Monolithic White Light-emitting Diode Research Articles

State-of-the-art and prospects for intense red radiation from core\u2013shell InGaN/GaN nanorods

Core–shell nanorods (NRs) with InGaN/GaN quantum wells (QWs) are promising for monolithic white light-emitting diodes and multi-color displays. Such applications, however, are still a challenge because intensity of the red band is too weak compared with blue and green. To clarify this problem, we measured photoluminescence of different NRs, depending on power and temperature, as well as with time resolution. These studies have shown that dominant emission bands come from nonpolar and semipolar QWs, while a broad yellow-red band arises mainly from defects in the GaN core. An emission from polar QWs located at the NR tip is indistinguishable against the background of defect-related luminescence. Our calculations of electromagnetic field distribution inside the NRs show a low density of photon states at the tip, which additionally suppresses the radiation of polar QWs. We propose placing polar QWs inside a cylindrical part of the core, where the density of photon states is higher and the well area is much larger. Such a hybrid design, in which the excess of blue radiation from shell QWs is converted to red radiation in core wells, can help solve the urgent problem of red light for many applications of NRs.

High-color-rendering-index phosphor-free InGaN-based white light-emitting diodes by carrier injection enhancement via V-pits

We herein report the growth of phosphor-free InGaN-based white light-emitting diodes (LEDs) by metalorganic vapor-phase epitaxy. The active region consists of blue and red InGaN quantum wells (QWs). To improve the current injection and generate broadband emission, the V-pit structures in the LEDs were fabricated intentionally before growing the QWs. The monolithic white LEDs emit in the range of 410–770 nm and, by tuning the injection current, can cover correlated color temperature (CCT) values corresponding to warm white, natural white, and cool white. The color-rendering index (CRI) of the white LEDs reaches 88 at an injection current of 10 mA. At an injection current of 30 mA, the white LEDs exhibit the chromaticity coordinates of (0.320 and 0.334) in the Commission Internationale de l'Eclairage 1931 chromaticity diagram, a CRI of 78, and a CCT of 6110 K.

Electrically driven, polarized, phosphor-free white semipolar (20-21) InGaN light-emitting diodes grown on semipolar bulk GaN substrate.

We demonstrate a simple method to fabricate efficient, electrically driven, polarized, and phosphor-free white semipolar (20-21) InGaN light-emitting diodes (LEDs) by adopting a top blue quantum well (QW) and a bottom yellow QW directly grown on (20-21) semipolar bulk GaN substrate. At an injection current of 20 mA, the fabricated 0.1 mm2 size regular LEDs show an output power of 0.9 mW tested on wafer without any backside roughing, a forward voltage of 3.1 V and two emission peaks located at 427 and 560 nm. A high polarization ratio of 0.40 was measured in the semipolar monolithic white LEDs, making them promising candidates for backlighting sources in liquid crystal displays (LCDs). Furthermore, a 3dB modulation bandwidth of 410 MHz in visible light communication (VLC) was obtained in the micro-size LEDs (µLEDs) with a size of 20×20 µm2 and 40×40 µm2, which could overcome the limitation of slow frequency response of yellow phosphor in commercial white LEDs combing blue LEDs and yellow phosphor.

Monolithic Inorganic ZnO/GaN Semiconductors Heterojunction White Light-Emitting Diodes.

Monolithic light-emitting diodes (LEDs) that can generate white color at the one-chip level without the wavelength conversion through packaged phosphors or chip integration for photon recycling are of particular importance to produce compact, cost-competitive, and smart lighting sources. In this study, monolithic white LEDs were developed based on ZnO/GaN semiconductor heterojunctions. The electroluminescence (EL) wavelength of the ZnO/GaN heterojunction could be tuned by a post-thermal annealing process, causing the generation of an interfacial Ga2O3 layer. Ultraviolet, violet-bluish, and greenish-yellow broad bands were observed from n-ZnO/p-GaN without an interfacial layer, whereas a strong greenish-yellow band emission was the only one observed from that with an interfacial layer. By controlled integration of ZnO/GaN heterojunctions with different postannealing conditions, monolithic white LED was demonstrated with color coordinates in the range (0.3534, 0.3710)-(0.4197, 0.4080) and color temperatures of 4778-3349 K in the Commission Internationale de l'Eclairage 1931 chromaticity diagram. Furthermore, the monolithic white LED produced approximately 2.1 times higher optical output power than a conventional ZnO/GaN heterojunction due to the carrier confinement effect at the Ga2O3/n-ZnO interface.

