• 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.
    [BibTeX] [Abstract] [Download PDF]

    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.
    [BibTeX] [Abstract] [Download PDF]

    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.
    [BibTeX] [Abstract] [Download PDF]

    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/}
    }

  • I. A. Ajia, Y. Yamashita, K. Lorenz, M. M. Muhammed, L. Spasevski, D. Almalawi, J. Xu, K. Iizuka, Y. Morishima, D. H. Anjum, N. Wei, R. W. Martin, A. Kuramata, and I. S. Roqan, “GaN/AlGaN multiple quantum wells grown on transparent and conductive (-201)-oriented β-Ga₂O₃ substrate for UV vertical light emitting devices,” Applied Physics Letters, vol. 113, iss. 8, p. 82102, 2018.
    [BibTeX] [Abstract] [Download PDF]

    GaN/AlGaN multiple quantum wells (MQWs) are grown on a (-201)-oriented β-Ga₂O₃ substrate. The optical and structural characteristics of the MQW structure are compared with those of a similar structure grown on sapphire. Scanning transmission electron microscopy and atomic force microscopy images show that the MQW structure exhibits higher crystalline quality of well-defined quantum wells when compared to a similar structure grown on sapphire. X-ray diffraction rocking curve and photoluminescence excitation analyses confirm the lower density of dislocation defects in the sample grown on a β-Ga₂O₃ substrate. A detailed analysis of time-integrated and time-resolved photoluminescence measurements shows that the MQWs grown on a β-Ga₂O₃ substrate are of higher optical quality. Our work indicates that the (-201)-oriented β-Ga₂O₃ substrate can be a potential candidate for UV vertical emitting devices.

    @Article{strathprints65395,
    author = {I. A. Ajia and Y. Yamashita and K. Lorenz and M. M. Muhammed and L. Spasevski and D. Almalawi and J. Xu and K. Iizuka and Y. Morishima and D. H. Anjum and N. Wei and R. W. Martin and A. Kuramata and I. S. Roqan},
    title = {GaN/AlGaN multiple quantum wells grown on transparent and conductive (-201)-oriented {β-Ga₂O₃} substrate for UV vertical light emitting devices},
    journal = {Applied Physics Letters},
    year = {2018},
    volume = {113},
    number = {8},
    pages = {082102},
    month = {August},
    abstract = {GaN/AlGaN multiple quantum wells (MQWs) are grown on a (-201)-oriented β-Ga₂O₃ substrate. The optical and structural characteristics of the MQW structure are compared with those of a similar structure grown on sapphire. Scanning transmission electron microscopy and atomic force microscopy images show that the MQW structure exhibits higher crystalline quality of well-defined quantum wells when compared to a similar structure grown on sapphire. X-ray diffraction rocking curve and photoluminescence excitation analyses confirm the lower density of dislocation defects in the sample grown on a β-Ga₂O₃ substrate. A detailed analysis of time-integrated and time-resolved photoluminescence measurements shows that the MQWs grown on a β-Ga₂O₃ substrate are of higher optical quality. Our work indicates that the (-201)-oriented β-Ga₂O₃ substrate can be a potential candidate for UV vertical emitting devices.},
    keywords = {GaN/AlGaN multiple quantum wells, MQWs, UV, ultraviolet, photoluminescence excitation, Optics. Light, Physics and Astronomy (miscellaneous)},
    url = {https://strathprints.strath.ac.uk/65395/}
    }

  • 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.
    [BibTeX] [Abstract] [Download PDF]

    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/}
    }

  • S. Magalhães, N. Franco, I. M. Watson, R. W. Martin, K. P. O’Donnell, H. P. D. Schenk, F. Tang, T. C. Sadler, M. J. Kappers, R. A. Oliver, T. Monteiro, T. L. Martin, P. A. J. Bagot, M. P. Moody, E. Alves, and K. Lorenz, “Validity of Vegard’s rule for AlₓIn₁₋ₓN (0.08 < x < 0.28) thin films grown on GaN templates," Journal of Physics D: Applied Physics, vol. 50, iss. 20, p. 205107, 2017.
    [BibTeX] [Abstract] [Download PDF]

    In this work, comparative x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements allow a comprehensive characterization of AlₓIn₁₋ₓN thin films grown on GaN. Within the limits of experimental accuracy, and in the compositional range 0.08  <  x <  0.28, the lattice parameters of the alloys generally obey Vegard's rule, varying linearly with the InN fraction. Results are also consistent with the small deviation from linear behaviour suggested by Darakchieva et al (2008 Appl. Phys. Lett. 93 261908). However, unintentional incorporation of Ga, revealed by atom probe tomography (APT) at levels below the detection limit for RBS, may also affect the lattice parameters. Furthermore, in certain samples the compositions determined by XRD and RBS differ significantly. This fact, which was interpreted in earlier publications as an indication of a deviation from Vegard's rule, may rather be ascribed to the influence of defects or impurities on the lattice parameters of the alloy. The wide-ranging set of AlₓIn₁₋ₓN films studied allowed furthermore a detailed investigation of the composition leading to lattice-matching of AlₓIn₁₋ₓN/GaN bilayers.

    @Article{strathprints60823,
    author = {S Magalh{\~a}es and N Franco and I M Watson and R W Martin and K P O'Donnell and H P D Schenk and F Tang and T C Sadler and M J Kappers and R A Oliver and T Monteiro and T L Martin and P A J Bagot and M P Moody and E Alves and K Lorenz},
    title = {Validity of Vegard's rule for {AlₓIn₁₋ₓN (0.08 < x < 0.28)} thin films grown on GaN templates},
    journal = {Journal of Physics D: Applied Physics},
    year = {2017},
    volume = {50},
    number = {20},
    pages = {205107},
    month = {April},
    note = {This is an author-created, un-copyedited version of an article accepted for publication in Journal of Physics D: Applied Physics. The publisher is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1361-6463/aa69dc},
    abstract = {In this work, comparative x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements allow a comprehensive characterization of AlₓIn₁₋ₓN thin films grown on GaN. Within the limits of experimental accuracy, and in the compositional range 0.08  <  x <  0.28, the lattice parameters of the alloys generally obey Vegard's rule, varying linearly with the InN fraction. Results are also consistent with the small deviation from linear behaviour suggested by Darakchieva et al (2008 Appl. Phys. Lett. 93 261908). However, unintentional incorporation of Ga, revealed by atom probe tomography (APT) at levels below the detection limit for RBS, may also affect the lattice parameters. Furthermore, in certain samples the compositions determined by XRD and RBS differ significantly. This fact, which was interpreted in earlier publications as an indication of a deviation from Vegard's rule, may rather be ascribed to the influence of defects or impurities on the lattice parameters of the alloy. The wide-ranging set of AlₓIn₁₋ₓN films studied allowed furthermore a detailed investigation of the composition leading to lattice-matching of AlₓIn₁₋ₓN/GaN bilayers.},
    keywords = {comparative x-ray diffraction, Rutherford backscattering spectrometry , Vegard's rule, lattice parameters, gallium, thin films, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/60823/}
    }

  • 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.
    [BibTeX] [Abstract] [Download PDF]

    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.
    [BibTeX] [Abstract] [Download PDF]

    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/}
    }

  • A. Redondo-Cubero, K. Lorenz, E. Wendler, S. Magalhães, E. Alves, D. Carvalho, T. Ben, F. M. Morales, R. García, K. P. O'Donnell, and C. Wetzel, "Analysis of the stability of InGaN/GaN multiquantum wells against ion beam intermixing," Nanotechnology, vol. 26, iss. 42, p. 425703, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Ion-induced damage and intermixing was evaluated in InGaN/GaN multi-quantum wells (MQWs) using 35 keV N+ implantation at room temperature. In situ ion channeling measurements show that damage builds up with a similar trend for In and Ga atoms, with a high threshold for amorphization. The extended defects induced during the implantation, basal and prismatic stacking faults, are uniformly distributed across the quantum well structure. Despite the extremely high fluences used (up to 4 {$\times$}1016 cm-2), the InGaN MQWs exhibit a high stability against ion beam mixing.

    @Article{strathprints55103,
    author = {A Redondo-Cubero and K Lorenz and E Wendler and S Magalh{\~a}es and E Alves and D Carvalho and T Ben and F M Morales and R Garc{\'i}a and K P O'Donnell and C Wetzel},
    title = {Analysis of the stability of {InGaN/GaN} multiquantum wells against ion beam intermixing},
    journal = {Nanotechnology},
    year = {2015},
    volume = {26},
    number = {42},
    pages = {425703},
    month = {October},
    abstract = {Ion-induced damage and intermixing was evaluated in InGaN/GaN multi-quantum wells (MQWs) using 35 keV N+ implantation at room temperature. In situ ion channeling measurements show that damage builds up with a similar trend for In and Ga atoms, with a high threshold for amorphization. The extended defects induced during the implantation, basal and prismatic stacking faults, are uniformly distributed across the quantum well structure. Despite the extremely high fluences used (up to 4 {$\times$}1016 cm-2), the InGaN MQWs exhibit a high stability against ion beam mixing.},
    keywords = {implantation, InGaN, ion beam mixing, quantum wells, Solid state physics. Nanoscience, Bioengineering, Chemistry(all), Electrical and Electronic Engineering, Mechanical Engineering, Mechanics of Materials, Materials Science(all)},
    url = {http://strathprints.strath.ac.uk/55103/}
    }

  • D. Carvalho, F. M. Morales, T. Ben, R. García, A. Redondo-Cubero, E. Alves, K. Lorenz, P. R. Edwards, K. P. O'Donnell, and C. Wetzel, "Quantitative chemical mapping of InGaN quantum wells from calibrated high-angle annular dark field micrographs," Microscopy and Microanalysis, vol. 21, iss. 4, p. 994–1005, 2015.
    [BibTeX] [Abstract] [Download PDF]

    We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.

