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Professor Kevin O'Donnell

Physics

Publications

Eu-Mg defects and donor-acceptor pairs in GaN : photodissociation and the excitation transfer problem
Singh A.K., O'Donnell K.P., Edwards P.R., Lorenz K., Leach J.H., Boćkowski M.
Journal of Physics D: Applied Physics, (2017)
Luminescence of Eu3+ in GaN(Mg, Eu) : transitions from the 5D1 level
Singh A.K., O'Donnell K.P., Edwards P.R., Cameron D., Lorenz K., Kappers M.J., Boćkowski M., Yamaga M., Prakash R.
Applied Physics Letters, pp. 1-14, (2017)
Validity of Vegard's rule for Al1-xInxN (0.08<x<0.28) thin films grown on GaN templates
Magalhães S, Franco N, Watson I M, Martin R W, O'Donnell K P, Schenk H P D, Tang F, Sadler T C, Kappers M J, Oliver R A, Monteiro T, Martin T L , Bagot P A J , Moody M P, Alves E, Lorenz K
Journal of Physics D: Applied Physics Vol 50, (2017)
http://dx.doi.org/10.1088/1361-6463/aa69dc
Hysteretic photochromic switching of Eu-Mg defects in GaN links the shallow transient and deep ground states of the Mg acceptor
Singh A. K., O'Donnell K. P., Edwards P. R., Lorenz K., Kappers M. J., Bokowski M.
Scientific Reports Vol 7, (2017)
http://dx.doi.org/10.1038/srep41982
Crystalfield symmetries of luminescent Eu3+ centers in GaN : the importance of the 5D0 to 7F1 transition
O'Donnell K. P., Edwards P. R., Yamaga M., Lorenz K., Kappers M. J., Boćkowski M.
Applied Physics Letters Vol 108, (2016)
http://dx.doi.org/10.1063/1.4939631
Analysis of the stability of InGaN/GaN multiquantum wells against ion beam intermixing
Redondo-Cubero A, Lorenz K, Wendler E, Magalhães S, Alves E, Carvalho D, Ben T, Morales F M, García R, O'Donnell K P, Wetzel C
Nanotechnology Vol 26, (2015)
http://dx.doi.org/10.1088/0957-4484/26/42/425703

