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
Amari H
(2011)
Accurate calibration for the quantification of the Al content in AlGaN epitaxial layers by energy-dispersive X-ray spectroscopy in a Transmission Electron Microscope
in Journal of Physics: Conference Series
Ashraf H
(2010)
Reduction of the dislocation density in HVPE-grown GaN epi-layers by an in situ SiNx treatment
in Journal of Crystal Growth
Badcock T
(2011)
Properties of surface-pit related emission in a -plane InGaN/GaN quantum wells grown on r -plane sapphire
in physica status solidi c
Badcock T
(2010)
Carrier dynamics in non-polar GaN/AlGaN quantum wells intersected by basal-plane stacking faults
in physica status solidi c
Badcock T
(2008)
Optical properties of GaN/AlGaN quantum wells grown on nonpolar substrates
in Applied Physics Letters
Badcock T
(2010)
Characterising the degree of polarisation anisotropy in an a -plane GaN film
in physica status solidi c
Badcock T
(2009)
Optical polarisation anisotropy in a -plane GaN/AlGaN multiple quantum well structures
in physica status solidi c
Badcock T
(2011)
The effect of indium concentration on the optical properties of a -plane InGaN/GaN quantum wells grown on r -plane sapphire substrates
in physica status solidi (a)
Badcock T
(2011)
The Effect of Dislocation Density and Surface Morphology on the Optical Properties of InGaN/GaN Quantum Wells Grown on r-Plane Sapphire Substrates
in Japanese Journal of Applied Physics
Badcock T
(2010)
Effect of overgrowth conditions on the optical properties of lateral epitaxially overgrown a -plane GaN
in physica status solidi c
Badcock T
(2007)
Low threshold current density and negative characteristic temperature 1.3µm InAs self-assembled quantum dot lasers
in Applied Physics Letters
Badcock T
(2009)
Optical polarization anisotropy of a-plane GaN/AlGaN multiple quantum well structures grown on r-plane sapphire substrates
in Journal of Applied Physics
Barnard J
(2010)
The role of rough surfaces in quantitative ADF imaging of gallium nitride-based materials
in Journal of Physics: Conference Series
Bennett S
(2011)
Atom probe tomography assessment of the impact of electron beam exposure on InxGa1-xN/GaN quantum wells
in Applied Physics Letters
Bennett S
(2010)
Mg dopant distribution in an AlGaN/GaN p-type superlattice assessed using atom probe tomography, TEM and SIMS
in Journal of Physics: Conference Series
Bennett S
(2009)
Atom Probe Tomography Studies of GaN-Based Semiconductor Materials
in Microscopy and Microanalysis
Bennett S
(2010)
Atom probe extended to AlGaN: three-dimensional imaging of a Mg-doped AlGaN/GaN superlattice
in physica status solidi c
Bennett SE
(2010)
Imaging dislocations in gallium nitride across broad areas using atomic force microscopy.
in The Review of scientific instruments
Bennett SE
(2011)
Atom probe tomography and transmission electron microscopy of a Mg-doped AlGaN/GaN superlattice.
in Ultramicroscopy
Caliebe M
(2016)
Influence of trench period and depth on MOVPE grown ( 11 2 ¯ 2 ) GaN on patterned r-plane sapphire substrates.
in Journal of Crystal Growth
Caliebe M
(2016)
Growth and coalescence studies of (112?2) oriented GaN on pre-structured sapphire substrates using marker layers Studies of (112?2) oriented GaN using marker layers
in physica status solidi (b)
Cerezo A
(2007)
Atom probe tomography today
in Materials Today
Chan C
(2013)
Optical studies of the surface effects from the luminescence of single GaN/InGaN nanorod light emitting diodes fabricated on a wafer scale
in Applied Physics Letters
Chang T
(2010)
Inclined dislocation arrays in AlGaN/AlGaN quantum well structures emitting at 290 nm
in Journal of Applied Physics
Charash R
(2009)
Carrier distribution in InGaN/GaN tricolor multiple quantum well light emitting diodes
in Applied Physics Letters
Cherns P
(2008)
Microscopy of Semiconducting Materials 2007
Christian G
(2016)
Room temperature PL efficiency of InGaN/GaN quantum well structures with prelayers as a function of number of quantum wells
in physica status solidi c
Collins D
(2010)
Q-factor measurements on planar nitride cavities
in physica status solidi c
Collins D
(2009)
Non-linear excitation and correlation studies of single InGaN quantum dots
in physica status solidi c
Corbett B
(2008)
High brightness near-ultraviolet resonant LEDs
in physica status solidi c
Das Bakshi S
(2009)
The influence of coalescence time on unintentional doping in GaN/sapphire
in Journal of Crystal Growth
Datta R
(2006)
Mechanisms of bending of threading dislocations in MOVPE-grown GaN on (0001) sapphire
in physica status solidi c
Datta R
(2008)
Origin of additional threading dislocations in AlGaN grown on GaN using AlN as an interlayer
in physica status solidi c
Davies M
(2014)
High excitation density recombination dynamics in InGaN/GaN quantum well structures in the droop regime
in physica status solidi c
Davies M
(2016)
Comparative studies of efficiency droop in polar and non-polar InGaN quantum wells
in Applied Physics Letters
Davies M
(2014)
The effects of varying threading dislocation density on the optical properties of InGaN/GaN quantum wells
in physica status solidi c
Davies M
(2014)
Effects of an InGaN prelayer on the properties of InGaN/GaN quantum well structures
in physica status solidi c
Dawson P
(2016)
The nature of carrier localisation in polar and nonpolar InGaN/GaN quantum wells
in Journal of Applied Physics
Dawson P
(2008)
Effects of resonant LO phonon assisted excitation on the photoluminescence spectra of InGaN/GaN quantum wells
in physica status solidi c
De?Sousa?Pereira S
(2008)
Controlled Integration of Nanocrystals in Inverted Hexagonal Nano-Pits at the Surface of Light-Emitting Heterostructures
in Advanced Materials
Dunn A
(2016)
Investigating efficiency droop in InGaN/GaN quantum well structures using ultrafast time-resolved terahertz and photoluminescence spectroscopy
in physica status solidi c
Emiroglu D
(2008)
High resolution Laplace deep level transient spectroscopy studies of electron and hole traps in n-type GaN
in physica status solidi c
Founta S
(2007)
Anisotropic strain relaxation in a-plane GaN quantum dots
in Journal of Applied Physics
Fraser I
(2007)
Compositional contrast in AlxGa1-xN/GaN heterostructures using scanning spreading resistance microscopy
in Applied Surface Science
Fu W
(2011)
Dislocation Climb in c -Plane AlN Films
in Applied Physics Express
Galtrey M
(2008)
Atom probe reveals the structure of In x Ga 1- x N based quantum wells in three dimensions
in physica status solidi (b)
Galtrey M
(2007)
Three-dimensional atom probe studies of an InxGa1-xN/GaN multiple quantum well structure: Assessment of possible indium clustering
in Applied Physics Letters
Galtrey M
(2008)
Compositional inhomogeneity of a high-efficiency InxGa1-xN based multiple quantum well ultraviolet emitter studied by three dimensional atom probe
in Applied Physics Letters
Galtrey M
(2008)
Three-dimensional atom probe analysis of green- and blue-emitting InxGa1-xN/GaN multiple quantum well structures
in Journal of Applied Physics
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 | 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 | £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 | 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 | £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 | 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 |