• D. Cameron, K. P. O’Donnell, P. R. Edwards, M. Peres, K. Lorenz, M. J. Kappers, and M. Boćkowski, “Acceptor state anchoring in gallium nitride,” Applied Physics Letters, vol. 116, p. 102105, 2020.

The dual nature of the magnesium acceptor in gallium nitride results in dynamic defect complexes. Europium spectator ions reveal switching between two spectrally unique metastable centres, each corresponding to a particular acceptor state. By ion co-implantation of europium and oxygen into GaN(Mg), we produce, in addition, an anchored state system. In doing so we create an abundance of previously unidentified stable centres which we denote as “Eu0(Ox)”. We introduce a microscopic model for these centres with oxygen substituting for nitrogen in the bridging site.

@Article{strathprints71643,
author = {D. Cameron and K. P. O'Donnell and P. R. Edwards and M. Peres and K. Lorenz and M. J. Kappers and M. Bo{\'c}kowski},
journal = {Applied Physics Letters},
title = {Acceptor state anchoring in gallium nitride},
year = {2020},
month = {February},
pages = {102105},
volume = {116},
abstract = {The dual nature of the magnesium acceptor in gallium nitride results in dynamic defect complexes. Europium spectator ions reveal switching between two spectrally unique metastable centres, each corresponding to a particular acceptor state. By ion co-implantation of europium and oxygen into GaN(Mg), we produce, in addition, an anchored state system. In doing so we create an abundance of previously unidentified stable centres which we denote as "Eu0(Ox)". We introduce a microscopic model for these centres with oxygen substituting for nitrogen in the bridging site.},
keywords = {rare earth (RE) ions, europium, gallium nitride, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {https://strathprints.strath.ac.uk/71643/},
}

• A. K. Singh, K. P. O’Donnell, P. R. Edwards, K. Lorenz, J. H. Leach, and M. Boćkowski, “Eu-Mg defects and donor-acceptor pairs in GaN : photodissociation and the excitation transfer problem,” Journal of Physics D: Applied Physics, vol. 51, p. 65106, 2018.

We have investigated temperature-dependent photoluminescence (TDPL) profiles of Eu³⁺ ions implanted in an HVPE-grown bulk GaN sample doped with Mg and of donor-acceptor pairs (DAP) involving the shallow Mg acceptor in GaN(Mg) (unimplanted) and GaN(Mg):Eu samples. Below 125 K, the TDPL of Eu³⁺ in GaN(Mg):Eu correlates with that of the DAP. Below 75 K, the intensity of Eu³⁺ emission saturates, indicating a limitation of the numbers of Eu-Mg defects available to receive excitation transferred from the host, while the DAP continues to increase, albeit more slowly in the implanted than the unimplanted sample. Prolonged exposure to UV light at low temperature results in the photodissociation of Eu-Mg defects, in their Eu1(Mg) configuration, with a corresponding increase in shallow DAP emission and the emergence of emission from unassociated EuGa (Eu2) defects.

@Article{strathprints62526,
author = {A.K. Singh and K.P. O'Donnell and P.R. Edwards and K. Lorenz and J.H. Leach and M. Bo{\'c}kowski},
title = {Eu-Mg defects and donor-acceptor pairs in GaN : photodissociation and the excitation transfer problem},
journal = {Journal of Physics D: Applied Physics},
year = {2018},
volume = {51},
pages = {065106},
month = {December},
abstract = {We have investigated temperature-dependent photoluminescence (TDPL) profiles of Eu³⁺ ions implanted in an HVPE-grown bulk GaN sample doped with Mg and of donor-acceptor pairs (DAP) involving the shallow Mg acceptor in GaN(Mg) (unimplanted) and GaN(Mg):Eu samples. Below 125 K, the TDPL of Eu³⁺ in GaN(Mg):Eu correlates with that of the DAP. Below 75 K, the intensity of Eu³⁺ emission saturates, indicating a limitation of the numbers of Eu-Mg defects available to receive excitation transferred from the host, while the DAP continues to increase, albeit more slowly in the implanted than the unimplanted sample. Prolonged exposure to UV light at low temperature results in the photodissociation of Eu-Mg defects, in their Eu1(Mg) configuration, with a corresponding increase in shallow DAP emission and the emergence of emission from unassociated EuGa (Eu2) defects.},
keywords = {temperature-dependent photoluminescence, photodissociation, Physics, Physics and Astronomy(all)},
url = {https://strathprints.strath.ac.uk/62526/}
}

