Near-ultraviolet Light-emitting Diodes Research Articles

Broadband near-ultraviolet excited La2Mo2O9:Eu3+ red-emitting phosphors with high color purity for solid-state lighting

A series of different Eu3+ ions incorporated La2Mo2O9 phosphors exhibiting photoluminescence (PL) peaks in the red region (613 nm; 5D0→7F2) were successfully synthesized by a citrate sol-gel method. Under the 351 nm excitation, the optimal doping concentration of Eu3+ ions was identified to be 25 mol% with higher PL emission intensity and further the concentration quenching mechanism was governed by electric diploe-diploe interaction. More importantly, the optimized phosphor was demonstrated high color purity while the Commission Internationale de I'Eclairage chromaticity coordinates were (0.661, 0.339). The intrinsic radiative transition lifetime for the optimized red phosphor was calculated to be 530.7 μs and the local structure environment of Eu3+ ions in the La2Mo2O9 host matrix was also analyzed by the Judd-Ofelt theory. Moreover, a red-emitting light-emitting diode (LED) device was fabricated by combing a near-ultraviolet LED chip with the optimized red phosphor, which exhibits a dazzling red light. Based on the obtained results, the Eu3+ ions activated La2Mo2O9 phosphors are expected to be as a promising red-emitting phosphor for solid-state lighting.

CaAl2Si2O8: Ce3+, Tb3+: A novel high-efficiency luminescent material for white light-emitting diodes

A novel rare earth activated phosphor Ca1-x-yAl2Si2O8:xCe3+, yTb3+ were prepared under a weak reducing atmosphere by conventional solid-state reaction method and their luminescence properties under 351 nm excitation were investigated in detail. The results show that the CaAl2Si2O8: Tb3+ shows excited at short-UV region, the long-UV absorption peaks is very weak, which cannot be effectively excited by near ultraviolet light-emitting diodes (NUV-LEDs). However, by incorporation of Ce3+, in the excitation spectra, strong and broad excitation bands at ∼ 351nm were observed, which could well match with the emission wavelength of the NUV-LEDs chip. Simultaneity, the green characteristic emission of Tb3+ is obviously enhanced, and the emission intensity of CaAl2Si2O8:Ce3+, Tb3+ phosphor is 10 times higher than that of the green phosphor CaAl2Si2O8:Tb3+ phosphor. The results indicates an efficient energy transfer from Ce3+ to Tb3+ was found and the energy transfer mechanism of Ce3+ →Tb3+ was studied in detail.

Al2O3/AlN/Al-based backside diffuse reflector for high-brightness 370-nm AlGaN ultraviolet light-emitting diodes

We report an Al2O3/AlN/Al-based backside diffuse reflector for near-ultraviolet light-emitting diodes (NUV LEDs), and its superiority compared with conventional reflectors. After fabrication of LEDs, a sapphire (Al2O3) surface was UV-irradiated to utilize the diffuse reflection effect by roughening the surface. Then, a thermally treated AlN layer was deposited on it, to obtain a mirror-like surface, before deposition of Al. The reflectance increased by 1.3% and 3.9% for these Al2O3/AlN/Al reflectors without and with surface treatment, respectively, compared with an Al reflector. These reflectors were then used with 370 nm AlGaN NUV LEDs, and the performances of LEDs without and with an Al reflector were compared. As a result, the NUV LED with both thermally and UV-treated AlN/Al reflector exhibited the best performance, and its output power and electroluminescence intensity were higher by 82.7% and 41.9%, respectively, than those of the LED without a reflector.

Suppression of unintentional carbon incorporation in AlGaN-based near-ultraviolet light-emitting diode grown on Si

AlGaN-based near-ultraviolet light-emitting diodes (NUV-LEDs) emitting at 370 nm were grown on Si(111) substrates by metal-organic chemical vapor deposition. The effect of growth parameters of Si-doped n-type AlGaN thick layer on the material quality and optical performance was studied. Photoluminescence measurements showed that the near-band-edge emission of n-AlGaN was greatly increased and the yellow luminescence (YL) was substantially reduced, when the n-AlGaN layer was grown at a high temperature, a high chamber pressure, and a low growth rate. It was found that the reduced unintentional carbon incorporation in the n-AlGaN layer under those growth conditions was responsible for the improved optical property. The NUV-LED employing the optimized growth parameters of n-AlGaN showed an enhanced light output power and a suppressed YL emission, as well as a better color purity, as compared with the reference one. The results indicate that performance of NUV-LED can be significantly improved by suppressing unintentional carbon incorporation and the defects-related absorption/re-emission in the n-AlGaN.

