Lab profile: Rachel Oliver, University of Cambridge

Gallium nitride has revolutionized lighting, as well as other optoelectronic applications, over the past decade. Low-energy, highly efficient lighting based on GaN technology, with its enormous environmental implications, is now so prevalent that traditional light bulbs are increasingly obsolete in many settings. But gallium nitride also holds promise for another revolution, this time as single-photon sources for quantum communications and technologies.

Rachel Oliver has devoted her career to exploring nitride semiconductor materials from a materials science perspective. Only by understanding the structure of materials in exquisite detail, she believes, can the performance and fabrication of devices based on those materials truly be optimized.

After receiving a DPhil in materials science and engineering from the University of Oxford, Oliver moved to the University of Cambridge in 2003 to work on GaN. Since 2018, she has led the Cambridge Centre for Gallium Nitride.

Rachel Oliver talked to Materials Today about her current research and future plans.

How long has the lab been running?

The Cambridge Centre for Gallium Nitride has existed since the year 2000, originally under the leadership of Colin Humphreys.  I joined the group in 2003 and took over as Director last March when Professor Humphreys retired from the role.

How many staff makes up the lab?

The group includes two permanent members of academic staff: David Wallis, who holds a joint appointment with Cardiff University, and myself. In addition, there are currently seven postdocs and six PhD students, plus several Masters level students on short- or long-term project placements.   

What are the major themes of research in your group?

We’re all about gallium nitride! More seriously, we work on a range of nitride semiconductor and related materials, and take an in depth materials science approach linking the nanoscale structure of materials to their properties and how this affects the fabrication and performance of devices. That means everything we do is informed by state-of-the-art materials characterization techniques, so we spend quite a lot of time developing new approaches to microscopy and other methodologies.

However, there’s not much point examining materials in minute detail without an aim in terms of what you want to make with them and what improvements need to be made. The nitride devices on which we focus include light emitting diodes (for low energy light bulbs, communications, water purification, and other applications) and transistors (for power handling, sensing, and radio frequency applications). In many cases, these devices are already on the market, but we work to understand and improve their performance. 

We also work on single photon sources, which have applications in future quantum communications and quantum technologies, but are rather further from real world deployment.

How and why did you come to work in these areas?

I originally studied engineering and materials science, which included a lot of electrical engineering alongside learning about crystal structures, phase equilibria, and other basic building blocks of materials science. That juxtaposition gave me an unusual insight into the importance of materials optimization in making reliable working electronic and optoelectronic devices, which is something I’ve carried into my academic career. 

What facilities and equipment does the lab have?

The heart of the Cambridge Centre for Gallium Nitride is our materials growth capability. We currently have three systems for metal-organic vapor phase epitaxy (MOVPE) of nitride materials. That means we can deposit the thin layers of nitride materials on which all nitride devices are based. 

Once we’ve grown the materials, we need to characterize them and we are lucky to have access to a very wide range of departmental characterization facilities including X-ray diffraction, electron microscopy, and atomic force microscopy.

The Department of Materials Science and Metallurgy has a particularly strong capability in transmission electron microscopy (TEM), and this technique lets us access the really small-scale structure of materials. We can look at crystals and mistakes in crystals right down at the atomic level. Our characterization capability ranges from the atomic level, right up to the performance of full devices, via a device characterization probe station, and also photoluminescence systems to examine light emission from materials.

Do you have a favorite piece of kit or equipment?

My current favorite hasn’t quite arrived yet! We recently won a Strategic Equipment grant from the EPSRC for a new time-resolved cathodoluminescence (CL) system in a scanning electron microscope (SEM). In SEM-CL, you use the electron beam from the SEM to excite light emission from a sample, which allows you to study the impact of small-scale structure on the optical properties of the material – down to about 10 nm resolution. That’s already pretty cool, but it has been possible for a long time. What is amazing about our new microscope is that it has a pulsed electron gun, which allows us to inject a pulse of electrons within a period of about 10 picoseconds (i.e. 10^-11 seconds). Using the pulsed mode, we can measure how long after the electron injection light is emitted, which tells us a huge amount about how charge carriers move about inside a material and the mechanisms by which they combine to emit light.

