I. S. Roqan

University of Strathclyde, Glasgow, Scotland, United Kingdom

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Publications (15)20.29 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: GaN epilayers were implanted with Eu to fluences of 1×1013 Eu/cm2 and 1×1015 Eu/cm2. Post-implant thermal annealing was performed in ultra-high nitrogen pressures at temperatures up to 1450 ºC. For the lower fluence effective structural recovery of the crystal was observed for annealing at 1000 ºC 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 ºC 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.
    Proc SPIE 02/2012;
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    ABSTRACT: Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, EuGa, is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N2. 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 EuGa in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the 5D0 to 7F2 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.
    Optical Materials 05/2011; 33(7):1063–1065. · 1.92 Impact Factor
  • Optical Materials. 01/2011; 33(7):1063-1065.
  • Physical review. B, Condensed matter 01/2010; 81(8).
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    Applied Physics Letters 01/2010; 97(11). · 3.79 Impact Factor
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    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 Eu <sup>3+</sup> emission linewidth and key structural parameters of In <sub>x</sub> Al <sub>1-x</sub> N films on GaN in the composition range near lattice matching (x∼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. Eu <sup>3+</sup> emission from In <sub>0.13</sub> Al <sub>0.87</sub> N : Eu also shows significantly less thermal quenching than GaN:Eu. InAlN films are therefore superior to GaN for RE optical doping.
    Journal of Applied Physics 11/2009; · 2.21 Impact Factor
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    ABSTRACT: GaN was implanted with 300 keV Eu ions over a wide fluence range from 1 × 1013 to 1 × 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 × 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.
    Journal of Physics D Applied Physics 07/2009; 42(16):165103. · 2.53 Impact Factor
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    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 (≪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 <110> 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.
    Journal of Applied Physics 07/2009; · 2.21 Impact Factor
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    physica status solidi (b) 01/2008; 245(1):170-173. · 1.49 Impact Factor
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    ABSTRACT: During molecular beam epitaxy of GaN:Tm films, substrate temperature strongly influences the rare earth incorporation, surface morphology and luminescence spectrum. The Tm incorporation into films grown between 730 and 830 degrees C was estimated by wavelength-dispersive X-ray (WDX) spectroscopy. Comparative WDX, atomic force microscopy (AFM) and cathodoluminescence (CL) mappings reveal that at an optimal growth temperature between 775 and 780 degrees C, a high Tm content (similar to 2.2 at%) and a smooth surface morphology can be obtained, leading to an intense sharp TM3+ emission. For lower substrate temperatures, Ga droplets and large (similar to 8-15 mu m) circular pits mar the sample surface; for higher temperatures, the sharp CL lines disappear due to low Tm content (<= 0.8 at%). (C) 2008 Elsevier B.V. All rights reserved.
    Journal of Crystal Growth 01/2008; 310(18):4069-4072. · 1.55 Impact Factor
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    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 1D2→3F4 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. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Physica Status Solidi (A) Applications and Materials 12/2007; 205(1):34 - 37. · 1.46 Impact Factor
  • Superlattices and Microstructures 10/2006; 40(4-6):445-451. · 1.56 Impact Factor
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    ABSTRACT: Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted AlxGa1−xN with different AlN contents (in the range 0≤x≤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 AlxGa1−xN:Tm peaks in intensity for an AlN content of x∼0.11. The emission is enhanced by up to a factor of 50 times with an increase of annealing temperature from 1000 to 1300 ∘C. The blue emission from In0.13Al0.87N:Tm, annealed at 1200 ∘C, is more than ten times stronger than that from AlxGa1−xN:Tm, x≤0.2. However, the intensity decreases significantly as the InN fraction increases from 0.13 to 0.19.
    Superlattices and Microstructures 01/2006; 40:445-451. · 1.56 Impact Factor
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    ABSTRACT: Integrated AIN nanocaps are used to protect gallium nitride epilayers during high temperature annealing treatments following high-energy implantation of rare earth (RE) ions. Cracks formed in thicker caps due to the lattice mismatch between AIN and GaN lead to the creation of microscopic surface defects at annealing temperatures higher than around 1200 degrees C. GaN dissociates locally to produce holes in the caps. Simultaneous cathodoluminescence/wavelength dispersive X-ray microanalysis in a modified electron probe microanalyzer allows study of the compositional and light emission variations near these microscopic defects. The intensity of the D-5(0) - F-7(2) transition related emission is enhanced and spectral changes can be observed, which indicate changes in the structure and/or composition of a very thin layer that forms the walls of holes in the caps. We also report some preliminary observations on the influence of the annealing atmosphere (nitrogen or ammonia) on cap damage.
    01/2006: pages 625-630;
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    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)-> 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.
    01/2006: pages 599-604;