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Träfflista för sökning "WFRF:(DenBaars Steven P.) "

Sökning: WFRF:(DenBaars Steven P.)

  • Resultat 1-12 av 12
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1.
  • Liuolia, Vytautas, et al. (författare)
  • Dynamics of polarized photoluminescence in m-plane InGaN/GaN quantum wells
  • 2010
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 108:2, s. 023101-
  • Tidskriftsartikel (refereegranskat)abstract
    • Spectrally-, polarization-, and time-resolved photoluminescence (PL) experiments have been performed on 2.5 nm thick m-plane single InGaN quantum wells. It has been found that PL decay is mainly determined by nonradiative recombination through several types of recombination centers, while PL rise is largely affected by exciton transfer into localization minima. Prolonged PL rise times and time-dependent spectral shift were used to study exciton transfer into the localization centers. CharacteristiC time of the exciton transfer is 80-100 ps at lower temperatures and about 50 ps at room temperature, which corresponds to the exciton diffusion length of 200-500 nm. Degree of PL linear polarization was found to decrease at a similar rate. Decreased PL polarization for the localized excitons suggests that the localization centers are related to areas with relaxed strain. (C) 2010 American Institute of Physics. [doi:10.1063/1.3460278]
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2.
  • Marcinkevičius, Saulius, et al. (författare)
  • Dynamics of carrier injection through V-defects in long wavelength GaN LEDs
  • 2024
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 124:18
  • Tidskriftsartikel (refereegranskat)abstract
    • The efficiency of high-power operation of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on the realization of uniform hole distribution between the QWs. In long wavelength InGaN/GaN QW LEDs, the thermionic interwell hole transport is hindered by high GaN barriers. However, in polar LED structures, these barriers may be circumvented by the lateral hole injection via semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects. The efficiency of such carrier transfer depends on the transport time since transport in the semipolar QWs is competed by recombination. In this work, we study the carrier transfer from the semipolar to polar QWs by time-resolved photoluminescence in long wavelength (green to red) LEDs. We find that the carrier transfer through the semipolar QWs is fast, a few tens of picoseconds with the estimated room temperature ambipolar diffusion coefficient of ∼5.5 cm2/s. With diffusion much faster than recombination, the hole transport from the p-side of the structure to the polar QWs should proceed without a substantial loss, contributing to the high efficiency of long wavelength GaN LEDs.
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3.
  • Marcinkevičius, Saulius, et al. (författare)
  • High internal quantum efficiency of long wavelength InGaN quantum wells
  • 2021
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 119:7
  • Tidskriftsartikel (refereegranskat)abstract
    • Time-resolved and quasi-cw photoluminescence (PL) spectroscopy was applied to measure the internal quantum efficiency (IQE) of c-plane InGaN single quantum wells (QWs) grown on sapphire substrates using metal-organic chemical vapor deposition. The identical temperature dependence of the PL decay times and radiative recombination times at low temperatures confirmed that the low temperature IQE is 100%, which allowed evaluation of the absolute IQE at elevated temperatures. At 300 K, the IQE for QWs emitting in green and green-yellow spectral regions was more than 60%. The weak nonradiative recombination in QWs with a substantial concentration of threading dislocations and V-defects (similar to 2 x 10(8) cm(-2)) shows that these extended defects do not notably affect the carrier recombination.
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4.
  • Marcinkevicius, Saulius, et al. (författare)
  • Interwell carrier transport in InGaN/(In)GaN multiple quantum wells
  • 2019
  • Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 114:15
  • Tidskriftsartikel (refereegranskat)abstract
    • Uniform carrier distribution between quantum wells (QWs) of multiple QW light emitting diodes (LEDs) and laser diodes is important for the efficiency of device operation. In lasers, the uniform distribution ensures that all the QWs contribute to lasing; in LEDs, it enables high power operation with minimal Auger losses and a maximal efficiency. The carrier distribution between the QWs takes place via interwell (IW) transport. In polar GaN-based structures, the transport might be hindered by the strong carrier confinement and the internal electric fields. In this work, we study the IW transport in InGaN/(In)GaN multiple QW structures typical for ultraviolet-emitting devices with different well and barrier parameters. Experiments have been performed by means of time-resolved photoluminescence. We find that the IW transport rate is limited by the hole thermionic emission, which for InGaN/GaN QWs produces long transport times, similar to 1 ns per well, and a nonuniform IW carrier distribution. However, adding 5% In to the barriers completely changes the situation with the transport time decreasing by a factor of four and the hole thermionic emission energy from 200 meV to 70 meV. This study shows that using InGaN barriers is a promising pathway toward efficient high power InGaN LEDs. Published under license by AIP Publishing.
