. Up to now challenges associated to the different lattice parameters and atomic structures of these materials have hindered technological development.
Cardiff University hosts a unique Low Energy Electron Microscope combined with a III-As Molecular Beam Epitaxy reactor. This unique experimental system enables us to record videos with atomic resolution in the z direction and 5-10 nm resolution in the x/y plane of the III-As samples. The understanding of dynamic mechanisms associated to the growth of III-As on Silicon will provide an advantage to achieve full integration and understand the limits of epitaxial approaches.
This project consists of the study of growth of III-As thin films and nanostructures on Silicon substrates and will explore innovative alternatives to integration via epitaxial growth. During the PhD project the student will become an expert in Ultra High Vacuum systems, Molecular beam epitaxy and thin film growth and electron microscopy, as well as work on the modelling and simulation of growth processes. The project will suit more experimentally minded students.
MOVPE growth of Sb-containing alloys for strategic short-wave and mid-wave infrared applicationsors
Dr Qiang Li, Cardiff University
IQE is interested in supporting a student in the following typical areas of interest where it believes Cardiff University has a strong background for underpinning research into Antimony-containing materials and also those involving Indium Phosphide (InP). These materials are prevalent in strategic Short-Wave and Mid-Wave Infrared detector and laser devices used in many sensing, imaging and spectroscopy technologies. Such technologies are prevalent in any defence, security and environmental applications, to name but a few. The project will involve supporting a student strongly associated with the MOVPE tool at IQE-Si, using a technique complementary to the existing MBE process already established within the university. We anticipate the research will target areas, similar to these: InP processes for Quantum Technology/SWIR-related products, QCLs in the MIR; Sb-based mixed alloy systems Al(Ga)AsSb on InP/GaSb for e.g. detectors, SPADs; or any Sb-related MOVPE development/improvement technology.
MBE-grown 1550-nm III-V quantum-dot materials and devices on Si substrate for Si photonic systems
Professor Huiyun Liu, University College London
Silicon microelectronics has been the engine of the modern information for almost 50 years. In our everlasting quest to process more and more data at faster speeds, while using the smallest components the $100 billion silicon industry has successfully overcome many critical issues. The next critical problem in the evolution of modern information system is to overcome the limitations of metal interconnectors. Recently high performance electrically pumped 1300-nm quantum-dot lasers have been successfully demonstrated on Si substrate between Cardiff and UCL. Pushing the lasing wavelength of III-V/Si quantum-dot laser to most desirable telecom wavelength of 1550 nm will be next challenge for epitaxial growth. 1550-nm quantum-dot laser will be exploited in this project based on our techniques on Si-based and Ge-based 1300-nm InAs/GaAs quantum-dot lasers, GaAs-based GaAsSb metamorphic 1550-nm quantum dots [Appl. Phys. Lett. 92, 111906 (2008)] and conventional InP-based 1550-nm quantum dots. Once practical 1550-nm III-V/Si quantum-dot device having been demonstrated, the integration of photonic component with Si waveguides will be studied for Si Photonic System, hence further integration with Silicon microelectronics. The methodology will focus on exploiting novel growth techniques by using new epitaxial facility, Molecular Beam Epitaxy on InGaAsP/InP, InAlGaAs/InP, and GaAsSb/GaAs quantum-dot system at UCL EEE department, device-processing facilities at LCN, and study the optical and structural properties by ATM and STM at UCL.
New Quantum Phenomena with Applications in Quantum Computation
Professor Sir Michael Pepper, University College London
This project is based around new physical effects which we have discovered in the past 18 months. Using semiconductor nanostructures we have made two discoveries with important implications for quantum information processing and the development of new quantum phenomena. The first is our demonstration that using quantum wires in which electrons are confined by gates the spins line up as in a magnet. This will allow the creation of new spintronic functions and also studies of integrated quantum circuits in which both spin and charge can be controlled. The second is our discovery of fractional quantisation of conductance in the absence of a magnetic field. There have been numerous theoretical suggestions that this might occur, more or less since the discovery of the magnetic field induced Fractional Quantum Hall Effect, but the surprising feature is that we found it in a system never considered before, the relaxation of one-dimensional electrons in a second dimension. The strong repulsion causes a line of electrons to split into two or more and then form a particular configuration which we are studying both experimentally and theoretically. This new configuration corresponds to the creation of a new charge entity comprising a number of electrons which behave as if it was a new particle with an effective fractional charge. This opens up a new area with applications in quantum computation, which may resist the decoherence which scrambles the information. In this project, experimental samples based on III-V semiconductors (GaAs, InGaAs, InAs, etc) will be processed in the cleanroom and measurements will be performed at UCL.
