Results 1 - 10 of 36
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[en] A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1-x(SiO5)x has been developed for solid state white lighting applications. An examination of the host lattice, and the local structure around the Ce3+ activator ions through a combination of density functional theory, synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The maximum emission wavelength can be tuned from green (λem = 523 nm) to yellow (λem = 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 ± 5% for some of the examples in the series. Excellent thermal properties were found for the x = 0.5 sample, with the photoluminescence intensity at 160 C only decreased to 82% of its room temperature value. Phosphor-converted LED devices fabricated using an InGaN LED (λmax = 400 nm) exhibit high color rendering white light with Ra = 70 and a correlated color temperature near 7000 K. The value of Ra could be raised to 90 by the addition of a red component, and the correlated color temperature lowered to near 4000 K.
[en] We have developed a method for photoelectrochemical etching of p-type semiconductors, including GaN, that relies on the built-in bandbending already inherent to optical devices. Electron-hole pairs are generated by filtered light in a buried small bandgap layer, and a pn junction separates the charge. Electrons are sent into the n-type layer, where they are extracted, and holes to the surface, where they participate in etching reactions. This technique is rapid and inexpensive, and it requires no applied bias or elevated temperatures. This technique has widespread applications to GaN optical devices where ion-damage-free etching or wide tunability of etch parameters is desired.
[en] We have fabricated nonpolar GaN/InGaN microdisks using band-gap selective photoelectrochemical etching. These microdisks have a smoother optical cavity than our previous c-plane microdisks, and they support whispering gallery modes with quality factors as high as 2000 after a focused ion beam treatment to the quantum wells. Because of the lack of a Stokes shift in the quantum wells of these m-plane disks, absorption losses play a much more significant role than in our earlier c-plane microdisks, and the light which couples into the modes is emission from the InGaN post rather than the quantum wells within the cavity.
[en] Ohmic contacts to both c-plane and n-GaN are demonstrated using a pure aluminum layer which was vacuum annealed to prevent oxidation. Specific contact resistivities of 4.4 × 10−7 and 2.3 × 10−5 Ωcm2 were obtained without annealing for c-plane and samples respectively. A reflectivity of over 85% at 450 nm was measured for both samples. After a 300 °C anneal specific contact resistivities of 1.5 × 10−7 and 1.8 × 10−7 Ωcm2 were obtained for c-plane and samples respectively and the reflectivities remained higher than 80%. (paper)
[en] We discuss a proposed experimental technique for correlating optical excitation intensities in photoluminescence measurements with electrical excitation intensities in electroluminescence measurements for optoelectronic device characterization purposes. The technique utilizes the reverse saturation current under optical excitation as a measure of the optical excitation intensities and translates it to electrical excitation intensities as the forward current of the same amount. InGaN-based blue and green light-emitting diodes were examined to demonstrate the validity of the proposed technique. 4 W cm−2 of optical excitation intensity was determined to be approximately the same as a sub-mA forward current under the present experimental conditions. Photoluminescence and electroluminescence characteristics were compared by applying the determined excitation intensity; spatial uniformity of luminescence was found to differ from each other in green-emitting devices
[en] We demonstrate low resistance Ohmic contacts to ( semipolar p-type GaN using a thin p++–GaN contact layer. This layer was optimized by varying the V/III ratio, Cp2Mg flow, thickness, and growth rate which produced low forward voltage devices with specific contact resistivities estimated to be 4 × 10−4 Ω cm2 and 5 × 10−4 Ω cm2 for palladium contacts and indium tin oxide (ITO) contacts respectively. Forward voltages of 2.89 V and 3.47 V were obtained at 20 A cm−2 and 800 A cm−2 respectively for light emitting diodes (LEDs) emitting at 435 nm with palladium contacts. LEDs with ITO contacts had forward voltages of 2.94 V and 3.55 V at 20 A cm−2 and 800 A cm−2 respectively. (paper)
[en] Spontaneously polarized light emission from m-plane InGaN/GaN light-emitting diodes was studied as a function of In composition in the InGaN single quantum-well layer. Emission wavelength was varied between 394 and 472 nm. A strong correlation was confirmed between optical polarization and In composition; the higher the In composition, the stronger the optical polarization. The photon-energy difference between the emission spectra associated with the two polarizations, ΔM, was evaluated as a function of current. ΔM exhibited a negative monotonic current dependence for the 394 nm emitting sample and the dependence was changed to positive monotonic as the wavelength became longer towards 472 nm. This change was tentatively attributed to the valence band mixing and the crystal momentum conservation that became relevant with the band filling. ΔM and optical polarization exhibited only a moderate correlation; the Fermi-Dirac function has been used to explain the weakened optical polarization under increased current injection.
[en] The present communication reports a study on polarized light emission from sidewalls of light-emitting-diode (LED) dies with future interest in color display applications, e.g., liquid-crystal display (LCD) backlighting. Commercial LED dies with three primary colors were subjected to polarization characterization via a confocal microscope. AlInGaP red LED dies (cube shape) were shown to emit polarized light with a polarization ratio of 0.47. InGaN-based blue (sapphire substrate) and green (SiC substrate) LED dies were revealed to possess polarization ratios as high as 0.9 on side emission from epitaxial layers. These results ensure the potential of commercially available LED dies for LCD backlighting experiments. Discussions were made on improvement in polarization characteristics. (rapid communication)
[en] Thin AlN interlayers are widely used in (In,Al,Ga)N based high-electron-mobility transistors to improve the mobility of the two-dimensional electron gas forming at the GaN/(In,Al,Ga)N interface. AlN layers grown by metal-organic chemical vapor deposition, however, were recently shown to contain high amounts of gallium caused by carry over reactions, resulting in Al_xGa_1_−_xN layers with x ∼ 0.5 under typical deposition conditions. By modifying the AlN growth conditions in this study, layers with an Al mole fraction up to 0.78 were obtained. The unintentional Ga incorporation had a negligible effect on the electronic properties of GaN/AlN/AlGaN structures with nominally 0.7 nm thick AlN interlayer and sheet carrier densities in the order of 1 × 10"1"3 cm"−"3. It resulted, however, in low electron mobility values for samples with thicker nominal AlN layers and/or higher sheet carrier densities. (paper)
[en] This paper discusses radiative recombination efficiency in electroluminescence of InGaN-based light-emitting diodes prepared on the (1 0 1-bar 0) plane. Radiative efficiency was studied over a wide range of temperatures and drive currents on four types of LED samples with different InGaN active-layer thicknesses. Efficiency was minimally affected by active-layer thickness, yet was a strong function of temperature and current. Efficiency reduction at high current was observed on these LEDs, which confirms strain-induced electric polarization fields are not a dominant mechanism. Luminescence intensity was found to be proportional to the square root of current at low temperature. Acceptor freeze-out was suggested to induce hole depletion at increased current; shortage of holes resulted in reduced efficiency and triggered off electron injection into the p-type layer to sustain total current. Injected electrons were shown to lead to the square-root relationship by solving rate equations. (fast track communication)