Materials World Network to Optimize the Growth of InGaN Quantum Dots within High Quality Optical Micro-Cavities

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

Abstract

Materials scientists have been studying crystals - large and small - for many years. However, very tiny crystals - crystals only a few atoms across - exhibit some really surprising properties, which we are only just starting to understand. In terms of their optical properties, these very small crystals, which we call quantum dots, exhibit behaviour more similar to that of an individual atom, than that of a large crystal. This surprising observation - which is a consequence of the confinement (or trapping) of charge carriers within a very small region - is more than just a weird academic curiosity. Scientists hope to exploit quantum dots to allow improved performance in light sources such as laser diodes, and to develop completely new light sources which might be used in novel computers or in secure communication. For light sources emitting in the red or infra-red, researchers are already starting to realise some of these goals using a material called indium gallium arsenide. However, for light emission in the blue - which is particularly relevant to applications such as high density data storage and satellite-based communications networks - quantum dots made from different materials are required. For light emission in the blue spectral region, quantum dots made from indium gallium nitride (or InGaN) could be used. Quite apart from their convenient wavelength of emission, InGaN quantum dots might be rather flexible, since their emission can be adjusted by applying an external electric field. Also, by surrounding the InGaN quantum dots with an optimal matrix material, it may be possible to force them to exhibit their peculiar properties at room temperature, whereas quantum dots emitting in the red usually have to be cooled down to temperatures more than 200 degrees below freezing before they work properly. Unfortunately, InGaN quantum dots also have disadvantages. They are usually formed on top of layers of another semiconductor - gallium nitride. Gallium nitride is quite difficult to make, and contains many mistakes, or defects, in the crystal. The defects may become electrically charged, and the presence of this charge alters the properties of the quantum dot. Since the electrical charge on the defect varies with time, so does the behaviour of the quantum dot - leading to problems with the operation of a quantum dot device. In order to try to understand the properties of the InGaN quantum dots more thoroughly, and to improve the properties of quantum dot devices, we have decided to incorporate the quantum dots into optical cavities. An optical cavity is a structure within which light may be confined. By trapping the light emitted by the quantum dot within a small volume, we can force the quantum dot and the light to interact strongly, and this can lead to more efficient emission from the quantum dot. By understanding the interactions between the light and the quantum dot, we can also use the cavity as a tool to probe the details of the quantum dot's behaviour and its interactions with any defects in its immediate surroundings. We hope to use the cavities to tailor the quantum dots' properties so that they are easier to exploit in future applications. However, making the cavities is very challenging, particularly since we have to find routes to do this which do not damage the quantum dot. Since this is a very complex problem, we have set up an international collaboration in order to attack it more effectively. Two British research groups with expertise in InGaN quantum dots will collaborate with an American research group which has world-leading capability in cavity fabrication. Together, we hope to be able to develop quantum dot - cavity systems which allow very strong interactions between the quantum dot and the cavity. In the future such systems will be used not only as a probe to study the quantum dot properties but as a major building block of novel light sources.

Planned Impact

One key aim of this study is to achieve efficient single photon emission from InGaN QDs at room temperature - or at least at temperatures accessible by Peltier cooling. Hence, as well as potential providing a compact and convenient single photon source for use in the laboratory, this technology may potentially be transferred into the wider world, where it might be used in free-space quantum cryptography. Such quantum cryptographic applications might be short-range (involving communication between a mobile electronic device and an ATM for example), or long range (involving communication between satellites outside the earth's atmosphere). In the long-range situation in particular, using blue or UV wavelengths would give the lowest beam divergence for the currently available, weight-limited telescopes, minimizing losses from the quantum key distribution system. These envisaged applications of our technology would lead to many members of society benefiting from enhanced security of data communication whether in a financial or a defence context. This illustrates the potentially broad impact of our work. Additionally, this project has specific educational benefits, both to those who will be directly involved in the research and to the wider community. The opportunities provided by the Materials World Network scheme will allow UK scientists to spend time in a renowned US lab, learning skills and concepts in cavity fabrication. The arrangement of a buddying scheme between researchers on opposite sides of the Atlantic will facilitate exchange of both scientific and cultural concepts and provide a broad range of prospects for mentoring and career development. Hence, post-doctoral researchers and graduate students involved in the programme will have a broad set of both scientific and transferrable skills with which to seek further employment. Younger researchers working on Nitride Semiconductors in both Oxford and Cambridge have previously gone on to employment in industry in the UK, in both larger companies and SMEs, and we envisage that personnel trained under this programme will form a particularly useful resource to UK industry. Lastly, we aim to extend the educational benefits of our program to undergraduate students and to the general public. We will start modestly, recruiting a few undergraduate students to carry out summer research projects in the labs of the PIs in either the U.S. or U.K., but also providing the opportunity for U.K. students to spend 1-2 weeks in the U.S. lab, and similarly for the U.S. students to spend time in a lab in the U.K.. We would schedule the exchange so that all participants in the program could get together for a mini-convocation or exchange of research results. In a more ambitious program, which we aim to develop later in the MWN project, having accessed other funding sources, we propose to extend this exchange to high school (or sixth-form) students, also providing them with a research and learning framework during the summer, and culminating in a short visit to the other host country, under the guidance of a teacher who is also a participant in the program's activities. We hope to be able to accommodate students with local host families during their stays. Also, the US and UK investigators intend to collaborate in developing educational materials which will allow them to clearly and effectively explain their research to the wider community, in particular to children and young people. By working together and sharing experiences of educational outreach, the investigators hope to be better able to inspire the next generation of researchers.