    @Article{strathprints53715,
    author = {Daniel Carvalho and Francisco M. Morales and Teresa Ben and Rafael Garc{\'i}a and Andr{\'e}s Redondo-Cubero and Eduardo Alves and Katharina Lorenz and Paul R. Edwards and Kevin P. O'Donnell and Christian Wetzel},
    title = {Quantitative chemical mapping of {InGaN} quantum wells from calibrated high-angle annular dark field micrographs},
    journal = {Microscopy and Microanalysis},
    year = {2015},
    volume = {21},
    number = {4},
    pages = {994--1005},
    month = {August},
    abstract = {We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.},
    keywords = {chemical mapping, energy dispersive X-ray spectroscopy, InGaN/GaN quantum well , HAADF-STEM, RBS, EDX, Physics, Instrumentation},
    url = {http://strathprints.strath.ac.uk/53715/}
    }

  • 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.
    [BibTeX] [Abstract] [Download PDF]

    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.
    [BibTeX] [Abstract] [Download PDF]

    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/}
    }

  • S. Magalhães, I. M. Watson, S. Pereira, N. Franco, L. T. Tan, R. W. Martin, K. P. O'Donnell, E. Alves, J. P. Araújo, T. Monteiro, and K. Lorenz, "Composition, structure and morphology of Al1-xInxN thin films grown on Al1-yGayN templates with different GaN contents," Journal of Physics D: Applied Physics, vol. 48, iss. 1, p. 15103, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Four nominally Al0.85In0.15N thin films with different strains were grown simultaneously on Al1-yGayN (y = 1, 0.93, 0.87 and 0.69) templates by metal organic chemical vapour deposition. The nominal InN content of {$\sim$}15\% was chosen to achieve close lattice matching of Al1-xInxN with the templates of intermediate GaN molar fractions, a small tensile strain for growth on GaN, and compressive strain for the template with the lowest GaN fraction. The film deposited on GaN reveals the highest structural quality, the lowest surface roughness and a homogeneous composition with depth. For growth on the Al1-yGayN ternary templates, the film roughness and the surface pit density both increase with decreasing GaN content, in line with the roughening of the growth templates themselves. Detailed study indicates that the structural and morphological qualities of the templates influence not only the quality of the Al1-xInxN films but also their composition profile. Results suggest that surface roughness of the template and growth on the inclined facets lead to compositional gradients due to a decreased InN incorporation on these facets and to the formation of V-pits.

    @Article{strathprints53524,
    author = {S. Magalh{\~a}es and I. M. Watson and S. Pereira and N. Franco and L. T. Tan and R. W. Martin and K. P. O'Donnell and E. Alves and J. P. Ara{\'u}jo and T. Monteiro and K. Lorenz},
    title = {Composition, structure and morphology of {Al1-xInxN} thin films grown on {Al1-yGayN} templates with different {GaN} contents},
    journal = {Journal of Physics D: Applied Physics},
    year = {2015},
    volume = {48},
    number = {1},
    pages = {015103},
    month = {January},
    abstract = {Four nominally Al0.85In0.15N thin films with different strains were grown simultaneously on Al1-yGayN (y = 1, 0.93, 0.87 and 0.69) templates by metal organic chemical vapour deposition. The nominal InN content of {$\sim$}15\% was chosen to achieve close lattice matching of Al1-xInxN with the templates of intermediate GaN molar fractions, a small tensile strain for growth on GaN, and compressive strain for the template with the lowest GaN fraction. The film deposited on GaN reveals the highest structural quality, the lowest surface roughness and a homogeneous composition with depth. For growth on the Al1-yGayN ternary templates, the film roughness and the surface pit density both increase with decreasing GaN content, in line with the roughening of the growth templates themselves. Detailed study indicates that the structural and morphological qualities of the templates influence not only the quality of the Al1-xInxN films but also their composition profile. Results suggest that surface roughness of the template and growth on the inclined facets lead to compositional gradients due to a decreased InN incorporation on these facets and to the formation of V-pits.},
    keywords = {AlGaN, AlInN, RBS, XRD, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Acoustics and Ultrasonics, Surfaces, Coatings and Films},
    url = {http://strathprints.strath.ac.uk/53524/}
    }

  • 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.
    [BibTeX] [Abstract] [Download PDF]

    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/}
    }

  • E. Taylor, M. D. Smith, T. C. Sadler, K. Lorenz, H. N. Li, E. Alves, P. J. Parbrook, and R. W. Martin, "Structural and optical properties of Ga auto-incorporated InAlN epilayers," Journal of Crystal Growth, vol. 408, p. 97–101, 2014.
    [BibTeX] [Abstract] [Download PDF]

    InAlN epilayers deposited on thick GaN buffer layers grown by metalorganic chemical vapour deposition (MOCVD) revealed an auto-incorporation of Ga when analysed by wavelength dispersive x-ray (WDX) spectroscopy and Rutherford backscattering spectrometry (RBS). Samples were grown under similar conditions with the change in reactor flow rate resulting in varying Ga contents of 12-24\%. The increase in flow rate from 8000 to 24 000 sccm suppressed the Ga auto-incorporation which suggests that the likely cause is from residual Ga left behind from previous growth runs. The luminescence properties of the resultant InAlGaN layers were investigated using cathodoluminescence (CL) measurements.

    @Article{strathprints49667,
    author = {E. Taylor and M.D. Smith and T.C. Sadler and K. Lorenz and H.N. Li and E. Alves and P.J. Parbrook and R.W. Martin},
    title = {Structural and optical properties of {Ga} auto-incorporated {InAlN} epilayers},
    journal = {Journal of Crystal Growth},
    year = {2014},
    volume = {408},
    pages = {97--101},
    month = {December},
    abstract = {InAlN epilayers deposited on thick GaN buffer layers grown by metalorganic chemical vapour deposition (MOCVD) revealed an auto-incorporation of Ga when analysed by wavelength dispersive x-ray (WDX) spectroscopy and Rutherford backscattering spectrometry (RBS). Samples were grown under similar conditions with the change in reactor flow rate resulting in varying Ga contents of 12-24\%. The increase in flow rate from 8000 to 24 000 sccm suppressed the Ga auto-incorporation which suggests that the likely cause is from residual Ga left behind from previous growth runs. The luminescence properties of the resultant InAlGaN layers were investigated using cathodoluminescence (CL) measurements.},
    keywords = {metalorganic chemical vapour deposition, wavelength dispersive x-ray, Rutherford backscattering spectrometry, InAlN, InAlGaN, Ga incorporation, MOCVD, Physics, Chemical engineering, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/49667/}
    }

  • 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.
    [BibTeX] [Abstract] [Download PDF]

    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.
    [BibTeX] [Abstract] [Download PDF]

    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/}
    }

  • M. D. Smith, E. Taylor, T. C. Sadler, V. Z. Zubialevich, K. Lorenz, H. N. Li, J. O'Connell, E. Alves, J. D. Holmes, R. W. Martin, and P. J. Parbrook, "Determination of Ga auto-incorporation in nominal InAlN epilayers grown by MOCVD," Journal of Materials Chemistry. C, vol. 2, iss. 29, p. 5787–5792, 2014.
    [BibTeX] [Abstract] [Download PDF]

    We report on the consistent measurement of gallium incorporation in nominal InAlN layers using various complimentary techniques, underpinned by X-ray diffraction. Nominal InAlN layers with similar growth conditions were prepared, and the change in unintended Ga content in the group III sublattice ranged from similar to 24\% to similar to 12\% when the total reactor flow rate was increased from 8000 to 24 000 standard cubic centimetres per minute. Ultra-thin InAlN/GaN HEMT layers were grown in a clean reactor to minimize Ga auto-incorporation, and measured using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The implications of Ga incorporation in InAlN layers within optoelectronic and power devices is discussed.

    @article{strathprints51372,
    volume = {2},
    number = {29},
    title = {Determination of Ga auto-incorporation in nominal InAlN epilayers grown by MOCVD},
    author = {M. D. Smith and E. Taylor and T. C. Sadler and V. Z. Zubialevich and K. Lorenz and H. N. Li and J. O'Connell and E. Alves and J. D. Holmes and R. W. Martin and P. J. Parbrook},
    year = {2014},
    pages = {5787--5792},
    journal = {Journal of Materials Chemistry. C},
    keywords = {auto-incorporation, epilayers, gallium, InAlN, GaN, Physical and theoretical chemistry, Solid state physics. Nanoscience, Physical and Theoretical Chemistry},
    url = {http://strathprints.strath.ac.uk/51372/},
    abstract = {We report on the consistent measurement of gallium incorporation in nominal InAlN layers using various complimentary techniques, underpinned by X-ray diffraction. Nominal InAlN layers with similar growth conditions were prepared, and the change in unintended Ga content in the group III sublattice ranged from similar to 24\% to similar to 12\% when the total reactor flow rate was increased from 8000 to 24 000 standard cubic centimetres per minute. Ultra-thin InAlN/GaN HEMT layers were grown in a clean reactor to minimize Ga auto-incorporation, and measured using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The implications of Ga incorporation in InAlN layers within optoelectronic and power devices is discussed.}
    }

  • 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.
    [BibTeX] [Abstract] [Download PDF]

    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/}
    }

  • E. Taylor, F. Fang, F. Oehler, P. R. Edwards, M. J. Kappers, K. Lorenz, E. Alves, C. McAleese, C. J. Humphreys, and R. W. Martin, "Composition and luminescence studies of InGaN epilayers grown at different hydrogen flow rates," Semiconductor Science and Technology, vol. 28, iss. 6, p. 65011, 2013.
    [BibTeX] [Abstract] [Download PDF]

    Indium gallium nitride (In(x)Ga(1-x)N) is a technologically important material for many optoelectronic devices, including LEDs and solar cells, but it remains a challenge to incorporate high levels of InN into the alloy while maintaining sample quality. A series of InGaN epilayers was grown with different hydrogen flow rates (0-200 sccm) and growth temperatures (680-750 ?C) to obtain various InN fractions and bright emission in the range 390-480 nm. These 160-nm thick epilayers were characterized through several compositional techniques (wavelength dispersive x-ray spectroscopy, x-ray diffraction, Rutherford backscattering spectrometry) and cathodoluminescence hyperspectral imaging. The compositional analysis with the different techniques shows good agreement when taking into account compositional gradients evidenced in these layers. The addition of small amounts of hydrogen to the gas flow at lower growth temperatures is shown to maintain a high surface quality and luminescence homogeneity. This allowed InN fractions of up to {\texttt{\char126}}16\% to be incorporated with minimal peak energy variations over a mapped area while keeping a high material quality.

