Identification of point defects in GaN materials and their impact on device performance

The project proposes to extend the electrical characterisation techniques used for smaller gap materials by the Manchester group for decades to measure the electrical properties of defects in wide band gap semiconductors. The most important wide gap materials at the present time are GaN and InGaN. They are used to make low energy LED lighting and highpower transistors. Today's generation of these devices does not function as well as would seem possible from the properties of the materials and at the present time functionality and performance is limited. This is due, at least in part, to the presence of point defects in the component materials and devices. Eg, In LEDs so called Shockley-Reed-Hall recombination due to gap states is still a significant efficiency loss mechanism. The student will use deep level transient spectroscopy and photoluminescence techniques to identify the nature of the point defects in these materials and how they impact on real LED and transistor devices. Particular defects and impurities will be introduced, either by the industrial collaborator during growth or by using eg implantation or irradiation. The resulting defect energy levels will be then characterised using the techniques described, and the impact of device performance assessed. The nitride materials are still their infancy as semiconductors, they contain high levels of point defects, vacancies, interstitials and impurities such as carbon. The electronic behaviour of these defects is almost completely unknown. What is known is that real world devices show undesirable properties such as current collapse in transistors and anomalous efficiency droop in LEDs. These effects are believed to be due to defect states, but as of today there is no definitive demonstration of what defect is responsible. This project represents a tremendous opportunity to perform world class, high impact, research in this area.

Through our students, following consideration of the consequences of their research and appropriate action informed by their Responsible Innovation training, impact will fall into one of 3 strands:

SOCIETAL:

As a Key Enabling Technology, Compound Semiconductors (CS) bring benefit to society in general through developing the connected society, e.g. the 5G network, the smart phones that use it, satellite communications systems and data server infrastructure;

they contribute to reducing our carbon footprint through e.g. photovoltaics, new energy efficient lighting, and, power electronics for the next generation of electric vehicles.

CS sensor technology is at the heart of early medical diagnosis and CS based light sources are essential for both cosmetic treatments, such as hair removal, and life-saving treatments such as Photodynamic Therapy.

CS based magnetic sensors are being developed for security screening and next generation secure communication.

In total these technologies support our connected world, our health, our security and the environment.

ECONOMIC: The global market for CS is large, currently worth around $33.7Bn, with a compound annual growth rate of 17.3%.

The vision of the CS cluster was first defined in 2015, to build on existing academic and industrial assets, capability and manufacturing excellence to create Europe's 5th Semiconductor Cluster and the first in the world dedicated to Compound Semiconductor Technologies. To date the cluster has secured commitments of >£500M private and public investment with a suite of innovation assets and critical manufacturing infrastructure and a purpose to drive UK growth in the CS sector.

It is absolutely critical to recognise that the formation of clusters need ongoing nurturing, cross fertilisation of people and ideas and most importantly the supply of skilled staff to support rapid growth in order to reach critical mass for sustainability. The predicted PhD level jobs increase in just the current local cluster companies would more than use all of the minimum underwritten CDT output over the next 5 years, and we need to do much more. Our CDT is essential to support the development of key elements of the rapidly emerging Compound Semiconductor Cluster and drive new linkages within the wider UK industrial supply chain. Thus addressing the issue of bringing manufacturing supply chains back to the UK - a key element of the Government's Industrial Strategy.

The EPSRC CS roadmap document , June 2012, identified a concern that the UK CS Research Community is missing an exploitation link that can provide a route to impact and economic leverage EPSRC's >£20M pa CS research investment. Many technological solutions work well in the research laboratory or as one-off demonstrators but fail to translate to industrial production or commercial success. The CDT will directly address this issue by changing the mind-set of the next generation of researchers so that they start from solutions that allow rapid translation to production.

OUTREACH:

It is critical that the public and our politicians understand the excellence and importance of CS manufacturing in the UK. Our CDT cohort will undergo training in elevator pitches and media interactions to influence decision makers and will develop videos explaining how Compound Semiconductors are made and what they can do. They will inform a diverse set of people using a range of innovative formats such as performance and theatre production skills.

A crucial part of the people pipeline, which will support our future manufacturing excellence, is the motivation of our young people. Our CDT cohort will develop a Schools programme and an Undergraduate programme.

This will ensure we attract the very best and widest range of applicants and, most importantly, inform and excite the next generation about the opportunities that CS technology and Manufacturing offers them.