External Quantum Efficiency Of Light-emitting Diodes Research Articles

Three-Method Hybrid Numerical Simulation for Surface-Plasmon-Enhanced GaInN-Based Light-Emitting Diodes with Metal-Embedded Nanostructures

In this paper, a hybrid numerical simulation tool is introduced and performed for GaInN-based light-emitting diodes (LEDs) with metal-embedded nanostructure to theoretically predict external quantum efficiency (EQE), which composed of finite-difference time-domain, rigorous coupled wave analysis, and ray tracing. The advantage is that the proposed method provides results supported by sufficient physical background within a reasonable calculation time. From the simulation results, the EQE of LED with Ag-nanoparticles embedded nanostructure is expected to be enhanced by as high as ∼1.6 times the conventional LED device in theory.

(Invited) The Effects of Zinc-Tin-Oxide Nanosphere Monolayer on Improvement of Light Extraction in Light-Emitting Diodes

Over the last two decades, III-nitride-based light-emitting diodes (LEDs) have been widely used for our daily life, such as illumination, various backlight units for full-color displays, and automotive headlamps. In spite of the versatile advantages of GaN-LEDs, relatively low external quantum efficiency is still a problem to expand their application fields. For the viable solution to unravel pending issues in solid state light-emitting research fields, many efforts have been conducted. In this study, we report on the applications of zinc-tin-oxide (ZTO) nanoparticles and monolayer as a light extraction layer for GaN-based LEDs. The ZTO layers formed on top of an LED epi-structure with a variable Sn-ratio, which was deposited by the spin coating method. The transmission spectra of the ZTO layers with Zn to Sn ratios of 1:1 (ZTO-I) and 1:5 (ZTO-II) exhibited optical transmittances of 98% and 88% in the visible region, respectively. The electroluminescence (EL) and light power-current-voltage (L-I-V) measurements show that double ZTO layers consisting of various Zn to Sn ratios led to enhanced light extraction from blue LEDs. Finally, we will discuss the effects of ZTO monolayer transferred onto the p-GaN surface on the external quantum efficiency of LEDs. The ZTO monolayers were prepared by using the ZTO nanospheres synthesized by precipitation solution method. The transferred ZTO nanosphere monolayers on top surface of LED are very effective way to improve the light extraction efficiency.

The relationship between the defectness of emitting nanoheterostructures of green InGaN/GaN LEDs and their threshold current values

It is shown that the threshold current of green InGaN/GaN light-emitting diodes (LEDs) can be used for evaluating quality of their light-emitting nanoheterostructures.. The threshold current exhibits correlation with the value of operating current corresponding to the maximum of the external quantum efficiency of LEDs. It has been found that LEDs with high threshold currents tested in the dc regime exhibit faster degradation than devices with lower threshold currents.

Investigating the origin of efficiency droop by profiling the temperature across the multi-quantum well of an operating light-emitting diode

Performance degradation resulting from efficiency droop during high-power operation is a critical problem in the development of high-efficiency light-emitting diodes (LEDs). In order to resolve the efficiency droop and increase the external quantum efficiency of LEDs, the droop's origin should be identified first. To experimentally investigate the cause of efficiency droop, we used null-point scanning thermal microscopy to quantitatively profile the temperature distribution on the cross section of the epi-layers of an operating GaN-based vertical LED with nanoscale spatial resolution at four different current densities. The movement of temperature peak towards the p-GaN side as the current density increases suggests that more heat is generated by leakage current than by Auger recombination. We therefore suspect that at higher current densities, current leakage becomes the dominant cause of the droop problem.

Pattern Design of and Epitaxial Growth on Patterned Sapphire Substrates for Highly Efficient GaN-Based LEDs

This work represents a cost and time effective approach for pattern design of patterned sapphire substrates (PSS) for highly efficient GaN-based light emitting diodes (LEDs). Simulation is used to study how external quantum efficiency changes with the change in parameters of the unit hemisphere for LEDs fabricated on hemispherical PSS. Through a series of comparisons on simulation results, the most effective pattern to improve external quantum efficiency of LEDs on hemispherical PSS is revealed. The subsequent crystal growth of LED wafers demonstrates that both photoluminescence and electroluminescence intensities dramatically increase by about a half for LEDs grown on this pattern-optimized PSS compared to that of LEDs on non-PSS, which straight-forwardly proves the high efficiency of the optimized hemispherical patterns of PSS for improving LED efficacy.

Improvement of GaN-based light-emitting diodes using p-type AlGaN/GaN superlattices with a graded Al composition

We investigated the effect of graded Al composition in the p-type AlGaN/GaN superlattices (SLs) of InGaN/GaN multiple quantum well light-emitting diodes (LEDs) to improve their performance. The light output power and external quantum efficiency (EQE) of LEDs with Al composition grading was increased compared with those of LEDs without Al grading, indicating that the efficiency droop was reduced. The improved output power and EQE of LEDs with a graded Al composition was attributed to the increased hole injection by the reduced AlGaN barrier height and the suppression of potential spikes between the graded AlGaN and GaN layers in SLs.

Enhancing extraction efficiency from GaN-based LED by using an omni-directional reflector and photonic crystal

To enhance the light extraction efficiency of traditional light-emitting diodes (LEDs) by reducing the total internal reflection, an omni-directional reflector (ODR) and photonic crystal are adopted in the paper. The structures of photonic crystal and the ODR are designed by diffraction theory and finite difference time domain (FDTD) method. The photonic crystal is employed in the p-GaN layer and the ODR composed of TiO2/SiO2 is designed between the active region and substrate. The simulation results indicate that the light extraction of LEDs can be enhanced by 11.6 times, and the external quantum efficiency of LEDs will be effectively improved.

Features of the recombination processes in InGaN/GaN based LEDs at high densities of injection current

It is established that the low-frequency noise density S in InGaN/GaN based light-emitting diodes (LEDs) operating a current densities j > 20 A/cm2 depends on the current density as S ∼ j3. This dependence is indicative of the formation of new nonradiative recombination centers, which may account for a drop in the external quantum efficiency of LEDs operating at high current densities.

Dependence of SiC Blue Light-Emitting Diode Efficiency on the p-Type Layer Growth Temperature

Dependence of SiC blue light-emitting diode (LED) efficiency on the p-type layer growth temperature in the liquid-phase epitaxial process was investigated. The external quantum efficiency of LEDs fabricated at a lower growth temperature was higher than that of those fabricated at a conventional growth temperature of 1720° C and maximized at a growth temperature of 1530° C. In this study, luminous intensity of 32 mcd was obtained at a forward current of 20 mA (λ P=467 nm).