    @Article{strathprints43861,
    author = {E Taylor and F Fang and F Oehler and P R Edwards and M J Kappers and K Lorenz and E Alves and C McAleese and C J Humphreys and R W Martin},
    title = {Composition and luminescence studies of InGaN epilayers grown at different hydrogen flow rates},
    journal = {Semiconductor Science and Technology},
    year = {2013},
    volume = {28},
    number = {6},
    pages = {065011},
    month = {May},
    abstract = {Indium gallium nitride (In(x)Ga(1-x)N) is a technologically important material for many optoelectronic devices, including LEDs and solar cells, but it remains a challenge to incorporate high levels of InN into the alloy while maintaining sample quality. A series of InGaN epilayers was grown with different hydrogen flow rates (0-200 sccm) and growth temperatures (680-750 ?C) to obtain various InN fractions and bright emission in the range 390-480 nm. These 160-nm thick epilayers were characterized through several compositional techniques (wavelength dispersive x-ray spectroscopy, x-ray diffraction, Rutherford backscattering spectrometry) and cathodoluminescence hyperspectral imaging. The compositional analysis with the different techniques shows good agreement when taking into account compositional gradients evidenced in these layers. The addition of small amounts of hydrogen to the gas flow at lower growth temperatures is shown to maintain a high surface quality and luminescence homogeneity. This allowed InN fractions of up to {\texttt{\char126}}16\% to be incorporated with minimal peak energy variations over a mapped area while keeping a high material quality.},
    keywords = {composition and luminescence studies, InGaN epilayers, hydrogen flow rates, optoelectronic devices, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/43861/}
    }

  • V. Kachkanov, I. Dobnya, K. O'Donnell, K. Lorenz, S. M. de Sousa Pereira, I. Watson, T. Sadler, H. Li, V. Zubialevich, and P. Parbrook, "Characterisation of III-nitride materials by synchrotron X-ray microdiffraction reciprocal space mapping," Physica Status Solidi C, vol. 10, iss. 3, p. 481–485, 2013.
    [BibTeX] [Abstract] [Download PDF]

    X-ray Reciprocal Space Mapping (RSM) is a powerful tool to explore the structure of semiconductor materials. However, conventional lab-based RSMs are usually measured in two dimensions (2D) ignoring the third dimension of diffraction-space volume. We report the use of a combination of X-ray microfocusing and state-of-the-art 2D area detectors to study the full volume of diffraction?space while probing III-nitride materials on the microscale.

    @Article{strathprints42828,
    author = {V. Kachkanov and Igor Dobnya and Kevin O'Donnell and Katharina Lorenz and de Sousa Pereira, Sergio Manuel and Ian Watson and Thomas Sadler and Haoning Li and Vitaly Zubialevich and Peter Parbrook},
    title = {Characterisation of III-nitride materials by synchrotron X-ray microdiffraction reciprocal space mapping},
    journal = {Physica Status Solidi C},
    year = {2013},
    volume = {10},
    number = {3},
    pages = {481--485},
    month = {March},
    abstract = {X-ray Reciprocal Space Mapping (RSM) is a powerful tool to explore the structure of semiconductor materials. However, conventional lab-based RSMs are usually measured in two dimensions (2D) ignoring the third dimension of diffraction-space volume. We report the use of a combination of X-ray microfocusing and state-of-the-art 2D area detectors to study the full volume of diffraction?space while probing III-nitride materials on the microscale.},
    keywords = {X-ray microdiffraction, eciprocal space mapping, synchrotron radiation, nitride materials, Physics, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/42828/}
    }

  • K. O'Donnell, M. A. D. Maur, A. D. Carlo, and K. Lorenz, "It's not easy being green : strategies for all-nitrides, all-colour solid state lighting," Physica Status Solidi (RRL) - Rapid Research Letters, vol. 6, iss. 2, p. 49–52, 2012.
    [BibTeX] [Abstract] [Download PDF]

    The design strategy presently employed to obtain ?white? light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ?true? red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1) the green gap in the internal quantum efficiency (IQE) of LEDs; (2) the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wavefunction overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.

    @article{strathprints43970,
    volume = {6},
    number = {2},
    month = {February},
    author = {Kevin O'Donnell and Matthias Auf Der Maur and Aldo Di Carlo and Katharina Lorenz},
    title = {It's not easy being green : strategies for all-nitrides, all-colour solid state lighting},
    journal = {Physica Status Solidi (RRL) - Rapid Research Letters},
    pages = {49--52},
    year = {2012},
    keywords = {solid state lighting, band engineering, nitrides, Solid state physics. Nanoscience, Materials Science(all), Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/43970/},
    abstract = {The design strategy presently employed to obtain ?white? light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ?true? red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1) the green gap in the internal quantum efficiency (IQE) of LEDs; (2) the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wavefunction overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.}
    }

  • K. P. O'Donnell, A. D. M. Maur, D. A. Carlo, K. Lorenz, and S. Consortium, "It's not easy being green : strategies for all-nitrides, all-colour solid state lighting," Physica Status Solidi (RRL) - Rapid Research Letters, vol. 6, iss. 2, p. 49–52, 2012.
    [BibTeX] [Abstract] [Download PDF]

    The design strategy presently employed to obtain 'white' light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining 'true' red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1. the green gap in the internal quantum efficiency (IQE) of LEDs; 2. the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wave-function overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.

    @Article{strathprints42405,
    author = {K. P. O'Donnell and M. Auf Der Maur and A. Di Carlo and K. Lorenz and SORBET Consortium},
    journal = {Physica Status Solidi (RRL) - Rapid Research Letters},
    title = {It's not easy being green : strategies for all-nitrides, all-colour solid state lighting},
    year = {2012},
    month = {February},
    number = {2},
    pages = {49--52},
    volume = {6},
    abstract = {The design strategy presently employed to obtain 'white' light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining 'true' red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1. the green gap in the internal quantum efficiency (IQE) of LEDs; 2. the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wave-function overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.},
    keywords = {III-nitrides, light emitting diodes, bandgap engineering, solid state lighting, Physics, Materials Science(all), Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/42405/},
    }

  • V. Kachkanov, I. P. Dolbnya, K. P. O'Donnell, K. Lorenz, S. Pereira, R. W. Martin, P. R. Edwards, and I. M. Watson, "Characterization of InGaN and InAlN epilayers by microdiffraction X-Ray reciprocal space mapping," MRS Online Proceedings Library, vol. 1396, p. mrsf11-1396-o06-11, 2012.
    [BibTeX] [Abstract] [Download PDF]

    We report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that ?seed? InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of ?seeds? on strain fields generated by the a-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth.

    @Article{strathprints40397,
    author = {V. Kachkanov and I. P. Dolbnya and K. P. O'Donnell and K. Lorenz and S. Pereira and R. W. Martin and P. R. Edwards and I. M. Watson},
    title = {Characterization of InGaN and InAlN epilayers by microdiffraction X-Ray reciprocal space mapping},
    journal = {MRS Online Proceedings Library},
    year = {2012},
    volume = {1396},
    pages = {mrsf11-1396-o06-11},
    month = {February},
    abstract = {We report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that ?seed? InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of ?seeds? on strain fields generated by the a-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth.},
    keywords = {microstructure, nitride, x-ray diffraction (XRD), Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/40397/}
    }

  • K. K. Lorenz, S. M. C. S. M. C. Miranda, J. E. E. J. Alves, I. S. I. S. Roqan, K. P. K. P. O'Donnell, and M. ł, "High pressure annealing of Europium implanted GaN," in Proceedings of SPIE - The International Society for Optical Engineering, SPIE, 2012, vol. 8262, p. 82–87.
    [BibTeX] [Abstract] [Download PDF]

    GaN epilayers were implanted with Eu to fluences of 1{$\times$}1013 Eu/cm2 and 1{$\times$}1015 Eu/cm2. Post-implant thermal annealing was performed in ultra-high nitrogen pressures at temperatures up to 1450 ?C. For the lower fluence effective structural recovery of the crystal was observed for annealing at 1000 ?C while optical activation could be further improved at higher annealing temperatures. The higher fluence samples also reveal good optical activation; however, some residual implantation damage remains even for annealing at 1450 ?C which leads to a reduced incorporation of Eu on substitutional sites, a broadening of the Eu luminescence lines and to a strongly reduced fraction of optically active Eu ions. Possibilities for further optimization of implantation and annealing conditions are discussed.

    @incollection{strathprints42833,
    booktitle = {Proceedings of SPIE - The International Society for Optical Engineering},
    volume = {8262},
    title = {High pressure annealing of Europium implanted GaN},
    author = {Katharina K. Lorenz and S. M C S.M.C. Miranda and E. Jorge E.J. Alves and Iman S. I.S. Roqan and Kevin Peter K.P. O'Donnell and Micha{\l} X. M.X. Bo{\'c}kowski},
    publisher = {SPIE},
    year = {2012},
    pages = {82--87},
    keywords = {Europium implanted GaN , GaN epilayers , crystals, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Applied Mathematics, Computer Science Applications},
    url = {http://strathprints.strath.ac.uk/42833/},
    abstract = {GaN epilayers were implanted with Eu to fluences of 1{$\times$}1013 Eu/cm2 and 1{$\times$}1015 Eu/cm2. Post-implant thermal annealing was performed in ultra-high nitrogen pressures at temperatures up to 1450 ?C. For the lower fluence effective structural recovery of the crystal was observed for annealing at 1000 ?C while optical activation could be further improved at higher annealing temperatures. The higher fluence samples also reveal good optical activation; however, some residual implantation damage remains even for annealing at 1450 ?C which leads to a reduced incorporation of Eu on substitutional sites, a broadening of the Eu luminescence lines and to a strongly reduced fraction of optically active Eu ions. Possibilities for further optimization of implantation and annealing conditions are discussed.}
    }

  • 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.
    [BibTeX] [Abstract] [Download PDF]

    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.
    [BibTeX] [Abstract] [Download PDF]

    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.
    [BibTeX] [Abstract] [Download PDF]

    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]}
    }

  • K. Lorenz, S. Magalhaes, N. Franco, N. P. Barradas, V. Darakchieva, E. Alves, S. Pereira, M. R. Correia, F. Munnik, R. W. Martin, K. P. O'Donnell, and I. M. Watson, "AlₓIn₁₋ₓN/GaN bilayers: Structure, morphology, and optical properties," Physica Status Solidi B, vol. 247, iss. 7, p. 1740–1746, 2010.
    [BibTeX] [Abstract] [Download PDF]

    High quality Al1-xInxN/GaN bilayers, grown by metal organic chemical vapor deposition (MOCVD), were characterized using structural and optical techniques. Compositional analysis was performed using Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA). The InN molar fraction x decreased approximately linearly with increasing growth temperature and ranged from x = 0.13 to 0.24. Up to x = 0.20 the layers grow pseudomorphically to GaN with good crystalline quality. These layers show a smooth surface with V-shaped pits. Two layers with InN contents around 24\% showed partial strain relaxation. However, the mechanisms leading to relaxation of compressive strain are very different in the two samples grown both at similar temperature but with different growth rates. One sample shows a decreased c/a ratio, as expected for relaxation of the compressive strain, while In was shown to be homogeneously distributed with depth. The other sample started to grow with x = 0.24 but relaxed mainly by reduction of the incorporated InN content towards the lattice-match composition of x similar to 0.17. Both samples have an increased surface roughness. All samples show strong Al1-xInxN band edge luminescence with large bowing parameter and Stokes' shifts. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

