Study of semi-polar and non-polar nitride based structures for opto-electronic device applications

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

Abstract

Over the last 10 years gallium nitride based light emitting diodes (LEDs) have found widespread use as the active light emitting element of various optical displays, ranging from traffic lights to large area displays in, for example, sports stadia. This revolution in display technology has occurred because gallium nitride LEDs have not only the ability to generate blue and green light but also very efficiently, both of these attributes were not previously possible with other types of LED. Despite this revolutionary leap forward in display technology gallium nitride LEDs offer still further opportunities of developing not only even more efficient displays that can be used in televisions but also very efficient lighting systems, so called Solid State Lighting (SSL).
At the heart of most modern televisions is a liquid crystal display unit that is capable of displaying today's high definition programs. The liquid crystal display works by either transmitting or absorbing light when an electrical signal is applied to the crystal. For this to occur the light that is shone from the back of the crystal towards the viewer has to be polarised in a particular direction, i.e. the maxima and minima that make up the light wave light lie in a particular direction. Conventional light sources, including the latest generation of LED, emit unpolarised light so to make the light suitable for use in a liquid crystal based television means that the light has to be passed through a light polariser thus rejecting approximately 50% of the emitted light. Clearly this is an inefficient system and the overall efficiency of television displays would be greatly improved if an efficient light source could emit polarised light. By growing the nitride based LEDs on new forms of template, so-called semi-polar and non-polar crystals, it is possible to fabricate polarised light sources offering us the possibility of significant energy savings. At the moment the fundamental scientific questions that govern not only how well the light is polarised but also efficiency of the light generation process are not understood. In this program we will investigate these issues by making a comprehensive study of both the materials and underlying physics that will enable the fabrication of a new generation of liquid crystal based displays for inclusion in low power consumption televisions.
SSL is viewed as the most likely replacement for incandescent light bulbs and the current generation of compact fluorescent lamps. From this application alone the scale of the potential for energy saving can be judged by the following: "By 2025, SSL could reduce the global amount of electricity used for lighting by 50%. In the US alone this would alleviate the need for 133 new power stations (1000 MW each), eliminate 255 million metric tons of CO2 and save $115 billion of electricity costs." (The Promise of Solid State Lighting for General Illumination, US Department of Energy, 2000). The basis for SSL systems is that white light can produced by either using the combined output of a blue light emitting LED and a yellow light emitting phosphor or be combining the output from blue, green and red light emitting LEDs. The highest light generation efficiency achieved so far is ~70%, while in its self is a remarkably high figure for SSL to be employed in our offices requires efficiencies approaching ~90%. This step forward has so far proved impossible and it is widely believed that this is due to intrinsic reductions in the rate of light emission caused by internal electric fields. These fields can be reduced or eliminated by the growth of LEDs on semi-polar or non-polar templates. The promise of highly efficient LEDs using this methodology remains unfulfilled principally due to the difficulties of growing crystals of the required quality. We anticipate by using novel and improved methods of crystal growth that these problems can be overcome allowing the promise of SSL to be fulfilled.

Publications

10 25 50
 
Description We have found that the growth of InGaN QWs on sapphire can result in prohibitively high densities of defects that limit the polarisation properties of non polar LEDs. This problem can be overcome by growth on GaN substrates leading to very strongly linearly polarised light. Different non-polar planes have surprisingly different optical properties however. We have performed the first atom probe tomography investigation of non-polar quantum wells in order to investigate this issue, and found startling evidence for non-random indium clustering in InGaN quantum wells.
We have also shown that a phenomenon of efficiency droop, whereby LED efficiency drops as current increases, occurs in non-polar nitrides at similar carrier densities to those at which it occurs in polar nitrides, suggesting that the droop phenomenon is intrinsic and not a product of the electric fields.
Exploitation Route Our work has resulted in improved understanding of the issues facing the growth of non and semi polar QWs on non native substrates. Following on from our successful fabrication of non polar InGaN QWs we are currently testing the design of a polarised light LED, which could be incorporated into display technologies, as a low energy solution.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Environment

 
Description IoP Photonics Roadmapping Workshop
Geographic Reach National 
Policy Influence Type Participation in a national consultation
URL http://www.iopblog.org/roadmapping-the-photonics-sector/
 
Description EPSRC Responsive Mode
Amount £497,100 (GBP)
Funding ID EP/M011682/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom of Great Britain & Northern Ireland (UK)
Start 01/2015 
End 12/2017
 
Description EPSRC Responsive Mode
Amount £467,349 (GBP)
Funding ID EP/M012379/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom of Great Britain & Northern Ireland (UK)
Start 02/2015 
End 01/2018
 
