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Professor of Materials Science

MEng University of Oxford
DPhil University of Oxford

Dr Rachel Oliver has benefitted from the University's policies on flexible working, in line with the Department's commitments to the Athena Swan Charter. She was interviewed about her experiences by Chemistry World.

Nitrides at the nanoscale

Working in the Cambridge GaN centre, my research focuses on the characterization and exploitation of nanoscale structures in GaN-based materials. The broad aim of my work is to achieve improved performance in GaN-based optoelectronic devices and to develop and implement novel device concepts.

Nanoscale structure in nitride light emitting diodes and laser diodes

Nitride optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs) are in common use in a range of applications from bike lights to DVD players.  However, particularly in the context of efficient home and workplace lighting, further improvements are required to maximise the potential of this technology in reducing the energy costs (and associated greenhouse gas emissions) of general illumination.  By engineering the nanoscale structure of such devices, we can achieve not only improved efficiency, but also new functionality, such as polarised light emission (which is relevant for LED application in backlighting displays).  My work in this area focusses on the characterisation and engineering of the structure of the light emitting active region of the devices and understanding how the nanoscale structure can profoundly influence macroscopic device performance.  

Novel microscopy techniques for nitride semiconductors

To improve the performance of GaN-based devices we need to understand their structure and electronic properties on a micro- to nano-metre scale. New techniques are being developed to meet the demands of this unusual semiconductor.  One of my current goals is to combine multiple microscopy techniques all focussed on the same defect or nanostructure in a nitride device.  The microscopes applied range from techniques commonly used on metals (such as atom-probe tomography) to techniques which focus exclusively on semiconductors (such as scanning capacitance microscopy).  My work requires the development of new approaches to the application of these techniques, to allow the same nanoscale regions of material to be assessed in multiple microscopes, so that the structure and composition of a specific nanostructure may be linked directly and unambiguously to its electrical and optical properties.  Overall, the aim is to provide a more complete picture of nitride materials science than has previously been achieved, and to apply this new understanding to engineering improved materials for nitride optoelectronic devices. 

GaN-based single photon sources

Early single-photon sources emitting in the visible spectral region were based on heavy attenuation of a laser; such sources are intrinsically unreliable, and may emit multiple photons. In contrast, we aim to build a single-photon source, based on InGaN quantum dots, that is reliable and easy to operate. Such a device would find broad application in quantum cryptography and quantum computing, particularly as the emission wavelength of the InGaN dots is rather convenient in terms of available detectors. However, the high defect density and unusual electrical properties of GaN make realising the device a challenge.

Atom probe tomography can be used to record atom maps of devices such as this commercial laser diode structure, providing extraordinarily detailed information about local compositional variations. Here, orange dots are individual indium atoms, and turquoise dots are individual aluminium atoms.
  • Oliver, R. A.; Massabuau, F. C-P; Kappers, M. J.; Phillips, W. A.; Thrush, E. J.; Tartan, C. C.; Blenkhorn, W. E.; Badcock, T. J.; Dawson, P.; Hopkins, M. A.; Allsopp, D. W. E.; Humphreys, C. J., "The impact of gross well width fluctuations on the efficiency of GaN-based light emitting diodes", Applied Physics Letters, 103 No.141114 (2013), DOI:10.1063/1.4824193.
  • Zhu, T.; Oehler, F; Reid, B. P. L.; Emery, R. M.; Taylor, R. A.; Kappers, M. J.; Oliver, R. A., "Non-polar (11-20) InGaN quantum dots with short exciton lifetimes grown by metal-organic vapor phase epitaxy", Applied Physics Letters, 102 No.251905 (2013), DOI:10.1063/1.4812345.
  • Massabuau, F. C. -P.; Trinh-Xuan, L.; Lodie, D.; Thrush, E. J.; Zhu, D.; Oehler, F.; Zhu, T.; Kappers, M. J.; Humphreys, C. J.; Oliver, R. A., "Correlations between the morphology and emission properties of trench defects in InGaN/GaN quantum wells", Journal of Applied Physics, 113 No.073505 (2013), DOI:10.1063/1.4792505.
  • Bennett, S. E.; Smeeton, T. M.; Saxey, D. W.; Smith, G. D. W.; Hooper, S. E.; Heffernan, J.; Humphreys, C. J.; Oliver, R. A., "Atom probe tomography characterisation of a laser diode structure grown by molecular beam epitaxy", Journal of Applied Physics, 111 No.053508 (2012), DOI:10.1063/1.3692569.