Novel architecture and fabrication processes for single epitaxial growth distributed-feedback lasers for sensing and communication
Dr Samuel Shutts, Cardiff University
Specialised Compound semiconductor (CS) lasers with Distributed Feedback (DFB) are used for high-speed datacom networks and in LiDAR (Light Detection And Ranging) for navigation and obstacle avoidance by autonomous vehicles and robots. Applications require DFB lasers which feature high optical powers and narrow linewidths, that operate in a range of environments.
Current DFB lasers are manufactured in multiple steps by specialised suppliers at different sites: CS base layer is grown onto wafer-scale substrates, then sent to another supplier for a custom grating pattern to be defined using electron beam lithography, before being sent back to the CS material supplier to complete the structure. This extended supply chain increases manufacturing lead-times from weeks to months, and additional material handling increases chip-scale defects, lowering yield and increasing costs.
For the market to accommodate the number of devices required, the technology needs to be commoditised to drive a cost reduction of ~10x. Innovation is needed to replace the current multi-step approach. This project will focus on developing novel device architectures, associated material-scale product which will increase yield, lower production lead-times and costs of manufacture. The methods used will be verified by testing laser performance and reliability.
Optical Sources for Atomic Sensors
Dr Samuel Shutts, Cardiff University
This project will focus on developing optical sources required for alkali-vapour based chip-scale atomic sensors, including clocks and magnetometers.
The precise measurement of time is fundamental to the effective functioning of services such as transport links, mobile communications, financial transactions, security and reliable energy sources. Currently, Global Navigation Satellite Systems (GNSS) provide the timing signal for these critical infrastructure services but is vulnerable to disruption. As such, there is a need for wide-spread miniaturised atomic clocks to provide a timing reference in the event of an outage.
Measurement of magnetic fields, to pico-tesla precision is required for applications such as geophysical mapping, underwater ordinance detection and monitoring electrical pulses generated within the human body. Current magnetometer are bulky and dissipate substantial power in operation, due largely to the gas discharge lamp. Compound semiconductor light sources could reduce this power consumption significantly, whilst reducing the size and weight.
Next generation chip-scale atomic clocks and magnetometers, based on an effect know as coherent population trapping, require a reliable optical source with specific output characteristics. Vertical Cavity Surface Emitting Lasers (VCSELs) are the ideal candidates to meet the required specification and have the advantage of being compact and energy efficient. The student will have the opportunity to take part in this wide-reaching activity and develop novel ways to ensure that target specifications of VCSELs can be achieved. This will involve understanding the physics of atomic sensors, designing, implementing and testing methods to produce lasers with required characteristics, for example, single-mode emission, reduced spectral linewidth and polarisation stability.
Performance reliability of Vertical Cavity Surface Emitting Lasers
Dr Samuel Shutts, Cardiff University
There is an increasing need to develop sophisticated long-term reliability testing and performance verification of Vertical Cavity Surface Emitting Lasers (VCSELs), due to growing demand for their use in a range of diverse applications. These include, consumer technologies such as face/gesture recognition used in Smart Phones; laser detection and ranging (LiDAR) in the automotive industry; optical sources in datacentres; high-performance computing and are being considered in quantum applications such as miniaturized atomic clocks for ubiquitous precious timing.
Producing the required VCSEL specification is achieved by careful design of the constituent materials and fabricated device architecture. However, methods employed require verification, testing against performance specifications, e.g. optical power, spectral linewidth and beam profile. Importantly these requirements need to be adhered to for a sufficient length of time to be viable. Over time degradation occurs, the point at which the VCSEL no longer meets the required specification signals its ‘life-time’ or failure point.