Design analysis of phosphor-free monolithic white light-emitting-diodes with InGaN/ InGaN multiple quantum wells on ternary InGaN substrates

Phosphor-free monolithic white light emitting diodes (LEDs) based on InGaN/ InGaN multiple quantum wells (MQWs) on ternary InGaN substrates are proposed and analyzed in this study. Simulation studies show that LED devices composed of multi-color-emitting InGaN/ InGaN quantum wells (QWs) employing ternary InGaN substrate with engineered active region exhibit stable white color illumination with large output power (∼ 170 mW) and high external quantum efficiency (EQE) (∼ 50%). The chromaticity coordinate for the investigated monolithic white LED devices are located at (0.30, 0.28) with correlated color temperature (CCT) of ∼ 8200 K at J = 50 A/cm2. A reference LED device without any nanostructure engineering exhibits green color emission shows that proper engineered structure is essential to achieve white color illumination. This proof-of-concept study demonstrates that high-efficiency and cost-effective phosphor-free monolithic white LED is feasible by the use of InGaN/ InGaN MQWs on ternary InGaN substrate combined with nanostructure engineering, which would be of great impact for solid state lighting.

White light-emitting diode based on fluorescent SiC

A monolithic white light-emitting diode (LED,) comprising a combination of a fluorescent-SiC (f-SiC) substrate and a nitride-based near-UV LED stack, is proposed. On the basis of the recombination of donor and acceptor pairs, the f-SiC substrate works as a phosphor for visible light emission. By employing the Fast Sublimation Growth Process method, the high-quality f-SiC substrate doped with N and B exhibited a nonradiative carrier lifetime of 55μs and an internal quantum efficiency (IQE) of 40%. With increasing donor and acceptor doping concentrations, a high IQE was estimated even at a high excitation level.

Antireflective sub-wavelength structures for improvement of the extraction efficiency and color rendering index of monolithic white light-emitting diode

We have theoretically investigated the influence of antireflective sub-wavelength structures on a monolithic white light-emitting diode (LED). The simulation is based on the rigorous coupled wave analysis (RCWA) algorithm, and both cylinder and moth-eye structures have been studied in the work. Our simulation results show that a moth-eye structure enhances the light extraction efficiency over the entire visible light range with an extraction efficiency enhancement of up to 26 %. Also for the first time to our best knowledge, the influence of sub-wavelength structures on both the color rendering index (CRI) and the correlated color temperature (CCT) of the monolithic white LED have been demonstrated. The CRI of the monolithic white LED could be improved from 92.68 to around 94 by applying a cylinder structure, and the CCT could be modified in a very large range with appropriate design of the cylinder structure.

Fluorescent SiC and its application to white light-emitting diodes

Fluorescent-SiC (f-SiC), which contains donor and acceptor impurities with optimum concentrations, has high conversion efficiency from NUV to visible light caused by donor-acceptor-pair (DAP) recombination. This material can be used as a substrate for a near UV light-emitting diode (LED) stack, and leads to monolithic white LED device with suitable spectral property for general lighting applications. In this paper, we describe basic technologies of the white LED, such as optical properties of f-SiC substrate, and epitaxial growth of NUV stack on the f-SiC substrate.

Luminescence properties of blue and green dual wavelength InGaN/GaN multi-quantum well light-emitting diode

Blue and green dual wavelength InGaN/GaN multi-quantum well (MQW) light-emitting diode (LED) has wide applications in full color display, monolithic white LED and solid state lighting, etc. Blue and green dual wavelength LEDs, which consist of InGaN strain-reduction layer, green InGaN/GaN MQWand blue InGaN/GaN MQW, were grown by metal-organic chemical vapor deposition (MOCVD), and the luminescence properties of dual wavelength LEDs with different well arrangements were studied by photoluminescence and electroluminescence. The experimental results indicated that well position played an important role on the luminescence evolvement from photoluminescence to electroluminescence.

Monolithic white LED based on AlxGa1−x N/InyGa1−yN DBR resonant-cavity

A monolithic white light-emitting diode (LED) with blue and yellow light active regions has been designed and studied. With the AlxGa1−xN/InyGa1−yN distributed Bragg reflector (DBR) resonant-cavity, the extraction efficiency and power of the yellow light are enhanced so that high quality white light can be obtained.

Monolithic InGaN-based white light-emitting diodes with blue, green, and amber emissions

We demonstrated a monolithic white light-emitting diodes (LEDs) epitaxial structure with blue, green, and amber emissions by introducing the blue InGaN∕GaN five quantum wells (QWs) and InGaN∕GaN single quantum well (SQW) with In-phase separated green/amber emissions as an active layer. Three wavelength emissions were developed by increasing the thickness of InGaN SQW grown on blue InGaN five QWs. From high resolution transmission electron microscope, In-phase separation was clearly observed in a 3.5-nm-thick InGaN SQW. In-phase separation would be generated by the spinodal decomposition which was promoted by the composition pulling effect related to the increment of well thickness. Therefore, white lighting LEDs with three wavelengths for blue emission from InGaN∕GaN five QWs and green/amber emissions were achieved by the In-phase separation in InGaN SQW.