Time-resolved cathodoluminescence scanning electron microscope

Lead Research Organisation: University of Cambridge
Department Name: Materials Science & Metallurgy

Abstract

This proposal aims to bring to the UK an amazing microscope which will provide new and powerful capability in understanding the properties of light emitting materials and devices. These materials are key to many technologies, not only technologies that utilise the light emission from materials directly (such as energy efficient light bulbs based on light emitting diodes) but also a range of other devices which utilise the same family of materials such as solar cells and electronic devices for power conversion. Some of these technologies are in current use, but their efficiency and performance can be enhanced by achieving a better understanding of the relevant materials. Other target technologies are further from the market, but may represent the building blocks of our future security and prosperity. For example, the new microscope will provide information about light sources which emit one and only one fundamental particle of light (photon) on demand. Such "quantum light sources" are a potential building block for quantum computers and for quantum cryptography schemes which represent the ultimate in secure data transfer.

How will the new microscope allow us to advance the development of all these technologies? It is based on a scanning electron microscope, which utilises an electron beam incident on a sample surface to achieve resolutions almost three orders of magnitude better than can be achieved using a standard light microscope. It thus accesses the nanometre scale, which is vital to addressing modern day electronic devices. Standard electron microscopy accesses the topography of a surface, but the incoming electron beam also excites some of the electrons within the material under examination into states with a higher energy. When these electrons relax back down to their usual low energy state, light may be given out, and the colour and intensity of that light is incredibly informative about the properties of the material under examination. This light emission can be mapped on a scale of ~10 nanometres so that nanoscale structures ranging from defects to deliberately engineered quantum objects can be addressed. This technique is known as cathodoluminescence, and has been in use for many years.

The new capability of our proposed system is that it will map not only the colour and intensity of the light emission, but also allow us to measure the timescales on which an electron relaxes back down to its low energy state. We use the phrase "in the blink of an eye" to describe something that happens extraordinarily quickly. A real eye blink takes at least 100 milliseconds, whereas the relevant timescales for the electron to return to its low energy state could be almost 10 billion times quicker than this! The new microscope will be able to measure processes occurring on this time scale, by addressing how long after an electron pulse excites the material a photon is emitted. It will even be able to distinguish between photons with different wavelengths (or colours) being emitted on different time scales. Crucially, coupling this time-resolved capability with the ability to vary the temperature, we will be able to infer not only the time scales on which electrons relax to low energy sites emitting a photon, but also the time scales by which electrons reduce their energy by other, non-light-emitting routes. These non-light-emitting processes are what limit the efficiency of light emitting diodes, for example. Overall, across a broad range of materials, we will build up an understanding of how electrons interact with nanoscale structure to define a material's electrical and optical properties and hence what factors limit or improve the performance of devices.

The proposed system will be the most advanced in the world, and will give UK researchers working on these hugely important photonic and electronic technologies a global advantage in developing new materials, devices and ultimately products.

Planned Impact

This is a proposal for a piece of equipment intended to be used by a very broad range of academic and industrial researchers. It focuses primarily on materials for photonic, optoelectronic and electronic device applications, with the aim of accelerating the development of current devices and the invention of new devices based on novel or emerging materials systems. Within this broad range of research the system will enable, we give examples below of potential impacts on the economy, society, and the people pipeline, but note that the long term impact of the system, which is expected to have a lifetime of more than 20 years could be enormously broader and deeper than anything that can currently be envisaged:

(1) Economy
Research on semiconductor devices using the TRCL system can benefit the UK economy by either helping existing UK companies or by helping researchers to spin out new companies, generating employment and economic growth. Examples of such opportunities would include improvements to GaN LED technology benefitting Plessey Semiconductors Ltd, who have an established collaboration with the PI. New spin out companies or profitable IP might arise from any of our focus research areas, although perhaps the most freedom to operate is available in the technologies which are furthest from commercialisation such as defect-based quantum light sources. Other benefits to the economy will accrue from the direct access UK industry will have to the TRCL system to perform their own research and development with appropriate support from University personnel. This industrial use of the system will also help facilitate university-industry collaboration with the potential for further economic benefit.

(2) Society
Individual technologies which will be researched using the TRCL system will have specific societal benefits. For example, the development of cheaper solar cells with higher efficiencies could be facilitated by TRCL studies and will allow improvements to sustainable energy generation. Beyond these specific advantages, the beautiful images generated by the CL system present a wonderful opportunity for enhanced public engagement with science. The PI is an experienced science communicator who has experience using microscopy data to engage audiences from very small children to pubs full of (slightly inebriated) adults. Across the board, these opportunities are not only enriching for society but provide the opportunity to explain the scientific method and the way that scientists use evidence, to help foster trust in the research community in the general public.

(3) People pipeline
This project will support the UK people pipeline at a number of different levels. The public engagement activities described above will, where they involve young people, promote careers in science. The fact that this proposal is led by an all-female investigator team gives us a specific opportunity to promote science career opportunities to young women, particularly since the PI is the Chair of the Robinson College Women in Science Festival which annually attracts more than 100 female A-level students. Beyond these young people, the TRCL in and of itself offers unique opportunities for training. Users will develop skills in both advanced microscopy and time-resolved spectroscopy as well as an enhanced knowledge of the materials they are studying. The broad user base will provide us with an opportunity to train a significant body of multi-skilled users, and the relevance of the technique to a range of Centres for Doctoral Training (for example the NanoDTC at Cambridge) will ensure students as well as post-docs can access this opportunity. Moreover, we will provide specific training workshops to enhance user knowledge and understanding and to foster interdisciplinary links. Overall, the user base of this instrument will provide the UK people pipeline with people with an unprecedented range of skills and knowledge.
 
Description This award funded a new time-resolved cathodoluminescence scanning electron microscope. The microscope has now been installed in Cambridge, and very interesting results are starting to emerge on a range of materials including cubic GaN and hybrid perovskites.
Exploitation Route Ongoing work.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Energy

 
Description St Gobain- Lumilog 
Organisation Saint-Gobain Lumilog
Country France 
Sector Private 
PI Contribution Characterisation of samples to investigate leakage pathways in GaN electronics
Collaborator Contribution Provision of samples for characterisation
Impact A report will be made to the relevant industry
Start Year 2020
 
Description Time resolved cathodoluminescence grand opening 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact An open day was held to announce the availability of our new cathodoluminescence microscope and to advertise it to academia and industry.
Year(s) Of Engagement Activity 2019
URL https://www.gan.msm.cam.ac.uk/news/time-resolved-cathodoluminescence-is-ready