• V. Kachkanov, K. P. O’Donnell, R. W. Martin, J. F. W. Mosselmans, and S. Pereira, “Local structure of luminescent InGaN alloys,” Applied Physics Letters, vol. 89, iss. 10, p. 101908, 2006.
    [BibTeX] [Abstract] [Download PDF]

    Comparative Ga and In K-edge extended x-ray absorption fine structure studies provide the first direct evidence of an inequality of mean In-Ga and Ga-In next-nearest neighbor separations in InGaN alloys. The degree of inequality increases with decreasing InN fraction x in the range accessible to extended x-ray absorption fine structure analysis of alloys (0.9 {\ensuremath{<}}x {\ensuremath{<}}0.1). Its concurrence with an increase of luminescence efficiency in this composition range suggests that the breakdown of In/Ga randomness in InGaN is correlated with efficient radiative recombination in blue-green light emitting devices.

    @Article{strathprints31047,
    author = {V. Kachkanov and K. P. O'Donnell and R. W. Martin and J. F. W. Mosselmans and S. Pereira},
    title = {Local structure of luminescent InGaN alloys},
    journal = {Applied Physics Letters},
    year = {2006},
    volume = {89},
    number = {10},
    pages = {101908},
    month = {September},
    abstract = {Comparative Ga and In K-edge extended x-ray absorption fine structure studies provide the first direct evidence of an inequality of mean In-Ga and Ga-In next-nearest neighbor separations in InGaN alloys. The degree of inequality increases with decreasing InN fraction x in the range accessible to extended x-ray absorption fine structure analysis of alloys (0.9 {\ensuremath{<}}x {\ensuremath{<}}0.1). Its concurrence with an increase of luminescence efficiency in this composition range suggests that the breakdown of In/Ga randomness in InGaN is correlated with efficient radiative recombination in blue-green light emitting devices.},
    keywords = {luminescence, absorption, InGaN alloys, Optics. Light, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/31047/}
    }

  • V. Katchkanov, J. F. W. Mosselmans, K. P. O'Donnell, E. Nogales, S. Hernandez, R. W. Martin, A. Steckl, and D. S. Lee, "Extended X-ray absorption fine structure studies of GaN epilayers doped with Er," Optical Materials, vol. 28, iss. 6-7, p. 785–789, 2006.
    [BibTeX] [Abstract] [Download PDF]

    The structural properties of Er doped GaN epilayers were studied by means of extended X-ray absorption fine structure (EXAFS) measured at the Er L-III and Ga K-edges. The samples were doped with Er in-situ during growth by molecular beam epitaxy (MBE). The Ga local structure was found to be the same in all samples studied. Er L-III-edge EXAFS showed that when growth conditions were gradually changed from Ga-rich to Ga-poor, an increase in Er concentration from 0.15 at.\% to 0.64 at.\% is accompanied by the sequential formation of ErGaN, ErGaN clusters with locally high Er content and finally a pure ErN component. This study indicates that Er incorporation into GaN is enhanced under Ga-poor conditions, at the expense of the formation of Er-rich clusters and ErN precipitates. (c) 2005 Elsevier B.V. All rights reserved.

    @article{strathprints31051,
    volume = {28},
    number = {6-7},
    month = {May},
    author = {V Katchkanov and J F W Mosselmans and K P O'Donnell and E Nogales and S Hernandez and R W Martin and A Steckl and D S Lee},
    note = {Meeting of the European-Materials-Research-Society, Strasbourg, FRANCE, MAY 30-JUN 03, 2005},
    title = {Extended X-ray absorption fine structure studies of GaN epilayers doped with Er},
    year = {2006},
    journal = {Optical Materials},
    pages = {785--789},
    keywords = {absorption, optical materials, molecular 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/31051/},
    abstract = {The structural properties of Er doped GaN epilayers were studied by means of extended X-ray absorption fine structure (EXAFS) measured at the Er L-III and Ga K-edges. The samples were doped with Er in-situ during growth by molecular beam epitaxy (MBE). The Ga local structure was found to be the same in all samples studied. Er L-III-edge EXAFS showed that when growth conditions were gradually changed from Ga-rich to Ga-poor, an increase in Er concentration from 0.15 at.\% to 0.64 at.\% is accompanied by the sequential formation of ErGaN, ErGaN clusters with locally high Er content and finally a pure ErN component. This study indicates that Er incorporation into GaN is enhanced under Ga-poor conditions, at the expense of the formation of Er-rich clusters and ErN precipitates. (c) 2005 Elsevier B.V. All rights reserved.}
    }

