Surface-textured Light-emitting Diodes Research Articles

Efficiency Enhancement of White Light-Emitting Diodes via Nano-Textured Silicone Encapsulant

We textured the surface of a silicone encapsulant to increase the extraction efficiency of white light-emitting diodes (LEDs) by using a plasma treatment. Here, the surface morphology could be controlled by changing the plasma condition and texturing morphology of the silicone encapsulant were proportion to the increased ratio of white LEDs. The luminous efficacy of the surface textured LEDs were increased 9.70% relative to the reference LED. Furthermore, the Fourier transform infrared spectroscopy spectrum showed that the chemical bonds of the silicone encapsulant were not changed by the argon-nitrogen plasma treatment, thereby reducing degradation of the optical characteristics and improving the reliability of LEDs.

Improved Light Extraction in AlGaInP-Based LEDs Using a Roughened Window Layer

Surface-textured light-emitting diodes (LEDs) have been fabricated using a combination of natural lithography and dry etching techniques. Here, the randomly positioned polystyrene spheres are used as a mask for dry etching. The p-GaP window layer was successfully etched to achieve nanopillars on its surface. It was found that the surface-textured LEDs with the p-GaP nanopillars revealing a diameter of , a density of , and a depth of have maximum luminance intensity. Output power of the surface-textured LED increased about 70% compared with the planar-surface LED at a forward current of . The LED window with nanopillar diameter (, half of wavelength) and depth (, ) resulting in much light extraction compared with flat-surface LEDs or deeper nanopillars has been demonstrated by experiment and simulation. After a aging test, luminance intensity and voltage fluctuation do not exceed . These results indicated that AlGaInP-based LEDs with a roughened surface did not exhibit deteriorated performance.

Light Extraction Simulation of Surface-Textured Light-Emitting Diodes by Finite-Difference Time-Domain Method and Ray-Tracing Method

The light extraction efficiency (ηl) of surface-textured light-emitting diodes (LEDs) is calculated using a simulation based on the combination of the finite-difference time-domain (FDTD) method and the ray-tracing method. Considering the transmittance and light-scattering effects, which depend on the size of the surface-textured structure (STS), we investigated the dependence of ηl on the size of the STS including the period and aspect ratio. As a result, a clear dependence on the size of the STS has been shown. Furthermore, by optimizing the size of the STS, we found ηl to be about 4 times larger than that of the surface flat structure (SFS). In addition, we demonstrated that the absorption coefficient of the GaN layer (αGaN) affected ηl of the STS more strongly than that of the SFS. We suggest that both wide light-scattering and a high GaN crystalline quality are important points for improving ηl when texturing the top surface of LEDs.

Light-extraction mechanisms in high-efficiency surface-textured light-emitting diodes

We present a detailed quantitative analysis of the light extraction and loss mechanisms in high-efficiency GaAs-AlGaAs surface-textured thin-film light-emitting diodes (LEDs). The analysis is based on a Monte Carlo simulation. Most input parameters, including scattering of photons at the textured surface, sub-bandgap absorption, and absorption at the metal mirror are obtained from experiments or from literature. The simulation also takes into account the effect of photon recycling and the realistic geometry of the diodes. The only remaining fitting parameter is the internal quantum efficiency, which is deduced to be about 80% at room temperature for the experimentally realized 850-nm LEDs with an external quantum efficiency of 44%. The analysis shows further that the most important loss mechanism is reabsorption in the active layer, and in particular in those parts of the active layer that are not electrically pumped. This conclusion is also valid for other types of high-efficiency LEDs. We could furthermore verify the validity of the Monte-Carlo simulation results by conducting experiments at low temperatures, where nonradiative recombination processes are reduced, resulting in the internal quantum efficiency approaching unity. The measured external quantum efficiency at 90 K is 68%, which is close to the theoretically predicted efficiency for a perfect active layer. The results demonstrate that the light extraction from surface-textured LEDs is fully understood and can be quantitatively modeled by a simple raytracing algorithm.

Impact of texture-enhanced transmission on high-efficiency surface-textured light-emitting diodes

The transmission properties of semiconductor surfaces can be changed by surface texturing. We investigate these changes experimentally and find that an enhancement of the angle-averaged transmission by a factor of 2 can be achieved with optimum texturing parameters. This enhanced transmission provides an additional light extraction mechanism for high-efficiency surface-textured light-emitting diodes. External quantum efficiencies of 46% and 54% are demonstrated before and after encapsulation, respectively.

40% efficient thin-film surface-textured light-emitting diodes by optimization of natural lithography

In conventional light-emitting diodes (LEDs), the external efficiency is limited by total internal reflection at the semiconductor-air interface. The problem can be overcome by the concept of the nonresonant cavity LED, which is an LED with a textured top surface and a rear reflector. The surface is textured using natural lithography. A monolayer of randomly positioned polystyrene spheres acts as a mask for dry etching. We present details about the optimization of the parameters of the texturing process for GaAs/AlGaAs LEDs. The studied parameters are the size of the spheres, the distribution of the spheres on the surface and the etching depth. Using optimized texturing conditions, we have realized un-encapsulated top-emitting oxide-confined GaAs/AlGaAs nonresonant cavity LEDs with an external quantum efficiency of 40%.

Micro-lensed gigabit-per-second high-efficiencyquantum-well light-emitting diodes

The authors present highly efficient low-power surface-textured light-emitting diodes (LEDs) designed for optical communication at bit rates up to 1.3 Gbit/s. LEDs with an external quantum efficiency > 30% are demonstrated to perform at bit rates of 1 Gbit/s. The application of spherical glass micro-lenses improves the angular light output for enhanced coupling into optical fibres.