A. Koo

Publications (14)8.76 Total impact

• Article: Optical Response of DyN

  M. Azeem, B. J. Ruck, Binh Do Le, H. Warring, N. M. Strickland, A. Koo, V. Goian, S. Kamba, H. J. Trodahl
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  ABSTRACT: We report measurements of the optical response of polycrystalline DyN thin films. The frequency-dependent complex refractive index in the near IR-visible-near UV was determined by fitting reflection/transmission spectra. In conjunction with resistivity measurements these identify DyN as a semiconductor with 1.2 eV optical gap. When doped by nitrogen vacancies it shows free carrier absorption and a blue-shifted gap associated with the Moss-Burstein effect. The refractive index of 2.0+/-0.1 depends only weakly on energy. Far infrared reflectivity data show a polar phonon of frequency 280 cm-1 and dielectric strength delta epsilon= 20.
  07/2012;
• Source

  Available from: preston.co.nz

  Article: Photocurrent diffusion length in disordered GaN

  A. Koo, F. Budde, B. J. Ruck, H. J. Trodahl, A. Bittar, A. R. H. Preston
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  ABSTRACT: The diffusion length of carriers in semiconductors is a significant parameter determining the suitability of a material for device applications. Here we describe a simple new technique to measure diffusion length and apply it to a study of two types of disordered GaN. We find the diffusion length to be of the order of microns in nanocrystalline GaN and hundreds of microns in amorphous GaON. Experimentally the method involves the defocussing of a laser spot between two contacts. The results are supported by numerical modelling of the carrier concentrations in the experiment.
  Journal of Materials Science Materials in Electronics 09/2007; 18:107-110. · 1.08 Impact Factor
• Source

  Available from: preston.co.nz

  Article: Photoconductivity in nanocrystalline GaN and amorphous GaON

  A. Koo, F. Budde, B. J. Ruck, H. J. Trodahl, A. Bittar, A. Preston, A. Zeinert
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  ABSTRACT: In this work we present a study of the optoelectronic properties of nanocrystalline GaN (nc-GaN) and amorphous GaON (a‐GaON) grown by ion-assisted deposition. The two classes of film show very distinct photoconductive responses; the nc-GaN has a fast small response while the a‐GaON films have a much larger response which is persistent. To describe the observed intensity, wavelength, and temperature dependence of the photoconductivity in each class of film, we build a model which takes into account the role of a large density of localized states in the gap. The photoconductivity measurements are supplemented by thermally stimulated conductivity, measurement of the absorption coefficient, and determination of the Fermi level. Using the model to aid our interpretation of this data set, we are able to characterize the density of states in the gap for the two materials.
  Journal of Applied Physics 02/2006; 99(3):034312-034312-7. · 2.17 Impact Factor
• Article: Optical conductivity and x-ray absorption and emission study of the band structure of MnN films

  S. Granville, B. J. Ruck, F. Budde, A. Koo, J. E. Downes, H. J. Trodahl, A. Bittar, N. Strickland, G. V. M. Williams, W. R. L. Lambrecht, Timothy Learmonth, Kevin E. Smith, V. J. Kennedy, A. Markwitz, Thorsten Schmitt
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  ABSTRACT: The band structure of MnN films prepared by ion assisted deposition has been investigated by optical conductivity and x-ray absorption and emission spectroscopies. X-ray diffraction and extended x-ray absorption fine structure show the films to be nanocrystalline but phase pure and exhibiting the known antiferromagnetic distorted rocksalt phase. X-ray emission spectroscopy of the N K-edge and x-ray absorption near edge spectroscopy of both the N K- and Mn L-edges are used to probe the occupied and empty densities of states, which compare well with the N(2p) and Mn(3d) partial densities of states calculated using the linearized muffin-tin orbital band structure method. A similar comparison is made between the measured optical conductivity and the calculated contribution from interband transitions. It is possible to associate the main features in the measured spectrum with corresponding ones in the calculated optical function. The major differences between calculated and measured spectra can be understood on the basis of a limited electron mean-free-path in these nanocrystalline films, which broadens the features in the joint density of states and relaxes the momentum conservation requirement. The calculated optical functions are analyzed in detail in terms of their dominant band-to-band contributions and in addition the polarization dependence is predicted. Temperature dependent conductivity measurements are also reported and show a clear metallic behavior and a weak Kondo-like low temperature anomaly.
  Phys. Rev. B. 11/2005; 72(20).
• Article: Raman spectroscopy of nanocrystalline and amorphous GaN