• P. R. Edwards, K. P. O’Donnell, A. K. Singh, D. Cameron, K. Lorenz, M. Yamaga, J. H. Leach, M. J. Kappers, and M. Boćkowski, “Hysteretic photochromic switching (HPS) in doubly doped GaN(Mg):Eu–a summary of recent results,” Materials, vol. 11, iss. 10, p. 1800, 2018.

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.

@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/}
}

• A. K. Singh, K. P. O’Donnell, P. R. Edwards, D. Cameron, K. Lorenz, M. J. Kappers, M. Boćkowski, M. Yamaga, and R. Prakash, “Luminescence of Eu³⁺ in GaN(Mg, Eu) : transitions from the ⁵D₁ level,” Applied Physics Letters, vol. 111, p. 241105, 2017.

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.

@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/}
}

• A. K. Singh, K. P. O’Donnell, P. R. Edwards, K. Lorenz, M. J. Kappers, and M. Boćkowski, “Hysteretic photochromic switching of Eu-Mg defects in GaN links the shallow transient and deep ground states of the Mg acceptor,” Scientific Reports, vol. 7, p. 41982, 2017.

Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a spectator ion’ to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on switchdown’ from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm2 to (an estimated) several hours at 50 K, 1 mW/cm². Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.

@Article{strathprints59279,
author = {A. K. Singh and K. P. O'Donnell and P. R. Edwards and K. Lorenz and M. J. Kappers and M. Bo{\'c}kowski},
title = {Hysteretic photochromic switching of {Eu-Mg} defects in {GaN} links the shallow transient and deep ground states of the {Mg} acceptor},
journal = {Scientific Reports},
year = {2017},
volume = {7},
pages = {41982},
month = {February},
abstract = {Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a spectator ion' to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on switchdown' from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm2 to (an estimated) several hours at 50 K, 1 mW/cm². Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.},
keywords = {photoluminescence spectroscopy, hysteretic photochromic switching, Lany-Zunger model, energy barrier, shallow transient state, deep ground state, acceptor state, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/59279/}
}

• K. P. O’Donnell, P. R. Edwards, M. Yamaga, K. Lorenz, M. J. Kappers, and M. Boćkowski, “Crystalfield symmetries of luminescent Eu³⁺ centers in GaN : the importance of the ⁵D₀ to ⁷F₁ transition,” Applied Physics Letters, vol. 108, iss. 2, p. 22102, 2016.

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.

@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/}
}

• M. Yamaga, H. Watanabe, M. Kurahashi, K. P. O’Donnell, K. Lorenz, and M. Boćkowski, “Indirect excitation of Eu³⁺ in GaN codoped with Mg and Eu,” Journal of Physics: Conference Series, vol. 619, iss. 1, p. 12025, 2015.

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.

@Article{strathprints54146,
author = {M Yamaga and H Watanabe and M Kurahashi and K P O'Donnell and K Lorenz and M Bo{\'c}kowski},
title = {Indirect excitation of {Eu³⁺} in {GaN} codoped with {Mg} and {Eu}},
journal = {Journal of Physics: Conference Series},
year = {2015},
volume = {619},
number = {1},
pages = {012025},
month = {June},
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/}
}

• M. A. Sousa, T. C. Esteves, N. B. Sedrine, J. Rodrigues, M. B. Lourenço, A. Redondo-Cubero, E. Alves, K. P. O’Donnell, M. Bockowski, C. Wetzel, M. R. Correia, K. Lorenz, and T. Monteiro, “Luminescence studies on green emitting InGaN/GaN MQWs implanted with nitrogen,” Scientific Reports, vol. 5, p. 9703, 2015.

