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
Moram M
(2008)
The effect of oxygen incorporation in sputtered scandium nitride films
in Thin Solid Films
Moram M
(2008)
Very low dislocation density, resistive GaN films obtained using transition metal nitride interlayers
in physica status solidi (a)
Oliver R
(2008)
High resolution transmission electron microscopy and three-dimensional atom probe microscopy as complementary techniques for the high spatial resolution analysis of GaN based quantum well systems
in Materials Science and Technology
Cherns P
(2008)
Microscopy of Semiconducting Materials 2007
Hollander J
(2008)
Improvements in a-plane GaN crystal quality by a two-step growth process
in Applied Physics Letters
Salcianu C
(2008)
Palladium-based on-wafer electroluminescence studies of GaN-based LED structures
in physica status solidi c
Sahonta S
(2008)
Microstructure of defects in InGaN/GaN quantum well heterostructures
in Journal of Physics: Conference Series
Oliver R
(2008)
Advances in AFM for the electrical characterization of semiconductors
in Reports on Progress in Physics
Oliver R
(2008)
Gross well-width fluctuations in InGaN quantum wells
in physica status solidi c
Rao DV
(2008)
A simple technique for the estimation of the voltage of a transmission electron microscope.
in Journal of electron microscopy
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
Jarjour A
(2008)
Experimental and theoretical study of the quantum-confined Stark effect in a single InGaN/GaN quantum dot under applied vertical electric field
in Superlattices and Microstructures
Humphreys C
(2008)
Microscopy of Semiconducting Materials 2007
Corbett B
(2008)
High brightness near-ultraviolet resonant LEDs
in physica status solidi c
Zhang Y
(2008)
Structural features in GaN grown on a Ge(111) substrate
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
Johnston C
(2008)
Characterisation of non-polar (11-20) gallium nitride using TEM techniques
in Journal of Physics: Conference Series
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
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
Kappers M
(2008)
Properties of non-polar a-plane GaN/AlGaN quantum wells
in Journal of Crystal Growth
Xiu H
(2008)
Degradation of III-nitride laser diodes grown by molecular beam epitaxy
in physica status solidi c
Badcock T
(2008)
Optical properties of GaN/AlGaN quantum wells grown on nonpolar substrates
in Applied Physics Letters
Holec D
(2008)
A theoretical study of ELNES spectra of Al x Ga 1 - x N using Wien2k and Telnes programs
in Computational Materials Science
Zhao L
(2008)
Degradation of GaN-based quantum well light-emitting diodes
in Journal of Applied Physics
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)
Ketteniss N
(2008)
The role of strain in controlling the surface morphology of AlxGa1-xN following in situ treatment with SiH4 and NH3
in Applied Surface Science
Oliver R
(2008)
Growth and assessment of InGaN quantum dots in a microcavity: A blue single photon source
in Materials Science and Engineering: B
Jiang N
(2008)
Observation of long-range compositional fluctuations in glasses: implications for atomic and electronic structure.
in Micron (Oxford, England : 1993)
Holec D
(2008)
Electron energy loss near edge structure (ELNES) spectra of AlN and AlGaN: a theoretical study using the Wien2k and Telnes programs.
in Micron (Oxford, England : 1993)
Oliver R
(2008)
Insights into the growth mechanism of InxGa1-xN epitaxial nanostructures formed using a silane predose
in Journal of Crystal Growth
Jarjour A
(2008)
Electrically driven single InGaN/GaN quantum dot emission
in Applied Physics Letters
Moram M
(2008)
Growth of epitaxial thin films of scandium nitride on 100-oriented silicon
in Journal of Crystal Growth
Sumner J
(2008)
Assessment of scanning spreading resistance microscopy for application to n-type GaN
in physica status solidi c
Leys M
(2008)
Growth and characterization of unintentionally doped GaN grown on silicon(111) substrates
in Journal of Crystal Growth
Oliver R
(2008)
The origin and reduction of dislocations in Gallium Nitride
in Journal of Materials Science: Materials in Electronics
Oliver R
(2009)
Morphological changes of InGaN epilayers during annealing assessed by spectral analysis of atomic force microscopy images
in Journal of Applied Physics
Oliver R
(2009)
Scanning capacitance microscopy as a tool for the assessment of unintentional doping in GaN
in physica status solidi c
Sahonta S
(2009)
Mechanism of compositional modulations in epitaxial InAlN films grown by molecular beam epitaxy
in Applied Physics Letters
Collins D
(2009)
Non-linear excitation and correlation studies of single InGaN quantum dots
in physica status solidi c
Moram M
(2009)
On the origin of threading dislocations in GaN films
in Journal of Applied Physics
Charash R
(2009)
Carrier distribution in InGaN/GaN tricolor multiple quantum well light emitting diodes
in Applied Physics Letters
Johnston C
(2009)
Assessment of defect reduction methods for nonpolar a-plane GaN grown on r-plane sapphire
in Journal of Crystal Growth
Moram M
(2009)
X-ray diffraction of III-nitrides
in Reports on Progress in Physics
Moram M
(2009)
Structural properties of wurtzitelike ScGaN films grown by NH3-molecular beam epitaxy
in Journal of Applied Physics
Bennett S
(2009)
Atom Probe Tomography Studies of GaN-Based Semiconductor Materials
in Microscopy and Microanalysis
Hertkorn J
(2009)
Highly conductive modulation doped composition graded p-AlGaN/(AlN)/GaN multiheterostructures grown by metalorganic vapor phase epitaxy
in Journal of Applied Physics
Moram M
(2009)
Investigating stacking faults in nonpolar gallium nitride films using X-ray diffraction
in Physica B: Condensed Matter
Johnston C
(2009)
Microstructural evolution of nonpolar (11-20) GaN grown on (1-102) sapphire using a 3D-2D method
in Journal of Applied Physics
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
Liu, Lewis Z.-Y.
(2009)
High-Resolution Electron Holography Study Of Basal-Plane Stacking Faults In (11-20) Gan
| 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 | 09/2008 |
| End | 02/2011 |
| 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 | 03/2009 |
| End | 10/2012 |
| 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 | £1,447,635 (GBP) |
| Funding ID | EP/G042330/1 |
| Organisation | Engineering and Physical Sciences Research Council (EPSRC) |
| Sector | Public |
| Country | United Kingdom |
| Start | 03/2009 |
| 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 | 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 | 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 |