  H. J. Trodahl, F. Budde, B. J. Ruck, S. Granville, A. Koo, A. Bittar
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  ABSTRACT: We report Raman measurements on thin films of strongly disordered GaN and GaN:O prepared by ion-assisted deposition. The incident photon energies used in the experiments ranged from 1.95 to 3.8 eV, spanning the interband edge. Under subgap excitation the signal resembles the crystalline GaN vibrational density-of-modes, with significant broadening as expected for disordered material. There is a strong resonant behavior at the interband edge of the same mode for which a strong resonance is found in crystalline GaN, with a width suggesting that the entire vibrational branch contributes to the signal. Even nanocrystalline material is found to display Raman spectra characteristic of very short-range (<1 n) translational symmetry, in agreement with x-ray diffraction evidence for the random stacking nature of the 3 nm diameter crystallites. The presence of oxygen at even 25 at.% has only a subtle effect on Raman spectra at the network vibrational frequencies below 800 cm−1, but its presence is signaled by the appearance of an oxygen mode at 1000 cm−1. An N2 line at 2360 cm−1 correlates with a nitrogen excess introduced during growth.
  Journal of Applied Physics 04/2005; 97(8):084309-084309-5. · 2.17 Impact Factor
• Article: Evolution of the local structure in GaN:O thin films grown by ion‐assisted deposition with film thickness

  M. J. Ariza, A. Koo, H. J. Trodahl, B. J. Ruck, A. Bittar, D. J. Jones, B. Bonnet, J. Rozière, J. Moscovici
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  ABSTRACT: Optical and optoelectronic properties of gallium nitride strongly depend on the synthesis procedure, which may be related to specific structural characteristics of GaN inherent to each preparation condition. Amorphous and nanocrystalline GaN films have been prepared by ion-assisted deposition (IAD). The films prepared at 10−5 Torr for <50 min have shown exploitable optoelectronic properties, in spite of the high concentration of oxygen of these films (up to 25 at.%). We study here the evolution of the local structure around Ga atoms as the deposition time increases. Five IAD GaN films of thickness ranging between 140 and 450 nm on silicon substrates were analysed by x-ray absorption fine structure (XAFS) at the Ga K-edge. The first and second shells of neighbouring atoms are clearly identified in the radial distribution functions at approximately 1.9 and 3.2 Å, respectively. In all of the films, Ga seems to be tetrahedrally coordinated to four nitrogen atoms, some of which may be substituted by oxygen. For deposition times <50 min, analysis of both x-ray adsorption near-edge structure (XANES) and extended x-ray adsorption fine structure (EXAFS) regions indicates that the material is highly amorphous. Above this threshold, a peak corresponding to the first coordination sphere of Ga atoms becomes discernible and increases in intensity for longer deposition times, indicating that the second shell of atoms is now more ordered. The pseudo Debye–Waller factor of the Ga shell is used for monitoring the average degree of amorphization in an ∼100 nm thick top layer, which seems to be related to the film oxygen content. The XAFS results are compatible with a layered distribution of crystallinity, as has been suggested previously for these films. Copyright © 2005 John Wiley & Sons, Ltd.
  Surface and Interface Analysis 01/2005; 37(3):273 - 280. · 1.18 Impact Factor
• Article: Filled and empty states of disordered GaN studied by x-ray absorption and emission

  B. J. Ruck, A. Koo, U. D. Lanke, F. Budde, H. J. Trodahl, G. V. M. Williams, A. Bittar, J. B. Metson, E. Nodwell, T. Tiedje, A. Zimina, S. Eisebitt
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  ABSTRACT: X-ray absorption and emission spectroscopies are used to study the effects of short-ranged ordering on the electronic states of disordered GaN. Nanocrystalline samples with crystallites as small as 3 nm exhibit an electronic structure resembling a broadened version of that in crystalline GaN. The electronic structure is even more heavily broadened in amorphous GaN films containing oxygen impurities or excess gallium. The oxygen containing films show an additional peak in the density of states just above the conduction band edge, and a downward shift of the valence band edge. The signature of molecular nitrogen trapped within the films is evident in both the absorption and emission spectra.
  Journal of Applied Physics 09/2004; 96(6):3571-3573. · 2.17 Impact Factor
• Source

  Available from: James B Metson

  Article: Quantitative study of molecular N_2 trapped in disordered GaN:O films

  B. J. Ruck, A. Koo, U.D. Lanke, F. Budde, S. Granville, H. J. Trodahl, A. Bittar, J. B. Metson, V. J. Kennedy, A Markwitz
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  ABSTRACT: The structure of disordered GaN:O films grown by ion-assisted deposition is investigated using x-ray absorption near-edge spectroscopy and Raman spectroscopy. It is found that between 4 and 21 % of the nitrogen in the films is in the form of molecular N_2 that interacts only weakly with the surrounding matrix. The anion to cation ratio in the GaN:O host remains close to unity, and there is a close correlation between the N_2 fraction, the level of oxygen impurities, and the absence of short-range order in the GaN:O matrix. Comment: 5 pages, 3 figures
  08/2004;
• Conference Proceeding: Nanostructured amorphous group III nitrides: potential for refractive index engineering

  H. J. Trodahl, B. J. Ruck, F. Budde, A. Koo, S. Granville, A. Bittar, G. V. M. Williams, C.E.A. Grigorescu, R. Notonier, A. Tonetto
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  ABSTRACT: Amorphous and nanocrystalline group III nitrides can be formed with relative ease in comparison with fully single crystalline material, and they show at least some properties in common with the crystalline form. Here we report experiments with the preparation of these materials, focussing especially on the potential to form patterned films with structures whose dimensions are comparable to visible wavelengths. There are clear opportunities to form both self-assembled and controlled patterns.
  Transparent Optical Networks, 2004. Proceedings of 2004 6th International Conference on; 08/2004
• Source