    @Article{strathprints29009,
    author = {K. Lorenz and S. Magalhaes and N. Franco and N. P. Barradas and V. Darakchieva and E. Alves and S. Pereira and M. R. Correia and F. Munnik and R. W. Martin and K. P. O'Donnell and I. M. Watson},
    title = {{AlₓIn₁₋ₓN/GaN} bilayers: Structure, morphology, and optical properties},
    journal = {Physica Status Solidi B},
    year = {2010},
    volume = {247},
    number = {7},
    pages = {1740--1746},
    month = {July},
    abstract = {High quality Al1-xInxN/GaN bilayers, grown by metal organic chemical vapor deposition (MOCVD), were characterized using structural and optical techniques. Compositional analysis was performed using Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA). The InN molar fraction x decreased approximately linearly with increasing growth temperature and ranged from x = 0.13 to 0.24. Up to x = 0.20 the layers grow pseudomorphically to GaN with good crystalline quality. These layers show a smooth surface with V-shaped pits. Two layers with InN contents around 24\% showed partial strain relaxation. However, the mechanisms leading to relaxation of compressive strain are very different in the two samples grown both at similar temperature but with different growth rates. One sample shows a decreased c/a ratio, as expected for relaxation of the compressive strain, while In was shown to be homogeneously distributed with depth. The other sample started to grow with x = 0.24 but relaxed mainly by reduction of the incorporated InN content towards the lattice-match composition of x similar to 0.17. Both samples have an increased surface roughness. All samples show strong Al1-xInxN band edge luminescence with large bowing parameter and Stokes' shifts. (C) 2010 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
    keywords = {photoluminescence, Rutherford backscattering spectrometry, surface structure, III-V semiconductors, field-effect transistor, alinn, alloys, growth, layers, movpe, GaN, optics, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/29009/}
    }

  • 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.
    [BibTeX] [Abstract] [Download PDF]

    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/}
    }

  • I. S. Roqan, K. P. O'Donnell, R. W. Martin, C. Trager-Cowan, V. Matias, A. Vantomme, K. Lorenz, E. Alves, and I. M. Watson, "Optical and structural properties of Eu-implanted InₓAl₁₋ₓN," Journal of Applied Physics, vol. 106, iss. 8, p. 83508, 2009.
    [BibTeX] [Abstract] [Download PDF]

    Off-axis implantation of 80 keV Eu ions into epitaxial c-plane InAlN/GaN bilayers confines rare-earth (RE) doping largely to the InAlN layer. Rutherford backscattering spectrometry and x-ray diffraction show good correlations between the Eu3+ emission linewidth and key structural parameters of InxAl1?xN films on GaN in the composition range near lattice matching (x {$\sim$} 0.17). In contrast to GaN:Eu, selectively excited photoluminescence (PL) and PL excitation spectra reveal the presence of a single dominant optical center in InAlN. Eu3+ emission from In0.13Al0.87N:Eu also shows significantly less thermal quenching than GaN:Eu. InAlN films are therefore superior to GaN for RE optical doping.

    @Article{strathprints18902,
    author = {I.S. Roqan and K.P. O'Donnell and R.W. Martin and C. Trager-Cowan and V. Matias and A. Vantomme and K. Lorenz and E. Alves and I.M. Watson},
    title = {Optical and structural properties of Eu-implanted InₓAl₁₋ₓN},
    journal = {Journal of Applied Physics},
    year = {2009},
    volume = {106},
    number = {8},
    pages = {083508},
    month = {October},
    abstract = {Off-axis implantation of 80 keV Eu ions into epitaxial c-plane InAlN/GaN bilayers confines rare-earth (RE) doping largely to the InAlN layer. Rutherford backscattering spectrometry and x-ray diffraction show good correlations between the Eu3+ emission linewidth and key structural parameters of InxAl1?xN films on GaN in the composition range near lattice matching (x {$\sim$} 0.17). In contrast to GaN:Eu, selectively excited photoluminescence (PL) and PL excitation spectra reveal the presence of a single dominant optical center in InAlN. Eu3+ emission from In0.13Al0.87N:Eu also shows significantly less thermal quenching than GaN:Eu. InAlN films are therefore superior to GaN for RE optical doping.},
    keywords = {aluminium compounds, europium, III-V semiconductors, indium compounds, ion implantation, photoluminescence, quenching (thermal), Rutherford backscattering, semiconductor doping, semiconductor epitaxial layers, wide band gap semiconductors, X-ray diffraction, Optics. Light, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/18902/}
    }

  • K. Wang, K. P. O'Donnell, B. Hourahine, R. W. Martin, I. M. Watson, K. Lorenz, and E. Alves, "Luminescence of Eu ions in AlₓGa₁₋ₓN across the entire alloy composition range," Physical Review B, vol. 80, iss. 12, p. 125206, 2009.
    [BibTeX] [Abstract] [Download PDF]

    Photoluminescence (PL) and PL excitation (PLE) spectra of Eu-implanted AlxGa1-xN are obtained across the whole alloy composition range. The dominant D-5(0)-F-7(2) emission band broadens and then narrows as x increases from 0 to 1 while the peak shifts monotonically. This behavior is surprisingly similar to the broadening of excitons in a semiconductor alloy caused by composition fluctuations [E. F. Schubert et al., Phys. Rev. B 30, 813 (1984). PLE spectra reveal a steplike AlxGa1-xN band-edge absorption and two "subgap" bands X-1,X-2:X-1 peaks at 3.26 eV in GaN and shifts linearly to 3.54 eV in AlN. For x {\ensuremath{>}} 0.6, X-2 emerges approximately 1 eV higher in energy than X-1 and shifts in a similar way. We propose that X-1,X-2 involve creation of core-excitonic complexes of Eu emitting centers.

    @Article{strathprints18942,
    author = {K. Wang and K.P. O'Donnell and B. Hourahine and R.W. Martin and I.M. Watson and K. Lorenz and E. Alves},
    title = {Luminescence of Eu ions in AlₓGa₁₋ₓN across the entire alloy composition range},
    journal = {Physical Review B},
    year = {2009},
    volume = {80},
    number = {12},
    pages = {125206},
    month = {September},
    abstract = {Photoluminescence (PL) and PL excitation (PLE) spectra of Eu-implanted AlxGa1-xN are obtained across the whole alloy composition range. The dominant D-5(0)-F-7(2) emission band broadens and then narrows as x increases from 0 to 1 while the peak shifts monotonically. This behavior is surprisingly similar to the broadening of excitons in a semiconductor alloy caused by composition fluctuations [E. F. Schubert et al., Phys. Rev. B 30, 813 (1984). PLE spectra reveal a steplike AlxGa1-xN band-edge absorption and two "subgap" bands X-1,X-2:X-1 peaks at 3.26 eV in GaN and shifts linearly to 3.54 eV in AlN. For x {\ensuremath{>}} 0.6, X-2 emerges approximately 1 eV higher in energy than X-1 and shifts in a similar way. We propose that X-1,X-2 involve creation of core-excitonic complexes of Eu emitting centers.},
    keywords = {rare-earth ions, iii-v semiconductors, tm-doped alxga1-xn, implanted gan, optical-properties, temperature, aln, photoluminescence, intensities, excitation, Optics. Light, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/18942/}
    }

  • K. Lorenz, N. P. Barradas, E. Alves, I. S. Roqan, E. Nogales, R. W. Martin, K. P. O'Donnell, F. Gloux, and P. Ruterana, "Structural and optical characterization of Eu-implanted GaN," Journal of Physics D: Applied Physics, vol. 42, iss. 16, p. 165103, 2009.
    [BibTeX] [Abstract] [Download PDF]

    GaN was implanted with 300 keV Eu ions over a wide fluence range from 1 {$\times$} 1013 to 1 {$\times$} 1016 Eu cm?2 at room temperature (RT) or 500 ?C. Detailed structural and optical characterizations of the samples were performed using Rutherford backscattering spectrometry and channelling, transmission and scanning electron microscopy, wavelength dispersive x-ray emission and RT cathodoluminescence (CL) spectroscopy. RT implantation results in a sigmoidal-shaped damage build-up curve with four regimes that were correlated with the formation of specific kinds of defects. After annealing at 1000 ?C only samples implanted to fluences below 0.8 {$\times$} 1015 Eu cm?2 showed near complete recovery of the crystal. Implantation at elevated temperature significantly decreases the implantation damage and increases the fraction of Eu incorporated on substitutional Ga-sites. The improved structural properties of samples implanted at elevated temperature are reflected in a higher intensity of Eu-related red light emission after annealing at 1000 ?C. The RT CL intensity is correlated with the number of Eu ions on substitutional Ga-sites after annealing. Furthermore, a detailed study of optical activation shows that the optimum annealing temperature depends on the implantation fluence due to the sensitive balance of defects removed and created during high temperature annealing.