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5.
  • Marcinkevičius, Saulius, et al. (författare)
  • Multimode scanning near-field photoluminescence spectroscopy of InGaN quantum wells
  • 2018
  • Ingår i: 2018 IEEE RESEARCH AND APPLICATIONS OF PHOTONICS IN DEFENSE CONFERENCE (RAPID). - : IEEE. ; , s. 93-95
  • Konferensbidrag (refereegranskat)abstract
    • Multimode scanning near-field photoluminescence spectroscopy was developed and applied to study carrier localization and dynamics in m-plane InGaN quantum wells. The study showed that localized hole states maintain properties of extended bands, radiative and nonradiative carrier lifetimes are spatially nonuniform, and hole diffusion is anisotropic.
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6.
  • Marcinkevicius, Saulius, et al. (författare)
  • Optical properties and carrier dynamics in m-plane InGaN quantum wells
  • 2014
  • Ingår i: physica status solidi c. - : Wiley. - 1610-1642.
  • Konferensbidrag (refereegranskat)abstract
    • Scanning near-field and time-resolved photoluminescence spectroscopy have been applied to characterize single m-plane InGaN QW structures. The PL spectra were found to be formed by emission from the extended and the localized states. Lateral dimensions of uniform potential regions were placed around a few tens of nm. It was shown that optimization of the template for the QW growth results in a more uniform emission spectrum and a reduced effect of carrier localization. The radiative recombination time at low temperatures was found to be short, about 0.5 ns. At room temperature, nonradiative recombination via efficient recombination centres was prevailing. Complexes of Ga vacancies with O were suggested to play this role.
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7.
  • Marcinkevicius, Saulius, et al. (författare)
  • Spatial variations of optical properties of semipolar InGaN quantum wells
  • 2015
  • Ingår i: Gallium Nitride Materials and Devices X. - : SPIE - International Society for Optical Engineering. - 9781628414530
  • Konferensbidrag (refereegranskat)abstract
    • Spatial variations of band potentials and properties of carrier recombination were examined in semipolar (20 (2) over bar1) plane InGaN/GaN single quantum wells by scanning near-field photoluminescence (PL) spectroscopy. The quantum wells had In content from 0.11 to 0.36 and were emitting from violet to yellow-green. Near-field scans showed small PL peak energy and linewidth variations with standard deviations below 10 meV, which confirms small alloy composition variations in the quantum wells. The scans revealed large, similar to 5 to 10 mu m size areas of similar PL parameter values, as opposed to 100 nm scale variations, often reported for InGaN wells. With increased excitation power, an untypical photoluminescence peak energy shift to lower energies was observed. The shift was attributed to density dependent carrier redistribution between nm-scale sites of different potentials. The experimental results show that in the (20 (2) over bar1) plane InGaN quantum wells the localization potentials are shallow and the recombination properties are spatially rather uniform, which confirms the high potential of these QWs for photonic applications.
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8.
  • Mensi, Mounir, et al. (författare)
  • Direct Measurement of Nanoscale Lateral Carrier Diffusion : Toward Scanning Diffusion Microscopy
  • 2018
  • Ingår i: ACS Photonics. - : AMER CHEMICAL SOC. - 2330-4022. ; 5:2, s. 528-534
  • Tidskriftsartikel (refereegranskat)abstract
    • A multimode scanning near-field optical microscopy technique that allows the mapping of surface morphology, photoluminescence (PL) spectra in illumination and illumination-collection modes, and PL dynamics, all in one scan, has been developed along with a method to use it for evaluation of carrier diffusion. The method allows measuring diffusion lengths as small as similar to 100 nm and their anisotropy and spatial distribution, parameters remaining inaccessible to conventional far-field techniques. The procedure has been applied to study ambipolar carrier diffusion in a nonpolar m-plane InGaN/GaN quantum well. The diffusion was found to be highly anisotropic with diffusion coefficients along and perpendicular to the wurtzite c axis equal to 0.4 and 1.9 cm(2)/s, respectively. The large diffusion anisotropy confirms band structure calculations that suggest that the topmost valence band in an m-plane InGaN quantum well is highly anisotropic.