Publications

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Aharonovich I (2013) Low threshold, room-temperature microdisk lasers in the blue spectral range in Applied Physics Letters

 
Description In this project we have developed materials that allow nitride quantum dots (QDs) to be combined with optical cavities and have investigated their properties, aiming to optimise the dots and cavities for application in single photon sources for quantum computation and quantum cryptography.

We have developed methods to grow nitride quantum dots within heterostructures that permit optical isolation of optical cavity structures, and allow external electric control. We have then used these QD-cavity systems as a sensitive diagnostic to probe the optical behaviour of the InGaN QDs and also of InGaN QWs and their interaction with the environment. In particular we have shown that dislocations are one cause of spectral diffusion in these systems and that these defects also degrade cavity quality factors. We have investigated structures in which the barrier to carrier escape from the QDs is increased to try to achieve higher temperature operation of the QDs. In particular, we successfully integrated InGaN quantum dots with AlGaN barriers. We have developed methods to tune InGaN quantum dots into resonance with cavities, including a novel method involving etching in water under ultra-violet light.

All the above experiments were performed on quantum dots on a polar plane, which exhibit large internal electric fields. This leads to relatively inefficient emission and exacerbates the problems with spectral diffusion. We have recently developed a new method for quantum dot growth on an alternative crystal plane which results in greatly reduced internal electric fields. We have demonstrated that these new quantum dots are more efficient emitters and that they exhibit a key quantum phenomenon, Rabi oscillations, which indicates they are very promising for incorporation into future quantum technologies. This suggestion is further supported by the fact that these QDs show better temperature stability than our original structures.
Exploitation Route Initial exploitation of single photon sources is likely to be in the laboratory context, but looking further into the future, they may also be used in quantum key distribution at "quantum ATMs" allowing many people to use personal electronic devices to safely exchange secret "keys" which are then used to securely encrypt internet financial transactions. We would anticipate that these quantum structures could be exploited as single photon sources in the context of quantum information processing or quantum cryptography. However, significant further development work is required and we are currently finalising a new proposal for a new project on which Toshiba will be a partner in order to take these structures forwards toward exploitation.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Security and Diplomacy

 
Description Building upon the work performed in this project, we have now under a later EPSRC project submitted two patents. The patents are mainly based on that later project and this and other relevant impacts will thus be described elsewhere.
First Year Of Impact 2017
Impact Types Economic

 
Description EPSRC Responsive Mode
Amount £467,349 (GBP)
Funding ID EP/M012379/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 02/2015 
End 01/2018
 
Description EPSRC Responsive Mode
Amount £497,100 (GBP)
Funding ID EP/M011682/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2015 
End 12/2017
 
Description ERC Starting Grant: A multi-microscopy approach to the characterisation of Nitride semiconductors
Amount £1,122,605 (GBP)
Funding ID 279361 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 12/2011 
End 11/2016
 
Description RAEng/Leverhulme Senior Research Fellowship
Amount £50,276 (GBP)
Organisation Royal Academy of Engineering 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2015 
End 08/2016
 
Description Harvard 
Organisation Harvard University
Country United States 
Sector Academic/University 
PI Contribution Supply of samples
Collaborator Contribution Processing and characterisation of samples
Impact Multiple publications.
Start Year 2006
 
Company Name PORO TECHNOLOGIES LTD 
Description Poro Technologies is an early stage spinout from the Cambridge Centre for Gallium Nitride focussed on porous GaN technologies for performance enhancement in LEDs 
Year Established 2018 
Impact Poro Technologies won the 2018 Cambridge Enterprise Postdoc Business Plan competition and is currently in discussion with venture capatalists about seed funding.
Website https://www.enterprise.cam.ac.uk/news/poro-technologies-wins-2018-postdoc-business-plan-competition/
 
Description Communication using light - Hatchend School 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact A workshop on the uses of light in communications, delviered to multiple classes in thee 14-16 age group and their teachers.
Year(s) Of Engagement Activity 2018
 
Description Little Light Sources With Big Ideas (#RobinSTEM) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact A short talk on quantum dots and single photon sources as part of the Robinson College Women in Science Festival, which I organise.
Year(s) Of Engagement Activity 2018
URL https://www.robinson.cam.ac.uk/news/women-stem-festival-2018
 
Description Quantum Dots: Artifical atoms in a Quantum World 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Seminar, Queen's college, Oxford.

Dates may not be accurate

Unknowm
Year(s) Of Engagement Activity 2013
 
Description Secrets and Lights - Cafe Scientifique Bishops Stortford 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact A lecture on single photon sources and quantum cryptography
Year(s) Of Engagement Activity 2018
 
Description Thinking inside the box - cavity quantum electrodynamics 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Invited seminar, Wadham College Graduate Research forum, Oxford.

Dates may not be accurate

Unknown
Year(s) Of Engagement Activity 2013