We studied the optical properties of metalorganic chemical vapour deposited (MOCVD) InGaN/GaN multiple quantum wells (MQW) subjected to nitrogen (N) implantation and post-growth annealing treatments. The optical characterization was carried out by means of temperature and excitation density-dependent steady state photoluminescence (PL) spectroscopy, supplemented by room temperature PL excitation (PLE) and PL lifetime (PLL) measurements. The as-grown and as-implanted samples were found to exhibit a single green emission band attributed to localized excitons in the QW, although the N implantation leads to a strong reduction of the PL intensity. The green band was found to be surprisingly stable on annealing up to 1400?C. A broad blue band dominates the low temperature PL after thermal annealing in both samples. This band is more intense for the implanted sample, suggesting that defects generated by N implantation, likely related to the diffusion/segregation of indium (In), have been optically activated by the thermal treatment.

@Article{strathprints53469,
author = {Marco A. Sousa and Teresa C. Esteves and Nabiha Ben Sedrine and Joana Rodrigues and M{\'a}rcio B. Louren{\c c}o and Andr{\'e}s Redondo-Cubero and Eduardo Alves and Kevin P. O'Donnell and Michal Bockowski and Christian Wetzel and Maria R. Correia and Katharina Lorenz and Teresa Monteiro},
title = {Luminescence studies on green emitting {InGaN/GaN MQW}s implanted with nitrogen},
journal = {Scientific Reports},
year = {2015},
volume = {5},
pages = {9703},
month = {April},
abstract = {We studied the optical properties of metalorganic chemical vapour deposited (MOCVD) InGaN/GaN multiple quantum wells (MQW) subjected to nitrogen (N) implantation and post-growth annealing treatments. The optical characterization was carried out by means of temperature and excitation density-dependent steady state photoluminescence (PL) spectroscopy, supplemented by room temperature PL excitation (PLE) and PL lifetime (PLL) measurements. The as-grown and as-implanted samples were found to exhibit a single green emission band attributed to localized excitons in the QW, although the N implantation leads to a strong reduction of the PL intensity. The green band was found to be surprisingly stable on annealing up to 1400?C. A broad blue band dominates the low temperature PL after thermal annealing in both samples. This band is more intense for the implanted sample, suggesting that defects generated by N implantation, likely related to the diffusion/segregation of indium (In), have been optically activated by the thermal treatment.},
keywords = {multi quantum wells, luminescence , light emitting diodes, Physics, Optics. Light, General, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/53469/}
}

• B. N. Sedrine, T. C. Esteves, J. Rodrigues, L. Rino, M. R. Correia, M. C. Sequeira, A. J. Neves, E. Alves, M. Boćkowski, P. R. Edwards, K. P. O’Donnell, K. Lorenz, and T. Monteiro, “Photoluminescence studies of a perceived white light emission from a monolithic InGaN/GaN quantum well structure,” Scientific Reports, vol. 5, p. 13739, 2015.

In this work we demonstrate by photoluminescence studies white light emission from a monolithic InGaN/GaN single quantum well structure grown by metal organic chemical vapour deposition. As-grown and thermally annealed samples at high temperature (1000 ?C, 1100 ?C and 1200 ?C) and high pressure (1.1 GPa) were analysed by spectroscopic techniques, and the annealing effect on the photoluminescence is deeply explored. Under laser excitation of 3.8 eV at room temperature, the as-grown structure exhibits two main emission bands: a yellow band peaked at 2.14 eV and a blue band peaked at 2.8 eV resulting in white light perception. Interestingly, the stability of the white light is preserved after annealing at the lowest temperature (1000 ?C), but suppressed for higher temperatures due to a deterioration of the blue quantum well emission. Moreover, the control of the yellow/blue bands intensity ratio, responsible for the white colour coordinate temperatures, could be achieved after annealing at 1000 ?C. The room temperature white emission is studied as a function of incident power density, and the correlated colour temperature values are found to be in the warm white range: 3260?4000 K.