The experiments we will be able to do will be really transformative in terms of understanding how different defects and features in a material affects how it performs. Imagine studying a car engine but you can only look under the bonnet when the engine is off, or watch the car moving at the macroscopic scale when the engine is on. Time-resolved CL is like being able to look under the bonnet and watch the motion of all the different components of the engine with the engine running. I’m really excited about that!

What do you think has been your most influential work to date?

My work on the structure of the active region of light emitting diodes (LEDs) is well known. The active region – the bit that actually emits the light – consists of very thin layers (a few nanometers thick) of a nitride alloy called indium gallium nitride, sandwiched between thicker layers of gallium nitride. For a long time, the whole nitride community thought that the performance of nitride LEDs relied on the existence of nonuniformities in the indium gallium nitride layer. We believed that rather than the indium and gallium being randomly mixed across the metal sites in the crystal, the indium tended to cluster together. However, the concept became controversial once researchers realized that the technique being used to examine indium gallium nitride – TEM – might actually be causing the clusters to form. I decided to find a new technique to address this problem and was lucky enough to be able to set up a collaboration with the Oxford Atom Probe Group to look at these materials using Atom Probe Tomography (APT). In APT, we evaporate a needle of material atom by atom, and use some clever detectors to identify the mass of each ion and the position from which it originated on the needle. We can then build up a three dimensional map of the composition of the material at a sub-nanometer scale. That experiment showed that indium gallium nitride in LEDs is a random alloy and LEDs don’t need indium clustering to work.

Our work was influential in two ways. Firstly, it changed people’s understanding of LEDs. Prior to this work, crystal growers spent a lot of time trying to optimize clustering, which was thought to be key to making bright devices. Now, when clustering is present in materials, which can occasionally happen, we worry about whether it is damaging the performance of the device. There’s been a real shift in thinking. Secondly, the fact that we managed to do the measurement was influential in itself. APT has traditionally been used on metals (something I am familiar with due to my background in materials science) and when we started the experiment, it was generally assumed that it wouldn’t work with materials with limited conductivity, like gallium nitride. Since we started getting beautiful data on nitride materials, other researchers have taken courage from that and are attempting to use atom probes to study a vast range of low conductivity materials – including sapphire and even chocolate!

What is the key to running a successful group?

There’s obviously an awful lot of management and fundraising skills involved in running a modern group, but none of that is worth anything if you’re not open to new and unusual ideas. One of my postdocs had an idea a little while ago about making porous GaN, which I thought was frankly mad, but I let him have a go at it. As a result he’s opened up a whole new sphere of research to us and developed a technology for which we are setting up a spinout company. 

How do you plan to develop your group in the future?

A lot of the strength of what we do is in collaborations. The work we’ve done with the Oxford Atom Probe Group is a good example of that. At the moment, I’m building some new collaborations from outside the nitride sphere. I’m developing relationships with groups working on a range of complementary materials, including hybrid perovskites, gallium oxide, and synthetic diamond, to work towards devices that integrate these diverse systems, or even composites made from these different materials with (hopefully) novel properties. Our porous GaN is providing us with an interesting platform for the fabrication of such composites. We’re excited to apply our expertise in characterization to these new combinations of materials and learn how to engineer their properties and performance. 

Key publications

1. F. Tang, K. B. Lee, I. Guiney, M. Frentrup, J. S. Barnard, G. Divitini, Z. H. Zaidi, T. L. Martin, P. Bagot, M. P. Moody, C. J. Humphreys, P. A. Houston, R. A. Oliver, and D. J. Wallis. Nanoscale structural and chemical analysis of F-implanted enhancement-mode InAlN/GaN heterostructure field effect transistors. J. Appl. Phys. 123 (2018) 024902. 