    @Article{strathprints19030,
    author = {K. Lorenz and N.P. Barradas and E. Alves and I.S. Roqan and E. Nogales and R.W. Martin and K.P. O'Donnell and F. Gloux and P. Ruterana},
    title = {Structural and optical characterization of Eu-implanted GaN},
    journal = {Journal of Physics D: Applied Physics},
    year = {2009},
    volume = {42},
    number = {16},
    pages = {165103},
    month = {July},
    abstract = {GaN was implanted with 300 keV Eu ions over a wide fluence range from 1 {$\times$} 1013 to 1 {$\times$} 1016 Eu cm?2 at room temperature (RT) or 500 ?C. Detailed structural and optical characterizations of the samples were performed using Rutherford backscattering spectrometry and channelling, transmission and scanning electron microscopy, wavelength dispersive x-ray emission and RT cathodoluminescence (CL) spectroscopy. RT implantation results in a sigmoidal-shaped damage build-up curve with four regimes that were correlated with the formation of specific kinds of defects. After annealing at 1000 ?C only samples implanted to fluences below 0.8 {$\times$} 1015 Eu cm?2 showed near complete recovery of the crystal. Implantation at elevated temperature significantly decreases the implantation damage and increases the fraction of Eu incorporated on substitutional Ga-sites. The improved structural properties of samples implanted at elevated temperature are reflected in a higher intensity of Eu-related red light emission after annealing at 1000 ?C. The RT CL intensity is correlated with the number of Eu ions on substitutional Ga-sites after annealing. Furthermore, a detailed study of optical activation shows that the optimum annealing temperature depends on the implantation fluence due to the sensitive balance of defects removed and created during high temperature annealing.},
    keywords = {cathodoluminescence, electron microscopy, dipersive spectrometry, ion implantation, gallium nitride, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/19030/}
    }

  • K. Lorenz, I. S. Roqan, N. Franco, K. P. O'Donnell, V. Darakchieva, E. Alves, C. Trager-Cowan, R. W. Martin, D. J. As, M. Panfilova, P. (. Fundacao para a Ciencia e Tecnologia (FCT), H. C. (Funder), and G. S. F. (. (Funder), "Europium doping of zincblende GaN by ion implantation," Journal of Applied Physics, vol. 105, iss. 11, p. 113507, 2009.
    [BibTeX] [Abstract] [Download PDF]

    Eu was implanted into high quality cubic (zincblende) GaN (ZB-GaN) layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction (XRD) and Rutherford backscattering/channeling spectrometry. A low concentration ({\ensuremath{<}}10\%) of wurtzite phase inclusions was observed by XRD analysis in as-grown samples with their (0001) planes aligned with the \{111\} planes of the cubic lattice. Implantation of Eu causes an expansion of the lattice parameter in the implanted region similar to that observed for the c-lattice parameter of wurtzite GaN (W-GaN). For ZB-GaN:Eu, a large fraction of Eu ions is found on a high symmetry interstitial site aligned with the {\ensuremath{<}} 110 {\ensuremath{>}} direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal annealing, but an increase in the wurtzite phase fraction was observed at the same time. Cathodoluminescence, photoluminescence (PL), and PL excitation spectroscopy revealed several emission lines which can be attributed to distinct Eu-related optical centers in ZB-GaN and W-GaN inclusions.

    @Article{strathprints19085,
    author = {K. Lorenz and I.S. Roqan and N. Franco and K.P. O'Donnell and V. Darakchieva and E. Alves and C. Trager-Cowan and R.W. Martin and D.J. As and M. Panfilova and Fundacao para a Ciencia e Tecnologia (FCT), Portugal (Funder) and HOYA Corporation (Funder) and German Science Foundation (DFG) (Funder)},
    journal = {Journal of Applied Physics},
    title = {Europium doping of zincblende GaN by ion implantation},
    year = {2009},
    month = {June},
    number = {11},
    pages = {113507},
    volume = {105},
    abstract = {Eu was implanted into high quality cubic (zincblende) GaN (ZB-GaN) layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction (XRD) and Rutherford backscattering/channeling spectrometry. A low concentration ({\ensuremath{<}}10\%) of wurtzite phase inclusions was observed by XRD analysis in as-grown samples with their (0001) planes aligned with the \{111\} planes of the cubic lattice. Implantation of Eu causes an expansion of the lattice parameter in the implanted region similar to that observed for the c-lattice parameter of wurtzite GaN (W-GaN). For ZB-GaN:Eu, a large fraction of Eu ions is found on a high symmetry interstitial site aligned with the {\ensuremath{<}} 110 {\ensuremath{>}} direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal annealing, but an increase in the wurtzite phase fraction was observed at the same time. Cathodoluminescence, photoluminescence (PL), and PL excitation spectroscopy revealed several emission lines which can be attributed to distinct Eu-related optical centers in ZB-GaN and W-GaN inclusions.},
    keywords = {EU-doped gan, cubic gan, optical activiation, lattice location, photoluminescence, growth, films, spectroscopy, epilayers, epitaxy, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/19085/},
    }

  • K. Lorenz, N. Franco, E. Alves, S. Pereira, I. M. Watson, R. W. Martin, and K. P. O'Donnell, "Relaxation of compressively strained AlInN on GaN," Journal of Crystal Growth, vol. 310, iss. 18, p. 4058–4064, 2008.
    [BibTeX] [Abstract] [Download PDF]

    Epitaxial layers of wurtzite-phase Al1?xInxN, 120 nm thick with (0 0 0 1) orientation, were grown by metal organic chemical vapour deposition on GaN buffer layers at setpoint temperatures between 760 and 840 ?C. For growth temperatures 800 ?C, the AlInN layers grew with uniform composition, pseudomorphic with the underlying GaN buffer layer. In the temperature range studied, the InN fractions are a linear function of the setpoint temperature and straddle the near-lattice-match composition around Al0.83In0.17N. Lowering the growth temperature to 760 ?C caused a compositional grading, a marked change in surface morphology, and a reduction in AlInN crystal quality. The resulting AlInN layer consists of a compressively strained interfacial layer with a composition of Al0.76In0.24N, and a mostly relaxed near-surface layer with a composition of Al0.81In0.19N. Atomic force microscopy suggests that a transition to a three-dimensional growth mode accompanies the structural relaxation and change in composition.

    @article{strathprints8120,
    volume = {310},
    number = {18},
    month = {August},
    author = {K. Lorenz and N. Franco and E. Alves and S. Pereira and I.M. Watson and R.W. Martin and K.P. O'Donnell},
    title = {Relaxation of compressively strained AlInN on GaN},
    journal = {Journal of Crystal Growth},
    pages = {4058--4064},
    year = {2008},
    keywords = {characterization, stresses, metal organic vapour phase epitaxy, nitrides, semiconducting III?V materials, semiconducting ternary compounds, Optics. Light, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/8120/},
    abstract = {Epitaxial layers of wurtzite-phase Al1?xInxN, 120 nm thick with (0 0 0 1) orientation, were grown by metal organic chemical vapour deposition on GaN buffer layers at setpoint temperatures between 760 and 840 ?C. For growth temperatures 800 ?C, the AlInN layers grew with uniform composition, pseudomorphic with the underlying GaN buffer layer. In the temperature range studied, the InN fractions are a linear function of the setpoint temperature and straddle the near-lattice-match composition around Al0.83In0.17N. Lowering the growth temperature to 760 ?C caused a compositional grading, a marked change in surface morphology, and a reduction in AlInN crystal quality. The resulting AlInN layer consists of a compressively strained interfacial layer with a composition of Al0.76In0.24N, and a mostly relaxed near-surface layer with a composition of Al0.81In0.19N. Atomic force microscopy suggests that a transition to a three-dimensional growth mode accompanies the structural relaxation and change in composition.}
    }

  • K. Wang, R. W. Martin, D. Amabile, P. R. Edwards, S. Hernandez, E. Nogales, K. P. O'Donnell, K. Lorenz, E. Alves, V. Matias, A. Vantomme, D. Wolverson, and I. M. Watson, "Optical energies of AllnN epilayers," Journal of Applied Physics, vol. 103, iss. 7, p. 73510, 2008.
    [BibTeX] [Abstract] [Download PDF]

    Optical energy gaps are measured for high-quality Al1?xInxN-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of 6 eV and differences with earlier reports are discussed. Very large Stokes' shifts of 0.4-0.8 eV are observed in the composition range 0.13{\ensuremath{<}}x{\ensuremath{<}}0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field

    @Article{strathprints14005,
    author = {K. Wang and R. W. Martin and D. Amabile and P. R. Edwards and S. Hernandez and E. Nogales and K. P. O'Donnell and K. Lorenz and E. Alves and V. Matias and A. Vantomme and D. Wolverson and I. M. Watson},
    title = {Optical energies of AllnN epilayers},
    journal = {Journal of Applied Physics},
    year = {2008},
    volume = {103},
    number = {7},
    pages = {073510},
    month = {April},
    abstract = {Optical energy gaps are measured for high-quality Al1?xInxN-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of 6 eV and differences with earlier reports are discussed. Very large Stokes' shifts of 0.4-0.8 eV are observed in the composition range 0.13{\ensuremath{<}}x{\ensuremath{<}}0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field},
    keywords = {aluminium compounds, indium compounds, photoluminescence, semiconductor epitaxial layers, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/14005/}
    }

  • I. S. Roqan, K. P. O'Donnell, C. Trager-Cowan, B. Hourahine, R. W. Martin, K. Lorenz, E. Alves, D. J. As, M. Panfilova, and I. M. Watson, "Luminescence spectroscopy of Eu-implanted zincblende GaN," Physica Status Solidi B, vol. 245, iss. 1, p. 170–173, 2008.
    [BibTeX] [Abstract] [Download PDF]

    Cathodoluminescence (CL) and Photoluminescence (PL) of Eu-implanted zincblende-GaN (ZB-GaN:Eu) and wurtzite-GaN (W-GaN:Eu) are compared in order to investigate the optical activation of GaN by Eu. The EU3+ emission spectrum depends critically on the crystal structure of the GaN host; implantation and post-annealing at 800 degrees C partially converts implantation-damaged ZB-GaN:Eu to W-GaN:Eu. Selective excitation of PL at wavelengths below the ZB-GaN band edge reveals a new sharp emission line at 627 nm, together with a number of satellites, which we ascribe to ZB-GaN: Eu.

    @article{strathprints13953,
    volume = {245},
    number = {1},
    month = {January},
    author = {I.S. Roqan and K.P. O'Donnell and C. Trager-Cowan and B. Hourahine and R.W. Martin and K. Lorenz and E. Alves and D.J. As and M. Panfilova and I.M. Watson},
    title = {Luminescence spectroscopy of Eu-implanted zincblende GaN},
    journal = {Physica Status Solidi B},
    pages = {170--173},
    year = {2008},
    keywords = {luminescence spectroscopy, spectroscopy, zincblende GaN, cathodoluminescence, photoluminescence, Optics. Light, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/13953/},
    abstract = {Cathodoluminescence (CL) and Photoluminescence (PL) of Eu-implanted zincblende-GaN (ZB-GaN:Eu) and wurtzite-GaN (W-GaN:Eu) are compared in order to investigate the optical activation of GaN by Eu. The EU3+ emission spectrum depends critically on the crystal structure of the GaN host; implantation and post-annealing at 800 degrees C partially converts implantation-damaged ZB-GaN:Eu to W-GaN:Eu. Selective excitation of PL at wavelengths below the ZB-GaN band edge reveals a new sharp emission line at 627 nm, together with a number of satellites, which we ascribe to ZB-GaN: Eu.}
    }

  • K. Lorenz, E. Alves, I. S. Roqan, R. W. Martin, C. Trager-Cowan, K. P. O'Donnell, and I. M. Watson, "Rare earth doping of III-nitride alloys by ion implantation," Physica Status Solidi A, vol. 205, iss. 1, p. 34–37, 2008.
    [BibTeX] [Abstract] [Download PDF]

    The implantation damage and rare earth (RE) luminescence in the wide band gap ternaries AlGaN and AlInN were studied and compared to GaN. For both ternaries lower damage levels were observed and in contrast to GaN, no surface amorphisation occurs during the implantation. Damage recovery of RE implanted GaN was studied for post implant annealing at temperatures between 800 ?C and 1300 ?C. The blue and IR Tm related luminescence intensity is seen to increase strongly with the annealing temperature. The two Tm lines observed at 478 nm and 465 nm are assigned to the 1G4 3H6 and the 1D23F4 transition, respectively. For GaN the line at 465 nm is fully quenched at RT while it becomes the dominant line for the ternaries. The blue luminescence intensity in the ternaries is significantly stronger than in GaN. Furthermore, AlInN shows a very high ratio of blue/IR luminescence.