@Article{strathprints54228,
author = {N. Ben Sedrine and T. C. Esteves and J. Rodrigues and L. Rino and M. R. Correia and M. C. Sequeira and A. J. Neves and E. Alves and M. Bo{\'c}kowski and P. R. Edwards and K.P. O'Donnell and K. Lorenz and T. Monteiro},
title = {Photoluminescence studies of a perceived white light emission from a monolithic {InGaN/GaN} quantum well structure},
journal = {Scientific Reports},
year = {2015},
volume = {5},
pages = {13739},
abstract = {In this work we demonstrate by photoluminescence studies white light emission from a monolithic InGaN/GaN single quantum well structure grown by metal organic chemical vapour deposition. As-grown and thermally annealed samples at high temperature (1000 ?C, 1100 ?C and 1200 ?C) and high pressure (1.1 GPa) were analysed by spectroscopic techniques, and the annealing effect on the photoluminescence is deeply explored. Under laser excitation of 3.8 eV at room temperature, the as-grown structure exhibits two main emission bands: a yellow band peaked at 2.14 eV and a blue band peaked at 2.8 eV resulting in white light perception. Interestingly, the stability of the white light is preserved after annealing at the lowest temperature (1000 ?C), but suppressed for higher temperatures due to a deterioration of the blue quantum well emission. Moreover, the control of the yellow/blue bands intensity ratio, responsible for the white colour coordinate temperatures, could be achieved after annealing at 1000 ?C. The room temperature white emission is studied as a function of incident power density, and the correlated colour temperature values are found to be in the warm white range: 3260?4000 K.},
keywords = {light emitting diodes, LEDs, photoluminescence, Physics, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/54228/}
}

• K. P. O’Donnell, P. R. Edwards, M. J. Kappers, K. Lorenz, E. J. Alves, and M. X. Boćkowski, “Europium-doped GaN(Mg) : beyond the limits of the light-emitting diode,” Physica Status Solidi C, vol. 11, iss. 3–4, p. 662–665, 2014.

Rare-earth doped III-N semiconductors have been studied for decades on account of their possible application in visible light-emitting diodes (LED) with built-in utility as red (e.g. Eu), green (Er) and blue (Tm) monochromatic sources (O’Donnell and Dierolf (eds.), Topics in Applied Physics, Vol. 124 (Springer, Dordrecht, 2010) [1]). However, to date, no commercial devices have been introduced on the basis of these materials. Recently, we discovered thermally activated hysteresis in the emission spectrum of p-type GaN thin films that were co-doped with Mg and Eu (O’Donnell et al., Proc. ICPS31, Zurich, July 2012 [2]). We have also reported an unexpected Zeeman splitting and induced magnetic moment of Eu3+ ions in GaN (Kachkanov et al., Scientific Rep. 2, 969 (2012) and MRS Proc. 1290?i03?06 (2011) [3, 4]). These findings encourage speculation on taking the study of RE-doped III-N beyond the limited goal of improving LED efficiency into the realm of novel magneto-optic and quantum-optical devices. In particular we will describe in this presentation the spectroscopy of ion-implanted and annealed GaN(Mg): Eu samples and the possible exploitation of the Mg acceptor in GaN as a qubit.

@Article{strathprints47513,
author = {K.P. O'Donnell and P.R. Edwards and M.J. Kappers and K. Lorenz and E.J. Alves and M.X. Bo{\'c}kowski},
title = {Europium-doped {GaN(Mg)} : beyond the limits of the light-emitting diode},
journal = {Physica Status Solidi C},
year = {2014},
volume = {11},
number = {3--4},
pages = {662--665},
month = {April},
abstract = {Rare-earth doped III-N semiconductors have been studied for decades on account of their possible application in visible light-emitting diodes (LED) with built-in utility as red (e.g. Eu), green (Er) and blue (Tm) monochromatic sources (O'Donnell and Dierolf (eds.), Topics in Applied Physics, Vol. 124 (Springer, Dordrecht, 2010) [1]). However, to date, no commercial devices have been introduced on the basis of these materials. Recently, we discovered thermally activated hysteresis in the emission spectrum of p-type GaN thin films that were co-doped with Mg and Eu (O'Donnell et al., Proc. ICPS31, Zurich, July 2012 [2]). We have also reported an unexpected Zeeman splitting and induced magnetic moment of Eu3+ ions in GaN (Kachkanov et al., Scientific Rep. 2, 969 (2012) and MRS Proc. 1290?i03?06 (2011) [3, 4]). These findings encourage speculation on taking the study of RE-doped III-N beyond the limited goal of improving LED efficiency into the realm of novel magneto-optic and quantum-optical devices. In particular we will describe in this presentation the spectroscopy of ion-implanted and annealed GaN(Mg): Eu samples and the possible exploitation of the Mg acceptor in GaN as a qubit.},
keywords = {GaN, rare earth doping, luminescence hysteresis, qubit, Physics, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/47513/}
}

