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We study the effect of nitrogen on the GaAs0.9-xNxSb0.1 (x = 0.00, 0.65%, 1.06%, 1.45%, and 1.90%) alloy dielectric function by spectroscopic ellipsometry in the energy range from 0.73 to 4.75 eV. The compositional dependences of the critical points energies for the GaAs0.9-xNxSb0.1 are obtained. In addition to the GaAs intrinsic transitions E-1, E-1+ Delta(1), and E-0, the nitrogen-induced Gamma-point optical transitions E-0 and E+, together with a third transition E-#, are identified. We find that with increasing the N content, the E-0 transition shifts to lower energies while the E+ and (E)# transitions shift to higher energies. We suggest that the origin of the E-0, E+, and E-# transitions may be explained by the double band anticrossing (BAC) model, consisting of a conduction BAC model and a valence BAC model.Original Publication:N. Ben Sedrine, C. Bouhafs, J.C. Harmand, R. Chtourou and Vanya Darakchieva, Effect of nitrogen on the GaAs0.9-xNxSb0.1 dielectric function from the near-infrared to the ultraviolet, 2010, Applied Physics Letters, (97), 20, 201903.http://dx.doi.org/10.1063/1.3518479Copyright: American Institute of Physicshttp://www.aip.org

Eu activation in beta-Ga_2O_3 MOVPE thin films by ion implantation

In this work, we have established the effects of Eu implantation and annealing on beta-Ga_2O_3 thin films grown by metal organic vapor phase epitaxy (MOVPE) on sapphire substrate. The study is based on the combined information from structural and optical techniques: X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), cathodoluminescence (CL), photoluminescence (PL), and photoluminescence excitation (PLE). The thin films were implanted with a fluence of 1 x 10^15 Eu.cm^-2 and annealed at 900 degrees C. Neither significant changes in peak width or position nor additional peaks related to Eu complexes were detected in the XRD 2 theta-omega scans. RBS results and SRIM simulation are in good agreement, revealing that no Eu diffusion to the surface occurs during annealing. For the used implantation/annealing conditions, the Eu ion penetration depth reached similar to 130 nm, with a maximum concentration at similar to 50 nm. Furthermore, CL and PL/PLE results evidenced the optical activation of the Eu^3+ in the beta-Ga_2O_3 host. The detailed study of the Eu^3+ intra-4f shell transitions revealed that at least one active site is created by the Eu implantation/annealing in beta-Ga_2O_3 thin films grown on sapphire. Independently of the beta-Ga_2O_3 film thickness, well controlled optical activation of implanted Eu was achieved

Eu-Doped AlGaN/GaN Superlattice-Based Diode Structure for Red Lighting: Excitation Mechanisms and Active Sites

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Multiple optical centers in Eu-implanted AlN nanowires for solid-state lighting applications

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Photoluminescence studies of a perceived white light emission from a monolithic InGaN/GaN quantum well structure

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

Quantum well intermixing and radiation effects in InGaN/GaN multi quantum wells

K. Lorenz, A. Redondo-Cubero, M. B. Lourenço, M. C. Sequeira, M. Peres, A. Freitas, L. C. Alves, E. Alves, M. P. Leitão, J. Rodrigues, N. Ben Sedrine, M. R. Correia, T. Monteiro, "Quantum well intermixing and radiation effects in InGaN/GaN multi quantum wells", Gallium Nitride Materials and Devices XI, Proc. SPIE 9748,97480L (26 February 2016); doi: 10.1117/12.2211429. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibitedProceeding of the XI Gallium Nitride Materials and Devices Conference (San Francisco, California, United States)Compositional grading of InGaN/GaN multi quantum wells (QWs) was proposed to mitigate polarization effects and Auger losses in InGaN-based light emitting diodes [K. P. O'Donnell et al., Phys. Status Solidi RRL 6 (2012) 49]. In this paper we are reviewing our recent attempts on achieving such gradient via quantum well intermixing. Annealing up to 1250 °C resulted in negligible interdiffusion of QWs and barriers revealing a surprising thermal stability well above the typical MOCVD growth temperatures. For annealing at 1400 °C results suggest a decomposition of the QWs in regions with high and low InN content. The defect formation upon nitrogen implantation was studied in detail. Despite strong dynamic annealing effects, which keep structural damage low, the created defects strongly quench the QW luminescence even for low implantation fluences. This degradation could not be reversed during thermal annealing and is hampering the use of implantation induced quantum well intermixing in InGaN/GaN structures.We acknowledge funding by FCT Portugal (PTDC/FIS-NAN/0973/2012, UID/CTM/50025/2013, and RECI/FISNAN/0183/2012 (FCOMP-01-0124-FEDER-027494 and individual grants SFRH/BD/111733/2015 (MCS), SFRH/BPD/111285/2015 (MP), SFRH/BD/76300/2011 (JR), and Investigador FCT (KL)). ARC acknowledges Juan de la Cierva grant (under contract number JCI-2012-14509, Spai

Analyzing ab initio infrared spectra and electronic properties of polyethylenimine water complexes in the solid state

International audienceWe focused our study on the hydrated polyethylenimine (PEI) crystals which are characterized by the presence of 0.5, 1.5 or 2 water molecules per ethylenimine monomer unit. We propose a theoretical study in the framework of periodic linear combination of atomic orbitals (LCAO) using different Hamiltonians based on density functional theory (DFT) and hybrid Hartree-Fock (HF) - DFT level of theory. Once hydrogen atoms added when missing and redundant atoms withdrawn in the PEI structures, ab initio calculations yielded relaxed geometries at the ground state. The closest structural parameters to the experimental values are observed for hybrid Hamiltonians and the use of the 6-31++G* basis set reduces this difference. We provide the IR vibration modes at the Gamma point, with assignments of the bands. The electronic study reveals that these PEI hydrates have an insulating character with a quasi constant band gap whatever the number of water molecules. From the projected density of states (PDOS), we have found that the nitrogen orbital has the major contribution to the valence band, while the conduction band is mainly constructed from hydrogen bondings

Photoluminescence investigations of ZnO micro/nano structures

Zinc oxide (ZnO) is probably one of the most researched wide bandgap semiconductors in the last decades due to its unique characteristics in terms of low production cost, high availability, bioinertness, and especially its interesting optical properties. Although this semiconductor is considered an ‘old’ material and is known to possess such unique properties for more than three decades, the interest was renewed because of the advances in nanotechnology and the possibility to be produced in a vast number of nanostructures with tunable properties. An adequate knowledge of the nanomaterials’ optical response is mandatory for assessing and optimizing their functionalities towards different applications. Although the photoluminescence properties of ZnO bulk materials have been known from several decades, quite a number of open questions remains, namely regarding the nature of defects responsible for the broad luminescence bands frequently observed in the visible spectral region. With the effects of reducing the dimensionality of the material to the nanoscale, changes may arise in the luminescence outcome due to the role of the surface/interface characteristics. Indeed, the surface phenomena can strongly affect the nanostructure properties and can be used to tailor them, consequently having a profound influence on the performance of the devices where the nanostructures are employed. Hence, in this article, an overview of the fundamental properties of ZnO, with emphasis on the main optical recombination mechanisms, both in bulk and at the nanoscale, is provided to disclose some of the current knowledge in this subject. In addition, some examples of the myriad of applications where this semiconductor has been exploited are also discussed.publishe