InGaN/GaN ultraviolet LED with a graphene/AZO transparent current spreading layer

We report an approach of using an interlayer of single layer graphene (SLG) for electroluminescence (EL) enhancement of an InGaN/GaN-based near-ultraviolet (NUV) light-emitting diode (LED) with an aluminum-doped zinc oxide (AZO)-based current spreading layer (CSL). AZO-based CSLs with and without a SLG interlayer were fabricated on the NUV LED epi-wafers. The current-voltage (I-V) characteristic and the EL intensity were measured and compared. We find that the LED without the SLG interlayer can possess a 40% larger series resistance. Furthermore, a 95% EL enhancement was achieved by the employment of the SLG interlayer.

Reverse leakage current characteristics of InGaN/GaN multiple quantum well ultraviolet/blue/green light-emitting diodes

We investigated the reverse leakage current characteristics of InGaN/GaN multiple quantum well (MQW) near-ultraviolet (NUV)/blue/green light-emitting diodes (LEDs). Experimental results showed that the NUV LED has the smallest reverse leakage current whereas the green LED has the largest. The reason is that the number of defects increases with increasing nominal indium content in InGaN/GaN MQWs. The mechanism of the reverse leakage current was analyzed by temperature-dependent current–voltage measurement and capacitance–voltage measurement. The reverse leakage currents of NUV/blue/green LEDs show similar conduction mechanisms: at low temperatures, the reverse leakage current of these LEDs is attributed to variable-range hopping (VRH) conduction; at high temperatures, the reverse leakage current of these LEDs is attributed to nearest-neighbor hopping (NNH) conduction, which is enhanced by the Poole–Frenkel effect.

Luminescent properties of novel red-emitting M7 Sn(PO4 )6 :Eu3+ (M = Sr, Ba) for light-emitting diodes.

Novel red-emitting phosphors, Eu3+ -activated M7 Sn(PO4 )6 (M = Sr, Ba), were synthesized at 1200°C by conventional solid-state reaction method. The luminescent properties of M7 Sn(PO4 )6 :Eu3+ (M = Sr, Ba) phosphors were investigated, and the critical concentration of the activator (Eu3+ ) concentration were found to be 0.175mol and 0.21mol per formula unit for Sr7-x Sn(PO4 )6 :xEu3+ and Ba7-x Sn(PO4 )6 :xEu3+ , respectively. These phosphors presented red luminescence under the excitation of 395 or 465nm, perfectly matching with the emissions wavelength of near-ultraviolet (UV) light-emitting diodes (LEDs) and InGaN blue LED.

Luminescence properties and energy transfer of co-doped Ba3GdNa(PO4)3F:Ce3+,Tb3+ green-emitting phosphors

Phase pure Ce3+ and Tb3+ singly doped and Ce3+/Tb3+ co-doped Ba3GdNa(PO4)3F samples have been synthesized via the high temperature solid-state reaction. The crystal structures, photoluminescence properties, fluorescence lifetimes, thermal properties and energy transfer of Ba3GdNa(PO4)3F:Ce3+,Tb3+ were systematically investigated. Rietveld structure refinement indicates that Ba3GdNa(PO4)3F crystallizes in a hexagonal crystal system with the space group P-6. For the co-doped Ba3GdNa(PO4)3F:Ce3+,Tb3+ samples, the emission color can be tuned from blue to green by varying the doping concentration of the Tb3+ ions. The intense green emission was realized in the Ba3GdNa(PO4)3F:Ce3+,Tb3+ phosphors on the basis of the highly efficient energy transfer from Ce3+ to Tb3+. Also the energy transfer mechanism has been confirmed to be quadrupole–quadrupole interaction, which can be validated via the agreement of critical distances obtained from the concentration quenching (13.84 A). These results show that the developed phosphors may possess potential applications in near-ultraviolet pumped white light-emitting diodes.

