@article{strathprints88432,
month = {March},
title = {Crystal-field analysis of photoluminescence from orthorhombic Eu centers and energy transfer from host to Eu in GaN co-doped with Mg and Eu},
year = {2024},
journal = {Journal of Luminescence},
url = {https://strathprints.strath.ac.uk/88432/},
issn = {0022-2313},
abstract = {Gallium nitride co-doped with magnesium and europium shows great potential for active layers in red light emitting diode structures due to strong and sharp luminescence emission around 620 nm. In this work, sharp and intense Eu3+ luminescence lines from the excited states of the 5DJ (J=0, 1) multiplets to the ground states of the 7FJ (J=0, 1, 2) multiplets have been analyzed using a C2v crystal-field equivalent operator Hamiltonian. A model of Eu centers with the C2v symmetry has been proposed to be an Eu3+ complex accompanied by either a pair of nitrogen and gallium vacancies (VN-VGa) or a pair consisting of a nitrogen vacancy and magnesium impurity (VN-MgGa) in the vicinity of the Eu ion based on the crystal-field analysis, the selection rules and the observed polarization of the Eu3+ luminescence lines. Energy transfer from the host to the Eu ions under band-to-band excitation occurs through electron-hole recombination between VN with the electron-like state and VGa or MgGawith the hole-like state; these may be associated with the shallow-trapped or deep-trapped states, respectively, proposed as the energy transfer mechanism in previous literature.},
author = {Yamaga, Mitsuo and Singh, Akhilesh K. and Cameron, Douglas and Edwards, Paul R. and Lorenz, Katharina and Kappers, Menno J. and Bo{\'c}kowski, Michal}
}

@Article{strathprints65532,
author = {Paul R. Edwards and Kevin P. O'Donnell and Akhilesh K. Singh and Douglas Cameron and Katharina Lorenz and Mitsuo Yamaga and Jacob H. Leach and Menno J. Kappers and Michal Bo{\'c}kowski},
title = {Hysteretic photochromic switching (HPS) in doubly doped GaN(Mg):Eu{--}a summary of recent results},
journal = {Materials},
year = {2018},
volume = {11},
number = {10},
pages = {1800},
month = {September},
abstract = {Europium is the most-studied and least-well-understood rare earth ion (REI) dopant in GaN. While attempting to increase the efficiency of red GaN light-emitting diodes (LEDs) by implanting Eu+ into p-type GaN templates, the Strathclyde University group, in collaboration with IST Lisbon and Unipress Warsaw, discovered hysteretic photochromic switching (HPS) in the photoluminescence spectrum of doubly doped GaN(Mg):Eu. Our recent work, summarised in this contribution, has used time-, temperature- and light-induced changes in the Eu intra-4f shell emission spectrum to deduce the microscopic nature of the Mg-Eu defects that form in this material. As well as shedding light on the Mg acceptor in GaN, we propose a possible role for these emission centres in quantum information and computing.},
keywords = {gallium nitride, rare earth ions, europium, photoluminescence, photochromism, qubit, Physics, Materials Science(all), Physics and Astronomy(all)},
url = {https://strathprints.strath.ac.uk/65532/}
}

@Article{strathprints62516,
author = {A.K. Singh and K.P. O'Donnell and P.R. Edwards and D. Cameron and K. Lorenz and M.J. Kappers and M. Bo{\'c}kowski and M. Yamaga and R. Prakash},
title = {Luminescence of Eu³⁺ in GaN(Mg, Eu) : transitions from the ⁵D₁ level},
journal = {Applied Physics Letters},
year = {2017},
volume = {111},
pages = {241105},
month = {November},
abstract = {Eu-doped GaN(Mg) exemplifies hysteretic photochromic switching between two configurations, Eu0 and Eu1(Mg), of the same photoluminescent defect. Using above bandgap excitation, we studied the temperature dependence of photoluminescence (TDPL) of transitions from the excited ⁵D₁ level of Eu³⁺ for both configurations of this defect. During sample cooling, ⁵D₁→⁷F₀,₁,₂ transitions of Eu0 manifest themselves at temperatures below ~200 K, while those of Eu1(Mg) appear only during switching. The observed line positions verify crystal field energies of the ⁷F₀,₁,₂ levels. TDPL profiles of ⁵D₁→⁷F₁ and ⁵D₀→7FJ transitions of Eu0 show an onset of observable emission from the ⁵D₁ level coincident with the previously observed, but hitherto unexplained, decrease in the intensity of its ⁵D₀→⁷FJ emission on cooling below 200 K. Hence the ⁵D₀→⁷FJ TDPL anomaly signals a back-up of ⁵D₁ population due to a reduction in phonon-assisted relaxation between ⁵D₁ and ⁵D₀ levels at lower temperatures. We discuss this surprising result in the light of temperature-dependent transient luminescence measurements of Eu0.},
keywords = {photoluminescence, bandgap, temperature, Physics, Physics and Astronomy(all)},
url = {https://strathprints.strath.ac.uk/62516/}
}

@Article{strathprints55347,
author = {K. P. O'Donnell and P. R. Edwards and M. Yamaga and K. Lorenz and M. J. Kappers and M. Bo{\'c}kowski},
title = {Crystalfield symmetries of luminescent {Eu³⁺} centers in {GaN} : the importance of the {⁵D₀} to {⁷F₁} transition},
journal = {Applied Physics Letters},
year = {2016},
volume = {108},
number = {2},
pages = {022102},
month = {January},
abstract = {Eu-doped GaN is a promising material with potential application not only in optoelectronics but also in magneto-optical and quantum optical devices ?beyond the light emitting diode?. Its interesting spectroscopy is unfortunately complicated by spectral overlaps due to ?site multiplicity?, the existence in a given sample of multiple composite centers in which Eu ions associate with intrinsic or extrinsic defects. We show here that elementary crystalfield analysis of the 5D0 to 7F1 transition can critically distinguish such sites. Hence, we find that the center involved in the hysteretic photochromic switching (HPS) observed in GaN(Mg):Eu, proposed as the basis of a new solid state qubit material, is not in fact Eu1, as previously reported, but a related defect, Eu1(Mg). Furthermore, the decomposition of the crystalfield distortions of Eu0, Eu1(Mg) and Eu1 into axial and non-axial components strongly suggests reasonable microscopic models for the defects themselves.},
keywords = {crystal defects, III-V semiconductors, visible spectra, rare earth ions, emission spectra, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials},
url = {http://strathprints.strath.ac.uk/55347/}
}

@Article{strathprints54146,
author = {M Yamaga and H Watanabe and M Kurahashi and K P O'Donnell and K Lorenz and M Bo{\'c}kowski},
journal = {Journal of Physics: Conference Series},
title = {Indirect excitation of {Eu³⁺} in {GaN} codoped with {Mg} and {Eu}},
year = {2015},
month = {June},
number = {1},
pages = {012025},
volume = {619},
abstract = {Temperature-dependent Eu3+ luminescence spectra in GaN(Mg):Eu can be assigned to, at least, two distinct Eu3+ centres, denoted by Eu0 and Eu1. The splitting energy levels of the 7FJ (J=1,2) multiplets for the Eu0 and Eu1 centres have been calculated using the equivalent operator Hamiltonian for C3v crystal field with the addition of an odd parity distortion.},
keywords = {GaN films, crystal structure, optical spectroscopy, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/54146/},
}