  • V. Katchkanov, K. P. O'Donnell, J. F. W. Mosselmans, S. Hernandez, R. W. Martin, Y. Nanishi, M. kurochi, I. M. Watson, W. van der Stricht, and E. Calleja, "Extended x-ray absorption fine structure studies of InGaN epilayers," MRS Online Proceedings Library, vol. 831, p. 203–207, 2005.
    [BibTeX] [Abstract] [Download PDF]

    The local structure around In atoms in InGaN epilayers grown by Molecular Beam Epitaxy (MBE) and by Metal-Organic Chemical Vapour Deposition (MOCVD) was studied by means of Extended X-ray Absorption Fine Structure (EXAFS). The averaged In fraction of MOCVD grown samples ranged from 10\% to 40\% as estimated by Electron Probe Microanalysis (EPMA). The In fraction of MBE grown samples spanned the range from 13\% to 96\%. The In-N bond length was found to vary slightly with composition, both for MBE and MOCVD grown samples. Moreover, for the same In content, the In-N bond lengths in MOCVD samples were longer than those in MBE grown samples. In contrast, the In-In radial separations in MOCVD and MBE samples were found to be indistinguishable for the same In molar fraction. The In-Ga bond length was observed to deviate from average cation-cation distance predicted by Vegard's law for MBE grown samples which indicates alloy compositional fluctuations.

    @article{strathprints9984,
    volume = {831},
    title = {Extended x-ray absorption fine structure studies of InGaN epilayers},
    author = {V. Katchkanov and K.P. O'Donnell and J.F.W. Mosselmans and S. Hernandez and R.W. Martin and Y. Nanishi and M. kurochi and I.M. Watson and W. van der Stricht and E. Calleja},
    year = {2005},
    pages = {203--207},
    journal = {MRS Online Proceedings Library},
    keywords = {physics, materials research, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/9984/},
    abstract = {The local structure around In atoms in InGaN epilayers grown by Molecular Beam Epitaxy (MBE) and by Metal-Organic Chemical Vapour Deposition (MOCVD) was studied by means of Extended X-ray Absorption Fine Structure (EXAFS). The averaged In fraction of MOCVD grown samples ranged from 10\% to 40\% as estimated by Electron Probe Microanalysis (EPMA). The In fraction of MBE grown samples spanned the range from 13\% to 96\%. The In-N bond length was found to vary slightly with composition, both for MBE and MOCVD grown samples. Moreover, for the same In content, the In-N bond lengths in MOCVD samples were longer than those in MBE grown samples. In contrast, the In-In radial separations in MOCVD and MBE samples were found to be indistinguishable for the same In molar fraction. The In-Ga bond length was observed to deviate from average cation-cation distance predicted by Vegard's law for MBE grown samples which indicates alloy compositional fluctuations.}
    }

  • K. P. O'Donnell, V. Katchkanov, K. Wang, R. W. Martin, B. Hourahine, P. R. Edwards, E. Nogales, J. F. W. Mosselmans, and B. De-Vries, "Site multiplicity of rare earth ions in III-nitrides," MRS Online Proceedings Library, vol. 831, p. 527–535, 2004.
    [BibTeX] [Abstract] [Download PDF]

    This presentation reviews recent lattice location studies of RE ions in GaN by electron emission channelling (EC) and X-ray absorption fine structure (XAFS) techniques. These studies agree that RE ions at low concentrations (whether they are incorporated during growth or introduced later by ion implantation) predominantly occupy Ga substitutional sites, as expected from considerations of charge equivalence. We combine this result with some examples of the welldocumented richness of optical spectra of GaN:RE3+ to suggest that the luminescence of these materials may be ascribed to a family of rather similar sites, all of which feature the REGa defect.

