Top transmission grating GaN LED simulations for light extraction improvement

Abstract

We study the top transmission grating's improvement on GaN LED light extraction efficiency. We use the finite difference time domain (FDTD) method, a computational electromagnetic solution to Maxwell's equations, to measure light extraction efficiency improvements of the various grating structures. Also, since FDTD can freely define materials for any layer or shape, we choose three particular materials to represent our transmission grating: 1) nonlossy p-GaN, 2) lossy indium tin oxide (ITO), and 3) non-lossy ITO (�=0). We define a regular spacing between unit cells in a crystal lattice arrangement by employing the following three parameters: grating cell period (�), grating cell height (d), and grating cell width (w). The conical grating model and the cylindrical grating model are studied. We also presented in the paper directly comparison with reflection grating results. Both studies show that the top grating has better performance, improving light extraction efficiency by 165%, compared to that of the bottom reflection grating (112%), and top-bottom grating (42%). We also find that when grating cells closely pack together, a transmission grating maximizes light extraction efficiency. This points our research towards a more closely packed structure, such as a 3-fold symmetric photonic crystal structure with triangular symmetry and also smaller feature sizes in the nano-scale, such as the wavelength of light at 460 nm, half-wavelengths, quarter wavelengths, etc.

Further enhancement of light extraction efficiency from light emitting diode using triangular surface grating and thin interface layer

“…Random surface texturing or roughening [3][4][5], periodic surface modification in the form of nanopyramids [6] or surface gratings [7], and exploiting photonic crystals (PCs) [8][9][10][11][12][13] or surface plasmon resonance [14,15] are some of the interesting approaches aimed at increasing the LEE of LEDs. Interesting reports [16] are also made on improving the efficiency by using top transmission gratings only.…”

Scale-Dependent Light Scattering Analysis of Textured Structures on LED Light Extraction Enhancement Using Hybrid Full-Wave Finite-Difference Time-Domain and Ray-Tracing Methods

“…Therefore, to substantially enhance the LEE of the LED, various textured structures have been proposed to avoid the occurrence of total internal reflection, including rough surfaces, patterned substrates, diffraction gratings, and photonic crystals. Experiments have proven that these textured structures can enhance the LEE, and that they all appear to function in the same manner by using light scattering effects to increase the probability of photon escape [3][4][5][6][7][8][9]. However, because most of the research is conducted using trial and error in the manufacturing process, it is not easy to obtain the optimal structural parameters; nor is it possible to further understand the impact of the light scattering factor on the LEE.…”

Development of a cost effective surface-patterned transparent conductive coating as top-contact of light emitting diodes

“…A transparent polymer can be affixed over the device to create a larger escape angle to air. 8 Another way to improve light output is to roughen or to pattern transmission grating 5,[9][10][11][12][13] on the emission surface that offers the trapped light more angles of escape. Grating structures of various shapes like pyramidal, spherical, conical, sinusoidal, cylindrical, and so on can be studied, but only a few can be fabricated with great success.…”