2. C. C. Kocher, T. J. Puchtler, J. C. Jarman, T. Zhu, T. Wang, L. Nuttall, R. A. Oliver, R. A. Taylor. Highly polarized electrically driven single-photon emission from a non-polar InGaN quantum dot. Appl. Phys. Lett. 111 (2017) 251108. 

3. T. Wang, T. J. Puchtler, T. Zhu, J. C. Jarman, L. P. Nuttall, R. A. Oliver, R. A. Taylor. Polarisation-controlled single photon emission at high temperatures from InGaN quantum dots. Nanoscale 9 (2017) 9421-9427. 

4. T. Wang, T. J. Puchtler, S. K. Patra, T. T. Zhu, J.C. Jarman, R. A. Oliver, S. Schulz, R. A. Taylor. Deterministic optical polarisation in nitride quantum dots at thermoelectrically cooled temperatures. Nature: Scientific Reports 7 (2017) 12067. 

5. F. C-P. Massabuau, S. L. Rhode, M. K. Horton, T. J. O'Hanlon, A. Kovacs, M. S. Zielinski, M. J. Kappers, R. E. Dunin-Borkowski, C. J. Humphreys, R. A. Oliver. Dislocations in AlGaN: Core Structure, Atom Segregation, and Optical Properties. Nano Letters 17 (2017) 4846- 4852. 

6. M. Pristovsek, A. Bao, R. A. Oliver, T. Badcock, M. Ali, A. Shields. Effects of Wavelength and Defect Density on the Efficiency of (In, Ga) N-Based Light-Emitting Diodes. Phys. Rev. Appl. 7 (2017) 64007. 

7. T. Zhu, Y. Liu, T. Ding, W. Y. Fu, J. Jarman, C. X. Ren, R. V. Kumar, R. A. Oliver. Wafer-scale Fabrication of Non-Polar Mesoporous GaN Distributed Bragg Reflectors via Electrochemical Porosification. Nature: Scientific Reports 7 (2017) 45344. 

8. F. C-P. Massabuau, P. Chen, M. K. Horton, S. L. Rhode, C. X. Ren, T. J. O'Hanlon, A. Kovacs, M. J. Kappers, C. J. Humphreys, R. E. Dunin-Borkowski, R. A. Oliver. Carrier localization in the vicinity of dislocations in InGaN. J. Appl. Phys. 121 (2017) 13104. 

9. S. Hammersley, D. Watson-Parris, P. Dawson, M. J. Godfrey, T. J. Badcock, M. J. Kappers, C. McAleese, R. A. Oliver, C. J. Humphreys. The consequences of high injected carrier densities on carrier localization and efficiency droop in InGaN/GaN quantum well structures. J. Appl. Phys. 111 (2012) 83512. 

10. D. Watson-Parris, M. J. Godfrey, P. Dawson, R. A. Oliver, M. J. Galtrey, M. J. Kappers, C. J. Humphreys. Carrier localization mechanisms in InxGa1-xN/GaN quantum wells. Phys. Rev. B 83 (2011) 115321. 

11. M. J. Galtrey, R. A. Oliver, M. J. Kappers, C. J. Humphreys, D. J. Stokes, P. H. Clifton, A. Cerezo. Three-dimensional atom probe studies of an InxGa1-xN/GaN multiple quantum well structure: Assessment of possible indium clustering. Appl. Phys. Lett. 90 (2007) 61903. 

12. R. A. Oliver, M. J. Kappers, J. Sumner, R. Datta, C. J. Humphreys. Highlighting threading dislocations in MOVPE-grown GaN using an in situ treatment with SiH₄ and NH₃. J. Crystal Growth 289 (2006) 506-514. 

13. R. A. Oliver, G. A. D. Briggs, M. J. Kappers, C. J. Humphreys, S. Yasin, J. H. Rice, J. D. Smith, R. A. Taylor. InGaN quantum dots grown by metalorganic vapor phase epitaxy employing a post-growth nitrogen anneal. Appl. Phys. Lett. 83 (2003) 755-757.