Preparation and photoluminescence properties of red-emitting phosphor ZnAl2O4:Eu3+ with an intense 5D0 → 7F2 transition

A series of ZnAl2−xO4:xEu3+ phosphors was synthesized by a modified co-precipitation method. The phase structure and photoluminescence properties were examined and extensively discussed. Composition-optimized ZnAl1.97O4:0.03Eu3+ exhibited sharp, intense red characteristic emissions with the Commission Internationale de l’Eclairage coordinates of (0.62, 0.33) peaking at 620 nm under an excitation at 394 nm, corresponding to the 5D0 → 7F2 transition of Eu3+. The quenching concentration of Eu3+ in ZnAl2−xO4:xEu3+ phosphors was approximately 0.03. In addition, the concentration quenching mechanism, fluorescence decay curves, and thermally stable luminescence properties of ZnAl2–xO4:xEu3+ phosphors were investigated. The results indicated that ZnAl2–xO4:xEu3+ phosphors demonstrate potential applications in near-ultraviolet light-emitting diode pumped white light lamps.

Performance enhancement of AlGaN-based 365 nm ultraviolet light-emitting diodes with a band-engineering last quantum barrier.

In this Letter, the characteristics of the AlGaN-based near-ultraviolet light-emitting diodes with a band-engineering last quantum barrier (LQB) were analyzed experimentally and numerically. The experimental results show that the peak wavelengths of UV-LEDs are around 368nm with a full width at half-maximum of 12-14nm, and the optical and electrical properties are improved by using an AlxGa1-xN LQB with a gradually decreasing Al content. The designed LQB can reduce the forward voltage from 4.35 to 4.29V and markedly enhance LOP by 47.4% at an injection current of 200mA, compared with the original structure. These improvements are mainly attributed to less electron leakage and higher hole injection efficiency, resulting from the weakened polarization field in the electron-blocking layer (EBL) and LQB, as well as the alleviation of the band bending at the EBL/LQB interface.

A novel Sm3+ singly doped LiCa3ZnV3O12 phosphor: a potential luminescent material for multifunctional applications.

The Sm3+ ion singly doped LiCa3ZnV3O12 (LCZV) phosphors were synthesized by a traditional high-temperature solid-state method. The luminescence characteristics of the as-prepared samples were studied by photoluminescence excitation and emission spectra. Under 343 nm excitation, the synthesized phosphors exhibited color controllable emission induced by the efficient energy transfer from VO43− groups to Sm3+ ions. Besides, the energy transfer efficiency was revealed to be about 58.4% when the dopant concentration was 4 mol%. By utilizing the as-prepared LCZV:Sm3+ phosphors, commercial BaMgAl10O7:Eu2+ blue-emitting phosphors and a near-ultraviolet light-emitting diode (LED) chip, a white LED device was fabricated. Under an injection current of 100 mA, the packaged LED device emitted pure white light with high color rendering index (88.4) and proper correlated color temperature (4320 K). By analyzing the thermal quenching behavior of the VO43− groups and Sm3+ ions, the optical thermometric behavior of the Sm3+-doped LCZV compounds was investigated. The maximum absolute sensor sensitivity and relative sensor sensitivity were found to be 0.25 K−1 and 1.8% K−1, respectively. Additionally, the emitting color of the studied samples was dependent on the temperature. Ultimately, the Sm3+-doped LCZV phosphors were potential candidates for indoor illumination, optical thermometry and safety signals in high temperature environments.

High-performance vertical GaN-based near-ultraviolet light-emitting diodes on Si substrates

High-performance vertical GaN-based near-ultraviolet (UV) light-emitting diodes (LEDs) on Si substrates with an electroluminescence emission wavelength of 395 nm have been fabricated by designing epitaxial structures to reduce the dislocation density and enhance the electron confinement and hole injection.

Tunable luminescent spectra via energy transfers between different lattice sites in Ce3+, Mn2+ codoped Ba9Lu2Si6O24 phosphors for NUV-based warm white LED applications

A series of Ce3+–Mn2+ codoped Ba9Lu2Si6O24 (BLS) were synthesized by high-temperature solid-state reactions. The 380–410 nm excitation band of Ce3+ at the Lu sites (Ce(1)) matches well with the emission light of commercial near-ultraviolet (NUV) light-emitting diode (LED) chips. Under the Ce(1) excitation, BLS:Ce3+, Mn2+ exhibited a tunable emission from blue–green to yellow–orange via energy transfers (ETs) from Ce(1) to Mn2+. The ET was demonstrated to be of the resonant type via a dipole–quadrupole mechanism. At room temperature (RT), the optimal internal and external quantum efficiencies (QEs) of BLS:Ce3+, Mn2+ were determined as 79 and 42%. At 150 °C, 85% of the RT QE still can be remained, showing a high thermal stability. A warm white LED (WLED) with a color rendering index of 84 and a correlated color temperature of 3660 K was obtained by combining a 395 nm NUV chip with the phosphor and CaAlSiN3:Eu2+. The efficiency reaches 17 lm W−1 at 20 mA, which is better than the value of most NUV-based WLEDs. These results indicate the promising application of the phosphor as an attractive candidate for WLEDs.