• S. M. C. Miranda, P. R. Edwards, K. P. O’Donnell, M. Boćkowski, E. Alves, I. S. Roqan, A. Vantomme, and K. Lorenz, “Sequential multiple-step europium ion implantation and annealing of GaN,” Physica Status Solidi C, vol. 11, iss. 2, p. 253–257, 2014.

Sequential multiple Eu ion implantations at low fluence (1{$\times$}1013 cm-2 at 300 keV) and subsequent rapid thermal annealing (RTA) steps (30 s at 1000 ?C or 1100 ?C) were performed on high quality nominally undoped GaN films grown by metal organic chemical vapour deposition (MOCVD) and medium quality GaN:Mg grown by hydride vapour phase epitaxy (HVPE). Compared to samples implanted in a single step, multiple implantation/annealing shows only marginal structural improvement for the MOCVD samples, but a significant improvement of crystal quality and optical activation of Eu was achieved in the HVPE films. This improvement is attributed to the lower crystalline quality of the starting material, which probably enhances the diffusion of defects and acts to facilitate the annealing of implantation damage and the effective incorporation of the Eu ions in the crystal structure. Optical activation of Eu3+ ions in the HVPE samples was further improved by high temperature and high pressure annealing (HTHP) up to 1400 ?C. After HTHP annealing the main room temperature cathodo- and photoluminescence line in Mg-doped samples lies at {$\sim$} 619 nm, characteristic of a known Mg-related Eu3+ centre, while after RTA treatment the dominant line lies at {$\sim$} 622 nm, typical for undoped GaN:Eu.

@Article{strathprints47160,
author = {S. M. C. Miranda and P. R. Edwards and K. P. O'Donnell and M. Bo{\'c}kowski and E. Alves and I. S. Roqan and A. Vantomme and K. Lorenz},
title = {Sequential multiple-step europium ion implantation and annealing of {GaN}},
journal = {Physica Status Solidi C},
year = {2014},
volume = {11},
number = {2},
pages = {253--257},
month = {February},
abstract = {Sequential multiple Eu ion implantations at low fluence (1{$\times$}1013 cm-2 at 300 keV) and subsequent rapid thermal annealing (RTA) steps (30 s at 1000 ?C or 1100 ?C) were performed on high quality nominally undoped GaN films grown by metal organic chemical vapour deposition (MOCVD) and medium quality GaN:Mg grown by hydride vapour phase epitaxy (HVPE). Compared to samples implanted in a single step, multiple implantation/annealing shows only marginal structural improvement for the MOCVD samples, but a significant improvement of crystal quality and optical activation of Eu was achieved in the HVPE films. This improvement is attributed to the lower crystalline quality of the starting material, which probably enhances the diffusion of defects and acts to facilitate the annealing of implantation damage and the effective incorporation of the Eu ions in the crystal structure. Optical activation of Eu3+ ions in the HVPE samples was further improved by high temperature and high pressure annealing (HTHP) up to 1400 ?C. After HTHP annealing the main room temperature cathodo- and photoluminescence line in Mg-doped samples lies at {$\sim$} 619 nm, characteristic of a known Mg-related Eu3+ centre, while after RTA treatment the dominant line lies at {$\sim$} 622 nm, typical for undoped GaN:Eu.},
keywords = {crystal quality, europium, gallium nitride, ion implantation, multiple-step , GaN, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/47160/}
}

• K. P. O’Donnell, R. W. Martin, P. R. Edwards, K. Lorenz, E. Alves, and M. Bockowski, “Temperature-dependent hysteresis of the emission spectrum of Eu-implanted, Mg-doped HVPE GaN,” in The Physics of Semiconductors, T. Ihn, C. Rössler, and A. Kozikov, Eds., AIP Conference Proceedings, 2013, vol. 1566, p. 63.

