InGaN-based LEDs Research Articles

Y 4MgSi 3O 13:RE 3+ (RE=Ce, Tb and Eu) nanophosphors for a full-color display

Nanophosphors of Y 4MgSi 3O 13:RE 3+ (RE=Ce, Tb and Eu) were synthesized by a facile sol–gel method. X-ray diffraction, scanning electron microscopy and photoluminescence excitation and emission spectroscopy were employed to systematically characterize the samples. The results show that the nanophosphors are of single-phase hexagonal Y 4MgSi 3O 13 with average size distribution of 50 nm in diameter. Photoluminescence spectra indicate that Y 4MgSi 3O 13:Ce 3+ emits strong blue light centered at 430 nm attributed to the transition from 5d to 4f of Ce 3+ ions. Y 4MgSi 3O 13:Tb 3+ gives intense green emission at 542 nm originating from the 5D 4→ 7F 5 transition of Tb 3+ ions, while Y 4MgSi 3O 13:Eu 3+ shows red emission with a maximum at 614 nm corresponding to the 5D 0→ 7F 2 transition of Eu 3+ ions. The excellent Commission International de I’Eclairage (CIE) chromaticity coordinates of Y 4MgSi 3O 13:RE 3+ (RE=Ce, Tb and Eu) nanophosphors indicate that they are good candidates for the application of InGaN-based white LEDs.

Thermal stable La 2Ti 2O 7:Eu 3+ phosphors for blue-chip white LEDs with high color rendering index

High concentration Eu 3+ doped La 2Ti 2O 7 phosphor with good crystallinity was prepared via solid-state method. This phosphor can emit intense red light with a peak around 612 nm corresponding to the 5D 0 → 7F 2 transitions of Eu 3+. The luminescent intensity excited at 465 nm, which corresponds to the wavelength of InGaN-based LEDs, is comparable with the intensity excited at 396 nm. The emission intensity increased with increasing Eu 3+ concentration up to 30 mol%, and then decreased due to the concentration quenching. The White LED (WLED) fabricated by coating a mixture of Y 3Al 5O 12:Ce 3+ and La 2Ti 2O 7:Eu 3+ onto a blue InGaN chip showed higher color rendering index than the WLED fabricated by coating Y 3Al 5O 12:Ce 3+(Y 3Al 5O 12:Ce 3+-WLED). This phosphor also has excellent thermal stability, and the luminous intensity can remain 75.46% when it was heated to 200°. By optimizing the Eu 3+ concentration and calcination temperature, we demonstrated that La 2Ti 2O 7:Eu 3+ was a promising red phosphor under blue light (465 nm) for high-power commercial Y 3Al 5O 12:Ce 3+-WLEDs.

Promising red phosphors LaNbO 4:Eu 3+, Bi 3+ for LED solid-state lighting application

La 0.75NbO 4:Eu 3+ 0.25 and La 0.65NbO 4:Eu 3+ 0.25, Bi 3+ 0.10 phosphors were synthesized by solid-state reaction method, and their photoluminescence properties were discussed in detail. With the increased incorporation of the co-activator Bi 3+, the charge transfer (CT) bands of Nb 5+→O 2− and Eu 3+→O 2− (−280 nm) weakened and a new and significant broad band Bi 3+−O 2−(−330 nm) appeared, while the peaks at 395 and 466 nm assigned to f–f transitions of Eu 3+ was slightly changed. Compared with the commercial phosphor Y 2O 2S:0.05Eu 3+, La 0.75Nb 5O 4:Eu 3+ 0.25 and La 0.65Nb 5O 4:Eu 3+ 0.25, Bi 3+ 0.10 phosphors emit brighter red light at 615 nm with line spectra upon the excitation of near UV light (395 nm) and exhibited the Commission International d'Eclairage (CIE) chromaticity coordinates ( x=0.658, y=0.342) and ( x=0.657, y=0.342) that are closer to the standard of national television standard committee (NTSC). The results showed that La 0.75Nb 5O 4:Eu 3+ 0.25 and La 0.65Nb 5O 4:Eu 3+ 0.25, Bi 3+ 0.10 phosphors might be as promising red phosphors for near UV InGaN-based LED solid-state lighting application.

Defect studies with positrons: What could we learn on III-nitride heterostructures?