Project outline: develop robust statistical methods to monitor degradation of VCSELs, making use of testing equipment available at the CSA-Catapult. Identifying failure mechanisms will be a key part of this project and the student will have the opportunity to explore and apply modifications to VCSEL designs, improving their longevity.
Quantum dot photonic devices in III-V and III-V on silicon platforms
Professor Michael J. Wale, UCL
Over the last twenty years, quantum dot (QD) structures have demonstrated unique capabilities in semiconductor lasers, offering low threshold currents and temperature-independent operation. Furthermore, they are the basis of the world-leading development of III-V semiconductor lasers grown epitaxially on silicon, offering prospects for large-scale, low-cost manufacture on large (200-300mm) substrates. In contrast, development of quantum dot structures for integrated optical modulators, detectors and other functions is still at an early stage, despite early demonstrations of strongly enhanced electro-optic effects in QD media. We perceive great potential in QD devices in high-functionality integrated optical platforms based on III-V semiconductors on silicon, both in terms of performance (high speed, small size and low drive Voltage) and in facilitating the growth of effective, reliable device elements on lattice-mismatched substrates.
This project will explore the potential of III-V quantum dots for high functionality integrated photonics platforms, complementing and adding to the laser capability already demonstrated. It will embrace the development of epitaxial material and device structures on native substrates (GaAs, InP) and III-V materials on silicon, and will explore the limits of performance in this new device technology, particularly for very high speed (>100GHz) applications in high bit-rate communications, mm-wave and THz systems.
Quantum dot lasers for next generation of datacenters and hard drives
Dr Nicolás Abadía, Cardiff University
Seagate has recently demonstrated a groundbreaking hard drive that can store up to 20TB and is ready for commercialization in late 2020. Behind this achievement lies the Heat Assisted Magnetic Recording (HAMR) technology . By temporarily heating the magnetic material at the time of recording, the areal density capacity of the hard disk drive can be significantly increased up to 2 Tbpsi . To heat the recording material, a laser is used inside the read/write head of HAMR drives. The goal of the project is to develop quantum dot lasers and coupling mechanism to excite the Near Field Transducers for HAMR drives. The PhD student will design and characterize a quantum dot laser and the coupling mechanism suitable for HAMR applications. Based on our previous research [1,2], the work will be carried out in close collaboration with the world-leading American company Seagate and University College London.  C. Zhong, N. Abadía, et al. ‘Effective Heat Dissipation in an Adiabatic Near-Field Transducer for HAMR’. Optics Express (2018). DOI:10.1364/OE.26.018842
 N. Abadía, C. Zhong, et al. ‘Optical and Thermal Analysis of the Light-Heat Conversion Process Employing an Antenna-Based Hybrid Plasmonic Waveguide for HAMR’. Optics Express (2018). DOI:10.1364/OE.26.001752
Quantum Integrated Circuits and Electrons Spins in Semiconductor Nanostructures
Professor Sir Michael Pepper, University College London
Proposed schemes of quantum information and computation utilise the spin of the electron either singly or in the form of double/triple quantum dots with more complex spin textures. It is proposed to investigate nanostructures based on GaAs, InGaAs and InAs which integrate a zero-dimensional quantum dot with a quantum wire which can act either as a spin polarizer or as a spin detector depending on the configuration. Single, Double and Triple Quantum Dots can act as qubits in quantum information schemes and the resistance of the quantum wire is sensitive to electron distribution nearby and so can act as a readout mechanism of the spin configuration such as singlet, triplet and a more general spin state. Simulations will be performed on the spin states of different quantum dot configurations and the accuracy of reading out the spin states with a quantum wire detector and possible applications as qubits in a quantum information scheme. In order to accomplish this the operating limits such as temperature, geometry and factors affecting the spin polarization will be investigated and calculated. For example, as the temperature is raised, or the carrier concentration in the wire is lowered, the spin direction becomes increasingly ill-defined and the incoherent regime is reached. At this stage the nature of transmission through the dot will be investigated as it is an open question as to how the electrons will be transmitted if the spin is not a good quantum parameter. The project will involve compound device fabrication and measurements at UCL.