A red emission site (hereafter, Eu0), with its main 5D0 to 7F2 peak at 619 nm, is observed by photoluminescence (PL) spectroscopy of Eu-implanted, Mg-doped GaN, in samples annealed at high temperature and pressure (up to 1400 ?C, 1 GPa) in order to remove lattice damage. The PL spectrum is strongly temperature-hysteretic between room temperature and {$\sim$}20 K: below 30 K, photochromic switching occurs between Eu0 and the usually dominant Eu1 center; upon warming the sample, the Eu0 signal does not recover until the temperature reaches {$\sim$}150 K. Photobleaching of Eu1 takes place at low temperatures after cooling, while photo-enhancement of Eu0 takes place at high temperatures after re-warming. These observations suggest a microscopic model of charge-driven defect interconversion in p-type GaN:Eu, Mg.

@InCollection{strathprints46604,
author = {K. P. O'Donnell and R. W. Martin and P. R. Edwards and K. Lorenz and E. Alves and M. Bockowski},
title = {Temperature-dependent hysteresis of the emission spectrum of Eu-implanted, Mg-doped HVPE GaN},
booktitle = {The Physics of Semiconductors},
publisher = {AIP Conference Proceedings},
year = {2013},
editor = {Thomas Ihn and Clemens R{\"o}ssler and Aleksey Kozikov},
volume = {1566},
pages = {63},
month = {December},
abstract = {A red emission site (hereafter, Eu0), with its main 5D0 to 7F2 peak at 619 nm, is observed by photoluminescence (PL) spectroscopy of Eu-implanted, Mg-doped GaN, in samples annealed at high temperature and pressure (up to 1400 ?C, 1 GPa) in order to remove lattice damage. The PL spectrum is strongly temperature-hysteretic between room temperature and {$\sim$}20 K: below 30 K, photochromic switching occurs between Eu0 and the usually dominant Eu1 center; upon warming the sample, the Eu0 signal does not recover until the temperature reaches {$\sim$}150 K. Photobleaching of Eu1 takes place at low temperatures after cooling, while photo-enhancement of Eu0 takes place at high temperatures after re-warming. These observations suggest a microscopic model of charge-driven defect interconversion in p-type GaN:Eu, Mg.},
keywords = {temperature dependent hysteresis, emission spectrum, eu-implanted, mg-doped, hvpe gan, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/46604/}
}

• K. P. O’Donnell, I. S. Roqan, K. Wang, K. Lorenz, E. Alves, and M. Bockowski, “The photoluminescence/excitation (PL/E) spectroscopy of Eu-implanted GaN,” Optical Materials, vol. 33, iss. 7, p. 1063–1065, 2011.

Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, Eu(Ga), is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N(2). According to structural determinations, such samples exhibit an essentially complete removal of lattice damage caused by the implantation process. A second centre, Eu1, probably comprising Eu(Ga) in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the (5)D(0) to (7)F(2) spectral region near 620 nm. We can readily distinguish Eu1 and Eu2 by their excitation spectra, in particular through their different sensitivities to above-gap and below-gap excitation. The present study extends recent work on photoluminescence/excitation (PL/E) spectroscopy of Eu1 and Eu2 to arrive at an understanding of these mechanisms in terms of residual optically active defect concentrations. We also report further on the ‘host-independent’ excitation mechanism that is active in the case of a prominent minority centre. The relevance of this work to the operation of the red GaN:Eu light-emitting diode is discussed.