Facile synthesis of bifunctional Eu3+-activated NaBiF4 red-emitting nanoparticles for simultaneous white light-emitting diodes and field emission displays

A series of novel Eu3+-activated NaBiF4 nanoparticles were synthesized by an ultra-fast chemical precipitate method at room temperature. Under the excitation of 394 nm, all the compounds emitted glaring red emissions originating from the intra-4f transitions of Eu3+ ions. The optimal doping concentration was found to be 40 mol% and the concentration quenching mechanism was dominated by dipole–dipole interaction. The temperature-dependent photoluminescence emission spectra revealed that the resultant nanoparticles possessed admirable thermal stability with an activation energy of 0.237 eV. The internal quantum efficiency of the synthesized nanoparticles was estimated to be as high as 73.1%. Furthermore, the optical transition parameters of Eu3+ ions were calculated using a facile method based on the Judd-Ofelt theory to explore the local structure environment around the Eu3+ ions in the NaBiF4 host lattice. Additionally, the packaged light-emitting diode (LED) device, which was composed of a near-ultraviolet LED chip, synthesized nanoparticles and commercial blue-emitting/green-emitting phosphors, exhibited brightly white light with high color rendering index of 88.2 and proper correlated color temperature of 6851 K. Ultimately, the studied nanoparticles also had splendid cathodoluminescence properties. These results suggest that the Eu3+-activated NaBiF4 nanoparticles were promising bifunctional materials for simultaneous white LEDs and field emission displays.

Nanocomposite Phosphor Consisting of CaI2:Eu2+ Single Nanocrystals Embedded in Crystalline SiO2.

High luminescence efficiency is obtained in halide- and chalcogenide-based phosphors, but they are impractical because of their poor chemical durability. Here we report a halide-based nanocomposite phosphor with excellent luminescence efficiency and sufficient durability for practical use. Our approach was to disperse luminescent single nanocrystals of CaI2:Eu2+ in a chemically stable, translucent crystalline SiO2 matrix. Using this approach, we successfully prepared a nanocomposite phosphor by means of self-organization through a simple solid-state reaction. Single nanocrystals of 6H polytype (thr notation) CaI2:Eu2+ with diameters of about 50 nm could be generated not only in a SiO2 amorphous powder but also in a SiO2 glass plate. The nanocomposite phosphor formed upon solidification of molten CaI2 left behind in the crystalline SiO2 that formed from the amorphous SiO2 under the influence of a CaI2 flux effect. The resulting nanocomposite phosphor emitted brilliant blue luminescence with an internal quantum efficiency up to 98% upon 407 nm violet excitation. We used cathodoluminescence microscopy, scanning transmission electron microscopy, and Rietveld refinement of the X-ray diffraction patterns to confirm that the blue luminescence was generated only by the CaI2:Eu2+ single nanocrystals. The phosphor was chemically durable because the luminescence sites were embedded in the crystalline SiO2 matrix. The phosphor is suitable for use in near-ultraviolet light-emitting diodes. The concept for this nanocomposite phosphor can be expected to be effective for improvements in the practicality of poorly durable materials such as halides and chalcogenides.

Sol-gel derived barium orthovanadate phosphors for white light-emitting diodes

The self-luminescent Ba3(VO4)2 materials were synthesized by a sol-gel method. The structural and morphological properties of the prepared materials were analyzed by X-ray diffraction (XRD) pattern and scanning electron microscope images. The XRD pattern shows that the sample crystallized in the rhombohedral phase with a space group of R3¯ m (166). The XRD pattern is consistent with the Joint Committee on Powder Diffraction Standards card values without any other impurity peaks. The self-luminescent properties of the samples were investigated by measuring the photoluminescence (PL) excitation and emission spectra by using a spectrofluorometer. The PL spectra revealed that the materials can give visible bluish-green emission under ultraviolet irradiation. The materials were further employed in making near-ultraviolet excitable white light-emitting diodes. The Ba3(VO4)2 phosphor along with a red emitting phosphor was mixed in silicone epoxy and applied to the near-ultraviolet light-emitting diode to obtain a bright white-light emission with a color rendering index of 80 and a correlated color temperature of 4075 K.

Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors with ultrabroad excitation band for white light-emitting diodes

A series of novel Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors have been successfully prepared by a citrate-assisted sol-gel process. Both photoluminescence excitation and emission spectra suggest that the resultant products have the strong ultrabroad absorption band ranging from 220 to 450 nm. Under the excitation of 379 nm, the characteristic emissions of Eu3+ ions corresponding to the 5D0 → 7FJ transitions are observed in the doped samples. The optimal doping concentration for Eu3+ ions is found to be 12 mol% and the quenching mechanism is attributed to the dipole-dipole interaction. A theoretical calculation based on the Judd-Ofelt theory is carried out to explore the local structure environment around the Eu3+ ions. The studied samples exhibit a typical thermal quenching effect with a T0.5 value of 338 K and the activation energy is determined to be 0.427 eV. A near-ultraviolet (NUV)-based white light-emitting diode (LED) is packaged by integrating a mixture of resultant phosphors, commercial blue-emitting and green-emitting phosphors into an NUV LED chip. The fabricated LED device emits glaring white light with high color rendering index (84.6) and proper correlated color temperature (6492 K). These results demonstrate that the Eu3+-activated La2MoO6-La2WO6 compounds are a promising candidate for indoor lighting as red-emitting phosphors.

Enhanced Light Output of Near-Ultraviolet LEDs With Ta2O5/SiO2 Hybrid DBR Reflector

We designed and fabricated AlGaN-based near-ultraviolet light-emitting diodes (LEDs) operating at 368 nm with high output power using Ta2O5/SiO2 hybrid distributed Bragg reflectors (DBRs). The output powers of LEDs with eight-pair DBR and hybrid-DBR configurations were increased by 18% and 25% when compared with that of an LED without any DBR. The reflectance of the eight-pair DBR and hybrid-DBR were as high as 99%. In addition, the reflectance of the fabricated hybrid-DBR showed a slight dependence on the incident light wavelength and the angle of incidence. In comparison to the eight-pair DBR, the bandwidth of the hybrid-DBR’s reflectance band was broadened by up to 74 nm. Under current injection at 350 mA, output powers of 88, 104, and 110 mW were measured for LEDs without any DBR reflector, with the eight-pair DBR, and with the hybrid-DBR, respectively.

Effect of device package on optical, spectral, and thermal properties of InGaN/GaN near-ultraviolet lateral light-emitting diodes

We investigated the package effect on the temperature-dependent optical and spectral characteristics of InGaN/GaN near-ultraviolet (NUV) lateral light-emitting diodes (LLEDs) on the metal heatsink (MH) and package (PKG) in the injection current range of 0 - 500 mA at 298 and 358 K. For the NUV LLEDs on the MH, the device characteristics reflected directly its chip performance. For the NUV LLEDs on the PKG, the rapidly varied spectral shift as well as the reduced device efficiency was observed due to the increased number of layers with relatively low thermal conductivities. The junction temperature (T j ) and thermal resistance of the NUV LLEDs on the PKG were also significantly increased compared to the NUV LLEDs on the MH. The three-dimensional heat transfer simulations for both the devices were carried out to obtain the temperature distributions by finite element method. The theoretically calculated T j values showed a good agreement with the experimentally measured T j values.

High-performance metal mesh/graphene hybrid films using prime-location and metal-doped graphene

We introduce high-performance metal mesh/graphene hybrid transparent conductive layers (TCLs) using prime-location and metal-doped graphene in near-ultraviolet light-emitting diodes (NUV LEDs). Despite the transparency and sheet resistance values being similar for hybrid TCLs, there were huge differences in the NUV LEDs’ electrical and optical properties depending on the location of the graphene layer. We achieved better physical stability and current spreading when the graphene layer was located beneath the metal mesh, in direct contact with the p-GaN layer. We further improved the contact properties by adding a very thin Au mesh between the thick Ag mesh and the graphene layer to produce a dual-layered metal mesh. The Au mesh effectively doped the graphene layer to create a p-type electrode. Using Raman spectra, work function variations, and the transfer length method (TLM), we verified the effect of doping the graphene layer after depositing a very thin metal layer on the graphene layers. From our results, we suggest that the nature of the contact is an important criterion for improving the electrical and optical performance of hybrid TCLs, and the method of doping graphene layers provides new opportunities for solving contact issues in other semiconductor devices.