We have applied positron annihilation spectroscopy to study 400 – 500 nm InGaN-based LED structures, as well as InGaN and AlGaN materials with varying In and Al contents. We find that the effect of adding In to GaN on the annihilation parameters obeys the Vegard's law, while in the case of AlGaN the possible effect of Al is completely screened by efficient formation of cation vacancies. The results obtained in the InGaN LED structures are indistinguishable from defect-free GaN, suggesting that the positrons annihilate preferentially in the barriers of the MQW system.

Effect of carrier spillover and Auger recombination on the efficiency droop in InGaN-based blue LEDs

We have investigated efficiency droop in InGaN-based blue LEDs by considering radiative, nonradiative, and carrier spillover processes in the context of internal quantum efficiency (IQE) vs. injection current. If relied on fitting only, both the Auger recombination and an empirical formula for carrier spillover are consistent with experiments. However, the dependence of IQE on quantum well parameters and lack of droop in optical pumping experiments support the notion that carrier spillover is the main mechanism in play.

Improved Output Power of 380 nm InGaN-Based LEDs Using a Heavily Mg-Doped GaN Insertion Layer Technique

High-performance InGaN-based 380 nm UV LEDs are fabricated by using a heavily Mg-doped GaN insertion layer (HD-IL) technique. Based on the transmission electron microscopy, etch pit density, and cathodoluminescence results, the HD-IL technique can substantially reduce the defect density of GaN layer. The double-crystal X-ray diffraction results are in good agreement with those observations. The internal quantum efficiency of LED sample with an HD-IL shows around 40% improvement compared with the LED sample without the use of HD-IL. When the vertical-type LED chips (size: 1 mm times 1 mm) are driven by a 350 mA current, the output powers of the LEDs with and without an HD-IL are measured to be 203.4 and 158.9 mW, respectively. As much as 28% increased light output power is achieved.

Phosphor-Free Monolithic White-Light LED

Phosphor-free monolithic InGaN-based white-light LED has the advantages of simpler device process and potentially higher efficiency. Several techniques have been developed for implementing such white-light LEDs. Among them, the key issue is the growth of a high-quality high-indium InGaN/GaN quantum well (QW). An underlying InGaN layer growth technique is introduced for enhancing the crystal quality of a high-indium QW. To demonstrate the superior properties of a QW grown with this technique, a green LED is fabricated based on the underlying growth technique to compare with another LED of the same emission wavelength based on the conventional growth method. Then, the underlying growth technique is used to grow three yellow-emitting QWs of high efficiency. The yellow photons mix with blue light from an overgrown blue-emitting QW to produce white light. The improved properties of the phosphor-free monolithic white-light LED are discussed in detail.

Novel Epitaxial Nanostructures for the Improvement of InGaN LEDs Efficiency

We demonstrated that the efficiency of an InGaN LED can be improved by using a novel epitaxial nanostructure, namely, the nanostructured semipolar (NSSP) gallium nitride (GaN). The NSSP GaN template was fabricated on a c-plane GaN surface using a standard GaN metal-organic chemical vapor deposition tool on c-plane sapphire substrates. We showed that the surface of NSSP GaN consisted of two semipolar orientations: (10-11) and (11-22). InGaN/GaN multiple quantum wells (MQWs) fabricated on NSSP GaN exhibited negligible quantum-confined Stark effect (QCSE) and a 30% improvement in internal quantum efficiency as compared to planar c-plane InGaN/GaN MQWs. Using time-resolved photoluminescence (PL), a considerable improvement in radiative recombination lifetime was also observed. We fabricated and characterized semipolar InGaN LEDs on NSSP GaN that emitted at 543 nm and showed negligible QCSE. The NSSP GaN structure can also be applied to improve the photon extraction efficiency of InGaN-based LEDs. The surface texturing was performed in situ together with the LED epitaxy without additional ex situ etching processes. The in situ surface texturing improved the PL intensity by a factor of two. An electrical injection LED structure employing in situ surface texturing was also demonstrated.

