Materials Challenges in GaN-based Light Emitting Structures

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

Abstract

Gallium nitride (GaN) is an amazing material that can emit brilliant light. GaN light emitting diodes (LEDs) first became available about ten years ago, and are already used in a wide range of applications, including interior lighting in cars, buses and planes; traffic lights, large full-colour displays and backlighting in mobile phones. GaN blue lasers are about to be sold for next-generation DVD players, in which the DVDs will contain up to ten times the amount of music or pictures as existing DVDs. Looking to the future, GaN may make possible high-quality, high efficiency white lighting which will produce major energy savings. Another exciting development could be high-efficiency deep ultra-violet LEDs for water purification, particularly in the developing world.Unfortunately, we are currently unable to make the high-efficiency white lighting and deep-UV LEDs referred to above because there are some key scientific problems that remain to be solved. To successfully surmount these challenges requires a detailed understanding of the complex processes involved in the fabrication of the light emitting regions of the LED. These consist of thin layers of an alloy called InGaN, which are sandwiched between thicker layers of GaN to make structures called quantum wells. These quantum wells are 50,000 times thinner than a human hair. We must also understand the processes that limit light emission and optimise the electrical conductivity of the many other semiconductor layers in an LED. Following on our highly successful work on GaN of the last five years which has put us into an internationally competitive position, we have put together a team of leading researchers from different universities and industry to attack the critical factors that limit the performance of GaN-based LEDs.One key limitation to our understanding is the reason why GaN blue LEDs emit brilliant light even though they are full of defects called dislocations that should quench the light emission arising from the quantum wells. This is hotly debated and in 2005 two major international conferences had special sessions devoted to discussing this topic. Our theory is that the light-emitting InGaN quantum wells have atomic scale thickness fluctuations on a nanometre lateral scale, and thus the light emission is mainly localised in tiny nanometre-scale regions away from the dislocations. However, this localisation is much weaker for UV LEDs, and so unfortunately dislocations strongly quench the light emission in these devices.A major thrust of our research is to understand how the electrical carriers whose interaction is responsible for the light emission are localised, and kept away from defects which would otherwise quench the light emission, and then to optimise this localisation. This may be achieved by engineering the growth of the quantum wells. To understand the quantum wells we will not only examine the light they emit, but use microscopes that allow us to visualise objects far smaller than the wavelength of light to image detailed, atomic-scale variations within the light emitting regions. Quantum structures made from GaN also have strong internal electric fields which can reduce the light emission. We will use specialist microscopy techniques to measure these fields, and study ways of reducing them.Another focus is to develop new methods of reducing the density of defects in crystals called dislocations. Additionally, we will study the electrical properties of the GaN material which surrounds the quantum wells in an LED, in order to understand what defects prevent electrical conduction and reduce their occurrence. Our research involves crystal growers, electron microscopists, experts in optical and electrical characterisation techniques, theoretical and experimental physicists, chemists, and materials scientists. Only this type of integrated approach can solve the challenging problems in GaN-based technology.

Publications

10 25 50

 
Description We have discovered that GaN LEDs can be grown on large area Silicon substrates. We were the first group in the world to demonstrate fully processed GaN LEDs on a 6-inch Si substrate
Exploitation Route They have been taken forward and Plessey is now manufacturing LEDs based on our technology at its factory in Plymouth. This year so far it has made 1.8 million LEDs
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology

URL http://www.gan.msm.cam.ac.uk
 
Description This was a key grant which enabled us to file our first patent for the growth of low-cost GaN LEDs on large area Si substrates and to set up a spin-off company, CamGaN, in 2010.
First Year Of Impact 2010
Sector Digital/Communication/Information Technologies (including Software),Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology
Impact Types Societal,Economic

 
Description Science Policy (Round Table meetings at No. 10 and BIS)
Geographic Reach National 
Policy Influence Type Participation in advisory committee
Impact Increased funding for materials research has resulted. This will have economic impacts and on the quality of life
 
Description EPSRC
Amount £1,447,635 (GBP)
Funding ID EP/G042330/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2009 
End 10/2012
 
Description EPSRC
Amount £148,698 (GBP)
Funding ID TS/G001383/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2008 
End 02/2011
 
Description EPSRC
Amount £826,111 (GBP)
Funding ID EP/H019324/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 11/2009 
End 10/2014
 
Description EPSRC
Amount £6,330,270 (GBP)
Funding ID EP/I012591/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 12/2010 
End 11/2015
 
Description EPSRC
Amount £1,447,635 (GBP)
Funding ID EP/G042330/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 04/2009 
End 10/2012
 
Description EPSRC
Amount £6,330,270 (GBP)
Funding ID EP/I012591/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 12/2010 
End 11/2015
 
Description EPSRC
Amount £826,111 (GBP)
Funding ID EP/H019324/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 11/2009 
End 10/2014
 
Description Sharp Laboratories Of Europe Ltd
Amount £27,000 (GBP)
Funding ID RG50526 
Organisation Sharp Laboratories of Europe Ltd 
Sector Private
Country United Kingdom
Start 11/2006 
End 09/2010
 
Description Aixtron 
Organisation Aixtron Limited
Country United Kingdom 
Sector Private 
PI Contribution We grew world class GaN device structures on our Aixtron reactor(s), thus increasing Aixtron sales.
Collaborator Contribution They donated to us a senior scientist for 25% of his time. They provided free servicing and maintenance of our growth reactor.
Impact Increased sales of Aixtron growth reactors.
 
Description Forge Europa (International Headquarters 
Organisation Forge Europa
Country United Kingdom 
Sector Private 
PI Contribution Expertise. Solving a major problem with the reliability of some Forge Europa LEDs
Collaborator Contribution Advice. Testing. Market forecasts.
Impact Improved reliability and lifetimes of Forge Europa LED based products.
 
Description QinetiQ 
Organisation Qinetiq
Country United Kingdom 
Sector Private 
PI Contribution We supplied GaN-on-Si LED structures to QinetiQ for processing into devices
Collaborator Contribution They processed Cambridge grown device structures.
Impact Joint publications. A major EU grant. Multi-disciplinary: physics, materials, electronics.
 
Description Thomas Swan Scientific Equipment Ltd 
Organisation Thomas Swan and Co Ltd
Country United Kingdom 
Sector Private 
Start Year 2006
 
Description BBC Breakfast TV and BBC Radio "You and Yours" 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Interview of Prof Humphreys on BBC Breakfast TV, and on the BBC Radio "You and Yours" on low-cost LEDS sparked a lot of discussions

Increased public awareness of LEDs
Year(s) Of Engagement Activity 2009
 
Description Big Bang Fair (London) 
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 Encouraged school pupils to study science

Schools reported increased interest in science and increased numbers studying science
Year(s) Of Engagement Activity 2013,2014
 
Description Chelterham Science Festival 
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 More school pupils studying science

Schools reported greater interest in science.
Year(s) Of Engagement Activity 2013,2014