    @article{strathprints13951,
    volume = {205},
    number = {1},
    month = {January},
    author = {K. Lorenz and E. Alves and I.S. Roqan and R.W. Martin and C. Trager-Cowan and K.P. O'Donnell and I.M. Watson},
    title = {Rare earth doping of III-nitride alloys by ion implantation},
    journal = {Physica Status Solidi A},
    pages = {34--37},
    year = {2008},
    keywords = {rare earth luminescence, wide band gap, AlGaN, AlInN, GaN, Optics. Light, Materials Chemistry, Surfaces, Coatings and Films, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/13951/},
    abstract = {The implantation damage and rare earth (RE) luminescence in the wide band gap ternaries AlGaN and AlInN were studied and compared to GaN. For both ternaries lower damage levels were observed and in contrast to GaN, no surface amorphisation occurs during the implantation. Damage recovery of RE implanted GaN was studied for post implant annealing at temperatures between 800 ?C and 1300 ?C. The blue and IR Tm related luminescence intensity is seen to increase strongly with the annealing temperature. The two Tm lines observed at 478 nm and 465 nm are assigned to the 1G4 3H6 and the 1D23F4 transition, respectively. For GaN the line at 465 nm is fully quenched at RT while it becomes the dominant line for the ternaries. The blue luminescence intensity in the ternaries is significantly stronger than in GaN. Furthermore, AlInN shows a very high ratio of blue/IR luminescence.}
    }

  • I. S. Roqan, K. Lorenz, K. P. O'Donnell, C. Trager-Cowan, R. W. Martin, I. M. Watson, and E. Alves, "Blue cathodoluminescence from thulium implanted AlₓGa₁₋ₓN and InₓAl₁₋ₓN," Superlattices and Microstructures, vol. 40, iss. 4-6, p. 445–451, 2006.
    [BibTeX] [Abstract] [Download PDF]

    Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted AlxGa1-xN with different AlN contents (in the range 0 {\ensuremath{<}}= x {\ensuremath{<}}= 0.2) and from implanted InxAl1-xN with different InN contents (x = 0.13 and 0.19) close to the lattice match with GaN. The Tm3+ emission spectrum depends critically on the host material. The blue emission from AIxGal-xN:Tm peaks in intensity for an AlN content of x similar to 0.11. The emission is enhanced by up to a factor of 50 times with an increase of annealing temperature from 1000 to 1300 degrees C. The blue emission from In0.13Al0.87N:Tm, annealed at 1200 degrees C, is more than ten times stronger than that from AlxGa1-xN:Tm, x {\ensuremath{<}}= 0.2. However, the intensity decreases significantly as the InN fraction increases from 0.13 to 0.19.

    @Article{strathprints42400,
    author = {I. S. Roqan and K. Lorenz and K. P. O'Donnell and C. Trager-Cowan and R. W. Martin and I. M. Watson and E. Alves},
    title = {Blue cathodoluminescence from thulium implanted AlₓGa₁₋ₓN and InₓAl₁₋ₓN},
    journal = {Superlattices and Microstructures},
    year = {2006},
    volume = {40},
    number = {4-6},
    pages = {445--451},
    month = {December},
    abstract = {Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted AlxGa1-xN with different AlN contents (in the range 0 {\ensuremath{<}}= x {\ensuremath{<}}= 0.2) and from implanted InxAl1-xN with different InN contents (x = 0.13 and 0.19) close to the lattice match with GaN. The Tm3+ emission spectrum depends critically on the host material. The blue emission from AIxGal-xN:Tm peaks in intensity for an AlN content of x similar to 0.11. The emission is enhanced by up to a factor of 50 times with an increase of annealing temperature from 1000 to 1300 degrees C. The blue emission from In0.13Al0.87N:Tm, annealed at 1200 degrees C, is more than ten times stronger than that from AlxGa1-xN:Tm, x {\ensuremath{<}}= 0.2. However, the intensity decreases significantly as the InN fraction increases from 0.13 to 0.19.},
    keywords = {rare earth, GaAlN, InAlN, implantation, cathodoluminescence, Physics, Materials Science(all), Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/42400/}
    }

  • D. Pastor, S. Hernandez, R. Cusco, L. Artus, R. W. Martin, K. P. O'Donnell, O. Briot, K. Lorenz, and E. Alves, "UV-Raman scattering study of lattice recovery by thermal annealing of Eu⁺-implanted GaN layers," Superlattices and Microstructures, vol. 40, iss. 4-6, p. 440–444, 2006.
    [BibTeX] [Abstract] [Download PDF]

    Lattice recovery of Eu-implanted GaN has been studied by means of Raman scattering under UV excitation. GaN epilayers implanted at 300 keV with doses ranging from 2 x 10(14) to 4 x 10(15) cm(-2) and subsequently annealed at 1000 degrees C for 20 min show an increasing degree of disorder as the implantation dose increases. Higher temperature annealings up to 1300 degrees C were also investigated in samples having an AlN capping layer. Disorder related modes, observed in samples annealed at 1000 degrees C, disappear at higher annealing temperatures, indicating an incomplete lattice recovery at 1000 degrees C. The Raman scattering spectra show resonant A(1)(LO) multiphonon scattering up to sixth order, whose relative intensities depend on the implantation dose. The intensity ratios between multiphonon peaks observed for the highest implantation doses suggest a spread of the resonance, which could be related to a heterogeneous strain distribution, also indicative of incomplete lattice recovery. (c) 2006 Elsevier Ltd. All rights reserved.

    @Article{strathprints31063,
    author = {D. Pastor and S. Hernandez and R. Cusco and L. Artus and R. W. Martin and K. P. O'Donnell and O. Briot and K. Lorenz and E. Alves},
    title = {UV-Raman scattering study of lattice recovery by thermal annealing of Eu⁺-implanted GaN layers},
    journal = {Superlattices and Microstructures},
    year = {2006},
    volume = {40},
    number = {4-6},
    pages = {440--444},
    month = {October},
    note = {Symposium on Material Science and Technology of Wide Bandgap Semiconductors held at the 2006 Spring Meeting of the EMRS, Nice, FRANCE, MAY 29-JUN 02, 2006},
    abstract = {Lattice recovery of Eu-implanted GaN has been studied by means of Raman scattering under UV excitation. GaN epilayers implanted at 300 keV with doses ranging from 2 x 10(14) to 4 x 10(15) cm(-2) and subsequently annealed at 1000 degrees C for 20 min show an increasing degree of disorder as the implantation dose increases. Higher temperature annealings up to 1300 degrees C were also investigated in samples having an AlN capping layer. Disorder related modes, observed in samples annealed at 1000 degrees C, disappear at higher annealing temperatures, indicating an incomplete lattice recovery at 1000 degrees C. The Raman scattering spectra show resonant A(1)(LO) multiphonon scattering up to sixth order, whose relative intensities depend on the implantation dose. The intensity ratios between multiphonon peaks observed for the highest implantation doses suggest a spread of the resonance, which could be related to a heterogeneous strain distribution, also indicative of incomplete lattice recovery. (c) 2006 Elsevier Ltd. All rights reserved.},
    keywords = {rare earth doping, ion beam implantation, Raman scattering, Physics, Materials Science(all), Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/31063/}
    }

  • K. Wang, R. Martin, E. Nogales, V. Katchkanov, K. O'Donnell, S. Hernandez, K. Lorenz, E. Alves, S. Ruffenach, and O. Briot, "Optical properties of high-temperature annealed Eu-implanted GaN," Optical Materials, vol. 28, iss. 6-7, p. 797–801, 2006.
    [BibTeX] [Abstract] [Download PDF]

    A 10 nm thick epitaxially grown AIN cap has been used to protect the surface of a GaN epilayer both during Eu ion implantation and the subsequent high-temperature annealing. The 15 K photoluminescence (PL) intensity of the intra-4f Eu transition increases by two orders of magnitude when the annealing temperature is increased from 1000 to 1300 degrees C. High-resolution PL spectra reveal that the emission lines due to the D-5(0)-F-7(2) transition exhibit different dependencies on the annealing temperature in the studied annealing range. PL excitation measurements demonstrate band edge absorption by the GaN host at 356 nm, together with a broad excitation band centred at similar to 385 nm. The PL spectra of the D-5(0)-F-7(2) transition selectively excited by above band-gap absorption and by this broad excitation band are noticeably different. The first peak at 620.8 nm is suppressed when exciting below the GaN band gap. This demonstrates differing energy transfer processes for the different Eu luminescent peaks and is direct evidence for at least two kinds of different Eu sites in the host with distinct optical activation. Temperature dependent PL and PLE demonstrate that one of the two Eu-centres does not contribute to the room temperature luminescence. (c) 2005 Elsevier B.V. All rights reserved.

