Light Extraction Of GaN-based Light-emitting Diodes Research Articles

Research on Light Extraction of GaN-based LED with Surface/Interface Coarsening

Light-emitting diodes (LEDs) are a highly versatile light source that have been widely employed in various fields. In order to maximize its quantum efficiency, it is essential to boost the efficiency of light extraction. Internal photon reflection can have a distinct negative impact on the luminous output power of GaN-based LEDs, which can be addressed by surface and interface texturing of different LED layers. This article discusses the mechanisms of light extraction losses in GaN-based LEDs and summarizes various surface and interface coarsening techniques proposed for improving the light extraction rate, including a range of physical and chemical etching methods.

Polymeric Nano-Pyramids Structure for Enhancing Light Extraction in GaN-Based LEDs

A hexagonal pyramids nanostructure made of a UV curable polymer was implemented on the ITO electrode layer of a GaN-based light-emitting diodes (LEDs) to improve the light extraction efficiency. Length of a diagonal of polymeric hexagonal pyramid structure was adjusted from 180 nm to 570 nm and successfully transferred to the top of GaN LEDs. GaN LED with diagonal of 570nm showed transmittance of 82.9% as well as haze of 77.4%, leading to enhancement of light output power by 26% compared to those with flat ITO. The finite-difference time-domain (FDTD) simulation provides the evidence that nanostructures play a critical role in enhancing light output by eliminating total internal reflection as well as by extracting confined waveguide mode in LEDs.

Effects of Mesa Size on Current Spreading and Light Extraction of GaN-Based LEDs

The mesa size effect on light extraction efficiency (LEE) of light-emitting diodes (LEDs) was studied in this work. The mesa area size of three kinds of LEDs that were evaluated include: 350×950 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (small-size embedded electrodes: GaN LED, S-LED), 500×950 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (medium-size embedded electrodes: GaN LED, M-LED), 950×950 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (large-size embedded electrodes : GaN LED, L-LED). This paper not only discusses LEE, but current density and heat dissipation performance as well. The output power and light extraction efficiency at 700 mA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for S-LED, M-LED, and L-LED are 555, 485, and 432 mW and 38.1%, 33.4%, and 29.7%, respectively. The best output power and LEE of S-LED is due to the electron-hole recombination rate increasing. This phenomenon is caused by the greatest current spread and heat dissipation potential.

Less strained and more efficient GaN light-emitting diodes with embedded silica hollow nanospheres

Light-emitting diodes (LEDs) become an attractive alternative to conventional light sources due to high efficiency and long lifetime. However, different material properties between GaN and sapphire cause several problems such as high defect density in GaN, serious wafer bowing, particularly in large-area wafers, and poor light extraction of GaN-based LEDs. Here, we suggest a new growth strategy for high efficiency LEDs by incorporating silica hollow nanospheres (S-HNS). In this strategy, S-HNSs were introduced as a monolayer on a sapphire substrate and the subsequent growth of GaN by metalorganic chemical vapor deposition results in improved crystal quality due to nano-scale lateral epitaxial overgrowth. Moreover, well-defined voids embedded at the GaN/sapphire interface help scatter lights effectively for improved light extraction, and reduce wafer bowing due to partial alleviation of compressive stress in GaN. The incorporation of S-HNS into LEDs is thus quite advantageous in achieving high efficiency LEDs for solid-state lighting.

Micro/nano structures induced by femtosecond laser to enhance light extraction of GaN-based LEDs

Surface texturing has been widely adopted to enhance the light extraction efficiency of light-emitting diodes (LEDs), and chemical etching is a technique commonly used to produce surface texturing. This study employed femtosecond lasers to apply ITO films directly onto the surface of LEDs to generate periodic micro/nanostructures and roughen the surface without contact or chemical substances. As a result, photons emitted in the active region escape into the free space, due to the scattering effect produced by texturing. This study discovered that light-emitting efficiency increases with surface roughness, and achieved an improvement of 18%. Caution regarding laser fluence was required during laser processing to avoid damaging the LED beneath the ITO film, which could detract from the electrical characteristics.

Improved Light Extraction of GaN-based LEDs with Nano-roughened p-GaN Surfaces

p-GaN surfaces are nano-roughened by plasma etching to improve the optical performance of GaN-based light emitting diodes (LEDs). The nano-roughened GaN present a relaxation of stress. The light extraction of the LEDs with nano-roughened surfaces is greatly improved when compared with that of the conventional LEDs without nano-roughening. PL-mapping intensities of the nano-roughened LED epi-wafers for different roughening times present two to ten orders of enhancement. The light output powers are also higher for the nano-roughened LED devices. This improvement is attributed to that nano-roughened surfaces can provide photons multiple chances to escape from the LED surfaces.