@article{strathprints35792,
volume = {33},
number = {7},
month = {May},
author = {K. P. O'Donnell and I. S. Roqan and Ke Wang and K. Lorenz and E. Alves and M. Bockowski},
title = {The photoluminescence/excitation (PL/E) spectroscopy of Eu-implanted GaN},
journal = {Optical Materials},
pages = {1063--1065},
year = {2011},
keywords = {ion implantation, excitation mechanism, europium, luminescence, photoluminescence, gallium nitride, photoluminescence/excitation , PL/E, spectroscopy, Eu-implanted GaN , Physics, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics, Electrical and Electronic Engineering, Computer Science(all)},
url = {http://strathprints.strath.ac.uk/35792/},
abstract = {Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, Eu(Ga), is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N(2). According to structural determinations, such samples exhibit an essentially complete removal of lattice damage caused by the implantation process. A second centre, Eu1, probably comprising Eu(Ga) in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the (5)D(0) to (7)F(2) spectral region near 620 nm. We can readily distinguish Eu1 and Eu2 by their excitation spectra, in particular through their different sensitivities to above-gap and below-gap excitation. The present study extends recent work on photoluminescence/excitation (PL/E) spectroscopy of Eu1 and Eu2 to arrive at an understanding of these mechanisms in terms of residual optically active defect concentrations. We also report further on the 'host-independent' excitation mechanism that is active in the case of a prominent minority centre. The relevance of this work to the operation of the red GaN:Eu light-emitting diode is discussed.}
}

• V. Kachkanov, K. P. O’Donnell, C. Rice, D. Wolverson, R. W. Martin, K. Lorenz, E. Alves, and M. Bockowski, “Zeeman splittings of the ⁵D₀–⁷F₂ transitions of Eu³⁺ ions implanted into GaN,” MRS Online Proceedings Library, vol. 1290, p. mrsf10-1290-i03-06, 2011.

We report the magnetic field splittings of emission lines assigned to the 5D0?7F2 transitions of Eu3+ centres in GaN. The application of a magnetic field in the c-axis direction (B{\ensuremath{|}}{\ensuremath{|}}c) leads to a splitting of the major lines at 621 nm, 622 nm and 622.8 nm into two components. The Zeeman splitting is linear with magnetic field up to 5 Tesla for each line. In contrast, a magnetic field applied in the growth plane (B?c) does not influence the photoluminescence spectra. The estimated g-factors vary slightly from sample to sample with mean values of g{\ensuremath{|}}{\ensuremath{|}} {\texttt{\char126}}2.8, {\texttt{\char126}}1.5 and {\texttt{\char126}}2.0 for the emission lines at 621 nm, 622 nm and 622.8 nm respectively.

@Article{strathprints35791,
author = {V. Kachkanov and K. P. O'Donnell and C. Rice and D. Wolverson and R. W. Martin and K. Lorenz and E. Alves and M. Bockowski},
title = {Zeeman splittings of the {⁵D₀}--{⁷F₂} transitions of Eu³⁺ ions implanted into GaN},
journal = {MRS Online Proceedings Library},
year = {2011},
volume = {1290},
pages = {mrsf10-1290-i03-06},
abstract = {We report the magnetic field splittings of emission lines assigned to the 5D0?7F2 transitions of Eu3+ centres in GaN. The application of a magnetic field in the c-axis direction (B{\ensuremath{|}}{\ensuremath{|}}c) leads to a splitting of the major lines at 621 nm, 622 nm and 622.8 nm into two components. The Zeeman splitting is linear with magnetic field up to 5 Tesla for each line. In contrast, a magnetic field applied in the growth plane (B?c) does not influence the photoluminescence spectra. The estimated g-factors vary slightly from sample to sample with mean values of g{\ensuremath{|}}{\ensuremath{|}} {\texttt{\char126}}2.8, {\texttt{\char126}}1.5 and {\texttt{\char126}}2.0 for the emission lines at 621 nm, 622 nm and 622.8 nm respectively.},
keywords = {Eu, ion-implantation, nitride, TIC - Bionanotechnology, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/35791/}
}

• K. Lorenz, E. Alves, I. S. Roqan, K. P. O’Donnell, A. Nishikawa, Y. Fujiwara, and M. Bockowski, “Lattice site location of optical centers in GaN : Eu light emitting diode material grown by organometallic vapor phase epitaxy,” Applied Physics Letters, vol. 97, iss. 11, p. 111911, 2010.