Title XRD determination of alloy content semipolar nitride layer 
Description New method to compute the alloy content of InGaN of AlGaN material by X-ray diffraction. 
Type Of Material Improvements to research infrastructure 
Year Produced 2013 
Provided To Others? Yes  
Impact Allow correct alloy content to be computed, thus modelling of quantum well can be done accurately and the theory can be tested appropriately. 
URL http://dx.doi.org/10.1063/1.4817422
 
Title Research data supporting "Local carrier recombination and associated dynamics in m-plane InGaN/GaN quantum wells probed by picosecond cathodoluminescence" 
Description Research data in support of the publication "Local carrier recombination and associated dynamics in m-plane InGaN/GaN quantum wells probed by picosecond cathodoluminescence". We have included the original data (tab-separated text files) as plotted for the quantum wells, measured by spatially- and time-resolved cathodoluminescence. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Description Tyndall 
Organisation University College Cork
Department Tyndall National Institute
Country Ireland, Republic of 
Sector Academic/University 
PI Contribution Provision of data on the structural and optical properties of non and semi-polar quantum wells
Collaborator Contribution Theoretical modelling of non- and semi-polar quantum wells
Impact Publication: DOI: 10.1063/1.4868692
Start Year 2010
 
Description Cambridge Science Festival 2015 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Over 100 people attended an interactive talk which sparked questions and discussions afterwards. The science festival had positive feedback that attendees enjoyed the event and we have been asked to put on a larger event in 2016.
Year(s) Of Engagement Activity 2015
URL http://www.sciencefestival.cam.ac.uk/about/past-festivals/2015-cambridge-science-festival
 
Description Chesterton Science Club (Communication using Light) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact An interactive talk on "Communication using Light" was delivered to the Chesterton Community College Science Club, incorporating elements on LiFi using LEDs and quantum communication using single photon sources.
Year(s) Of Engagement Activity 2016
 
Description EPSRC Stand - Big Bang 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Schools
Results and Impact Many school children increased their awareness of energy efficiency issues

This activity was coordinated by the EPSRC and they would be more able to comment on such impacts.
Year(s) Of Engagement Activity 2013
 
Description EPSRC Stand - Cheltenham 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Schools
Results and Impact Many school children had increased awareness of energy efficiency issues.

This activity was coordinated by the EPSRC, who would be more able to comment on this than me.
Year(s) Of Engagement Activity 2013
 
Description EYFS and KS1 workshops at Cambridge Science Festival 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact A series of workshops on light and electricity was held for early years foundation stage and key stage 1 children as part of the Cambridge Science Festival. This sparked discussion with the children's parents also. This series of workshops will be repeated at Cambridge Science Festival this year and in local schools.
Year(s) Of Engagement Activity 2016
 
Description GaN Group Stand - Cheltenham 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Schools
Results and Impact Many schoolchildren have increased awareness of energy efficiency issues and enjoyed an opportunity to experience some simple lab work.

Visitors mentioned that the information on energy efficient lighting might change what light bulbs they would buy. This may result in changed buying patterns and decreased energy usage.
Year(s) Of Engagement Activity 2014
 
Description Researcher profiles website 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Schools
Results and Impact A website has been set up with career profiles for members of the Cambridge Gallium Nitride Centre as a resource for school and University students considering a career in research.
Year(s) Of Engagement Activity 2016,2017
URL http://www.gan.msm.cam.ac.uk/resources/profiles
 
Description Robinson Science Residential 
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 Schools
Results and Impact Approximately 40 sixth formers took part in a 2 day even at Robinson College to which I contributed a talk, tutorial content and a practical session. Robinson reported increased student applications in related subject areas.
Year(s) Of Engagement Activity 2015
URL https://www.robinson.cam.ac.uk/access-and-outreach
 
Description Robinson Women in Science Festivale 
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 Over 120 female sixth formers took part in a day festival aimed at encouraging participation in the Physical Sciences. In addition to organising the day, I gave a presentation on my research, chaired a student talk session and organised a practical session. Feedback from the sixth formers was very positive, several have been influenced to apply to study science at University and a second event is being organised.
Year(s) Of Engagement Activity 2015
URL https://www.robinson.cam.ac.uk/access-and-outreach
 
Description Science festival talk (How Can GaN Change Your Life?) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact An interactive talk was delivered to a large audience at the Cambridge Science Festival, outlinging a number of different uses for Gallium Nitride from lighting to quantum communications.
Year(s) Of Engagement Activity 2016
 
Description Talk to visiting school group (Communications using light) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact An interactive talk on "Communication using Light" was delivered to a group of sixth formers visiting the department, incorporating elements on LiFi using LEDs and quantum communication using single photon sources.
Year(s) Of Engagement Activity 2016