  Available from: James B Metson

  Article: Stabilization of amorphous GaN by oxygen

  F. Budde, B. J. Ruck, A. Koo, S. Granville, H. J. Trodahl, A. Bittar, G. V. M. Williams, M J Ariza, B Bonnet, D J Jones, J. B. Metson, S. Rubanov, P. Munroe
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  ABSTRACT: Ion assisted deposition (IAD) has been investigated for the growth of GaN, and the resulting films studied by x-ray diffraction and absorption spectroscopy and by transmission electron microscopy. IAD grown stoichiometric GaN consists of random-stacked quasicrystals of some 3 nm diameter. Amorphous material is formed only by incorporation of 15% or more oxygen, which we attribute to the presence of non-tetrahedral bonds centered on oxygen. The ionic favourability of heteropolar bonds and its strikingly simple constraint to even-membered rings is the likely cause of the instability of stoichiometric a-GaN. Comment: 4 pages, 3 figures
  07/2004;
• Article: Conductivity, photoconductivity and optical properties of amorphous GaN films

  A. Koo, U. D. Lanke, B. J. Ruck, S. A. Brown, R. Reeves, I. Liem, A. Bittar, H. J. Trodahl
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  ABSTRACT: It has been predicted that amorphous GaN has a low density of states in the gap, and therefore has potential as a useful opto-electronic material in the blue-green spectral region. We have synthesised amorphous GaN films on various substrates by ion assisted deposition and investigated the effects of sample preparation conditions on the conducting and optical properties. The room temperature resistivity ρ0 of stoichiometric (Ga:N of 1:1) films is above 105 Ω cm, and these films exhibit a complex form for the temperature dependence of the resistivity. Films having an excess of Ga show a much lower ρ0. The optical absorption shows ar 0 band-gap of 3 eV, with the gap falling below that value when the amorphous network incorporates homopolar (Ga-Ga) bonds. The best films are thus transparent across the visible region with a low density of gap states, undetectable in optical absorption. The photoluminescence spectra obtained from these a-GaN films consist of a broad green light emission peaking at 528 nm. Preliminary photoconductivity measurements show sensitivity in the UV.
  MRS Proceedings. 12/2000; 693.
• Article: Compositional and structural studies of amorphous GaN grown by ion-assisted deposition

  U. D. Lanke, A. Koo, B. J. Ruck, H. K. Lee, A. Markwitz, V. J. Kennedy, M. J. Ariza, D. J. Jones, J. Rozière, A. Bittar, H. J. Trodahl
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  ABSTRACT: Amorphous GaN films have been deposited onto various substrates by ion-assisted deposition. The films were deposited at room temperature using nitrogen ion energies in the range 40-900 eV. Rutherford backscattering spectroscopy and nuclear reaction analysis show that the Ga:N atomic ratio is approximately one for films grown with ion energy near 500 eV; these films have the highest transparency. Films grown with ion energies below 300 eV are Ga rich, and show reduced transparency across the visible. Raman spectroscopy, x-ray diffraction, and transmission electron microscopy confirm the amorphous nature of the films. Annealing studies on a-GaN establish that the films begin to crystallise at a temperature of about 700 C. To investigate the local bonding environment of the Ga or N atoms, we have measured the extended x-ray absorption fine structure (EXAFS) of the transparent GaN films. The EXAFS results indicate that the films are dominated by heteropolar tetrahedral bonding, with a low density of homopolar bonds.
  MRS Proceedings. 12/2000; 693.
• Conference Proceeding: Amorphous GaN: Optoelectronic properties and device potential

  H. J. Trodahl, B. J. Ruck, A. Koo, U.D. Lanke, A. Bittar
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  ABSTRACT: This paper describes the preparation of an amorphous form of hetero-polar tetrahedrally bonded GaN by ion-assisted deposition. The structure, including composition and bonding configurations, has been subjected to thorough investigation to establish the optimum deposition conditions. The optoelectronic properties of the best films have been measured to evaluate their potential in UV-blue detectors and/or emitters. Currently the best films have shown clear photoconductivity, with sensitivity peaking in the UV, but no useful luminescence has yet been found.
  Transparent Optical Networks, 2003. Proceedings of 2003 5th International Conference on;
• Article: Semiconducting ground state of GdN thin films

  S. Granville, B. J. Ruck, F. Budde, A. Koo, D. J. Pringle, F. Kuchler, A. R. H. Preston, D. H. Housden, N. Lund, A. Bittar, G. V. M. Williams, H. J. Trodahl
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  ABSTRACT: We report the growth of GdN thin films and a study of their structure and magnetic and conducting properties. It is demonstrated that they are semiconducting at ambient temperature with nitrogen vacancies the dominant dopant. The films are ferromagnetic below 68 K, and a significant narrowing of the band gap is signaled by more than a doubling of its conductivity. The conductivity in the low-temperature ferromagnetic state remains typical of a doped semiconductor, supporting the view that this material is semiconducting in its ground state and that no metal-insulator transition occurs at the Curie temperature.
  Phys. Rev. B. 73(23).