Novel red phosphor of Eu3+ , Bi3+ coactivated double tungstates

Eu3+, Bi3+ coactivated double-tungstate phosphors were prepared by the solid-state reaction technique, and their photoluminescent properties were investigated. No concentration quenching of Eu3+ was observed in the series samples of NaLa1–x (WO4)2:x Eu3+. With the introducing of Bi3+ions, the intensities of the excitation and emission of these phosphors were increased. The phosphor NaEu0.70(WO4)2:0.30Bi3+ showed a broadened excitation band around 400 nm and intensely red emissions under ∼400 nm light excitation, with the CIE chromaticity coordinates (x = 0.66, y = 0.34). Hence, the novel phosphor NaEu0.70(WO4)2:0.30Bi3+ may find application in the fabrication of near-UV InGaN-based LEDs. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Sr 9R 2− xEu xW 4O 24 (R=Gd and Y) red phosphor for near-UV and blue InGaN-based white LEDs

Eu 3+-activated phosphors, Sr 9R 2− x Eu x W 4O 24 (R=Gd and Y ), were prepared by the conventional solid-state reaction method and their photoluminescent properties were studied. The phosphors show intense red emission under 395 and 465 nm light excitation, which is matched with the light-emitting wavelength of a near-UV-emitting and a blue-emitting InGaN chips, respectively. Bright red-light-emitting diodes (LEDs) and white-light-emitting diodes (WLEDs) were fabricated by coating Sr 9Y 0.4Eu 1.6W 4O 24 phosphor onto ∼395 nm-emitting InGaN chips and ∼460 nm blue-emitting InGaN chips, respectively. The good performances of the LEDs demonstrate that the tungstates are suitable for application of near-UV and blue InGaN-based WLEDs.

Injection current‐dependent quantum efficiency of InGaN‐based light‐emitting diodes on sapphire and GaN substrates

AbstractWe studied the nonthermal mechanism of quantum efficiency (QE) roll‐off in InGaN‐based multiple‐quantum‐well LEDs on sapphire and bulk GaN substrates with In content ranging from 1% to 28%. Both the efficiency evolution and peak energy shift appear to be strongly dependent on the In content, but nearly independent of the dislocation density in the active region. The QE of the green LED peaks at a current density as low as 1.4 A/cm2, and decreases dramatically as current is increased, whereas the UV LED has a nearly constant QE at currents up to 1 kA/cm2. In contrast to minimal current‐induced peak shift and spectral broadening in the near‐UV and UV LED, the green LED has a monotonic peak blueshift of a total amount of ∼110 meV and a spectral broadening by a factor of 2. These results lead to a conclusion that carrier delocalization followed by nonradiative recombination at misfit defects is the major nonthermal mechanism of the efficiency roll‐off in InGaN‐based visible LEDs. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence properties of Eu 2+- and Ce 3+-doped CaAl 2S 4 and application in white LEDs

The Eu 2+- and Ce 3+-doped CaAl 2S 4 phosphors were comparatively synthesized by conventional solid-state reaction and the evacuated sealed quartz ampoule. The X-ray diffraction (XRD) patterns show that the sample with better crystalline quality was prepared by the evacuated sealed quartz ampoule, resulting in the enhancement of the emission intensity of Eu 2+ ion by a factor of 1.7. The intensive green LEDs were also fabricated by combining CaAl 2S 4:Eu 2+ with near-ultraviolet InGaN chips ( λ em=395 nm). The dependence of as-fabricated green LEDs on forward-bias currents shows that it presents good chromaticity stability and luminance saturation, indicating that CaAl 2S 4:Eu 2+ is a promising green-emitting phosphor for a near-UV InGaN-based LED. In addition, the optical properties of CaAl 2S 4:Ce 3+ were systematically investigated by means of diffuse reflectance, photoluminescence excitation and emission, concentrating quenching and the decay curve.

Enhancement of InGaN-Based Vertical LED With Concavely Patterned Surface Using Patterned Sapphire Substrate

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> To improve the external quantum efficiency, we have proposed a new method utilizing surface roughening of vertical-type light-emitting diodes (VT-LEDs) fabricated on hemispherical patterned sapphire substrate by using a laser lift-off technique. The advantages of this method are simple and reproducible in transferring the well-defined patterns on sapphire into GaN layer. The VT-LED with concavely patterned surface showed a nearly twofold increase in the output power compared to the normal planar surface. This improvement in the VT-LED performances is attributed to the increase in the escaping probability of photons from the LED surface. </para>

Fabrication and Characterization of InGaN-Based Green Resonant-Cavity LEDs Using Hydrogen Ion-Implantation Techniques