Surface Activated Wafer Bonding for the next generation of hybrid devices
Prof Oliver A Williams, Cardiff University
Compound semiconductor devices are often thermally limited under high power conditions due to their modest thermal conductivity. Some piezoelectric compounds have relatively low acoustic wave velocities that limit the frequencies they can operate at, precluding their exploitation in 5G networks. Many of these problems can be alleviated by a hybrid materials approach, that is by combining materials with complementary properties. For example, diamond has been grown on GaN to yield three times the power handling capabilities or power transistors. Diamond layers integrated with Aluminium Nitride have produced the highest frequency performance of Surface Acoustic Wave filters which are currently be investigate for 5G base station filters. Growing one material on another is always limited by thermal expansion coefficients, as materials are generally grown at elevated temperatures. One solution to this is to grow the two materials independently and bond them together. In this project we will develop a Surface Activated Wafer Bond (SAWB) that will in principal allow any two materials to be bonded together. This process is carried out by polishing both materials atomically flat, irradiating them with an atom beam to create dangling bonds and then fusing them together under high vacuum at room temperature. We will focus on bonding diamond (grown and polished in house), Ga2O3, GaAs, LiNbO3 and GaN. Once the process is working it can be easily applied to different materials.
Theoretical modelling of optical nonlinearity in semiconductor quantum dot devices
Dr Sang Soon Oh, Cardiff University
Photoluminescent semiconductor quantum dots (QD) have been commercially used for the light emitting devices and also used as single photon emitting sources in quantum optic devices. However, there are still many issues to be resolved such as balanced and efficient carrier injection and long-term stability. Together with the optimization of electrical properties, the optical design of devices is essential to achieve required or better light extraction efficiency. In this project, the PhD student will study optical nonlinear processes in QDs of different materials in particular optical down conversion and dynamic excitation of carriers. To predict the optical properties of semiconductor QD devices, both numerical modelling on optical modes and analytical modelling for carrier and photon dynamics will be carried out. The developed models will be applied to improve the design of commercial on-chip quantum dot LEDs and analyse the optical properties of QD embedded single photon emitters.
Short-gate GaN HEMTs for mm-wave integrated circuits
Professor Khaled Elgaid, Cardiff University
There is an explosion of demand for high frequency circuits able to perform efficiently at mm-wave, driven by applications such as automotive radar, imaging, 5G, security. From a device manufacturing point of view, higher frequency of operation requires to target aggressive gate length, being careful to address simultaneously other specifications avoiding to jeopardize other figures of merit.
This project, supported by NWF, will target the fabrication, characterization, and optimization of GaN HEMTs with <100nm gate length for W- and D- band applications. The candidate will use both the University and company premises to optimize the process steps to achieve the needed process accuracy and repeatability. Advanced characterization will be used to assess the progress of the technique developed.
Short-Gate Length GaN-on-Diamond transistor technology for High Frequency MMIC Power Amplifier Applications
Professor Khaled Elgaid, Cardiff University
Microwave power amplifiers require a mix of sophisticated electrical design, electromagnetic insight and thermal considerations, all based on the most modern semiconductor technologies. GaN is one of those semiconductor materials that can be used to fabricate transistors with high voltage and high current capability and sufficient gain to operate at high frequencies (>10 GHz). The combination of high voltage and high current, however, leads to high dissipation. To prevent overheating, the active areas need either a large physical separation or the output power necessarily has to be reduced. Improved cooling by bringing diamond, both an isolator as well as an excellent thermal conductor, directly underneath the active area would allow to reduce the separation and/or junction temperature and thus increase the power that can be extracted out of a transistor within a specific area.
The research target, in one sentence, is hence to design, fabricate and characterize short gate-length GaN-on-diamond transistors, capable of state-of-the-art power generation, and preferably combine several of them in an MMIC-like manner into a fully integrated power amplifier.