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9.
  • Mounir, Mensi, et al. (författare)
  • Direct measurement of nanoscale lateral carrier diffusion : toward scanning diffusion microscopy
  • 2017
  • Ingår i: ACS Photonics. - 2330-4022.
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • A multimode scanning near-field optical microscopy technique allowing to map surface morphology, photoluminescence (PL) spectra in illumination and illumination-collection modes, as well as PL dynamics, all in one scan, has been developed along with a method to use it for evaluation of carrier diffusion. The method allows to measure diffusion lengths as small as ~100 nm, their anisotropy and spatial distribution, parameters remaining inaccessible to conventional far-field techniques. The procedure has been applied to study ambipolar carrier diffusion in a nonpolar m-plane InGaN/GaN quantum well. The diffusion was found to be highly anisotropic with diffusion coefficients along and perpendicular to the wurtzite c axis equal to 0.4 and 1.9 cm2/s, respectively. The large diffusion anisotropy confirms band structure calculations that suggest that the top-most valence band in m-plane InGaN quantum well is highly anisotropic
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10.
  • Nijikovsky, Boris, et al. (författare)
  • Microstructure of ZnO films synthesized on MgAl2O4 from low-temperature aqueous solution: growth and post-annealing
  • 2013
  • Ingår i: Journal of Materials Science. - : Springer Verlag (Germany). - 0022-2461 .- 1573-4803. ; 48:4, s. 1614-1622
  • Tidskriftsartikel (refereegranskat)abstract
    • The microstructure of ZnO films synthesized from low-temperature (90 degrees C) aqueous solution on (111) MgAl2O4 single crystal substrates was characterized by X-ray diffraction, high-resolution scanning electron microscopy, conventional and high-resolution transmission electron microscopy. To examine the thermally activated microstructural evolution of the ZnO, both as-deposited and annealed films were characterized. The ZnO films were confirmed to have a ZnO[10 (1) over bar0](0001)parallel to MgAl2O4[011](1 (1) over bar1) orientation relationship, with Zn polarity normal to the surface. Despite their highly oriented nature, the ZnO films have a columnar grain structure with low-angle (less than2.5 degrees) grain boundaries. In addition to lattice dislocations forming low-angle grain boundaries, threading dislocations were observed, emanating from the interface with the substrate. In annealed films, thermally generated voids were observed and appeared to preferentially form at grain boundaries and dislocations. Based on these characterization results, mechanisms are proposed for film growth and microstructural evolution. Finally, the diffusion coefficient of vacancies via dislocations at grain boundaries in the produced ZnO films was estimated.
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11.
  • Yapparov, Rinat, et al. (författare)
  • Carrier diffusion in long wavelength InGaN quantum well LEDs after injection through V-defects
  • 2024
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 125:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The efficiency of operation of GaN-based light emitting diodes (LEDs) to a large degree relies on realization of a uniform hole distribution between multiple quantum wells (QWs) of the active region. Since the direct thermionic transport between the QWs is inefficient, the hole injection through semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects has been suggested as an alternative approach. However, for an efficient LED operation, the carrier distribution should be uniform not only vertically, between the QWs but also laterally, within individual QWs. In this work, the lateral carrier distribution in long wavelength InGaN/GaN QW LEDs is studied by the scanning near-field optical microscopy. The measurements have shown that emission is concentrated around the V-defect injectors. At high currents, the diffusion length of holes in polar QWs was found to be ∼0.6-1 μm and the hole diffusion coefficient ∼0.6 cm2/s. The obtained data should aid design of the V-defect injectors for a laterally uniform carrier distribution in the active region QWs.
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12.
  • Yapparov, Rinat, et al. (författare)
  • Engineering of quantum barriers for efficient InGaN quantum well LEDs
  • 2022
  • Ingår i: Novel Optical Materials and Applications, NOMA 2022. - : Optica Publishing Group (formerly OSA).
  • Konferensbidrag (refereegranskat)abstract
    • Ways to improve efficiency of high-power LEDs based on InGaN/(In)GaN multiple quantum wells are explored by studying interwell carrier transport and recombination. Best results are achieved for InGaN barriers with thin GaN or AlGaN interlayers.
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  • Resultat 1-12 av 12

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