    @article{strathprints3024,
    volume = {831},
    month = {December},
    title = {Site multiplicity of rare earth ions in III-nitrides},
    author = {K.P. O'Donnell and V. Katchkanov and K. Wang and R.W. Martin and B. Hourahine and P.R. Edwards and E. Nogales and J.F.W. Mosselmans and B. De-Vries},
    year = {2004},
    pages = {527--535},
    journal = {MRS Online Proceedings Library},
    keywords = {site multiplicity, rare earth ions, III-nitrides, nanoscience, Solid state physics. Nanoscience, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/3024/},
    abstract = {This presentation reviews recent lattice location studies of RE ions in GaN by electron emission channelling (EC) and X-ray absorption fine structure (XAFS) techniques. These studies agree that RE ions at low concentrations (whether they are incorporated during growth or introduced later by ion implantation) predominantly occupy Ga substitutional sites, as expected from considerations of charge equivalence. We combine this result with some examples of the welldocumented richness of optical spectra of GaN:RE3+ to suggest that the luminescence of these materials may be ascribed to a family of rather similar sites, all of which feature the REGa defect.}
    }

  • K. P. O'Donnell, S. Pereira, R. W. Martin, P. R. Edwards, M. J. Tobin, and J. F. W. Mosselmans, "Wishful physics: Some common misconceptions about InGaN," Physica Status Solidi A - Applications and Materials Science, vol. 195, iss. 3, p. 532–536, 2003.
    [BibTeX] [Abstract] [Download PDF]

    All III-N visible light emitting devices contain ultrathin active layers of InGaN. Although this material has been widely studied during the last ten years or so, opinion is still divided as to its nature. Most researchers would agree with the proposition that III-nitride "alloys" are a mess, at least when compared with analogous III-As materials. It may be further argued that the quality of InGaN samples is at present too variable to allow general statements to be made about the material. We repudiate this misconception. The similarities between luminescent InGaN samples from different laboratories outweigh the differences. Any differences that do occur can be confidently accounted for, in terms of a peculiar growth habit of III-nitrides. We also briefly discuss the status of accidental InN quantum dots.

    @article{strathprints30541,
    volume = {195},
    number = {3},
    month = {February},
    author = {K P O'Donnell and S Pereira and R W Martin and P R Edwards and M J Tobin and J F W Mosselmans},
    title = {Wishful physics: Some common misconceptions about InGaN},
    journal = {Physica Status Solidi A - Applications and Materials Science},
    pages = {532--536},
    year = {2003},
    keywords = {photoluminescence excitation spectroscopy, absorption fine-structure, molecular-beam epitaxy, X-ray-diffraction, exciton localization, quantum-wells, stokes shift, epilayers, layers, dependence, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/30541/},
    abstract = {All III-N visible light emitting devices contain ultrathin active layers of InGaN. Although this material has been widely studied during the last ten years or so, opinion is still divided as to its nature. Most researchers would agree with the proposition that III-nitride "alloys" are a mess, at least when compared with analogous III-As materials. It may be further argued that the quality of InGaN samples is at present too variable to allow general statements to be made about the material. We repudiate this misconception. The similarities between luminescent InGaN samples from different laboratories outweigh the differences. Any differences that do occur can be confidently accounted for, in terms of a peculiar growth habit of III-nitrides. We also briefly discuss the status of accidental InN quantum dots.}
    }

  • K. P. O'Donnell, J. F. W. Mosselmans, R. W. Martin, S. Pereira, and M. E. White, "Structural analysis of InGaN epilayers," Journal of Physics: Condensed Matter, vol. 13, iss. 32, p. 6977–6991, 2001.
    [BibTeX] [Abstract] [Download PDF]

    {The structural properties of InGaN have attracted interest on account of the recent widespread use of the material in visible light-emitting devices. A key topic has been the indirect determination of the composition by x-ray diffraction (XRD). We examine critically the several levels of approximation involved in this procedure. It is shown by extended x-ray absorption fine structure (EXAFS) measurements that the local structure of InGaN is independent of the composition, in the range of InN fraction, from about 15 to 40\%, that corresponds to blue to infrared light emission from this material. EXAFSdetermined ratios of the numbers of indium and gallium atoms in the first metal co-ordination shell, M1, show very good agreement with the composition measured by established techniques, both structural and chemical, on similar samples. On the other hand, the atomic separations deviate markedly from values calculated using Vegard's law. In particular, the average radial separations

