We run a Call for Projects annually. Supervisors submit projects which are rigorously screened for fit to the remit and aims of the CDT, and the quality of the research and student experience on offer. Students choose from a ‘catalogue’ of these projects made available in December in the first term of the MSc year. They have a few weeks in which they research the projects that interest them most to make sure they match their interests, before indicating their four preferred options in early February. Projects are allocated in early March. These are then worked up into the 12 week MSc research projects which run from June and which then continue as the PhD project from October. Below are examples from previous years:
III-V quantum dot lasers grown on patterned silicon and SOI substrates by MBE – 2021
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 – 2022
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 – 2021
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 – 2022
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 – 2021
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 – 2021
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 to 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.
Dilute nitride and other approaches to 1300nm VCSELs – 2021
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 – 2022
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.
Engineering interaction effects in semiconductor nanostructures – 2022
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.
Epitaxy approach to achieve InGaN/GaN based green VCSEL laser diodes for display applications – 2022
Professor Tao Wang, University of Sheffield
It is expected that a pico-projector using three coherent and compact laser diodes (Red, Blue and Green), which can be built into a smart-phone, is the next electronic “must-have” gadget. However, green LDs with satisfied performance for the particular application are missing at the moment. A vertical cavity surface emitting laser (VCSEL) exhibits a number of major advantages compared with any other laser diodes, such as low threshold current, circular beam, which would be best for the fabrication of microdisplay or pic-projectors with ultra-high resolution and ultra-high efficiency.
There exist a number of great challenges in achieving green VCSELs with satisfied performance. Firstly, a DBR with a reflectivity approaching 100% is required. Furthermore, excellent performance of InGaN/GaN multiple quantum wells with high indium content is required, which is particularly challenging due to the intrinsic nature of III-nitrides. In addition, device fabrication is another major challenge due to the well-known current spreading issue.
In order to address these challenges, the Sheffield team has developed selective overgrowth of III-nitride emitters with high performance on a microscale. Furthermore, the Sheffield team has also well-established electro-chemical etching processes, allowing them to achieve lattice-matched DBRs with a high reflectivity in a wide spectral range from blue to red. The project proposes to further epitaxially integrate both techniques, aiming to simplify device fabrication processes and to enhance optical performance of InGaN/GaN MQWs in the green spectral region.
Exploring the nanoscale optoelectronic properties of low-dimensional materials via terahertz spectroscopy and near-field terahertz microscopy – 2021
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).
Graphene in III-V Semiconductor Photonics – 2022
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.
High-performance optical sources for photonic integrated platforms – 2021
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.
Integrated Colloidal Quantum Dot (QD) smart optoelectronic devices for the Internet of Things (IoT) – 2021
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.
Modelling and optimising inductively coupled plasma etching tools – 2022
Dr Jerome Cuenca, Dr Jonny Lees and Professor Oliver Williams, Cardiff University
The inductively coupled plasma (ICP) is a fundamentally important reactive ion etching (RIE) tool in semiconductor wafer processing. Gas precursors in a vacuum chamber are dissociated by a radio frequency (RF) plasma, creating a soup of electrons, ions, free radicals and various other species that diffuse to and etch the surface of a wafer. Understanding this whole process and how the ion density is affected by practical control parameters is key to optimising semiconductor etch processes.
In this project, we will investigate methods of simulating the plasma ion density in a typical ICP-RIE chamber. This will be achieved by developing a finite element model of a typical ICP-RIE chamber and the results will be compared with experimental measurements. Several parameters will be investigated including RF input power, chamber pressure, geometry, temperature and gas flow. The outputs of this project will provide valuable insight into the etch processes employed in state-of-the-art ICP-RIE systems.
MOVPE growth of Sb-containing alloys for strategic short-wave and mid-wave infrared applications – 2021
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 – 2021
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.
Monolithically integration of GaN-based n-channel and p-channel hetero-junction field effect transistors for power electronics applications. – 2022
Dr Kean Boon Lee, University of Sheffield
The power electronics market has seen rapid growth and is currently forecast to reach $21bn by 2024 as a results of emergence of applications such as hybrid/electric vehicles in automotive sector. Realising low losses semiconductor devices which are used as electronic switches at the heart of a power electronics system are critical. Gallium nitride (GaN) related semiconductor devices with higher critical field and the ability to produce highly conductive two dimensional electron gas channels offer significant improvements in efficiency and switching frequency. Despite these advantages, GaN electronic devices market adoption remains relatively low. One of the main reasons is the lack of efficient driver circuits specifically tailored for GaN transistors. This project focuses on the realisation of novel integrated driver circuits with power transistors using a GaN complementary n- and p-channel platform to produce unprecedented power loss savings in the DC-DC converters. The monolithically integration approach will reduce parasitic and enable ultra-high switching frequency (100s MHz) DC-DC conversions for future RF tele-communications systems and electric cars in the automotive sector.
Novel architecture and fabrication processes for single epitaxial growth distributed-feedback lasers for sensing and communication – 2021
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.
Quantum Integrated Circuits and Electrons Spins in Semiconductor Nanostructures – 2021
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.
Short-gate GaN HEMTs for mm-wave integrated circuits – 2021
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.