more publications

Projects

EPSRC DOCTORAL TRAINING GRANT | Taylor-Shaw, Elaine
Martin, Robert (Principal Investigator) O'Donnell, Kevin (Co-investigator) Taylor-Shaw, Elaine (Research Co-investigator)
Period 01-Oct-2011 - 02-Oct-2015
Lighting the future | Wallace, Michael
Martin, Robert (Principal Investigator) O'Donnell, Kevin (Co-investigator) Wallace, Michael (Research Co-investigator)
Period 01-Mar-2011 - 14-Oct-2016
Hysteretic photochromic switching (HPS) of europium-magnesium defects in gallium nitride: a potential route to a new solid-state qubit
O'Donnell, Kevin (Principal Investigator) Edwards, Paul (Co-investigator)
"Doping is the incorporation of chosen atomic impurities to make a material behave better or differently. When Shuji Nakamura developed a method of producing electrically conducting GaN by activating magnesium (Mg) atoms, he continued a tradition fundamental to all modern electronic devices. Mg doping of GaN allowed production of p-n junctions for today's ubiquitous 'white' light-emitting diodes (LED) and won Nakamura a share in the 2014 Physics Nobel prize. In the same way, europium (Eu) doping of oxide phosphors provided the necessary red optical emission in the 'fluorescent' lamps of a previous lighting revolution. We now propose to take the science of Eu-doped GaN beyond the limited goal of improving red III-nitride LEDs. We aim to explore the potential of hysteretic photochromic switching (HPS), recently discovered by us in GaN co-doped with Eu and Mg, to form the basis of a new solid state qubit or quantum bit.
First trials of rare earth (RE-) doped semiconductors, carried out in the late 1980's, suggested that materials with a wider band gap would show better high-temperature performance, thus favouring II-VI materials and III-nitrides over conventional semiconductors like silicon. However it was not until the present century that III-N semiconductors, grown as high-quality epitaxial thin films on sapphire, were good enough to test this conjecture; another decade passed before Fujiwara demonstrated an LED based on GaN doped with Eu during growth (2010).
Extensive comparative studies of Eu doping methods by the proposer and coworkers in the decade 2001-2011 established that, while such thick GaN:Eu samples could produce brighter overall emission, material produced by ion implantation, followed by annealing, was actually more efficient per dopant ion, by up to 400 times at low temperatures. We also showed that the defect responsible for the GaN:Eu red LED emission was the 'prime' defect, Eu2, consisting of an isolated Eu ion on a Ga lattice site. The commoner Eu1 defect has a more complex emission spectrum, suggesting a Eu atom perturbed by a lattice defect, such as a vacancy or interstitial atom. The total number of such complex centres reported in the GaN:Eu literature is larger than 10.
While attempting to improve the light emission advantage further by implanting Eu in p-type or n-type GaN templates, we discovered hysteretic photochromic switching (HPS) in GaN(Mg):Eu: p-type, Mg-doped GaN samples implanted with Eu ions and annealed. The HPS shows itself in the temperature dependence of the photoluminescence spectrum. At room temperature, the dominant emission, due to the centre Eu0, shows a sharp line at 619 nm. For comparison, Eu1 has a peak at 622 nm and Eu2 at 621 nm. On cooling the sample, the Eu0 intensity increases, as expected, until about 230 K, when it appears to saturate. Below 30 K, we observe a surprising rapid decline of Eu0 as the temperature decreases towards the base temperature of the cooling system. At the same time, an Eu1-like spectrum emerges and effectively replaces Eu0 at 11 K. We deduce that Eu0 somehow switches to Eu1 on cooling over a narrow temperature range. This switching does not reverse if the temperature is then increased from 11 K through 30 K. In fact, Eu1 fades rather slowly, allowing Eu0 to reappear only above ~ 100 K; this is hysteresis. Sample emission is maximum at about 200 K and then fades, reversibly, between 230 K and room temperature. The occurrence of photochromic switching near 20 K on cooldown followed by luminescence hysteresis on warming is given the acronym HPS (hysteretic photochromic switching).
The surprises continue: for samples cooled in the dark, switching from Eu0 to Eu1 can be seen in the time domain; and a resonance line appears at an intermediate wavelength between Eu0 and Eu1. The proposed project aims to determine if the resonance is an actual superposition of Eu0 and Eu1, promising a novel and simple solid state qubit based on Mg acceptor defects."
Period 01-Dec-2015 - 30-Mar-2019
EPSRC Science and Innovation Nanometrology for Molecular Science, Medicine and Manufacture
Birch, David (Principal Investigator) Dawson, Martin (Co-investigator) Faulds, Karen (Co-investigator) Graham, Duncan (Co-investigator) Martin, Robert (Co-investigator) O'Donnell, Kevin (Co-investigator) Rolinski, Olaf (Co-investigator) Smith, William (Co-investigator)
The lack of capacity for advancing the emerging field of nanometrology can only be addressed through stra-tegic interdisciplinary collaborations that provide a stimulating and innovative research environment to catalyse and sustain a new dimension in UK research capability. In a major strategic initiative, Strathclyde University (SU) has founded a Centre for Molecular Nanometrology (2005 - to our knowledge, the first in the world) with facilities supported by the Wolfson Foundation and the Science Research Infrastructure Fund. This Centre has the ultimate goal of recording real-time images of dynamical interactions of single molecules in-situ. With the award of a Science and In-novation Award the Centre will facilitate the high quality, innovative, multidisciplinary research environment required to nurture and develop the extra capacity needed to make the UK a leader in nanometrology. A Science and Innovation Award will also bring together the Centre and medical collaborators at King's College London (KCL), bridging the molecular measurement gap to innovation in emerging areas of strategic impor-tance such as disease pathology, diagnostic tools in nanomedicine, the design of new drug treatments and new structural materials while facilitating knowledge transfer into the healthcare and chemical industries.
Period 01-Aug-2008 - 31-Jan-2012
STIMSCAT
Martin, Robert (Principal Investigator) Dawson, Martin (Co-investigator) O'Donnell, Kevin (Co-investigator)
Period 01-Sep-2005 - 30-Nov-2008
Rainbow (FP7 MC ITN)
Trager-Cowan, Carol (Principal Investigator) Hourahine, Benjamin (Co-investigator) Martin, Robert (Co-investigator) O'Donnell, Kevin (Co-investigator)
Period 01-Oct-2008 - 30-Sep-2012

more projects