The InGaN-based green resonant-cavity light-emitting diodes (RCLEDs) have been fabricated using hydrogen ion-implantation and laser liftoff techniques. The RCLEDs structure consisted of an multiple-quantum-well active layer between the top (5 pairs) and bottom (7.5 pairs) dielectric distributed Bragg reflectors with optical reflectance of 85% and 99.9%, respectively. The insulation layers of the RCLEDs with and without implantation were formed by the hydrogen ion-implantation layers of concentration and film, respectively. The corresponding forward turn-on voltage at dc current density injection were about and for the RCLEDs with and without implantation. The light output intensity of the RCLEDs with implantation is higher by a factor of 1.4 as compared to that of the similar structure without implantation at a current density of . The directionality of RCLEDs with implantation is superior to that of RCLEDs without implantation.

A triphenylamine derivative as an efficient organic light color-conversion material for white LEDs

A novel efficient ( Φ=0.44) organic yellow-emitting dye, 3-(4-(diphenylamino)phenyl)-1-phenylprop-2-en-1-one (DPPO), was synthesized. The compound was characterized by means of 1H NMR and differential scanning calorimetry (DSC) and analyzed by quantum chemistry method. It was found that DPPO could be effectively excited by the InGaN-based blue LED. Photo-durability data of DPPO were studied. Bright white light of Commission International del’ Eclairage (CIE) x=0.30, y=0.33 was obtained by using DPPO as a light color-conversion material (CCM).

Carrier transport studies of dichromatic InGaN‐based LEDs with spacer bandgap dependence

Carrier transport of dichromatic InGaN-based LEDs with AlGaN spacer bandgap dependence has been studied. TREL measurements show that carrier dynamics could be well explained by the combined effects of carrier effective mass, carrier mobility, quantum confinement, and device structures. The experimental results provide important information for device designs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

TAG:Ce3+ Phosphors Prepared by a Novel Sol-combustion Method for Application in InGaN-based White LEDs

Abstract Tb3Al5O12:Ce3+ (TAG:Ce3+) phosphors were prepared by a novel sol-combustion (SC) method, and the samples showed improved luminescent properties than those by conventional solid-state (SS) method. Bright white LEDs were fabricated and the performances of the LEDs confirm that TAG:Ce3+ prepared by SC method is a good phosphor for white LEDs with low color temperature for indoor lighting.

Dibarium Magnesium Diphosphate Yellow Phosphor Applied in InGaN-based LEDs

Abstract An efficient yellow-emitting phosphor, Eu2+-doped Ba2Mg(PO4)2, is reported. Its excitation band is extending from 250 to 450 nm, which is adaptable to the emission band of near-UV LED chips (350–420 nm). Upon 367-nm light excitation, the phosphor exhibits a strong yellow emission centered at 576 nm. A bright yellow LED was fabricated by incorporating the phosphor with an InGaN-based NUV chip. All the characteristics suggest that Ba2Mg(PO4)2:Eu2+ is a good yellow phosphor candidate for creating white light in phosphor converted white LEDs.

Development of Yellow and White LED's Using InGaN-based Multi-Quantum well Structures

In recent years, InGaN-based LED's emitting in the blue and green spectral regions have emerged into widespread use. However, due to fundamental material issues, achieving yellow and red emission from InGaN-based structures has presented a unique challenge. We have developed conditions for the MOCVD growth of a series of InGaN-based MQW devices that emit over a wide spectral range (420nm - 600nm). It has been found that the emission properties of these monochromatic devices depend critically on the growth rate, growth temperature, QW width, barrier growth conditions, and p-type capping layer growth conditions. Additionally, we have demonstrated white electroluminescence through the proper mixing of complementary colors.

Light Enhancement Al2O3Passivation in InGaN/GaN based Blue Light-emitting Diode Lamps

In this study, sputtered thin films were evaluated as a passivation layer in the process of InGaN-based blue LEDs in order to improve the brightness of LED lamps. In terms of packaged LED lamps, lamps with passivation layer emanated higher brightness than those with passivation layer, and LED lamps with 90 nm passivation layer were the brightest among four kinds of lamps. Although lamps with passivation had a slight increase in operating voltage, their brightness was improved about 13.6 % compare to the lamps made of conventional LEDs without the changes of emitting wavelength.