    @article{strathprints31052,
    volume = {28},
    number = {6-7},
    month = {May},
    author = {K Wang and RW Martin and E Nogales and V Katchkanov and KP O'Donnell and S Hernandez and K Lorenz and E Alves and S Ruffenach and O Briot},
    note = {Meeting of the European-Materials-Research-Society, Strasbourg, FRANCE, MAY 30-JUN 03, 2005},
    title = {Optical properties of high-temperature annealed Eu-implanted GaN},
    year = {2006},
    journal = {Optical Materials},
    pages = {797--801},
    keywords = {photoluminescence , luminescence, optics, lasers, beams, Optics. Light, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics, Electrical and Electronic Engineering, Computer Science(all)},
    url = {http://strathprints.strath.ac.uk/31052/},
    abstract = {A 10 nm thick epitaxially grown AIN cap has been used to protect the surface of a GaN epilayer both during Eu ion implantation and the subsequent high-temperature annealing. The 15 K photoluminescence (PL) intensity of the intra-4f Eu transition increases by two orders of magnitude when the annealing temperature is increased from 1000 to 1300 degrees C. High-resolution PL spectra reveal that the emission lines due to the D-5(0)-F-7(2) transition exhibit different dependencies on the annealing temperature in the studied annealing range. PL excitation measurements demonstrate band edge absorption by the GaN host at 356 nm, together with a broad excitation band centred at similar to 385 nm. The PL spectra of the D-5(0)-F-7(2) transition selectively excited by above band-gap absorption and by this broad excitation band are noticeably different. The first peak at 620.8 nm is suppressed when exciting below the GaN band gap. This demonstrates differing energy transfer processes for the different Eu luminescent peaks and is direct evidence for at least two kinds of different Eu sites in the host with distinct optical activation. Temperature dependent PL and PLE demonstrate that one of the two Eu-centres does not contribute to the room temperature luminescence. (c) 2005 Elsevier B.V. All rights reserved.}
    }

  • S. Hernandez, R. Cusco, L. Artus, E. Nogales, R. W. Martin, K. P. O'Donnell, G. Halambalakis, O. Briot, K. Lorenz, and E. Alves, "Lattice order in thulium-doped GaN epilayers : in situ doping versus ion implantation," Optical Materials, vol. 28, iss. 6-7, p. 771–774, 2006.
    [BibTeX] [Abstract] [Download PDF]

    We have investigated the crystalline quality of thulium-doped GaN obtained either by in situ doping during MBE growth or by ion-beam implantation of MOCVD GaN layers. Both types of samples display the typical sharp intra-4f shell emission lines of Tm3+ ions in the blue and infrared spectral regions. The Raman spectra of the MBE samples indicate a good crystalline quality, showing the narrow E-2 and A(1) phonon peaks characteristic of GaN, even for the highest Tin concentrations. In contrast, Raman peaks associated with vacancy-related defects, as well as low-frequency bands due to disorder-activated modes can be observed in the Raman spectra of the implanted samples. These results indicate that, for the implantation doses required to achieve Tm emission, some residual disorder remains in the implanted GaN layers which is not observed in Tm-doped MBE samples with higher Tm concentration.

    @article{strathprints31050,
    volume = {28},
    number = {6-7},
    month = {May},
    author = {S Hernandez and R Cusco and L Artus and E Nogales and R W Martin and K P O'Donnell and G Halambalakis and O Briot and K Lorenz and E Alves},
    note = {Meeting of the European-Materials-Research-Society, Strasbourg, FRANCE, MAY 30-JUN 03, 2005},
    title = {Lattice order in thulium-doped GaN epilayers : in situ doping versus ion implantation},
    year = {2006},
    journal = {Optical Materials},
    pages = {771--774},
    keywords = {GaN epilayers , lattices, raman spectra, raman scattering, Optics. Light, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics, Electrical and Electronic Engineering, Computer Science(all)},
    url = {http://strathprints.strath.ac.uk/31050/},
    abstract = {We have investigated the crystalline quality of thulium-doped GaN obtained either by in situ doping during MBE growth or by ion-beam implantation of MOCVD GaN layers. Both types of samples display the typical sharp intra-4f shell emission lines of Tm3+ ions in the blue and infrared spectral regions. The Raman spectra of the MBE samples indicate a good crystalline quality, showing the narrow E-2 and A(1) phonon peaks characteristic of GaN, even for the highest Tin concentrations. In contrast, Raman peaks associated with vacancy-related defects, as well as low-frequency bands due to disorder-activated modes can be observed in the Raman spectra of the implanted samples. These results indicate that, for the implantation doses required to achieve Tm emission, some residual disorder remains in the implanted GaN layers which is not observed in Tm-doped MBE samples with higher Tm concentration.}
    }

  • K. Lorenz, U. Wahl, E. Alves, E. Nogales, S. Dalmasso, R. W. Martin, K. P. O'Donnell, M. Wojdak, A. Braud, T. Monteiro, T. Wojtowicz, P. Ruterana, S. Ruffenach, and O. Briot, "High temperature annealing of rare earth implanted GaN films : structural and optical properties," Optical Materials, vol. 28, iss. 6-7, p. 750–758, 2006.
    [BibTeX] [Abstract] [Download PDF]

    GaN epilayers grown by MOCVD were implanted with Tm and Eu under different implantation conditions, in order to optimize the implantation parameters of fluence, implantation temperature and energy. Rutherford backscattering spectrometry in the channelling mode (RBS/C), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL) and room temperature cathodoluminescence (CL) were used to study structural and optical properties of the samples. Different annealing methods are compared. According to RBS/C measurements, Tm at low fluences is found to incorporate entirely on substitutional Ga-sites while for higher fluences the substitutional fraction decreases. Increasing the fluence leads to increasing damage levels and finally to the formation of a partially amorphous, nanocrystalline surface layer. The luminescence intensity increases with fluence at first. However., for fluences in excess of 2 x 10(15) Tm/cm(2) it decreases again, showing a strong correlation between structural and optical properties. Implanting at higher temperature inhibits the formation of the nanocrystalline layer and increases both the substitutional fraction and the luminescence intensity. We also found that the presence of a thin AIN cap protects the sample during post-implant high temperature annealing and prevents the formation of a nanocrystalline surface layer during the implantation even for high fluences. The intensity of Eu-related CL near 622 nm at room temperature increases by a factor of 40 within the studied annealing range from 1000 to 1300 degrees C. (c) 2005 Elsevier B.V. All rights reserved.

    @article{strathprints31049,
    volume = {28},
    number = {6-7},
    month = {May},
    author = {K. Lorenz and U. Wahl and E. Alves and E Nogales and S Dalmasso and R W Martin and K P O'Donnell and M Wojdak and A Braud and T Monteiro and T Wojtowicz and P Ruterana and S Ruffenach and O Briot},
    note = {Meeting of the European-Materials-Research-Society, Strasbourg, FRANCE, MAY 30-JUN 03, 2005},
    title = {High temperature annealing of rare earth implanted GaN films : structural and optical properties},
    year = {2006},
    journal = {Optical Materials},
    pages = {750--758},
    keywords = {GaN films , electron microscopy , nanocrystalline surface , backscattering , spectrometry, Optics. Light, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics, Electrical and Electronic Engineering, Computer Science(all)},
    url = {http://strathprints.strath.ac.uk/31049/},
    abstract = {GaN epilayers grown by MOCVD were implanted with Tm and Eu under different implantation conditions, in order to optimize the implantation parameters of fluence, implantation temperature and energy. Rutherford backscattering spectrometry in the channelling mode (RBS/C), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL) and room temperature cathodoluminescence (CL) were used to study structural and optical properties of the samples. Different annealing methods are compared. According to RBS/C measurements, Tm at low fluences is found to incorporate entirely on substitutional Ga-sites while for higher fluences the substitutional fraction decreases. Increasing the fluence leads to increasing damage levels and finally to the formation of a partially amorphous, nanocrystalline surface layer. The luminescence intensity increases with fluence at first. However., for fluences in excess of 2 x 10(15) Tm/cm(2) it decreases again, showing a strong correlation between structural and optical properties. Implanting at higher temperature inhibits the formation of the nanocrystalline layer and increases both the substitutional fraction and the luminescence intensity. We also found that the presence of a thin AIN cap protects the sample during post-implant high temperature annealing and prevents the formation of a nanocrystalline surface layer during the implantation even for high fluences. The intensity of Eu-related CL near 622 nm at room temperature increases by a factor of 40 within the studied annealing range from 1000 to 1300 degrees C. (c) 2005 Elsevier B.V. All rights reserved.}
    }

  • I. Roqan, C. Trager-Cowan, B. Hourahine, K. Lorenz, E. Nogales, K. P. O'Donnell, R. W. Martin, E. Alves, S. Ruffenach, and O. Briot, "Characterization of the blue emission of Tm/Er co-implanted GaN," in GaN, AIN, InN and Related Materials, M. Kuball, T. Mukai, T. H. Myers, and J. M. Redwing, Eds., Warrendale: Materials Research Society, 2006, p. 599–604.
    [BibTeX] [Abstract] [Download PDF]

    Comparative studies have been carried out on the cathodoluminescence (CL) and photoluminescence (PL) properties of GaN implanted with Tin and GaN co-implanted with Tin and a low concentration of Er. Room temperature CL spectra were acquired in an electron probe microanalyser to investigate the rare earth emission. The room temperature CL intensity exhibits a strong dependence on the annealing temperature of the implanted samples. The results of CL temperature dependence are reported for blue emission (similar to 477 nm) which is due to intra 4f-shell electron transitions ((1)G(4)-{\ensuremath{>}} H-3(6)) associated with Tm3+ ions. The 477 nm blue CL emission is enhanced strongly as the annealing temperature increases up to 1200 degrees C. Blue PL emission has also been observed from the sample annealed at 1200 degrees C. To our knowledge, this is the first observation of blue PL emission from Tin implanted GaN samples. Intra-4f transitions from the D-1(2) level (similar to 465 nm emission lines) of Tm3+ ions in GaN have been observed in GaN:Tm films at temperatures between 20-200 K. We will discuss the temperature dependent Tm3+ emission in both GaN:Tm,Er and GaN:Tm samples.

    @incollection{strathprints36884,
    month = {March},
    author = {Iman Roqan and Carol Trager-Cowan and Ben Hourahine and Katharina Lorenz and Emilio Nogales and Kevin P. O'Donnell and Robert W. Martin and Eduardo Alves and S. Ruffenach and Olivier Briot},
    series = {Materials Research Society Symposium Proceedings},
    note = {(c) 2006 Cambridge University Press.},
    booktitle = {GaN, AIN, InN and Related Materials},
    editor = {M. Kuball and T. Mukai and T.H. Myers and J.M. Redwing},
    title = {Characterization of the blue emission of Tm/Er co-implanted GaN},
    address = {Warrendale},
    publisher = {Materials Research Society},
    year = {2006},
    pages = {599--604},
    keywords = {tm-doped alxga1-xn, photoluminescence, cathodoluminescence, ions, EU, ER , ion-implantation, nitride, luminescence, Physics, Electronic, Optical and Magnetic Materials, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/36884/},
    abstract = {Comparative studies have been carried out on the cathodoluminescence (CL) and photoluminescence (PL) properties of GaN implanted with Tin and GaN co-implanted with Tin and a low concentration of Er. Room temperature CL spectra were acquired in an electron probe microanalyser to investigate the rare earth emission. The room temperature CL intensity exhibits a strong dependence on the annealing temperature of the implanted samples. The results of CL temperature dependence are reported for blue emission (similar to 477 nm) which is due to intra 4f-shell electron transitions ((1)G(4)-{\ensuremath{>}} H-3(6)) associated with Tm3+ ions. The 477 nm blue CL emission is enhanced strongly as the annealing temperature increases up to 1200 degrees C. Blue PL emission has also been observed from the sample annealed at 1200 degrees C. To our knowledge, this is the first observation of blue PL emission from Tin implanted GaN samples. Intra-4f transitions from the D-1(2) level (similar to 465 nm emission lines) of Tm3+ ions in GaN have been observed in GaN:Tm films at temperatures between 20-200 K. We will discuss the temperature dependent Tm3+ emission in both GaN:Tm,Er and GaN:Tm samples.}
    }

  • E. Nogales, R. W. Martin, K. P. O'Donnell, K. Lorenz, E. Alves, S. Ruffenach, and O. Briot, "Failure mechanism of AlN nanocaps used to protect RE-implanted GaN during high temperature annealing," Applied Physics Letters, vol. 88, p. 31902, 2006.
    [BibTeX] [Abstract] [Download PDF]

    The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300 ?C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1-2 m which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates.