    @article{strathprints3022,
    volume = {13},
    number = {32},
    month = {August},
    author = {K.P. O'Donnell and J.F.W. Mosselmans and R.W. Martin and S. Pereira and M.E. White},
    title = {Structural analysis of InGaN epilayers},
    journal = {Journal of Physics: Condensed Matter},
    pages = {6977--6991},
    year = {2001},
    keywords = {InGaN, light-emitting devices, x-ray diffraction, nanoscience, Solid state physics. Nanoscience, Materials Science(all), Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/3022/},
    abstract = {The structural properties of InGaN have attracted interest on account of the recent widespread use of the material in visible light-emitting devices. A key topic has been the indirect determination of the composition by x-ray diffraction (XRD). We examine critically the several levels of approximation involved in this procedure. It is shown by extended x-ray absorption fine structure (EXAFS) measurements that the local structure of InGaN is independent of the composition, in the range of InN fraction, from about 15 to 40\%, that corresponds to blue to infrared light emission from this material. EXAFSdetermined ratios of the numbers of indium and gallium atoms in the first metal co-ordination shell, M1, show very good agreement with the composition measured by established techniques, both structural and chemical, on similar samples. On the other hand, the atomic separations deviate markedly from values calculated using Vegard's law. In particular, the average radial separations, In-N1 = 2.11(2) {\^i}.. and In-M1 = 3.28(3) {\^i}.., do not vary significantly with In/Ga ratio in the examined composition range. We conclude with some brief comments on the uncertain but challenging topic of InGaN nanostructure.}
    }

  • K. P. O'Donnell, R. W. Martin, M. E. White, M. J. Tobin, J. F. W. Mosselmans, I. M. Watson, B. Damiliano, and N. Grandjean, "Luminescence and structural properties of InGaN epilayer, quantum well and quantum dot samples using synchrotron radiation," MRS Online Proceedings Library, vol. 639, p. G9.11, 2001.
    [BibTeX] [Abstract] [Download PDF]

    The Daresbury synchrotron radiation source (SRS) provides bright, tunable x-rays for scattering and absorption probes of local structure. Scanning confocal microscopy and luminescence decay measurements employ the SRS in alternative ways, as a tunable luminescence excitation engine and as a source of weak, 160 ps pulses with a large pulse-topulse separation, respectively. This report first describes local atomic structure studies of InGaN epilayers by extended x-ray absorption fine structure (EXAFS). In addition, we report photoluminescence (PL) imaging, PL microspectroscopy and photoluminescence decay studies of various nitride samples, including tailored InGaN quantum wells and discs.

    @article{strathprints38541,
    volume = {639},
    title = {Luminescence and structural properties of InGaN epilayer, quantum well and quantum dot samples using synchrotron radiation},
    author = {K.P. O'Donnell and R.W. Martin and M.E. White and M.J. Tobin and J.F.W. Mosselmans and I.M. Watson and B. Damiliano and N. Grandjean},
    year = {2001},
    pages = {G9.11},
    journal = {MRS Online Proceedings Library},
    keywords = {InGaN epilayer , quantum well, quantum dot samples , synchrotron radiation, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/38541/},
    abstract = {The Daresbury synchrotron radiation source (SRS) provides bright, tunable x-rays for scattering and absorption probes of local structure. Scanning confocal microscopy and luminescence decay measurements employ the SRS in alternative ways, as a tunable luminescence excitation engine and as a source of weak, 160 ps pulses with a large pulse-topulse separation, respectively. This report first describes local atomic structure studies of InGaN epilayers by extended x-ray absorption fine structure (EXAFS). In addition, we report photoluminescence (PL) imaging, PL microspectroscopy and photoluminescence decay studies of various nitride samples, including tailored InGaN quantum wells and discs.}
    }