Eu-doped GaN was grown by organometallic vapor phase epitaxy at temperatures from 900 to 1100 degrees C. Eu incorporation is influenced by temperature with the highest concentration found for growth at 1000 degrees C. In all samples, Eu is incorporated entirely on substitutional Ga sites with a slight displacement which is highest (similar to 0.2 angstrom) in the sample grown at 900 degrees C and mainly directed along the c-axis. The major optical Eu3+ centers are identical for in situ doped and ion-implanted samples after high temperature and pressure annealing. The dominant Eu3+ luminescence lines are attributed to isolated, substitutional Eu. (c) 2010 American Institute of Physics. [doi:10.1063/1.3489103]

@article{strathprints30172,
volume = {97},
number = {11},
month = {September},
author = {K. Lorenz and E. Alves and I. S. Roqan and K. P. O'Donnell and A. Nishikawa and Y. Fujiwara and M. Bockowski},
title = {Lattice site location of optical centers in GaN : Eu light emitting diode material grown by organometallic vapor phase epitaxy},
journal = {Applied Physics Letters},
pages = {111911},
year = {2010},
keywords = {annealing, doping profiles, europium, gallium compounds, III-V semiconductors, ion implantation, luminescence, MOCVD, semiconductor epitaxial layers, semiconductor growth, vapour phase epitaxial growth, wide band gap semiconductors, Physics, Physics and Astronomy (miscellaneous)},
url = {http://strathprints.strath.ac.uk/30172/},
abstract = {Eu-doped GaN was grown by organometallic vapor phase epitaxy at temperatures from 900 to 1100 degrees C. Eu incorporation is influenced by temperature with the highest concentration found for growth at 1000 degrees C. In all samples, Eu is incorporated entirely on substitutional Ga sites with a slight displacement which is highest (similar to 0.2 angstrom) in the sample grown at 900 degrees C and mainly directed along the c-axis. The major optical Eu3+ centers are identical for in situ doped and ion-implanted samples after high temperature and pressure annealing. The dominant Eu3+ luminescence lines are attributed to isolated, substitutional Eu. (c) 2010 American Institute of Physics. [doi:10.1063/1.3489103]}
}

• I. S. Roqan, K. P. O’Donnell, R. W. Martin, P. R. Edwards, S. F. Song, A. Vantomme, K. Lorenz, E. Alves, and M. Boćkowski, “Identification of the prime optical center in GaN:Eu³⁺,” Physical Review B, vol. 81, iss. 1, p. 85209, 2010.

We identify a dominant light-emitting center in ion-implanted GaN:Eu3+ for which the lattice damage has been completely healed, according to x-ray diffraction and Rutherford backscattering spectrometry measurements, by high-temperature, high-pressure annealing. This center is likely to be the isolated substitutional EuGa defect. It lacks a ‘subgap’ excitation band and therefore has no state in the GaN band gap, shows threefold splitting of its 7F2 level, with two sublevels nearly degenerate, and exhibits a long, single-exponential luminescence decay. Competing luminescent centers of GaN:Eu involve this prime center with intrinsic lattice defects, one of which may also be responsible for the GaN yellow band.

@Article{strathprints26804,
author = {I.S. Roqan and K.P. O'Donnell and R.W. Martin and P.R. Edwards and S.F. Song and A. Vantomme and K. Lorenz and E. Alves and M. Bo{\'c}kowski},
title = {Identification of the prime optical center in GaN:Eu³⁺},
journal = {Physical Review B},
year = {2010},
volume = {81},
number = {1},
pages = {085209},
month = {February},
abstract = {We identify a dominant light-emitting center in ion-implanted GaN:Eu3+ for which the lattice damage has been completely healed, according to x-ray diffraction and Rutherford backscattering spectrometry measurements, by high-temperature, high-pressure annealing. This center is likely to be the isolated substitutional EuGa defect. It lacks a 'subgap' excitation band and therefore has no state in the GaN band gap, shows threefold splitting of its 7F2 level, with two sublevels nearly degenerate, and exhibits a long, single-exponential luminescence decay. Competing luminescent centers of GaN:Eu involve this prime center with intrinsic lattice defects, one of which may also be responsible for the GaN yellow band.},
keywords = {optics, GaN, Eu3+, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
url = {http://strathprints.strath.ac.uk/26804/}
}