    @Article{strathprints3008,
    author = {E. Nogales and R.W. Martin and K.P. O'Donnell and K. Lorenz and E. Alves and S. Ruffenach and O. Briot},
    title = {Failure mechanism of AlN nanocaps used to protect RE-implanted GaN during high temperature annealing},
    journal = {Applied Physics Letters},
    year = {2006},
    volume = {88},
    pages = {31902},
    month = {January},
    abstract = {The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300 ?C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1-2 m which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates.},
    keywords = {failure mechanism, AlN nanocaps, RE-implanted GaN, high temperature annealing, nanoscience, Solid state physics. Nanoscience, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/3008/}
    }

  • K. Wang, R. W. Martin, E. Nogales, P. R. Edwards, K. P. O'Donnell, K. Lorenz, E. Alves, and I. M. Watson, "Cathodoluminescence of rare earth implanted AlInN," Applied Physics Letters, vol. 89, iss. 13, p. 131912, 2006.
    [BibTeX] [Abstract] [Download PDF]

    AlInN layers implanted with europium and erbium ions are systematically studied and compared with similarly implanted GaN. Cathodoluminescence from four series of annealed samples shows that the Eu/Er emissions from AlInN are considerably broader than those from GaN, while the peak positions only change slightly. The rate of increase of cathodoluminescence intensity with annealing temperature, up to 1300 ?C, is analyzed for all four series. For Eu the increase exceeds 10{$\times$} in both hosts. Although some decomposition is observed for annealing at 1200 ?C, well above the growth temperature, AlInN is shown to be a surprisingly robust host for rare earth ions.

    @Article{strathprints37517,
    author = {K. Wang and R.W. Martin and E. Nogales and P.R. Edwards and K.P. O'Donnell and K. Lorenz and E. Alves and I.M. Watson},
    title = {Cathodoluminescence of rare earth implanted AlInN},
    journal = {Applied Physics Letters},
    year = {2006},
    volume = {89},
    number = {13},
    pages = {131912},
    abstract = {AlInN layers implanted with europium and erbium ions are systematically studied and compared with similarly implanted GaN. Cathodoluminescence from four series of annealed samples shows that the Eu/Er emissions from AlInN are considerably broader than those from GaN, while the peak positions only change slightly. The rate of increase of cathodoluminescence intensity with annealing temperature, up to 1300 ?C, is analyzed for all four series. For Eu the increase exceeds 10{$\times$} in both hosts. Although some decomposition is observed for annealing at 1200 ?C, well above the growth temperature, AlInN is shown to be a surprisingly robust host for rare earth ions.},
    keywords = {cathodoluminescence, aluminium compounds, indium compounds, gallium compounds, wide band gap semiconductors, rare earth, AllnN, implanted, Physics, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/37517/}
    }

  • K. Lorenz, N. Franco, E. Alves, I. M. Watson, R. W. Martin, and K. P. O'Donnell, "Anomalous ion channeling in AlInN/GaN bilayers: Determination of the strain state," Physical Review Letters, vol. 97, p. 85501, 2006.
    [BibTeX] [Abstract] [Download PDF]

    Monte Carlo simulations of anomalous ion channeling in near-lattice-matched AlInN/GaN bilayers allow an accurate determination of the strain state of AlInN by Rutherford backscattering or channeling. Although these strain estimates agree well with x-ray diffraction (XRD) results, XRD composition estimates are shown to have limited accuracy, due to a possible deviation from Vegard's law, which we quantify for this alloy. As the InN fraction increases from 13\% to 19\%, the strain in AlInN films changes from tensile to compressive with lattice matching predicted to occur at [InN]=17.1\%.

    @article{strathprints4214,
    volume = {97},
    title = {Anomalous ion channeling in AlInN/GaN bilayers: Determination of the strain state},
    author = {K. Lorenz and N. Franco and E. Alves and I.M. Watson and R.W. Martin and K.P. O'Donnell},
    year = {2006},
    pages = {085501},
    journal = {Physical Review Letters},
    keywords = {photonics, lasers, optics, quantum electronics, physics, Optics. Light, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/4214/},
    abstract = {Monte Carlo simulations of anomalous ion channeling in near-lattice-matched AlInN/GaN bilayers allow an accurate determination of the strain state of AlInN by Rutherford backscattering or channeling. Although these strain estimates agree well with x-ray diffraction (XRD) results, XRD composition estimates are shown to have limited accuracy, due to a possible deviation from Vegard's law, which we quantify for this alloy. As the InN fraction increases from 13\% to 19\%, the strain in AlInN films changes from tensile to compressive with lattice matching predicted to occur at [InN]=17.1\%.}
    }

  • K. Lorenz, U. Wahl, E. Alves, S. Dalmasso, R. W. Martin, K. P. O'Donnell, S. Ruffenach, and O. Briot, "High temperature annealing and optical activation of Eu implanted GaN," Applied Physics Letters, vol. 85, iss. 14, p. 2712–2714, 2005.
    [BibTeX] [Abstract] [Download PDF]

    Europium was implanted into GaN through a 10 nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing. Employing this AlN layer prevents the formation of an amorphous surface layer during the implantation. Furthermore, no dissociation of the crystal was observed by Rutherford backscattering and channeling measurements for annealing temperatures up to 1300?C. Remarkably, the intensity of the Eu related luminescence, as measured by cathodoluminescence at room temperature, increases by one order of magnitude within the studied annealing range between 1100 and 1300?C.

    @article{strathprints3006,
    volume = {85},
    number = {14},
    month = {October},
    author = {K. Lorenz and U. Wahl and E. Alves and S. Dalmasso and R.W. Martin and K.P. O'Donnell and S. Ruffenach and O. Briot},
    title = {High temperature annealing and optical activation of Eu implanted GaN},
    journal = {Applied Physics Letters},
    pages = {2712--2714},
    year = {2005},
    keywords = {high temperature annealing, optical activation, Eu implanted GaN, nanoscience, Solid state physics. Nanoscience, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/3006/},
    abstract = {Europium was implanted into GaN through a 10 nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing. Employing this AlN layer prevents the formation of an amorphous surface layer during the implantation. Furthermore, no dissociation of the crystal was observed by Rutherford backscattering and channeling measurements for annealing temperatures up to 1300?C. Remarkably, the intensity of the Eu related luminescence, as measured by cathodoluminescence at room temperature, increases by one order of magnitude within the studied annealing range between 1100 and 1300?C.}
    }

  • K. Wang, R. W. Martin, K. P. O'Donnell, V. Katchkanov, E. Nogales, K. Lorenz, E. Alves, S. Ruffenach, and O. Briot, "Selectively excited photoluminescence from Eu- implanted GaN," Applied Physics Letters, vol. 87, iss. 11, p. 112107, 2005.
    [BibTeX] [Abstract] [Download PDF]

    The intensity of Eu-related luminescence from ion-implanted GaN with a 10 nm thick AlN cap, both grown epitaxially by metal organic chemical vapor deposition (MOCVD) is increased markedly by high-temperature annealing at 1300 ?C. Photoluminescence (PL) and PL excitation (PLE) studies reveal a variety of Eu centers with different excitation mechanisms. High-resolution PL spectra at low temperature clearly show that emission lines ascribed to 5D0-7F2 ({\texttt{\char126}}622 nm), 5D0-7F3 ({\texttt{\char126}}664 nm), and 5D0-7F1 ({\texttt{\char126}}602 nm) transitions each consist of several peaks. PL excitation spectra of the spectrally resolved components of the 5D0-7F2 multiplet contain contributions from above-bandedge absorption by the GaN host, a GaN exciton absorption at 356 nm, and a broad subedge absorption band centred at {\texttt{\char126}}385 nm. Marked differences in the shape of the 5D0-7F2 PL multiplet are demonstrated by selective excitation via the continuum/exciton states and the below gap absorption band. The four strongest lines of the multiplet are shown to consist of two pairs due to different Eu3+ centers with different excitation mechanisms.

    @Article{strathprints3004,
    author = {K. Wang and R.W. Martin and K.P. O'Donnell and V. Katchkanov and E. Nogales and K. Lorenz and E. Alves and S. Ruffenach and O. Briot},
    title = {Selectively excited photoluminescence from Eu- implanted GaN},
    journal = {Applied Physics Letters},
    year = {2005},
    volume = {87},
    number = {11},
    pages = {112107},
    month = {September},
    abstract = {The intensity of Eu-related luminescence from ion-implanted GaN with a 10 nm thick AlN cap, both grown epitaxially by metal organic chemical vapor deposition (MOCVD) is increased markedly by high-temperature annealing at 1300 ?C. Photoluminescence (PL) and PL excitation (PLE) studies reveal a variety of Eu centers with different excitation mechanisms. High-resolution PL spectra at low temperature clearly show that emission lines ascribed to 5D0-7F2 ({\texttt{\char126}}622 nm), 5D0-7F3 ({\texttt{\char126}}664 nm), and 5D0-7F1 ({\texttt{\char126}}602 nm) transitions each consist of several peaks. PL excitation spectra of the spectrally resolved components of the 5D0-7F2 multiplet contain contributions from above-bandedge absorption by the GaN host, a GaN exciton absorption at 356 nm, and a broad subedge absorption band centred at {\texttt{\char126}}385 nm. Marked differences in the shape of the 5D0-7F2 PL multiplet are demonstrated by selective excitation via the continuum/exciton states and the below gap absorption band. The four strongest lines of the multiplet are shown to consist of two pairs due to different Eu3+ centers with different excitation mechanisms.},
    keywords = {photoluminescence, Eu- implanted GaN, organic chemical vapor deposition, nanoscience, Solid state physics. Nanoscience, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/3004/}
    }